The Chemistry of Tris(phosphino)borate Supported Iron-Nitrogen Multiply-Bonded Linkages Citation Brown, Steven Douglas (2005) The Chemistry of Tris(phosphino)borate Supported Iron-Nitrogen Multiply-Bonded Linkages. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/46DN-YH09. https://resolver.caltech.edu/CaltechETD:etd-05312005-201150 Abstract The metallation of FeX 2 (X = Cl, Br, I) salts with the strong-field [PhBP 3 ] ([PhBP 3 ] = PhB(CH 2 PPh 2 ) 3 - ) ligand is presented. The resulting four-coordinate, 14-electron species, [PhBP 3 ]FeX, have been thoroughly characterized and feature high-spin ( S = 2) electronic ground-states. X-ray diffraction analysis of [PhBP 3 ]FeCl establishes a monomeric structure in the solid state. The one electron reduction of [PhBP 3 ]FeCl in the presence of a triphenylphosphine cap affords a rare example of four-coordinate iron(I). This species, [PhBP 3 ]Fe(PPh 3 ), serves as a synthetic surrogate to a low-valent "[PhBP 3 ]Fe(I)" subunit that is readily oxidized in the presence of organic azides. The resulting S = 1/2 iron(III) imides of general formula [PhBP 3 ]Fe≡NR may be subsequently reduced by one electron to yield the anionic S = 0 derivatives. Exposure of the former to an atmosphere of CO results in cleavage of the Fe≡NR linkage to yield [PhBP 3 ]Fe(CO) 2 and free isocyanate (O=C=N-R). Dicarbonyl [PhBP 3 ]Fe(CO) 2 is itself an imide precursor and is gradually converted back to [PhBP3]Fe≡NR upon exposure to excess organic azide. Tolyl imide [PhBP 3 ]Fe?N- p -tolyl readily reacts with H 2 under mild conditions to undergo a step-wise Fe-N x bond scission process to ultimately release free p -toluidine. Initially formed is the S = 2 iron(II) anilide, [PhBP 3 ]Fe(N(H)- p -tolyl), which has been independently prepared and shown to release p -toluidine in the presence of H 2 . In benzene solvent the final iron containing product of the hydrogenation process is diamagnetic [PhBP 3 ]Fe(? 5 -cyclohexadienyl), which is presumably formed from benzene insertion into a low-valent iron-hydride intermediate. Reduction of the ferromagnetically coupled dimer, {[PhBP 3 ]Fe(N 3 )} 2 , yields the bridging nitride species, [{[PhBP 3 ]Fe} 2 ( μ -N)][Na(THF) 5 ]. This compound features two high-spin iron(II) metal centers that are so strongly antiferromagnetically coupled that a diamagnetic S = 0 ground-state is exclusively populated at room temperature. X-ray diffraction analysis reveals a bent Fe-N-Fe linkage that quantitatively releases ammonia in the presence of excess protons. Reactivity with CO and H 2 is also presented, and for the latter, complete rupture of the Fe-N-Fe manifold is not observed as the presence of an additional metal center (when compared with the iron(III) imides) favors the formation of the diamagnetic bridging imide-hydride species, [{[PhBP 3 ]Fe} 2 ( μ -NH)( μ -H)][Na(THF) 5 ]. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Iron; Multiple Bonds; Multiply Bonded Ligands; Nitrogen Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Peters, Jonas C. Thesis Committee: Stoltz, Brian M. (chair) Gray, Harry B. Bercaw, John E. Peters, Jonas C. Defense Date: 24 May 2005 Record Number: CaltechETD:etd-05312005-201150 Persistent URL: https://resolver.caltech.edu/CaltechETD:etd-05312005-201150 DOI: 10.7907/46DN-YH09 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 2330 Collection: CaltechTHESIS Deposited By: Imported from ETD-db Deposited On: 01 Jun 2005 Last Modified: 08 Nov 2023 00:44 Thesis Files Preview PDF (ThesisCh0.pdf) - Final Version See Usage Policy. 306kB Preview PDF (ThesisCh1.pdf) - Final Version See Usage Policy. 270kB Preview PDF (ThesisCh2.pdf) - Final Version See Usage Policy. 1MB Preview PDF (ThesisCh3.pdf) - Final Version See Usage Policy. 516kB Preview PDF (ThesisCh4.pdf) - Final Version See Usage Policy. 1MB Preview PDF (ThesisCh5.pdf) - Final Version See Usage Policy. 838kB Repository Staff Only: item control page

The Chemistry of Tris(phosphino)borate Supported Iron-Nitrogen Multiply-Bonded Linkages Thesis by Steven Douglas Brown In partial fulfillment of the requirements for the degree of Doctor of Philosophy California Institute of Technology Pasadena, California 2005 (Defended May 24, 2005) ii © 2005 Steven Douglas Brown All Rights Reserved iii Acknowledgements It seems surreal that the opportunity for me to acknowledge those individuals who have helped me throughout my graduate experience has finally arrived. First and foremost I must thank my graduate advisor, Professor Jonas C. Peters, whose intensity and passion for inorganic chemistry was apparent from our very first phone conversation. The dedication and standard of excellence that he demanded from me were crucial for achieving my goals of a productive and timely graduate career, and for that I am indebted to him. I also owe a great deal of gratitude to the remaining members of my committee, Professor John E. Bercaw, Professor Brian M. Stoltz, and Professor Harry B. Gray. Each of these individuals played an integral role in keeping me focused on my scientific endeavors—especially during my second year. I would also like to thank Professor Thomas G. Richmond at the University of Utah for exposing me to inorganic chemistry as an undergraduate student, and for personally teaching me the techniques required for the manipulation of air sensitive compounds. I couldn’t have asked for a better group of co-workers than those with whom I have been fortunate enough to share 304 Noyes. Each day with these individuals was always a new adventure in which at least five minutes of my day were guaranteed to be devoted to laughter. To the senior members of the lab, Dr. J. Christopher Thomas, Dr. Cora MacBeth, Dr. Bruce MacKay, and Dr. Mark Mehn, I thank you for sharing your scientific intellect and for providing me with sound advice during my graduate career. Seth “Powder” Harkins, Christine Thomas, and Connie Lu were always a pleasure to be around in the office. To the new group members, Dr. Xile “Exile” Hu, Matt Whited, and Neal Mankad, I have enjoyed getting to know each of you and wish you the best with iv your continued studies in the Peters lab. Many thanks go to Kathleen Hand for keeping the day-to-day operations running smoothly and for providing a bottomless candy jar. I want to personally thank Dr. Theodore “T-Bone” Betley for being my best friend over the past five years; rarely does one have the opportunity to befriend an individual with so many common interests and so much character. He is the brother that I wish I always had. It’s hard to believe that our “Boston marriage” is almost over, and I have thoroughly enjoyed sharing an office, dry-box, and apartment with you. I wish you the best of luck in your impending marriage with The Beve, and I hope that the future brings plenty of prosperity for the both of us. Several individuals have provided technical support and deserve recognition. Larry Henling, Dr. Mike Day, and Dr. Betley were instrumental in assisting with X-ray crystallography. Solid-state magnetic data (SQUID) were collected by Dr. Mehn and Dr. David Jenkins, while EPR spectroscopy would not have been possible if it weren’t for the efforts of Dr. Angel di Bilio. Dr. Scott Ross is gratefully acknowledged for aiding with 15 N NMR experiments. For constantly reminding me that there is life outside of science, I wish to thank my friends. To Team Justice, I enjoyed our Saturdays and Sundays filled with football and Carl’s Jr. It is imperative that we institute an annual Las Vegas Justice weekend to ensure that our Justice bonds are never broken. My adventures in Old Town with Dr. Bruce Lambert have provided a lifetime of unforgettable, and often entertaining, memories. And finally, I wish to thank my family. There are too many of you to list individually, but know that I love you all and I hope that I continue to make you proud. Your unconditional love and support has molded me into the person that I am today. v Abstract The metallation of FeX2 (X = Cl, Br, I) salts with the strong-field [PhBP3] ([PhBP3] = PhB(CH2PPh2)3-) ligand is presented. The resulting four-coordinate, 14electron species, [PhBP3]FeX, have been thoroughly characterized and feature high-spin (S = 2) electronic ground-states. X-ray diffraction analysis of [PhBP3]FeCl establishes a monomeric structure in the solid state. The one electron reduction of [PhBP3]FeCl in the presence of a triphenylphosphine cap affords a rare example of four-coordinate iron(I). This species, [PhBP3]Fe(PPh3), serves as a synthetic surrogate to a low-valent “[PhBP3]Fe(I)” subunit that is readily oxidized in the presence of organic azides. The resulting S = ½ iron(III) imides of general formula [PhBP3]Fe≡NR may be subsequently reduced by one electron to yield the anionic S = 0 derivatives. Exposure of the former to an atmosphere of CO results in cleavage of the Fe≡NR linkage to yield [PhBP3]Fe(CO)2 and free isocyanate (O=C=N-R). Dicarbonyl [PhBP3]Fe(CO)2 is itself an imide precursor and is gradually converted back to [PhBP3]Fe≡NR upon exposure to excess organic azide. Tolyl imide [PhBP3]Fe≡N-p-tolyl readily reacts with H2 under mild conditions to undergo a step-wise Fe-Nx bond scission process to ultimately release free p-toluidine. Initially formed is the S = 2 iron(II) anilide, [PhBP3]Fe(N(H)-p-tolyl), which has been independently prepared and shown to release p-toluidine in the presence of H2. In benzene solvent the final iron containing product of the hydrogenation process is diamagnetic [PhBP3]Fe(η5-cyclohexadienyl), which is presumably formed from benzene insertion into a low-valent iron-hydride intermediate. vi Reduction of the ferromagnetically coupled dimer, {[PhBP3]Fe(N3)}2, yields the bridging nitride species, [{[PhBP3]Fe}2(µ-N)][Na(THF)5]. This compound features two high-spin iron(II) metal centers that are so strongly antiferromagnetically coupled that a diamagnetic S = 0 ground-state is exclusively populated at room temperature. X-ray diffraction analysis reveals a bent Fe-N-Fe linkage that quantitatively releases ammonia in the presence of excess protons. Reactivity with CO and H2 is also presented, and for the latter, complete rupture of the Fe-N-Fe manifold is not observed as the presence of an additional metal center (when compared with the iron(III) imides) favors the formation of the diamagnetic bridging imide-hydride species, [{[PhBP3]Fe}2(µ-NH)(µ-H)][Na(THF)5]. vii Table of Contents Acknowledgements………………………………………………………………….. iii Abstract……………………………………………………………………………… v Table of Contents……………………………………………………………………. vii List of Figures……………………………………………………………………….. xi List of Tables………………………………………………………………………... xv List of Abbreviations and Nomenclature……………………………………………. xvi Dedication…………………………………………………………………………… xxii Chapter 1. Introduction and Background to Tris(phosphino)borate Iron Chemistry……………………………………………………………………………… 1 1.1 Tris(phosphino)borates…………………………………………………….. 2 1.2 Iron Imides and Iron Nitrides………………………………………………. 5 1.3 Chapter Summaries………………………………………………………… 8 References Cited……………………………………………………………….. 9 Chapter 2: Low-Spin d5 Iron Imides: Nitrene Capture by Low-Coordinate Iron(I) Provides the 4-Coordinate Fe(III) Complexes [PhB(CH2PPh2)3]Fe≡NR... 15 2.1 Introduction……………………………………………………………….. 16 2.2 Results…………………………………………………………………….. 17 2.2.1 Synthesis and Characterization of [PhBP3]FeX (X = Cl, Br, I)… 17 2.2.2 Synthesis and Characterization of [PhBP3]Fe(PPh3)…………… 19 2.2.3 Synthesis and Characterization of Terminal Fe(III) Imides via Oxidative Group Transfer…………………………………... 21 viii 2.2.4 Imide Synthesis via Metathesis…………………………………. 28 2.2.5 Raman Characterization of the Fe≡NR Linkage………………... 30 2.2.6 Initial Reactivity Studies of the Fe≡NR Linkage……………….. 35 2.3 Discussion………………………………………………………………… 37 2.4 Conclusions……………………………………………………………….. 43 2.5 Experimental Section……………………………………………............... 43 2.5.1 General Considerations…………………………………………. 43 2.5.2 Magnetic Measurements………………………………............... 44 2.5.3 EPR Measurements……………………………………............... 45 2.5.4 Electrochemical Measurements…………………………………. 45 2.5.5 DFT Calculations……………………………………………….. 45 2.5.6 Raman Measurements…………………………………............... 46 2.5.7 Starting Materials and Reagents………………………………… 46 2.5.8 Synthesis of Compounds………………………………............... 47 2.5.9 X-ray Experimental Data……………………………………….. 55 References Cited……………………………………………………………… 58 Chapter 3: Hydrogenolysis of [PhB(CH2PPh2)3]Fe≡N-p-tolyl: Probing the Reactivity of an Iron Imide with H2…………………...…………………………… 64 3.1 Introduction……………………………………………………………….. 65 3.2 Results and Discussion……………………………………………………. 65 3.3 Conclusions……………………………………………………………….. 72 ix 3.4 Experimental Section……………………………………………………... 73 3.4.1 General Considerations…………………………………………. 73 3.4.2 Magnetic Measurements……………………………................... 73 3.4.3 Starting Materials and Reagents………………………………… 74 3.4.4 Synthesis of Compounds………………………………………... 74 3.4.5 X-ray Experimental Data……………………………………….. 79 References Cited……………………………………………………………… 81 Chapter 4: Ground-State Singlet L3Fe-(µ-N)-FeL3 and L3Fe(NR) Complexes Featuring Pseudotetrahedral Fe(II) Centers……………………………………... 84 4.1 Introduction……………………………………………………………… 85 4.2 Results and Discussion…………………………………………………... 86 4.3 Conclusions……………………………………………………………… 105 4.4 Experimental Section……………………………………………………. 108 4.4.1 General Considerations………………………………………... 108 4.4.2 Magnetic Measurements………………………………………. 108 4.4.3 Electrochemical Measurements………………………………... 108 4.4.4 DFT Calculations……………………………………………… 108 4.4.5 Starting Materials and Reagents……………………………….. 109 4.4.6 Synthesis of Compounds………………………………………. 109 4.4.7 X-ray Experimental Data……………………………………… 113 References Cited…………………………………………………………….. 116 x Chapter 5: Heterolytic Activation of H2 with an L3Fe-(µ-N)-FeL3 Linkage: Characterization of a Diiron System Featuring a µ-NH, µ-H Manifold………... 121 5.1 Introduction……………………………………………………………… 122 5.2 Results and Discussion…………………………………………………... 122 5.3 Molecular Orbital Considerations……………………………………….. 138 5.4 Conclusions……………………………………………………………… 141 5.5 Experimental Section……………………………………………………. 141 5.5.1 General Considerations………………………………………... 141 5.5.2 Magnetic Measurements………………………………………. 141 5.5.3 EPR Measurements……………………………………………. 141 5.5.4 Electrochemical Measurements………………………………... 141 5.5.5 Starting Materials and Reagents……………………………….. 142 5.5.6 Synthesis of Compounds………………………………………. 142 5.5.7 X-ray Experimental Data……………………………………… 144 References Cited…………………………………………………………….. 145 xi List of Figures Chapter 1. Figure 1.1. Examples of mono, bis, and tris(phosphino)borates…………….. 2 Figure 1.2. Molecular orbital splitting diagram for the [PhBP3]Co platform and the anticipated electronic structure of a [PhBP3]FeIII(NR) species……... 4 Figure 1.3. Examples of known iron imides………………………………… 6 Figure 1.4. Examples of known iron nitrides………………………………... 7 Chapter 2. Figure 2.1. 50% thermal ellipsoid representation of [PhBP3]FeCl………….. 18 Figure 2.2. Cyclic voltammetry of [PhBP3]FeCl……………………………. 19 Figure 2.3. 50% thermal ellipsoid representation of [PhBP3]Fe(PPh3)…...… 20 Figure 2.4. 50% thermal ellipsoid representation of [PhBP3]Fe≡N-p-tolyl…. 22 Figure 2.5. a) SQUID magnetization data for the complexes [PhBP3]FeCl, [PhBP3]Fe(PPh3), and [PhBP3]Fe≡N-p-tolyl; b) EPR spectrum of [PhBP3]Fe≡N-p-tolyl; and c) cyclic voltammetry of [PhBP3]Fe≡N-p-tolyl… 23 Figure 2.6. Uv-vis data for the specified imides…………………………….. 26 Figure 2.7. 50% thermal ellipsoid representation of [PhBP3]Fe≡NtBu……... 27 Figure 2.8. EPR spectrum of [PhBP3]Fe≡NtBu……………………………... 27 Figure 2.9. Cyclic voltammetry of [PhBP3]Fe≡N-p-tolyl, [PhBP3]Fe≡NtBu, and [PhBP3]Fe≡N(1-Ad)…………………………………………………….. 28 Figure 2.10. Resonance Raman spectra of (A) [PhBP3]Fe≡NPh, (B) [PhBP3]Fe≡15NPh, (C) [PhBP3]Fe≡NPh-d5, (D) [PhBP3]Fe≡15NPh-d5, and (E) [PhBP3]Fe≡N-p-tolyl……………………………………………………. 31 xii Figure 2.11. Resonance Raman spectra of (A) [PhBP3]Fe≡NtBu, (B) 1:1 [PhBP3]Fe≡NtBu:[PhBP3]Fe≡15NtBu, and (C) [PhBP3]Fe≡NtBu-d9………... 33 Figure 2.12. 50% thermal ellipsoid representation of [PhBP3]Fe(CO)2…….. 36 Figure 2.13. (left) Qualitative d-orbital splitting diagram illustrating the decent in symmetry from Td to axially distorted C3v. (right) Qualitative d-orbital splitting diagram illustrating the Jahn-Teller distortion observed for [PhBP3]CoI ……………………………………………………………… 38 Figure 2.14. Theoretically predicted geometry and electronic structure for S = ½ [PhBP3]Fe≡NtBu…………………………………………………….... 40 Figure 2.15. Qualitative d-orbital splitting diagram for a d5 imide and a hypothetical d5 oxo…………………………………………………………... 41 Chapter 3. Figure 3.1. 50% thermal ellipsoid representation of [PhBP3]Fe(N(H)-p-tolyl)…………………………………………………….. 67 Figure 3.2. 50% thermal ellipsoid representation of [PhBP3]Fe(η5-cyclohexadienyl)……………………………………………... 68 Figure 3.3. Kinetic data for the H2 or D2 promoted consumption of [PhBP3]Fe≡N-p-tolyl in C6D6 ………………………………………………. 69 Figure 3.4. 50% thermal ellipsoid representation of [PhBP3]Fe(BH4)……… 71 Figure 3.5. Plot of χM*T vs. T for anilide [PhBP3]Fe(N(H)-p-tolyl)……….. 73 Figure 3.6. Plot of χM-1 vs. T for anilide [PhBP3]Fe(N(H)-p-tolyl)…………. 74 xiii Chapter 4. Figure 4.1. 50% thermal ellipsoid representation of {[PhBP3]Fe(µ-1,3-N3)}2……………………………………………………... 89 Figure 4.2. SQUID magnetometry data for {[PhBP3]Fe(µ-1,3-N3)}2……….. 90 Figure 4.3. 50% thermal ellipsoid representation of [{[PhBP3]Fe}2(µ-N)][Na(THF)5]……………………………………………. 92 Figure 4.4. 50% displacement ellipsoid representations of {[PhBP3]Fe≡N(1-Ad)}{nBu4N} (left) and [PhBP3]Fe≡N(1-Ad) (right)…...... 95 Figure 4.5. Electronic absorption spectrum of [PhBP3]FeCl (inset, blue), [{[PhBP3]Fe}2(µ-N)][Na(THF)5] (green), and {[PhBP3]Fe≡N(1-Ad)} {nBu4N} (red) in THF-d8…………………………………………………….. 97 Figure 4.6. Theoretically predicted geometry and electronic structure for S = 0 {[PhBP3]Fe≡N(tBu)}- .………………………………………………… 99 Figure 4.7. Cyclic voltammetry of [PhBP3]Fe≡N(1-Ad), top, and [{[PhBP3]Fe}2(µ-N)][Na(THF)5], bottom…………………………………... 100 Figure 4.8. (left) Qualitative d-orbital splitting diagrams of various [BP3]Fe≡Nx species. (right) Comparison of the d-orbital splitting diagrams anticipated under idealized three-fold and four-fold symmetry…………….. 107 Chapter 5. Figure 5.1. 15N and 1H NMR data for [([PhBP3]Fe)2(µ-NH)(µ-H)] [Na(THF)5]-(50% 15N)……………………………………………………… 124 Figure 5.2. 50% thermal ellipsoid representation of [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5]…………………………………….. 126 xiv Figure 5.3. Cyclic voltammetry of [([PhBP3]Fe)2(µ-N)][Na(THF)5], top, and [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5], bottom……………………….. 128 Figure 5.4. 50% thermal ellipsoid representation of ([PhBP3]Fe)2(µ-N)……. 129 Figure 5.5. X-band EPR spectrum of ([PhBP3]Fe)2(µ-N)…………………… 131 Figure 5.6. SQUID magnetometry data for ([PhBP3]Fe)2(µ-N)……………... 133 Figure 5.7. NIR data for [{[PhBP3]Fe}2(µ-N)][Na(THF)5] and ([PhBP3]Fe)2(µ-N)…………………………………………………………… 134 Figure 5.8. NIR data for [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] (5.1), and [([PhBP3]Fe)2(µ-NH)(µ-H)] (5.3)…………………................................. 135 Figure 5.9. Experimental (blue, solid line) and simulated (red, broken line) X-band EPR spectrum of ([PhBP3]Fe)2(µ-NH)(µ-H)……………………….. 137 Figure 5.10. SQUID magnetometry data for ([PhBP3]Fe)2(µ-NH)(µ-H)…… 138 Figure 5.11. Qualitative d-orbital splitting diagram for [{[PhBP3]Fe}2(µ-N)][Na(THF)5] in C2v symmetry………………………….. 139 Figure 5.12. Qualitative d-orbital splitting diagram for [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] in C2v symmetry………………….. 140 xv List of Tables Chapter 2. Table 2.1. Metal-imido vibrational data……………………………………... 34 Table 2.2. Crystallographic data for [PhBP3]FeCl, [PhBP3]Fe(PPh3), [PhBP3]Fe≡N-p-tolyl, [PhBP3]FeNtBu, and [PhBP3]Fe(CO)2………………. 56 Chapter 3. Table 3.1. Crystallographic data for [PhBP3]Fe(N(H)-p-tolyl), [PhBP3]Fe(η5-cyclohexadienyl), and [PhBP3]Fe(BH4)……………………… 80 Chapter 4. Table 4.1. Crystallographic data for {[PhBP3]Fe(µ-1,3-N3)}2, [{[PhBP3]Fe}2(µ-N)][Na(THF)5], {[PhBP3]Fe≡N(1-Ad)}{nBu4N}, and [PhBP3]Fe≡N(1-Ad)…………………………………………………………. 114 Chapter 5. Table 5.1. Comparative structural data for the compounds [{[PhBP3]Fe}2(µ-N)][Na(THF)5], [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5], and ([PhBP3]Fe)2(µ-N)……………………………………………………..... 130 Table 5.2. Crystallographic data for [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] and ([PhBP3]Fe)2(µ-N)…………… 144 xvi List of Abbreviations and Nomenclature [PhBP3] [PhB(CH2PPh2)3]- [PhBPiPr3] [PhB(CH2P iPr2)3]- [BP3] general tris(phosphino)borate ligand Cp cyclopentadienyl Cp* pentamethylcyclopentadienyl [PhTptBu] phenyl tris(3-tert-butylpyrazolyl)borate [Tp] general hydrotris(pyrazolyl)borate ligand [TpMe2] hydrotris(3,5-dimethylpyrazolyl)borate {1H} hydrogen-1 decoupled ° degrees, in measure of angles °C degrees Celcius 1 H hydrogen-1 11 boron-11 15 nitrogen-15 19 fluorine-19 31 P phosphorus-31 Å Angstrom, 10-10 m ACN acetonitrile Ad 1-adamantyl B N F Anal. Calcd. elemental analysis calculated atm atmosphere av average Ax xvii EPR coupling constant where X is the nucleus coupling to the unpaired electron B3LYP Becke three-parameter function with Lee-Yang-Parr correlation br broad C6H6 benzene C6D6 benzene-d6 C3, C2v, C1 Schoenflies symmetry designations CCD charge-coupled device cm centimeter(s) cm-1 inverse centimeters or wavenumbers cm3 cubic centimeters cont. continued d doublet dbabh 2,3:5,6-dibenzo-7-aza bicycle[2.2.1]hepta-2,5-diene DC direct current Dcalcd calculated density dd doublet of doublets dn d-electron count of n-electrons for a transition metal DFT density functional theory dt doublet of triplets dtbpe 1,2 bis(di-tert-butylphosphino)ethane E an atom or functional group forming a metal-ligand multiple bond EPR electron paramagnetic resonance xviii Eq equation equiv. equivalents ESI/MS electrospray ionization mass spectrometry eV electron volt fac facial coordination Fc/Fc+ ferrocene/ferrocenium g gram(s) G Gauss GC/MS gas chromatography mass spectrometry GHz gigahertz g g-factor h hour(s) H applied magnetic field HOMO highest occupied molecular orbital Hz hertz In nuclear spin of atom n IR infrared K degrees in Kelvin kcal kilocalories kHz kilohertz L dative ligand for a transition metal LACVP Los Alamos core valence potential LUMO lowest unoccupied molecular orbital xix m multiplet M general transition metal mg milligram(s) MHz megahertz, 106 Hertz min minute(s) mL milliliter(s) mmol millimole(s) MO molecular orbital mol mole(s) MS mass spectrometry mT millitesla(s) mV millivolt(s) mW milliwatt(s) n in NMR spectroscopy, coupling constant between nuclei A and Z over n JA-Z bonds (n, A, or Z omitted if not known) NIR near-infrared nm nanometer(s) NMR nuclear magnetic resonance [nacnac] general β-diketiminate ligand OTf -OSO2CF3 p- para position of an aryl ring Ph phenyl ppm parts per million xx q quartet R general alkyl or aryl substituents rt room temperature s second(s) S spin SOMO singly occupied molecular orbital SQUID superconducting quantum interference device sqrt square root t triplet T temperature THF tetrahydrofuran THF-d8 tetrahydrofuran-d8 tmeda tetramethylethylenediamine TMS trimethylsilyl tolyl -C6H4CH3 TPP tetraphenylporphyrnato triphos H3CC(CH2PPh2)3 UV-vis ultraviolet-visible V volume X monoanionic atom or group, such as halide XRD X-ray diffraction δ delta, chemical shift ε extinction coefficient in M-1 cm-1 xxi η n hapticity of order n λ wavelength λmax wavelength of maximum absorption µ absorption coefficient (XRD) µ-A bridging atom µB Bohr magnetons µeff effective magnetic moment µL microliter(s) υ frequency χ magnetic susceptibility χM molar magnetic susceptibility xxii Dedication This work is dedicated to my parents, Steven Jay Brown and Elizabeth Ann Blake.  1 Chapter 1: Introduction and Background to Tris(phosphino)borate Iron Chemistry 2 1.1 Tris(phosphino)borates Since their inception in 1999,1 the family of (phosphino)borate ligands has emerged as a versatile class of strong-field ligands for both coordination chemistry2 and catalysis.3 In general, these ligands feature one, two, or three phosphine donor arms linked to an anionic borate center. As a result of their modular synthesis, a variety of ligand architectures with varying steric and electronic properties may be achieved through substitutions at both the borate and phosphine functionalities.4 Selected examples of (phosphino)borates are shown in Figure 1.1. PPh2 P Ph P Ph Ph B Ph Ph Ph Ph P P Ph Ph P P B Ph B Ph Ph Ph Ph P Ph P P Ph Ph P B B Ph Ph [PhBP3] [PhBPiPr3] Figure 1.1. Examples of mono, bis, and tris(phosphino)borates. The tris(phosphino)borates, first generation [PhBP3] ([PhBP3] = [PhB(CH2PPh2)3]-) and second generation [PhBPiPr3] ([PhBPiPr3] = [PhB(CH2PiPr2)3]-), are isolobal to the facially coordinating L2X architectures that include the well-known families of tris(pyrazolyl)borate ([Tp]), tris(thioether)borate,5 cyclopentadienyl (Cp), and pentamethylcyclopentadienyl (Cp*) ligands. Comparative structural and electronic studies from the Tilley group between Cp*, TpMe2 (TpMe2 = hydrotris(3,5- dimethylpyrazolyl)borate), and [PhBP3] suggest that of these three ligands, [PhBP3] is the most electron rich and sterically encumbering.6 Additionally, the anionic charge of 3 [PhBP3] likely renders a more electron-rich scaffold than that of the structurally similar, but charge-neutral triphos ligand (triphos = CH3C(CH2PPh2)3).7 Recent studies with [PhBPiPr3] suggest that it is even more electron rich than [PhBP3].2f From a molecular orbital perspective, the tris(phosphino)borates preserve a threefold symmetric environment about a ligated metal center. The resulting e set of orbitals with π-symmetry along the borate-metal vector is therefore well suited for the formation of two energetically equivalent π-bonds with a ligand in the fourth coordination site of the metal center.8,9 As long as the resulting π* orbitals remain vacant (i.e., no more than 6 d-electrons), terminal M≡E (E = electronegative element) linkages with these auxiliary scaffolds are anticipated to be stable.2c Indeed a brief survey of the literature demonstrates that threefold symmetry has been exploited for the synthesis of a variety of M≡E linkages.10 The potential for the tris(phosphino)borate framework to engender novel properties at a transition metal center was demonstrated in dramatic fashion with the report of [PhBP3]CoI, an unprecedented example of low-spin, pseudotetrahedral Co(II).11 Of additional interest was the installation of an imido ligand (NR2-) onto this [PhBP3]Co platform.12 The resulting 4-coordinate, terminal Co(III) imide is diamagnetic and features a triple bond (1σ + 2π) between the Co-N linkage. This chemistry is in contrast to what is typically observed for low-coordinate (4-coordinate or lower), late-transition metal ions of the first-row: an increased preference for the population of high-spin electronic ground-states (and thus π* orbitals),13,14 and the formation of bridging species with electronegative elements.15 Ensuing work by the groups of Meyer16 and Theopold17 established the ability for tris(carbene) and tris(pyrazolyl)borate ligands, respectively, to 4 also facilitate the formation of 4-coordinate, Co(III) imides. Of the three systems, however, the tris(phosphino)borate-supported is the most stable. Electronically, the typical 3 over 2 molecular orbital diagram anticipated for a 4coordinate tetrahedral species does not provide for an accurate description of the [PhBP3]Co(III) imides. More consistent is a 2 over 3 splitting diagram, similar to that obtained for a d6 sandwich compound.18 Consequently, it was hypothesized that a lowspin, [PhBP3] supported terminal Fe(III) imide should be stable, considering the relatively non-bonding nature of the anticipated SOMO (Figure 1.2). Furthermore, the inherent metalloradical character of this system may yield enhanced chemical reactivity when compared with its closed shell Co(III) congener. a" e e a1 e a1 e a' a' a" a' R R I N N II Ph Co Ph Ph2P P PPh2 III Ph Co Ph Ph2P P PPh2 III Ph Fe Ph Ph2P P PPh2 B B B Ph Ph Ph doublet singlet doublet Figure 1.2. (left) Qualitative molecular orbital splitting diagram deduced for the [PhBP3]Co platform. Adapted from reference 12. (right) Anticipated electronic structure of a [PhBP3]FeIII(NR) species. 5 1.2 Iron Imides and Iron Nitrides As alluded to above, the [PhBP3] ligand has the potential to serve as an ideal scaffold for the preparation and reactivity studies of iron-nitrogen multiply bonded linkages. The importance of such linkages is highlighted by their implicated intermediacy in both biological19 and industrial20 nitrogen fixation processes. Establishing their reactivity patterns and spectroscopic signatures in well-defined synthetic systems is therefore of considerable interest. Additionally, any catalytic group-transfer processes with these linkages, such as aziridination,21,22 would be of practical importance as iron is an abundant and inexpensive transition metal.23 Holland’s work with low-coordinate iron has established N2 binding and activation,24 in addition to both N=N25 and N-N26 hydrazine bond cleavage processes.27 Missing from these “[nacnac]Fe” platforms, however, are examples of compounds featuring multiply-bonded iron-nitrogen linkages (nacnac = general β-diketiminate ligand28). Similarly, multiply-bonded intermediates have yet to be characterized during iron-facilitated N2 binding, activation, and protonation processes that ultimately yield hydrazine and ammonia.29 Precedence for a 4-coordinate iron center featuring a terminal NR2- linkage was provided in 2000 with Lee’s report of [Fe4(µ3-NtBu)4(NtBu)Cl3].30 Obtained in very low yields (~ 1 – 2%), a detailed magnetic study was not forwarded although it was assumed that three iron(III) centers and one iron(IV) center were present on the basis of charge balance. Prior to this work, azide decomposition with [Fe(salen)] units (salen = N,N’-ethane-1,2-diylbis(salicylaldiminato)) was reported to yield bridging iron(III) imides. Magnetic studies with these linkages established the presence of antiferromagnetically coupled, high-spin iron(III) centers,31 similar to what has been reported for [Fe(salen)]2O 32 6 and [Fe(salen)]2S.33 The preparation of well-defined [PhBP3]Fe≡NR imides would therefore be of considerable interest as they would provide the first examples of mononuclear, terminal iron imides. Additionally, their anticipated low-spin electronic configurations would be unprecedented for pseudotetrahedral Fe(III). N Fe N N Fe Fe Cl N t O Bu N N N Fe Cl N N Fe Cl estimates 3 iron(III) & 1 iron(IV) ~ 1 - 2% yield O Fe O Ar N O Antiferromagnetically coupled S = 5/2 iron(III) centers ~ 95% yield Figure 1.3. Examples of known iron imides. The chemistry of the iron-nitride (N3-) linkage is similarly underdeveloped. When considering terminal linkages, solid-state structural data is not yet available as these systems are typically generated at low temperature (~ 30 K) via photolysis of the corresponding azide and decompose upon warming.34 Alternatively, the thermal decomposition of a [PhBPiPr3]Fe(II) amide was recently reported by Betley and Peters to yield a terminal iron(IV) nitride that is stable at 0 °C.35 However, efforts to obtain a solidstate structure are hampered by a bimetallic reduction process to form N2, although ammonia production was noted in the presence of excess protons and electrons. Bridging iron nitrides, which have been characterized in the solid state, typically feature five- or six-coordinate iron centers in mid- to high-valent oxidation states (i.e., Fe(III) and Fe(IV)).36 Although these systems are well characterized, little data has been forwarded concerning their reactivity. 7 (L)Fe(diolate) N N (TPP)Fe N Fe(TPP) N N H N Fe N NH O Fe O O N N + N Ph HN Ph N N Fe Fe N N Ph P P P B Ph Ph O S = 1/2 S = 3/2 S=0 Formed at ~ 30 K via Photolysis Formed at ~ 30 K via Photolysis Forms at 273 K Reductively Couples at 298 K Figure 1.4. Examples of known iron nitrides (TPP = tetraphenylporphyrnato). The preparation and investigation into the chemical reactivity of [PhBP3] supported iron nitrides is therefore of considerable interest. Of immediate concern is to assess whether this scaffold is capable of supporting a terminal nitride that may be characterized in the solid state. Bridging systems, which are anticipated to be more stable, may also facilitate unique reaction chemistry with small molecules as a consequence of the coordinatively unsaturated nature of each metal center. Indeed, the recent discovery of an interstitial atom, presumably a nitride, within the active site of nitrogenase enzymes highlights the potential biological importance of low-coordinate “Fe-(µ-N)-Fe” linkages.37 Reactivity with H2 is also of interest, as no iron nitride (or iron imide) has been shown to react directly with H2, which is presumably a key mechanistic step in the Haber-Bosch synthesis of ammonia.20 The following chapters demonstrate that through the use of an anionic, strongfield tris(phosphino)borate ligand, several unique features concerning the chemistry of 8 Fe-Nx linkages are revealed. Foremost among these is the isolation and characterization of stable, mononuclear low-spin iron(III) imides that may be hydrogenated under mild conditions to promote a complete Fe-Nx bond scission process. Also presented is the first 4-coordinate, low-valent bridging nitride of iron, [{[PhBP3]Fe}2(µ-N)][Na(THF)5]. This species features two high-spin antiferromagnetically coupled iron(II) centers and reacts with H2 under mild conditions to generate an unprecedented L3FeII-(µ-NH)(µ-H)-FeIIL3 manifold.38 1.3 Chapter Summaries Chapter 2 introduces the [PhBP3]Fe framework and presents characterization data for a number of four-coordinate Fe(I), Fe(II), and Fe(III) compounds. Of particular interest is the preparation of [PhBP3]Fe(PPh3), which reacts with organic azides to yield terminal Fe(III) imides. Initial reactivity studies with the imides demonstrate their enhanced reactivity in group transfer processes when compared with their closed-shell cobalt(III) congeners. Whether this is in fact a reflection of the metalloradical character of the iron system is worth considering. Density functional theory calculations and solid state studies corroborate the assignment of a triple bond between the iron-nitrogen linkages. Chapter 3 presents the room temperature hydrogenation chemistry of [PhBP3]Fe≡N-p-tolyl. Surprisingly, a complete Fe-N bond scission process is observed to yield free amine and a solvent-activated product as the final iron containing species. The reaction appears to proceed in a stepwise fashion, as an independently prepared iron(II) anilide, [PhBP3]Fe(N(H)-p-tolyl), is observed during the course of the hydrogenation reaction. Mechanistically, the rate-limiting step for the disappearance of the iron(III) 9 imide is consistent with a high degree of H-H bond-breaking character as a kinetic isotope effect (kH/kD) of 5.6 is observed for this process. Chapter 4 discusses the preparation and characterization of ground-state singlet L3Fe-(µ-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral iron(II) centers. In contrast to the low-spin mononuclear iron(II) imide, spectroscopic data suggests that the anionic bridging nitride is best described as two high-spin iron(II) centers that are sufficiently antiferromagnetically coupled at room temperature to yield a singlet-ground state. Protonation of the L3Fe-(µ-N)-FeL3 linkage with an excess of acid liberates ammonia in isolated yields greater than 80%. Chapter 5 describes the hydrogenation and oxidation chemistry of the L3Fe-(µ-N)FeL3 linkage presented in Chapter 4. In contrast to [PhBP3]Fe≡N-p-tolyl, a complete FeN bond scission process is not observed when [{[PhBP3]Fe}2(µ-N)][Na(THF)5] is exposed to an atmosphere of H2. Rather, the presence of an additional metal center favors the formation of the bridging imide-hydride species, [{[PhBP3]Fe}2(µ-NH)(µH)][Na(THF)5]. This compound may be chemically oxidized by one electron to yield {[PhBP3]Fe}2(µ-NH)(µ-H), which may independently be prepared from the hydrogenation of {[PhBP3]Fe}2(µ-N). References Cited 1 (a) Barney, A. A.; Heyduk, A. F.; Nocera, D. G. Chem. Commun. 1999, 2379. (b) Thomas, J. C.; Peters, J. C. Inorg. Synth. 2004, 34, 8. 2 See for example: (a) Lu, C. C.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 15818. (b) Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2004, 23, 2488. (c) MacBeth, C. E.; Thomas, J. C.; Betley, T. A.; Peters, J. C. Inorg. Chem. 2004, 43, 4645. (d) Daida, 10 E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474. (e) Thomas, J. C.; Peters, J. C. Polyhedron 2004, 23, 489. (f) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074. (g) Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2003, 22, 4629. (h) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 11162. (i) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782. (j) Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21, 4065. (k) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100. 3 (a) Thomas, C. M.; Peters, J. C. Inorg. Chem. 2004, 43, 8. (b) Betley, T. A.; Peters, J. C. Angew. Chem. Int. Ed. 2003, 42, 2385. (c) Lu. C. C.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 5272. 4 See for example: Thomas, J. C.; Peters, J. C. Inorg. Chem. 2003, 42, 5055. 5 For examples of tris(thioether)borate ligands see: (a) Schebler, P. J.; Riordan, C. G.; Guzei, I. A.; Rheingold, A. L. Inorg. Chem. 1998, 37, 4754. (b) Schebler, P. J.; Riordan, C. G.; Liable-Sands, L.; Rheingold, A. L. Inorg. Chim. Acta 1998, 270, 543. (c) Ohrenberg, C.; Ge, P.; Schebler, P.; Riordan, C. G.; Yap, G. P. A.; Rheingold, A. L. Inorg. Chem. 1996, 35, 749. (d) Ge, P.; Haggerty, B. S.; Rheingold, A. L.; Riordan, C. G. J. Am. Chem. Soc. 1994, 116, 8406. 6 Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21, 4050. 7 Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 8870. 8 Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy: An Introduction to Vibrational and Electronic Spectroscopy; Dover Publications, Inc.: New York, 1989. 9 A similar π-manifold has been ascribed to Schrock’s triamidoamine ligands. See: Schrock, R. R. Acc. Chem. Res. 1997, 30, 9. 11 10 See for example: (a) Stephens, F. H.; Figueroa, J. S.; Diaconescu, P. L.; Cummins, C. C. J. Am. Chem. Soc. 2003, 125, 9264. (b) Brask, J. K.; Dura-Vila, V.; Diaconescu, P. L.; Cummins, C. C. Chem. Commun. 2002, 902. (c) Peters, J. C.; Odom, A. L.; Cummins, C. C. Chem. Commun. 1997, 1995. (d) Mösch-Zanetti, N. C.; Schrock, R. R.; Davis, W. M.; Wanninger, K.; Seidel, S. W.; O’Donoghue, M. B. J. Am. Chem. Soc. 1997, 119, 11037. (e) Herrmann, W. A.; Baratta, W.; Herdtweck, E.; Angew. Chem., Int. Ed. 1996, 35, 1951. (f) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861. (g) Hay-Motherwell, R. S.; Wilkinson, G. Polyhedron 1993, 12, 2009. (h) Cotton, F. A.; Schwotzer, W.; Shamshoum, E. S. Organometallics 1984, 3, 1770. (i) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. Inorg. Chem. 1983, 22, 2903. 11 (a) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 15336. (b) Shapiro, I. R.; Jenkins, D. M.; Thomas, J. C.; Day, M. W.; Peters, J. C. Chem. Commun. 2001, 2152. 12 Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 11238. 13 Transition metals of the first row form high-spin complexes more readily than metals of the second or third row. See: Miessler, L. G.; Tarr, D. A. Inorganic Chemistry, 2nd Ed.; Prentice Hall: New Jersey, 1999; pg. 323. 14 First-row ions in square-planar geometries typically feature intermediate- and low-spin electronic configurations in order to prevent population of the strongly σ-antibonding dx2-y2 orbital. For examples of the former, see: Hawrelak, E. J.; Bernskoetter, W. H.; Lobkovsky, E.; Yee, G. T.; Bill, E.; Chirik, P. J. Inorg. Chem. 2005, 44, 3101. For examples of the latter, see: Heinicke, J.; He, M.; Dal, A.; Klein, H.-F.; Hetche, O.; Keim, W.; Flörke, U.; Haupt, H.-J. Eur. J. Inorg. Chem. 2000, 431. 12 15 See for example: (a) Murray, K. S. Coord. Chem. Rev. 1974, 12, 1. (b) Mukherjee, R. N.; Stack, T. D. P.; Holm, R. H. J. Am. Chem. Soc. 1988, 110, 1850. (c) Dai, X.; Kapoor, P.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 4798. (d) Que, L.; Tolman W. B. Angew. Chem., Int. Ed. 2002, 41, 1114 and references therein. (e) Kurtz, D. M., Jr. Chem. Rev. 1990, 90, 585. 16 Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322. 17 Shay, D. T.; Yap, G. P. A.; Zakharov, L. N.; Rheingold, A. L.; Theopold, K. H. Angew. Chem., Int. Ed. 2005, 44, 1508. 18 Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J. Am. Chem. Soc. 1971, 93, 3603. 19 See for example: (a) Huniar, U.; Ahlrichs, R.; Coucouvanis, D. J. Am. Chem. Soc. 2004, 126, 2588. (b) Hinnemann, B.; Nørskov, J. K. J. Am. Chem. Soc. 2004, 126, 3920. (c) Lee, H.-I.; Benton, P. M. C.; Laryukhin, M.; Igarashi, R. Y.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem. Soc. 2003, 125, 5604. (d) Schimpl, J.; Petrilli, H. M.; Blochl, P. E. J. Am. Chem. Soc. 2003, 125, 15772. (e) Cao, Z.; Zhou, Z.; Wan, H.; Zhang, Q.; Thiel, W. Inorg. Chem. 2003, 42, 6986. (f) Hinnemann, B.; Nørskov, J. K. J. Am. Chem. Soc. 2003, 125, 1466. (g) Dance, I. Chem. Commun. 2003, 324. 20 Ertl, G. Chem. Rec. 2001, 1, 33. 21 Though not yet catalytic, aziridination chemistry has been reported with a nickel(II) imide. See: Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2003, 125, 13350. 22 Iron porphyrin systems are known to catalyze aziridination. See: (a) Mansuy, D.; Mahy, J.-P.; Dureault, A.; Bedi, G.; Battioni, P. J. Chem. Soc., Chem. Commun. 1984, 1161. (b) Mahy, J.-P.; Bedi, G.; Battoni, P.; Mansuy, D. J. Chem. Soc., Perkin Trans. 2 1988, 1517. 13 23 Chirik et al. have established the ability of 4-coordinate iron(II) species to catalyze the polymerization of ethylene. See: Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.; Chirik, P. J. Organometallics 2004, 23, 237. 24 Smith, J. M.; Lachicotte, R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K. R.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222. 25 Smith, J. M.; Lachicotte, R. L.; Holland, P. L. J. Am. Chem. Soc. 2003, 125, 15752. 26 Vela, J.; Stoian, S.; Flaschenriem, C. J.; Münck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522. 27 Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589. 28 For a recent review of β-diketiminatometal complexes see: Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031. 29 (a) George, T. A.; Rose, D. J.; Change, Y.; Chen, Q.; Zubieta, J. Inorg. Chem. 1995, 34, 1295. (b) Leigh, G. J.; Jimenez-Tenorio, M. J. Am. Chem. Soc. 1991, 113, 5862. 30 Verma, A. K.; Nazif, T. N.; Achim, C.; Lee, S. C. J. Am. Chem. Soc. 2000, 122, 11013. 31 Nichols, P. J.; Fallon, G. D.; Murray, K. S.; West, B. O. Inorg. Chem. 1988, 27, 2795. 32 Wollmann, R. G.; Hendrickson, D. N. Inorg. Chem. 1977, 16, 723. 33 Dorfman, J. R.; Girerd, J.-J.; Simhon, E. D.; Stack, T. D. P.; Holm, R. H. Inorg. Chem. 1984, 23, 4407. 34 (a) Meyer, K. M.; Eckhard, B.; Mienert, B.; Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 4859. (b) Wagner, W.-D.; Nakamoto, K. J. Am. Chem. Soc. 1989, 111, 1590. 35 Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252. 14 36 (a) Justel, T.; Muller, M.; Weyhermuller, T.; Kressl, C.; Bill, E.; Hildebrandt, P.; Lengen, M.; Grodzicki, M.; Trautwein, A. X.; Nuber, B.; Wieghardt, K. Chem.-Eur. J. 1999, 5, 793. (b) Kienast, A.; Homborg, H. Z. Anorg. Allg. Chem. 1998, 624, 233. (c) Moubaraki, B.; Benlian, D.; Baldy, A.; Pierrot, M. Acta Crystallogr., Sect. C 1989, 45, 393. (d) Scheidt, W. R.; Summerville, D. A.; Cohen, I. A. J. Am. Chem. Soc. 1976, 98, 6623. (e) Summerville, D. A.; Cohen, I. A. J. Am. Chem. Soc. 1976, 98, 1747. 37 Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696. 38 For [Cp2Ti](µ-NH)2(µ-H) see: Armor, J. N. Inorg. Chem. 1978, 17, 203.  15 Chapter 2: Low-Spin d5 Iron Imides: Nitrene Capture by LowCoordinate Iron(I) Provides the 4-Coordinate Fe(III) Complexes [PhB(CH2PPh2)3]Fe≡NR The text in this chapter is reproduced in part with permission from: Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 322. Copyright 2003 American Chemical Society 16 2.1 Introduction While metalloenzymes such as nitrogenases and hydrogenases are far from mechanistically well understood, it seems plausible that low-valent, sulfur-ligated Fe(II) and Fe(I) species play a critical role in their small-molecule reduction chemistry.1 The careful preparation and study of the redox reactivity of low-coordinate Fe(I) species is therefore of concern.2,3 Iron(I) is an unusual oxidation state whose coordination chemistry, particularly for low-coordinate examples (4-coordinate or lower), is poorly developed. This point was underscored recently in Parkin's report of the 4-coordinate Fe(I) complex [PhTptBu]Fe(CO) ([PhTptBu] = phenyl tris(3-tert-butylpyrazolyl)borate).4 Knowledge of the elementary reactivity patterns of Fe(I) will guide mechanistic postulates for its possible role in biological systems. This chapter describes metallation of the [PhBP3] ligand ([PhBP3] = [PhB(CH2PPh2)3]-)5 to generate the 4-coordinate Fe(II) halide species, [PhBP3]FeX (X = Cl, Br, I). Of particular interest is the one electron reduction of the chloride derivative to yield the rare, 4-coordinate Fe(I) species, [PhBP3]Fe(PPh3), which serves as a synthetic surrogate to the tripodal "[PhBP3]Fe(I)" subunit. This system undergoes rapid oxidation in the presence of organic azides to produce mononuclear, terminally bound Fe(III) imides, [PhBP3]Fe≡NR (R = alkyl, aryl). Synthetically rare iron oxo and imide/nitrene species continue to receive intense scrutiny due to their postulated role in catalysis.6,7,8 Recent work by Lee and co-workers provided important precedent for the "Fe=NR" multiple-bond linkage.6c However, Lee's system, [Fe4(µ3-NtBu)4(NtBu)Cl3], is synthetically ill-defined and electronically complex owing to its cluster nature, which features four magnetically coupled iron centers. The title complexes reported herein 17 provide distinctly simple, mononuclear 4-coordinate Fe(III) imides. Their S = 1/2 groundstate electronic configurations are also distinct in that, to the best of our knowledge, lowspin examples of tetrahedral or pseudotetrahedral Fe(III) were previously unknown. 2.2 Results 2.2.1 Synthesis and Characterization of [PhBP3]FeX (X = Cl, Br, I) High-yield (>80%) entry into the "[PhBP3]Fe" system proceeded from metallation of FeX2 salts (X = Cl, Br, I) by [Tl][PhBP3]9 to produce the yellow, 4-coordinate highspin halide derivatives [PhBP3]FeCl (2.1), [PhBP3]FeBr (2.2), and [PhBP3]FeI (2.3) (Equation 2.1). All three halides give rise to characteristic, yet paramagnetically shifted, 1 H NMR spectra. Room temperature Evans method magnetic measurements10 yielded µeff values of 5.04 µB, 5.20 µB, and 5.03 µB, respectively, consistent with four unpaired electrons (S = 2). UV-vis data correlates with the halide ligand field strengths of the spectrochemical series (λmax = 416 nm (2.1), 424 nm (2.2), and 438 nm (2.3)).11 Ph Tl Ph Ph2P P PPh2 Cl FeX2 B -TlX Ph X = Cl, Br, I Ph Fe Ph2P P Br Ph PPh2 B Ph Ph Fe Ph2P P I Ph PPh2 B 2.1 Ph Ph Fe Ph2P P Ph PPh2 (2.1) B 2.2 Ph 2.3 Chloride 2.1 was characterized in the solid state by X-ray crystallography (Figure 2.1) and is rigorously monomeric despite being coordinatively unsaturated and a 14electron species. This is in direct contrast with the analogous [PhBP3]CoCl12 and [PhBP3]RuCl13 species, for which X-ray diffraction experiments yielded dimeric structures. Chloride 2.1 features an Fe-Cl bond distance of 2.2048(10) Å, with Fe-P bond distances of approximately 2.43 Å. The P-Fe-P bond angles average 92°, which is consistent with the description of [PhBP3] as a facially capping ligand that occupies one 18 half of an octahedral coordination sphere. The electronic consequences of such a 4coordinate environment versus that for a rigorously tetrahedral system (i.e., L-M-L = 109.5°) are discussed in Section 2.3. Other threefold symmetric ligand environments such as tris(pyrazolyl)borate,14 tris(thioether)borate,15 and triphos (CH3C(CH2Ph2)3)16 are also known to ligate Fe(II) in a facially capping orientation, but only with bulky tris(pyrazolyl)borate ligands have 4-coordinate systems been realized.4,17 Riordan’s tris(thioether)borates, for example, form octahedral species in which the metal center is coordinated by two ligands.15a Cl Fe P3 P1 P2 Figure 2.1. 50% thermal ellipsoid representation of [PhBP3]FeCl (2.1). For the image on the right, all phosphino phenyl groups have been removed for clarity. For both images, all hydrogen atoms and a benzene solvent molecule have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe-Cl, 2.2048(10); Fe-P1, 2.4318(11); Fe-P2, 2.4168(11); Fe-P3, 2.4268(9); P1-Fe-P2, 94.29(4); P1Fe-P3, 89.76(3); P2-Fe-P3, 93.19(4); Cl-Fe-P1, 129.60(4); Cl-Fe-P2, 110.57(4); Cl-Fe-P3, 129.63(4). 19 Little structural difference is observed between 2.1 and the second generation high-spin chloride, [PhBPiPr3]FeCl ([PhBPiPr3] = [PhB(CH2PiPr2)3]-; Fe-Cl = 2.220(1) Å; Fe-Pavg. = 2.42 Å; P-Fe-Pavg. = 94°).13 The largest deviation is manifested in the Cl-Fe-P bond angles, which are approximately 110°, 129°, and 129° for 2.1, and 122° for all three angles of [PhBPiPr3]FeCl. Electronically, a comparative cyclic voltammetry study between the two species reveals a significant difference in the potential of the FeI/II redox couple. As shown in Figure 2.2, a fully reversible, one-electron redox event is observed at -1.65 V versus Fc/Fc+ for 2.1. This potential is approximately 320 mV more positive than that observed for [PhBPiPr3]FeCl,13 which is presumably due to the weaker σ-donating and better π-accepting properties of aryl versus alkyl phosphine ligands. -1000 -1500 -2000 + Potential (mV vs. Fc/Fc ) Figure 2.2. Cyclic voltammetry of [PhBP3]FeCl (2.1) (THF, 0.4 M [TBA][PF6], scan rate = 50 mV/s). 2.2.2 Synthesis and Characterization of [PhBP3]Fe(PPh3) As anticipated from the aforementioned electrochemical data, chloride 2.1 was readily reduced by one electron with a sodium/mercury amalgam (THF) in the presence of a triphenylphosphine cap. The desired iron(I) product, [PhBP3]Fe(PPh3) (2.4), was obtained as an orange, crystalline solid in 62% yield. Repeating the reduction in the 20 absence of triphenylphosphine invariably led to decomposition18 while the use of PMe3 resulted in low yields of the desired Fe(I) adduct, impeding its use as a readily available synthetic precursor. Complex 2.4 features characteristic resonances in the 1H NMR spectrum and a high-spin electronic ground-state at room temperature (µeff = 3.88 µB, S = 3/2). X-ray crystallography (Figure 2.3) established a 4-coordinate, pseudotetrahedral geometry in which the Pborate-Fe-Pborate bond angles average 96°. The triphenylphosphine ligand was located at a distance of 2.2889(9) Å from the metal center, while the observed Fe-Pborate distances of approximately 2.34 Å are ~ 0.1 Å shorter than those of 2.1. P4 Fe P2 P3 P1 Figure 2.3. 50% thermal ellipsoid representation of [PhBP3]Fe(PPh3) (2.4). For the image on the right, all phosphino phenyl groups have been removed for clarity. For both images, all hydrogen atoms and disordered solvent molecules have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe-P1, 2.3350(10); Fe-P2, 2.3400(10); Fe-P3, 2.3341(10); Fe-P4, 2.2889(9); P1-Fe-P2, 97.54(3); P1-Fe-P3, 95.58(3); P2-Fe-P3, 96.19(3); P4-Fe-P1, 121.60(4); P4-FeP2, 118.79(3); P4-Fe-P3, 121.30(4). 21 2.2.3 Synthesis and Characterization of Terminal Fe(III) Imides via Oxidative Group Transfer As demonstrated in Equation 2.2, complex 2.4 underwent clean oxidation by ptolyl azide at room temperature in benzene to provide the forest-green, d5 imido complex [PhBP3]Fe≡N-p-tolyl (2.5), which could be crystallized (50% isolated) by vapor diffusion of petroleum ether into the reaction solution. The overall reaction appeared to be quantitative by 1H and 31P NMR spectroscopies: a stoichiometric equivalent of p-tolylN=PPh3 was produced as the only detectable byproduct. Complex 2.5 was thoroughly characterized by X-ray diffraction (Figure 2.4), SQUID magnetization (Figure 2.5a), EPR spectroscopy (Figure 2.5b), and cyclic voltammetry (Figure 2.5c). [PhBP3]Fe(PPh3) 2.4 2 p-tolyl-N=N=N - p-tolyl-N=PPh3 - 2 N2 [PhBP3]Fe N (2.2) 2.5 The Fe(1)-N(1)-C(46) bond angle in 2.5 is 169.96(2)°, and the Fe(1)-N(1) bond distance of 1.6578(2) Å is much shorter than typical Fe-amido linkages19 and is similar to that reported by Lee (1.635(4) Å).6c These structural data are consistent with strong πbonding in the Fe(1)-N(1) linkage: 2.5 is best represented with a triple bond. The axial πinteractions destabilize two orbitals of dxz and dyz parentage to provide the low-spin S = 1/2 configuration observed for 2.5, as indicated from its SQUID magnetization data. For comparison, SQUID data for 2.1 and 2.4 are also presented in Figure 2.5a. All three compounds display temperature independent data in the region 10 to 310 K that obeys the Curie-Weiss law,20 with effective moments of 5.00, 4.11, and 2.07 µB for the S = 2, S = 3 /2, and S = ½ systems, respectively.21 22 C46 N1 Fe1 P1 P3 Figure 2.4. 50% thermal ellipsoid representation of [PhBP3]Fe≡N-p-tolyl (2.5). For the image on the right, all phosphino phenyl groups have been removed for clarity. For both images, all hydrogen atoms and ½ of a benzene solvent molecule have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe1-N1, 1.6578(19); N1-C46, 1.383(3); Fe1-P1, 2.2734(7); Fe1-P2, 2.2235(7); Fe1-P3, 2.2229(7); C46-N1-Fe1, 169.96(18); P1-Fe1-P2, 91.77(3); P1-Fe1-P3, 88.46(3); P2-Fe1-P3, 86.89(3); N1-Fe1-P1, 136.89(7); N1-Fe1-P2, 119.83(7); N1-Fe-P3, 119.38(7). P2 23 2.5a 3.5 complex 2.1 (S = 2) χ MT (cm3·K/mol) 3.0 2.5 complex 2.4 (S = 3/2) 2.0 1.5 1.0 complex 2.5 (S = 1/2) 0.5 0.0 0 50 100 150 200 250 300 T (K) 2.5b g1 g2 200 300 g3 400 Field (mT) 2.5c 100 -100 -300 -500 -700 -900 -1100 -1300 -1500 -1700 Potential (mV) vs Ag/AgNO3 Figure 2.5. (a) SQUID magnetization data for complexes 2.1, 2.4, and 2.5. (b) EPR spectrum of 2.5 (glassy toluene at 30 K, X-band, 9.474 GHz). (c) Cyclic voltammetry of 2.5 (THF, 0.3 M [TBA][PF6], scan rate = 30 mV/s). 24 The rhombic EPR spectrum for 2.5 shows g1 in the region 2.61, suggesting the unpaired spin resides in an orbital orthogonal to the Fe(1)-N(1) vector, and g2 and g3 in the region 2.0. While the three components g1, g2, and g3 are well resolved at 30 K, g2 and g3 were not resolved at 77 K. Assignment of the electronic configuration of 2.5 can be made cautiously by assuming approximate three-fold symmetry and placing the molecular z-axis along the Fe-N bond vector. Using this coordinate system we suggest that a half-filled dxy orbital sits slightly above two lower-lying and filled dx2-y2 and dz2 orbitals to provide the ground-state configuration (dz2)2(dx2-y2)2(dxy)1(dxz)0(dyz)0.22 This electronic model suggests that it should be possible to reduce 2.5 by one electron, in accord with its cyclic voltammetry. A well-behaved and fully reversible FeII/III couple is observed at -1.35 V, whereas an irreversible oxidation is observed at ~ -300 mV in the CV (Figure 2.5c). That 2.5 can be reversibly reduced to the d6 anion “{PhBP3]Fe≡NAr}-” is plausible, given the stability of its isolobal and isoelectronic d6 relative [PhBP3]Co≡NAr.23 The chemical reduction of 2.5 by one electron occurs readily in the presence of a sodium/mercury amalgam to yield the anionic imide, {[PhBP3]Fe≡N-ptolyl}{Na(THF)2} (2.6), which was isolated in high-yield (88%) and features a singlet resonance at δ 87 ppm in its 31P NMR spectrum. The difficulty in obtaining suitable crystals for X-ray diffraction analysis impeded a comparative structural study with 2.5.24 As shown in Equation 2.3, the reaction between 2.4 and a variety of aryl- and alkyl-substituted organic azides may be utilized for the synthesis of a number of terminal Fe(III) imides. Curiously, however, no reaction was observed with trimethylsilyl azide despite the facile reactivity of 2.4 with tert-butylazide.25 The phosphaimide byproducts 25 N3-p-tolyl [PhBP3]Fe N 2.5 N3Ph [PhBP3]Fe N 2.7 N3Ph-p-CF3 [PhBP3]Fe N CF3 N t N NMe2 2.8 [PhBP3]Fe(PPh3) N3Ph-p-tBu [PhBP3]Fe Bu (2.3) 2.9 2.4 N3Ph-p-NMe2 [PhBP3]Fe 2.10 N3tBu [PhBP3]Fe N 2.11 N3(1-Adamantyl) [PhBP3]Fe N 2.12 from these reactions may be removed by thorough washing with petroleum ether for compounds 2.8, 2.11, and 2.12. For the remaining imides, purification by crystallization or precipitation from benzene solution with petroleum ether is required. All of the aryl imides are green in color and with the exception of 2.10, feature absorptions in the UVvis region that range from 630 to 638 nm with ε values from 2300 to 3400 cm-1 M-1. Also present is a distinct shoulder at higher energy (~ 400 nm). These electronic transitions are presumably charge-transfer in nature, and the data obtained for aryl imides 2.5–2.10 are shown in Figure 2.6. Alkyl imides 2.11 (λ = 418, 506 nm; ε = 1300, 830 cm-1 M-1) and 2.12 (λ = 422, 510 nm; ε = 1600, 1050 cm-1 M-1) are brown in color and feature less intense absorptions than their aryl congeners. 26 18000 16000 2.5 2.6 14000 2.7 2.8 2.9 10000 2.10 -1 -1 ε (cm M ) 12000 8000 6000 4000 2000 0 350 400 450 500 550 600 650 700 750 800 Wavelength (nm) Figure 2.6. Uv-vis data for imides 2.5–2.10. Structural analysis of 2.11 (Figure 2.7) revealed a more C3 symmetric structure than that observed for 2.5. Nearly idealized sp-hybridization was observed for the imide nitrogen as evidenced from the Fe(1)-N(1)-C(46) bond angle of 179.17(13)° (for 2.5 Fe(1)-N(1)-C(46) = 169.96(18)°). Also present is a characteristically short iron-nitrogen linkage of 1.6346(13) Å, which is consistent with multiple bonding character. These structural parameters are very similar to those observed for [PhBiPr3]Fe≡N(1-Ad) (Fe-N-C = 176.2(0)°, Fe-N = 1.638(2)), which is prepared from the addition of two equivalents of tert-butylazide to a solution of {[PhBPiPr3]Fe}2(µ-N2).18 The increased symmetry of 2.11 is manifested in its axial EPR spectrum (Figure 2.8), in which the observed g⊥ signal in the region 1.95 was not resolved even at temperatures as low as 5 K. Similar to the EPR data obtained for 2.5, the g║ region at 2.91 suggests that the unpaired electron resides in an orbital orthogonal to the Fe-N vector. 27 C46 N1 Fe1 P2 P1 P3 Figure 2.7. 50% thermal ellipsoid representation of [PhBP3]Fe≡NtBu (2.11). For the image on the right, all phosphino phenyl groups have been removed for clarity. For both images, all hydrogen atoms have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe1-N1, 1.6346(13); N1-C46, 1.442(2); Fe1-P1, 2.2427(5); Fe1-P2, 2.2153(5); Fe1-P3, 2.2531(5); C46-N1-Fe1, 179.17(13); P1-Fe1-P2, 89.96(2); P1-Fe1-P3, 91.16(2); P2-Fe1-P3, 89.38(2); N1Fe1-P1, 125.80(5); N1-Fe1-P2, 117.83(5); N1-Fe-P3, 131.04(5). = g g⊥ 150 200 250 300 350 400 450 Field (mT) Figure 2.8. EPR spectrum of 2.11 (glassy toluene at 4 K, X-band, 9.378 GHz). 28 An electrochemical comparison between 2.5, 2.11, and 2.12 (Figure 2.9) demonstrates that more electron density is present at the metal center for the alkyl substituted imides, as evidenced by their more negative redox potentials for the FeII/III couple (E1/2 = -1206, -1350, and -1317 mV, respectively). The nature of the irreversible oxidative events at ~ -200 mV has not been rigorously established; possibilities include borate ligand oxidation or the formation of a cationic, high-valent FeIV species. 2.5 2.11 2.12 200 -300 -800 -1300 -1800 + Potential (mV vs. Fc/Fc ) Figure 2.9. Cyclic voltammetry of 2.5, 2.11, and 2.12 (THF, 0.3 M [TBA][PF6], scan rate = 30 mV/s for 2.5, 100 mV/s for 2.11 and 2.12). 2.2.4 Imide Synthesis via Metathesis Although the iron(III) imides presented above are readily synthesized with an iron(I) precursor and organic azides, their synthesis using more conventional metathesis is of interest. For example, Bergman’s family of d6 iridium imides with the general formula Cp*Ir≡NR is generated upon reacting [Cp*IrCl2]2 with four equivalents of [Li][N(H)R].26 Lee generated his tetranuclear imide via the reaction of FeCl3 with 2 29 equiv. of [Li][N(H) Bu] while Hillhouse employed a deprotonation-oxidation strategy t 6c for the preparation of (dtbpe)Ni=NAr (dtbpe = 1,2 bis(di-tert-butylphosphino)ethane; Ar = 2,6-di-iso-propylphenyl).27 Two metathetic strategies may be envisioned for the preparation of [PhBP3]Fe≡NR. As shown in Equation 2.4, the first involves the reaction of an iron(III) dihalide with two equivalents of a lithiated amide. This strategy has proven successful for the synthesis of [PhBP3]Co≡NR complexes.28 The second approach involves the reaction of iron(II) halides with lithiated amides, followed by deprotonation and oxidation. Cl R Cl Fe [PhBP3] + 2 [Li][N(H)R] N - 2 [Li][Cl] - NH2R Fe [PhBP3] - Or - 1. Deprotonate 2. Oxidize H Cl Fe [PhBP3] + [Li][N(H)R] - [Li][Cl] N (2.4) R Fe [PhBP3] 2.1 All attempts to synthesize an Fe(III) dihalide species have been unsuccessful. These include the oxidation of 2.1 in the presence of excess halide ions, or the metallation of FeBr3 with [Tl][BP3]. The lack of any isolable iron(III) product from these experiments is in agreement with the observed cyclic voltammetry of 2.1, for which only a reversible FeII/I couple was observed. However, 2.1 does react readily with a variety of lithiated amides to form the desired iron(II) amide. For example, the reaction between 2.1 and [Li][N(H)-p-tolyl] generates [PhBP3]FeN(H)-p-tolyl, whose detailed synthetic protocol, characterization, and reactivity are presented in Chapter 3. Once purified, the 30 deprotonation of [PhBP3]FeN(H)-p-tolyl to yield {[PhBP3]Fe≡N-p-tolyl}- readily occurs in the presence of a variety of bases, which was verified with 31P NMR spectroscopy. Problematic with this synthetic route, however, is the absence of a high-yielding oxidative protocol. Varieties of ferrocinium salts, as well as the tropylium cation, have been canvassed to effect the oxidation of {[PhBP3]Fe≡N-p-tolyl}-, but yields of 2.5 were never greater than ~ 10% and separation of the desired Fe(III) product from the reaction matrix proved difficult. 2.2.5 Raman Characterization of the Fe≡NR Linkage Multiply bonded iron-nitrogen species are proposed as key intermediates in a number of group-transfer processes,29 including industrial nitrogen fixation,30 and vibrational spectroscopy provides a means for their in situ detection. Therefore, providing a spectroscopic benchmark for low molecular weight Fe-Nx linkages may prove essential to the analysis of potential intermediates during catalytic processes. Analysis of 2.5 with infrared spectroscopy did not reveal the presence of any vibration(s) that could unambiguously be assigned to the Fe≡NR linkage. Having established the presence of an intense chromophore with the aryl imides (vida supra), and to a lesser extent the alkyl imides, a collaboration with Prof. Larry Que and Dr. Mark Mehn of the University of Minnesota was initiated to examine their vibrational properties with resonance Raman spectroscopy.31 Initial studies with the iron(III) system focused on the evaluation of 2.7 and its isotopically labeled derivatives: 2.7-15N, 2.7-(Ph-d5), and 2.7-(15N, Ph-d5). Excitation into the low-energy charge-transfer band of these complexes with 647.1 nm light affords a 31 Figure 2.10. Resonance Raman spectra of (A) [PhBP3]Fe≡NPh (2.7), (B) [PhBP3]Fe≡15NPh (2.7-15N), (C) [PhBP3]Fe≡NPh-d5 (2.7-(Ph-d5)), (D) [PhBP3]Fe≡15NPh-d5 (2.7-(15N, Ph-d5)), and (E) [PhBP3]Fe≡N-p-tolyl. All spectra were obtained at 77 K using a back-scattering geometry (647.1 nm excitation). Peaks due to solvent are marked with an ‘s’. 32 rich Raman spectrum in each case (Figure 2.10). For 2.7 (Figure 2.10A), several prominent non-solvent features are present. The vibrations at 958, 995, 1292, and 1309 cm-1 involve displacement of the nitrogen atom as a downshift in energy is observed upon 15 N substitution (2.7-(15N); Figure 2.10B). The two modes with the greatest isotopic displacement, 958 → 946 cm-1 (∆E = 12 cm-1) and 1309 → 1277 cm-1 (∆E = 32 cm-1), are candidates for the Fe≡N vibration as shifts of 26 and 35 cm-1, respectively, are expected from a diatomic harmonic oscillator model. The feature at 1309 cm-1 is also highly sensitive to deuteration of the aryl ring (2.7-(Ph-d5); Figure 2.10C), suggesting that it is predominantly of N-Ar character. Therefore the vibration at 958 cm-1 is assigned to the Fe≡N linkage. The doubly labeled sample, 2.7-(15N, Ph-d5) (Figure 2.10D), which is anticipated to decouple the Fe≡N and N-Ar modes, features a mode at 932 cm-1 which is the calculated value of 26 cm-1 lower in energy than that observed for the Fe≡N mode of naturally abundant 2.7. Further support for the Fe≡N assignment is obtained through analysis of 2.5 (Figure 2.10E), which features a mode at 962 cm-1 that is assigned as νFe≡N. In order to eliminate vibrational modes associated with the aromatic ring of 2.7,32 resonance Raman studies with alkyl imide 2.11, 2.11-d9, and a 1:1 mixture of 2.11:2.1115 N were pursued.33 As shown in Figure 2.11, the resulting data from excitation into the high-energy charge-transfer band with 406.7 nm light affords rich Raman spectra that, although less intense than those obtained for 2.7 and its labeled derivatives, are readily interpreted. Figure 2.11A features peaks at 1104 and 1233 cm-1 obtained for naturally abundant 2.11 that, upon analysis of a 1:1 mixture of 2.11:2.11-15N (Figure 2.11B), split into doublets as expected for the presence of equal amounts of 14N and 15N. 33 Figure 2.11. Resonance Raman spectra of (A) [PhBP3]Fe≡NtBu (2.11), (B) 1:1 [PhBP3]Fe≡NtBu : [PhBP3]Fe≡15NtBu (2.11:2.11-15N), and (C) [PhBP3]Fe≡NtBud9. All spectra were obtained at 77 K using a back-scattering geometry (406.7 nm excitation). Peaks due to solvent are marked with an ‘s’. The peak at 1104 cm-1 shifts to 1084 cm-1 (∆E = 20 cm-1), while the peak at 1233 cm-1 shifts to 1228 cm-1 (∆E = 5 cm-1). This demonstrates that admixed character of the Fe≡N and N-Alkyl modes is still present, but to a much weaker extent than in the aryl systems. A simple diatomic is expected to downshift 30 cm-1; therefore the 1104 cm-1 mode must possess significant Fe≡N character. Consistent with this assignment is the spectra obtained with 2.11-d9 (Figure 2.11C), in which the 1104 cm-1 feature observed for 34 naturally abundant 2.11 shifts to 1096 cm (∆E = 8 cm-1), while the 1233 cm-1 feature -1 shifts to 1219 cm-1 (∆E = 14 cm-1). Deuteration of the alkyl group has a greater influence on the energy of the latter mode, which is therefore assigned to the N-alkyl linkage. A summary of the vibrational data obtained for both the alkyl and aryl systems, in addition to a variety of other metal imido species, is presented in Table 2.1. Table 2.1. Metal-imido vibrational data (Resonance Raman in CH2Cl2 unless otherwise noted). νMN-R (cm-1) νM-NR (cm-1) Ref. Compound Iron N-Alkyls [PhBP3]Fe≡NtBu 1104 1233 [PhBP3]Fe≡15NtBu 1084 1228 1096 1219 1111 1214 958 1292 [PhBP3]Fe≡ NPh 946 1277 [PhBP3]Fe≡NPh-d5 936 1249 [PhBP3]Fe≡ NPh-d5 932 1236 [PhBP3]Fe≡NTol 962 1281 [PhBP3]Co≡NtBu 1103 1238 [PhBP3]Co≡15NtBu 1084 [PhBP3]Co≡NtBu-d9 1100 1226 31 [PhBP3]Co≡NPh 956 1307 31 [PhBP3]Co≡15NPh 944 1295 31 [PhBP3]Co≡NPh-d5 934 1252 31 [PhBP3]Co≡15NPh-d5 934 1242 31 [PhBP3]Co≡NTol 959 1317 31 Cp*2V≡NPh 934 1330 34 Cp*2V≡15NPh 923 1307 34 t [PhBP3]Fe≡N Bu-d9 t t a [Fe4(µ3-N Bu)4(N Bu)Cl3] 6c Iron N-Aryls [PhBP3]Fe≡NPh 15 15 Cobalt N-Alkyls 31 31 Cobalt N-Aryls Vanadium N-Arylsb a IR data, KBr b IR data, Nujol 35 2.2.6 Initial Reactivity Studies of the Fe≡NR Linkage Preliminary reactivity studies with imide 2.5 highlight its reactive nature by comparison to its cobalt congener [PhBP3]Co≡NAr, which required forcing conditions to release its "NAr" group to CO (70 °C, 12 days).12 Complex 2.5 reacted immediately and quantitatively at room temperature upon CO addition to release isocyanate (O=C=N-ptolyl, external integration standard) and form the golden dicarbonyl byproduct [PhBP3]Fe(CO)2 (2.13). Complex 2.13 (νCO = 1979, 1914 cm-1, cf. [PhTptBu]Fe(CO)4 = 1907 cm-1) has been structurally characterized as a distorted square pyramid (Figure 2.12) and independently generated by reaction of 2.4 with excess CO. In light of the recent nickel chemistry reported by Hillhouse and Mindiola,35 well-defined examples of nitrene transfer to CO with release of isocyanate are now established for first-row systems including Fe(III), Co(III), and Ni(II). Particular to the present system is that dicarbonyl 2.13 is itself a precursor to imide 2.5 (Equation 2.5). An NMR tube experiment demonstrated that the addition of 2 equivalents of p-tolyl azide to 2.13 slowly (rt, C6D6, 44 h) effected its conversion back to imide 2.5, along with the release of free isocyanate (80%, external standard). The extended reaction times characteristic of this reaction (when compared with the formation of 2.5 from 2.4 and p-tolyl azide) likely reflect slow dissociation of CO from iron if proceeding through a dissociative mechanism, or the formation of an electronically unfavorable 19 e- intermediate if proceeding through an associative mechanism. 2 p-tolyl-N=N=N / - p-tolyl-NCO [PhBP3]Fe N 2.5 xs CO [PhBP3]Fe(CO)2 2.13 + NCO (2.5) 36 Despite these extended reaction times, catalytic isocyanate production is observed when a reaction vessel is cycled with azide and CO as necessary although conditions for an efficient “one-pot” catalytic procedure have yet to be established. O1 O2 C46 C47 Fe1 P3 P1 P2 Figure 2.12. 50% thermal ellipsoid representation of [PhBP3]Fe(CO)2 (2.13). For the image on the right, all phosphino phenyl groups have been removed for clarity. For both images, all hydrogen atoms have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe1-C46, 1.751(3); Fe1-C47, 1.768(3); Fe1-P1, 2.3103(8); Fe1-P2, 2.2819(8); Fe1-P3, 2.2626(8); C46-Fe1-P1, 92.83(10); C46-Fe1-P2, 164.62(10); C46-Fe1-P3, 99.50(10); C47-Fe1-P1, 173.88(9); C47-Fe1-P2, 92.67(9); C47-Fe1-P3, 96.35(9). Although 2.5 does not react with acetonitrile at room temperature, group transfer chemistry is observed in the presence of isonitriles. Thus, exposure of 2.5 to excess t BuN≡C readily generates the five-coordinate iron(I) species, [PhBP3]Fe(C≡N-tBu)2 (2.14), with concomitant formation of the carbodiimide, tBu-N=C=N-p-tolyl. As a final note concerning the reactivity of 2.5, reaction conditions have yet to be established that 37 facilitate aziridination chemistry. This behavior contrasts that of Hillhouse’s d8 Ni(II) system, which is capable of aziridinating ethylene upon gentle warming.36 2.3 Discussion The preceding sections have presented structural, magnetic, and spectroscopic characterization for a number of 4-coordinate iron compounds supported by the first generation tris(phosphino)borate ligand, [PhBP3]. Despite being coordinatively and electronically unsaturated, these complexes remain monomeric in the solid state and feature iron in the +1, +2, and +3 oxidation states while facilitating coordination chemistry with both π-acidic (CO) and π-basic (NR2-) ligands. Compound 2.4 represents a rare example of 4-coordinate iron(I) that is readily oxidized by organic azides to yield terminal iron(III) imides.37 The potential for these Fe≡NR linkages to engage in group transfer chemistry is highlighted in their reactivity to generate isocyanates and carbodiimides upon exposure to CO and isonitrile, respectively. These systems are inherently more reactive than their Co(III) congeners, which likely reflects the metalloradical character of the Fe(III) electronic configuration. In contrast to the second generation “[PhBPiPr3]Fe” system, N2 coordination was never observed with 2.1 under a variety of reductive conditions. This is attributed to the attenuated amount of electron density at the metal center for the parent [PhBP3] system, which is manifested in a comparative electrochemical study between the respective chloride derivatives. Such density is necessary for π-backbonding with N2 and for stabilization of the B-C bonds with the ligand framework. Nevertheless, the steric and electronic properties of [PhBP3] that facilitate the formation of Fe≡NR linkages are topics of considerable interest. Recent studies in the Tilley lab with [PhBP3]Ir, Cp*Ir, and Me2 [Tp 38 ]Ir complexes suggest that of these auxiliary ligands, [PhBP3] is the most electron donating and sterically demanding.38 The synergistic effect of these properties in engendering unique coordination environments was first realized with the isolation of an unprecedented, low-spin pseudotetrahedral cobalt(II) compound, [PhBP3]CoI.9 As shown in Figure 2.13, this electronic anomaly is rationalized on the basis of an axial distortion (i.e., L-M-L ~ 90° versus 109.5° for a tetrahedral system) that is enforced by the [PhBP3] ligand, which results in a lowering of the a1 orbital in C3v symmetry. The extent to which this orbital is lowered, in combination with the destabilizing effect of the strong-field phosphine donors on the σ* orbitals of the upper e set, determines whether it is energetically favorable to form a high-spin species or spin-pair the electrons to form a low-spin species. For [PhBP3]CoI, the latter electronic situation is preferred and a JahnTeller elongation of one Co-P bond distance results from placing the seventh d-electron in the upper e set.12 The inability of [Tp] ligands to stabilize a low-spin electronic z Td C3v L Y C3v y x axial distortion M L L L M L L 90° L L L L L L e For low-spin Co(II): L M M L L Oh Y e L eg Jahn-Teller t2 e a1 + e e a1 e a1 e C3v Cs t2g Figure 2.13. (left) Qualitative d-orbital splitting diagram illustrating the decent in symmetry from Td to axially distorted C3v. (right) Qualitative d-orbital splitting diagram illustrating the Jahn-Teller distortion observed for [PhBP3]CoI. This figure is adapted from reference 12. 39 ground-state for pseudotetrahedral cobalt(II)17c,39 likely arises from two factors: (1) the weaker field splitting effects of the nitrogenous ligands (i.e., the upper e set that is σ* with respect to the pyrazolyl donors is not as destabilized as it is with phosphine donors), and (2) the geometrical rigidity of the [Tp] framework, which may not be capable of accommodating a Jahn-Teller distortion.40 Although halides 2.1–2.3 feature high-spin electronic configurations, the preceding electronic arguments suggest that under C3 symmetry, low-spin d5 and d6 iron species should be stable as the SOMO or HOMO, respectively, would be of relatively non-bonding character. One method for obtaining this low-spin configuration involves further destabilization of the upper e set through the installation of a π-basic ligand. This would make spin-pairing of the d-electrons energetically favorable when compared with placing them in relatively high-energy d-orbitals that now feature σ* and π* character. Such an electronic situation is in fact observed when NR2- ligands are installed in the fourth coordination site of the “[PhBP3]Fe” subunit. The electronic structures of transition metal imido compounds have been discussed in some detail,41 and the unique nature of the Fe≡NR linkages presented herein warrants further consideration of the d-orbital manifold. For Bergman’s Cp*Ir≡NR systems,26 the dz2 orbital is presumed to not interact too strongly with the Cp* ligand, a common feature of metallocenes.42 Consequently, the Ir-N σ-bond was largely attributed to interactions between the nitrogen σ-orbital and the Ir-pz orbital (with some admixed dz2 character), rendering the dz2 orbital essentially non-bonding. Whether the dz2 orbital (with some admixed s and pz character) actually lies lowest in energy for the [PhBP3]Fe≡NR systems was of particular interest, and a DFT study with alkyl imide 2.11 was examined. 40 A restricted DFT geometry optimization calculation using the X-ray coordinates of 2.11 as the initial HF guess demonstrates several salient features concerning its electronic structure. As shown in Figure 2.14, the highest-lying metal-based orbitals are of dxz and dyz parantage and are π* with respect to the imido nitrogen atom. This calculation is therefore consistent with solid-state data when ascribing triple bond character to the Fe≡NR linkage. Also evident is the relatively non-bonding nature of the orbital with dz2 parentage, which is only slightly higher in energy than the filled orbital of non-bonding character in the xy plane (dxy or dx2-y2). The SOMO has been ordered as the lowest-lying orbital, which is likely a consequence of the restricted nature of the openshell calculation and prevents the determination of a reliable SOMO-LUMO gap.43 MO 210 - SOMO + 3 MO 211 - SOMO + 4 MO 206 - SOMO + 1 MO 208 - SOMO + 2 X-Ray DFT o 177.5 1.630 Å 2.316 Å 2.335 Å 2.357 Å Fe-N-C Fe-N Fe-P Fe-P Fe-P Orbital 211 - SOMO + 4 210 - SOMO + 3 208 - SOMO + 2 206 - SOMO + 1 209 - SOMO o 179.2 1.635 Å 2.215 Å 2.243 Å 2.253 Å Energy (kcal/mol) - 28.9 - 29.7 - 127.1 - 133.4 - 168.7 MO 209 - SOMO Figure 2.14. Theoretically predicted geometry and electronic structure (DFT, JAGUAR 5.0, B3LYP/LACVP**) for S = ½ [PhBP3]Fe≡NtBu (2.11). Structural parameters are highlighted within the figure. 41 The fact that the unpaired electron is calculated to reside in the xy plane is in agreement with the g║ values of 2.61 and 2.91 observed in the EPR spectra obtained for 2.5 and 2.11, respectively, which demonstrate considerable angular momentum dependence. These electronic considerations suggest that an isoelectronic oxo ligand (O2-) should be compatible with the “[PhBP3]Fe” platform to generate an iron(III) oxo, [PhBP3]Fe(O). However, DFT studies with [PhBPiPr3]Fe≡N demonstrate that significant electronic changes are observed upon going from an M≡NR linkage to a terminal M≡N linkage.44 In fact, two properties were noted that significantly destabilize the orbital of a1 character in C3v symmetry. The first concerns the L-Fe-L angles, which are appreciably expanded (99–101°) to yield a structure more tetrahedral in nature. Additionally, the hybridization of the terminal atom allows for increased σ interactions with the a1 orbital, resulting in significant σ* character (a HOMO-LUMO gap of 3.90 eV is calculated for [PhBPiPr3]Fe≡N). As shown in Figure 2.15, an iron(III) system would place one electron in the now destabilized a1 orbital, which is anticipated to yield a highly reactive and/or unstable linkage. A cationic iron(IV) oxo45 would alleviate this situation, although this is an oxidation state that has yet to be observed with the [PhBP3]Fe system. R O N Fe Fe L L L L L L σ* (z2) nb (z2) Figure 2.15. Qualitative d-orbital splitting diagram for a d5 imide and a hypothetical d5 oxo. 42 Resonance Raman experiments with 2.7 and its isotopically labeled derivatives provide a preliminary assignment of 958 cm-1 for the Fe≡N vibration (Figure 2.10) and demonstrate that the charge transfer band observed at ~ 640 nm (see Figure 2.6) in the UV-vis spectrum is associated with the Fe≡N linkage. This value is higher in energy than that of 876 cm-1 reported by Nakamoto for (TPP)FeV(N), although Nakamoto’s value likely reflects attenuation of the Fe-N bond order resulting from occupation of the Fe-N π* orbitals by two electrons.46 The structurally related iron(IV) nitride, [PhBPiPr3]Fe≡N, features an Fe≡N vibration at 1034 cm-1.44 However, direct comparisons between the imide and nitride systems may not be warranted as the current study demonstrates that a significant degree of vibronic coupling exists between the Fe≡N and N-Ar vibrational modes (Figures 2.10 B, C, and D). A similar situation was reported for Cp*2V≡NPh,34 in addition to Lee’s tetranuclear iron imide (νFe≡N = 1111 cm-1).6c Thus for the imides under investigation, a pure Fe≡N vibration is not observed and the experimental value may actually reflect an average of the symmetric and anti-symmetric stretches of the Fe≡N-Ar linkage.47 Resonance Raman experiments with the [PhBP3]Co(III) aryl imide systems (see Table 2.1) are similar in that extensive coupling between the Co≡N and N-Ar linkages is observed. Resonance Raman data obtained for 2.11 and its isotopically labeled derivatives are significantly easier to interpret than those obtained for the aryl system (Figure 2.11). This is presumably due to the absence of any aryl vibrational modes, and from these data a value of 1104 cm-1 is proposed for νFe≡N. As alluded to above, direct comparisons with terminal systems are tenuous at best, as a pure Fe≡N vibration is not observed due to vibronic coupling between the Fe≡N and N-tBu linkage. 43 2.4 Conclusions In summary, this work shows that Fe(I) supported by a strong field and tripodal phosphine donor can undergo a facile, multielectron group transfer process to accept a strongly π-donating imide ligand. The imide can be subsequently released to the acceptor substrate CO. The method of formation of imide 2.5, its reactivity with CO, and its regeneration from 2.13 underscore the viability of a well-defined, Fe(I) → Fe(III)/Fe(III) → Fe(I) group transfer loop. Whether parallels to such group transfer processes occur in biological systems, such as the reducing FeS-clusters of certain metalloenzymes, is worth considering. 2.5 Experimental Section 2.5.1 General Considerations All manipulations were carried out using standard Schlenk or glove-box techniques under a dinitrogen atmosphere. Unless otherwise noted, solvents were deoxygenated and dried by thorough sparging with N2 gas followed by passage through an activated alumina column. Non-halogenated solvents were tested with a standard purple solution of sodium benzophenone ketyl in tetrahydrofuran in order to confirm effective oxygen and moisture removal. Elemental analyses were performed by Desert Analytics, Tucson, AZ. Deuterated benzene was purchased from Cambridge Isotope Laboratories, Inc. and was degassed and dried over activated 3 Å molecular sieves prior to use. A Varian Mercury-300 NMR spectrometer was used to record 1H, 31P, and 19F NMR spectra at ambient temperature. 1 H chemical shifts were referenced to residual solvent. 31P chemical shifts were referenced to 85% H3PO4 at δ 0 ppm, and 19F chemical shifts were referenced to C6F6 at δ -167.7 ppm. MS data for samples were obtained by 44 injecting a benzene solution into a Hewlett-Packard 1100MSD mass spectrometer equipped with an electrospray (ES) ionization chamber. UV-vis measurements were taken on a Hewlett Packard 8452A Diode Array Spectrometer using a quartz cell with a Teflon cap. IR measurements were obtained with a KBr solution cell using a Bio-Rad Excalibur FTS 3000 spectrometer controlled by Bio-Rad Merlin Software (v. 2.97) set at 4 cm-1 resolution. 2.5.2 Magnetic Measurements Measurements were recorded using a Quantum Designs SQUID magnetometer running MPMSR2 software (Magnetic Property Measurement System Revision 2). Data were recorded at 5000 G. Samples were suspended in the magnetometer in a clear plastic straw sealed under nitrogen with Lilly No. 4 gel caps. Loaded samples were centered within the magnetometer using the DC centering scan at 35 K and 5000 G. Data were acquired at 2–10 K (one data point every 2 K), 10–60 K (one data point every 5 K), and 60–310 K (one data point every 10 K). The magnetic susceptibility was adjusted for diamagnetic contributions using the constitutive corrections of Pascal’s constants. The molar magnetic susceptibility (χm) was calculated by converting the calculated magnetic susceptibility (χ) obtained from the magnetometer to a molar susceptibility (using the multiplication factor {(molecular weight)/[(sample weight)*(field strength)]}). CurieWeiss behavior was verified by a plot of χm-1 versus T. Effective magnetic moments were calculated using Equation 2.6. Solution magnetic moments were measured using Evans method.10 µeff = sqrt(7.997 χmT) (2.6) 45 2.5.3 EPR Measurements X-band EPR spectra were obtained on a Bruker EMX spectrometer (controlled by Bruker Win EPR Software v. 3.0) equipped with a rectangular cavity working in the TE102 mode. Variable temperature measurements were conducted with an Oxford continuous-flow helium cryostat (temperature range 3.6–300 K). Accurate frequency values were provided by a frequency counter built into the microwave bridge. Solution spectra were acquired in toluene. Sample preparation was performed under a dinitrogen atmosphere in an EPR tube equipped with a ground glass joint. 2.5.4 Electrochemical Measurements Electrochemical measurements were carried out in a glove-box under a dinitrogen atmosphere in a one-compartment cell using a BAS model 100/W electrochemical analyzer. A glassy carbon electrode and platinum wire were used as the working and auxiliary electrodes, respectively. The reference electrode was Ag/AgNO3 in THF. Solutions (THF) of electrolyte (0.3 M tetra-n-butylammonium hexafluorophosphate) and analyte (2 µM) were also prepared in a glove-box. 2.5.5 DFT Calculations A hybrid density functional calculation was performed for 2.11 using the Jaguar package (version 5.0, release 20).48 The calculation employed B3LYP with LACVP** (LACVP**++ for B) as the basis set.49 A geometry optimization was carried out using the X-ray coordinates for 2.11 as the initial HF guess. No symmetry constraints were imposed, and the calculation was performed assuming a doublet electronic ground state. Pictorial representations of the resulting molecular orbitals were generated using the gOpenMol software program. 46 2.5.6 Raman Measurements Resonance Raman spectra were collected on an Acton AM-506 spectrometer (1200 groove grating) using a Kaiser Optical holographic super-notch filter with a Princeton Instruments liquid N2 cooled (LN-1100PB) CCD detector with a 4 cm-1 spectral resolution. The laser excitation lines were obtained with a Spectra Physics 2030KR-V krypton laser. The Raman frequencies were referenced to indene, and the entire spectral range was obtained by collecting spectra at several different frequency windows and splicing the spectra together. The spectra were obtained at 77 K using a backscattering geometry on samples frozen on a gold-plated copper cold finger in thermal contact with a Dewar containing liquid N2. Typical accumulation times were 16–32 minutes per frequency window. Curve fits (Gaussian functions) and baseline corrections (polynomial fits) were carried out using Grams/32 Spectral Notebase Version 4.04 (Galactic). 2.5.7 Starting Materials and Reagents All reagents were purchased from commercial vendors and used without further purification unless otherwise noted. [Tl][PhBP3],9 p-tolylazide,50 phenylazide,50 N3(Ph-pCF3),51 N3(Ph-p-NMe2),52 and tert-butylazide53 were prepared according to literature procedures. N3(Ph-p-tBu) was prepared following the procedure outlined for p-tolylazide but with 4-tert-butylaniline. [Na][15NNN] was obtained from Cambridge Isotope Labs and used as received for the synthesis of tert-butylazide-15N. The 15N label resides in a 50:50 statistical distribution at the α and γ positions of the organic azide, resulting in 50% incorporation of the 15N label into the corresponding metal imide when synthesized via azide decomposition. tert-butanol-d9 was obtained from Cambridge and used as received 47 for the synthesis of tert-butylazide-d9. For Ph-15NNN and d5-PhN3, aniline-15N and aniline-2,3,4,5,6-d5 were obtained from Aldrich and used as received. The doubly labeled azide, d5-Ph-α-15NNN, was synthesized from d5-Ph-15NH2 which was prepared from the nitration of benzene-d6 (Cambridge) with [NH4][15NO3] (Cambridge)54 followed by reduction with H2 in the presence of 10 % Pd/C. 2.5.8 Synthesis of Compounds Synthesis of [PhBP3]FeCl, 2.1: FeCl2 (0.285 g, 2.25 mmol) was added to THF (80 mL) with stirring. A THF slurry (40 mL) of [Tl][PhBP3] (2.00 g, 2.25 mmol) was then added dropwise. During the addition, the reaction yellowed as TlCl precipitated from solution. After stirring overnight at room temperature, volatiles were removed under reduced pressure, and the crude solids were extracted with benzene (~ 30 mL). The benzene extract was then filtered over celite. Removal of the solvent under reduced pressure followed by washing with petroleum ether (3 x 30 mL) and drying afforded 2.1 as a yellow powder (1.45 g, 83 %). X-ray quality crystals were grown via vapor diffusion of petroleum ether into a benzene solution. 1H NMR (C6D6, 300 MHz): δ 40.8 (s); 19.8 (s); 18.3 (s); 7.01 (s); -12.2 (s); -37.7 (br, s). UV-vis (C6H6) λmax, nm (ε, M-1 cm1 ): 416 (580). SQUID (solid, average 10 – 310 K): µeff = 4.95 µB. Evans Method (C6D6): 5.04 B.M. ES-MS: calcd. for C45H41BClFeP3 (M)+ 777 m/z, found (M – Cl)+ 741 m/z. Anal. Calcd. for C45H41BClFeP3: C, 69.57; H, 5.32. Found: C, 69.69; H, 5.21. Synthesis of [PhBP3]FeBr, 2.2: FeBr2 (0.0240 g, 0.112 mmol) was added to THF (3 mL) with stirring. A THF slurry (3 mL) of [Tl][PhBP3] (0.100 g, 0.112 mmol) was then added dropwise. During the addition, the reaction changed from brown to yellow as TlBr precipitated from solution. After stirring overnight, volatiles were 48 removed under reduced pressure, and the crude solids were extracted with benzene (3 mL), filtered over Celite, lyophilized, and washed with petroleum ether (3 x 5 mL). The resulting yellow powder was dried to yield 2.2 (0.0775 g, 84 %). 1H NMR (C6D6, 300 MHz): δ 37.0 (s); 18.2 (s); 17.1 (s); 8.89 (s); -11.2 (s); -32.9 (br, s). UV-vis (C6H6) λmax, nm (ε, M-1 cm-1): 424 (620). Evans Method (C6D6): 5.20 µB. ES-MS: calcd. for C45H41BBrFeP3 (M)+ 821 m/z, found (M – Br)+ 741 m/z. Anal. Calcd. for C45H41BBrFeP3: C, 65.81; H, 5.03. Found: C, 65.42; H, 4.87. Synthesis of [PhBP3]FeI, 2.3: FeI2 (0.100 g, 0.323 mmol) was added to a THF/Et2O solution (3 mL/3 mL) with stirring. A THF slurry (3 mL) of [Tl][PhBP3] (0.287 g, 0.323 mmol) was then added dropwise. During the addition, the reaction gradually changed from purple to yellow as TlI precipitated from solution. After stirring overnight, volatiles were removed under reduced pressure, and the crude solids were extracted with benzene (5 mL), filtered over Celite, lyophilized, and washed with petroleum ether (3 x 5 mL). The resulting yellow powder was dried to yield 2.3 (0.237 g, 85 %). 1H NMR (C6D6, 300 MHz): δ 29.4 (s); 15.1 (s); 14.6 (s); 12.1 (s); -8.96 (s); -22.9 (br, s). UV-vis (C6H6) λmax, nm (ε, M-1 cm-1): 438 (650). Evans Method (C6D6): 5.03 µB. ES-MS: calcd. for C45H41BIFeP3 (M)+ 868 m/z, found (M – I)+ 741 m/z. Anal. Calcd. for C45H41BIFeP3: C, 62.25; H, 4.76. Found: C, 62.60; H, 4.72. Synthesis of [PhBP3]Fe(PPh3), 2.4: A 0.19 weight % Na/Hg amalgam (0.0296 g, 1.29 mmol of sodium dissolved in 15.6 g of mercury) was stirred in THF (50 mL). To this was added dropwise a THF solution (20 mL) of 2.1 (1.00 g, 1.29 mmol) and PPh3 (1.01 g, 3.86 mmol). During the addition, the reaction changed from yellow to orange and finally a brown color approximately 20 minutes after the addition was complete. 49 After 15 hours, the reaction solution was decanted from the mercury, and volatiles were removed under reduced pressure. The crude solids were extracted with benzene (50 mL) and filtered over Celite. The filtrate was then concentrated under reduced pressure to approximately 20 mL, at which point a small amount of precipitate was evident. The addition of petroleum ether (150 mL) resulted in the precipitation of orange solids, which were collected on a sintered glass frit and washed with petroleum ether (3 x 15 mL). After drying, 2.4 was obtained as an orange powder (0.795 g, 62 %). X-ray quality crystals were grown via vapor diffusion of petroleum ether into a benzene solution. 1H NMR (C6D6, 300 MHz): δ 13.1 (br, s); 11.1 (br, s); 8.51 (s); 7.49 (s); -1.09 (br, s); -2.68 (br, s). Evans Method (C6D6): 3.88 B.M. SQUID (solid, average 10 – 310 K): µeff = 4.09 µB. ES-MS: calcd. for C63H56BFeP4 (M)+ 1004 m/z, found (M – PPh3)+ 741 m/z. Anal. Calcd. for C63H56BFeP4: C, 75.39; H, 5.62. Found: C, 75.60; H, 5.61. Synthesis of [PhBP3]Fe≡N-p-tolyl, 2.5: Compound 2.4 (0.600 g, 0.598 mmol) was dissolved in benzene (15 mL) with stirring. A benzene solution (2 mL) of ptolylazide (0.159 g, 1.19 mmol) was added dropwise, during which time the reaction changed color from orange to an intense forest green. After 12 hours, volatiles were removed under reduced pressure, and the solids were washed with petroleum ether (3 x 20 mL) and collected on a sintered glass frit. Drying yielded 0.453 g of a forest green powder contaminated with the Ph3P=N(p-tolyl) by-product. Crystallization from petroleum ether into benzene yielded pure 2.5 (0.255 g, 50 %). Alternatively, the benzene reaction solution may be concentrated to ~ 1/3 the volume and with the slow addition of petroleum ether, 2.5 selectively precipitates. The purity of 2.5 obtained from this method is dependent on the benzene/petroleum ether ratio. With the appropriate combination, 50 pure 2.5 may be obtained. H NMR (C6D6, 300 MHz): δ 26.4 (br, s); 13.8 (s); 13.3 (s); 1 10.4 (br, s); 9.90 (t, J = 6.6 Hz); 9.08 (t, J = 6.9 Hz); 7.93 (br, s); 5.47 (d, J = 6.6 Hz); 3.14 (s); -0.75 (s). UV-vis (C6H6) λmax, nm (ε, M-1 cm-1): 640 (3300). SQUID (solid, average 10 – 310 K): µeff = 2.03 µB. Evans Method (C6D6): 1.85 µB. Anal. Calcd. for C52H48BFeNP3: C, 73.78; H, 5.72; N, 1.65. Found: C, 73.62; H, 5.84; N, 1.93. Synthesis of {[PhBP3]Fe≡N-p-tolyl}{Na(THF2)}, 2.6: Imide 2.5 was stirred in THF (~5 mL) with a 0.19 weight % Na/Hg amalgam. After 4 hours the THF was decanted from the amalgam, and volatiles were removed under reduced pressure. The crude solids were washed with petroleum ether (10 mL) and dried to obtain 2.6 as a green solid (0.105 g, 88 %). 1H NMR (C6D6, 300 MHz): δ 8.21 (d, J = 7.2 Hz, 2H); 7.89 (s, 12 H); 7.69 (m, 4H); 7.42 (t, J = 7.2 Hz, 1H); 6.90 (d, J = 8.4 Hz, 2H); 6.80 (m, 18H); 3.14 (m, 8H); 2.03 (s, 3H); 1.56 (s, 6H); 1.23 (m, 8H). 31P{1H} NMR (C6D6, 121.4 MHz): δ 87 (s). UV-vis (C6H6) λmax, nm (ε, M-1 cm-1): 630 (2300), 370 (8700). Anal. Calcd. for C60H64BFeNNaO2P3: C, 71.09; H, 6.36; N, 1.38. Found: C, 70.70; H, 6.13; N, 1.36. Synthesis of [PhBP3]Fe≡NPh, 2.7: Compound 2.4 (0.200 g, 0.199 mmol) was added to benzene (~5 mL) with stirring. A benzene solution (1 mL) of phenyl azide (0.0475 g, 0.399 mmol) was added dropwise, resulting in a rapid color change from orange to forest green. After 12 hours the reaction was concentrated to a volume of 2 mL under reduced pressure. The addition of petroleum ether (~18 mL) and stirring for an additional 30 minutes resulted in the precipitation of green solids, which were isolated on a medium porosity sintered glass frit. After washing with additional petroleum ether (3 x 10 mL), the green solids were dried under reduced pressure to yield 2.7 as a pure compound (0.119 g, 72 %). Crystals of 2.7 were grown via vapor diffusion of petroleum 51 ether into a benzene solution. H NMR (C6D6, 300 MHz): δ 14.0 (s); 13.3 (s); 10.1 (br, 1 s); 9.90 (t, J = 6.0 Hz); 9.42 (br, s); 9.09 (t, J = 7.5 Hz); 5.37 (d, J = 7.5 Hz); 3.21 (t, J = 6.0 Hz); -0.65 (br, s); -7.19 (s). UV-vis (C6H6) λ, nm (ε, M-1 cm-1): 632 (3000). Evans Method (C6D6): 1.87 µB. Anal. Calcd. for C51H46BFeNP3: C, 73.58; H, 5.57; N, 1.68. Found: C, 73.73; H, 5.68; N, 1.76. Synthesis of [PhBP3]Fe≡N(Ph-p-CF3), 2.8: Compound 2.4 (0.200 g, 0.199 mmol) was added to benzene (~5 mL) with stirring. A benzene solution (1 mL) of N3(Php-CF3) (0.0475 g, 0.399 mmol) was then added dropwise, resulting in a rapid color change from orange to forest green. After 12 hours the reaction was concentrated to a volume of 2 mL under reduced pressure. The addition of petroleum ether (~18 mL) and stirring for an additional 30 minutes resulted in the precipitation of green solids, which were isolated on a medium porosity sintered glass frit. After washing with additional petroleum ether (3 x 10 mL), the green solids were dried under reduced pressure to yield 2.8 as a pure compound (0.123 g, 69 %). Crystals of 2.8 were grown via vapor diffusion of petroleum ether into a benzene solution. 1H NMR (C6D6, 300 MHz): δ 14.8 (s); 13.2 (s); 9.86 (t, J = 6.0 Hz); 9.06 (t, J = 6.3 Hz); 8.71 (br, s); 8.54 (br, s); 5.21 (d, J = 6.3 Hz); 3.44 (t, J = 6.0 Hz); -0.41 (br, s). 19F NMR (C6D6, 282 MHz): δ -27.2 (s). UV-vis (C6H6) λ, nm (ε, M-1 cm-1): 632 (2800). Evans Method (C6D6): 1.80 µB. Anal. Calcd. for C52H45BF3FeNP3: C, 69.36; H, 5.04; N, 1.56. Found: C, 68.99.; H, 5.36; N, 1.70. Synthesis of [PhBP3]Fe≡N(Ph-p-tBu), 2.9: Compound 2.4 (0.200 g, 0.199 mmol) was added to benzene (~5 mL) with stirring. A benzene solution (1 mL) of N3(Ph-p-tBu) (0.0698 g, 0.399 mmol) was added dropwise, resulting in a rapid color change from orange to forest green. After 12 hours the reaction was concentrated to a 52 volume of 2 mL under reduced pressure. The addition of petroleum ether (~18 mL) and stirring for an additional 30 minutes resulted in the precipitation of green solids, which were isolated on a medium porosity sintered glass frit. After washing with additional petroleum ether (3 x 10 mL), the green solids were dried under reduced pressure to yield 2.9 as a pure compound (0.090 g, 51 %). Crystals of 2.9 were grown via vapor diffusion of petroleum ether into a benzene solution. 1H NMR (C6D6, 300 MHz): δ 13.7 (s); 13.3 (s); 11.3 (br, s); 9.92 (t, J = 6.0 Hz); 9.75 (br, s); 9.11 (t, J = 6.3 Hz); 5.43 (d, J = 6.3 Hz); 3.18 (t, J = 6.0 Hz); -0.70 (br, s). UV-vis (C6H6) λ, nm (ε, M-1 cm-1): 639 (3400). Evans Method (C6D6): 1.88 µB. Synthesis of [PhBP3]Fe≡N(Ph-p-NMe2), 2.10: Compound 2.4 (0.200 g, 0.199 mmol) was added to benzene (~5 mL) with stirring. A benzene solution (1 mL) of N3(Ph-p-NMe2) (0.0475 g, 0.399 mmol) was added dropwise, resulting in a rapid color change from orange to forest green. After 12 hours the reaction was concentrated to a volume of 2 mL under reduced pressure. The addition of petroleum ether (~18 mL) and stirring for an additional 30 minutes resulted in the precipitation of green solids, which were isolated on a medium porosity sintered glass frit. After washing with additional petroleum ether (3 x 10 mL), the green solids were dried under reduced pressure to yield 2.10 (0.119 g, 72 %) contaminated with traces of the phosphaimide byproduct as evidenced by 1H and 31P NMR spectroscopy. Crystallization of the crude solids via vapor diffusion of petroleum ether into benzene yielded pure 17 (0.050 g, 29 %). 1 H NMR (C6D6, 300 MHz): δ 17.1 (s); 12.8 (br, s); 10.5 (s); 9.70 (t, J = 6.0 Hz); 8.93 (t, J = 7.5 Hz); 6.00 (d, J = 6.0 Hz); 3.20 (s); 2.20 (br, s); -0.27 (br, s). UV-vis (C6H6) λ, nm (ε, 53 M cm ): 446 (17000); 699 (6,500). Evans Method (C6D6): 1.80 µB. Anal. Calcd. for -1 -1 C53H51BFeN2P3: C, 72.70; H, 5.87; N, 3.20. Found: C, 72.90; H, 6.05; N, 3.22. Synthesis of [PhBP3]Fe≡NtBu, 2.11: Compound 2.4 (0.200 g, 0.199 mmol) was added to benzene (~5 mL) with stirring. A benzene solution (1 mL) of tert-butylazide (0.0395 g, 0.398 mmol) was added dropwise, during which time the reaction changed color from orange to brown. After 12 hours volatiles were removed under reduced pressure. The resulting crude solids were washed with petroleum ether (3 x 20 mL) and dried under reduced pressure to yield 2.11 as a brown solid (0.113 g, 70 %). X-ray quality crystals were grown via slow evaporation of a benzene solution. 1 H NMR (C6D6, 300 MHz): δ 19.3 (br, s); 16.5 (br, s); 15.0 (s); 10.6 (t, J = 3.0 Hz); 9.61 (t, J = 6.8 Hz); 4.92 (d, J = 6.6 Hz); 2.39 (t, J = 6.6 Hz); -3.67 (br, s). UV-vis (C6H6) λ, nm (ε, M-1 cm-1): 418 (1300); 506 (830). Evans Method (C6D6): 1.96 µB. Anal. Calcd. for C49H50BFeNP3: C, 72.43; H, 6.20; N, 1.72. Found: C, 72.26; H, 6.11; N, 1.83. Synthesis of [PhBP3]Fe≡N(1-Ad), 2.12: Compound 2.4 (0.100 g, 0.0996 mmol) was added to benzene (~5 mL) with stirring. A benzene solution (1 mL) of 1- azidoadamantane (0.0353 g, 0.199 mmol) was added dropwise, during which time the reaction changed color from orange to brown. After 12 hours volatiles were removed under reduced pressure. The resulting crude solids were washed with petroleum ether (3 x 20 mL) and dried under reduced pressure to yield 2.12 as a brown solid (0.071 g, 80 %). X-ray quality crystals were grown via vapor diffusion of petroleum ether into a benzene solution. 1H NMR (C6D6, 300 MHz): δ 23.1 (br, s); 16.8 (br, s); 14.9 (s); 10.6 (t, J = 6.3 Hz); 9.61 (t, J = 7.2 Hz); 7.99 (d, J = 12.3 Hz); 6.50 (d, J = 11.1 Hz); 5.01 (d, J = 7.2 Hz); 3.48 (s); 2.48 (t, J = 6.3 Hz); -3.34 (br, s). UV-vis (C6H6) λ, nm (ε, M-1 cm-1): 422 (1600); 510 (1050). 54 Evans Method (C6D6): 1.98 µB. Anal. Calcd. for C55H56BFeNP3: C, 74.17; H, 6.34; N, 1.57. Found: C, 74.13; H, 6.26; N, 1.72. Synthesis of [PhBP3]Fe(CO)2, 2.13: A benzene solution (3 mL) of 2.4 (0.100 g, 0.0996 mmol) was pressurized with an atmosphere of CO. This resulted in a color change from yellow-orange to light brown. After 30 minutes at room temperature, volatiles were removed under reduced pressure, and the resulting tan solid was washed with petroleum ether (3 x 5 mL) and dried to yield 2.13 (0.0558 g, 70 %). X-ray quality crystals were grown from vapor diffusion of petroleum ether into a benzene solution. 1H NMR (C6D6, 300 MHz): δ 8.97 (br, s); 8.39 (br, s); 7.88 (s); 7.67 (s); 7.38 (s); 7.03 (s); 4.71 (br, s). IR (C6H6): 1979, 1914 cm-1. ES-MS: calcd. for C47H41BFeO2P3 (M)+ 797 m/z, found (M – 2 CO)+ 741 m/z. Evans Method (C6D6): 1.73 µB. Anal. Calcd. for C47H41BFeO2P3: C, 70.79; H, 5.18. Found: C, 70.46; H, 4.96. Synthesis of [PhBP3]Fe(C≡NtBu)2, 2.14: Compound 2.4 (0.100 g, 0.0996 mmol) was added to benzene (~5 mL) with stirring. A benzene solution (1 mL) of tBuN≡C (22.5 µL, 0.199 mmol) was added dropwise, during which time the reaction reddened in color. After 4 hours at room temperature, the reaction was concentrated under reduced pressure to a volume of ~ 2 mL. The addition of 15 mL of petroleum ether resulted in the precipitation of a red-orange solid that was isolated and dried to yield 2.14 (0.073 g, 81 %). X-ray quality crystals were grown from vapor diffusion of petroleum ether into a benzene solution. 1H NMR (C6D6, 300 MHz): δ 9.08 (br, s); 8.28 (br, s); 7.80 (s); 7.56 (s); 4.26 (br, s); 2.23 (br, s). IR (C6H6): 2058, 2019 cm-1. Evans Method (C6D6): 1.75 µB. Anal. Calcd. for C55H59BFeN2P3: C, 72.78; H, 6.55; N, 3.09 Found: C, 72.78; H, 6.39; N, 3.03. 55 Quantification of p-tolylisocyanate: A J. Young tube was charged with 2.5 (0.0107 g, 0.0126 mmol), hexamethylbenzene (0.0013 g, 0.0080 mmol), and C6D6 (~ 0.5 mL) under an atmosphere of N2. The addition of an additional atmosphere of CO resulted in a color change from green to purple, and finally yellow-brown within minutes. After allowing the reaction to proceed at room temperature for 10 minutes, 1H NMR integration versus the internal standard revealed that p-tolylisocyanate had been generated in approximately quantitative yield. Reaction of 2.13 with p-tolylazide: An NMR tube was charged with 2.13 (0.0100 g, 0.0125 mmol), p-tolylazide (0.0033 g, 0.025 mmol), ferrocene (0.0017 g, 0.0091 mmol), and C6D6 (~ 1.5 mL). The reaction gradually (hours) turns forest green and after 44 hours at room temperature, integration versus the internal ferrocene reference revealed that p-tolylisocyanate had been generated in 80% yield. The paramagnetic metal-containing product was identified as 2.5. 2.5.9 X-ray Experimental Data X-ray diffraction studies were carried out in the Beckman Institute Crystallographic Facility on a Bruker Smart 1000 CCD diffractometer under a stream of dinitrogen. Data were collected using the Bruker SMART program, collecting ω scans at 5 φ settings. Data reduction was performed using Bruker SAINT v6.2. Structure solution and structure refinement were performed using SHELXS-97 (Sheldrick, 1990) and SHELXL-97 (Sheldrick, 1997). All structural representations were produced using the Diamond software program. Crystallographic data are summarized in Table 2.1. 56 Table 2.2. Crystallographic data for [PhBP3]FeCl, 2.1; [PhBP3]Fe(PPh3), 2.4; [PhBP3]Fe≡N-p-tolyl, 2.5; [PhBP3]FeNtBu, 2.11; and [PhBP3]Fe(CO)2, 2.13. 2.1·C6H6 2.5·0.5C6H6 chemical formula C51H47BClFeP3 C71H69BFeP4 C55H51BFeNP3 fw 854.91 1112.80 885.54 T (°C) -177 -177 -177 λ (Å) 0.71073 0.71073 0.71073 a (Å) 10.9340(17) 20.2596(6) 10.9216(8) b (Å) 16.666(3) 17.7356(6) 16.9009(13) c (Å) 24.153(3) 33.4617(11) 25.2795(19) α (°) 90 90 85.0390(10) β (°) 102.752(13) 90 85.4520(10) γ (°) 90 90 77.0040(10) V (Å3) 4292.8(11) 12023.3(7) 4521.0(6) space group P2(1)/c Pbca P-1 Z 4 8 4 Dcalc (g/cm3) 1.323 1.230 1.301 µ(cm-1) 5.61 3.99 4.79 0.0713, 0.1170 0.0497, 0.0704 R1, wR2a (I > 2σ(I)) 0.0467, 0.0746 a 2.4·C5H10·0.5C6H6 R1 = Σ||Fo| - |Fc||/Σ|Fo|, wR2 = {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2 57 Table 2.2 cont. 2.11 2.13 chemical formula C49H50BFeNP3 C47H41BFeO2P3 fw 812.47 797.37 T (°C) -177 -177 λ (Å) 0.71073 0.71073 a (Å) 14.7124(13) 9.7323(9) b (Å) 19.0114(17) 17.2050(15) c (Å) 15.0887(13) 23.208(2) α (°) 90 90 β (°) 96.9378(17) 95.718(2) γ (°) 90 90 V (Å3) 4189.5(6) 3866.8(6) space group P2(1)/n P2(1)/n Z 4 4 Dcalc (g/cm3) 1.288 1.370 µ(cm-1) 5.10 5.54 R1, wR2a (I > 2σ(I)) 0.0429, 0.0848 0.0505, 0.0888 58 References Cited 1 (a) Einsle, O.; Tezcan, A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696. (b) Rees, D. C. Annu. Rev. Biochem. 2002, 71, 221. (c) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853. 2 (a) Seyferth, D.; Henderson, R. S.; Song, L. C. Organometallics 1982, 1, 125. (b) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.; Chiang, C. Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917. (c) Gloaguen, F.; Lawrence, J. D.; Schmidt, M.; Wilson, S. R.; Rauchfuss, T. B. J. Am. Chem. Soc. 2001, 123, 12518. 3 See: Lappert, M. F.; MacQuitty, J. J.; Pye, P. L. J. Chem. Soc., Dalton Trans. 1998, 1583 and references therein. 4 Kisko, J. L.; Hascall, T.; Parkin, G. J. Am. Chem. Soc. 1998, 120, 10561. 5 Thomas, J. C.; Peters, J. C. Inorg. Synth. 2004, 34, 8. 6 (a) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que, L., Jr. Science 2003, 299, 1037. (b) MacBeth, C. E.; Golombek, A. P.; Young, V. G., Jr.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938. (c) Verma, A. K.; Nazif, T. N.; Achim, C.; Lee, S. C. J. Am. Chem. Soc. 2000, 122, 11013. (d) Que, L., Jr.; Chen, K.; Costas, M.; Ho, R. Y. N.; Jensen, M. P.; Rohde, J. U.; Torelli, S.; Zheng, H. J. Inorg. Biochem. 2001, 86, 88. (e) Pistorio, B. J.; Chang, C. J.; Nocera, D. G. J. Am. Chem. Soc. 2002, 124, 7884. (f) Tshuva, E. Y.; Lee, D.; Bu, W.; Lippard, S. J. J. Am. Chem. Soc. 2002, 124, 2416. (g) Meunier, B., Ed. Biomimetic Oxidations Catalyzed by Transition Metal Complexes; Imperial College Press: London, 2000. 59 7 Mansuy and co-workers were the first to report heme-supported [FeNNR2]+ systems. Physical data for these systems suggests they are better represented as Fe(II) [Fe NNR2]. See: Mansuy, D.; Battioni, P.; Mahy, J. P. J. Am. Chem. Soc. 1992, 104, 4487. 8 Wieghardt and Meyer have provided spectroscopic evidence at 77 K for an Fe(V) nitride. See: Meyer, K.; Bill, E.; Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 4859. 9 Shapiro, I. R.; Jenkins, D. M.; Thomas, J. C.; Day, M. W.; Peters, J. C. Chem. Commun. 2001, 2152. 10 (a) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (b) Evans, D. F. J. Chem. Soc. 1959, 2003. 11 Miessler, L. G.; Tarr, D. A. Inorganic Chemistry, 2nd Ed.; Prentice Hall: New Jersey, 1999; pg. 343. 12 Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 15336. 13 Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074. 14 Trofimenko, S. Scorpionates: Polyporazolylborate Ligands and Their Coordination Chemistry; Imperial College Press: London, 1999. 15 (a) Ohrenberg, C.; Ge, P.; Schebler, P.; Riordan, C. G.; Yap, G. P. A.; Rheingold, A. L. Inorg. Chem. 1996, 35, 749. (b) Ge, P.; Haggerty, B. S.; Rheingold, A. L.; Riordan, C. G. J. Am. Chem. Soc. 1994, 116, 8406. 16 See for example: Dapporto, P.; Midollini, S.; Sacconi, L. Inorg. Chem. 1975, 14, 1643. 17 See for example: (a) Shirasawa, N.; Nguyet, T. T.; Hikichi, S.; Moro-oka, Y.; Akita, M. Organometallics 2001, 20, 3582. (b) Ito, M.; Amagai, H.; Fukui, H.; Kitajima, N.; 60 Moro-oka, Y. Bull. Chem. Soc. Jpn. 1996, 69, 1937. (c) Gorrell, I. B.; Parkin, G. Inorg. Chem. 1990, 29, 2452. (d) Brunker, T. J.; Hascall, T.; Cowley, A.R.; Rees, L. H.; O'Hare, D. Inorg. Chem. 2001, 40, 3170. 18 The reduction of [PhBiPr3]FeCl with a sodium/mercury amalgam under a dinitrogen atmosphere results in the formation of the FeI-N2 dimer, ([PhBiPr3]Fe)2(µ-N2). See: Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782. 19 For example, [PhBP3]Fe(N(H)-p-tolyl) features an Fe-N bond length of 1.9132(18) Å. See Chapter 3. 20 Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993; Ch. 2. 21 Spin-crossover has been observed in a number of [PhBP3]Co(II) systems. See: (a) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 11162. (b) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. In Press. 22 See Section 2.3 for a DFT study of [PhBP3]Fe≡NtBu. 23 Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 11238. 24 See Chapter 4 for a comparative structural study between terminal d5 and d6 iron imides supported by [PhBP3]. 25 Terminal cobalt trimethylsilyl imido species have been inferred as intermediates in ligand activation processes. See: Thyagarajan, S.; Shay, D. T.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc. 2003, 125, 4440. 26 Glueck, D. S.; Wu, J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1991, 113, 2041. 27 Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623. 61 28 Jenkins, D. M. Low spin pseudotetrahedral cobalt(trisphosphino)borate complexes, Caltech, Ph.D. Thesis, 2005. 29 See for example: (a) Bach, T.; Körber, C. Eur. J. Org. Chem. 1999, 1033. (b) Mahy, J.- P.; Battioni, P.; Mansuy, D. J. Am. Chem. Soc. 1986, 108, 1079. 30 Ertl, G. In Catalytic Ammonia Synthesis: Fundamentals and Practice; Jennings, J. R.; Ed.; Plenum Press: New York, 1991; pp 109-132. 31 Mehn, M. P.; Jensen, M. P.; Que, L., Jr.; Brown, S. D.; Jenkins, D. M.; Betley, T. A.; Peters, J. C. Manuscript in Preparation. 32 (a) Mukherjee, A.; McGlashen, M. L.; Spiro, T. G. J. Chem. Phys. 1995, 99, 4912. (b) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969. 33 Synthetic protocols afford 50% 15N incorporation. See Section 2.5.7. 34 Osborne, J. H.; Trogler, W. C. Inorg. Chem. 1985, 24, 3098. 35 Mindiola, D. J.; Hillhouse, G. L. Chem. Commun. 2002, 1840. 36 Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2003, 125, 13350. 37 For examples of five-coordinate, bridging iron(III) imides see: Nichols, P. J.; Fallon, G. D.; Murray, K. S.; West, B. O. Inorg. Chem. 1988, 27, 2795. 38 Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21, 4050. 39 See for example: (a) Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. Inorg. Chem. 1994, 33, 2306. (b) Rheingold, A. L.; Liable-Sands, L. M.; Golen, J. A.; Yap, G. P. A.; Trofimenko, S. Dalton Trans. 2004, 598. 40 Curiously, [PhBPiPr3]CoI features a high-spin electronic configuration in contrast to the low-spin configuration observed for [PhBPiPr3]CoI. This is counter intuitive for an 62 assumed stronger-field ligand, and one working hypothesis suggests that with bulky alkyl groups, the second generation tris(phosphino)borate is not able to distort to accommodate a Jahn-Teller distortion. See reference 13. 41 See for example: (a) Cundari, T. R. J. Am. Chem. Soc. 1992, 114, 7879. (b) Mayer, J. M. Comments Inorg. Chem. 1988, 8, 125. (c) Osborne, J. H.; Rheingold, A. L.; Trogler, W. C. J. Am. Chem. Soc. 1985, 107, 7945. (d) Nugent, W. A. Coord. Chem. Rev. 1980, 31, 123. 42 Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; Wiley-Interscience: New York, 1985; pp 387-389. 43 A DFT study with the closed-shell system {[PhBP3]Fe≡NtBu}- yields a HOMO- LUMO gap of 86 kcal/mol. See Chapter 4. 44 Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252. 45 For structurally characterized terminal iron oxos see references 6a and 6b. 46 Wagner, W. D.; Nakamoto, K. J. Am. Chem. Soc. 1988, 110, 4044. 47 For a detailed vibrational analysis of niobium and tantalum arylamido systems see: Heinselman, K. S.; Miskowski, V. M.; Geib, S. J.; Wang, L. C.; Hopkins, M. D. Inorg. Chem. 1997, 36, 5530. 48 Jaguar 5.0, Schrodinger, LLC, Portland, Oregon, 2002. 49 Lee, C.; Yang, W.; Parr R. G. Phys. Rev. B 1988, 37, 785. 50 Smith, P. A. S.; Brown, B. B. J. Am. Chem. Soc. 1951, 73, 2438. 51 Abramovitch, R. A.; Challand, S. R.; Scriven, E. F. V. J. Org. Chem. 1972, 37, 2705. 52 (a) Li, Y.; Kirby, J. P.; George, M. W.; Poliakoff, M.; Schuster, G. B. J. Am. Chem. Soc. 1988, 110, 8092. (b) Smith, P. A. S.; Hall, J. H. J. Am. Chem. Soc. 1962, 84, 480. 63 53 Bottaro, J. C.; Penwell, P. E.; Schmitt, R. J. Syn. Comm. 1997, 27, 1465. 54 Crivello, J. V. J. Org. Chem. 1981, 46, 3056.  64 Chapter 3: Hydrogenolysis of [PhB(CH2PPh2)3]Fe≡N-p-tolyl: Probing the Reactivity of an Iron Imide with H2 The text in this chapter is reproduced in part with permission from: Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538. Copyright 2004 American Chemical Society 65 3.1 Introduction It is generally accepted that low-coordinate iron nitride species are formed and hydrogenated on the surface of solid-state iron catalysts in the Haber-Bosch process.1 Moreover, it has been postulated that Fe-N (and/or Mo-N) multiple bonds may be formed from N2 and further reduced by hydrogen equivalents (i.e., H+/e-) during turnover in nitrogenase enzymes.2 Studying the reactivity patterns of hydrogen with molecular iron complexes featuring Fe-N multiple bond linkages is therefore of considerable interest but has been difficult to undertake due to the historical absence of well-defined Fe=NR and/or Fe≡N species.3 Our group has recently reported the synthesis of a number of mononuclear imides of iron and cobalt (e.g., [PhBPR3]M(NR) where [PhBPR3] = [PhB(CH2PPh2)3]- and [PhB(CH2PiPr2)3]-).4 These trivalent imides are characterized by relatively robust M≡NR triple bonds (1σ + 2π) but are nonetheless able to release their imide functionalities, an example being transfer to CO producing isocyanate.4a,b This group-transfer reactivity prompted us to survey their respective reactivities toward hydrogen. 3.2 Results and Discussion The low-spin cobalt complex [PhBP3]Co≡N-p-tolyl4b is stable to hydrogen pressure (1-3 atm) at modest temperatures (≤70 °C). By contrast, low-spin [PhBP3]Fe≡Np-tolyl (2.5)4a gives rise to a fascinating reaction profile upon exposure to 1 atm of H2 at room temperature. Both partial and complete hydrogenolysis of the Fe≡NR linkage is observed. Whereas H2-promoted reduction of an imide to its corresponding amide was first described by Wolczanski,5 reductive scission of a metal imide by hydrogen to release amine has not to our knowledge been previously reported.6 66 Fingerprint resonances for imide 2.5 are conveniently monitored by 1H NMR spectroscopy despite its paramagnetic S = ½ ground state.4a When a sealable NMR tube is charged with forest green 2.5 in C6D6 under an atmosphere of H2, its resonances fully decay within a period of 3 h at room temperature. During this time, the reaction solution remains homogeneous, and the color changes to an intense red/purple. The major reaction product at this stage is the paramagnetic anilido complex [PhBP3]Fe(N(H)-p-tolyl) (3.1) (Equation 3.1).7 Assignment of 3.1 from its paramagnetically shifted 1H NMR resonances is aided by comparison to a spectrum of an independently generated sample of 3.1 prepared by the reaction between [Li][N(H)-p-tolyl] and [PhBP3]FeCl.4a Ar Ph Fe Ph Ph2P P PPh2 B Ph H Ar N 2.5 1 atm H2 RT, hours benzene NH Fe [PhBP3] 3.1 1 atm H2 days benzene - H2N-p-tolyl H Fe [PhBP3] (3.1) 3.2 Anilide 3.1 features a weak N-H vibration at 3326 cm-1 (Nujol), a temperatureindependent magnetic moment of 5.20 µB (SQUID, S = 2),8 and relatively intense chargetransfer bands (450 nm, 3400 M-1 cm-1; 547 nm, 3000 M-1 cm-1). The solid-state structure of 3.1 (Figure 3.1) is related to that of 2.5 by the addition of a single H-atom at nitrogen. Although both 2.5 and 3.1 are 4-coordinate and pseudotetrahedral, 2.5 features an Fe-N bond distance (1.913(2) Å) and an Fe-N-C bond angle (127.4(2)°) very different from 2.5 (Fe-N = 1.6578(2) Å; Fe-N-C = 169.96(2)°).4a Of additional note are the Fe-P bond distances, which are appreciably expanded in the structure of 3.1 (Fe-Pavg for 2.5 = 2.24 Å; Fe-Pavg for 3.1 = 2.42 Å). These structural differences reflect the low- versus high-spin 67 configurations of 2.5 and 3.1, respectively, in addition to the diminished Fe-N π-bond character of 3.1 by comparison to 2.5. C46 N1 Fe1 P2 P3 P1 Figure 3.1. 50% thermal ellipsoid representation of [PhBP3]Fe(N(H)-p-tolyl) (3.1). For the image on the right, all phosphino phenyl groups have been removed for clarity. For both images, all hydrogen atoms have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe1-N1, 1.9132(18); Fe1-P1, 2.3928(7); Fe1-P2, 2.4520(7); Fe1-P3, 2.4267(7); Fe1-N1-C46, 127.44(16); P1-Fe1-P2, 90.29(2); P1-Fe1-P3, 94.44(2); P2-Fe1-P3, 92.64(2); N1-Fe1-P1, 114.15(6); N1Fe1-P2, 129.34(6); N1-Fe-P3, 126.19(6). Prolonged monitoring of the hydrogenation of 2.5 in C6D6 over a period of days establishes a new diamagnetic product (31P NMR: 52 ppm (s)) and the release of H2N-ptolyl (40% after 3 days). In a preparative-scale reaction, imide 2.5 was exposed to an atmosphere of H2 in benzene for 3 days in a sealed reaction flask. Extraction of the crude 68 solids into diethyl ether left behind the diamagnetic species as a bright orange powder in 25% yield. XRD analysis of a single crystal confirmed it to be the cyclohexadienyl complex [PhBP3]Fe(η5-cyclohexadienyl) (3.2) (Figure 3.2). Complex 3.1 was then isolated in 60% yield from the ethereal extract by crystallization. The combined isolated yield of 3.1 and 3.2 accounted for 85% of the total iron content of the reaction. Similar yields were provided by in situ monitoring and integration of 1H NMR spectra of sealed C51 C46 C50 C49 C47 C48 Fe P3 P1 P2 Figure 3.2. 50% thermal ellipsoid representation of [PhBP3]Fe(η5- cyclohexadienyl) (3.2). For the image on the right, only the L3Fe(η5cyclohexadienyl) core is shown. For both images, all hydrogen atoms have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe-P1, 2.2730(10); Fe-P2, 2.2701(10); Fe-P3, 2.2647(9); Fe-C47, 2.213(3); Fe-C48, 2.122(3); FeC49, 2.100(3); Fe-C50, 2.098(3); Fe-C51, 2.191(3); P1-Fe-P2, 90.29(2); P1-FeP3, 94.44(2); P2-Fe-P3, 92.64(2); C46-C47, 1.477(4); C47-C48, 1.394(4); C48C49, 1.415(4); C49-C50; 1.410(4); C50-C51, 1.385(4); C51-C46, 1.493(4); P1Fe-P2, 90.40(3); P1-Fe-P3, 90.92(4); P2-Fe-P3, 88.17(3); C46-C47-C48, 120.5(3); C50-C51-C46, 119.5(3); C47-C46-C51, 104.4(3). 69 NMR tube experiments after 3 days. Ill-defined paramagnetic resonances indicative of at least one side product account for the remaining iron material (~ 15%) in these sealedtube experiments. We suspect that the side-product(s) arise from the kinetically competitive degradation of 3.1 during the course of the reaction. As a control experiment, it is noted that the storage of 3.1 in benzene under N2 leads to some degradation after 3 days but does not produce 3.2.9 To ensure that the H-atoms being delivered to both benzene and the imide functionality arise from H2, the hydrogenation of 2.5 by D2 in C6D6 was examined. The consumption of 2.5 proceeds in this case much more slowly (krel = k(H2)/k(D2) = 5.6; Figure 3.3), and the product yields after 3 days are consequently much lower. The overall reaction is moreover less clean than for the case of H2 due to the increased role of kinetically competitive side reactions given the longer reaction time. Nonetheless, the [PhBP3]Fe≡N-p -tolyl Consumption in C6D6 ln (Integral) 2.5 2 D2 1.5 H2 Linear (D2) 1 Linear (H2) 0.5 0 y = -0.2474x + 1.9051 2 R = 0.997 -0.5 y = -1.3612x + 2.0608 -1 2 R = 0.9931 -1.5 -2 0 2 4 6 8 10 12 14 Time (h) Figure 3.3. Kinetic data for the consumption of 2.5 in C6D6 with H2 (yellow triangles) and D2 (turquoise squares). 70 expected products [PhBP3]Fe(N(D)-p-tolyl) (3.1b), [PhBP3]Fe(η5-cyclohexadienyl-d7) (3.2b), and D2N-p-tolyl can be identified. Compound 3.1b is identified on the basis of its 1 H NMR resonances and the isotopically shifted N-D stretch in its IR spectrum (Nujol: νND = 2462 cm-1 (w)). Diamagnetic 3.2b is assigned from its 31P and 1H NMR spectra. In particular, no cyclohexadienyl ring resonances are present in its 1H NMR spectrum due to complete deuteration. Compound 3.2b also exhibits the expected ES-MS molecular ion peak at 828 m/z. The organic byproduct, D2N-p-tolyl, is identified by GC-MS as the only amine-containing product of the reaction. The reduction of 2.5 to anilide 3.1 and then to 3.2 appears to proceed in a stepwise fashion (Equation 3.1). The detailed mechanism by which these steps occur is clearly of interest but somewhat difficult to unravel due to the paramagnetic nature of 2.5 and 3.1, in addition to the presence of at least one paramagnetic side product(s) that is formed during the course of the reaction. A reasonable mechanistic outline to suggest is as follows: The first step (2.5 + 1/2 H2 → 3.1) involves the addition of H2 to 2.5 to generate an unobservable species “[PhBP3]FeIII(H)(NHAr),” which, if formed, must bimolecularly release H2 to provide observable 3.1. A second addition of H2 would occur at 3.1 to generate an unobservable "FeII-H" source (with loss of H2N-p-tolyl) that adds to benzene via insertion.10,11 Evidence consistent with a reactive "FeII-H" intermediate comes from the following set of observations: First, the incubation of 3.1 under an atmosphere of H2 at 25 °C in C6D6 slowly generates 3.2 (30% after 3 days) with concomitant evolution of H2N-p-tolyl (50% after 3 days). Second, the addition of KHBEt3 to a benzene solution of [PhBP3]FeIICl generates 3.2 in high yield (77% isolated). Moreover, when this latter reaction is carried out in THF rather than benzene a 71 new and diamagnetic species is generated that can be assigned as the complex [PhBP3]FeII(HBEt3) (3.3) on the basis of its solution IR and NMR data in THF (νBH = 2448 cm-1; 31P NMR: δ 55 ppm (br s); 11B NMR: δ 25.5 ppm (br s, HBEt3), -12.8 ppm (s, PhB(CH2PPh2)3-)). Although crystals of thermally sensitive 3.3 were never obtained,12 the structurally related complex, [PhBP3]Fe(BH4) (3.4), was synthesized and fully characterized. The solid-state structure of the latter (Figure 3.4) is consistent with an η3borohydride adduct,13,14 while spectroscopic data is similar to that obtained for 3.3 (νBH = 2603, 2575 cm-1; 31P NMR: δ 76.2 ppm (s); 11B NMR: δ 22.4 ppm (br s, BH4), -12.9 ppm (s, PhB(CH2PPh2)3-)). H46 B2 H47 H48 H49 Fe1 P2 P1 P3 Figure 3.4. 50% thermal ellipsoid representation of [PhBP3]Fe(BH4) (3.4). For the image on the right, all phosphino phenyl groups have been removed for clarity. For both images, all hydrogen atoms have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe1-B2, 1.8550(15); Fe1-P1, 2.2119(4); Fe1-P2, 2.2088(4); Fe1-P3, 2.1977(4); Fe-H47, 1.570(17); Fe1-H48, 1.615(15); Fe1-H49, 1.612(15); P1-Fe1-P3, 89.53 (2); P2-Fe1-P3, 89.03(2); P1-Fe1-P2, 92.06(2). 72 Most importantly, complex 3.3 serves as a "[PhBP3]FeII-H" equivalent and is instantly converted to 3.2 with loss of BEt3 upon addition of benzene to a THF solution. We also note that the hydrogenation of 2.5 in CD2Cl2 generates H2N-p-tolyl and the chloride complex [PhBP3]FeIICl. Reactive metal hydrides are known to exchange with halocarbons,15 and it is reasonable to expect that a "[PhBP3]FeII-H" intermediate might behave similarly. Whereas iron cyclohexadienyl complexes structurally related to 3.2 are known,16,17,18 their formation from the insertion of benzene into a reactive Fe-H bond is, to our knowledge, unprecedented. A curious reactivity comparison to note in this context concerns Holland's 4-coordinate iron hydride dimer {LFeIIH}2, in which L represents a bulky β-diketiminate ligand.7 This low-coordinate hydride system is isolable and, while reactive toward certain unsaturated substrates (e.g., azobenzene), appears to be stable in aromatic solvents such as benzene. 3.3 Conclusions In summary, the low-spin iron(III) imide 2.5 undergoes partial and then complete hydrogenation under ambient conditions to release aniline in what appears to be a welldefined, stepwise process. We can directly observe the intermediate Fe(II) anilido species, 3.1, and have provided evidence for the subsequent intermediacy of a reactive FeII-H species that is trapped by benzene solvent to provide 3.2. Interesting mechanistic issues remain to be resolved and are currently under investigation. 73 3.4 Experimental Section 3.4.1 General Considerations General considerations are outlined in Section 2.5.1. 11B NMR data was acquired on a JOEL 400 MHz spectrometer and chemical shifts were referenced to neat BF3·OEt2 at δ 0 ppm. GC-MS data were obtained by injecting benzene solutions into an Agilent 6890 GC equipped with an Agilent 5973 mass selective detector (EI). IR samples were prepared as either a Nujol mull on CaF2 plates or as a solution in a sealed cell with KBr plates. Deuterated methylene chloride was purchased from Cambridge Isotope Laboratories, Inc. and was degassed and dried over activated 3 Å molecular sieves prior to use. 3.4.2 Magnetic Measurements SQUID data for anilide 3.1 was acquired from 2–250 K as outlined in Section 2.5.2, and the corresponding data are shown below. 4 3.5 3 χ M*T 2.5 2 1.5 1 0.5 0 0 50 100 150 Temp (K) 200 Figure 3.5. Plot of χM*T vs. T for [PhBP3]Fe(N(H)-p-tolyl) (3.1). 250 74 80 70 60 χM -1 50 40 30 20 10 0 0 50 100 150 200 250 Temp (K) Figure 3.6. Plot of χM-1 vs. T for [PhBP3]Fe(N(H)-p-tolyl) (3.1). 3.4.3 Starting Materials and Reagents Chloride 2.1 and imide 2.5 were prepared according to literature procedures.4a [Na][BH4] and [K][HBEt3] (1.0 M in THF) were obtained from Aldrich. The former was dried overnight under reduced pressure at 150 °C while the latter was used as received. [Li][N(H)-p-tolyl] was prepared upon the addition of one equivalent of nBuLi to ptoluidine in petroleum ether. The resulting solid was isolated and dried under reduced pressure. Cylinders of hydrogen and deuterium gas were obtained from Air Liquide and Cambridge Isotope Laboratories, respectively, and used as received. 3.4.4 Synthesis of Compounds Synthesis of [PhBP3]Fe(N(H)-p-tolyl), 3.1: Chloride 2.1 (0.372 g, 0.479 mmol) was added to benzene (10 mL) with stirring. A benzene slurry (2 mL) of [Li][N(H)-ptolyl] (0.0542 g, 0.479 mmol) was then added dropwise at room temperature, which 75 resulted in a color change from yellow to opaque red/purple. After stirring for two hours the reaction was filtered over celite and volatiles were removed under reduced pressure. The crude solids were then washed with petroleum ether (3 x 20 mL) and dried to afford a dark powder. A crude 1H NMR spectrum of the product demonstrated the presence of 3.1 along with minor paramagnetic impurities. The majority of the impurities were removed by precipitation upon the addition of petroleum ether into a benzene solution. After filtration over Celite the filtrate was chilled to -35 oC for three days to precipitate 0.194 g (48%) of a dark solid that was predominantly 3.1 but still contaminated by trace impurities; a crystal suitable for X-ray diffraction analysis was collected from the side of the chilled vial. Subsequent vapor diffusion of petroleum ether into a toluene solution of the nearly pure sample of 3.1 provided solids of high enough purity to provide satisfactory combustion analysis. Compound 3.1 was stored at -35 oC as it degraded over a period of days at room temperature as both a solid and as a benzene solution. 1H NMR (C6D6, 300 MHz): δ 139.3 (s); 75.7 (s); 38.3 (s); 18.8 (s); 17.6 (s); 6.63 (s); -11.8 (s); 34.4 (br, s). UV-vis (C6H6) λ, nm (ε, M-1 cm-1): 450 (3400), 547 (3000). SQUID (solid, average 10 – 250 K): µeff = 5.20 µB. IR (Nujol): 3326 cm-1 (w). Anal. Calcd. for C52H49BFeNP3: C, 73.69; H, 5.83; N, 1.65. Found: C, 73.29; H, 5.71; N, 1.60. Synthesis of [PhBP3]Fe(η5-cyclohexadienyl), 3.2: Chloride 2.1 (0.050 g, 0.0644 mmol) was added to benzene (3 mL) with stirring. A benzene solution (1 mL) of [K][HBEt3] (71 µL of a 1.0 M solution in THF, 0.0708 mmol) was then added dropwise. After stirring overnight the reaction was filtered over Celite, and volatiles were removed under reduced pressure. The resulting solids were washed with petroleum ether (1 x 5 mL) and dried under reduced pressure to yield 3.2 as an orange powder (0.036 g, 68%). 76 X-ray quality crystals may be obtained by vapor diffusion of petroleum ether into a THF solution. 1 H NMR (C6D6, 300 MHz): δ 8.01 (d, J = 7.0 Hz, 2H); 7.55 (t, J = 7.5 Hz, 2H); 7.34 (t, J = 7.5 Hz, 1H); 7.07 (br s, 12H), 6.94 (t, J = 7.5 Hz, 6H); 6.77 (t, J = 7.5 Hz, 12H); 5.98 (t, J = 4.5 Hz, 1H); 5.16 (t, J = 4.5 Hz, 2H); 2.66 (m, 2H); 2.29 (m, 2H); 1.65 (br s, 6H). 31P{1H} NMR (C6D6, 121.4 MHz): δ 52.0 ppm (s). ES-MS: calcd. for C51H48BFeP3 [M]+ 821 m/z, found [M]+ 821 m/z. Anal. Calcd. for C51H48BFeP3: C, 74.65; H, 5.90. Found: C, 74.32; H, 6.00. Synthesis of [PhBP3]Fe(HBEt3), 3.3: Chloride 2.1 (0.100 g, 0.129 mmol) was added to Et2O (10 mL) with stirring. To the stirring slurry of 2.1, an ethereal solution (2 mL) of [K][HBEt3] (129 µL of a 1 M solution in THF, 0.129 mmol) was added dropwise over a period of 5 minutes. By the time the addition was complete the reaction was midnight blue in color and precipitates were evident. After stirring at room temperature for 20 minutes, the crude reaction was filtered over Celite and the filtrate was cooled to -35 oC overnight. Approximately 0.060 g (55%) of midnight blue solid was isolated and dried under reduced pressure. Attempts to obtain crystals suitable for an X-ray diffraction experiment have been unsuccessful. The inability to obtain a satisfactory combustion analysis for 3.3 is presumably the result of its thermal instability. 1H NMR (THF-d8, 300 MHz): δ 7.61 (d, J = 7.2 Hz, 2H); 7.27 (br s, 12H); 7.15 (t, J = 7.2 Hz, 6H); 7.02 (m, 14H); 6.88 (t, J = 7.2 Hz, 1H); 1.44 (br d, J = 10.8 Hz, 6H); 0.91 (br s, 6H); 0.68 (t, J = 6.6 Hz, 9H). The hydride resonance was not located in the 1H NMR spectrum. 31 P{1H} NMR (THF-d8, 121.4 MHz): δ 55.0 ppm (br s). 11B{1H} NMR (THF-d8, 128.3 MHz): δ 25.5 ppm (br s, HBEt3); -12.8 ppm (s, PhB(CH2PPh2)3-). IR (KBr/THF): νBH = 2448 cm-1 (br m). 77 Synthesis of [PhBP3]Fe(BH4), 3.4: Chloride 2.1 (0.100 g, 0.129 mmol) was dissolved in THF (10 mL) with stirring. Solid [Na][BH4] (0.146 g, 3.86 mmol) was added in one portion, which resulted in a color change from yellow to red after stirring overnight. Volatiles were then removed under reduced pressure. The resulting crude solids were extracted with benzene (5 mL), filtered over celite, and lyophilized to yield 3.4 as a red solid (0.92 g, 95%). 1H NMR (300 MHz, C6D6): δ 8.10 (d, J = 7.5 Hz, 2H); 7.69 (t, J = 7.5 Hz, 2H); 7.49 (br s, 12H); 7.45 (overlaps with resonance at 7.49 ppm, t, J = 7.5 Hz, 1H); 6.72 (m, 18 H); 1.54 (s, 6H); -11.8 (br s, 4H). 31P{1H} NMR (THF-d8, 121.4 MHz): δ 76 (br s). 11B{1H} NMR (THF-d8, 128.3 MHz): δ 22.2 ppm (br s, BH4); 12.9 ppm (s, PhB(CH2PPh2)3-). IR (KBr/C6H6): νBH = 2575 cm-1 (m) with a shoulder at 2603 cm-1 (m). Anal. Calcd. for C45H45B2FeP3: C, 71.47; H, 6.00. Found: C, 71.07; H, 6.09. Reaction of [PhBP3]Fe≡N-p-tolyl with H2: Imide 2.5 (0.506 g, 0.598 mmol) was dissolved in benzene (30 mL) and loaded into a 250 mL sealable flask equipped with a stir bar. The flask was evacuated, flushed with 1 atm of H2, and sealed with a Teflon stopcock. After stirring at room temperature for 3 days, volatiles were removed under reduced pressure, and the resulting solids were stirred in Et2O (20 mL) for 3 hours. The ethereal suspension was filtered over a sintered glass frit and [PhBP3]Fe(η5cyclohexadienyl) (3.2) was isolated as an orange solid (0.121 g, 25%) from which X-ray quality crystals were grown via vapor diffusion of petroleum ether into a THF solution. The filtrate was collected and the volatiles were removed under reduced pressure. The resulting dark solids were washed with petroleum ether (3 x 20 mL) and dried to yield 0.205 g of 3.1, which was slightly contaminated with paramagnetic impurities (1H NMR 78 spectroscopy). NMR integration of this isolated sample versus an internal ferrocene standard demonstrated that 3.1 had been produced in 60% yield from 2.5 after 3 days. When the hydrogenation reaction was carried out in C6D6, the cyclohexadienyl resonances at δ 5.98, 5.16, and 2.29 ppm disappeared, while the multiplet at δ 2.66 ppm collapsed into a singlet. In C6D6 with D2, the singlet at δ 2.66 ppm disappeared. Reaction of [PhBP3]Fe≡N-p-tolyl with D2: Imide 2.5 (0.0114 g, 0.0135 mmol) was dissolved in benzene-d6 and loaded into a sealable NMR tube. The NMR tube was then evacuated and flushed with 1 atm of D2. After 3 days, NMR integration against an internal ferrocene reference demonstrated the following product yields: [PhBP3]Fe(N(D)p-tolyl), 10%; [PhBP3]Fe(η5-cyclohexadienyl-d7), 5%; D2N-p-tolyl, 33%. Additionally, no 1H NMR ring resonances were observed for the η5-cyclohexadienyl-d7 moiety of [PhBP3]Fe(η5-cyclohexadienyl-d7) (3.2b). Volatiles were then removed under reduced pressure, and IR analysis confirmed a weak vibration at 2462 cm-1 (Nujol) for 3.1b. ESMS of the crude solids verified an [M]+ ion at 828 m/z for 3.2b while GC-MS demonstrated that the only p-toluidine in solution was D2N-p-tolyl with a base peak 2 mass units greater than that for H2N-p-tolyl. Reaction of [PhBP3]Fe≡N-p-tolyl with H2 in CD2Cl2: Imide 2.5 (0.0098 g, 0.012 mmol) was dissolved in CD2Cl2 and loaded into a sealable NMR tube. The NMR tube was then evacuated and flushed with 1 atm of H2. After 3 days, NMR integration against an internal ferrocene reference demonstrated the following product yields: [PhBP3]Fe(N(H)-p-tolyl), 0%; [PhBP3]FeCl, 30%; H2N-p-tolyl, 80%. Unidentified diamagnetic as well as paramagnetic species were also present in the reaction matrix. 79 Reaction of [PhBP3]Fe(N(H)-p-tolyl) with H2 in C6D6: Anilide 3.1 (0.0083 g, 0.0098 mmol) was dissolved in C6D6 and loaded into a sealable NMR tube. The NMR tube was then evacuated and flushed with 1 atm of H2. After 3 days, NMR integration against an internal ferrocene reference demonstrated the following product yields: [PhBP3]Fe(η5-cyclohexadienyl-d6) 30%; H2N-p-tolyl, 50%. Unidentified paramagnetic species were also present in the reaction matrix. Kinetic Analysis for the Consumption of [PhBP3]Fe≡N-p-tolyl: Equal volumes of 0.018 M solutions of 2.5 were exposed to 1 atm of either H2 or D2 in a sealable NMR tube containing an internal ferrocene standard. The decay of 2.5 was then monitored by 1 H NMR spectroscopy for a minimum of 4 half lives (Figure 3.3). Analysis of this data provided a kinetic isotope effect (kH/kD) of 5.6, with rate constants of k(H) = 3.85 x 10-4 s1 and k(D) = 6.88 x 10-5 s-1. 3.4.5 X-ray Experimental Data Crystallographic procedures are outlined in Section 2.5.8. Crystallographic data are summarized in Table 3.1. For anilide 3.1, the location of the amide hydrogen atom was calculated. For borohydride adduct 3.4, the hydride hydrogen atoms were located in the difference map and refined as normal. 80 Table 3.1. Crystallographic data for [PhBP3]Fe(N(H)-p-tolyl), 3.1; [PhBP3]Fe(η5cyclohexadienyl), 3.2; and [PhBP3]Fe(BH4), 3.4. 3.1 3.4 chemical formula C52H49BFeNP3 C51H48BFeP3 C45H45B2FeP3 fw 847.49 820.46 756.19 T (°C) -177 -177 -177 λ (Å) 0.71073 0.71073 0.71073 a (Å) 10.0087(9) 13.8476(9) 39.226(4) b (Å) 14.8340(13) 14.5772(9) 13.0057(14) c (Å) 15.2850(13) 20.3589(13) 16.1408(18) α (°) 80.110(2) 90 90 β (°) 77.925(2) 90 110.695(4) γ (°) 87.176(2) 90 90 V (Å3) 2186.0(3) 4109.6(5) 7703.1(14) space group P-1 P2(1)2(1)2(1) C2/c Z 2 4 8 Dcalc (g/cm3) 1.288 1.326 1.304 µ(cm-1) 4.92 5.20 5.48 0.0478, 0.0629 0.0515, 0.0785 R1, wR2a (I > 2σ(I)) 0.0489, 0.0813 a 3.2 R1 = Σ||Fo| - |Fc||/Σ|Fo|, wR2 = {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2 81 References Cited 1 Smil, V. Enriching the Earth; MIT Press: Cambridge, MA, 2001. 2 (a) Howard, J. B.; Rees, D. C. Chem. Rev. 1996, 96, 2965 and references cited therein. (b) Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 2983. (c) Einsle, O.; Tezcan, A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696. (d) Thorneley, R. N. F.; Lowe, D. In Molybdenum Enzymes; Spiro, T. G., Ed.; Wiley-Interscience: New York, 1985. (e) Sellmann, D.; Sutter, J. Acc. Chem. Res. 1997, 30, 460. (f) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76. (g) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589. (h) Richards, R. L. Coord. Chem. Rev. 1996, 154, 83. 3 (a) Verma, A. K.; Nazif, T. N.; Achim, C.; Lee, S. C. J. Am. Chem. Soc. 2000, 122, 11013. (b) Wagner, W. D.; Nakamoto, K. J. Am. Chem. Soc. 1989, 111, 1590. (c) Meyer, K.; Bill, E.; Mienert, B.; Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 4859. (d) Jensen, M. P.; Mehn, M. P.; Que, L., Jr. Angew. Chem., Int. Ed. 2003, 42, 4357. 4 (a) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 322. (b) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 11238. (c) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782. 5 Cummins, C. C.; Baxter, S. M.; Wolczanski, P. T. J. Am. Chem. Soc. 1988, 110, 8731. 6 For recent examples of terminal imide H2 chemistry, see: (a) Schmidt, J. A. R.; Arnold, J. Organometallics 2002, 21, 3426. (b) Burckhardt, U.; Casty, G. L.; Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 3108. (c) Cameron, T. M.; Ortiz, C. G.; Ghiviriga, I.; Abboud, K. A.; Boncella, J. M. J. Am. Chem. Soc. 2002, 124, 922. 82 7 Anilido complexes of divalent iron have been reported recently from direct amination of an auxiliary ligand (see reference 3d) and by thermal degradation of an intermediate hydrazido complex of iron(II). See: Smith, J. M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc. 2003, 125, 15752. 8 See Section 3.4.2 for the SQUID magnetization data obtained for 3.1. 9 While 3.1 can be isolated and thoroughly characterized, including a satisfactory combustion analysis, its 1H NMR spectra invariably show minor impurities because of its tendency to slowly degrade upon dissolution. 10 Tilley has observed the intramolecular hydrogenation of an aromatic ring upon exposure of a d0 tantalum imide to hydrogen. This process is believed to occur via an intermediate hydride. See: Gavenonis, J.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 8536. 11 [PhBPiPr3]Fe-Hx species have been isolated and characterized. See: Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474. 12 For an example of a structurally characterized η3-superhydride adduct see: Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Organometallics 2003, 22, 4705. 13 The structurally related compound (triphos)Fe(H)(η2-BH4) has been reported. See: (a) Ghilardi, C. A.; Innocenti, P.; Midollini, S.; Orlandini, A. J. Chem. Soc., Dalton Trans. 1985, 605. (b) Ghilardi, C. A.; Innocenti, P.; Midollini, S.; Orlandini, A. J. Organomet. Chem. 1982, 231, C78. 14 For [TpMe2]Ni(η3-BH4) see: Desrochers, P. J.; LeLievre, S.; Johnson, R. J.; Lamb, B. T.; Phelps, A. L.; Cordes, A. W.; Gu, W.; Cramer, S. P. Inorg. Chem. 2003, 42, 7945. 15 Kaesz, H. D.; Saillant, R. B. Chem. Rev. 1972, 72, 231. 83 16 See for example: (a) Rigaut, S.; Delville, M. H.; Astruc, D. J. Am. Chem. Soc. 1997, 119, 11132.(b) Lee, S. S.; Lee, I. S.; Chung, Y. K. Organometallics 1996, 15, 5428. (c) Meng, W. D.; Stephenson, G. R. J. Organomet. Chem. 1989, 371, 355. (d) Whitesides, T. H.; Arhart, R. W. J. Am. Chem. Soc. 1971, 93, 5296. 17 Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2003, 22, 4627. 18 Metal hydride systems are known to catalyze the conversion of benzene to cyclohexane in the presence of hydrogen. For a recent lead reference, see: Suss-Fink, G.; Faure, M.; Ward, T. R. Angew. Chem., Int. Ed. 2002, 41, 99.  84 Chapter 4: Ground-State Singlet L3Fe-(µ-N)-FeL3 and L3Fe(NR) Complexes Featuring Pseudotetrahedral Fe(II) Centers The text in this chapter is reproduced in part with permission from: Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 1913. Copyright 2004 American Chemical Society 85 4.1 Introduction Low-coordinate iron sites that feature terminally bound or bridged nitride ligands may be relevant to various nitrogen reduction schemes.1 For example, the report of an interstitial µ6-ligand (presumably a nitride) within the MoFe-cofactor of nitrogenase2 has drawn recent attention to Fe-N-Fe structure types germane to biological nitrogen fixation.3 Schemes have also been forwarded to suggest that an N2 scission process on hot iron surfaces might lead to surface-bound iron nitrides that are reactive toward hydrogen during the Haber-Bosch ammonia synthesis.4 In this broad context, it is noteworthy that synthetically well-defined molecular iron nitrides, whether terminal or bridging, are uncommon.5,6 For instance, there are presently no examples of bimetallic Fe-(µ-N)-Fe species if one restricts consideration to iron centers that feature coordination numbers lower than five. Moreover, terminal nitrides of iron, regardless of coordination number, are exceptionally rare.6,7 Well-defined, low-coordinate iron nitrides that can be thoroughly characterized are therefore of synthetic interest, and we have begun to undertake their systematic development. As a preface to the current study, our laboratory has invested considerable effort in the synthesis and study of various monomeric pseudotetrahedral L3Fe-Nx platforms to assess what range of Nx-type ligands, oxidation states, and redox processes might be accommodated by a single pseudotetrahedral iron site. Using tris(phosphino)borate ligands as the L3 auxiliary,8 we have observed that an L3Fe-Nx center can display a remarkable range of redox flexibility, supporting π-acidic N29 and π-basic nitride6 (N3-) at the extremes, but also a range of intermediate Nx-type ligands that include amide,10 imide,9,11 and diazenido.9 A characteristic feature of these L3Fe-Nx species that feature 86 strong Fe-Nx π-bonding is their propensity to populate a low-spin ground-state electronic configuration. For instance, the complexes [PhBPiPr3]FeIV≡N and [PhBPiPr3]FeIII≡N(1Ad) are characterized by one σ and two π bonds at the Fe-Nx linkage and exhibit S = 0 and S = 1/2 ground states, respectively, despite the fact that the iron centers are pseudotetrahedral ([PhBPiPr3] = [PhB(CH2PiPr2)3]-).12 In this report, we describe a new bimetallic L3Fe-Nx-FeL3 complex in which the L3 scaffold is the parent [PhBP3] ligand ([PhBP3] = [PhB(CH2PPh2)3]-),13 and the Nx ligand is a bridging nitride group. As alluded to above, previously prepared bridged nitride diiron systems have featured five- or six-coordinate iron centers in relatively high oxidation states (e.g., FeIII(µ-N)FeIV, FeIV(µ-N)FeIV).5 The system described herein, [{[PhBP3]Fe}2(µ-N)][Na(THF)5] (4.2), is distinct from these complexes with respect to the coordination number and the formal oxidation state of each of its iron centers. The divalent iron sites in 4.2 are pseudotetrahedral iron(II), and the complex is diamagnetic in solution even at room temperature. A related mononuclear Fe(II) complex can be prepared that features a bona fide low-spin ground-state, {[PhBP3]FeII≡N(1Ad)}{nBu4N} (4.3). However, comparative spectroscopic data indicate that 4.2 is best described by two high-spin Fe(II) centers that exhibit such strong antiferromagnetic coupling that a singlet state is exclusively populated at 293 K. 4.2 Results and Discussion Whereas the reaction between [PhBPiPr3]FeCl and [Li][dbabh]14 (dbabh = 2,3:5,6dibenzo-7-aza bicycle[2.2.1]hepta-2,5-diene) cleanly generates an amide intermediate, [PhBPiPr3]Fe(dbabh), that subsequently decays to [PhBPiPr3]FeIV≡N,6 the reaction between [PhBP3]FeCl (2.1)11 and [Li][dbabh] is ill-defined, and no evidence for a 87 terminally bound nitride species is observed. To examine N-atom transfer to this latter system, we focused on the installation of azide (N3-) as a suitable N3- source.15 Access to the required iron(II) azide precursor is readily accomplished via metathesis between 2.1 and sodium azide in acetonitrile (Equation 4.1). Dissolution of yellow, paramagnetic 2.1 in CH3CN gives rise to a red solution in which complete consumption of 2.1 is evident by 1H and 31P{1H} NMR spectroscopy. The 31P{1H} NMR spectrum of this solution reveals a singlet at δ 44.5 ppm, consistent with the formation of a C3-symmetric solvento adduct that we formulate as {[PhBP3]Fe(CH3CN)3}{Cl}. A related species, {[PhBP3]Fe(CH3CN)3}{PF6}, has been isolated and characterized.16 The 1 H NMR spectrum (CD3CN) reveals, in addition to the major diamagnetic product, the presence of a minor paramagnetic species that we tentatively formulate as [PhBP3]Fe(CH3CN)(Cl). A similar five-coordinate iron-containing species has been characterized with the [PhBPiPr3] ligand.12b Upon addition of 1 equiv of [Na][N3] to a red acetonitrile solution of 2.1, the gradual precipitation of purple solids occurs over a period of hours. Extraction of these solids with copious amounts of benzene and subsequent filtration through a pad of celite yields {[PhBP3]Fe(µ-1,3-N3)}2 (4.1) as a reddish-brown solid in 85% yield. Installation of a terminal azide functionality is readily confirmed by solution-state infrared spectroscopy due to the presence of a strong vibration at 2077 cm-1.17 Repeating the synthesis with [Na][15NNN] yields 4.1-15N in which the azide Cl N3 Ph Fe Ph Fe Ph2P P B Ph PPh2 NaN3 CH3CN 1/2 Ph2P P N Ph Ph2P Fe Na/Hg P Ph THF PPh2 B Ph Ph PPh2 B Ph Ph 2.1 4.1 2 4.2 Na(THF)5 Ph Fe PPh2 Ph P Ph2P B Ph (4.1) 88 vibration is shifted to 2066 cm . Dilute solutions of 4.1 are transparent yellow in color -1 with optical data (λ = 410 nm, ε = 700 M-1 cm-1) similar to that observed for the monomeric chloride 2.1. Evans method18 magnetic measurements on crystalline samples of 2 dissolved in C6D6 repeatedly afforded magnetic moments of 4.50 (±0.04) µB, a value somewhat below the spin-only value of 4.89 µB for four unpaired electrons. Other pseudotetrahedral [PhBPiPr3]Fe(II) complexes, for example, [PhBPiPr3]FeMe, also exhibit moments below the spin-only value.19 We note that a monomer-dimer equilibrium process occurs for 4.1 in solution, as reflected by a color change from yellow to red upon cooling, and the dimeric form has been characterized in the solid state by X-ray diffraction and SQUID magnetometry. Red crystals of 4.1 suitable for an X-ray diffraction experiment were grown via a benzene/petroleum ether vapor diffusion chamber, and its solid-state structure is shown in Figure 4.1. Azide 4.1 crystallizes in the triclinic crystal system with a benzene solvent molecule (omitted from Figure 1 for clarity) and features a dimeric Fe2(µ-1,3-N3)2 core that possesses a crystallographic center of symmetry.20 The coordination geometry of each iron center is therefore identical and is best described as distorted square pyramidal (τ = 0.4)21 in which N1, N3, P2, and P3 comprise the basal plane. As a result of the vacant coordination site trans to P1, the Fe-P1 bond distance of 2.1610(5) Å is slightly shorter than that observed for Fe-P2 (2.2279(6) Å) and Fe-P3 (2.2101(6) Å). The N-N bond distances in each of the azide bridges are nearly identical at 1.181(2) Å (N1-N2 and N1A-N2A) and 1.175(2) Å (N2-N3A and N2A-N3). 89 P1 N1A N3 P2 P3 N2A Fe N1 N2 N3A Figure 4.1. 50% thermal ellipsoid representation of {[PhBP3]Fe(µ-1,3-N3)}2 (4.1). All hydrogen atoms and a benzene solvent molecule have been removed for clarity. Selected bond lengths (Å) and angles (°): Fe-N1: 2.0107(15); Fe-N3: 1.9516(16); N1-N2: 1.181(2); N2-N3A: 1.175(2); Fe-P1: 2.1610(5); Fe-P2: 2.2279(6); Fe-P3: 2.2101(6); P1-Fe-P2: 89.37(2); P1-Fe-P3: 92.23(2): P1-Fe-N1: 94.52(5); P1-Fe-N3: 116.91(5); P2-Fe-P3: 89.88(2); N3-Fe-N1: 87.06(6); N1-FeP2: 175.64(5); N1-Fe-P3: 91.92(5); P3-Fe-N3: 150.84(5). Solid-state magnetic susceptibility data for 4.1 were obtained from 4 to 290 K by SQUID magnetometry and are plotted, per dimeric unit, in Figure 4.2. The plot of χM*T verses temperature (Figure 4.2 top) is consistent with ferromagnetic coupling as a marked increase in the value of χM*T is observed as the sample is cooled. A maximum value of 8.31 cm3 K mol-1 is observed at 40 K, and further cooling of the sample results in a decrease in the value of χM*T, which is likely attributable to zero-field splitting.22 The 90 plot of µeff versus temperature (Figure 4.2 bottom) establishes a room-temperature magnetic moment of 4.3 µB for 4.1, while a maximum value of 8.2 µB is observed at 40 K. χMT (cm3·K·mol-1) 10 8 6 4 2 0 0 100 200 300 Temp (K) 10 µeff (µB) 8 6 4 2 0 0 50 100 150 200 250 300 Temp (K) Figure 4.2. SQUID magnetometry data for 4.1 plotted per dimeric unit as (top) χMT versus temperature and (bottom) µeff versus temperature. 91 As depicted in Equation 4.1, the chemical reduction of 4.1 is readily accomplished in THF with 1 equiv of a 0.2 weight % Na/Hg amalgam. A darkening of the reaction solution is accompanied by the effervescence of gas almost immediately upon exposure of 4.1 to the amalgam. After the mixture was stirred at room temperature for 8 h, the major product of the reaction, [{[PhBP3]Fe}2(µ-N)][Na(THF)5] (4.2), was isolated as a brown solid in ca. 50% yield. Crystals of 4.2 suitable for X-ray diffraction analysis were grown via a THF/hexanes vapor diffusion chamber, and its solid-state structure is shown in Figure 4.3A. Compound 4.2 crystallizes in the monoclinic crystal system with a free molecule of THF (omitted from Figure 4.3 for clarity). The nitride ligand was refined in two positions (Figure 4.3 B and C) and modeled satisfactorily with a population ratio of 1.86 to 1. Repeating the X-ray diffraction experiments on several independently synthesized and crystallized samples consistently afforded data in which the nitride ligand was located in two positions, including samples in which the sodium cation had been exchanged for a nBu4N+ cation. Combustion analysis data for 4.2 is consistent with a single µ-N ligand. In contrast to precursor 4.1, each iron center in the anion of 4.2 is crystallographically unique and exhibits a geometry that is best described as distorted tetrahedral. The relatively short Fe-Nav bond distance of 1.70 Å for both positions of the nitride ligand is consistent with multiple bonding character.9,11,23 One of the dramatic structural differences between 4.2 and other literature examples of diiron µ-nitride systems concerns the Fe-N-Fe bond angle of 4.2, which averages 135° for either position of the nitride ligand.24 All previous examples of structurally characterized bridging iron nitrides feature linear Fe-N-Fe linkages.5 In fact, to the best of our knowledge, the only 92 A P1 P3 Fe1 N1A P2 P4 Fe2 P5 Na P6 P1 B C P5 P3 P4 P3 Fe1 N1B P5 P1 Fe1 Fe2 P2 P4 N1A N1A Fe2 N1B P6 P2 P6 Figure 4.3. (A) 50% thermal ellipsoid representation of [{[PhBP3]Fe}2(µN)][Na(THF)5] 4.2 showing only the more populated position of the disordered nitride ligand. A free molecule of THF has been omitted for clarity. Selected bond distances (Å) and angles (°): Fe1-N1A: 1.675(5); Fe2-N1A: 1.705(5); Fe1-Fe2: ~ 3.13; Fe1-P1: 2.2295(10); Fe1-P2: 2.2338(10); Fe-P3: 2.2127(10); Fe2-P4: 2.2299(10); Fe2-P5: 2.2106(10); Fe2-P6: 2.2534(10); Fe1-N1A-Fe2: 135.9(3); P1-Fe1-P2: 92.10(4): P1-Fe-P3: 88.83(4); P2-Fe1-P3: 90.79(4); P2-Fe1-N1A: 143.93(16); P1-Fe1-N1A: 112.69(14); P3-Fe1-N1A: 114.45(15). (B) Side view of the Fe1-N-Fe2 core of 4.2 showing both positions of the disordered nitride ligand. (C) Top view of the Fe1-N-Fe2 core of 4.2 showing both positions of the disordered nitride ligand. 93 other report of a mono-bridged bimetallic nitride species featuring an appreciably bent M-N-M linkage is that from Floriani and co-workers in which the anion of [{p-tBucalix[4]-(O)4}2Nb2(µ-N)][Na(TMEDA)2] (TMEDA = N,N,N',N'- tetramethylethylenediamine) was reported to have a bond angle of 145.2(1)°.25 This type of bent structure is reminiscent of R3P=N=PR3 cations, which feature bridging nitride ligands sandwiched by 4-coordinate phosphorus centers. Examples of R3P=N=PR3 cations in which the P-N-P bond angle is approximately 135° and the two N-P bond distances are similar, but crystallographically distinct, have been reported.26 Direct Fe-Fe bonding interactions within 4.2 are unlikely given their separation by ca. 3.13 Å.27 Characteristic NMR resonances for triply recrystallized samples of nitride 4.2 are observed by 1H, 31P{1H}, and 15N NMR spectroscopy (a 50% 15N-enriched sample of 4.2 may be generated from the reduction of 4.1-15N), consistent with the assignment of a singlet ground state. The 31P{1H} spectrum features a singlet at δ 58.0 ppm, and the 1H NMR spectrum in THF-d8 displays, in addition to several equivalents of THF due to the sodium cation, one set of diamagnetic resonances corresponding to the [PhBP3] ligand. A singlet at δ 801 ppm (versus NH3 at δ 0 ppm) in the 15N NMR spectrum of 4.2 confirms the presence of the nitride functionality. We were unable to resolve 15N-31P coupling, similar to the case of the terminal nitride derivative [PhBPiPr3]FeIV≡N6 where an 15N NMR signal was observed at δ 952 ppm. The downfield nature of the nitride resonance in comparison with other mono-bridged systems28 likely reflects a paramagnetic contribution to the chemical shift from the antiferromagnetically coupled high-spin ferrous centers (vide infra). In accord with the assignment of a diamagnetic ground state, nitride 4.2 is X-band EPR silent at 4 K as a glassy 1-methyltetrahydrofuran solution. 94 Two limiting electronic descriptions for complex 4.2 need to be considered. It can be electronically described by two localized low-spin S = 0 Fe(II) centers, or alternatively as a fully delocalized system that features two high-spin S = 2 Fe(II) centers that are sufficiently antiferromagnetically coupled that a singlet ground state is populated even at room temperature. In accord with the latter description, Holland, Munck, and co-workers recently reported a sulfido-bridged diiron(II) compound that features paramagnetic solution chemical shifts in the 1H NMR spectrum at room temperature, but is assigned to an S = 0 ground state due to antiferromagnetic coupling on the basis of low-temperature Mossbauer data.29 In addition, Wieghardt, Trautwein, and co-workers have described an interesting FeIV(µ-N)FeIV complex that is diamagnetic in solution at room temperature, suggesting strong antiferromagnetic coupling and a lower | J | value limit of 250 cm-1.5a To further examine this issue, we prepared a mononuclear d6 iron(II) complex exhibiting a related pseudotetrahedral geometry with one π-bonded Nx type linkage for comparison. This complex, {[PhBP3]FeII≡N(1-Ad)}{nBu4N} (4.3), is diamagnetic and clearly establishes that an isolated pseudotetrahedral Fe(II) center can adopt a low-spin ground-state electronic configuration. Complex 4.3 was easily prepared by Na/Hg amalgam reduction of S = 1/2 [PhBP3]FeIII≡N(1-Ad) (2.12), followed by the addition of [nBu4N][Br]. The trivalent imide precursor 2.12 was itself readily prepared by the addition of 1-adamantyl azide to [PhBP3]Fe(PPh3) (2.4), a procedure analogous to that used in the preparation of the related imide [PhBP3]FeIII≡N(p-tolyl) (2.5).11 Complex 4.3 represents what is, to the best of our knowledge, the only reported example of a d6 iron imide30 and is moreover the first example of a monomeric, pseudotetrahedral S = 0 Fe(II) complex. 95 The solid-state structures of both 4.3 and 2.12 have been determined for comparison with one another and with nitride 4.2, their core structures are shown in Figure 4.4. Most striking is the overall similarity of the structures of 4.3 and 2.12 at {[PhBP3]FeII≡N(1-Ad)}{nBu4N} [PhBP3]FeIII≡N(1-Ad) C46 C46 N1 N1 Fe1 Fe1 P1 P2 P3 Fe1-N1-C46 = 178.57(2)° Fe1-N1 = 1.651(3) Å Fe-P1 = 2.133(6) Å Fe-P2 = 2.143(3) Å Fe-P3 = 2.150(6) Å P2 P3 P1 Fe1-N1-C46 = 176.33(1)° Fe1-N1 = 1.641(2) Å Fe-P1 = 2.217(1) Å Fe-P2 = 2.283(1) Å Fe-P3 = 2.263(1) Å Figure 4.4. Solid-state molecular structures of 4.3 (left) and 2.12 (right) showing 50% displacement ellipsoid representations for the P3Fe≡N(1-Ad) core. For 4.3, a THF solvent molecule and the nBu4N+ cation have been removed for clarity. Selected bond lengths and angles are highlighted within the figure. the Fe1-N1-C46 linkage, with respect to both the Fe1-N1 bond distance and the Fe1-N1C46 bond angle, despite their different spin states. Given this similarity, a structural distinction worth noting is that the average Fe-P bond distances in the divalent anion of 4.3 are ca. 0.1 Å shorter than the average of the Fe-P bond distances for neutral, trivalent 2.12. This difference is somewhat counterintuitive and may reflect the presence of an 96 unpaired spin in the case of 2.12, although this explanation is unsatisfying given the predominantly nonbonding nature of the orbital that would house the unpaired electron (dxy or dx2-y2, vide infra). An alternative possibility to further consider is that the anion of 4.3 more strongly π-back-bonds into σ* orbitals of the appropriate symmetry from the [PhBP3] ligand than for the case of 2.12 by virtue of its d6 (versus d5) electronic configuration.31 This scenario would give rise to shorter Fe-P interactions. Also consistent with a π-back-bonding argument is the observation that the P-C bonds of anionic 4.3 are, on average, 0.016 Å longer than those for neutral 2.12. A similar phenomenon has been observed for the complex [(PP3)Fe(C≡CPh)][BPh4] (PP3 = P(CH2CH2PPh2)3),32 in which the one-electron reduction of this species induces a contraction of the Fe-P bond distances by an average of 0.09 Å. The authors also attributed this structural anomaly to π-back-bonding. These explanations notwithstanding, the magnitude between the respective Fe-P bond distances in 4.3 and 2.12 is noteworthy. The average of the Fe-N bond distances in 4.2 is approximately 0.05 Å longer than that observed for 4.3 and 2.12. Additionally, the Fe-P bond distances observed for 4.2 (Fe-Pavg. = 2.228 Å) are intermediate to those observed for the structures of 4.3 (FePavg. = 2.142 Å) and 2.12 (Fe-Pavg. = 2.254 Å). By contrast, the recently communicated high-spin Fe(II) amide complex [PhBP3]Fe(N(H)-p-tolyl)(3.1)10 features diminished πbonding character at the Fe-N linkage in comparison to 4.2, 4.3, and 2.12 and exhibits expanded Fe-N and Fe-P distances due to its preferred S = 2 ground-state electronic configuration (Fe-N = 1.913 (2) Å, Fe-Pavg. = 2.424 Å). On the basis of these structural considerations, the tabulation of charges for 4.2, and the fact that it exhibits a 97 diamagnetic ground state, it might seem most reasonable to assign each of the iron centers of 4.2 as localized and low-spin Fe(II). A comparison of the electronic absorption spectra for compounds 2.1, 4.2, and 4.3, however, seems to suggest that this is not the best description. Electronic absorption spectra for compounds 2.1, 4.2, and 4.3 were collected from 350 to 2000 nm at room temperature in THF-d8, and the resulting spectra are shown in Figure 4.5. Chloride 2.1 (Figure 4.5, inset), which represents a classic example of highspin iron(II), exhibits an absorption feature at 1285 nm (7782 cm-1, ε = 185 M-1 cm-1) that is reflective of its expected low-energy d-d transitions.33 The d-d transitions of similar energy are not expected for a low-spin iron(II) center due to a large HOMO-LUMO gap, nor are charge-transfer transitions, which would be expected to occur at much higher 25000 -1 -1 ε (M cm ) 20000 4.2 15000 -1 -1 ε (M cm ) 1000 800 600 400 200 0 350 10000 2.1 550 750 950 1150 1350 1550 1750 1950 nm 5000 4.3 0 350 550 750 950 1150 1350 1550 1750 1950 Wavelength (nm) Figure 4.5. Electronic absorption spectrum of 2.1 (inset, blue), 4.2 (green), and 4.3 (red) in THF-d8. 98 energy. Indeed, no d-d type or charge-transfer type transitions are observed in the NIR region of the spectrum for the rigorously low-spin Fe(II) imide 4.3; a feature at ca. 510 nm (19 608 cm-1, ε = 2112 M-1 cm-1) is likely reflective of an LMCT transition. Inspection of the NIR region of the spectrum for complex 4.2, however, reveals a very broad and intense feature centered at approximately 1120 nm (8929 cm-1, ε = 3720 M-1 cm-1). This feature, which we assume arises from a charge-transfer transition associated with the bridged nitride ligand, displays an easily discernible shoulder on its low-energy side that reflects the low-energy d-d transitions characteristic of high-spin Fe(II) and observed in 2.1. Curve-fitting of the NIR data for 4.2 from 850 to 2000 nm using GRAMS v.3.2 (Thermo Galactic, see Supporting Information) resolves the two features into a transition centered at 1120 nm (8929 cm-1, ε = 2740 M-1 cm-1) and one centered at 1266 nm (7900 cm-1, ε = 943 M-1 cm-1). It is interesting to contemplate the cause of the low-spin configurations in complexes 4.3 and 4.3. According to the d-orbital splitting scheme that we have previously used to describe the d6 cobalt imide complex [PhBP3]Co≡N(p-tolyl),13b an appropriate electronic configuration for the simplest case, d6 4.3, is (dz2)2(dxy, dx20 2 4 2 y ) (dxz,dyz) . Under idealized C3v symmetry, these d-orbitals transform as a1 + 2e. The dz orbital is expected to lie low in energy, very close to the nearly degenerate pair of dorbitals that are oriented perpendicular to the Fe-N bond vector. A DFT calculation on the slightly simplified complex {[PhBP3]Fe≡N(tBu)}- provides a minimized geometrical (Figure 4.6A) and electronic structure fully consistent with this view. As shown in Figure 4.6B, a two-over-three splitting diagram is obtained in which dz2, dxy, and dx2-y2 are nearly degenerate and non-bonding, and a pair of unoccupied dxz, dyz orbitals that are both 99 179.45o 1.651 Å xz yz 2.08 eV 2.07 eV -1.69 eV 2 2 A HOMO x -y -1.68 eV xy HOMO - 2 dxy dz2 -1.73 eV 2 z B LUMO + 1 dxz C Figure 4.6. (A) Theoretically predicted geometry and (B) electronic structure (DFT, JAGUAR 5.0, B3LYP/LACVP**) for S = 0 {[PhBP3]Fe≡N(tBu)}-. (C) Lobal representations of the highest occupied molecular orbital (HOMO), the orbital of dz2 parentage (HOMO – 2), and the dxz orbital (LUMO + 1). Structural parameters: Fe-P = 2.209, 2.213, 2.213 Å; Fe-N = 1.651 Å; Fe-N-C = 179.45°; NFe-P = 125.08, 125.39, 125.63°; P-Fe-P = 89.72, 89.88, 89.97°. σ* and π* in character lie at much higher energy. The calculation predicts the orbital of dz2 parentage to lie lowest in energy (HOMO – 2), and the HOMO is therefore predicted to be either dxy or dx2-y2. Lobal representations for the HOMO, HOMO – 2, and LUMO + 1 orbitals are shown in Figure 4.6C. Hypothetical removal of one electron from this system 100 to generate Fe(III) is expected to give rise to a doublet ground-state species, which is observed for imide 2.12 (µeff = 1.98 µB). Because the electron is removed from an essentially nonbonding orbital, little structural change is expected at the Fe-N-C linkage, fully consistent with the solid-state structures of 4.3 and 2.12. That the Fe-P bond distances in fact contract on reducing 2.12 to 4.3 is not as readily explained (vide supra). The comparative cyclic voltammetry of 4.2 and 2.12 is of obvious interest and has been studied in THF (Fc/Fc+, 0.30 M [nBu4N][PF6], 50 mV/s; Fc = ferrocene). The data acquired are shown in Figure 4.7. Inspection of the electrochemical response of 4.2 reveals an irreversible oxidative wave at -350 mV, a fully reversible redox process centered at -1340 mV, and another redox process at -2520 mV that appears to be quasireversible. The two redox events observed at the more positive potentials are strikingly 2.12 4.2 100 -400 -900 -1400 -1900 -2400 -2900 Potential (mV vs. Fc/Fc+) Figure 4.7. Cyclic voltammetry of [PhBP3]FeIII≡N(1-Ad) (2.12), top, and [{[PhBP3]Fe}2(µ-N)][Na(THF)5] (4.2), bottom. Experimental parameters: 2.0 mM analyte, 0.30 M [nBu4N][PF6] electrolyte, scan rate = 50 mV/s. 101 similar to those observed for 2.12. In a previous communication, we have reported that a reversible FeIII/II redox process is available for [PhBP3]FeIII≡N-p-tolyl,11 and the reversible wave centered at -1315 mV in the cyclic voltammogram of 2.12 is consistent with this picture and with the fact that this species can be chemically reduced to produce anionic 4.3. The irreversible wave in the cyclic voltammogram of 2.12 has a peak maximum at -130 mV. We presume that this step represents an oxidation to generate a highly unstable “[PhBP3]FeIV≡N(1-Ad)+” species, or constitutes an oxidation of the borate ligand. We suggest that the reversible wave centered at -1.3 V in the cyclic voltammogram of 4.2 corresponds to an Fe(III)Fe(II)/Fe(II)Fe(II)- redox process. Several assignments are possible for the irreversible wave at -350 mV. It may correspond to an Fe(III)/Fe(IV) redox event at a single iron center consistent with the picture put forth for 2.12, to an Fe(III)Fe(II)/Fe(III)Fe(III)+ redox process, or to oxidative degradation of a borate ligand. The reduction wave at low potential (-2.5 V) is interesting and is not observed for complex 2.12. This wave is suggestive of an Fe(II)Fe(II)-/Fe(II)Fe(I)2- redox process in which the resulting reduced iron species appears to be relatively unstable under the conditions of the electrochemical experiment, as the anodic wave decreases and eventually vanishes when this redox event is isolated and cycled multiple times. Synthetic examination of this system using chemical reductants and oxidants may offer a better understanding of these redox events. The mechanism concerning the conversion of 4.1 to 4.2 deserves some consideration. Due to the heterogeneous reaction and the paramagnetic nature of the precursor complex and presumable intermediates, it is a difficult system to probe experimentally. As shown in Equation 2, we presume that Na/Hg amalgam reduction of 102 N III Ph Fe Ph Ph2P P PPh2 Na(THF)x B N3 ½ Ph Fe Ph2P P Ph PPh2 Na/Hg - NaN3 2 Ph 4.2 (solv) THF B Ph - N2 Ph Fe Ph2P P I (4.2) Ph PPh2 B Ph 4.1 to its corresponding anion releases N2 gas, consistent with the observation of rapid effervescence. A plausible iron byproduct from N2 release is the anionic nitride species “{[PhBP3]FeIII(N)}-.” Based upon the DFT calculations we have used to explore the electronic nature of a structurally related species, [PhBPiPr3]FeIV≡N,6 a single unpaired electron would populate a high-lying a1 orbital featuring significant Fe-N σ* character in {[PhBP3]FeIII(N)}-. This electronic situation should render the putative species highly reactive. It is therefore reasonable to suspect that if generated intermittently, such a species might attack the iron center of a secondary molecule of 4.1 to displace azide, or might attack the azide ligand itself.34 Both of these scenarios have been suggested by Summerville and Cohen in the context of the photolysis products of a tetraphenylporphyrnatoiron(III) azide,5e and also by Wieghardt, Trautwein, and coworkers.5a We note that repeating the reduction of 4.1 in the presence of excess PPh3 results in the formation of a substantial quantity of the previously reported Fe(I) complex [PhBP3]Fe(PPh3) (2.4),11 in addition to nitride 4.2. Because complex 4.1 itself does not react with PPh3, it appears likely that PPh3 traps an Fe(I) source that is generated in situ. 103 We can therefore put forth the notion that “{[PhBP3]FeIII(N)} -” rapidly condenses with an “[PhBP3]FeI(solv)” species to form isolable 4.2, but the speculative nature of this suggestion needs to be underscored. One way to identify {[PhBP3]FeIII(N)}- as an intermediate would involve trapping it by addition of an appropriate electrophile. Accordingly, we have tried to reduce the azide precursor 4.1 in the presence of trimethylsilyl chloride. While we do not know the nature of the reaction products, it is quite clear from spectroscopic data that [PhBP3]FeIII(NSiMe3), the simplest product we might expect to form via metathesis (and elimination of NaCl and N2), is not among these products. An intriguing possibility was that addition of CO would lead to disproportionation of the system with liberation of [PhBP3]Fe(CO)2 (2.13)11 and the elusive nitride species “{[PhBP3]FeIII(N)}-” to which we have alluded to above. Exposure of 4.2 to an atmosphere of CO results in its instantaneous consumption (Equation 4.3), but the major product of the reaction proved to be the FeI dicarbonyl, 2.13. A second iron-containing product, present in ca. 25% yield, was identified as the diamagnetic iron(II) isocyanate OC 4.2 CO Ph PPh2 Ph Ph2P P Ph Fe xs CO Ph2P THF, RT CO CO P + B NCO Fe Ph PPh2 B 2.13 Ph major Ph 4.4 minor, ~ 25% (4.3) CO CO Cl Fe Ph Ph Ph2P P B Ph PPh2 [K][NCO] THF, RT 4.5 4.4 104 complex [PhBP3]Fe(CO)2(NCO) (4.4), a presumed product of nitride capture. The identity of diamagnetic 4.4 was readily ascertained by comparison of the crude spectral data (IR, 1H, and 31P{1H}) from the reaction with that of an independently prepared sample of 4.4 generated by the reaction between [PhBP3]Fe(CO)2Cl (4.5) and [K][NCO]. We have not identified the fate of the sodium cation in this reaction, but it is worth noting that, based upon the electronic structure considerations mentioned above for “{[PhBP3]FeIII(N)}-,” we would expect it to be a powerful reductant if generated as an intermediate. Its oxidation product, [PhBP3]FeIV≡N, should be susceptible to attack by CO to produce isolable 4.4.35 While it is unclear which species serves as the formal electron acceptor in the production of 4.4, [PhBP3]Fe(CO)2 and 4.4 are the only ironcontaining products observed in the reaction. While more comprehensive reactivity studies are in the offing, we wish to note that the (µ-N) ligand of 4.2 can serve as a source of NH3 upon exposure to acid (Eq 4.4). Thus, the addition of 3 equiv. of HCl to a THF solution of 4.2 at -35 °C released NH3 in good yield (two independent experiments provided NH3 in yields of 80% and 95%),6,36 along with the chloride complex 2.1 as the predominant iron-containing byproduct (>80%). The yield of NH3 was determined by vacuum transfer of the reaction volatiles into an ethereal solution containing HCl, which results in the precipitation of ammonium chloride. Subsequent isolation and quantification of the [NH4][Cl] salt was accomplished by NMR spectroscopy in DMSO-d6 using a ferrocene integration standard.6 It is of future interest to add fewer proton equivalents, and perhaps H-atom equivalents (H+/e-) to 4.2 to 4.2 + 3 HCl → 2 [PhBP3]FeCl (2.1) + NH3 + NaCl (4.4) 105 try to elucidate the other protonated forms of the system prior to NH3 release (e.g., FeII(µNH)FeII and FeII-NH2 + FeIICl). 4.3 Conclusions The bimetallic bridged nitride complex 4.2 is unique in several regards. Foremost among these are the low coordination number and the low oxidation state of each of its two iron centers, and its severely bent Fe-N-Fe linkage. These features are unprecedented for bimetallic bridged iron nitrides and are very uncommon to bimetallic bridged nitrides more generally. Spectroscopic data for 4.2 suggest the presence of two high-spin ferrous centers that are sufficiently coupled to provide a singlet ground state even at room temperature. While bridged iron nitride species are relatively rare, they may be important to nitrogen fixation schemes. As mentioned at the outset, it is interesting to recall the recent finding that the FeMo-cofactor of nitrogenase contains an interstitial light atom, presumably a nitride, surrounded by six pseudotetrahedral iron centers.2 While this revised cofactor structure does little to clarify the mechanism of biological N2 reduction, it at least calls attention to a possible role for relatively low-valent, low-coordinate Fe-(µN)-Fe linkages within the active site of the enzyme.3 Likewise, similar structural motifs may be relevant to the industrial synthesis of ammonia mediated by catalytically active, low-valent iron sites.4 While these suggestions remain highly speculative due to the absence of structural data during catalytic turnover conditions, the bimetallic nitride we have described at least provides chemical support for well-defined, low-valent µ-iron nitride systems. Specifically, our study shows that low-valent iron(II) centers are able to support bridging µ2-nitride ligands, in this case with pseudotetrahedral systems supported 106 by relatively soft phosphine donors. The µ2-nitride functionality can serve a structural role under reducing chemical conditions. The cyclic voltammetry of 4.2, shown in Figure 4.7, adequately illustrates this point by revealing a pseudo-reversible Fe(II)Fe(II)/Fe(II)Fe(I)2- reduction process at low potential. It is of further note that the µ2-nitride ligand can serve as a source of NH3 upon addition of protons. Conceptually replacing one of the "[PhBP3]Fe(II)" units in the anion of 4.2 with an organic moiety results in a mononuclear, anionic iron(II) imide. We have described an example of such a species herein, {[PhBP3]FeII≡N(1-Ad)}{nBu4N} (4.3), that features a linear Fe-N-C bond angle, a short Fe-N bond length, and a diamagnetic ground state. These features are collectively indicative of significant Fe-N multiple bonding character in the complex. The DFT calculation presented for this system corroborates the d-orbital splitting scheme that we have proposed previously,11 placing three low-lying and largely non-bonding orbitals (dx2-y2, dxy, dz2) at similar energy and well separated from two highlying orbitals (dxz, dyz) with significant σ and π antibonding contributions (see Figure 8). This orbital arrangement favors a d6 electronic configuration and additionally accommodates a d5 configuration, as demonstrated by the relative stability of doublet imides such as complex 2.12. It is therefore evident that the imide ligand confers a lowspin electronic configuration to both pseudotetrahedral iron(II) and iron(III) centers, at least for [BP3]Fe systems. As we have recently shown, transforming the imide ligand to a terminal nitride ligand significantly alters the d-orbital splitting scheme.6 In the latter case, the a1 orbital of dz2 character is strongly destabilized due to favorable directional overlap with a 2pz orbital at the nitride N-atom. This arrangement favors the d4 configuration, in accord with our successful characterization of [PhBPiPr3]FeIV≡N. 107 Addition of one electron to this latter species produces the hypothetical d5 anion “[PhBPiPr3]Fe(N)-,” which would be characterized by a double (or partial triple) bond due to the population of a strongly antibonding orbital by one electron (Figure 4.8). Needless to say, such a species should be a potent reductant in its own right, and it is therefore not surprising that the reduction of azide 4.1 ultimately leads to an anionic bridged nitride rather than an anionic terminal nitride species. We emphasize that for the d6 and d5 imides, and for the d4 nitride, the Fe-N triple-bond formulation is appropriate under threefold symmetry. This situation sharply contrasts four-fold symmetric Fe=E species (E = O, N),37,38 where the population of π* antibonding orbitals prohibits the formation of a bona fide triple bond. Figure 4.8. (left) Qualitative d-orbital splitting diagrams illustrating the predicted ground-state electronic structures of various [BP3]Fe≡Nx species for d2, d3, and d4 configurations. Note that for the imide species an orbital of a1 symmetry with dz2 character lies in the non-bonding region, very close in energy to orbitals with dx22 6 5 y and dxy character. Therefore, d and d configurations are relatively stable. In contrast, the a1 orbital is strongly destabilized for a terminal nitride species, [BP3]Fe≡N. Therefore, the d4 configuration is relatively stable whereas the d5 configuration is destabilized due to population of a high-lying orbital. (right) 108 Comparison of the d-orbital splitting diagrams anticipated under idealized threefold and four-fold symmetry. Whereas an Fe≡E triple bond best describes the configurations shown under three-fold symmetry, an Fe=E double bond best describes the configurations shown under four-fold symmetry. 4.4 Experimental Section 4.4.1 General Considerations General considerations are outlined in Section 3.4.1. Deuterated THF was purchased from Cambridge Isotope Laboratories, Inc. and dried over fine alumina (THF) prior to use.15N NMR data was acquired on an Inova 500 MHz spectrometer and chemical shifts were referenced to CH3NO2 (380 ppm relative to liquid ammonia at 0 ppm). UV/vis/NIR measurements were taken in THF-d8 on a Cary 500 UV/vis/NIR spectrophotometer using a 0.1 cm quartz cell with a Teflon stopper. 4.4.2 Magnetic Measurements SQUID data for azide 4.1 was acquired from 4–290 K as outlined in Section 2.5.2. 4.4.3 Electrochemical Measurements Electrochemical procedures are outlined in Section 2.5.4. 4.4.4 DFT Calculations A hybrid density functional calculation was performed for {[PhBP3]Fe≡N(tBu)}using the Jaguar package (version 5.0, release 20).39 The calculation employed B3LYP with LACVP** (LACVP**++ for B) as the basis set.40 A geometry optimization was carried out using the X-ray coordinates for 2.12 (with all of the carbon atoms of the adamantyl group removed except for those directly attached to the imide nitrogen atom) 109 as the initial HF guess. No symmetry constraints were imposed and the calculation was performed assuming a singlet electronic ground state. 4.4.5 Starting Materials and Reagents All reagents were purchased from commercial vendors and used without further purification unless otherwise stated. [Na][N3], [Na][15NNN] (Cambridge Isotope Laboratories), and [K][NCO] were dried under reduced pressure at 120 °C overnight prior to use. The compounds [PhBP3]FeCl (2.1) and [PhBP3]Fe(PPh3) (2.4) were prepared according to literature procedures.11 Imide 2.12 was prepared according to Section 2.5.8. 4.4.6 Synthesis of Compounds Synthesis of {[PhBP3]Fe(µ-1,3-N3)}2, 4.1: Chloride 2.1 (1.00 g, 1.28 mmol) was dissolved in acetonitrile (10 mL) with stirring and added to a stirring acetonitrile (5 mL) slurry of [Na][N3] (0.0837 g, 1.28 mmol). The initial red color of the reaction gradually faded as a purple solid precipitated out of the solution. After 24 hours volatiles were removed under reduced pressure, and the solids were extracted with benzene (50 mL), filtered over Celite, and lyophilized. The resulting solid was isolated on a sintered glass frit, washed with ether (2 x 10 mL), petroleum ether (3 x 30 mL), and dried to yield 0.861 g (85%) of a reddish-brown powder. X-ray quality crystals were grown via vapor diffusion of petroleum ether into a benzene solution. IR (KBr/C6H6): ν(N3) = 2077 cm-1. UV-vis (C6H6) λ max, nm (ε, M-1 cm-1): 410 (700). SQUID: µeff = 4.3 µB at 290 K. Evans Method (C6D6): µeff = 4.50 µB. Anal. Calcd. for C45H41BFeN3P3: C, 68.99; H, 5.28; N, 5.36. Found: C, 68.66; H, 5.33; N, 5.34. For 4.1-15N: an analogous synthetic procedure employing [Na][15NNN] yielded 15N labeled 4.1. IR (KBr/C6H6): ν(N3) = 2066 cm-1. 110 Synthesis of [{[PhBP3]Fe}2(µ-N)][Na(THF)5], 4.2: Azide 4.1 (0.874 g, 1.12 mmol) was dissolved in THF (15 mL) with stirring. To this solution was added 1 equivalent of sodium in the form of a 0.20 weight % Na/Hg amalgam (0.0256 g Na dissolved in 13.5 g Hg). The addition of the amalgam resulted in the vigorous effervescence of N2 as the color of the reaction solution darkened. After ~ 5 minutes the effervescence had subsided, and the reaction was capped and allowed to stir at room temperature for 8 hours. After this time the reaction was decanted from the amalgam, and volatiles were removed under reduced pressure. The crude solids were extracted with a fresh aliquot of THF (40 mL) and filtered over Celite. Volatiles were again removed under reduced pressure, and the resulting solids were collected on a sintered glass frit and washed with copious amounts of benzene (~ 5 x 30 mL), petroleum ether (2 x 15 mL), and dried to obtain 0.546 g (52%) of a brown solid. X-ray quality crystals were grown from a THF/hexanes vapor diffusion chamber while samples for elemental and 1H NMR analysis were repeatedly crystallized from THF/petroleum ether. 1H NMR (THF-d8, 300 MHz): δ 7.72 (d, J = 6.0 Hz, 4H); 7.44 (br s, 24H); 7.12 (t, J = 7.2 Hz, 4H); 6.88 (t, J = 7.2 Hz, 2H); 6.70 (t, J = 7.5 Hz, 12H); 6.53 (t, J = 7.5 Hz, 24H); 3.60 (m, 20H, overlaps with residual solvent peaks); 1.78 (m, 20H, overlaps with residual solvent peaks); 1.56 (br s, 12H). 31 P{1H} NMR (THF-d8, 121.4 MHz): δ 58.0 (s). UV/vis/NIR (THF-d8) λ, nm (ε, M-1 cm-1): 1266 (943), 1120 (2740), 565 (11 520), 445 (20 530), 390 (21 300). Anal. Calcd. for C110H122B2Fe2NNaO5P6: C, 70.26; H, 6.54; N, 0.74. Found: C, 70.06; H, 6.35; N, 0.82. A sample of 15N enriched 4.2 (50%) was synthesized using an analogous synthetic procedure with 4.1-15N. 15N NMR (THF, 50.751 MHz): δ 801 (s). 111 Synthesis of {[PhBP3]Fe≡N(1-Ad)}{nBu4N}, 4.3: Compound 2.12 (0.168 g, 0.189 mmol) was dissolved in THF (5 mL) with stirring. A 0.20 weight % Na/Hg amalgam (0.0043 g Na dissolved in ca. 2.2 g Hg) was added to the THF solution of 2.12, and the reaction was allowed to stir at room temperature for two hours. The reaction solution was then transferred from the amalgam and stirred in a vial containing [nBu4N][Br] (0.0606 g, 0.189 mmol) for 4 hours. After this time the volatiles were removed under reduced pressure, and the crude solids were extracted with a fresh aliquot of THF (20 mL) and filtered over a pad of celite. Volatiles were again removed under reduced pressure, and the resulting brown solids were washed with benzene (3 x 10 mL) and dried to obtain 0.141 g (66%) of 4.3. Crystals suitable for an X-ray diffraction study were grown via vapor diffusion of petroleum ether into a THF solution. 1H NMR (THFd8, 300 MHz): δ 7.79 (br s, 12H); 7.57 (m, 2H); 7.04 (t, J = 7.2 Hz, 2H); 6.67 (m, 18H); 3.07 (m, 8H); 2.15 (s, 6H); 2.07 (s, 3H); 1.78 (m, 6H); 1.57 (m, 8H); 1.35 (m, 8H); 0.98 (t, J = 7.2 Hz, 12H, overlaps with ligand methylene resonance at δ 0.94); 0.94 (s, 6H, overlaps with nBu4N+ resonance at δ 0.98). 31P{1H} NMR (THF-d8, 121.4 MHz): δ 95.0 (s). 11B{1H} NMR (THF-d8, 128.3 MHz): δ -13.5 (s). UV/vis/NIR (THF-d8) λ, nm (ε, M-1 cm-1): 600 (815), 510 (2112). Anal. Calcd. for C71H92BFeN2P3: C, 75.26; H, 8.18; N, 2.47. Found: C, 75.15; H, 8.31; N, 2.78. Synthesis of [PhBP3]Fe(CO)2(NCO), 4.4: Compound 4.5 (0.050 g, 0.060 mmol) and [K][NCO] (0.049 g, 0.60 mmol) were stirred in THF (5 mL) for 24 hours. After this time volatiles were removed under reduced pressure, and the crude solids were extracted with benzene (5 mL), filtered over celite, and lyophilized to yield 0.038 g (75%) of a yellow solid. 1H NMR (C6D6, 300 MHz): δ 7.84 (d, J = 7.2 Hz, 2H); 7.69 (br s, 8H); 7.55 112 (t, J = 7.2 Hz, 2H); 7.36 (d, J = 7.2 Hz, 1H); 7.10 (m, 4H); 6.86 (br s, 14H); 6.72 (m, 4H); 1.83 (br d, J = 60 Hz, 4H); 1.56 (d, J = 14 Hz, 2H). 31P{1H} NMR (C6D6, 121.4 MHz): δ 50.4 (t, J = 58 Hz, 1P); 27.3 (d, J = 58 Hz, 2P). IR (KBr/C6H6): ν(CO) = 2043, 1999 cm-1; ν(NCO) = 2195, 2224 cm-1. Anal. Calcd. for C48H41BFeNO3P3: C, 68.68; H, 4.92; N, 1.67. Found: C, 69.01; H, 5.23; N, 2.01. Synthesis of [PhBP3]Fe(CO)2Cl, 4.5: Chloride 2.1 (0.200 g, 0.257 mmol) was stirred in benzene (10 mL) in a 50 mL Schlenk flask. The solution was frozen, and the atmosphere was evacuated and replaced with one atmosphere of CO. The reaction was then allowed to warm to room temperature and stirred for an additional 30 minutes. After this time the volatiles were removed under reduced pressure, yielding an essentially quantitative amount of a golden solid. 1H NMR (C6D6, 300 MHz): δ 7.87 (m, 10H); 7.56 (t, J = 7.2 Hz, 2H); 7.35 (t, J = 6.0 Hz, 1H); 7.20 (m, J = 7.2 Hz, 4H); 6.84 (m, 14H); 6.74 (m, 4H); 1.91 (br d, J = 57 Hz, 4H); 1.62 (d, J = 15 Hz, 2H). 31P{1H} NMR (C6D6, 121.4 MHz): δ 54.7 (t, J = 53 Hz, 1P); 25.2 (d, J = 53 Hz, 2P). 11B{1H} NMR (C6D6, 128.3 MHz): δ -13.9 (s). IR (KBr/C6H6): ν(CO) = 2039, 1997 cm-1. ES-MS: calcd. for C47H41BClFeO2P3 (M)+: 833 m/z, found (M – Cl)+ 797 m/z. Anal. Calcd. for C47H41BClFeO2P3: C, 67.78; H, 4.96. Found: C, 67.45; H, 5.13. The reaction of 4.2 with CO: Nitride 4.2 (0.0083 g, 0.004 mmol) was dissolved in THF in a sealable NMR tube. The resulting solution was frozen, and the N2 atmosphere was evacuated and replaced with one atmosphere of CO. The NMR tube was shaken periodically over a period of 10 minutes at which time a crude 31P NMR spectrum confirmed the presence of 4.4. After this time volatiles were removed under reduced pressure, and both 1H NMR (C6D6) and IR analysis (KBr/C6H6) confirmed the presence of [PhBP3]Fe(CO)2 11 113 as the major product and 4.4 as the minor product. Addition of a hexamethylbenzene internal standard to this 1H NMR sample demonstrated that 4.4 was produced in ca. 25% yield. The reaction of 4.2 with HCl: Nitride 4.2 (0.030 g, 0.0160 mmol) was dissolved in 1 mL of THF in a sealable NMR tube. The solution was cooled to -35 °C, and 3 equivalents of HCl (46 µL of a 1 M solution in Et2O diluted in 0.25 mL THF) cooled to -35 °C were added. The reaction was then allowed to warm to room temperature. After 30 minutes the reaction volatiles were vacuum transferred into 5 mL of a 1 M solution of HCl in Et2O. Once the transfer was complete, the HCl solution was stirred at room temperature for about 30 minutes. At this point a small amount of precipitate was evident, and the volatiles were removed under reduced pressure. The resulting residue was dissolved in DMSO-d6, and the presence of [NH4][Cl] was confirmed by a triplet at δ 7.15 ppm (t, J = 51 Hz, 4H), which was integrated versus an internal ferrocene standard.6 Two independent runs provided an 80% and a 95% yield of NH3, respectively. The remaining solids from the original reaction were dissolved in benzene-d6, and chloride 2.1 was observed to be the major (> 80%) iron-containing product by 1H NMR spectroscopy. 4.4.7 X-ray Experimental Data Crystallographic procedures are outlined in Section 2.5.8. Crystallographic data are summarized in Table 4.1. Crystals of imide 2.12 were obtained as outlined in Section 2.5.8. 114 Table 4.1. Crystallographic data for {[PhBP3]Fe(µ-1,3-N3)}2 (4.1), [{[PhBP3]Fe}2(µN)][Na(THF)5] (4.2), {[PhBP3]Fe≡N(1-Ad)}{nBu4N} (4.3), and [PhBP3]Fe≡N(1-Ad) (2.12). monomer 4.1·C6H6 2.12·THF C75H100BFeN2OP3 chemical formula C51H47BFeN3P3 fw 861.49 C114H130B2Fe2NNa O6P6 1952.32 T (°C) -177 -177 -177 λ (Å) 0.71073 0.71073 0.71073 a (Å) 13.0738(10) 16.7910(8) 12.173(3) b (Å) 14.2209(11) 13.7352(6) 14.051(4) c (Å) 14.6878(12) 43.299(2) 22.131(6) α (°) 118.2270(10) 90 71.610(4) β (°) 95.2290(10) 93.6530(10) 79.790(4) γ (°) 111.0900(10) 90 68.373(4) V (Å3) 2128.4(3) 9965.8(8) 3331.1(14) space group P-1 P2(1)/c P-1 Z 2 4 2 Dcalc (g/cm3) 1.344 1.301 1.202 µ(cm-1) 5.08 4.49 3.44 0.0650, 0.0940 0.0470, 0.0972 R1, wR2a (I > 2σ(I)) 0.0396, 0.0858 a 4.2·THF R1 = Σ||Fo| - |Fc||/Σ|Fo|, wR2 = {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2 1205.14 115 Table 4.1 cont. 2.12 chemical formula C55H56BFeNP3 fw 890.58 T (°C) -177 λ (Å) 0.71073 a (Å) 16.4143(15) b (Å) 14.1225(13) c (Å) 20.7686(19) α (°) 90 β (°) 109.186(2) γ (°) 90 V (Å3) 4547.0(7) space group P2(1)/n Z 4 Dcalc (g/cm3) 1.301 µ(cm-1) 4.76 R1, wR2a (I > 2σ(I)) 0.0489, 0.0982 116 References Cited 1 (a) Huniar, U.; Ahlrichs, R.; Coucouvanis, D. J. Am. Chem. Soc. 2004, 126, 2588. (b) Hinnemann, B.; Nørskov, J. K. J. Am. Chem. Soc. 2004, 126, 3920. (c) Lee, H.-I.; Benton, P. M. C.; Laryukhin, M.; Igarashi, R. Y.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem. Soc. 2003, 125, 5604. (d) Schimpl, J.; Petrilli, H. M.; Blochl, P. E. J. Am. Chem. Soc. 2003, 125, 15772. (e) Cao, Z.; Zhou, Z.; Wan, H.; Zhang, Q.; Thiel, W. Inorg. Chem. 2003, 42, 6986. (f) Hinnemann, B.; Nørskov, J. K. J. Am. Chem. Soc. 2003, 125, 1466. (g) Dance, I. Chem. Commun. 2003, 324. 2 Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696. 3 Lee, S. C.; Holm, R. H. Chem. Rev. 2004, 104, 1135. 4 Ertl, G. Chem. Rec. 2001, 1, 33. 5 For examples of structurally characterized Fe-N-Fe linkages, see: (a) Justel, T.; Muller, M.; Weyhermuller, T.; Kressl, C.; Bill, E.; Hildebrandt, P.; Lengen, M.; Grodzicki, M.; Trautwein, A. X.; Nuber, B.; Wieghardt, K. Chem.-Eur. J. 1999, 5, 793. (b) Kienast, A.; Homborg, H. Z. Anorg. Allg. Chem. 1998, 624, 233. (c) Moubaraki, B.; Benlian, D.; Baldy, A.; Pierrot, M. Acta Crystallogr., Sect. C 1989, 45, 393. (d) Scheidt, W. R.; Summerville, D. A.; Cohen, I. A. J. Am. Chem. Soc. 1976, 98, 6623. (e) Summerville, D. A.; Cohen, I. A. J. Am. Chem. Soc. 1976, 98, 1747. 6 Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252. 7 High-spin iron(V) terminal nitrides have been reported on the basis of spectroscopic data at low temperatures. See: (a) Meyer, K. M.; Eckhard, B.; Mienert, B.; 117 Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 4859. (b) Wagner, W.D.; Nakamoto, K. J. Am. Chem. Soc. 1989, 111, 1590. 8 (a) Thomas, J. C.; Peters, J. C. Inorg. Synth. 2004, 34, 8. (b) Barney, A. A.; Heyduk, A. F.; Nocera, D. G. Chem. Commun. 1999, 2379. 9 Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 10782. 10 Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538. 11 Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 322. 12 For other examples of chemistry featuring [PhBPiPr3], see: (a) Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2004, 23, 2488. (b) Betley, T. A.; Peters, J. C. Inorg. Chem. 2003, 42, 5074. (c) Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2003, 22, 4627. 13 For other examples of chemistry featuring [PhBP3], see: (a) Jenkins, D. M.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 11162. (b) Jenkins, D. M.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 11238. (c) Jenkins, D. M.; Di Bilio, A. J.; Allen, M. J.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2002, 124, 15336. (d) Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21, 4065. (e) Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21, 4050. 14 (a) Mindiola, D. J.; Cummins, C. C. Angew. Chem., Int. Ed. 1998, 37, 945. (b) Carpino, L. A.; Padykula, R. E.; Barr, D. E.; Hall, F. H.; Krause, J. G.; Dufresne, R. F.; Thoman, C. J. J. Org. Chem. 1988, 53, 2565. 15 (a) Du Bois, J.; Tomooka, C. S.; Hong, J.; Carreira, E. M. Acc. Chem. Res. 1997, 30, 364. (b) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley & Sons: New York, 1988; p 76. (c) LaMonica, G.; Cenini, S. J. Chem. Soc., Dalton Trans. 1980, 1145. 118 16 17 Duimstra, J.; Peters, J. C. Unpublished Results. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds. Part B: Applications in Coordination, Organometallic, and Bioinorganic Chemistry, 5th ed.; Wiley & Sons: New York, 1997; p 124. 18 (a) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (b) Evans, D. F. J. Chem. Soc. 1959, 2003. 19 Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474. Also see ref 12c for a [PhBPiPr3]FeII-SiR3 example. 20 To the best of our knowledge, examples of (µ-1,3) bridging azide systems for iron have yet to be reported. For examples of (µ-1,1) systems, see: (a) Clemente-Juan, J. M.; Mackiewicz, C.; Verelst, M.; Dahan, F.; Bousseksou, A.; Sanakis, Y.; Tuchagues, J.-P. Inorg. Chem. 2002, 41, 1478. (b) Reddy, K. R.; Rajasekharan, M. V.; Tuchagues, J.-P. Inorg. Chem. 1998, 37, 5978. (c) De Munno, G.; Poerio, T.; Viau, G.; Julve, M.; Lloret, F. Angew. Chem., Int. Ed. Engl. 1997, 36, 1459. (d) De Munno, G.; Poerio, T.; Viau, G.; Julve, M.; Lloret, F. Chem. Commun. 1996, 2587. 21 A τ value of 1 corresponds to an ideal trigonal bipyramidal geometry, while a τ value of 0 corresponds to an ideal square pyramidal geometry. See: Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rin, J.; Verschoor, G. C. J. Chem. Soc., Dalton Trans. 1984, 1349. 22 The decrease in χM*T below 16 K for [α-(Me3tacn)2(cyclam)NiMo2(CN)6]I2, which features a ferromagnetically coupled S = 4 electronic configuration, was also attributed to zero-field splitting. See: Shores, M. P.; Sokol, J. J.; Long, J. R. J. Am. Chem. Soc. 2002, 124, 2279. For a more detailed discussion concerning the magnetism of differous bridged 119 systems, see: Hendrich, M. P.; Day, E. P.; Wang, C.-P.; Synder, B. S.; Holm, R. H.; Munck, E. Inorg. Chem. 1994, 33, 2848 and references therein. 23 Verma, A. K.; Nazif, T. N.; Achim, C.; Lee, S. C. J. Am. Chem. Soc. 2000, 122, 11013. 24 We have considered the presence of a (µ-NH) functionality instead of a (µ-N) ligand. However, IR studies of 4.2 as both a solution (THF) and a solid (Nujol, KBr pellet) provide no evidence for an N-H vibration. Additionally, it is not possible to assign formal iron oxidation states with a (µ-NH) ligand and still maintain a diamagnetic manifold while accounting for all charges. 25 Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C.; Re, N.; Rizzoli, C.; Chiesi-Villa, A. J. Am. Chem. Soc. 2000, 122, 3652. 26 (a) Glidewell, C.; Holden, H. D. J. Organomet. Chem. 1982, 226, 171. (b) Canziani, F.; Garlaschelli, L.; Malatesta, M. C.; Albinati, A. J. Chem. Soc., Dalton Trans. 1981, 2395. 27 (a) Shannon, R. D. Acta. Crystallogr. 1976, A32, 751. (b) Shannon, R. D.; Prewitt, C. T. Acta. Crystallogr. 1969, B25, 925. 28 See, for example: (a) Cherry, J.-P. F.; Stephens, F. H.; Johnson, M. J. A.; Diaconescu, P. L.; Cummins, C. C. Inorg. Chem. 2001, 40, 6860. (b) Tsai, Y.-C.; Johnson, M. J. A.; Mindiola, D. J.; Cummins, C. C. J. Am. Chem. Soc. 1999, 121, 10426. (c) Banaszak Holl, M. M.; Kersting, M.; Pendley, B. D.; Wolczanski, P. T. Inorg. Chem. 1990, 29, 1518. 29 Vela, J.; Stoian, S.; Flaschenriem, C. J.; Munck, E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522. 30 For an example of a d6 iridium imide, see: Glueck, D. S.; Wu, J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc. 1991, 113, 2041. 31 Orpen, A. G.; Connelly, N. G. Organometallics 1990, 9, 1206. 120 32 Bianchini, C.; Laschi, F.; Masi, D.; Ottaviani, F. M.; Pastor, A.; Peruzzini, M.; Zanello, P.; Zanobini, F. J. Am. Chem. Soc. 1993, 115, 2723. 33 For example, the 5 E → 5 T2 transition for Fe(HMPA)42+ (HMPA = hexamethylphosphoramide) occurs at 6950 cm-1. See: Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984; p 461. 34 For an example of a bridged N4 ligand, see: Mass, V. W.; Kujanek, R.; Baum, G.; Dehnicke, K. Angew. Chem., Int. Ed. Engl. 1984, 23, 149. For an example of a terminal N4 ligand, see: Huynh, M. H. V.; Baker, R. T.; Jameson, D. L.; Labouriau, A.; Meyer, T. J. J. Am. Chem. Soc. 2002, 124, 4580. 35 Consistent with this suggestion is that the well-defined Fe(IV) nitride species [PhBPiPr3]Fe≡N (ref 6) is reduced by CO in benzene to produce a mixture of two diamagnetic iron(II) isocyanates: [PhBPiPr3]Fe(NCO)(CO) and [PhBPiPr3]Fe(NCO)(CO)2. This transformation and further reactivity studies pertaining to [PhBPiPr3]Fe≡N will be elaborated in due course. 36 Yandulov, D. V.; Schrock, R. R. J. Am. Chem. Soc. 2002, 124, 6252. 37 Rohde, J. U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Munck, E.; Nam, W.; Que, L., Jr. Science 2003, 299, 1037. See also ref 11. 38 Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. Rev. 2004, 104, 939. 39 Jaguar 5.0, Schrodinger, LLC, Portland, Oregon, 2002. 40 Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785-789.  121 Chapter 5: Heterolytic Activation of H2 with an L3Fe-(µ-N)-FeL3 Linkage: Characterization of a Diiron System Featuring a (µ-NH), (µ-H) Manifold 122 5.1 Introduction The interaction of dinitrogen-derived, surface-bound nitrides with H2 is likely responsible for ammonia production during turnover in Haber-Bosch catalysis.1 Despite this mechanistic postulate, the chemistry of iron compounds featuring either terminal2 or bridging3 nitride ligands has been slow to develop, and no system has been reported to react with H2.4 Previous work in our laboratory established that the terminal iron(III) imide, [PhBP3]Fe≡N-p-tolyl (2.5), is capable of complete H2-induced Fe-N bond scission at room temperature to yield free p-toluidine.5 Encouraged by this reactivity, we prepared a bridging nitride species, [([PhBP3]Fe)2N][Na(THF)5] (4.2), whose synthesis and ammonia production in the presence of excess protons has been established.6 Herein we wish to report on the facile heterolysis of H2 with 4.2 under mild conditions (room temperature, 1 atm H2). The resulting imide-hydride product, [([PhBP3]Fe)2(µ-NH)(µH)][Na(THF)5] (5.1), is itself unique in that it represents the first example of a bimetallic system bridged exclusively by these two ligands.7,8 5.2 Results and Discussion Incubation of a brown THF solution of 4.2 with an atmosphere of H2 at room temperature results in a color change to green within minutes. Isolation and characterization of the resulting product demonstrated that a formal 1,2 addition process of H2 had occurred to yield the bridging imide-hydride species, [([PhBP3]Fe)2(µ-NH)(µH)][Na(THF)5] (5.1) (Eq 5.1). Akin to nitride precursor 4.2, compound 5.1 features a diamagnetic manifold and was conveniently characterized by 1H, 31P, and 15N NMR 123 Na(THF)5 Ph B Ph2 P Ph2P Fe PPh2 N 4.2 Ph2 P Fe PPh 2 B Ph2P Na(THF)5 H2 Ph Ph B Ph2 Ph2 H Ph P N P Ph Fe Fe P P Ph H Ph B Ph2P PPh2 PCl3 Ph B Ph2 P Ph2P Fe PPh2 N 5.2 Ph2 P Fe PPh 2 B Ph2P [NO][PF6] H2 Ph Ph Ph 5.1 B Ph2 Ph2 H Ph P N P Ph Fe Fe P P Ph H Ph B Ph2P PPh2 (5.1) Ph 5.3 υ(NH) = 3319 cm-1 spectroscopy. The 31P NMR spectrum exhibits a singlet at δ 66 ppm, which is 8 ppm downfield from the resonance observed for 4.2. The 1H NMR spectrum features singlet resonances at δ +18.5 and -22.4 ppm for the (µ-NH) and (µ-H) ligands, respectively, which are absent when 4.2 is hydrogenated with D2. For the 15N NMR experiment, 5.1(50% 15N) was prepared from the hydrogenation of 4.2-(50% 15N) (δ 801 ppm (s)) and features a doublet at δ 406 ppm (1JN-H = 63 Hz) (Figure 5.1A). This chemical shift and 1 JN-H coupling constant are similar to those observed for the imide ligands of (Cp*Ti)3(µ- NH)3(µ-N) (δ 470 ppm, 1JN-H = 63 Hz).9 The 1H NMR spectrum of 5.1-(50% 15N) also exhibits a virtual triplet for the (µ-NH) resonance (Figure 5.1C), which is composed of a singlet from naturally abundant 14N (I = 1) and a doublet from the 15N label (I = 1/2). The (µ-H) resonance is unaffected by nitrogen labeling (Figure 5.1B). When an isolated sample of 4.2 is exposed to an atmosphere of D2, the hydride ligand undergoes deuterium exchange much more rapidly than the imide ligand. Thus on an NMR scale, the protio-hydride resonance is observed to fully decay within a few hours 124 of exposure to D2 . The protio-imide resonance also decays at a noticeable rate, though it is still detectable after 3 days. One plausible mechanistic scenario for isotope exchange involves the oxidative addition of D2 to a single iron center to form an iron(IV) bisdeuteride, mono-hydride intermediate. Hydride exchange would then result from reductive elimination of HD. Isotopic exchange at the imide ligand may be envisioned through an additional protonation/deprotonation event that precedes the reductive elimination step. It is worth noting that iron(IV) tri-hydride species supported by the second generation [PhBiPr3] ligand have been reported.10 Infrared spectroscopy of 5.1 failed to reveal any vibrations associated with either the imide or hydride ligands, regardless of how the sample was prepared for analysis (i.e., THF solution, KBr pellet, or Nujol mull). (A) (B) (C) * * Figure 5.1. (A) Proton coupled 15N NMR spectrum of 5.1-(50% 15N). (B) µ-H resonance observed in the 1H NMR spectrum of 5.1-(50% 15N). (C) µ-NH resonance observed in the 1H NMR spectrum of 5.1-(50% 15N). Peaks marked with an asterisk are due to coupling with the 15N nuclei. 125 Crystals of 5.1 suitable for an X-ray diffraction experiment were grown via a THF/hexanes diffusion chamber, and the resulting solid state structure is shown in Figure 5.2. Although the imide-hydride bridge between Fe1 and Fe2 is disordered over two positions as a result of the two ligands flipping positions randomly throughout the crystal, electron density attributable to both the imide hydrogen and hydride ligands was located in the difference map and successfully refined when the Fe-H distances were constrained to be equal. Despite this disorder, the structure of 5.1 was of sufficient quality to establish the connectivity of the diiron core and provide reliable structural data concerning the more populated position of the Fe-(µ-NH)-Fe linkage (~ 70% occupancy). In comparison with nitride 4.2, the Fe-N bond lengths for 5.1 have increased on average 0.118 Å (Fe-N = 1.675(5) Å, 1.705(5) Å for 4.2; 1.790(5) Å, 1.826(5) Å for 5.1), which is consistent with the loss of a bond order between the Fe-N linkages. The Fe-P bond distances of ~ 2.2 Å are similar to those of 4.2 while the Fe-Nx-Fe bond angle contracts from 135.9(3)° to 94.7(3)° upon addition of H2. An iron-iron bonding interaction is required for the observation of a diamagnetic manifold and is evidenced by the relatively short Fe-Fe bond distance of 2.6588(9) Å. This value is approximately 0.45 Å shorter than that for 4.2 and is similar to the iron-iron distances observed in hydrogenase model compounds.11 126 P4 Na1 P6 H200 Fe2 P5 P2 P3 N1A Fe1 H1C P1 A B P3 C H1D P5 N1B P2 P2 H200 P4 P4 H201 H201 Fe1 Fe2 P3 Fe1 N1A P1 H200 N1A P6 P1 H1C N1B H1C P6 Fe2 P5 Figure 5.2. (A) 50% thermal ellipsoid representation of [([PhBP3]Fe)2(µ-NH)(µH)][Na(THF)5] 5.1 showing only the more populated position of the disordered (µ-NH)(µ-H) linkage. Selected bond distances (Å) and angles (°): Fe1-N1A: 1.826(5); Fe2-N1A: 1.790(5); Fe1-H200: 1.508(14); Fe2-H200: 1.508(14); Fe1Fe2: 2.6595(9); Fe1-P1: 2.2121(13); Fe1-P2: 2.2024(13); Fe1-P3: 2.2274(13); Fe2-P4: 2.2247(13); Fe2-P5: 2.2306(13); Fe2-P6: 2.2411(13); Fe1-N1A-Fe2: 94.7(3); P1-Fe1-P2: 88.98(5): P1-Fe1-P3: 90.62(5); P2-Fe1-P3: 88.91(5); P2-Fe1N1A: 125.5(2); P1-Fe1-N1A: 85.77(18); P3-Fe1-N1A: 145.2(2). (B) Top view of the L3Fe-(µ-NH)(µ-H)-FeL3 core of 5.1 showing both positions of the disordered (µ-NH)(µ-H) linkage. (C) Side view of the L3Fe-(µ-NH)(µ-H)-FeL3 core of 5.1 showing both positions of the (µ-NH)(µ-H) linkage. 127 The comparative cyclic voltammetry of 5.1 and 4.2 is shown in Figure 5.3. The electrochemical response for 5.1 features an irreversible oxidation at -0.15 V and two fully reversible redox events at -1.25 V and -2.58 V (vs. Fc/Fc+). The event at -1.25 V is assigned to an Fe(II)Fe(II)-/Fe(II)Fe(III) redox process, which is similar to what has been proposed for 4.2 (-1.3 V vs. Fc/Fc+).6 That both 4.2 and 5.1 may be chemically oxidized by one electron (Eq 5.1) to yield ([PhBP3]Fe)2(µ-N) (5.2) and ([PhBP3]Fe)2(µ-NH)(µ-H) (5.3), respectively, is consistent with this assignment. The irreversible oxidations at ~ 150 (5.1) and -350 mV (4.2) may correspond to an Fe(III)/Fe(IV) redox event at a single iron center, to an Fe(III)Fe(II)/Fe(III)Fe(III)+ redox process, or to oxidative degradation of a borate ligand.12 The reduction waves at low potential (-2.5 V for 4.2, -2.6 for 5.1) are interesting and are not observed in the iron imide systems. These events are suggestive of an Fe(II)Fe(II)-/Fe(II)Fe(I)2- redox process that is fully reversible for 5.1; for 4.2 the anodic wave decreases in intensity and eventually vanishes when this redox event is isolated and cycled multiple times. The chemical reduction of both 4.2 and 5.1 with an excess of sodium mirror yields a number of diamagnetic products as evidenced by 31P NMR spectroscopy. 128 4.2 5.1 -100 -600 -1100 -1600 -2100 -2600 -3100 + Potential (mV vs. Fc/Fc ) Figure 5.3. Cyclic voltammetry of [{[PhBP3]Fe}2(µ-N)][Na(THF)5] (4.2), top, [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] and (5.1), bottom. Experimental parameters: 2.0 mM analyte, 0.30 M [nBu4N][PF6] electrolyte, scan rate = 50 mV/s. Crystals of 5.2 suitable for an X-ray diffraction experiment were grown via vapor diffusion of petroleum ether into a THF solution, and the resulting solid-state structure is shown in Figure 5.4. In contrast to parent nitride 4.2, no disorder was observed for the nitride ligand as it was satisfactorily refined in one position. Structurally, the Fe-N bond lengths of 1.668(2) Å and 1.683(2) Å are virtually identical to each other and are similar to those observed for anionic 4.2 (see Table 5.1). Also apparent is the increase in linearity from 135.9(3)° to 142.4(1)° for the Fe-N-Fe manifold of the oxidized species. Consistent with the observation of iron-phosphorus bond expansion in going from the closed shell 129 P1 P6 Fe1 P3 N P4 Fe2 P5 P2 A P1 P3 B Fe1 N C P3 P6 P4 Fe1 Fe2 N P4 Fe2 P5 P2 P5 P2 P1 P6 Figure 5.4. (A) 50% thermal ellipsoid representation of ([PhBP3]Fe)2(µ-N) 5.2. Two molecules of THF have been removed for clarity. Selected bond distances (Å) and angles (°): Fe1-N: 1.683(2); Fe2-N: 1.668(2); Fe1-Fe2: ~ 3.18; Fe1-P1: 2.2491(8); Fe1-P2: 2.3398(8); Fe1-P3: 2.3122(8); Fe2-P4: 2.2016(8); Fe2-P5: 2.3479(8); Fe2-P6: 2.2275(8); Fe1-N-Fe2: 142.41(12); P1-Fe1-P2: 88.37(3): P1Fe1-P3: 88.45(3); P2-Fe1-P3: 94.03(3); P2-Fe1-N: 134.60(7); P1-Fe1-N: 111.76(7); P3-Fe1-N: 125.35(7). (B) Side view of the L3Fe-(µ-N)-FeL3 core of 5.2. (C) Top view of the L3Fe-(µ-N)-FeL3 core of 5.2. For both B and C, elongated Fe-P bonds are represented in red. 130 (d ) to the open shell (d ) iron imides, 5.2 features two elongated Fe-P bonds on Fe1 6 5 (2.3122(8) Å and 2.3398(8) Å vs. 2.2491(8) Å) and one elongated Fe-P bond on Fe2 (2.3479(8) Å vs. 2.2016(8) Å and 2.2275(8) Å). Whether this is an inherent property of the metalloradical systems, or the result of decreased back-bonding into σ* orbitals of the [PhBP3] ligand,13 is still a matter of debate.14 That both metal centers in 5.2 feature elongated Fe-P bonds suggests that the SOMO is delocalized over the entire L3Fe-(µ-N)FeL3 linkage. Structural parameters for 4.2, 5.1, and 5.2 are summarized in Table 5.1. Table 5.1. Structural data for compounds [([PhBP3]Fe)2(µ-N)][Na(THF)5] (4.2), [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] (5.1), and ([PhBP3]Fe)2(µ-N) (5.2). Compound d Fe-Nx (Å) d Fe-P (Å) d Fe-Fe (Å) Angle Fe-Nx-Fe [([PhBP3]Fe)2N][Na(THF)5] (4.2)a 1.675(5) 2.2295(10) 2.2338(10) 2.2127(10) ~ 3.13 135.9(3) 1.705(5) 2.2299(10) 2.2106(10) 2.2534(10) 2.2121(13) 2.2024(13) 2.2274(13) 2.6595(9) 94.7(3) ~ 3.18 142.41(12) Formal oxidation states: Fe(II)Fe(II) [([PhBP3]Fe)2(µ-NH)(µH)][Na(THF)5] (5.1)a 1.826(5) Formal oxidation states: Fe(II)Fe(II) 1.790(5) ([PhBP3]Fe)2N (5.2) 1.683(2) Formal oxidation states: Fe(II)Fe(III) 1.668(2) 2.2247(13) 2.2306(13) 2.2411(13) 2.2491(8) 2.3398(8) 2.3122(8) 2.2016(8) 2.3479(8) 2.2275(8) a Structural data are tabulated for the more populated position of the bridging linkage. 131 X-band EPR analysis of 5.2 at 4 K (Figure 5.5) reveals an axial signal that is consistent with the presence of one unpaired electron.15 Also present is a weak, low-field signal (g ~ 10.1) that suggests 5.2 is best described electronically as a ground-state doublet arising from antiferromagnetic exchange between high-spin Fe(II) and Fe(III) nuclei. The extensive coupling observed for the axial signal likely arises from superhyperfine coupling of the unpaired electron with multiple phosphorus nuclei (31P: 100% abundance, I = ½), and possibly with the nitride ligand (15N: 0.365% abundance, I = ½; 14 N: 99.635% abundance, I = 1). 40 0 60 80 100 100 200 300 400 500 600 Field (mT) Figure 5.5. X-band EPR spectrum of ([PhBP3]Fe)2(µ-N) (5.2) (glassy toluene, 4 K, 9.378 GHz). (inset) Region from 40–100 mT highlighting the low-field signal. 132 Solid-state magnetic susceptibility data for 5.2 were obtained from 4 to 300 K by SQUID magnetometry and are plotted, per dimeric unit, in Figure 5.6. The plot of µeff versus temperature (Figure 5.6 top) is consistent with antiferromagnetic coupling as µeff decreases upon cooling of the sample. A maximum value of 3.40 µB is observed at 300 K, which suggests that a fraction of the molecules are populating higher spin-states16 as this value is considerably larger than the spin-only value for one unpaired electron (1.79 µB). At approximately 165 K, the µeff value of 2.52 µB begins to reflect that expected for one unpaired electron. A final value of 1.82 µB is obtained at 4 K, which is consistent with the ground-state doublet observed by EPR spectroscopy. A plot of χM versus T (Figure 5.6 bottom) demonstrates that the antiferromagnetic exchange in 5.2 is weak. As shown in the inset, a slight increase and then a decrease in χM is observed as the sample is cooled from 300 to 185 K.17 Although attempts to extract a J value for 5.2 from this data have thus far been unsuccessful, the relative magnitudes observed for 5.2 and 4.2 (| J | ≥ 250 cm-1)6 are intriguing considering the minimal structural differences observed between the Fe-N-Fe linkages. Geometrical considerations suggest that a larger coupling constant should be observed for the more linear Fe-N-Fe linkage of 5.2 as a result of better π-orbital overlap.18 Such a trend is typically observed for bridging iron oxos,19 although exceptions are known.20 One plausible explanation for the present system is that the reduced symmetry of 5.2 (C1) versus 4.2 (C2v) alters the σ and π-bonding character of the Fe-N-Fe linkage in such a manner as to yield similar structural parameters but weaker π-orbital overlap.21 133 3.5 µ eff (µ B) 3 2.5 2 1.5 0 50 100 150 200 250 300 200 250 300 Temp (K) χ M (cm 3*K/mol) 0.008 0.08 0.06 0.007 0.006 0.005 0.004 100 3 χ M (cm *K/mol) 0.1 150 Tem p (K) 0.04 0.02 0 0 50 100 150 200 250 300 Temp (K) Figure 5.6. SQUID magnetometry data for ([PhBP3]Fe)2(µ-N) (5.2) plotted per dimeric unit as (top) µeff versus temperature and (bottom) χMT versus temperature. (inset) Plot of χMT versus temperature highlighting the region from 100–300 K. 134 The low-energy d-d electronic transitions centered at 1266 nm in the NIR spectrum of diamagnetic 4.2 cemented its assignment as an antiferromagnetically coupled, high-spin system.6 For 5.2 these transitions are anticipated to blue-shift as a result of the increased oxidation state of the molecule.22 Comparative optical data are therefore of interest and are shown in Figure 5.7. Upon initial inspection it becomes apparent that the low-energy features observed for 4.2 are absent in 5.2. Present in the spectrum for 5.2 is a shoulder at ~ 650 nm and a broad feature that begins at ~ 1100 nm, which may correspond to the low-energy charge-transfer and d-d transitions, respectively, of 4.2. Coincidently, the feature at 650 nm is of the same energy that is observed for the lower-energy charge-transfer bands of the iron(III) imides (See Chapter 2). The features at ~ 1370 nm in both spectra are instrumental artifacts and have been observed in a number of NIR spectrum acquired under similar conditions. 25000 ε (M -1 cm -1) 15000 4000 3000 2000 1000 0 500 -1 -1 ε (M cm ) 20000 5000 4.2 1000 2000 Wavelength (nm ) 10000 5000 B 5.2 0 350 1500 B A 550 750 A 950 1150 1350 1550 1750 1950 Wavelength (nm) Figure 5.7. NIR data for [([PhBP3]Fe)2(µ-N)][Na(THF)5] (4.2) and ([PhBP3]Fe)2(µ-N) (5.2). Potential corresponding transitions are labeled with the letters “A” and “B.” 135 The electronic transitions of imide-hydride 5.1 were also examined with NIR spectroscopy (Figure 5.8). Although the feature at 850 nm tails off into the lower-energy regime where d-d transitions are expected, distinct ligand-field features similar to those observed for 4.2 (Figure 5.7) are not present. Consequently each metal center of 5.1 is assigned as low-spin. This is consistent with five- versus four-coordinate iron in which the presence of an additional hydride ligand contributes to a stronger ligand-field splitting in which spin pairing (versus populating strongly antibonding orbitals) is energetically favorable. This data, however, does not exclude the possibility of two exchange-coupled metal centers as a stronger ligand field may shift any observable d-d transitions to higher energy. 16000 14000 -1 5.1 -1 8000 ε (M cm ) -1 10000 -1 ε (M cm ) 12000 6000 4000 2000 1800 1600 1400 1200 1000 800 600 400 200 0 950 1150 1350 1550 1750 1950 Wavelength (nm) 2000 0 350 5.3 550 750 950 1150 1350 1550 1750 1950 Wavelength (nm) Figure 5.8. NIR data for [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] (5.1), and [([PhBP3]Fe)2(µ-NH)(µ-H)] (5.3). 136 The reactivity patterns exhibited by neutral 5.2 resemble those of anionic 4.2.6 For example, exposure to an atmosphere of CO produces equimolar amounts of [PhBP3]Fe(CO)2 (2.13) and [PhBP3]Fe(NCO)(CO)2 (4.4) while hydrogenolysis with an atmosphere of H2 at room temperature (Eq 5.1) results in the independent preparation of 5.3. In contrast to parent 5.1, IR spectroscopy (Nujol) of 5.3 reveals a readily discernable N-H vibration at 3319 cm-1. Solid-state structural analysis has thus far been impeded by the inability to obtain crystalline samples of suitable quality for X-ray diffraction experiments. Near-infrared analysis of 5.3 (Figure 5.8) reveals the presence of a broad transition centered at 1150 nm (ε = 540 M-1 cm-1) that is lower in energy than any transition observed for parent 5.1. As the ligand-field transitions of 4.2 are observed to increase in energy upon oxidation to form neutral 5.2 (Figure 5.7), this transition is assigned as an intervalence charge transfer. For comparison, it is noted that Wieghardt reported an intervalence charge-transfer transition at 866 nm (ε = 300 M-1 cm-1) for an octahedral Fe(III)/Fe(IV) µ-nitride.3a The 4 K X-band EPR spectrum of 5.3 (Figure 5.9, blue, solid line) features an axial signal attributable to an S = ½ electronic configuration. In contrast to 5.2, a lowfield signal is not observed, suggesting that at this temperature both iron centers are low spin. Delocalization of the unpaired electron over both metal centers is evidenced from weak phosphorus super-hyperfine coupling in the g⊥ region, which was simulated as arising from 6 phosphorus nuclei (Figure 5.9, red, broken line, A6P = 12 G). 137 Experim ental Sim ulated 320 325 330 335 340 Field (mT) 250 260 270 280 290 300 310 320 330 340 Field (m T) Figure 5.9. Experimental (blue, solid line) and simulated (red, broken line) Xband EPR spectrum of ([PhBP3]Fe)2(µ-NH)(µ-H) (5.3) (glassy toluene, 20 K, 9.374 GHz). (inset) Region from 320–340 mT highlighting the g⊥ region. Simulation parameters: g1 = 2.53, g2 = 2.052, g3 = 2.033; A6P = 12 G. Solid-state magnetic susceptibility data for 5.3 were obtained from 4 to 300 K by SQUID magnetometry and are plotted, per dimeric unit, in Figure 5.10. The plot of µeff versus temperature demonstrates that similar to 5.2, the magnetic moment is temperature dependent and is consistent with a doublet ground state at 4 K (µeff = 2.12 µB). This value increases in near linear fashion with increasing temperature and reaches a maximum of 3.30 µB at 300 K. As there is no evidence for antiferromagnetic coupling in the plot of χM versus T (not shown), this behavior likely reflects angular-momentum contributions23 and/or the population of high-spin electronic configurations. As a final note concerning the magnetic properties of 5.3, solution magnetic measurements (rt, Evans method) yield similar µeff values as those obtained by SQUID. 138 µ eff (µ B) 3.5 2.5 1.5 0 50 100 150 200 250 300 Temp (K) Figure 5.10. SQUID magnetometry data for ([PhBP3]Fe)2(µ-NH)(µ-H) (5.3) plotted per dimeric unit as µeff versus temperature. 5.3 Molecular Orbital Considerations The iron(III) imides are characterized by one σ- and two energetically equivalent π-bonds within the Fe-N linkage, resulting in a nearly linear Fe-N-C bond angle. The molecular orbital (MO) diagram for 4.2 is of interest as the descent from C3 to C2v symmetry has significant ramifications concerning the bonding within the Fe-N-Fe manifold. To describe the bonding in 4.2, the atomic orbitals for the nitride ligand were combined with the linear combinations of two C3v-symmetric L3Fe fragments24 in C2v symmetry (Figure 5.11). The resulting molecular orbitals reveal that the bonding within the Fe-N-Fe linkage of 4.2 is best described as 2σ + 1π, which is consistent with its bent structure. Deriving a bonding scheme for neutral 5.2 is complicated by the continued decent in symmetry as evidenced by the staggered nature of the two [PhBP3] ligands. 139 Na(THF)5 B Ph Ph2 P Ph2P Fe PPh2 N Ph2 P Fe PPh 2 B Ph2P 4.2 σ∗ σ∗ π∗ Ph 5a1 4b2 C2v C3v e 3b2 2a2 2b1 3a1 a 2b2 2a1 3b2 1b2 1a2 1b1 1a1 2b2 1a2 e 2a2 2b1 4a1 2a1 1b1 1b2 1a1 5.11. 1b2 1a1 C2v 3a1 2 [PhBP3]Fe Figure 1b1 3b1 Qualitative b1 b2 px py a1 s + pz N z π σ y σ d-orbital x splitting diagram for [([PhBP3]Fe)2(µ-N)][Na(THF)5] (4.2) in idealized C2v symmetry. Exact orbital energies are not known. For imide-hydride 5.1, one of the N pz + s orbitals is involved in bonding with the H atom of the bridging-imide ligand. As shown in Figure 5.12, the imide ligand itself is linked to each iron center through 2 σ-type molecular orbitals (2a1 and 1b2) and 1 πbonding molecular orbital (1b1). In comparison to nitride 4.2, however, the diminished Fe-N-Fe angle in 5.3 likely minimizes orbital overlap in the 1b2 molecular orbital to ultimately result in the observed solid-state elongation of the Fe-N linkages. Molecular orbital 1a1 is also consistent with solid-state data in that it forces the imide ligand to eclipse one phenyl arm on each of the two [PhBP3] ligands. For 4.2, the iron atomic 140 orbitals of this parentage are bonding with respect to the nitride ligand, resulting in a staggered configuration with the two [PhBP3] ligands. The addition of 20 electrons to Figure 5.12 in a low-spin configuration demonstrates that no strongly antibonding orbitals are populated. As a final comment concerning the nature of the postulated ironiron bond, molecular orbital 2a1 suggests that it is in fact a 3-centered interaction. B Ph Ph2 H N P Ph Fe P Ph H PPh2 Ph Fe P Ph Ph2P 5a1 σ∗ Na(THF)5 4a1 σ∗ Ph2 P 4b2 σ∗ B ∗ π Ph 1b1 3b1 5.1 C3v 3b2 2a2 2b1 3a1 e C2v 1b2 3b2 2a2 2a1 a 2b2 2a1 e 1b2 1a2 1b1 1a1 2b2 1a2 2b1 3a1 2 [PhBP3]Fe 1b1 1b2 2a1 1a1 1a1 C2v b1 b2 px py a1 s + pz a1 s N π σ σ H z y σ x Figure 5.12. Qualitative d-orbital splitting diagram for [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] (5.1) in idealized C2v symmetry. Exact orbital energies are not known. 141 5.4 Conclusions In summary, this work demonstrates that low-valent and low-coordinate bridging iron nitrides supported by [PhBP3] are capable of facile H2 activation under mild conditions (rt, 1 atm H2). In contrast to earlier work concerning the hydrogenation of lowspin Fe(III) imides, complete scission of the Fe-N linkage in the present systems is not observed. This is presumably due to the presence of an additional metal center that traps a putative imide-hydride intermediate to form a bimetallic species. Spectroscopic data for both anionic 5.1 and neutral 5.3 are consistent with the presence of two low-spin d6 iron centers. Whether related systems featuring (µ-NH) and/or (µ-H) linkages are relevant to dinitrogen reduction schemes is worth considering. Neutral 5.3, for example, may provide spectroscopic signatures for the identification of similar species, if formed, in biologically relevant systems such as nitrogenase.25 5.5 Experimental Section 5.5.1 General Considerations General considerations are outlined in Section 4.4.1. 5.5.2 Magnetic Measurements SQUID data was acquired from 4–300 K as outlined in Section 2.5.2. 5.5.3 EPR Measurements EPR data was acquired as outlined in Section 2.5.3. 5.5.4 Electrochemical Measurements Electrochemical data was acquired as outlined in Section 2.5.4. 142 5.5.5 Starting Materials and Reagents All reagents were purchased from commercial vendors and used without further purification unless otherwise noted. Compounds 4.2 and 4.2-(50% 15N) were prepared according to literature procedures. 5.5.6 Synthesis of Compounds Synthesis of [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5], 5.1: [([PhBP3]Fe)2(µN)][Na(THF)5] (4.2, 0.203 g, 0.108 mmol) was dissolved in THF (20 mL) and transferred to a 100 mL sealed flask equipped with a stirbar. The resulting brown solution was frozen in liquid nitrogen, and the N2 atmosphere was evacuated and replaced with an atmosphere of H2. The reaction was then allowed to warm to room temperature with stirring and within ten minutes had changed color from brown to green. After stirring for four hours at room temperature, volatiles were removed under reduced pressure, and the crude solids were washed with benzene (3 x 30 mL), petroleum ether (2 x 20 mL), and dried to yield 5.1 as a green powder (0.175 g, 86%). X-ray quality crystals were grown via a THF/hexanes vapor diffusion chamber. 1H NMR (THF-d8, 300 MHz): δ 18.5 (s, 1H); 7.56 (d, J = 3.0 Hz, 4H); 7.03 (t, J = 7.2 Hz, 4H); 6.90 (m, 26H); 6.80 (t, J = 7.8 Hz, 12H); 6.56 (t, J = 7.8 Hz, 24H); 3.60 (m, 20H); 1.78 (m, 20H); 1.31 (br s, 12H). 31P{1H} NMR (THF-d8, 121.4 MHz): δ 66.0 (s). Anal. Calcd. for C110H124B2Fe2NNa05P6: C, 70.19; H, 6.64; N, 0.74 Found: C, 69.82; H, 6.50; N, 0.83. For the 15N NMR data, a sample of 4.2-(50% 15N) was hydrogenated in a J. Young tube. 15N NMR (THF, 50.751 MHz): δ 406 (d, 1JN-H = 63 Hz). 1H NMR (THF-d8, 300 MHz): δ 18.5 (virtual triplet due to the 50% 14NH singlet and the 50% 15NH doublet (1JN-H = 63 Hz), 1H). 143 Synthesis of ([PhBP3]Fe)2(µ-N), 5.2: [([PhBP3]Fe)2(µ-N)][Na(THF)5] (4.2, 0.300 g, 0.160 mmol) was dissolved in THF (20 mL) with stirring. A THF solution (5 mL) of PCl3 (14 µL, 0.160 mmol) was added dropwise, which resulted in a rapid darkening of the reaction solution. After 2 hours volatiles were removed under reduced pressure, and the crude solids were extracted with benzene (50 mL), filtered over Celite, and lyophilized. The resulting solids were washed with Et2O (2 x 20 mL) and dried to obtain 5.2 as a dark solid (0.157 g, 66%). X-ray quality crystals were grown via vapor diffusion of petroleum ether into a THF solution. 1H NMR (C6D6, 300 MHz): δ 52.5 (s); 19.7 (s); 12.6 (s); 11.5 (t, J = 6.0 Hz); 6.14 (d, J = 6.0 Hz); -1.69 (s); -6.44 (s). µeff = 3.40 µB at 300 K (SQUID). Anal. Calcd. for C90H82B2Fe2NP6: C, 72.22; H, 5.52; N, 0.94 Found: C, 71.92; H, 5.56; N, 0.74. Synthesis of ([PhBP3]Fe)2(µ-NH)(µ-H), 5.3: [([PhBP3]Fe)2(µ-NH)(µ- H)][Na(THF)5] (5.1, 0.400 g, 0.213 mmol) was dissolved in THF (40 mL) with stirring. Solid [NO][BF4] (0.0248 g, 0.213 mmol) was added to this solution in one portion. The green color of 5.1 gradually fades and after two hours volatiles were removed under reduced pressure and the crude solids were extracted with benzene (~ 80 mL), filtered over Celite, and lyophilized. The resulting solids were washed with Et2O (2 x 20 mL) and dried to obtain 5.3 as a dark solid (0.248 g, 78%). 1 H NMR (THF-d8, 300 MHz): δ 21.4 (v br, s); 11.9 (s); 9.00 (s); 8.51 (t, J = 6.3 Hz); 7.11 (s); 3.53 (s, overlaps with residual solvent resonance); 1.03 (br, s). µeff = 3.13 µB at 300 K (SQUID). IR (Nujol): νNH = 3319 cm-1. Anal. Calcd. for C90H84B2Fe2NP6: C, 72.12; H, 5.65; N, 0.93 Found: C, 71.73; H, 5.76; N, 0.80. 144 5.5.7 X-ray Experimental Data Crystallographic procedures are outlined in Section 2.5.8. Crystallographic data are summarized in Table 5.2. Table 5.2. Crystallographic data for [([PhBP3]Fe)2(µ-NH)(µ-H)][Na(THF)5] (5.1) and ([PhBP3]Fe)2(µ-N) (5.2). 5.1 5.2·2 THF C98H98B2Fe2NO2P6 fw C110H124B2Fe2NNa O5P6 1882.23 T (°C) -177 -177 λ (Å) 0.71073 0.71073 a (Å) 13.712(2) 23.519(5) b (Å) 23.001(4) 13.524(3) c (Å) 30.221(5) 25.937(6) α (°) 90 90 β (°) 91.006(6) 95.178(4) γ (°) 90 90 V (Å3) 9530(3) 9965.8(8) space group P2(1)/c P2(1)/n Z 4 4 Dcalc (g/cm3) 1.312 1.327 µ(cm-1) 4.66 5.22 chemical formula R1, wR2a (I > 2σ(I)) 0.0666, 0.0941 a 1640.91 0.0501, 0.0726 R1 = Σ||Fo| - |Fc||/Σ|Fo|, wR2 = {Σ[w(Fo2 - Fc2)2]/Σ[w(Fo2)2]}1/2 145 References Cited 1 Ertl, G. Chem. Rec. 2001, 1, 33. 2 (a) Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252. (b) Meyer, K. M.; Eckhard, B.; Mienert, B.; Weyhermuller, T.; Wieghardt, K. J. Am. Chem. Soc. 1999, 121, 4859. (c) Wagner, W.-D.; Nakamoto, K. J. Am. Chem. Soc. 1989, 111, 1590. 3 (a) Justel, T.; Muller, M.; Weyhermuller, T.; Kressl, C.; Bill, E.; Hildebrandt, P.; Lengen, M.; Grodzicki, M.; Trautwein, A. X.; Nuber, B.; Wieghardt, K. Chem.-Eur. J. 1999, 5, 793. (b) Kienast, A.; Homborg, H. Z. Anorg. Allg. Chem. 1998, 624, 233. (c) Moubaraki, B.; Benlian, D.; Baldy, A.; Pierrot, M. Acta Crystallogr., Sect. C 1989, 45, 393. (d) Scheidt, W. R.; Summerville, D. A.; Cohen, I. A. J. Am. Chem. Soc. 1976, 98, 6623. (e) Summerville, D. A.; Cohen, I. A. J. Am. Chem. Soc. 1976, 98, 1747. 4 For ammonia production from a terminal iron nitride with hydrogen equivalents see reference 2a. 5 Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 4538. 6 Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2005, 127, 1913. 7 For (Cp2Ti)2(µ-NH)2(µ-H) see: Armor, J. N. Inorg. Chem. 1978, 17, 203. 8 For examples of bimetallic systems featuring µ-NH ligands see: (a) Wraage, K.; Schmidt, H.-G.; Noltemeyer, M.; Roesky, H. W. Eur. J. Inorg. Chem. 1999, 863. (b) Puff, H.; Hanssgen, D.; Beckermann, N.; Roloff, A.; Schuh, W. J. Organomet. Chem. 1989, 373, 37. (c) Edelblut, A. W.; Haymore, B. L.; Wentworth, R. A. D. J. Am. Chem. Soc. 1978, 100, 2250. (d) Abarca, A.; Martin, A.; Mena, M.; Yelamos, C. Angew. Chem., Int. Ed. 2000, 39, 3460. 9 Roesky, H. W.; Bai, Y.; Noltemeyer, M; Angew. Chem., Int. Ed. 1989, 28, 754. 146 10 Daida, E. J.; Peters, J. C. Inorg. Chem. 2004, 43, 7474. 11 See for example: (a) Georgakaki, I. P.; Miller, M. L.; Darensbourg, M. Y. Inorg. Chem. 2003, 42, 2489. (b) Zhao, X.; Chiang, C.-Y.; Miller, M. L.; Rampersad, M. V.; Darensbourg, M. Y. J. Am. Chem. Soc. 2003, 125, 518 and references therein. (c) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B.; Bénard, M.; Rohmer, M.-M. Inorg. Chem. 2002, 41, 6573. (d) Lawrence, J. D.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2002, 41, 6193. 12 The cyclic voltammagram of [NBu4][BPh4] in CH2Cl2 features an irreversible oxidation at ~ 0.75 V vs. S.C.E. (~ 0.26 V vs. Fc/Fc+). For complete experimental details see: Pal, P. K.; Chowdhury, S.; Drew, M. G. B.; Datta, D. New J. Chem. 2002, 26, 367. 13 Orpen, A. G.; Connelly, N. G. Organometallics 1990, 9, 1206. 14 A similar phenomenon has been observed for the complex [(PP3)Fe(C≡CPh)][BPh4] (PP3 = P(CH2CH2PPh2)3). The one-electron reduction of this species induced a contraction of the Fe-P bond distances by an average of 0.09 Å, which the authors attributed to π-backbonding. See: Bianchini, C.; Laschi, F.; Masi, D.; Ottaviani, F. M.; Pastor, A.; Peruzzini, M.; Zanello, P.; Zanobini, F. J. Am. Chem. Soc. 1993, 115, 2723. 15 For EPR analysis of the d5 iron imides see Chapter 2. Also see: Brown, S. D.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 322. 16 For antiferromagnetic coupling between S1 = 5/2 and S2 = 2 centers, the S = 3/2 spin state is expected to lie 3J higher in energy than the S = ½ spin state. See: Kurtz, D. M., Jr. Chem. Rev. 1990, 90, 585. 147 17 For examples of χM and µeff vs. T plots obtained for antiferromagnetically coupled Fe(II)/Fe(II) systems, see: Klose, A.; Solari, E.; Floriani, C.; Chiese-Villa, A.; Rizzoli, C.; Re, N. J. Am. Chem. Soc. 1994, 116, 9123. 18 (a) Hay, P. J.; Thibeault, J. C.; Hoffmann, R. J. Am. Chem. Soc. 1975, 97, 4884 and references therein. (b) Murray, K. S. Coord. Chem. Rev. 1974, 12, 1. 19 See for example: (a) Mukherjee, R. N.; Stack, T. D. P.; Holm, R. H. J. Am. Chem. Soc. 1988, 110, 1850. (b) Drew, M. G. B.; McKee, V.; Nelson, S. M. J. Chem. Soc., Dalton Trans. 1978, 80. 20 See for example: (a) Norman, R. E.; Holz, R. C.; Menage, S.; O’Connor, C. J.; Zhang, J. H.; Que, L., Jr. Inorg. Chem. 1990, 29, 4629. (b)Mabbs, F. E.; McLachlan, V. N.; McFadden, D.; McPhail, A. T. J. Chem. Soc., Dalton Trans. 1973, 2016. 21 Examples of C1 symmetric, µ-oxo iron species are known in which the coupling is stronger than in related C2v systems. See for example: Gomez-Romero, P.; Witten, E. H.; Reiff, W. M.; Jameson, G. B. Inorg. Chem. 1990, 29, 5211. 22 For example, ∆oct for Co(NH3)63+ and Co(NH3)62+ are 22,900 and 10,200 cm-1, respectively. See: Housecroft, C. E.; Sharpe, A. G. Inorganic Chemistry; 2nd Ed., Pearson Education Limited: England, 2005; p 559. 23 Octahedral Fe(III) compounds with low-spin t2g5 configurations have considerable orbital contributions to their magnetic moments, which are therefore intrinsically temperature dependent. Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochman, M. Advanced Inorganic Chemistry; 6th Ed., Wiley & Sons: New York, 1999; p 790. 24 Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions in Chemistry; Wiley & Sons: New York, 1985; p 383. 148 25 See for example: Igarashi, R. Y; Laryukhin, M.; Dos Santos, P. C.; Lee, H.-L.; Dean, D. R.; Seefeldt, L. C.; Hoffman, B. M. J. Am. Chem. Soc. 2005, 127, 6231.