Spectroscopy and Photochemistry of Pyrazolyl-Bridged Binuclear Iridium(I) Complexes Citation Marshall, Janet Layne (1987) Spectroscopy and Photochemistry of Pyrazolyl-Bridged Binuclear Iridium(I) Complexes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/63cx-a405. https://resolver.caltech.edu/CaltechTHESIS:02192019-111735898 Abstract Spectroscopic studies of a series of pyrazolyl-bridged and substituted-pyrazolyl-bridged binuclear iridium(I) complexes indicate that the description of the metal-metal interactions in previously studied D 4h d 8 -d 8 rhodium(I), iridium(I), and platinum(II) species may be extended to these lower symmetry (C 2v ) d 8 -d 8 molecules. Specifically, the 1 A 1 (dσ) 2 (dσ * ) 2 ground state exhibits weak metal-metal bonding, and the lowest excited states are a singlet ( 1 B 2 ) and triplet ( 3 B 2 ) derived from the (dσ) 2 (dσ * ) 1 (pσ) 1 electronic configuration. The 1 B 2 and 3 B 2 excited states, which are expected to feature strong metal-meta1 bonding, are 1uminescent at ambient temperature in fluid solution. Electronic absorption and emission spectroscopic studies and photophysical investigations of the emissive singlet and triplet excited states of bis(1,5-cyclooctadiene)bis(µ-pyrazolyl)diiridium(I), [Ir(µ-pz)(COD)] 2 , and analogous substituted-pyrazolyl complexes are presented in Chapter 2. The absorption spectrum of [Ir(µ-pz)(COD)] 2 exhibits an intense band attributable to 1 A 1 → 1 B 2 at 498 nm (ε = 8100 M -1 cm -1 ). Both fluorescence (λ max = 558 nm, Φ em = 0.0001, τ < 20 ps) and phosphorescence (λ max = 684 nm, Φ em = 0.0078, τ = 250 ns) from the 1 B 2 and 3 B 2 excited states, respectively, are observed at ambient temperature for this complex. The absorption and emission spectra of the substituted-pyrazolyl complexes show similar features. In addition, ground-state resonance Raman studies of these complexes suggest the presence of a reasonable metal-metal bonding interaction in the formally nonbonded binuclear center; excitation into the bands corresponding to the metal-metal localized 1 A 1 → 1 B 2 transition results in resonance-enhancement of vibrations at frequencies of of 58 cm -1 to 80 cm -1 that are assigned to ν(Ir-Ir ). The long lifetime of the 3 B 2 (dσ * pσ) excited state of [Ir(µ-pz)-(COD)] 2 implies that it should be able to participate in bimolecular reactions. The results presented in Chapter 3 show that this strongly reducing excited state undergoes photoinduced electron transfer with a variety of substrates including reversible electron transfer to one-electron acceptors such as methyl viologen and pyridinium monocations. For pyridinium acceptors with reduction potentials ranging from -0.67 V to -1.85 V vs. SSCE, the rates of electron-transfer quenching range from a diffusion-limited rate of 2.0 x 10 10 M -1 s -1 to 1.1 x 10 6 M -1 s -1 and obey Marcus-theory predictions for outer-sphere electron transfer in the "normal free-energy region." However, the rates do not decrease as predicted for the "inverted free-energy region." With acceptors such as halocarbons, the unproductive back-electron-transfer reaction can be circumvented, and net two-electron, photoinduced electron transfer yields iridium(II)-iridium(II) oxidative addition products. Chapter 4 focuses on spectroscopic and photophysical investigations of pyrazolyl-bridged binuclear iridium(I) complexes containing carbon monoxide ligands. Spectroscopic studies of tetracarbonylbis(µ-pyrazolyl)diiridium(I), tetracarbonylbis(µ-3-methylpyrazolyl)diiridium(I), and tetracarbonylbis(µ-3,5-dimethylpyrazolyl)diiridium(I) reveal interesting features in the electronic absorption spectra at ambient temperature and 77 K that may be assigned to dπ(xz,yz) → [σ(p z ), π * (CO)] transitions and predominantly metal-metal localized σ * (d z 2) → [σ(p z ),π * (CO)] transitions that reflect the degree of metal-meta1 interaction. Photophysical studies of the emissive 1,3 B 2 (dσ * pσ) excited states suggest that a higher-energy d-d excited state may provide a pathway for thermal deactivation of these states. Item Type: Thesis (Dissertation (Ph.D.)) Subject Keywords: Chemistry Degree Grantor: California Institute of Technology Division: Chemistry and Chemical Engineering Major Option: Chemistry Thesis Availability: Public (worldwide access) Research Advisor(s): Gray, Harry B. Thesis Committee: Bercaw, John E. (chair) Gray, Harry B. Dougherty, Dennis A. Grubbs, Robert H. Defense Date: 4 December 1986 Funders: Funding Agency Grant Number Union Carbide Company UNSPECIFIED Sun Oil Company UNSPECIFIED Atlantic Richfield Foundation UNSPECIFIED Record Number: CaltechTHESIS:02192019-111735898 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:02192019-111735898 DOI: 10.7907/63cx-a405 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 11397 Collection: CaltechTHESIS Deposited By: INVALID USER Deposited On: 19 Feb 2019 21:10 Last Modified: 16 Apr 2021 23:31 Thesis Files Preview PDF - Final Version See Usage Policy. 47MB Repository Staff Only: item control page

SPECTROSCOPY AND PHOTOCHEMISTRY OF PYRAZOL YL-BRIDGED BINUCLEAR IRIDIUM(!) COMPLEXES Thesis by Janet Layne Marshall In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California 1987 (Submitted December 4, 1986) 11 Acknowledgements I would like to thank my opportunity to pursue enthusiasm is greatly my advisor, graduate appreciated, Harry degree I and Gray, for giving me in his research group. hope that some his of the His knowl- edge has rubbed off on me. Several members of the Gray group have been especially helpful. I would like to thank Mike, AI, and Tom for a number of insightful discussions and special thanks thetic ear. study in for with various proofreading this intramolecular complexes getting me absorption on experiments. dividuals, I friends and have Tad for deserves having a sympa- working with me on the reactions of binuclear appreciate Les' patience the transient past year. I the laser and helping addition to the both scientific and the group. Special in Miriam always the In enjoyed co-workers and electron-trans[ er during started experiments. thesis Thanks to "my colleague" of ir id i u m(I) assistance with above-mentioned social in- interactions appreciation with goes to Catherine for providing a full pot of coffee and a comfortable chair. I wish to acknowledge collaboration on the Stobart providing for Woody \Voodruff Raman · experiments. samples of the I and also iridium Ed want Kober to complexes for thank their Stephen and getting us people from the enjoyed the started on this project. I have Bercaw, enjoyed Collins, my and friendships Grubbs good times with Leigh, thank my family for with a number groups, and I have here and at Carolina. both all the support, of especially encouragement, I would and like to education~! 111 assistance helping that me they have scientifically, provided. being a Jeff great deserves friend, and special thanks for me adopt letting l\1abel and l\1argaret. I Sun would Oil also Company, fellowships. like and to acknowledge the Atlantic the Union Richfield Carbide Foundation Company, for the graduate iv to Jeff v Abstract Spectroscopic studies of sti tu ted-pyrazol yl- bridged that the studied be d 8 -d 8 D4h extended cally, ( 3 B2 ) 2 to series of the lower iridi urn (I), symmetry ground and the derived 3B and metal-meta 1 lowest from excited excited 2 bonding, states, state states which 1uminescen t are d 8 -d 8 are at previously species molecules. rna y Specifi- exhibits weak metal-metal a ( 1B 2 ) and singlet electronic are sub- indicate in pia tinum(II) (C 2 v) and complexes interactions and (da) 2 (da *) 1(pa) 1 the pyrazolyl-bridged metal-metal rhodi urn (I), these of iridium(!) binuclear the bonding, 1B description a configuration. expected ambient triplet to The feature tempera turc strong 1n fluid studies and and triplet solution. Electronic absorption photophysical excited investigations states 2, presented Chapter in and of the 2. The Both fluorescence and phosphorescence ("-max 1B and 3B perature for 2 2 excited this s u bsti tu ted -p y razol yl ground-state absorption presence of a ("-max singlet 1A = 558 spectrum complex. nm, ~em complexes Raman reasonable absorption are 0.0001, T show similar studies of metal-metal at 20 ps) ambient spectra features. In bonding = 8100 < emission these 2 250 ns) from the observed and are [Ir(~-pz)(COD)] 1 1 -+ B 2 at 498 nm (E respectively, The complexes of = 684 nm, ~em = 0.0078, T states, resonance emissive su bsti tu ted-pyrazol yl analogous exhibits an intense band attributable to M -1 em -1) . spectroscopic bis( 1,5-cyclooctadiene )bis( ~ -pyrazol yl)d iiridi um(I), of [lr(~ -pz)(COD )] emission and complexes of the addition, suggest interaction tern- in the the VI formally non bonded sponding to the binuclear center; metal-metal resonance-enhancement of excitation into bands transition localized vibrations the of frequencies at corre- results cm- 1 58 in to 80 em - 1 that are assigned to v (Ir- Ir ). The long lifetime (COD))2 implies that reactions . The reducing excited variety of electron acceptors cations. For from -0.67 quenching 1.1 electron rates excited be able results presented in Chapter state undergoes photoinduced substrates including reversible such as pyridinium to range M- 1s- 1 acceptors -1.85 from V a and methyl vs. to 3 show with reduction the rates diffusion-limited rate of 2.0 obey Marcus-theory "normal free-energy decrease as With acceptors such as halocarbons, reaction can be circumvented, electron predicted for yields transfer this with to a onemono- potentials ranging of electron-transfer x 10 10 M- 1s- 1 to for region." outer-sphere However, "inverted the strongly pyridinium predictions not photoinduced the bimolecular transfer and SSCE, in (Ir(1J-pz)- transfer electron viologen of that electron In electron-transfer state participate transfer do region ." it 3 B (da • pa) 2 the should V 10 6 x of the free-energ y unproductive and net the back- two-electron , iridi um(II)-iridi um(II) oxida- ti ve addition products. Chapter tions of carbon 4 focuses on pyrazol yl- bridged monoxide ligands. spectroscopic binuclear veal and interesting photophysical investiga- iridium(!) complexes containing Spectroscopic pyrazol yl)diiridi um(I), di um(I), and studies of tetracarbon y lbis(IJ- tetra carbonyl bis(IJ -3-meth ylpyrazol yl)diir i- tetra carbonyl bis( IJ-3,5-dimeth ylpyrazolyl)diiridi um(I) re- features at in the electronic absorption spectra Vll ambient temper at u r e [a(pz),n-*(CO)] a *(dz2) ~ and 77 transitions [a(pz),n- *(CO)] K and interaction. Photophysical excited states that a may be assigned predominantly transitions meta 1 suggest that that studies to metal-metal reflect the of emissive higher-energy d n-( x z, y z) the d-d provide a pathway for thermal deactivation of these states. degree excited ~ localized of metal- l, 3 B 2(da *pa) state may viii Table of Contents Acknowledgements 11 Abstract v Chapter 1 Introduction References Chapter 2 Chapter 3 Chapter 4 9 Spectroscopic Studies of Bis(l ,5-cyclooctadiene )bis(IJ-pyrazolyl)diiridium(I) and Related Complexes 14 Introduction 15 Experimental 23 Results and Discussion 33 References and Notes 97 Electron-Transfer Reactivity of the 3 B 2 (da *pa) Excited State of [Ir(1J-pz)(COD)] 2 102 Introduction 103 Ex per imen ta 1 105 Rcsul ts and Discussion 116 References and Notes 157 Spectroscopic Studies of Tetracarbonylbis(IJ-pyrazolyl)diiridium(I) and Related Complexes 163 Introduction 164 Experimental 166 Results and Discussion 172 References and Notes 201 Chapter 1 Introduction 2 Interest in part, due of organic the the to chemistry of binuclear attractiveness of these inorganic and Binuclear complexes offer stra tes, m ultielectron redox capabilities, plexes, distinct given substrate. 4 ,s of binuclear While cooperative multiple that can numerous binding and for interact studies of the thermal reactivity have attention has been given to the inorganic molecules by the species those that can have different of the ground substrates by photoexcited thermall y. 1 0 1 1 ' In conversion of visible light portant the study of both the is an understanding to of binuclear and the state, this of these to of complex possibility binuclear regard, complexes consequences the states a and exists for that complexes are of the metal-metal bond-forming our has un- organic these com- a unique is from reactivity with is not observed promising and a properties energy. 1 2 - 1 4 photochemical of of physical complexes chemical been activation excited state chemical sites with less excited in terac- diff eren tl y substrates long-lived metal com- of a activation heteron uclear variety Since the in sub- a plexes. 6 -g been, for with electronic has for complexes dertaken, and centers complexes by substrates tions. 1 - 3 meta 1 complexes for the Especially im- thermal reactivity metal-metal bonding and bond-breaking re- actions. Since the mid-1970's, chemistry, photophysics, complexes of second and research efforts have our activity and of the ground and rhodium(!) and iridium(!) group been photochemistry third row focused of several transition on luminescent with interested the excited bridging in the In characterization isocyanide d 8 -d 8 binuclear metals. states thermal of particular, and complexes ligands and reof a 3 platinum(II) dimer with bridging pyrophosphite ligands. Spectroscopic studies of monomeric, plexes of rhodium(!) species to form dimers and tions. 1 5 -l 7 With this mind, plexes a variety with [Rh 2 (bridge) 4 ]2 +, of a filled la 2 u(dz2a *) of a the oligomers number bridging weak of orbitals isocyanide by ( 3 A 2 u) tronic configuration showed that state of predicted excited by states interaction mixing are luminescent at (8.5 of 79 with vibronically structured the band orbit- 20 21 ' the related rhodium ( 1A 2 u) and (da) 2 (da *) 1 (pa) 1 elec- singlet ambient temperature resonance Raman in studies excited with studies of unoccupied and excited-state studies anti bonding theory, spectroscopic 1,3- and orbital absorption the including stabilization metal-metal excited in com- from strongly em -l rhodium(!) the is 3A u 2 144 in low-energy from long-lived molecular the derived and stretching frequency cm- 1 that Ground-state the solu- ligands, [Rh 2 (bridge) 4 ]2 + of re vealed triplet solution. 1 9 - 2 2 concentrated spectroscopic configurational studies dimcrs these dimeric als of the same symmetries [2a 1gCPza) and 2a 2 u(Pza *)]. Add i tiona I of bonding metal-metal com- 2,5-d imeth yl-2,5-diisocyanohexa ne From metal-metal tendency in isocyanide and isocyanide d 8-d 8 dimers was shown to arise state of the the in showed higher prepared. 1 8 ,l 9 were ground iridium(!) (bridge) diisocyanopropa ne (TMB), and square-planar in 1A g ground 1 the statc .2 3 at - 670 as rhodium-rhodium state increasing Low-temperature, corroborated centered the bonding, to single-crystal this prediction. The nm, corresponding to a of 1A lg ~ Eu( 3 A u) transition, shows a progression in 2 frequency 4 - 150 em - 1 of 144 em - 1 obtained from consistent ' shape suggested that the the 3 A 2 u excited state. Studies it with the the Raman studies. thermal oxidatively adds substrates methyl iodide produces have added across photochemical interesting. [Rh 2 (TMB) 4 ] 2 + substrates produces a chemistry 0.3 A in [Rh 2 (bridge) 4 ] 2+ revealed that iodine, and chlorine iodide fragments distance decreases react as where the methyl the metal-metal axis of these excited by energy both irradiation bromine, dimer The hydrogen, band of such reactivity including Furthermore, frequency - 2 4 the also stretching An analysis of the rhodium-rhodium of The rhodium-rhodium of although and electron trans- isocyanide dimers is with variety of of transfer and a quenchers. 2 5 reductive [Rh 2 (bridge) 4 ]2 + in the involves mechanism give to rhodium states oxidative and HCI aqueous both solution thermal and photochemical production of hydrogen and is rather complex. 8 , 2 0 , 2 6 - 3 0 Studies the of [Ir (TMB) ] 2 + 2 4 reveal cally to similar As dimers. dramatically of 1 ground that the discussed in [lr (TI\1B) 4 ] 2 + 2 A lg ground the is structurally states of and electroni- "face-to-face" rhodium isocyanide above, the metal-metal interaction increases 1A u 2 and 3A u 2 rela ti vel y weak bonding the 3 0 3 1 ' complex excited square-planar luminescent from state. this 1uminescen t and This complex also (da *pa) excited interaction thermally states in the oxidatively adds halogens to give iridium(ll)-iridium(II) metal-metal bonded species. 3 2 The square-planar "face-to-face" dimer of platinum(II) with bridg- 5 ing pyrophosphite the same as the rhodium(!) and to the thermal oxidative rich [Pt 2 (pop) 4 ]4 - ligands, ground-state and excited-state properties iridium(!) isocyanide dimers. 3 3 ' 3 4 addition by linear arc semiconducting water, and confirm the metal-metal highly state reacts by tive of of abstracting this complex hydrogen has atoms spectroscopic ex- interaction in tin abstracts hydrogen atoms from a isopropanol to this bonding 3 The irradiation of [Pt 2 (pop) 4 ]4 - in two-electron, photoinduced oxida- Recently, shown to variety of a of A2u variety hydrides." 6 and resonance a been germanium, con verts of with net silicon, ca ta 1ytica 11 y studies state. A2u to that excited from leads complexes 3 substrates. 4 3 - 4 5 these of In transfer to oxidation increase and leads d 7 -d 7 substantia 1 luminescent electron halocarbons state a quenchers, 3 4 ' 4 2 addition excited in of and addition the excited-state [Pt2(pop)4]4- the presence and In the emission in reductive with and state and mixed-valence As reactivity triiodide absorption ground excited or and give single-crystal 1 interaction water, ground-state of Alg bromine solids. 3 8 ' 3 9 per imen ts 4 1 the to partial "-Pt(II)- Pt(III)- X-" chain both Raman 4 0 halogens ' chlorine novel of dimers 3 5 -s 7 pia tin um(III)-pla tin um(III) [Pt 2 (pop) 4 ]4 - (pop oxidative the the be extremely substrates, This excited number of alcohols acetone with visible reactive including state and and also photo- near- UV light.4 3 ,4 4 ,4 7 Our dimers, and interest the in the spectroscopic unique reactivity of photochemical reactions, and the the properties binuclear center metal-metal of these in both interactions thermal in the 6 ground and properties (C 2 v) excited states prompted and photochemica 1 binuclear iridium(!) dimers carbon monoxide, reactivity and examine of a the class photophysical of lower-symmetry Our and bis(JJ-SU bsti tu ted-pyrazol yl)-bridged ancillary with to complexes. bis(JJ -pyrazolyl)- bridged iridi urn us ligands phosphines studies have focused on su.ch as I ,5-cyclooctadiene, coordinated in a square-planar arrangement around each iridium atom. We have pz)(COD)] 2 particularly (pzH pyrazole, iridium(!) related first been published related in ligand a patent is able to for X-ray bridge two the short similar which a of centers weak .ground-state several was thereafter, of this complex that the wide range separation suggests of that square-planar the and [Ir(JJ- Shortly Given metals over a of [Ir(JJ -pz)(COD )] 2 structure iridium-iridium to properties I981. 4 8 in crystal iridium(!) the synthesis co-workers. 4 9 ' 50 and nonbonding electronically dimers and Stobart formally Spanish in I ,5-cyclooctadiene) The by distances, 5 1 - 53 COD dimers. preparation reported interested of was pyrazol yl intermetallic 3.2I6(I) this A for dimer is d8-d8 "face-to-face" metal-metal a interaction has been documented (vide supra). Stobart will and co-workers oxidatively symmetric iodide in the atoms. add of and its substrates Br 2 , products. [Ir(JJ-pz)(COD)] 2 methyl Because ox ida ti vel y to demonstrated (1 2 , halogens iridium(II)-iridium(II) methyl that add have iodide Cl 2 ) The parallels fragments structure, that bind [Ir(JJ-pz)(COD)h thermally oxidative that coordinate unique must that of to to give addition of [Rh 2 (bridge) 4 ]2 + opposing ~Ir(JJ-PZ)(COD)h to both metal metal can also centers such 7 as the activated alkynes hexafl uoro bu t-2-yne, methyl propiola te, and dimethylacetylenedicarboxylate.'' 9 , 54 ' 55 The su bsti tu tion chemistry [Ir(~-pz)(COD)] of 2 and related bis( 1,5-cyclooctadiene )bis(~ -su bsti tu ted-pyrazolyl)diiridium(I) plexcs 5 0 ' 56 that of provides also have is route rather [Ir(~-pz)(COD)] ture a rapid short carbonyl ligand from each trans Analogous separations. complexes The reaction to leads a thermal iridium-iridium binuclear and with to other atm. of carbon monoxide at ambient tempera- reacts has several 2 with complex which into com- formation variety of [Ir(~-pz)(C0) 2 ] 2 . 5 7 ' 58 of (PR 3 ), phosphines to stereochemistry. 5 9 These complexes reactions are with a observed give variety for one 3 2 center addition replacing [Ir(~-pz)(CO)(PR )] , metal oxidative This are also subject substrates. 5 6 ' 6 0 ' 6 1 of the su bsti tu ted-pyrazolyl binuclear ir idi um(I) cycloocta diene complexes. Our studies interactions of the the of interpreted in states ground are figuration terms of a on in investigating low-energy analogy to the and these the luminescent complexes c2 v of configuration. The terms of the ( da ) 2 (d a*) 1(pa) 1 and expected to be strongly have angular an shape. interactions metal-metal We as earlier, symmetry luminescent in the states "face-to-face" discussed described are excited square-planar pia tin um(II) metal-metal are weakly metal-metal bonding ground state with electronic Interestingly, "A-frame" and irid i um(I), interactions (da) 2(da *) 2 both By rhodi um(I), metal-metal the con centra ted complexes. 6 2 these dimers tn have well are as distance also interested in dependence in electronic metal-metal these the excited bonding. complexes by con- can virtue of the photochemical re- 8 activity of these dimeric complexes since the lower symmetry and shape, relative to the previously studied d 8-d 8 greater access to the metals potential substrates, by dimers, open allows for enhancing the possibility of m ul tielectron excited-state reactivity. Specifically, Chapter spectroscopic and cent states excited pyrazolyl activity a number the of spectroscopic carbonyl this photophysical Chapter pyridinium studies of the 2 and four addresses 3 excited acceptors three and related ground and, and lumines- substituted- electron-transfer re- of [Ir(~-pz)(COD)b with state Chapter 4 of the dimers iridium presents containing and consequently, excited complexes lead the states to 3-CH 3 pzH The wide metal-metal of unique these the the ligands: separations properties. ground of halocarbons. 3,5-dimethylpyrazole). iridium(!) several and pyrazole, the describes of (da *pa) triplet thesis properties [Ir(~-pz)(COD)] of dimers. of of 2 [Ir( ~- 3-methylpyrazole, range bonding of metal-metal interactions pyrazolyl-bridged spectroscopic and in binuclear photophysical 9 References 1. Poilblanc, R. Inorg. Chim. Acta 1982, 62, 75-86 and references J. Inorg. Chim. Acta 1982, 62, 31-37 and references C.L.; Robbins, therein. 2. Halpern, therein. 3. Poilblanc, R. Nouv. J. Chim. 1978, 2, 145-150. 4. McCleverty, J.A. Inorg. Chim. Acta 1982, 62, 67-73. 5. Chisholm, M.H. ACS Symp. Ser. 1980, 155, 17-39. 6. Wrighton, M.S.; Graff, J.L.; Luong, J.C.; Reichel, J.L. ACS Symp. Ser. 1980, 155, 85-110. 7. Nocera, D.G.; Maverick, A.W.; Winkler, J.R.; Che, C.-M.; Gray, Winkler, J.R.; Gray, H.B. ACS Symp. Ser. 1983, 211, 21-33. 8. Gray, H.B.; Maverick, A.W. Science 1981, 214, 1201-1205. 9. Maverick, H.B. A.W.; in Ra bani, Che, C.-M; "Photochemical J., Ed.; Nocera, Conversion Weizmann Science D.G.; and Storage of Solar Energy"; Press: Jerusalem, 1982; pp 161- Maestri, Top. Curr. 173. 10. Balzani, V.; Bolletta, F.; Gandolfi, M.T.; F. in Resources M. Chem. 1978, 7 5, 1-64. 11. Balzani, istry V.; and Scandola, Catalysis"; Gratzel, "Energy M., Ed.; Academic through · PhotochemPress: New York, Storage of Solar and Ca- 1983; Chapter 1. 12. Ra bani, J., Ed. "Photochemical Conversion and Energy"; Weizmann Science Press: Jerusalem, 1982. 13. Gratzel, M., Ed. "Energy Resources through Photochemistry 10 talysis"; Academic Press: New York, 1983. 14. Connolly, J .S., Ed. "Photochemical Con version and Storage of Solar Am. Chem. Soc. 1975, Gordon, J.G., Energy"; Academic Press: New York, 1981. 15. Mann, K.R.; Gordon, J.G., II; Gray, Lewis, N.S.; Williams, H.B. J. R.M.; Gray, 97, 3553-3555. 16. Mann, K.R.; H.B.; II Inorg. Chem. 1978, 17, 828-834. 17. Geoffroy, G.L.; Bradley, M.G.; Keeney, Inorg. M.E. Chem. 1978, 17, 777-779. 18. Lewis, N.S.; Mann, K.R.; Gordon, Am. Chem. J.G., II; Gray, H.B. J. Soc. 1976, 98, 7 461-7463. 19. Mann, K.R.; Thich, J.H.; Bell, R.A.; Coyle, C.L.; Gray, H.B. Kliger, D.S. D.S.; Hammond, G.S.; Chem. Soc. 100, Inorg. Chem. 1980, 19, 2462-2468. 20. Mann, K.R.; Gray, H.B. Adv. Chem. Ser. 1979, 17 3, 225-235. 21. Rice, S.F.; Milder, S.J.; Gray, H.B.; Goldbeck, R.A.; Coord. Chem. Rev. 1982, 43, 349-354. 22. Miskowski, V.M.; Nobinger, Lewis, Mann, K.R.; N.S.; G.L.; Gray, Kliger, H.B. J. Am. 1978, 485-488. 23. Dallinger, R.F.; Miskowski, V.M.; Gray, H.B.; Woodruff, W.H. J. J. Am. Am. Chem. Soc. 1981, 103, 1595-1596. 24. Rice, S.F.; Gray, H.B. J. Am. Chem. Soc. 1981, 103, 1593-1595. 25. Milder, S.J.; Goldbeck, R.A.; Kliger, D.S.; Gray, H.B. Chem. Soc. 1980, 102, 6761-6764. 26. Mann, K.R.; Lewis, N.S.; Miskowski, V.M.; Erwin, G.S.; Gray, H.B. J. Am. Chem. Soc. 1977, 99, 5525-5526. D.K.; Hammopd, 11 27. Gray, H.B.; Mann, K.R.; Lewis, N.S.; Thich, J.A.; Richman, Mann, K.R.; Gray, H.B.; R.M. Adv. Chem. Ser. 1978, 168, 44-56. 28. Miskowski, S.J.; V.M.; Sigal, I.S.; Milder, G.S.; Ryason, P.R. J. Am. Chern. Soc. 1979, 101' DiPierro, M.J.; Gill, T.P. J. Am. Chem. Soc. 1980, V.M.; Hammond, 4383-4385. 29. Mann, K.R.; 102, 3965-3967. 30. Gray, H.B.; Miskowski, A.W.; Buhr, J.D.; "Fundamental Milder, S.J.; Smith, T.P.; W.L.; Sigal, I.S.; Mann, K.R. in Tsutsui, M., Ed.; Gladfelter, Research in Homogeneous Catalysis"; Maverick, Plenum Press: New York, 1978; pp 819-834. 31. Smith, T.P. Ph.D. Dissertation, California Institute of Technol- ogy, Pasadena, 1982. 32. Miskowski, V.M.; Smith, T.P.; Loehr, T.M.; Gray, H.B. J. Am. Chem. Soc. 1985, 107, 7925-7934. 33. Sperline, R.P.; Dickson, M.K.; Roundhill, D.M. J. Chern. Soc. Chern. Soc., 1981, 103, Chem. Comrnun. 1977, 62-63. 34. Che, C.-M.; Butler, L.G.; Am. Gray, H.B. J. W.P.; Gray, H.B.; 7796-7797. 35. Che, C.-M.; Schaefer, Dickson, M.K.; Stein, P.B.; Roundhill, D.M. J. Am. Chem. Soc. 1982, 104, 4253-4255. 36. Bryan, S.A.; M.K.; Roundhill, D.M. J. L.G.; Grun thaner, P.J.; Gray, H.B. !nor g. Schaefer, W.P.; Marsh, R.E.; Gray, Dickson, Am. Chem. Soc. 1984,106, 1882-1883. 37. Che, C.-M.; Butler, Chem. 1985, 24, 4662-4665. 38. Che, C.-M.; Herbstein, F.H.; 12 H.B. J. Am. Chem. Soc. 1983, 105, 4604-4607. 39. Clark, R.J.H.; Kurmoo, M.; Dawes, H.M.; M.B. Inorg . . Woodruff, W.H. Hursthouse, Chem. 1986, 25, 409-412. 40. Che, C.-M.; Butler, L.G.; Gray, H.B.; Crooks, R.M.; J. Am. Chem. Soc. 1983, 105, 5492-5494. 41. Rice, S.F.; Gray, H.B. J. Am. Chern. Soc. 1983, 105, 4571-4575. 42. Heuer, W.B.; Totten, M.D.; Rodman, G.S.; Hebert, E.J.; Tracy, H.J.; Nagel, J.K. J. Am. Chem. Soc. 1984, 106, 1163-1164. 43. Marshall, J.L.; Stiegman, A.E.; Gray, ACS H .B. Symp. Ser. 1986, H.B., man- 307, 166-176. 44. Roundhill, D.M. J. Am. Chem. Soc. 1985, 107, 4354-4356. 45. Roundhill, D.M.; Atherton, S.F. Inorg. Chem. 1986, 25, 4071-4072. 46. Vlcek, A., Jr.; Gray, H.B. J. Am. Chem. Soc., submitted. 47. Harvey, E.L.; Stiegman, A.E.; Vlcek, A., Jr.; Gray, uscript in prcpara tion. 48. Uson Lacal, R.; Oro M.T.; Royo Macia, Giral, M.; L.A.; Pastor Ciriano Lopez, Martinez, E. M.A.; Spanish Pinillos, Patent ES 497,900, 1981; Chern. Abstr .. 1982, 97, 55306n. 49. Coleman, A.W.; Eadie, D.T.; Stobart, S.R.; Za worotko, M.J.; Atwood, J.L. J. Am. Chern. ·Soc. 1982, 104, 922-923. 50. Beveridge, K.A.; Bushnell, G.W.; Stobart, S.R.; Atwood, J.L.; Zaworotko, M.J. Organometallics 1983, 2, 1447-1451. 51. Stobart, S.R.; Dixon, K.R.; Eadie, D.T.; Atwood, M.J. Angew. Chem. Int. Ed. Engl. 1980, 19, 931-932. 52. Trofimenko, S. Chem. Rev. 1972, 72, 497-509. 53. Trofimenko, S. Pro g. /nor g. Chem. 1986, 34, 115-210. J.L.; Zaworotko, 13 54. Bushnell, R.; G.W.; Atwood, Decker, J.L.; M.J.; Eadie, Zaworotko, M.J. D.T.; Stobart, Organometallics S.R.; Vefghi, 4, 2106- Atwood, J.L.; Zaworotko, M.J.; 1985, 2111. 55. Coleman, S.R.; Stobart, Harrison, A.W.; D.G.; Zaworotko, M.J. J. Am. Chem. Soc., submitted. 56. Bushnell, G.W.; Fjeldsted, D.O.K.; Stobart, S.R.; Knox, S.A.R.; Macpherson, K.A. Organometallics 1985, 4, 1107-1114. 57. Nussbaum, S.; Rettig, S.J.; Storr, A.; Trotter, J. Chem. J. Can. J. Chem. 1985, 63, 692-702. 58. Harrison, D.G.; Stobart, S.R. Soc., Chem. Commwz. 1986, Dixon, K.R.; Eadie, D.T.; 285-286. 59. Beveridge, Stobart, K.A.; S.R.; G.W.; Bushnell, Atwood, J.L.; Zaworotko, M.J. J. Am. Bushnell, G.W.; Dixon, Chem. Soc. 1982, 104, 920-922. 60. Atwood, D.T.; J.L.; Stobart, Beveridge, S.R.; K.A.; Zaworotko, M.J. A.; S.C.; K.R.; Eadie, 23, 4050- Chim. Acta Inorg. Chem. Ng, W.W. Inorg. by Dr. Terrance 1984, 4057. 61. Powell, J.; Kuksis, Nyburg, 1982, 64, L211-L212. 62. Preliminary experiments were and are reported in reference 31. conducted P. Smith 14 Chapter 2 Spectroscopic Studies of Bis( 1,5-cyclooctadiene )bis(~-pyrazolyl)diiridium(I) and Related Complexes 15 Introduction Extensive studies of with bridging isocyanide ligands that the metal-metal state is weakly rhodium and (pop P 2 o 5 H 2 2 -) the 1A g(da) 2 (da *) 2 1 interaction of a iridium in single have bond. 1 - 7 A mation, the metal-metal nonbonding dimers since a 1g(pa) and a2u(pa *) accounts for the ground state. 2 ' 3 energy, in tense As band diagram this approaches that that 2.1. 8 To the ground state and * a 2 u(da) describes Figure interaction in the a 1g(da) mixing of these orbitals with the serves to stabilize the former metal-metal been the bonding absorption a first spectra to the approxi- expected orbitals are unoccupied set in these to filled. and 1 the relatively a of the is interaction well-documented, in and in symmetry is assigned at lower temperature 1A u(da *pa) 2 interaction bonding state of [Pt 2 (pop) 4 ]4 - and shown has in orbital complexes Alg low- "face-to-face" fully-allowed 1A g -+ 1 A weak band corresponding to the 1A 1g-+ 3 A 2 u transi- observed ambient is orbitals weak d 8 -d 8 dimers of D 4 h is molecular both configurational 1A u transition. 2 bin uc1ear excited these tion ground the in However, established (D 4 h) bonding be d 8 -d 8 spectroscopic in energy. fluid solution, phosphorescence and These complexes are and both the 3A u(da *pa) 2 from emissive fluorescence from excited at the states are observed. 1 - 3 ,s ' 7 -l 4 For similar lower-symmetry molecular fragments (C 2 v) orbital is shown in pyrazolyl-bridged diagram Figure 2.2. 7 for the Both iridium(!) interaction the of a 1(do) and dimers, two a ML 2 L ' 2 b 2 (da *) orbit- als are filled, and by analogy to the higher-symmetry d 8 -d 8 complexes, 16 Figure 2.1 Molecular orbital square-planar ct 8 diagram metal ligands (D 4 h symmetry). for ions the interaction complexed with of two isocyanide 17 x2-y2 ,.. ,.. ..,. ,.. ,..,... .,..., .,..., -- -- - .,..., Pz;rr*CNR ....... ---- ~ ...- z2 ....... ....... 02u .,..., .,..., b,g, b2u -- a,g - - 02u ~ ...- ~ ....... .............. ....... .............. a,g xz yz xy MONOMER DIMER 18 Figure 2.2 Molecular orbital diagram dium(I) dimers of C 2 v symmetry. for pyrazol yl- bridged Hl- 19 / * b2(pa-) / x2-y2 / / / Pz / / ....... b 1 ,a 2 / / ....... ....... ........ ....... ...... .......... ..... a,(pa-) ,. ,. b2(da-~) - a 1 (do-) / / / z2 / / / / ........._ xz ,yz - - - xy MONOMER ...... ....... ...... ...... ...... ..... - - - a 1,b 2 ___ b ,a 1 ___ a ,b 1 2 2 20 1 A (da) 2 (da *) 2 1 the ground state is expected to be weakly metal-metal bonding. The 1B 2 and 3 B 2 excited states derived from 1 A 1 by b 2 (da *) -+ a 1(pa) excitation luminescent, as square-planar of the are expected observed for "face-to-face" singlet and triplet to be strongly 1A u 2 and 3A u 2 d 8-d 8 dimers. The electronic (da *pa) excited states of the metal-metal excited bonding and states the of structure d 8 -d 8 these bi- nuclear complexes is illustrated in Figure 2.3. 1 5 Our spectroscopic recent pyrazol yl)diiridi um(I) and studies sever a 1 stituted-pyrazolyl)diiridium(I) complexes emissive these of a excited weakly metal-metal excited of metal-metal bonding higher-symmetry absorption states and states, bis( 1,5-cyclooctadiene )bis( ~- of bis( 1,5-cyclooctadiene) bis( 1-1 -subsuggest complexes can (da) 2 (da *) 1(pa) 1 excited states, d 8 -d 8 dimers. emission spectra, photophysical ground-state plexes are presented herein. be bonding (da) 2 (da *) 2 ground (D 4 h) and that resonance the and described in state enhanced by studies and analogy Accordingly, Raman ground the of spectra to the electronic the of terms emissive these com- 21 Figure 2.3 Electronic structure of d 8-d 8 binuclear highlights the emissive excited states. complexes which 22 2(Pz' L) " TT• , po-• ' ..... po- , , "' '' ' d 2 z 2( '~ ) ' "' " ,. "' "' ..... '' I! j do-* !l do- L3[do-· po-] emission 23 Experimental Syntheses Materials. Petroleum ether (Baker), ethanol (U.S. Indus trial), 95% benzene (Burdick Jackson) were cation. from Jackson), absolute ethanol (U.S. and dichloromethane 95+%, -40 mesh) (Burdick and Jackson) was freeze-pump-thaw 4A lized petroleum a molecular reaction. ether was sieves Pyrazole before 3,4,5-trimethylpyrazole recrystal- 3,5-Dimethylpyrazole (Aldrich, chloride Organic, 99%), 1,1,1,- hydrazine sulfate (Baker), anhydrous potassium 99%) used (Baker), (Aldrich, received. Iridium trichloride trihydrate triethylamine (Aldrich, 99+%, Label), 60-IOO mesh), and bromine (Baker) degassed, was (Columbia Gold 1,2- 98%) use. I,5-cyclooctadiene use. (Aldrich, sodium and to vacuum- sodium (Baker), prior nitrogen and 98%), (Baker), under purifi- Carbide), (Aldrich, hydroxide distilled and (Union trifluoro-2,4-pentanedione cal, (Burdick (Aldrich, into carbonate (Baker), Industrial), hydride transferred 99%), methanol Science) Linde from (Baker), (EM Dichloroethane over ether reagent grade or better and used without further Tetrahydrofuran calcium stored and diethyl were were Matthey), (Johnson Florisil used as (Spectrum Chemi- received. Iodine as (MCB) was sublimed before use. Ligands. 3-(Trifluorometh yl)-5-meth ylpyrazole tion of published I, I, I -trifl uoro-2,4-pen tanedione procedure for the was synthesized hydrazine with preparation of by condensa- following a 3,5-dimethylpyrazole. 1 6 24 Anal. Calcd c 5 H 5 N 2 F 3: for 40.01; C, H, 3.36; N, 18.66. Found: C, 40.04; H, 3.40; N, 18.66. Inorganic Complexes. Di-~-chloro-bis( 1,5-cyclooctadiene )diiridium(I), was synthesized the yield to bottomed by modification of a procedure, 1 7 published 90% from the Schlenk flask was charged with mL), water (17 mL), and mmol), 95% ethanol (34 mL). The neck the denser, the of sidearm of reported [lr(COD)(Cl)] 2 , flask the yield was flask of A 250 IrC1 3 • 3H 2 0 fitted was 72%. with attached improving mL, (2.00 roundg, 5.67 1,5-cyclooctadiene (6.0 a to water-jacketed a nitrogen con- or argon source, and a slow stream of inert gas was passed through the flask a mineral tion oil was refluxed, orange-to-red allowed bubbler attached to with product cool, tion, washed Yield: 1.71 g the top of the for h stirring, precipitated and well to the with (90%). solid 48 from Calcd during solution. [Ir(COD)(Cl)] 2 ice-c.o ld · methanol, Anal. condenser. was and time the solution was collected dried c 16 H 24 c1 21ri for The solu- which The C, to by filtra- under vacuum. 28.61; H, 3.60; Cl, 10.56. Found: C, 28.64; H, 3.68; Cl, 10.45. Bis( 1,5-cyclooctadiene )bis(~ -pyrazolyl)diiridi um(I), pz)(COD)] 2 , argon was prepared atmosphere a!. 1 8 ,l 9 by a [lr(COD)(Cl)] 2 hydrofuran (50 mL) using to procedure 0.744 give an orange-red g, 1.62 pyrazole (0.11 0 mL, 5.2 mmol) tetrahydrofuran to the solution [Ir(COD)(Cl)] 2 . an of Sto bart, et was dissolved in tetra- solution. A colorless solu- triethylamine (0.7 3 to mmol) mmol) (10 under related g, of of Schlenk (0.500 tion in standard [Ir( ~- and mL) The techniques that excess was transferred via cannula pyrazole/triethylamine residue 25 remaining in the flask the reaction also added to deep red upon was addition washed with flask. The the pyrazole of tetrahydrofuran reaction mixture solution and (5 mL) and began to turn stirred at was ambient temperature under argon for 1 h during which time a solid precipitated. The solvent was removed under vacuum, and the residue was transferred to a nitrogen atmosphere flush box. [Ir(~-pz)(COD)] The 2 was extracted into a minimum of benzene (80 mL) and filtered through a short column of evaporation of air-stable crystals Yield: 0.510 Florisil the with benzene benzene with a stream [Ir(~ -pz)(COD )] of g (93%). Anal. as Calcd the of eluting argon 2 that were for c 22 H 30 N 4 Iri solvent. yielded dried Slow dark red, under vacu urn. 35.95; H, C, 4.11; N, 7.62. Found: C, 36.24; H, 4.13; N, 7.64. Bis( 1,5-cyclooctadiene )bis(~ -3,5-dimeth ylpyrazolyl)diiridi um(I), [Ir(~ -3,5-(CH 3 )2 pz)(COD)] 2 , prepared was using standard Schlenk techniques under an argon atmosphere by a procedure related to that of Stobart, et a/. 2 0 A solution of 3,5-dimethylpyrazole (0.250 mmol) and excess triethylamine (1.0 mL, 7.2 mmol) in tetrahydrofuran cannula to an orange-red tetrahydrofuran (50 mL) was transferred solution (50 mL) of via with additional mixture. The residue tetrahydrofuran reaction solution 2.60 (0.865 g, 1.29 mmol). remaining in the flask was washed added to the reaction argon at ambient [Ir(COD)(Cl)] 2 methylpyrazole/triethylamine g, (1 0 mL) and was stirred under The di- temperature for 48 h during which time the solution became deep purple and a solid precipitated. Since the complex is air-stable in solu- tion, the reaction work-up was continued in the air by removal of the solvent on a rotary evaporator. The residue was washed with absolute 26 ethanol to remove the triethylammonium chloride [lr(IJ-3,5-(CH 3 )2 pz)(COD)] 2 was dissolved in by-product. The crude a 1:1 (v /v) mixture of on dichloromethane/ethanol (200 mL). The dichloromethane was removed a crystallizing the and well rotary solid evaporator, was filtered washed methane/ ethanol crystallization was black, air-stable crystals the under vacuum. Yield: 0.925 39.48; H, N, 4.84; purple-black of ethanol. The dichloro- repeated, giving small purple- product that were for 39.37; H, 4.80; N, [lr(~-.~-3,5-(CH ) pz)(COD)b were grown C, dried c 26 H 38 N 4 Iri C, Calcd Found: This ethanol. 3 2 of the with desired g (91 %). from Anal. 7.08. needles product 7.10. Large by slow evaporation of a 1:1 (v /v) dichloromethane/ethanol solution. Bis( 1,5-cyclooctadiene )bis(IJ -3,4,5-trimethylpyrazol yl)diiridi um(I), and bis( 1,5-cyclooctadiene )- bis(IJ -3-( tr ifl uorometh yl)-5-meth ylpyrazolyl)diiridi um(l), 5-CH 3 pz)(COD)b, Sto bart, were a!. 2 0 et prepared using by Florisi1 procedures as the the eluting solvent, dichlorometha ne/ ethanol solution as benzene Since the as compounds were done in the air. N, 6.84. Found: C26H32N4F 61ri C, C, are air-stable [Ir(IJ-3-CF 3to that chromatographic a and the of material, 1:1 (v/v) recrystallization solvent. solution, in similar the reaction work-ups Anal. Calcd for C28H42N4Ir2: C, 41.06; H, 5.17; 40.30; H, H, 3.59; 34.74; 4.92; N, N, 6.23. 6.65. Found: Anal. C, Calcd for H, 3.64; 34.81; N, 6.26. Bis( 1,5-cyclooctadiene )bis(IJ -3,5- bis( trifl uoromethyl)pyrazolyl)diiridium(!), Stephen R. Stobart and used as received. was obtained from Professor 27 Bis( 1,5-cyclooctadiene )bis(IJ -pyrazolyl)( diiodide )diiridi um(II), [Ir(IJ·Pz)(COD)(I)] 2 , under an argon was prepared atmosphere solution of iodine (0.037 transferred (0.1 00 via g, 0.136 tetrahydrofuran (COD)(I)]2 began to hexane (30 The air-stable mL), and dried C22H30N 4 I2Ir2: in deep stirring purple mL) was was under C, a precipitate. solid added filtered, H, (20 Upon mL). small washed N, techniques et a/. 2 1 ' 2 2 of A [lr(IJ·Pz)(COD)] 2 crystals solution further Yield: 3.06; Stobart, solution the to Schlenk tetrahydrofuran (5 mL) was and After vacuum. 26.72; of g, 0.146 mmol) in mmol) turned standard method to solution min, the cannula reaction 15 by using well mixing, of was precipitate the [Ir(1J·pz)- stirred the for product. with hexane (4 x 10 (89%). Anal. Calcd for H, 3.03; 0.120 g 5.67. Found: C, 26.88; N, 5.84. Bis( 1,5-cyclooctadiene )bis(IJ -pyrazol yl)( di bromide )diiridi um(II), [Ir(IJ·pz)(COD)(Br)) 2 , under an argon was prepared atmosphere pz)(COD)(I))2.2 1 A using standard Schlenk techniques a method analogous to that for of bromine 0.1 mmol) by solution (5 1-1L, [Ir(IJin tetrahydrofuran (5 mL) was added to a stirring tetrahydrofuran (20 mL) solution of [Ir(IJ·pz)(COD)] 2 reaction solution darkened precipitate. The reaction min and hexane (30 (0.075 g, and solid was 0.102 stirred mL) was added The air-stable, brown solid was under vacuum. Yield: 0.078 g (85%). mmol). Upon [Ir( IJ·pz)(COD )(Br )] 2 at to washed ambient further well Anal. mixing, the began to temperature precipitate the with Calcd hexane for for 15 product. and dried c 22 H 30 N 4 Br 2 Ir 2: C, 29.53; H, 3.38; N, 6.26. Found: C, 28.74; H, 3.22; N, 6.00. Bis( 1,5-cycloocta d iene) bis( 1-1 -pyrazol y 1)( dichloride )diiridi um(II), 28 [lr(~-pz)(COD)(Cl)b, tolysis was prepared [Ir(~-pz)(COD)] of 2 by the method of in 1,2-dichloroethane. C, 32.08; c 22 H 30 N 4 Cl 2 Ir2 • 0.75 C 2 H 4 Cli H, 3.77; Caspar 2 8 by pho- Anal. Calcd for 6.39. Found: C, N, 31.82; H, 3.60; N, 6.72. Bis( I ,5-cyclooctadiene)bis(~ -pyrazolyl)dirhodi um(I), (COD)] 2 , was obtained from Dr. Jonathan V. [Rh(~-pz)- Caspar and used as received. Physical Measurements Materials. Solvents emission, for transient measurements five the spectroscopic absorption, absorption, resonance Raman) purified, necessary, degassed were freeze-pump-thaw if cycles < 1o- 3 torr), and storage flasks equipped stored over ICN Nutritional either (electronic on a bulb-to-bulb with alumina high-vacuum distilled Teflon (Woelm Biochemicals), and with line into vacuum emission a valves. Iif etime minimum (limiting glass electronic of pressure round-bottomed The solvents were N, Activity Grade 1, obtained from Linde 4A molecular sieves (Union Carbide), or sodium metal (Baker)/benzophenone (MCB). The alumina and molecular sieves were activated by heating under dynamic vacuum (< 103 torr) for cyclohexane Jackson) 24 h. (Aldrich, were used Acetonitrile (Burdick and Jackson, UV 99+%, La bel), and benzene (Burdick as Gold received and stored over and molecular Tetrahydrofuran (EM sieves, and sodium/benzophenone, Science) was distilled under nitrogen from calcium 95+%, -40 mesh) and stored over sodium/benzophenone. . 2- the for Methyltetrahydrofuran (Aldrich), respectively. alumina, Grade), purified by hydride method (Aldrich, outlined 29 tetrahydrofuran, 2 4 was cuprous chloride (Allied), followed by hydroxide pellets "(Baker), and distilled before first vacuum-transfer deep purple-blue onto color tetrahydrofuran, distilled of and from then a 0.5 % distillationfrom sodium/benzophenone. the suspension benzophenone from potassium calcium hydride The characteristic in the benzene, storage flasks ketyl 2-methyltetrahydrofuran of solvents' signified that the solvents were deoxygenated and dry. For the measurement molar extinction stable in under nitrogen of several coefficients solution, either from of of the the iridium tetrah ydrofuran calcium hydride absorption or spectra and the dimers that are air- was freshly that cyclohexane that distilled was from a freshly opened bottle of Gold Label grade were used. Ruthenium tris(2,2 / -bipyridine) (bpy)3](PF 6 )2 , was obtained from tallized a solution of from di(hexafluoroph.o sphate), Dr. D. Michael acetonitrile [Ru- Hopkins and and Jackson) (Burdick recrysand toluene (Baker) before use. Electronic Absorption Spectroscopy. Electronic absorption spectra or a Hewlett-Packard of solutions torr) in quartz on a of a I 0 mL cuvette, and Teflon vacuum ate storage to the measurements, flask to a cell. Several were measured the solvent calibrated spectra of of Spectra were obtained < 10- 3 pressure Pyrex bulb, a em pathlength For molar extinction valve. distilled volumetric samples 17 (limiting was the Cary line high-vacuum consisting a using either a spectrophotometer. a cell coefficient solution prepared 8450A were measured complexes prepared from cylinder that in the the appropri- before transfer are air air-stable in in em 30 pa thlength quartz cu vettes. The absorption spectra of these complexes an emission in aerated solvents are identical to those in degassed solvents. Electronic Emission Spectroscopy. Electronic emission spectrophotometer constructed viously. 5 Spectra recorded solutions that < 1 o- 3 torr) pathlength were in quartz at at prepared a were measured using Cal tech that been spectra cell ambient on a consisting fluorescence has temperature high-vacuum of cell, 10 mL and a Teflon pre- obtained for were line a described (limiting Pyrex bulb, vacuum pressure a em valve. For the measurements at 77 K, solutions were prepared in the same manner in a glass NMR tube attached to a Teflon bulb-to-bulb distilled from appropriate the were used tion of for into the cell solvent the quantum [Ru(bpy) 3 ](PF 6 ) 2 , Solvent was or tube containing the compound storage flask. Highly dilute solutions yield which vacuum valve. measurements. 2 5 has a quantum was used An acetonitrile soluyield for emission of •, 0.062 at temperature, 2 6 ambient as the standard. For these measurements, the excitation wavelength was the 436 nm Hg line from a medium-pressure Hg/Xe interference filter. tubes held were emission For in intensities point-by-point a arc. lamp that was the K emission 77 filtered liquid-nitrogen-filled were multiplication corrected of for analog with . an Oriel 5645 measurements, the NMR dewar. The response by quartz finger spectrometer spectra by the appropriate correction factors. 2 7 Emission Lifetime and Transient Absorption Measurements. Emission lifetime measurements were conducted with a Nd:Y AG pulsed laser system that has been described previously 2 8 using 532 nm excita- 31 tion. The solutions for the prepared by the procedure solutions for the variable measurements at used ambient for the emission temperature and 77 K temperature were measurements. The lifetime measurements were prepared in NMR tubes by the same procedure. All of the emission intensity at lives. decays ex hi bi ted the measurements For liquid-nitrogen-filled ture measurements kinetics at the NMR tubes were dewar. For the variable tempera- continuous-flow nitrogen quartz 77 K, finger between gas dewar was used. bra ted first-order 190 and 300 K, over a least 3 ha If- held in a The sample temperature was monitored with a cali- copper /constantan thermocouple. The laser was opera ted at a low power output to minimize local sample heating. The transient absorption laser system with experiments were performed using in described changes the optical where. 2 8 The spectra were obtained region of the spectrum except at nm). The solutions pressure < 10- 3 torr) in a em quartz cuvette, and on a near Teflon vacuum in the the laser high-vacuum line cell consisting of a a as point-by-point wavelengths prepared were train 10 mL Pyrex by valve the same elsevisible line (532 (limiting bulb, a bulb-to-bulb 1 dis- tillation of the solvent into the cell containing the iridium complex. Resonance Raman Spectroscopy. Resonance Spex 14024 ings, a Raman spectra spectrometer thermoelectric obtained equipped cooled at Caltech with Hamamatsu 2400 were lines/mm R955 recorded with holographic photomultiplier a gra t- tube, and a Spex SCAMP data system. Laser excitation was provided by a Spectra- Physics 171-18 (Ar+) laser. Typical scans. Spectra were recorded of signal averaging used solutions prepared on 9, a 16 or 25 high-vacuum 32 line pressure (limiting equipped with a 1o- 3 < Teflon in a by distillation iridium dimers torr) vacuum valve pathlength cell of solvent into improved re- cm- 1) were mm the cell from the appropriate storage flask. Resonance solution at Raman low spectra frequencies of (for the .611 between 50 with and 100 measured by Dr. Edward M. Kober at Los Alamos National La bora tory. Additional Measurements. Elemental Caltech (Cl)]2 analyses Analytical was Michigan. obtained were obtained Laboratory. The by Spang by Mr. Larry Henling analysis for [Ir(COD)- Laboratory, Eagle Harbor, elemental Microanalytical at the 33 Results and Discussion synthesis The and bridged plexes and thermal chemistry substituted-pyrazolyl-bridged have been investigated in Stobart and through extensive studies of carbon monoxide, 1,5-cylooctadiene, complexes metal-metal demands of can be several of these are interaction diene ), reveals that a of separation number of dimers, the and trifluoromethyl because of this of 2 A, as groups, have repulsions ligands bridges. 2 0 ' 3 3 Table and 2.1 The 1A pareffort containing by since the the steric crystal complexes consistent ground 1 this interesting X-ray short, In ligands. 1 8 - 2 2 , 3 2 -a 8 dramatically pyrazole, adopts a structure reveal with state. that a weak The struc- shown the Figure ligands that heterocyclic shorter between the lists with in even the an similar 2.4.2 1 ' 3 3 are ring hydrogen on iridi um-iridi urn to iridium-iridium Related su bsti tu ted by methyl metal-metal substituents the 1,5-cycloocta- COD conf or rna tion complexes 3 9 pyrazol yl of to com- bis( 1,5-cyclooctadiene )bis(~-pyrazolyl)- complex 5-positions cyclooctadiene the (pzH containing steric in "A-frame" 3.216(1) ir idi urn 3- that rather pyrazolyl- complexes phosphine binuclear of [Ir(~-pz)(COD)] these influenced of distances of especially ligands. 2 0 , 3 3 bonding diiridium(l), are of iridium(!) contributed and bridging characterization tural number the iridi um-iridi urn metal-metal a variety Ia bora tories. 2 9 - 3 1 have cyclooctadiene separation determinations the of co-workers a binuclear several ticular, The of in or separations atoms of the pyrazolyl distances several of these complexes that have been structurally characterized. the of 34 Figure 2.4 Structure of [Ir(1J-pz)(COD)] 2 . 35 36 Table 2.1 Metal-metal distances for pyrazol yl- bridged binuclear iridium(!) cyclooctadiene complexes. Iridium Complex Ir-Ir Distance (A) Reference [Ir(J.l·pz)(COD)b 3.216(1) 21, 33 [Ir(l.l-3,4,5-(CH 3 ) 3 pz)(COD)] 2 3.096( 1) 20 [Ir(l.l-3-CF 3 -5-CH 3 pz)(COD)] 2 3.066( 1) 33 [Ir(J.l-3,5-(CF 3 )2 pz)(COD)b 3.073( I) 33 pzH = pyrazole 3,4,5-(CH 3 ) 3 pzH = 3,4,5-trimethylpyrazole 3-CF 3 -5-CH 3 pzH = 3-(trifluoromethyl)-5-methylpyrazole 3,5-(CF 3 ) 2 pzH = 3,5-bis(trifluoromethyl)pyrazole COD = 1,5-cyclooctadiene 37 The four iridium complex that has these The five coefficients are listed plexes qualitatively in in the similar; in solution purple. iridium The complexes analogy by and molar 2.2. 4 0 The spectra of intense the band observed for all of sponding triplet 1A 1 ~ of bands to these bridge, from splitting those between these 3B around • metal-metal shorter most likely the higher-energy is these com- 498 529 to 2 (da observed band • ~ for to the correshift substitution of the is with and consistent pyrazolyl the a 1(pa) orbitals for the The features at higher pa transitions, separations. d1r(xz,yz) ~ spectral features of the as rhodium(!) the energy with 2, pa) lower at assigned in extinction general to correspond and shown The energy b 2 (da ) features weak 1B ~ a bsorp- ~ pa) transition. [Ir(~ -pz)(COD )] of A complexes (da • 2 lower the with spectral dimers.1 -8 ,s ,1-1 o ,1 8 "face-to-face" d8-d8 is similar to are maxima nm in each spectrum is logically assigned to the lA 1 transition, electronic band Table [lr(~-3,5- characterized, and binuclear corresponding similar 1,5- deep of a = pink spectra and COD from to 2.1 3,5-dimethylpyrazole, colored tion are = intensely ranging Table structurally all state, 2.5. been in are solid Figure not listed (3,5-(CH 3 )2 pzH (CH 3 ) 2 pz)(COD)] 2 cyclooctadiene), complexes smaller complexes energy proposed and for iridium(!) isocyanide complexes. 7 -g These binuclear iridium(!) phosphorescence at 1B (da *pa) 2 3 B (da *pa) 2 spectra are and shown quantum yields and ambient in complexes exhibit temperature excited Figure 2.6, in both fluid fluorescence solution from and the The emission corresponding emission states, respectively. and the band maxima for the luminescent excited states are 38 Figure 2.5 Electronic (b) absorption [Ir(IJ-3,5-(CH 3 ) 2 pz)(COD)] 2 , pz)(COD)]2, (d) of (a) (c) [Ir(1J-pz)(COD)] 2 , [lr(IJ-3,4,5-(CH 3 ) 3 - [Ir(IJ-3-CF 3 -5-CH 3 pz)(COD)] 2 , 3,5-(CF 3 ) 2 pz)(COD)b 2°C). spectra (tetrahydrofuran (e) solutions, [Ir(IJ22 ± 39 --------------------------------------------~-. ~~ L I le£8 ,.... E ....,c ~ l ee; ~ I I I I I I I I i i~t I I ~I I I I I I I j_ II I r l i I i Sl (T') c-J s C"J -. m an . m 3JNVB~osev _j_ lliil In 83 s 155 Ietz ffi ~< :. 40 0Z8 'E ..5 :r: ~ t:l CE w > < _I 2~; =- I I Llt I raE ~~~ --~~-----~~-----~+------~~-----~~-----~~-----~._-----~~~2 ~ ~ ~ ~ N ~ -· m ~ 3JNV9~0S8V ~ ~ ~ ~ N ~ - ~ 41 (.) 3JNVBHOS8V 42 0 I B2L t~g i L ,.... E ~ :r ~ t!) :z. LL.! ~; · BeE l I ~ cs:i ~NVB~OSBV t5) t5) N ui ~ 0tZ £:! > < :a 43 UJ ezL. I I I J ez3 'Et: ....... .--L"l :I: z w a~; I i LL -- .L.......cc=--_....._ _ _ _~ ---~ -. -. .. -. 3:Jt-N8~0S8V rj > < :. 44 Table 2.2 Electronic absorption spectral data for pyrazolyl- bridged binuclear iridium(!) cyclooctadiene complexes. Iridium Complex [Ir(~J -pz)(COD )]2 585 150 498 8100 381 2500 274 13,600 625 180 528 9700 390 1500 274 10,700 630 180 529 9800 392 1700 277 11,800 610 180 524 10,900 390 1800 276 13,200 45 a. The band maxima of the are difficult to 1A determine 1 610 160 522 9700 388 1600 266 13,600 -+ 3B 2 transition for precisely because were measured of the these complexes low intensity and broad shape of the bands. b. The ex tinction coefficients solutions of the complexes and are accurate to :!: 5%. for tetrahydrofuran 46 Figure 2.6 Corrected emission spectra [Ir(JJ -3,5-(CH 3 )2 pz)(COD)] 2 , (COD)]2, (d) of (c) 3 3 (a) [Ir(J,J-pz)(COD)] 2 , (b) [Ir(JJ -3,4,5-(CH 3 ) 3 pz)- 2 [Ir(~-.~-3-CF -5-CH pz)(COD)] , (e) (CF 3 )2 pz)(COD)] 2 (acetonitrile solutions, 22 :!: 2°C). [Ir(JJ-3,5- 47 8 Cl) E c ...< ~----------------------~8 t{) 48 0 0 co 0 0 1'- E c: ,.<_ 0 0 lO 49 0 0 ro --E c ...< 0 0 <..0 0 0 L[) 50 8 (l) 0 0 f'- E c ...< 8tO L---------------------------------------------------~ 8 L{) 51 w 0 0 00 0 0 1'- E c ,<.. 8 lO 0 ~--------------------------------------~0 L[) 52 listed in 2.3. 4 1 Table The small Stokes shifts between the absorption lifetimes (vide infra), and the properties of and emission bands ( 1A 1 em - 1), 2500 ilarities excited-state between the physical and the 1, 3 A u(da *pa) 2 dimers of rhodium(!), of the states d8 the these luminescent excited states of the iridium(!), and platinum(II) excited states of dimers are sufficiently changes in simexcited d 8- "face-to-face" support this assignment. Several iridium(!) solution state 3 B (da *pa) 2 cyclooctadiene to by transient observe the nanosecond absorption laser The lifetimes of the the spectra 3 A u(da *pa) 2 of blue of the spectra pyrazolyl-bridged long-lived from in ground at ambient temperature. of [Ir(1J-pz)(COD)] 2 and [Ir(IJ- Figure 2.7. solutions are shown in transients coincide with the lifetimes excited these two state where two ground-state states. complexes The are transient very [Rh 2 (TMB) 4 ]2+ of broad singlet features da fluid the benzene excited 2,5-diisocyanohexane) the in 3 B (da *pa) 2 phosphorescent difference spectroscopy difference 3,4,5-(CH3)3pz)(COD)]2 absorbance the similar The of the absorption to that of (TMB 2,5-dimethyl- are observed also • -+ pa absorption.• 2 to Because of problems in measuring the changes in absorbance at wavelengths near the laser not measured for the transient cyclohexane at - excitation A. (532 > 480 nm. absorption solution nm), over the of [Ir(IJ-PZ)(COD)h was Recently, Dr. Jay R. Winkler measured difference a spectrum larger spectrum wavelength of this complex range, and a in bleaching 500 nm that corresponds to the singlet da * -+ pa transition of the ground state was observed. 4 2 53 Emission band maxima and quantum yields for the fluores- Table 2.3 1B (da *pa) 2 cent states of and phosphorescent pyrazol yl-bridged 3 B (da • pa) 2 excited iridium(!) cyclo- binuclear octadiene complexes. Emission Band Maxima (nm)b Quantum Yieldsc Iridium Complexa (~ -pz) 558 684 0.0001 0.0078 (~-3,5-(CH ) pz) 579 715 0.0001 0.0038 (~-3,4,5-(CH 3 ) 3 pz) 580 717 0.000 I 0.0035 3 -5-CH 3 pz) 567 705 0.0003 0.0003 579 705 0.0006 0.0006 3 2 (~-3-CF (~-3,5-(CF a. The 3 )2 pz) complex designations are abbreviated and represent [Ir(~­ pz)(COD)]2 and the substituted-pyrazolyl analogues. b. The emission band maxima and quantum yields were measured of acetonitrile solutions of these complexes. c. An yield acetonitrile for solution emission of of [Ru(bpy) 3 ](PF 6 )2 , 0.062, 2 6 numbers are accurate to ± 20%. was used which has as standard. the a quantum The 54 Figure 2.7 Transient absorption difference pz)(COD)b and [Ir(IJ-3,4,5-(CH 3 ) 3 pz)(COD)] 2 (b) spectra of solutions, 22 :t 2°C, [Ir 2 ] = 10- 4 M, i\ex = 532 nm). (a) [lr(IJ- (benzene 55 0 0 1.0 • • • 0 • ~1.0 v • • • E c ..< • -.,... 0 • ~0 v • • • • • --• I 1 N 0 T Q 0 0 aJUDq .J OS q'\7' 'J 0 0 1.0 1'0 56 0 ~--------------------------------------------------------~r- I() 0 • • • 0 • ~I() v • • - • E ..5 • 0 • ~o v • • • • 0 ~--~ • • • I I I !Q ~ I() 0 0 88UDq.JOSq'\1 'J 0 0 ..-< 57 The states emission intensities of of these pyrazol yl- bridged dependent. Upon cooling bands sharpen and yields increase, state, in 1B (da • pa) 2 the iridi urn 3 B (da • pa) 2 and complexes to 77 K, the fluorescence shift to and the particular, shows higher phosphorescence a rected emission spectra of (COD)] 2 , measured at K, 77 slightly dramatic and from the temperature temperature phosphorescence energies. [Ir(JJ-pz)(COD)h are are excited The emission triplet excited dependence. Cor- and representative of the spectral changes observed for all of the complexes at 77 K and are shown in Figure 2.8. The is enhancement paralleled in by the lifetime between iridium(!) complexes. of the the of the 3 B (da • pa) 2 excited increase in the excited-state luminescence substantial ambient triplet temperature and 77 K The lifetimes of the 3 B ( da • pa) 2 pyrazolyl-bridged iridium(!) dimers in for state these five excited states at ambient solutions temperature and glasses at 77 K are listed in Table 2.4. Both the emission yields excited states of between ambient temperature dependence constant the of for the for lifetimes nonradiative temperature states these the lifetimes of complexes increase dramatically and K, that temperature 77 implying primarily due to decay (k 0 r). In light of the lifetimes of of [Ir(J..l- and [Ir(J..l-3-CF 3 -5-CH 3 pz)(COD)h, (T obs), the quantum the yield relationship for emission between (t em), observation, [Ir(J..l-pz)(COD)b, 3,4,5-(CH3)3pz)(COD)]2, From rate excited iridium detail. the 3 B ( d a • pa) 2 the complexes, in this the in the variations of amined 3 B (da • pa) 2 the binuclear is dependences three and the and have observed the for radiative and nonradiative decay (kr, knr) of an excited state, rate been ex- lifetime constants 58 Figure 2.8 Corrected (b) emission spectra [lr(IJ-3,5-(CF 3 ) 2 pz)(COD)] 2 hydrofuran glasses). of (a) at [lr(IJ-PZ)(COD)] 2 77 K and (2-methyltetra- 59 0 ~----------------------------------------------------------.0 co E c 0 0 <..D L_______________________________________________________ ~ 0 0 1.{) 60 m~--------------------~--------------------.8 ro E c ....< 0 0 <..0 )( 0 1.{) 0 ~------------------------------------------------------------~0 1.{) ,<;~ISU8~U I UOISSIW3 61 Table 2.4 Lifetimes of the 3 B (da • pa) 2 excited states of pyrazolyl- bridged binuclear iridi um(I) cyclooctadiene complexes. Iridium Complex [Ir(IJ -pz)( COD )]2 T 245 ns (2-CH 3 THF) (77 K)b 2.68 IJS 250 ns (C 6 H 12 ) 280 ns (C 6 H 6 ) 250 ns (CH 3 CN) 640 ns (solid) 80 ns (2-CH 3 THF) 1.57 IJS I 00 ns (2-CH 3 THF) 1.52 ~s [Ir(IJ -3,4,5-(CH 3 ) 3 pz)(COD)h 100 ns (C 6 H 12 ) 100 ns (C 6 H 6) [Ir(IJ -3-CF 3 -5-CH 3 pz)(COD)]2 < 20 ns (2-CH 3 THF)c 1.73 ~s < 20 ns (2-CH THF)c 1.80 ~s 3 < 20 ns (C H 6 12 )c < 20 ns (CH CN)c 3 100 ns (solid) a. The lifetimes were measured in a variety of solvents (2-CH 3THF 2-methyltetrahydrofuran, C 6 H 12 = cyclohexane, CH 3 CN = aceto- 62 nitrile, C 6 H 6 = benzene) and are accurate to :!: 5%. b. The lifetimes were measured in 2-methyltetrahydrofuran of excited-state glasses and are accurate to :!: 5%. c. The measurement limited. this Iif etime is instrument- 63 given in equations (I) and (2), the values of kr and knr can be calcuIa ted for the triplet excited states of these three of the 3 B (da *pa) 2 complexes and are excited states are listed in Table 2.5. The lifetimes (T obs) approximately equal (1/knr), dependence of T obs be tion observed fit to and the data for the temperature for these three complexes, shown in Figure 2.9, can to a modified-Arrhenius (3). In this expression, expression knro and of the form given knr 'exp(-Ea/kbT) in equa- represent the nonradiative rate at low and high temperatures, respectively. The term Ea radiative the is the pathway calculated activation leading values [Ir(~-pz)(COD)b, of energy for the thermally-accessible non- to deactivation of the excited state, and Ea for 3 B (da *pa) 2 excited states of [lr(~-3-CF 3-5- the [Ir(~-3,4,5-(CH ) pz)(COD)]2, 3 3 and CH 3 pz)(COD)] 2 are 2600, 2500, and 2300 cm- 1, respectively. Although excited havior of the the state has been mechanism is not observed binuclear for understood, for rhodium(!) the the deactivation similar the 3 B (da *pa) 2 temperature-dependent 3 A u(da *pa) 2 bridging of excited state isocyanide be- of several complexes. 2 ,s In 64 Table 2.5 Calculated values (knr) decay states . of rate of the radiative constants selected for (kr) the and nonradia ti ve 3 B (da • pa) 2 pyrazolyl-bridged binuclear excited Hl- di um(l) cyclooctadiene complexes at 22 :!: 2° C. Iridium Complex [Ir(1J-pz)(COD)]2 3.1 X 10 4 4.0 X 10 6 [lr(IJ-3,4,5-(CH 3 ) 3 pz)(COD)] 2 3.5 x 1o4 1.0 X 10 7 1.5 x 1o4 > 5.0 X 10 7 [Ir(IJ-3-CF 3 -5-CH 3 pz)(COD)] 2 > 65 Figure 2.9 Arrhenius times for plots of the 3 B (da • pa) 2 [Ir(1J-pz)(COD)] 2 (ll), excited-state Iif e- [Ir(IJ -3,4,5-(CH 3 ) 3 pz)- (COD)] 2 (X), and [Ir(IJ-3-CF 3 -5-CH 3 pz)(COD)] 2 (•). 66 )( . <3ig • • <l 0 l() • • • • L{') <l • ...... )( ' )( <l • )( )( • • X <l )( • <l q ~ )( )( <l )( )( <l )( <l )( )( <l )( )( )( )( <l <l 0 ~ )( • • ,.., <l L{') r<i 67 contrast, the lifetime of excited state ambient temperature the of [Ir 2 (TMB) 4 ]2 + is and 77 K. 7 A nonradiative pathway that has been suggested to account for observations these excited state temperature independent involves between deactivation of the accessible d-d excited each metal through a thermally equilibrium geometry is D 2 d-distorted Furthermore, the temperature-independence 3 A u(da * pa) 2 state of deactivating state would around [Ir 2 (TMB) 4 ]2 + be much of the state this higher energy, whose center. 2 ,s ,4 3 lifetime supports in phosphorescent proposal, of the since this relative to the 3 A 2 u excited state, for iridium than for rhodium. The mechanism excited state related to of the that Alternatively, of the thermal pyrazolyl-bridged suggested this for deactivation the may activated delayed fluorescence. 4 4 '" 5 for the excited states activated delayed fluorescence the experimentally of certain measured complexes. occurring through thermally- This process been documented take has molecules.• 6 organic place activation in the energy For thermally- iridium complexes, the 3 B (da * pa) 2 for singlet-triplet in reasonable states of these temperature in solution in this state. correspond 2500 be be cm- 1 ' spectroscopic of isocyanide these from order rhodium For obtained the could data. should 3 B (da * pa) 2 the complexes the state on to of iridium to excited is deactivation complexes, splitting this splitting agreement with the complexes are very accord with the low Picosecond laser spectro- measured activation energies. The 1B (da*pa) 2 short-lived at excited ambient fluorescence quantum yields for scopic studies by Jay Winkler Dr. R. have iridium shown that the lifetime of 68 the 1B (da *pa) 2 excited state of [Ir(~-pz)(COD)] 2 is < 20 value is. much lower than that observed the lifetimes 1A u(da *pa) 2 [Rh 2 (TMB) 4 ]2 +, ns; 2 much state ([Rh 2 (bridge) 4 ]2 + dimers 1.1 excited lower T( fluorescence for two (bridge = 1A u) 2 of for the rhodium quantum yield. For the the 1B are similar, the dimer (0.000 1) implies that the is the of ps, given state on order d8-d8 2 if [Ir(~ -pz)(COD )] excited the T( 1A u) instance, 1A u state of [Rh (TMB) ]2+ and 2 2 4 iridium of = = 820 ps)• 2 but is consistent with the the the This isocyanide 1,3-diisocyanopropane), decay rates for 2 ps.• 2 lower 2 fluorescence radiative 2 state of quantum lifetime of that 1 the yield for the 1B 2 A2u excited dimers state of [Rh 2 (TMB) 4 ]2 + has a lifetime of 820 ps and +em = 0.046.• 2 Resonance Raman [Rh 2 (bridge) 4 ]2 + metal-metal ground of these bands corresponding these dimers complex to results vibrations, and interaction at Raman strong metal-metal the ct 8-ct 8 have shown that a is present in 1A g(da) 2(da *) 2 1 the complexes.• ' 6 binuclear substantial Excitation 1A g 1 the metal-localized in resonance-enhancement 1A u 2 -+ of the in to the transition in metal-metal which are observed at 79 cm- 1 for the cm- 1 for [Pt 2 (pop) 4 ]4 -. In addition, time-resolved 118 resonance of studies [Pt 2 (pop )4 ]4 - and bonding state stretching spectroscopic studies of these complexes bonding have confirmed interaction exists studies the rhodium that in a the 3 A u(da) 2 (da *) 1(pa) 1 excited state.• ' 6 2 Resonance iridium(!) interaction symmetry Raman spectroscopic cyclooctadiene is also complexes present d 8 -d 8 dimers. in Excitation suggest of that the ground in to the a metal-metal state bands pyrazolyl-bridged of these corresponding bonding lowerto the 69 metal-localized of vibrations at iridium stretch. in resonance the upon excitation metal-ligand results in may assigned transition low frequencies In addition, Raman into several spectra this stretching that other are bands and The are likely resonance (CF 3 ) 2 pz)(COD)] 2 in tional La bora tory plexes in Recently, Dr. Edward M measured the tetrahydrofuran resonance solutions and in Figure tetrahydrofuran benzene bee a use the candidates for spectra of [Ir(IJ-3,5- at Caltech and are Raman spectra obtained much of these better comresolu- The resonance Raman [Ir(iJ-3,5-(CH 3 ) 2 pz)(COD)] 2 tetrahydrofuran 1n observed Kober at Los Alamos Na- of shown are and tion in the low-frequency region of the spectrum. spectra iridium- Raman benzene solutions were measured shown in Figure 2.10. the resonance-enhanced [Ir(IJ-3,4,5-(CH 3 ) 3 pz)(COD)] 2 , [I r(1J-pz)(COD)] 2 , to that less-strongly transition vibrations. be resonance-enhancement solutions were also measured, and 2.11. 4 1 For the spectrum of the spectra of the the and solutions, of the Rayleigh spectra show a these spectra are [Ir(iJ-pz)(COD)] 2 in three iridium low-frequency features are poorly-resolved sea ttering of the line. Also, baseline slope because of emission to spectra of the substantial laser complexes all in of from the fluorescent excited state . Several iridium features complexes that are are common suggested to correspond iridium-alkene, and iridium-nitrogen listed Table 2.6. The is consistent in frequencies relative with the the to stretching trend in metal-metal the the binuclear iridium-iridium, are vibrations and metal-metal stretching separations plexes that have been structurally characterized. Specifically, the in the com- 70 Figure 2.10 9 I o- 4 496.5 nm, I6 tion; (b) [Ir(~-3,4,5-(CH ) pz)(COD)] , scans, of [Ir(~-pz)(COD)]2, Raman 9 spectra (a) Resonance (Ir 2 ] 3 3 [Ir 2 ] scans, = 7 x 3 3 2 (Ir(~-3,4,5-(CH ) pz)(COD)] , (lr 2] X 7 M, X I o- 4 benzene M, 2 M, i\ex Io- 4 M, benzene i\ex = 5I4.5 = benzene soiu- = 496.5 nm, solution; (c) nm, I6 [lr(~ -3,5- solution; 5I4.5 I6 scans, (Ir 2 ] = 6 x solution; nm, (d) scans, benzene (e) (COD)] 2 , i\ex = 496.5 nm, 25 scans, (Ir 2 ] = 6 x benzene solution. i\ex I0- 4 M, 71 < 0 ~~------~------~~-----r--~--~----~--r-----~~--~----0 'It 0 0 f'() I E ~ 0 0 N 0 Q 1 :::::0 <J 72 0 0 !'<") I E ~ 0 0 N 0 Q 1:::. <J 73 0 0 f(') I E ~ 0 0 N 0 Q I~ <J 74 0 0 f(') I E ~ 0 0 N 0 Q ·~ <J 75 0 0 ~ I E ~ 0 0 N 0 Q I~ <J 76 Figure 2.11 Resonance Raman 488.0 35 nm, spectra mW, 3,5-(CH 3 ) 2 pz)(COD)] 2 , tetrah ydrofuran (COD)h, ?\ex tion; (d) 30 mW, = 514.5 (a) [Ir(IJ-pz)(COD)h, tetrahydrofuran ?\ex nm, (c) 30 mW, [lr(IJ-3-CF 3-5-CH 3 pz)(COD)] 2, tetrah ydrofuran ?\ex = solution; (b) [lr(IJ- nm, 30 mW, 514.5 solution; (CF 3 ) 2 pz)(COD)b, ?\ex solution. of [lr(IJ -3,4,5-(CH 3 ) 3 pz)tetrah ydrofuran solu- = nm, Aex solution; (e) 514.5 [Ir(IJ-3,5- = 514.5 nm, 30 mW, tetrahydrofuran 77 A , l \ 50 100 150 200 250 6ii(cm- 1) 300 350 400 78 B - >. ·v; c (l) .£ (l) > ~ (l) 0:: 50 100 150 200 250 6 ii (cm- 1) 300 350 400 79 c 50 100 150 200 250 6ii(cm- 1) 300 350 400 80 D Q) > :2 Q) 0::: 50 100 150 200 250 ~ii{cm- 1 ) 300 350 400 81 E >. "in c: Q..) E Q..) > 50 100 150 200 250 6v(cm- 1) 300 350 400 82 Table 2.6 Vibrational Raman stretching spectroscopy frequencies for measured pyrazolyl-bridged by resonance binuclear dium(!) cyclooctadiene complexes. v (Ir-(7r-C=C)) Iridium Complex (em -I) [Ir(~-pz)(COD)b 58 263 337 [Ir(~ -3,5-(CH 76 264 336 77 266 343 76, 79 262 338 80 259 333 3 ) 2 pz)(COD )b [Ir(~-3,4,5-(CH [Ir(~-3-CF 3 ) 3 pz)(COD)] 2 3 -5-CH 3 pz)(COD)b [Ir(~-3,5-(CF 3 ) 2 pz)(COD)] 2 Iri- 83 3,5-d isu bsti tu ted-pyrazol yl-bridged Ir) 76 from quency 80 to for complexes whereas the is 58 [Ir(1J-pz)(COD)] 2 at all have values iridi um-iridi urn cm- 1, consistent v (Ir- of stretching fre- with sub- the stantially longer metal-metal separation in this complex. The spectrum closely-spaced of the features to diastereomers that The structure crystal at the correspond complex cm- 1 76 [Ir(IJ-3-CF 3 -5-CH 3 pz)(COD)] 2 shows cm- 1 likely and iridium-iridium arise from the 79 stretching asymmetry that vibrations in the this complex is CH 3 ; cF 3 site occupancy, 8 8 and this has been suggested co-crystallization diastereomers. and emission similar the spectral since no properties unusual disordered The of these are observed effects for with the ligand. 2 0 bridging of of most respect to result to from electronic absorption must extremely isomers in the be spectra and the emission lifetime decays are all single-exponential. The weak feature at - 2 v(Ir-Ir) that is observed in the resonance Raman spectrum of each of these complexes is probably the first overtone of the cm- 1 - 260 pyridine frared that is complexes of where bands iridium-alkene assigned at similar the to previous to The feature studies in the assigned to the iridium-nitrogen Likewise, the features at - 340 cm- 1 stretch. For to the stretch have suggested metal-alkene for the stretch.• 9 ?r-bonded allyl are a band Also, group in far-in- stretching reasonable for spectroscopic far-infrared that iridium- of frequencies example, at iridium-nitrogen been [Ir(COD)(Cl)] 2 corresponds is analogy iridium-alkene studies observed vibration. have spectra vibration.• 8 stretching by vibration, stretching the iridium-iridium at the 411 cm-l symmetric Ir(1r-C 3 H 5 )3 has 84 a frequency of 350 cm- 1, as measured by Raman spectroscopy. 5 0 The values v (Ir-Ir) of for these are suggestive of a weak bonding as predicted from configurational interaction b 2 (da ) • orbitals with the Analogous to the ground-state metal stretching less than that are consistent vibration with weak of metal-ligand spectra of these is consistent of for presence mixing unoccupied expected a iridium(!) of the a 1(pa) bonding • b 2 (pa ) interaction. orbitals. of dimers are bond 5 1 - 53 but addition, the In stretching vibrations in pyrazolyl-bridged iridium(!) cyclooctadiene complexes of rhodium-carbon stretching with the observation the the and studies iridium single state, a 1(da) Raman metal-metal complexes 1 ground filled and lower-symmetry full 1A the resonance these a in binuclear resonance Raman vibration in the resonance Raman spectra of [Rh 2 (bridge) 4 ]2 +." As mentioned gated the The complex stra tes, Stobart and co-workers dimers. 1 9 ' 2 2 ,s 7 [Ir(1J-pz)(COD)] 2 is easily oxidized by variety chlorine, and iodine, addition spectra bromine, x2 of The thermally oxidized investi- pyrazolyl-bridged are the are X-Ir(II)-Ir(II)-X intensely colored, in Figure shown with binuclear iodine, bromine, complexes. 7 53 5 ' ' " and sub- products of bonded [Ir(IJ-pz)(COD)(Br )] 2 , and their electronic The d8-d8 complexes also 2.12. chlorine The the of metal-metal [lr(IJ-pz)(COD )(I)] 2 , complexes are and a and react have these [lr(IJ-pz)(COD)(Cl)] 2 absorption 1, o·f dimers. 1 9 ' 2 2 and Chapter chemistry thermal including oxidative in to give doubly- 85 Figure 2.12 Electronic absorption spectra of (1)]2, €(562 nm) = 20,300 ~lcm-1 ' 1em-1 ' tetrah ydrofuran (Br )] 2 , €(484 nm) = 1em-1, [Ir(~-pz)(COD)- €(470 nm) = (b) 12,900 ~ [Ir(~-pz)(COD)- 17,000 M-1 em -1 , €(399 nm) = 8450 ~ (c) [Ir(~-pz)(COD)- 15,100 M-1 em -1 , €(351 nm) = 8670 M- tetrahydrofuran (Cl)] 2 , €(458 nm) = solution; (a) solution; 1em - 1, tetrah ydrofuran solution. 86 < 1-.----.- 1l I 1I I ! ees I I ~ eeL t I I I j I i ~ ees I ~ I ! II ,.... i ...... I :X: E ' I ~ (!) I I I r ~ 00S I I i ! I l I l I I ~ 00t I !I i I I i L LeE i l lSI In s s s sS s t-- c-.. c.c lSI In s s.... s ~t-,'V8~058V ~ s Rl 153 - m s s s I etz z UJ ti1 > < =- 87 tO eee I I I I I I ~ eeL. I I I I I ~ 1ea ~ I I ,... I .....! I i l I I I I ~ I ~ I I I!II!IS j I ~ jm» I I I ! eeE -. s s0') s I s s t'-. m m m m m 3JNVBHOSBV s • m s(T) m Rl m m si IJtZ £&1 > < :. 88 (.) r' eee I i I I I ! I I I - I I B~L. I l I ~ ees I l ,.... 5 ......t ~ (!) I I I 1ees II I lI I I I i i j I nt I I I JHE -. ~ . PJ - . m I ! ...L ~ ISS ISS sS 3JNVBHOSBV re sS ~z •I m ISS ffi ~ =-< 89 and the two bands in the absorption spectra of these dihalide • and d7r' · ~ da • transitions, with the higher-energy feature corresponding to da ~ da •.1 ,s 3 ,s ~ In complexes some have been respects, the iridium(II) ticular, dimers the two broad of bands in from those that the an the the halogen. spectra of the the spectra features in However, of da of the the these da bond relative (Ir-Ir that for separations. 5 3 of the "face-to-face" dimers, such si tion in the pyrazolyl-bridged complexes than the d?f da [Ir(~-pz)(COD)(X)h as • transition. complexes metal-metal da *), orbitals of da localized axial observed this for would not transition is ligand-to-metal the agrees with the shift of this halide ligands. If these of 2 the is A), 2 2 two Given long the for splitting D 4h band symmetry with more be reversed • tran- lower in energy the rather the da would be is case, the ~ da to transfer X-M(II)-M(II)-X ~ be Furthermore, su bstan tiall y transition complexes, 7 ,s 3 to higher energy low • transition in the unreasonable. expected charge relative da that be the dimers could the bands corresponding to the da the [Ir(~- of complexes.5 3 Presumably, those with par- 2 complexes are reversed the complexes of D 4 h from ~ intensities 3.085(3) = d1t" and mixed spectra [lr(~-pz)(COD)(I)] in the this In • orbitals of these c 2 v dimers · should be con- metal(II)-metal(II) energies for complexes. absorption of energies pyrazolyl-bridged the X-M(II)-M(II)-X distance single and siderably less than "normal" of [Ir(~-pz)(COD)(X)] the iridium-iridium the spectra higher-symmetry iridium-iridium between da = I, Br, Cl) shift to lower energy with the reducing pz)(COD)(X)h (X ability absorption resemble ~ to da assigned with (La ~ which less reducing 90 Several other pyrazolyl-bridged The rhodium Trofimenko this observations, iridium(!) dimers of [Ir(~-pz)(COD)] 1971 5 5 and an X-ray recently completed ' complex was [Rh(~-pz)(COD)] [Ir(~-pz)(COD)] , 2 with a In to the iridium contrast 2 The benzene solution is shown in luminescent in fluid solution singlet electronic and at ambient triplet absorption excited temperature. methyltetrahydrofuran studies of above, deserve mention. was by crystal structure determination of by Stobart the the rhodium 2.13. at as of 3.267(2) A. analogue is While ambient the iridium temperature 2 is This complex is luminescent at glass, but the emission is not 77 broad, the pyrazolyl-bridged interactions iridium in the analogous pz)(COD)] 2 are not as strong of the smaller radial relative to iridi urn. spectral properties of dimers. ground and those of as extension of This consistent the is the as the excited states dimers both in a 2- shown in for any metal-metal of [Rh(~- iridium complex because valence with pyrazolyl-bridged in emissive K Presumably the 2 from Figure 2.14, and bears no similarity to that observed at 77 K of lemon- [Rh(~-pz)(COD)] [Rh(~-pz)(COD)] states, conformation separation of co-workers. 3 3 and same spectrum Figure originally the prepared rhodium-rhodium yellow. are 2 adopts dimers, the to outlined analogue in Interestingly, related orbitals the of rhodium very different dirhodium(I) complex and cyclooctadiene com- the pyrazolyl-bridged diiridium(l) complexes. In plexes the summary, the pyrazolyl-bridged iridium(!) appear have the same types of metal-metal low-energy excited states as those ground to and interactions documented d8-d8 binuclear rhodium(!), iridium(!), and platinum(II) complexes. for in the 91 Figure 2.13 Electronic absorption spectrum (benzene solution, 22 :t 2°C). of [Rh(IJ-PZ)(COD)]2 92 .---------------------------------------------------------------------~--8 w 0 0 ~ 0 ~ ~ E c ~ ~ ~ G 0 ~ ~ > 0 ~ 93 Figure 2.14 Corrected emission spectrum of (2-methyl tetrah ydrofuran glass). [Rh(~-t-pz)(COD)h at 77 K 94 - >. . l/5 c Q) c c 0 .Ln (j) E w 500 550 600 650 A (nm) 700 750 BOO 95 Even though the (C 2 v) is plexes (D 4 h), describe the physical properties of higher-symmetry much the symmetry lower the than same electronic Raman studies presence of of the that the metal-metal these the pyrazolyl-bridged of absorption of a of iridium "face-to-face" bonding binuclear interactions and emission spectral complexes. Analogous to binuclear complexes, pyrazolyl-bridged reasonable complexes adequately and studies photofor ground-state two resonance iridium(!) dimers bonding interaction metal-metal com- suggest the in the formally non bonded binuclear center. Because interaction observed of the unique has both a from shape of distance and comparisons of the pyrazol y l-bridged of the IA 1 iridium(!) -+ I, 3 B bands by metal-metal the of localized metal these and and complexes separation, l, 3 B appear expected several pyrazolyl bridge. The complexes that have shorter lifetimes and for phosphorescence with plexes sidera bl y the lower 3B (cta *pa) 2 sively than 2 [Ir(~-pz)(COD)] . lifetimes excited either than methyl-substituted-pyrazolyl complexes. by the and determined photo- the substituents contain substituted- quantum yields the two com- ligands have con- quantum yields for unsubstituted This metal- the lower trifl uoromethyl-su bsti tu ted-pyrazol yl state of Furthermore, ambient-temperature (pa -+ da *) 1 predominantly on all -+ I A 2 for influenced of energies be largely seem ligands The to properties pyrazolyl strongly spectra 3,5-disu bsti tuted- complexes. physical the be emission the Interestingly, transitions. to as metal-metal absorption cyclooctadiene five the dependence, as has been (da * -+ pa) absorption and 2 emission dimers, an angular unsu bsti tu ted -pyrazol yl- bridged the these suggests or that excluthe 96 trifluoromethyl excited plexes ties ideally the influences Certainly, state. are of group suited • (da pa) pyrazolyl-bridged to the these deactivation pyrazolyl-bridged a study of excited states since iridi u m(I) of complexes ligand a are the 3 B (da • pa) 2 iridium(!) com- effects on the variety of substituted- easily proper- prepared. 2 0 97 References and Notes l. Mann, K.R.; Gray, H.B. Adv. Chern. Ser. 1979, 17 3, 225-235. 2. Rice, S.F.; Milder, S.J.; Gray, H.B.; Goldbeck, R.A.; Kliger, D.S. Coord. Chern. Rev. 1982, 43, 349-354. 3. Rice, S.F.; Gray, H.B. J. Am. Chern. Soc. 1981, 103, 1593-1595. 4. Dallinger, R.F.; Miskowski, V.M.; Gray, H.B.; Woodruff, W.H. J. Am. Chern. Soc. 1981, 103, 1595-1596. 5. Rice, S.F.; Gray, H.B. J. Am. Chern. Soc. 1983, 105, 4571-4575. 6. Che, C.-M; Butler, L.G.; Gray, H.B.; Crooks, R.M.; Woodruff, Institute of W.H. J. Am. Chern. Soc. 1983, 105, 5492-5494. 7. Smith, T.P. Ph.D. Dissertation, California Technol- ogy, Pasadena, 1982. 8. Rice, S.F. Ph.D. Dissertation, California Institute of Technology, Pasadena, 1982. 9. Mann, K.R.; Thich, J.A.; Bell, R.A.; Coyle, C.L.; Gray, H.B. D.S.; Hammond, G.S.; Chern. Soc. 1978, 100, D.M. J. Chern. Soc. , Inorg. Chern. 1980, 19, 2462-2468. 10. Miskowski, V.M.; Nobinger, Lewis, Mann, K.R.; Gray, H.B. Dickson, M.K.; Roundhill, N.S.; G.L.; Kliger, J. Am. 485-488. 11. Sperline, R.P.; Chern. Commun. 1977, 62-63. 12. Fordyce, W.A.; Brummer, J.G.; Crosby, G.A. J. Am. Chern. Soc. 1981, 103, 7061-7064. 13. Che, C.-M.; 7796-7797. Butler, L.S.; Gray, H.B. J. Am. Chern. Soc. 1981, 103, 98 14. Parker, W.L.; Crosby, G.A. Chern. Phys. Lett. 1984, 105, 544-546. 15. Nocera, D.G.; Maverick, A.W.; Winkler, J.R.; Che, C.-M; 1974, Gray, H.B. ACS Symp. Ser. 1983, 211, 21-33. 16. Wiley, R.H.; Hexner, P.E. Org. Syn. 1951, 31, 43-44. 17. Herde, J.L.; Lambert, J.C.; Senoff, C.V. lnorg. Syn. Bushnell, G.W.; Dixon, 15, 18- 20. 18. Atwood, D.T.; J .L.; Stobart, Beveridge, S.R.; K.A.; Zaworotko, MJ. Inorg. Chern. G.W.; Dixon, K.R.; 1984, Eadie, 23, 4050- Eadie, D.T.; 4057. 19. Beveridge, Stobart, K.A.; S.R.; Bushnell, Atwood, J.L. Za worotko, K.R.; MJ. J. Am. Chern. Soc. Zaworotko, M.J.; 1982, 104, 920-922. 20. Bushnell, G.W.; Fjeldsted, D.O.K.; Stobart, S.R.; Knox, S.A.R.; Macpherson, K.A. Organometallics 1985, 4, 1107-1114. 21. Coleman, A.W.; Eadie, D.T.; Stobart, S.R.; Za worotko, MJ.; Atwood, J.L; Atwood, J.L. J. Am. Chern. Soc. 1982, 104, 922-923. 22. Stobart, S.R.; Coleman, A.W.; Harrison, D.G.; Zaworotko, MJ. J. Am. Chern. Soc., submitted. 23. Caspar, J.V.; Gray, H.B. J. Am. Chern. Soc. 1984, 106, 3029-3030. 24. Gordon, A.J.; Ford, R.A. "The Chemist's Companion"; John Wiley and Sons: New York, 1972; p 436. 25. Demas, J.N.; Crosby, G.A . J. Phys. Chern. 1971, 75, 991-1024. 26. Caspar, J.V.; Meyer, T.J. J. Am. Chern. Soc. 1983, 105, 5583-5590. 27. Parker, C.A.; Rees, W.T. Analyst 1960, 85, 587-600. 28. Nocera, D.G.; Winkler, J.R.; Yocom, H.B. J. Am. Chern. Soc. 1984, 106, 5145-5150. K.M.; Bordignon, E.; Gray, 99 29. Powell, J.; Kuksis, A.; Nyburg, S.C.; Ng, W.W. Inorg. Chim. Acta J. Chern. 1982, 64, L211-L212. 30. Nussbaum, S.; Rettig, S.J.; Storr, A.; Trotter, J. Can. 1985, 63, 692-702. 31. Uson Lacal, R.; Oro MT.; Royo Macia, Giral, M· L.A.; Lopez, Martinez, Pastor ' Ciriano E. MA.; Spanish Pinillos, Patent ES Zaworotko, M.J. Atwood, J.L.; 497,900, 1981; Chern. Abstr. 1982, 97, 55306n. 32. Bushnell, G.W.; Fjeldsted, D.O.K.; Stobart, S.R.; J. Chern. Soc., Chern. Commun. 1983, 580-581. 33. Beveridge, K.A.; Bushnell, G.W.; Stobart, S.R.; Zaworotko, M.J. Organometallics 1983, 2, 1447-1451. 34. Bushnell, G.W.; Stobart, S.R.; Vefghi, R.; Zaworotko, M.J. J. Chern. Soc., Chern. Commun. 1984, 282-284. 35. Fjeldsted, D.O.K.; Stobart, S.R.; Zaworotko, MJ. J. S.R. J. Chern. Soc., Chern. Eadie, D.T.; Stobart, Am. Chern. Soc. 1985, 107, 8258-8259. 36. Fjeldsted, D.O.K.; Stobart, Commun. 1985, 908-909. 37. Bushnell, R.; G.W.; Atwood, Decker, J.L.; M.J.; Zaworotko, MJ. Organometallics S.R.; Vefghi, 4, 2106- Commun. 1986, 3543-3555 and 1985, 2111. 38. Harrison, D.G.; Stobart, S.R. D.M.; Hoffman, R. J. Chern. Soc., Chern. 285-286. 39. Hoffman, Inorg. Chern. 1981, 20, references therein. 40. The absorption spectra variety of solvents. of these complexes are the same in a An additional band is observed at 227 nm for 100 the pyrazolyl-bridged complex and at - 236 nm for the substituted- pyrazolyl-bridged solvents with low UV complexes are the same complexes in absorption, such as cyclohexane. 41. The emission such as spectra of acetonitrile, these cyclohexane, benzene, and T.L.; Gray, in solvents 2-methyltetra- hydrofuran. 42. Winkler, J.R.; Marshall, J.L.; Netzel, H.B. J. Am. Chern. Soc. 1986, 108, 2263-2266. 43. Milder, S.J. Inorg. Chern. 1985, 24, 3376-3378. 44. Turro, N.J. "Modern Molecular Photochemistry"; Benjamin/Cummings: Menlo Park, California, 1978; pp 146-148. 45. Birks, J.B. "Photophysics of Aromatic Molecules"; John Wiley and Sons: New York, 1970; pp 372-402. 46. Parker, C.A. Adv. Photochem. 1964, 2, 305-383. 4 7. The Raman spectrum of tetrahydrofuran - 280 cm- 1 (Strommen, D.P.; has a very weak Nakamoto, K. "Laboratory band at Raman Spectroscopy"; John Wiley and Sons: New York, 1984; p 117). 48. Clark, R.J.H.; Williams, C.S. Inorg. Chern. 1965, 4, 350-357. 49. Bennett, M.A.; Clark, R.J.H.; Milner, D.L. Inorg. 1967, Chern. 6, 1647-1652. 50. Andrews, D.C.; Davidson, G. J. Organomet. Chern. 1973, 55, 383-393. 51. Quicksall, C.O.; Spiro, T.G. /nor g. Chern. 1969, 8, 2011-2013. 52. Shriver, D.F.; Cooper, C.B., III Adv. Infrared Raman Spectrosc. Miskowski, V.M.; Smith, T.P.; Loehr, T.M.; Gray, H.B. J. Am. Chern. 1980, 6, 127-156. 53. Soc. 1985, 107, 7925-7934. 101 54. Lewis, N.S.; Mann, K.R.; Gordon, J.G., II; Gray, H.B. Soc. 1976, 98, 7461-7463. 55. Trofimenko, S. !nor g. Chern. 1971, 7, 13 72-13 76. J. Am. Chern. 102 Chapter 3 Electron-Transfer Reactivity of the 3 B 2 (da • pa) Excited State of [Ir(~-pz)(COD)] 2 103 Introduction An electronic excited state of a transition metal complex that is sufficiently long-lived to be in thermal equilibrium with surroundings is species with chemical and properties a new be quite ticular, the increased potential of that can an different that an ductant than the excited-state fluid state excited of the affinity and relative state to the should be corresponding complexes solution those electron excited prediction from can that have participate in ground a state. decreased ground state better of at par- lead to the and re- Con seq uen tl y, least bimolecular In ionization oxidant state. 1 ' 2 ground lifetimes physical its 0.1 to ns electron-transfer in reac- tions with appropriate electron donors and acceptors. 8 ,t Research in photoinduced complexes includes studies theoretical aspects of work concerning energy. 1 0 - 1 2 Along electron-transfer (bpy = the 2,2' -bipyridine), 1 ' 1 9 Clearly, understanding the developing and reactions, 5 -g of inorganic understanding the in to addition con version and storage of solar these lines, extensive investigations of the the excited states of and (II) and fundamental at reactions photochemical reactivity osmi urn aimed electron-transfer 2,2' -bipyridine) plexes, 1 8 - 1 7 electon-transfer related pol ypyrid yl complexes, 1 8 [Re 2 Clg] 2- 2 0 ' 2 1 studies factors ruthenium(II) that of this govern and may lead to systems in which the polypyridyl com- [Cr(bpy )3 ] 3 + (bpy been reported. have type [R u(bpy )3 ]2+ of are homogeneous important electron in transfer excited states of inorganic com- plexes are employed in energy con version and storage processes. 104 The relatively long [lr(J...l-pz)(COD)] 2 (pzH solution that suggests lifetime of pyrazole, COD it should be electron-transfer reactions donors. In light of the favorable of complex, we have been ities this for through the a in teres ted excited state for con version photoinduced been with in in outer-sphere of the context electron state to in in suitable electron acceptors absorption and interested light in to net or properties the possibil- chemical energy In addition, we have the electron-transfer reactivity of this of classical theory The process. bimolecular emission examining of fluid participate visible transfer. these goals are reported herein. excited 1,5-cyclooctadiene) able electron-transfer examining 3 B (da • pa) 2 the Marcus results of our predictions efforts toward 105 Experimental Syntheses .Materials. Acetone (EM Science), ethanol (U.S. Jackson), methanol (Baker), dichloroethane (Burdick (Baker), and N,N-dimethylacetamide better and used fluorophosphate Mahoning), without (Aldrich, chloride used as na te (Aldrich, (Aldrich, 4-Cyanopyridine (Aldrich, 98%), methyl (Burdick 4-picoline 2,6-lutidine sodium water), (Ozark- (MCB, 2,4,6-collidine in hexafluorophosphate iodoethane isonicotinamide (Aldrich, and (Aldrich, (Kodak), iodide and methyl were isonicoti- nicotinamide 2-methoxypyridine 98%), 4-tert-butylpyridine (ICN Pharmaceu- 2,6-dimethyl-/"-pyrone methylamine viologen 98%) 99%), Jackson), 99+%), (Baker), (Aldrich, 99%), 2,3,6-collidine (Aldrich, or hexa- sulfate ticals), solution ammonium ether Potassium dimethyl 99%), 99%), purification. 1,2- grade and (Aldrich, (Aldrich, reagent 97%), pyridine 98%), were diethyl 99%), 98%), 98%), Industrial), (Aldrich, (Aldrich, received. (Aldrich) further 98%), iodomethane benzyl (Aldrich, and absolute (Aldrich, dichloride hydrate 40 wt. % (Aldrich) were used as received. Pyridinium Hexafluorophospha tes. the The general method outlined pyridini urn hexafl uorophospha tes, ly 3-5 g of of alkylating below pyridine or substituted acetone/ethanol agent were solution. except mixture in 80-100 was for where pyridine and dissolved The was used the preparation of noted. Approximate- a 4-fold molar excess mL of a (v/v) refluxed from 1:1 4 - 24 h, 106 depending halide upon would the difficulty precipitate during of alkylation. the reaction Often the upon cooling, or pyridinium but if not, the solvents were removed on a rotary evaporator. The halide salt was isola ted dissolved ra ted, and aqueous ammoni urn solution of hexafl uorophospha te the pyridini urn well with yields minimum either of water. potassium A with stirring, hexafl uorophospha te. This salt was and recrystallized solution the pyridinium to from give a filtered, sa tu- hexafluorophospha te added, water, of a was water /acetone/ ethanol The in to hexafluorophosphates precipitate filtered, washed 2:1:1 (v/v/v) crystalline solid. a white, or varied from 20-80%. The 1H NMR spectra of the pyridinium hexafluorophosphates in acetoned6 agree with related data reported in the literature. 2 2 ' 2 3 4-Cyano-N-methylpyridinium from 4-cyanopyridine Anal. Calcd 31.84; H, iodomethane C 7 H 7N 2 PF 6 : for 2.67; and N, 1H 10.63. hexafluorophosphate C, by 31.83; NMR the H, above 2.67; (acetone-d 6 ): was synthesized general procedure. 10.61. Found: (1) N, 5 9.43 (d, hexafluorophosphate (2) 2 C, H, 2,6- was syn- H), 8.60-8.80 (m, 2 H, 3,5-H), 4. 77 (s, 3 H, CH 3 ). 4-Carbomethoxy-N-methylpyridinium thesized from methyl general procedure. Found: C, 4.71. (d, 2 H, 2,6-H), isonicotinate and iodomethane by the above Anal. Calcd for C 8 H 10 N0 2 PF 6: C, 32.34; H, 3.39; N, 32.32; H, 3.38; N, 4.72. 1H NMR (acetone-d 6 ): 5 9.33 8.55-8.75 (m, 2 H, 3,5-H), 4.73 (s, 3 H, NCH 3 ), 4.07 (s, 3 H, COOCH 3 ). 4-Amido-N-ethylpyridinium from isonicotinamide Anal. Calcd for and hexafluorophosphate iodoethane C 8 H 11 N 2 0PF 6: C, (3) by the above 32.45; H, 3.74; N, was synthesized general procedure. 9.46. Found: C, I07 IH NMR (acetone-d 6 ): 5 9.33 (d, 2 H, 2,6-H), 32.45; H, 3.73; N, 9.47. 8.50-8.67 (m, 2 H, 3,5-H), 4.97 (q, H, 2 CH 2 ), 1.80 (t, was (4) 3 H, CH 3 ), unable to locate the resonance for the amide protons. 3-Amido-N-benzylpyridinium hexafluorophosphate from nicotinamide and benzyl chloride Anal. Calcd 43.61; H, for by the above general procedure. C I 3H I 3 N 2 0PF 6: C, 43.59; H, 3.66; N, 3.67; N, 7.82. IH 9.03-9.37 (m, 2 H, 4,6-H), c 6 H 5), 6.I3 (s, 2 H, NMR 8.40 CH 2 ), synthesized 5 (acetone-d 6 ): (dd, H, unable to 9.63 5-H), locate 7.82. Found: C, (s, I H, 7.43-7.77 the (m, resonance 2-H), 5 H, for the amide protons. 3-Amido-N-methylpyridinium from nicotinamide Anal. Calcd 29.85; H, 9.00-9.23 for and iodomethane C 7 H 9 N 2 0PF 6: C, 3.23; N, 9.94. (m, H, 4,6-H), 2 hexafluorophosphate IH by the above 29.80; H, 3.22; NMR 8.33 (dd, I H, 5-H), synthesized general procedure. N, .9.93. 5 (acetone-d 6 ): was (5) 9.50 (s, 4.67 (s, Found: C, H, 2-H), H, CH ), 3 synthesized from 3 broad resonances at 7.30-8.IO for th·e amide protons. N-Ethylpyridinium pyridine and iodoethane for C 7 HioNPF 6: C, N, 5.49. 4-H), hexafluorophosphate IH 33.2I; by the H, above 3.98; NMR (acetone-d 6 ): 8.13-8.40 (m, 2 H, was (6) general 5 9.I3 (d, 2 H, 2,6-H), 8.73 (t, H, (q, 2 H, C, CH 2 ), 33.25; Calcd 3.90; 4.85 Found: Anal. H, 3,5-H), N, 5.53. procedure. 1.75 (t, 3 H, CH 3). 2-Methoxy-N-methylpyridinium sized a slight methoxypyridine and nate. using The hexafluorophospha te modification dimethyl pyridinium methyl of a sulfate published instead sulfate salt was of (7) was procedure 2 • methyl dissolved in synthefrom 2- fluorosulfoa minimum 108 of water and filtered, The meta thesized saturated, precipitated with well aqueous 3.75; water, 5.20. N, (acetone-d 6 ): 6 solution pyridinium water/acetone/ethanol H, to the hexafl uorophospha te by addition of a ammonium hexafluorophosphate and recrystallized solution. Anal. Found: 8.43-8.70 of 31.23; 2 H, (m, was for H, washed 2:1:1 (v /v /v) C 7H 10 NOPF 6 : C, 31.24; 1H NMR H, 4,5-H), 3.69; 3,6-H), filtered, a from Calcd C, hexafluorophosphate. N, 5.22. 7.50-7.87 (m, 2 hexafluorophosphate (8) was 4.40 (s, 3 H, OCH 3 ), 4.20 (s, 3 H, NCH 3 ). 4-Methyl-N-methylpyridinium sized from Anal. Calcd 4-picoline and for iodomethane C 7H 10 NPF 6 : C, by 33.21; the H, above 3.98; synthe- general procedure. 5.53. Found: N, C, 1H NMR (acetone-d ): 6 8.83 (d, 2 H, 2,6-H), 6 33.26; H, 3.96; N, 5.52. 8.03 (d, 2 H, 3,5-H), 4.50 (s, 3 H, NCH 3 ), 2.73 (s, 3 H, 4-CH 3). 4-tert-Butyl-N-ethylpyridinium sized from procedure. 4-tert-butylpyridine Anal. Calcd hexafluorophosphate and iodoethane C 11 H 18 NPF 6: for C, (9) was synthe- by the above general 42.73; H, 5.87; N, 4.53. Found: C, 42.63; H, 5.74; N, 4.47. 1H NMR (acetone, d ): 6 9.02 (d, 2 6 H, 4.72 2,6-H), 8.28 (d, 2 H, 3,5-H), (q, 2 H, CH 2 ), 1.72 (t, 3 H, NCH 2 CH3 ), 1.40 (s, 9 H, C(CH 3 ) 3). 2,6-Dimethyl-N-methylpyridinium thesized procedure. from Anal. 2,6-1 u tidine Calcd Found: C, 35.84; H, 4.41; and for N, hexafluorophosphate iodomethane c 8H 12 NPF 6: 5.23. C, was (10) syn- by the above 35.97; H, 4.53; N, 6 8.40 (t, 1H NMR (acetone-d ): 6 general 5.24. 1 H, 4-H), 7.93 (d, 2 H, 3,5-H), 4.30 (s, 3 H, NCH 3), 2.97 (s, 6 H, 2,6- 2,3,6-Trimethyl-N-methylpyridinium hexafl uorophospha te (11) was 109 synthesized from 2,3,6-collidine procedure. Anal. Calcd Found: 38.45; H, C, H, 4-H), 7.77 (d, H, iodomethane c 9 H 14 NPF 6 : for 4.97; and N, 1H 4.98. 5-H), 4.25 C, NMR (s, 3 H, by the above 38.44; H, 5.02; (acetone-d 6 ): NCH 3 ), 2.88 t> general N, 8.23 4.98. (d, 1 (s, 3 H, 2-CH 3 or 6-CH 3 ), 2.82 (s, 3 H, 2-CH 3 or 6-CH 3 ), 2.55 (s, 3 H, 3-CH ). 3 2,4,6-Trimethyl-N-methylpyridinium synthesized from 2,4,6-collidine procedure. Anal. Calcd Found: 38.67; H, H, C, 3,5-H), 4.20 (s, for and N, H, NCH 3 ), 3 iodomethane C 9 H 14 NPF 6 : 4.89; 1H 4.95. (12) hexafl uorophospha te 2.90 C, NMR (s, by the above 38.44; H, 5.02; N, t> 7.78 (s, 2 2.60 (s, 3 H, (acetone-d ): 6 H, 6 was 2,6-CH ), 3 general 4.98. 4-CH 3 ). 2,6-Dimethyl-4-methoxy-N-methylpyridinium was prepared by treating methoxy-N-methylpyridinium and Ozog, 2 5 with a water, and solution. Found: C, aqueous iodide, filtered, hexafl uorophospha te. methylpyridinium an Calcd 36.45; H, for 4. 7 4; by aqueous precipitated hexafluorophosphate Anal. solution synthesized saturated, The recrystallized hexafl uorophospha te from a N, 4. 72. the of of King ammonium 2,6-dimethyl-4-methoxy-Nfiltered, 2:1:1 (v/v/v) 1H 2,6-dimethyl-4- method solution was c 9 H 14 NOPF 6 : of (13) C, NMR washed well with water/acetone/ethanol 36.37; H, (acetone-d 6 ): 4.75; N, t> 7.45 4.71. (s, 2 H, 3,5-H), 4.25 (s, 3 H, NCH 3 or OCH 3 ), 4.20 (s, 3 H, NCH 3 or OCH 3 ), 2.85 (s, 6 H, 2,6-CH 3 ). Additional Compounds. Bis( 1,5-cyclooctadiene )bis(IJ -pyrazol yl)diiridi um(I), [lr(IJ-pz)- (COD)] , was prepared by the method outlined in Chapter 2. 2 N,N '-Dimethyl-4,4 '-bipyridinium di(hexafl uorophospha te ), methyl 110 viologen di(hexafl uorophospha te) treating an with filtered, a aqueous fluorophospha te. well solution The with of saturated, methyl aqueous and water /acetone/ethanol solution c 12 H 14N 2 P 2 F 12: 30.27; viologen solution MV 2 +(PF 6 -) 2 dichloride hexa- was filtered, washed 2:1:1 (v /v /v) to white needles. 5.88. Found: 2.96; N, hydrate ammonium from H, by of recrystallized give prepared was 6 2' precipitated water, C, MV 2 +(PF -) or a Anal. C, Calcd for H, 2.93; 30.32; N, 5.84. 2,6-Dimethyl-N-methylpyridinium ing 2,6-lutidine 0.13 mol) The pyridinium iodide was cooled, and the product dimethylacetamide to give (25%). in a Anal. iodide (3.0 mL, 0.026 1:1 (v/v) acetone/ethanol Calcd mol) precipitated for was a was and synthesized excess by iodomethane solution (60 mL) during -the reaction. filtered and recrystallized white, C 8 H 12 NI: crystalline C, H, 38.58; solid. 4.86; reflux(8.0 for The mL, 24 h. solution from N,N- Yield: 1.6 5.62. Found: N, g C, 38.63; H, 4.74; N, 5.68. Physical Measurements Materials. Solvents and and steady-state liquid photolysis degassed with a vacuum line (limiting tilled into glass minimum the experiments of five quenching, were flash purified, freeze-pump-thaw high- bulb-to-bulb dis- and round-bottomed storage flasks equipped alumina (Woelm N, solvents Activity Linde 4A and Grade molecular reagents 1, were obtained sieves (Union on with Teflon over either stored from necessary, a torr), The photolysis, if cycles < 1o· 3 valves. or for pressure vacuum Biochemicals) reagents ICN Carbide) Nutritional that were Ill activated by heating under Acetonitrile (Burdick and dynamic Label) were used as molecular sieves, Label) from calcium molecular and received respectively. (Aldrich, The and stored (Aldrich, 95+%, halocarbon -40 for 24 h. over alumina and (Aldrich, 98%) mesh) solvents' torr) cyclohexane (Aldrich, I ,2-Dibromoethane 1-bromo-2-chloroethane hydride sieves. Io- 3 (< Jackson, UV Grade) and 99+%, Gold Gold vacuum 99+%, were and distilled stored over storage flasks were for electrochemi- wrapped with foil to prevent exposure to room light. Acetonitrile (Burdick and Jackson, UV Grade) cal experiments was obtained from a freshly the opened bottle and used as received. Tetra-n-butylammonium n-butylammonium (Aldrich, 98%) iodide hexafluorophosphate (Kodak) and by addition of a filtered, saturated, iodide salt to sium salt. The was recrystallized precipitated a was prepared potassium saturated, tetra- hexafluorophosphate aqueous aqueous from solution solution tetra-n-butylammonium of of the the potas- hexafluorophosphate from absolute ethanol (U.S. Industrial) to give Tetra-n-butylammonium iodide (Kodak) was recrystallized from white needles. acetone (EM Science) (Southwestern Analytical Gold and Label), before Tetra-n-bu tylammonium use. Chemicals), tetramethylsilane acetone-d 6 (Aldrich, (Aldrich, 99.9+%, NMR conducted with fl uorobora te 99+atom% D, Grade) were used as received. Electrochemical ~Ieasurements. Cyclic voltammetric measurements were a Princeton Applied Research (PAR) 173 potentiostatjgalvanostat and a PAR 175 uni- 112 versa! programmer using a Pt a sodium auxiliary electrode, and the reference. Cyclic as button working saturated voltammograms were measured that 0.1 tetra-n-butylammonium contained supporting electrolyte and 5-7 [lr(~-pz)(COD)b mograms of solutions M 5 containing for Pt M wire (SSCE) pyridinium solutions hexa- under argon hexafluorophosphate as compound. voltam- Cyclic the were measured under the same conditions on 10- 4 x the acetonitrile 1o- 3 x a calomel- electrode of fluorophosphates electrode, M complex with 0.1 M tetra-n-butyl- ammonium fluoroborate as the supporting electrolyte. Stern-Volmer Quenching Procedures. For the experiments and the pyridinium tions of [lr(~-pz)(COD)] 2 with (1 (~ (limiting < 10- 3 o- 4 0.1 10- 5 M) were torr) in tometric cell consisting of a to from the its storage evacuated cell di(hexafluorophosphate) quenchers, M) and prepared a acetonitrile solu- t e t ra":'n- b u t y I ammontum · on a high-vacuum line two-compartment spectropho- 1 em pa thlength square cuvette of quartz or Pyrex and a 10 mL Pyrex bulb. 2 6 tilled viologen hexafluorophosphate hexafluorophosphate pressure methyl flask to a containing Acetonitrile was first vacuum-discalibrated the iridium volumetric complex cylinder and then electrolyte. Addition of solid quencher to one compartment of the cell was followed by re-evacuation the experiment. the Rate cell to maintain constants for the the oxygen-free the were determined [Ir(~-pz)(COD)b ( 3 [Ir(~-pz)(COD)] 2 •) measuring its lifetime function a of quencher triplet concentration. The emission lifetime measurements were conducted with a Nd:Y AG laser system that has been described of of of as conditions quenching state excited by of previously 2 1 using pu~sed 532 nm 113 excitation. Rate constants for the quenching 3[Ir(J,J-pz)(COD)] • 2 of with 2,6- dimethyl-N-methylpyridinium iodide and 2,6-dimethyl-N-methylpyridinium hexafl uorophospha te absence of added mined by measuring concentration. an emission been described change the its The with not in emission emission intensity spectrophotometer previously. 2 8 with measured from Sample orientation intensity the as electrolyte a function measurements constructed Since the at quencher addition, emission band maxima of instrument stability were and of Caltech the conducted which has shape did band emission deter- quencher were luminescence the were intensities were uncorrected spectra. monitored insure to confidence in the quenching data. The method of preparation of these solutions described was the same as that for the samples containing 0.1 M tetra-n-butylammonium hexafluorophosphate. Absorption spectra of the solutions were spectrophotometer insure that thermal or with a Cary 17 quenching experiments to before and after quenching was reversible and photochemical, was occurring between the the measured that no reaction, either [Ir(J,J-pz)(COD)] 2 and the quenchers during the course of the experiments. Microsecond Flash Photolysis Procedures. Flash structed photolysis at Caltech noted modifications. 2 7 citation filtering. 10-4 methyl M}, experiments were conducted on which been described previously 2 9 has viologen solutions of apparatus con- with the was used for ex- [Ir(J,J-pz)(COD}] 2 (1 x M}, and A Corning 3-73 cut-off filter Acetonitrile an di(hexafluorophosphate) (5 x 10- 3 tetra-n-butylammonium hexafluorophosphate (JJ = 0.1 M) were prepared on II4 a high ing of vacuum a diameter) torr) in (I 5 em pathlength, (limiting cylindrical Pyrex compartment a mL connected to pressure < I0- 3 line 50 Pyrex bulb used a for cell consist0.8 em condensation of solvent and a I em pathlength square quartz cuvette with each compartment oriented 90° from its its storage flask into a distillation into the evacuated distilled from followed by iridi urn complex, quencher, neighbor. and Acetonitrile was calibrated electrolyte. first vacuum- volumetric cylinder cell The containing solution the was then blanketed with argon. Steady-State Photolysis Procedures. Steady-state prepared on a photolysis high-vacuum two-compartment pathlength tilled cell square into the appropriate solvent in cell, the line consisting (limiting of a I0 cuvette. 2 6 quartz storage flask pressure mL followed were Io- 3 < Pyrex on bulb was by reactants and solid reactants were added re-evacuation of the of [lr(~-pz)(COD)h to cell one to a in of dis- from the distilled the reactant into the the cell oxygen-free con- compartment maintain a 1 em bulb-to-bulb addition bulb-to-bulb solutions torr) [Ir(~-pz)(COD)h the Liquid by conducted Solvent containing cell were dark. and followed experiments of di tions. Acetonitrile solutions methyl-N-methylpyridinium iodide (2 x x (I 10· 2 M), and 10- 4 M), 2,6-di- (n-C 4 H 9 ) 4 NI (2 x 1o· 2 M) were irradiated for at least 24 h with i\ex > 420 nm and showed no visible absorbance changes. Solutions of [Ir(~-pz)(COD)] 2 dibromoethane, in either neat I,2-dibromoethane acetonitrile cyclohexane, were irradiated or with i\ex or > and 1,2- diluted with 450 and nm 115 showed dark of rapid showed formation no formation [Ir(~-pz)(COD)] 2 [Ir(~-pz)(COD)(Br)] . 2 of of and the oxidative Solutions addition in product. the Solutions in either or cyclohexane, were irradiated with 'A > 450 nm and showed rapid formation of either [Ir(~­ bromo-2-chloroethane pz)(COD)(Br)]2 diluted 2 2 acetonitrile 2 the dark showed tion products. The fied by absorption their with Ir (~-pz) (COD) (Br)(Cl) or Solutions kept in or 1-bromo-2-chloroethane, kept iridium as discussed no formation oxidative spectra; the in of the addition 1- the text. oxidative addi- products organic neat were products identi- were not identified. Either a 1000 W high-pressure Hg/Xe arc lamp or a 200 W medium- pressure Hg/Xe arc lamp equipped with Corning cut-off filters was used for the tolysis irradiations. Spectrophotometric thermal-blank experiments and was monitoring done of with either both a pho- Cary 17 or Hewlett-Packard 8450A spectrophotometer. Additional Measurements. 1H NMR spectra spectrometer. were Chemical recorded shifts at are 90 MHz on reported a Varian EM 390 in ppm (6) vs. Larry Henling at the tetramethylsilane. Elemental analyses were Caltech Analytical Laboratory. obtained by Mr. 116 Results and Discussion A long-lived excited enhanced redox properties entropy difference between reduction and rna ted from E 0 (M/M-)) state oxidation the and redox of a relative to the ground the metal ground complex state. has If the and excited state is small, the the excited state can be esti- (E 0 (M+ /M) and potentials of potentials of ·the ground energy of the excited state. The dis- an excited state relative to the Stokes shift between absorption and the spectroscopic solvation) tortion (size, shape, ground state is reflected in the If this is small, emission. 3 0 transition shift of the state entropy difference is usually neglected, and the spectroscopic energy may be taken as a free energy. In this situation, redox the potentials the · of excited state (E 0 (~ /M*) and E 0 (M* /~)) can be estimated using equations (I) and (2) where E 0 _0 (M/M*) is the spectroscopic energy of the 0-0 transi tion. 1 ,• difficult to of approximations. the evaluate, electronic spectra the of Stokes corrections entropy the Since the Stokes From transitions [lr(IJ-pz)(COD)h shift for the the tn to these equations is used to shift band maxima in the acetonitrile 3B (da *pa) 2 excited gauge the corresponding absorption at are ambient state is and very validity to the emission temperature, 117 suggesting that equations of the excited-state 3 B (da *pa) 2 excited try however, is 2 can The be to solution (Ir 2 ) used to spectroscopic estimated acetonitrile [Ir(~-pz)(COD)] of (2) potentials. state ambient-temperature and (I) obtain estimates energy of the eV from the be 2.05 spectra . The in acetonitrile is electrochemis- rather since the oxidation to the mono-cation and this species to the di-cation are near the potential plica ted by E 0 / (Ir 2 +/o) comof suggested. No reduction of 2 is observed to -1.8 V vs. SSCE. These estimates for terms that 0.30 0.05 :!: in V vs. equations SSCE E 0 (Ir 2 + ; 31r 2 *) with for Eo/ (Ir2 o/-) < 0.25 v vs. SSCE, Since the long-lived pz)(COD)] 2 is predicted studied is and (I) [Ir(~-pz)(COD)] 2 of limit is are estimate a Given and of an state acetonitrile oxidation Therefore, = various excited coordination. 3 1 acetonitrile [Ir(~-pz)(COD)] the same the complex, (2) suggest ( 3 Ir 2 *) about is -1.7 3 B (da *pa) 2 the a powerful reductant V to V the value of < -1.8 V, implying that this (250 ns) 3B (da *pa) 2 to be a its electron-transfer -1.8 excited vs. in SSCE. E 0 ( 3 Ir */Ir -) 2 2 of state is a poor of [Ir( ~- oxidant. we initially methyl-4,4 / -bipyridinium solution, the quenched in very (methyl phosphorescence strong viologen, of presence of MV 2 +(PF 6 -) 2 . too e V) 3 3 is tive with electron-trans[ er energy transfer can transfer is obtained high be from for a reductant in quenching acetonitrile, reactivity with N,N/ -di- MV 2 +). 3 2 In acetonitrile is dramatically 2* The triplet energy of Mv 2 + energy-transfer quenching to be 3 [Ir(~-pz)(COD)] of out. Further measurement of ruled state 3 [Ir(~-pz)(COD)] the (3.1 0 excited the competi- 2 *, and evidence for rate the quenching of electron 118 reaction shown in equation (3). (3) The obeys Stern- Volmer quenching kinetics 8 4 reaction moni taring methyl the lifetime viologen of the triplet concentration. By excited analyzing and state the as data lifetimes presence of of the 3 B (da *pa) (Q), quencher rate constant (kq) can excited 2 respectively, state the function of according in value by a Stern- Volmer equation, shown below in equation (4), where the is studied T the of be extracted from the slope of a plot for to the T are and 0 absence the and quenching plot of (T 0 /T) vs. [Q]. The Stern- Volmer pz)(COD)] 2 of the 1.6 The for the reaction 0.3 x 10 10 rate of ± quenching the of 3 • by methyl viologen in acetonitrile solution (22 ± 2°C, 0.1 M (n-C 4 H 9 ) 4 NPF 6 ) is shown stant data acetonitrile, 8 6 the which is ± 8.7 M- 1s - 1 Figure The 3.1. 10 9 ~1 correction for 0.9 after reaction is in is X near reasonable for s the -1 ' measured which ~ = con- increases to diffusional effects. 8 5 diffusion limit electron-transfer between 3[Ir(~-pz)(COD)h * and methyl viologen, since reduced by one electron (E 1; 2 (MV 2 +/+) = -0.45 vs. SSCE, V rate [Ir(~- quenching Mv 2 + (n-C 4 H 9 ) 4 NPF 6 , CH 3 CN). 8 7 Conventional microsecond flash-photolysis studies show that the for is easil y ~ = 0.1 M 119 Figure 3.1 Stern- Volmer plot 3 B (da *pa) 2 excited viologen for the state oxidative of di(hexafl UOrophospha te) quenching [lr(1J-pz)(COD)] 2 ((Jr 2] = = 0.1 M (n-C 4 H 9 )4 NPF 6 , CH 3 CN, 22 :!: 2°C). 5 X of the by methyl 10- 5 M, 1J 120 \ • N • Q ~ E .. r--""'1 Cl) 0 lD 0 • \• ~ 0 N 0 0 L() ~ r0 (~I o~) N ob 0 L......-...1 121 redox expected products Mv 2+ are action between the viologen, shown in produced. electron-transfer for An investigation oxidized equation quenching quenching electron-transfer of of by back-electron-transfer the iridium of dimer and reduced re- methyl (5), provides very strong evidence for the excited state. Also, the rate of after the back electron transfer to give ground-state products can be measured. I r 2+ + MV+ -+ Ir 2 + MV 2 + The flash, transient shows quenching. difference the expected spectrum, redox (5) recorded products 160 IJS following electron-transfer The difference spectrum, shown in Figure 3.2, is dominated by in tense absorptions located at - 390 nm and - 600 nm that are characteristic of MV+. 3 8 The bleaching between 475 nm and 520 nm is due to absorption the tion ground-state Well-defined The [Ir(IJ -pz)(COD)] 2. The weak a bsorp- 450 nm is probably due to the transient [lr(IJ-pz)(COD)] 2+. around order, of decay of the kinetics equal-concentration solution is redox photochromic over transients that for 2-3 half -lives many flashes second- exhibits and is no observed. build-up of Ir 2+ and products is noted. The rate of the back-electron-transfer reaction between MV+ is measured from a plot of I I !J.A vs. t for the transient decay at 605 nm. The plot of the data, shown in > 2.5 half -lives and corresponds order, equal-concentration Figure to the decay kinetics. 3 9 The rate 3.3, is expected constant linear for for reaction (kb) can be calculated from the slope (m) of this plot using for secondthe back 122 Figure 3.2 Transient difference spectrum observed 160 IJS flash photolysis for the back-electron- microsecond transfer reaction between [Ir(1J-pz)(COD)] 2 + after and the MY+ ([Ir 2 ] = 1 x 10- 4 M, [MV 2 +] = 5 x 10- 3 M, 1..1 = 0.1 M (n- c4H9)4NPF 6 , CH 3CN, 22 2°C, i\ex > 420 nm). between 475 shown. nm and :!: 520 nm have -l10.D.s and The data are not 123 0 f2 • • •• •• • •• • • I • • • •• • • I •• •• •• •• I I •I aJ lO ~ N 0 0 0 0 ·a "O 'V - •• • • •• ••• ••• • • • • I - 0 l.[) lO - 8 lO • • I ••• ••• •• 0 l.[) l.[) E c ...<. - 0 0 - ~ - 8 l.[) ~ I 0 0 ~ 0 l.[) r0 124 Figure 3.3 Plot of l/l1A vs. transfer reaction Figure 3.2). t at 605 between 11A is nm for the [lr(JJ-pz)(COD)] 2 + calculated back-electronand MY+ the following from (see equation !1A = At - A 0 = -log(It/1 0 ) where I0 = -log(I 0 /Iref) A0 are before the and absorbance At = -log(It/Iref ). and intensity, the flash, At and It are intensity at time t after the flash, arbitrary value the intensity assigned beam. 10 = 450 mY. to A0 and respectively, the . absorbance and I ref of the and is an probe 125 12 10 8 6 4 2 o~----------~----------~------------~---4- o 500 1000 t (fLS) 1500 126 equation (6) ,g, where is the cell pathlength (15 em) and is ll£ the absolute . value of the difference between the sum of the redox product extinction coefficients and the sum of the extinction coefficients of the reactants at 605 nm as given in equation (7). kb = _g, • ll£ • m (6) The transient decay signal at 605 nm is dominated by the absorption of MV+, so ll£ 13,000 :t = £(MV+). From recent work by Kosower, et a/., 4 0 t(MV+) = ~ 1em - 1 at 605 nm in acetonitrile, and a linear least- 600 squares fit of the data gives kb = 1.4 :t 0.2 x 109 M- 1s- 1.'' 1 The observation of the redox 31r * 2 and pz)(COD)]2 MV 2 + shows can participate that products of electron the in suitable acceptors. In tron transfer, shown equation redox products in equation that of the rapidly (5), to these undergo give diffusion excited bimolecular with in 3 B (da *pa) 2 is state the diffusion limited, electron transfer, ground-state products with No build-up of a elec- and as the illustrated rate products [Ir(~­ reactions photoinduced back limit. between of electron-transfer experiments, (3), transfer also is near observed since the rapid back reaction of the redox products Ir 2 + and MY+ gives Consequently, this the ground state of [Ir(~-pz)(COD)h and Mv 2 +. photoinduced electron transfer does not lead to net chemistry. Our of interests in further 3[Ir(~-pz)(COD)h * and studying examining this the electron-transfer reactivity in the reactivity context of 127 classical Marcus-theory prompted us state to with a investigate series The potentials. predictions of the p,c (A +lo) outer-sphere the 3 B (da • pa) 2 excited acceptors of variable reduction and the measured values of one-electron reductions of these acceptors are shown in Table 3.1. Because of a reversible reduction couples of electrochemically irreversible quenching study. Table for 3.1 E p,c (A+ I 0 ) pyridinium Therefore, these the pyridiniums study by Meyer phate and 3-amido-N-methylpyridinium (A +lo) 0.03 V and -1.19 :!: in the were cathodic gives of for the reported in peak potentials 0.03 V vs. SCE (IJ = 0.1 using the method Olmstead, The values of E p,c (A+ I 0 ) by Meyer and reduction Espenson 4 6 shows a potentials, parameter a Based on these tials for the in good both a E correlation measured 112 (A +lo) of and linear-free-energy studies, the electrochemically relative several E p,c -1.07 potentials by a of (A +lo) analysis trends reversible of al., 4 5 et Furthermore, co-workers. the :!: M (C 2 H 5 ) 4 NCIO 4 ), respec- These of for hexafluorophos- hexafluorophosphate solution. 1 8 by estimates acceptors seem to be in agreement with the estimated E d uction number used 3-amido-N-benzylpyridinium acetonitrile elsewhere. 1 5 given a potentials co-workers E 112 for and of estimated acceptors are of acceptors with potentials, reduction values tively, lack measured at a scan rate of 200 m VIs under the same experi- recent A appropriate transfer the quenchers for electron of quenching pyridinium pyridinium E for and of in ' as us 112 with described for these (A +lo) values recent the are study by pyridinium re- the Hammett substituent effects. the reduction irreversible poten- pyridinium 128 Reduction potentials of the pyridinium quenchers. Table 3.1 E(A +lo), V vs. SSCEb Quench era (I) 4-cyano-N-methylpyridinium -0.67 (2) 4-carbomethoxy-N-rnethylpyridinium -0.78 (3) 4-arnido-N -ethylpyridini urn -0.93 (4) 3-arnido-N-benzylpyridiniurn -1.07 (5) 3-amido-N -methylpyridini urn -1.14 (6) N -eth ylpyridinium -1.36 (7) 2-rnethoxy-N-rnethylpyridinium -1.48 (8) 4-methyl-N-methylpyridiniurn -1.49 (9) 4-te rt- bu ty 1-N -eth ylpyridini urn -1.52 (10) 2,6-dimethyl-N-methylpyridiniurn -1.52 (11) 2,3,6-tr irneth y 1-N -rnethylpyridini urn -1.57 (12) 2,4,6-trimethyl-N-rnethylpyridiniurn -1.67 (13) 2,6-dirnethyl-4-methoxy-N-rnethylpyridiniurn -1.85 a. All of the compounds are hexafluorophosphate salts. b. For pyridiniums 1-3, E(A +lo) the reductions are irreversible; are the cathodic peak = E 112 (A +lo). potentials, therefore, For the Ep,c(A +lo), pyridiniums 4-13, values of E(A +lo) measured at a scan rate of 200 rn VIs. Both E 112 (A +I 0 ) and Ep,c(A +I 0 ) were measured by cyclic (22 voltarnrnetry acetonitrile solution. :!: 2°C, JJ 129 acceptors are probably electron-donating or self-consistent and electron-withdrawing agree with the qualitative effects of the substituents on the pyridinium ring . number A havior of of studies several of these reduced radicals. 4 0 ' 4 2 ' 4 3 acceptors does quenching studies. not trolled, so be competitive a transfer with to irreversible is due this to the interfere oxidized of process. electrochemical dimerization reaction with back-electron-transfer dimerization the the However, The pyridiniums and that pyridiniums appear reduced the show of our are reduced pyridiniums Other studies of excited reduced between the diffusion con- is unlikely • photoinduced states of the electron-transfer the luminescent of the reactions iridium dimer be- to electron ruthenium(II) and osmium(II) polypyridyl complexes have used some of the same pyridinium acceptors and likewise report no complications from this dimeriza tion reaction. 1 8 ' 4 4 The kinetics of the 3[Ir(1J-pz)(COD)] • 2 and (8), studied by and data were viologen, the electron-transfer the the pyridinium same were methods analyzed quenching reactions acceptors, shown used for the using the Stern-Volmer between in equation quencher methyl equation given in equation (4). The quenching reactions were done in acetonitrile solution at 22 t IJ = 0.1 strength. The with 2°C M (n-C 4 H 9 )4 NPF 6 added to maintain psuedo-constant ionic Stern-Volmer plots of ( T 0 /T) vs. [Q] for the data are 130 linear plot over of the the range of quencher concentrations. for the quenching of data trimethyl-N-methylpyridinium 3.4. The measured were calculated from the rate constants slopes of representative 3[Ir(1J-pz)(COD)] • 2 hexafluorophosphate quenching A is (kq), shown shown by 2,3,6- in Figure in linear-least-squares Table fits 3.2, of the reactions approach the constants need be data and are uncorrected for diffusional effects. Since the most diffusion-controlled corrected for stants. 2 ,l 7 (9) corrected for limit, of the diffusional The equation rapid kq is diffusional quenching measured effects relationship where the to rate obtain the activated to rate con- between these rate constants is given defined above, k , is rate constant effects, and q k0 is the the in diffusion-limited rate constant in the medi urn of interest. The values iridium of complex k0 for and the electron-transfer pyridiniums the are reactions between the calculated using the Smol uchowski equation, given in equation (I 0), where r 1 and r 2 are the radii of the reactants, '1 is the viscosity of the solution, and the other terms have their usual definitions." 7 The pyridini urn radii are calculated by averaging the van radii along the three molecular axes and vary from 3.2 A to 4.6 A. 4 8 der Waals 131 Figure 3.4 Stern-Volmer plot 3 B (da • pa) 2 excited for the state trimethyl-N-methylpyridinium oxidative of quenching [Ir(IJ-pz)(COD)] 2 of by the 2,3,6- hexafluorophosphate = 8 x 10-S M, 1-1 = 0.1 M (n-C 4 H 9 )4 NPF 6 , CH 3CN, 22 t 2°C). ([Ir 2 ] 132 \ • • ~ 0 C\J • E ... ~ 0 ~ 1.{) 0 • \• 1.{) 0 0 133 Table 3.2 Rate constants for 3[Ir(JJ-pz)(COD)] • 2 the by electron-transfer pyridinium quenching acceptors in of aceto- nitrile solution at 22 :t 2°C. Pyridinium Acceptora kq, (M"ls-1)b kq,, (M"ls-1 )c 1 9.7 :t 0.9 X 10 9 2.0 :t 0.4 X 10 10 2 7.6 :t 0.8 X 10 9 1.3 :t 0.2 X 10 10 3 8.2 :t 0.6 X 10 9 1.5 :t 0.2 X 10 10 4 4.2 :t 0.3 X 10 9 5.5 :t 0.5 X 10 9 5 4.6 :t 0.3 X 10 9 6.2 :t 0.6 X 10 9 6 2.2 :t 0.1 X 10 9 2.5 :t 0.1 X 10 9 7 7.7 :t 0.4 X 10 8 8.1 :t 0.4 X 10 8 8 6.4 :t 0.3 X 10 8 6.6 :t 0.3 X 10 8 9 6.3 :t 0.3 X 10 8 6.5 :t 0.3 X 10 8 10 5.6 :t 0.3 X 10 8 5.8 :t 0.3 X 10 8 11 2.8 :t 0.1 x 1o8 2.9 :t 0.1 X 10 8 12 2.2 :t 0.2 X 10 7 2.2 :t 0.2 X 10 7 13 1.1 :t 0.3 X 10 6 1.1 :t 0.3 X 10 6 a. The acceptors are the pyridinium hexafluorophosphates listed in corrected for Table 3.1. b. The measured quenching rate constants (kq) are not diffusional effects. c. The quenching rate constants effects as discussed in the text. (kq ') are corrected for diffusio.nal I34 [Ir(~-pz)(COD)] An average value of r = 5.0 A is calculated for the structure data.'' 9 crystal calculated equation from equation A (10) (9), values of k q , the pyridinium value for of all I 0 I 0 M"" I s-I = 1.9 ·x kD the quenchers are calculated from kq 2 from studied. and are is Using given in Table 3.2. For from -0.67 quenching V to acceptors -1.85 V 3 [Ir(~-pz)(COD)] of vs. with SSCE, 2• range = that reduce. the consistent as with et Meyer, most difficult to electron-transfer a!., have one-electron shown acceptors the from k q' = 2. 0 x I 0 10 M- 1s- 1 to k q ' is reduction rates a electron-transfer rate of 1.1 x I 0 6 M-I s- 1 for the p yr i din i u m The several with of ranging diffusion-limited trend quenching. that potentials the in these Furthermore, rates is studies by of these pyridiniums function excited states of ·. other inorganic complexes. 1 8 ' 4 4 Since tials these but generally structure, theory rate this Following the luminescent by Meyer, et state variable reduction size, shape, charge, quenching study lends itself to of the dependence the free-energy Marcus- change of the excited state of the quenching electron-transfer [Ru(bpy) 3]2 + quenching reaction. 3 ' 7 quenching of (bpy = 2,2 '-bipyridine) of the analysis of the quenching [Ir(~-pz)(COD)] by the pyridinium 2 classical electron-transfer for an electronic the used al., 1 5 a poten- and of treatment of have same on the acceptors the treatment constant excited pyridinium 3B (da • pa) 2 acceptors is dis- cussed below. The kinetic scheme for the quenching of 3 [Ir(~-pz)(COD)] 2 • by an acceptor A+ is shown in Figure 3.5 and is based on a scheme originally 135 Figure 3.5 Kinetic scheme for the electron-transfer 3[lr(IJ-pz)(COD)] • by pyridinium acceptors. 2 quenching of 136 31* r2 ' + 0 l r2 + A A+ 137 proposed by Rehm and Weller for the fluorescence quenching of a series of organic states. 5 0 ' 5 1 excited the In this formation and rate constants for the precursor complex ( 3Ir 2 *,A+) unimolecular prior scheme, k 12 and dissociation, to k 23 is the quenching of the excited state within constant for back electron transfer in regenerate the excited state k 32 . The rate from the successor is combined rate constant for lead net quenching including to processes constant the the respectively, of for The successor electron rate electron-transfer precursor complex. the back are transfer. constant to rate electron k 21 The rate (Ir 2 +,A 0 ) complex constant k 30 complex transfer to is a which give the 3.5 gives ground-state reactants. the A steady-state treatment expression the for of the activated kinetic scheme Stern-Volmer in Figure quenching rate constant (kq") shown in equation ( 11 ). ( 11) Assuming that formation of the precursor complex is rapid and electron transfer within this complex is i.e., rate-determining, k 12 >> k23' rate constant equation (11) reduces to the expression given in equation (12). k , ( 12) q The quenching rate constant (kq ") is for electron transfer within the proportional precursor complex to the (k 23 ) and the equi- 138 li bri urn constant term [~ events that the for for rna tion 30 ;(k 30 + k 32 )] lead excited quenching to net state from is the of this represents the quenching. If slow relative complex fraction back to k 12 ;k 21 ). = of The electron-transfer electron processes transfer to give net that lead to k 30 >> k 32 , equation theory for adiabatic electron terms of the complex, 5 2 successor (K 12 i.e., (12) reduces to the expression given in equation (13). In the transfer, classical the rate energy of motion (v 23 ) approach constant activation for Marcus k 23 can be expressed in • and an effective frequency (.6G 23 ) the of electron transfer converting for the free nuclear precursor complex into the successor complex, as shown in equation (14). 8 ,e According to this model, for the electron transfer successor complex (.6G 23 ) where transfer ?\/4 is .6G 23 • is related to the free-energy change converting by the the precursor expression given the contribution to the activation involving reorganization of the inner and spheres prior to electron transfer. 5 3 ,s 4 ~G • 23 ( 15) complex to the in equation (15), barrier for outer electron coordination 139 The term A is organization the sum energies for of the the inner (Ai) and electron-transfer outer (A 0 ) process, sphere re- the ex- and pressions for Ai and J\ 0 are based upon a harmonic oscillator model for the inner-sphere respectively. 6 ,s 5 solvent, into equation relates vibrations the (13) rate and a dielectric Substitution of gives the expression constant k , q to the continuum model equations (14) and (15) equation (16) that shown free-energy in for the ~G 23 for the related quenchers change electron-transfer quenching of the excited state. Equation (16) expressed in logarithmic form gives equation (17). For a series of structurally and electronically such as the pyridinium acceptors of Table 3.1, K 12 , v 23 , and A should be reasonably triplet excited constant for state [Ir(~-pz)(COD)] . of electron-transfer 2 with reactions Therefore, the the expression [RTlnv 23 K 12 - (A/4)] can be substituted by the term RTlnkq' (0) where kq' (0) is the quencher with activated ~G 23 = 0. quenching rate constant for a hypothetical The expression of equation (17) can then be simplified by the expression given in equation (18). RT Ink ' q ( 18) 140 I l1G 23 I When << 2i\, 56 the expression in equation (18) reduces to that of equation (19). The importance of equation can be couple calculated E 0 / (lr 2 +; 3Ir 2*) electrostatic of from the barriers quenching is related and (19) is apparent since values of !1G 23 the redox the acceptors involved reaction in are change for the excited-state provided reactants that and the products Specifically, l1G 23 the quenching reaction (!1G 0 / ) (8) as shown in equation (20) electrostatic work term The the included. of equation terms. for E 0 /(A+ I 0 ), bringing together to the free-energy potentials !1G 0 / where wp and is related to wr are the the potentials of the reactants as shown below. 7 l1G 23 = !1G 0 / + wp - wr Therefore, the values of .6G 23 for the (20) oxidative quenching of 3[Ir(IJ- • pz)(COD)] 2 by the pyridinium acceptors are given by equation (22). The work terms can be evaluated using the Debye-Huckel expression 1 where w P and w r are directly proportional to the products of the ionic 141 charges of transfer the products and reactants, reaction. In the electron-transfer pz)(COD)] 2 * by the actants neutral equation include Substitution gi v es an of pyridinium expression respectively, of the quenching of mono-cations, both the species, so both wP and wr (22) into equation (19) with relating the experimentally electron- products are equal and re- to zero. = wp determined 0 values of equation (23) kq" to the reduction potentials E 0 "(A+ I 0 ) as shown in equation (23 ). of Both the R Tlnkq " (0) terms 112[E o" (Ir 2 + I 3 Ir 2 *)] and of are constants. If the pyridinium obeys electron-transfer acceptors quenching follows Marcus-theory the plot of the the 3 B (da *pa) 2 is shown in data, a excited state of [Ir(1J-pz)(COD)] 2 by Figure 3.6. Because E 0 "(A +lo) are unavailable for values of RTlnkq" cellent that the follows the quenching classical are plotted linear agreement the with of region the the potentials majority vs. E of the of the and pyridiniums in of 1-13 finding values pyridiniums, so of the 112 CA +lo) and Ep,c(A +lo). The slope this curve (quenchers theoretical value of 0.5. 3 B (da *pa) 2 excited state of Marcus-theory 3.5 quenching discussed, the the E 0 "(A +lo) vs. difficulty as by I .O.G 23 I << 2i\. 5 6 for reduction Table of Figure R Tlnkq" 3.3, reversible in of in with 0.48 plot of presented acceptors of scheme 0.5· in the region where should give a line of slope This kinetic predictions, 3 [1r(IJ-pz)(COD)] * 2 of predictions for 7-13) This is in ex- suggests [Ir(1J-pz)(COD)] 2 outer-sphere electron 142 Table 3.3 R T1nkq' vs. E(A +/o) for the quenching of 3[Ir(ll-PZ)- (COD)] 2 * by pyridinium acceptors. Pyridinium Acceptora a. The RTlnkq', V 1 0.609 -0.67 2 0.598 -0.78 3 0.602 -0.93 4 0.576 -1.07 5 0.579 -1.14 6 0.556 -1.36 7 0.527 -1.48 8 0.522 -1.49 9 0.522 -1.52 10 0.518 -1.52 11 0.500 -1.57 12 0.434 -1.67 13 0.357 -1.85 acceptors are the pyridinium hexafluorophosphates Table 3.1. b. The values of E(A +I 0 ) are given in Table 3.1. listed in 143 Figure 3.6 Plot of R Tlnkq" transfer pyridinium (V) vs. quenching of acceptors in E(A +I 0 ) (V) for the 3[Ir(JJ-pz)(COD)] * 2 Table 3.1 (JJ electronby 0.1 the M (n- 144 I{) 0 I '·eN eJ"(') ~ I <:r ·~ + eiO w u Cf) Cf) +N ~ H vj -~r ~ + > > &;{ ~' ~~ -w *~ H rt') ~ C\J I 145 transfer. Furthermore, 3[Ir(1J-pz)(COD)] * 2 and the by rate constants with reductant the prediction that in (12), the E 0 (Ir 2 +; 3 Ir 2 •) on with the quenching 2,6-dimethyl-4-methoxy-N-methylpyridinium 2,4,6-trimethyl-N-methylpyridinium sistent for excited particular, state the order of -1.7 is a to -1.8 of (13) are con- very strong V vs. SSCE in acetonitrile. • on ~G 23 for a constant value of A is such that as ~G 23 becomes more negative, ~G 23 • at first decreases until -~G 23 exceeds A and ~G 23 • begins to increase. As given by equation (15), the dependence of ~G 23 ~G As depicted in Figure 3. 7, the region of the curve where called the verted region." of region" As given and by electron transfer is In quenching study, with In "normal our increasing practice, been the observed Several that predicted the to rate behavior predicted a but for this have diffusion of the for intramolecular where the "inverted region" since the quenching rates are to these questions in iridium(!) dimer with designed probe pyrazolyl-bridged electron-transfer more negative k , does q decrease diffusion-limited. "inverted region" the illustrated observed has not reaction. 8 ' 7 ' 57 ' 58 limits experiments in seem inorganic metal-metal including the value of the Figure 3.7. in organic to diffusion-limited. 5 8 -s 0 an not becomes presented, 8 ' 7 as been rate with been Recent "in- decrease electron-transfer constant, 5 8 ,s 9 the the the quenching called (16), reactants not is -A equation constant observed has < > -A is in instead bimolecular rate 23 expression force, explanations suggestion the driving in ~G where 23 systems support this Experiments system localized using a excited 146 Figure 3.7 Plot of the and quenching logarithm rate constants force in energy region, 6G 23 verted free-energy The dashed the as > -A, region, horizontal reaction, rate a and constant from references 2, 5 and 6). is ~G activation-con trolled electron-transfer (kdiff) classical kact > kdiff when ~G 23 observed the diffusion-limited driving con trolled of function of increasing model. The normal free- on the left, and in- < -A, is on the for a diffusion- 23 curve is the case the illustrated right. is for -A; under these conditions, the will be equal to kdiff (taken 147 6G >-A 23 6G =-A 23 148 states by attaching the pyridinium acceptor to the complex, are cur- rently underway in our laboratory. 6 1 As noted in Chapter singlet both 3 B (da *pa) 2 and triplet excited state molecular electron-transfer fluorescent 1B ( da *pa) 2 recent experiments that this [lr(~-pz)(COD)] 2, is states. sufficiently quenching, excited using excited excited state long-lived we have has a lifetime laser phosphorescent The as picosecond state 2 exhibits luminescence from to undergo bi- demonstrated. The of < 20 and spectroscopy stongl y does not participate of light to in ps, suggest intermolecular electron-transfer reactions. 6 2 Although conversion the reactivity electron-transfer photochemical the utilize the strong nonproductive Several [Ir(~-pz)(COD)b • rapidly products back-electron-transfer the because 3 of reducing power designed electron-transfer quenching acceptors to leads the to 3 transient is starting reactions are very 3 B (da *pa) 2 the reaction needs this [Ir(~-pz)(COD)h * mono-cation via rather to accomplish of energy return of back-electron-transfer experiments chemical the facile, rna terials, rapid. To excited state, to inhibited. be the have been tried. by the pyridinium [Ir(~-pz)(COD)] 2 +. The In an attempt to trap this cation by an added ligand, the quenching reaction of 3[Ir(~-pz)(COD)] 2 * been studied. ing the by 2,6-dimethyl-N-methylpyridinium The quenching rate decrease in intensity constant phosphorescence with are analyzed the The data equation shown below where I0 and a given and presence of by the concentration. absence measured of q uencher the kq is iodide I are amount using the of emission quencher has monitorincreasing Stern-Volmer intensities (Q), in respec- 149 ti v el Y, and is T0 the I if etime of the excited state in the absence of quencher (250 ns for 3 [Ir(JJ-pz)(COD)] • in CH CN). 2 3 This rate has also been measured for the related meth y l- N -meth ylpyridini urn hexafl uorophospha te. Both in solution in added constants are acetonitrile quenching ± 0.3 rate 1o8 M"" 1s- 1). X suggesting iodide excess Furthermore, transient in this experiment. [Ir( 1J -pz)(COD )] 2 , ion in of within no build-up done electrolyte. The error of products is is trapped not (6.0 observed by solution con- 2 ,6-dimethyl-N -methylpyridinium iodide, and form an are acetonitrile the Also, studies 2,6-di- experimental [lr(JJ-pz)(COD)] 2 + the iodide absence identical that ion taining the quencher (n-C 4 H ) 4 NI 9 of shows no visible absorbance change upon long-term irradiation. As Caspar and Gray have shown, an effective way to inhibit the unproductive back-electron-transfer thermally unstable excited of state to give For example, pz)(COD)]2 after the reaction initial [lr(JJ·pz)(COD)] 2 reacts [lr(JJ-pz)(COD)(Cl)] 2 in the dichloromethane. 6 3 presence In irradiation of dibromoethane results a in with is formed acceptors of halocarbons oxida ti ve-addi tion products. upon of a number irradiation is formed clean and upon Caspar's [Ir(JJ-pz)(COD)] 2 rapid are 3 B (da • pa) 2 1,2-dichloroethane, to that The transfer. of addition ha locarbons, with electron Ir(II)-Ir(II) two-electron net is in and the [lr(JJ- complex irradiation results the with presence photoreaction with to these of 1,2- give the 150 dibromo dimer spectral changes [lr(~-pz)(COD)(Br)] 2 and observed the irradiation are the oxidative during ethylene. The qualitative shown in Figure 3.8. mechanism A halocarbons such that as for accounts 1,2-dichloroethane and addition 1,2-dibromoethane of has been proposed. 6 3 ' 6 4 This mechanism involves the oxidative quenching triplet excited state as the primary anion species that dissociates a halide ion, thereby producing dissociated halide ion adds the partially lr(I)-Ir(II)-X intermedi- organic radical. oxidized metal dimer ate. light of In "extreme The recent instability, anion," the viewed better electron transfer formation. organic to if formation as an mixed-valence a studies not by the of the intermediate (presumably a a/., 6 5 R-X bond cleavage then second, RX-· intermediate atom-transfer radical- a an concerning the or would in give to of Ir(I)-Ir(II)-X with to et Sa vean t, inexistence inner-sphere concomitant This radical form photoprocess of the the radical might process be involving and Ir-X bond react further with the thermal electron transfer) to distribution found for At high form the final X-Ir(II)-Ir(II)-X dihalide dimer. This the mechanism photochemical concentrations dibromide tions is supported of of reactions this with substrate, the 1-bromo-2-chloroethane, This in Figure product l-bromo-2-chloroethane. the resulting however, result 3.9 with is a iridium exclusively. [lr( ~-pz)(COD)(Br )] 2 , species, mechanism shown by yield the concerted breaking, lr-Br forming step leading to the lr(I)-Ir(II)-Br dimer Low mixed predicted 6 4 by is the concentrahalide the dimer proposed electron-transfer, R-Br 151 Figure 3.8 Spectral changes upon [lr(~-pz)(COD)b tion of 3 1o- 5 M, [C2H4Br2] X irradiation and of a cyclohexane 1,2-dibromoethane = 2 M, 22 :!: sol u- ([Ir 2 ] 2°C, A > 450 nm, spectra recorded approximately 3 minutes apart). 152 .; DOL II I I ! i 009 1 ODS I I I I i ~ DOE ! i I 0 (X) 0 t-- 0 lD IJ1 '¢ 0 0 (T) 0 N 0 d cl cl cl ci ci d d 0 ~N¥8~8\' _j 002 0 d :r t- l!) z w _J w > < =- 153 Figure 3.9 Proposed pathways for 1-bromo-2-chloroethane induced ox ida ti ve addition to [lr(IJ -pz)(COD )] 2 . photo- 154 hv Br-M-M-Br 155 intermediate, than the consistent bond .6 6 C-CI pz)2(COD)2(Br) as the bromo-2-chloroethane in high d8 molecule, radical path way general, of dimers becomes the the as yield photoinduced the final bonded dimers. In particular, been documented the at suggesting that reducing is weaker formation of Ir2(~- the dimer, mixed low from another 1- latter species is or can react species. The it halocarbon concentrations. of electron the same with halide addition being this bond react dibromide platinum(II) the strongly long-lived, when yield product in does arising with C-Br can oxidative photochemistry p O H 2-)6 4 ,6 1 ,6 a 2 5 2 the to the that it competitive for that results intermediate chloroethane latter fact Photolysis to the typical the concentration, with In with halocarbons transfer d 8- from d 7 -d 7 stable is metal-metal type of dimer [Pt2(pop)4]4- photochemisty has (pop this reaction may be excited states of this reducing 3 B (da • pa) 2 excited state of electron transfer with a general for electronic configuration. In conclusion, [Ir(~-pz)(COD)h variety of such methyl as quenchers the strongly photoinduced undergoes Electron substrates. viologen or pyridinium that have reduction SSCE.6 4,6 9 As has been structurally related this sphere tron reactivity electron transfer chemistry. With obeys for one-electron mono-cations potentials shown pyridini urn to transfer the as readily of hexafl uorophospha tes Marcus-theory occurs with -1.8 V vs. electronically and Table 3.1, negative series predictions for acceptors as in adiabatic, transfer. With these acceptors, however, back to ground-state products prevents net give acceptors such as halocarbons that are outerelecphoto- thermally un- 156 stable after the electron-transfer photoinduced initial reaction electron tive addition products. electron can be transfer transfer, circumvented, yields the unproductive and net back- two-electron, iridi um(II)-iridi um(II) ox ida- 157 References and Notes 1. Balzani, V.; Bolletta, F.; Gandolfi, M.T.; Maestri, M Top. Curr. Ch em. 1978, 75, 1-64. 2. Sutin , N.; Creutz, C. J. Chem. Ed. 1983, 60, 809-814. 3. Balzani, istry V.; and Scandola, Catalysis"; F. in Gratzel, "Energy M, Resources Ed.; through Academic Photochem- Press: New York, Pure Appl. 1983; Chapter 1. 4. Julliard, M; Chanon, M. Chem. Brit. 1982, 18, 558-562. 5. Sutin, N. Accts. Chem. Res. 1982, 15, 275-282. 6. Sutin, N. Prog. Inorg. Chem. 1983, 30, 441-498. 7. Meyer, T.J. Prog. Inorg. Chem. 1983, 30, 389-441. 8. Balzani, V.; Bolletta, F.; Scandola, F. in F.; Ballardini, R. Chem. 1979, 51, 299-311. 9. Balzani, of Solar V.; Scandola, Energy"; Connolly, "Photochemical J.S., Ed.; Conversion Academic and Storage New York, of Solar Press: 1981; Chapter 4. 10. Ra bani, J ., Ed. "Photochemical Con version and Storage Energy"; Weizmann Science Press: Jerusalem, 1982. 11. Gra tzel, M., Ed. "Energy Resources through Photochemistry and Ca- talysis"; Academic Press: New York, 1983. 12. Connolly, J .S., Ed. "Photochemical Conversion and Storage of Solar 46, 159-244 and ref- Energy"; Academic Press: New York, 1981. 13. Kalyanasundaram, K. Coord . Chem. Rev. 1982, erences therein. 14. Watts, R.J. J. Chem. Ed. 1983, 60, 834-842 and references therein. 158 15. Bock, C.R.; Connor, Sullivan, D.G.; J.A.; B.P.; Gutierrez, Nagle, J.K. J. A.R.; Meyer, T.J.; Am. Chern. . Soc. Whitten, 1979, 101, 4815-4824 and references therein. 16. Sutin, N. J. Photochem. 1979, 10, 19-40 and references therein. 17. Whitten , D.G. Accts. Chern. Res. 1980, 13, 83-90 and references W.J.; Sullivan, B.P.; 2717-2738 and therein. 18. Kober, E.M.; Marshall, J .L.; Dressick, Caspar, J .V.; Meyer, T.J. lnorg. Chern. 1985, 24, 2755-2763. 19. Sutin, N.; Creutz, C. Pure Appl. 52, 1980, Chern . references therein. 20. Nocera, D.G.; Gray, H.B. J. Am. Chern. Soc. 1981, 103, 7349-7350. 21. Nocera, D.G.; Gray, H.B. Inorg. Chern. 1984, 23, 3686-3688. 22. Batterham, T.J. "NMR Spectra of Simple Heterocycles"; John Wiley and Sons: New York, 197 3; Chapter 2, Part I. 23. Biellmann, J.F.; Callot, H. Bull. Soc. Chim. Fr. 1967, 397-402. 24. Beak, P.; Mueller, D.S.; Lee, J. J. Am. Chern. 1974, Soc. 96, 3867- 3874. 25. King, L.C.; Ozog, F.J. J. Org. Chern. 1955, 20, 448-454. 26. For a see: Marshall, description J.L.; of the two-compartment Hopkins, MD.; Gray, spectrophotometric cell H.B. ACS Symp. Ser., Bordignon, E.; Gra y, submitted. 27. Nocera, D.G.; Winkler, J.R.; Yocom, K.M.; H.B. J. Am. Chern . Soc. 1984, 106, 5145-5150. 28. Rice, S.F.; Gray, H.B. J. Am. Chern. Soc. 1983, 105, 4571-4575. 29. Milder, S.J.; Goldbeck, R.A.; Chern. Soc. 1980, 102, 6761-6764 . Kliger, D.S.; Gray, H.B. J. Am. 159 30. Forster, A.W.; L.S. in "Concepts Fleischauer, P.D., of Inorganic Eds.; John Photochemistry"; Wiley and Sons: Adamson, New York, 197 5; Chapter 1. 31. Am. Chern. Soc. 1986, 108, G.S.; Mann, K.R. J. Boyd, D.C.; Rodman, 1779-1784. 32. Bird, C.L.; Kuhn, A.T. Chern. Soc. Rev. 1981, 10, 49-82. 33. Ledwith, A. Accts. Chern. Res. 1972, 5, 133-139. 34. Balzani, V.; Moggi, L.; Manfrin, MF.; Bolletta, F.; Laurence, G.S. Coord. Chern. Rev. 1975, 15, 321-433. 35. The correction transfer for diffusional quenching rate constant effects for the using r(MV 2+) measured 7.8 = electron- A and r(Ir 2) = 5.0 A is discussed Ia ter in this chapter. 36. Turro, N.J. "Modern Molecular Photochemistry"; Benjamin/Cummings: Menlo Park, California, 1978; pp 311-316. 37. Bock, C.R.; Meyer, T.J.; Whitten, J. D.G. Am. Chern. 1974, Soc. 96, 4710-4712. 38. Kosower, E.M.; Cotter, J.L. J. Am. Chern. Soc. 1964, 86, 5524-5527. 39. Espenson, J.H. "Chemical Kinetics and Reaction Mechanisms"; McGraw-Hill: New York, 1981; pp 16-21. 40. J.; Hermolin, M· ' Levin, lkegami, Y .; Sa wa yanagi, M.; previous! y reported Kosower, E.M. J. Am. Chern. Soc. 1981, 103, 4795-4800. 41. This value of kb is higher than that ence 69). An updated value of e(MV+) nm was value of used to e(MV+) calculate = 10,700 kb = 13,000 :t instead ~ 1 cm- 1 ence in the two values of kb reflects this. of the (reference (refer- 600 M- 1cm- 1 at 605 previously 38), and the reported differ- 160 42. Janik, B.; Elving, P.J. Chern. 1968, Rev. 295-319 68, and refer- ences therein. 43. Kosower, E.M. Top. Curr. Chern. 1983, 117-162 112, and references therein. 44. Nagle, J.K.; Dressick, J. Am. Chern. Soc. 1979, Nicholson, R.S. Anal. Chern. 1969, /nor g. Chern. Meyer, W.J.; T.J. 101, 3993-3995. 45. Olmstead, M.L.; Hamilton, R.G.; 41, 260-267. 46. Connolly, Espenson, P.; J.H.; Bakac, A. 1986, 25, 2169-2175. 47. Smoluchowski, M. Z. Phys. Chern. 1917, 92, 129-168. 48. Gordon, A.J.; Ford, R.A. "The Chemist's Companion"; John Wiley and Sons: New York, 1972; pp 107-109. 49. Beveridge, K.A.; Bushnell, G.W.; Stobart, S.R.; Atwood, J.L.; Zaworotko, M.J. Organometallics 1983, 2, 1447-1451. 50. Rehm, D.; Weller, A. Isr. J. Chern. 1970, 8, 259-271. 51. Rehm, D.; Weller, A. Ber. Bunsenges. Chern. 1969, 73, 834-839. 52. Electron-transfer (bpy quenching 2,2' -bipyridine) complexes by methyl quenchers listed in scheme presented ground states of the tron transfer erences of 15, to 18, of and the a number viologen Table give and states of osmium(II) several of to back electron transfer metal complexes more the excited states, and 44). By analogy, 3[Ir(1J-pz)(COD)] 2 • by methyl pyridinium follow rapidly i.e., k 30 the and kinetic to give the than back elec- >> k 32 (ref- electron-transfer viologen polypyridyl the known with [Ru(bpy) 3]2 + of is here 3.1 excited the quenching pyridinium 161 acceptors is expected this is that the has a slope to exhibit linear region 0.48, If this were not region of this plot should behavior. of the consistent k 32 . this the plot of with a slope proof that k 30 k 32 >> k 30 , I as discussed = of vs. E(A +I 0 ) R Tlnkq' assumption i.e., case, have the Further the >> linear in ref- erence 15. 53. Marcus, R.A. J. Chern. Phys. 1965, 43, 679-70 I. 54. Marcus, R.A. J. Chern. Ph ys. 1956, 24, 966-978. 55. Brunschwig, B.S.; Logan, J.; Newton, MD.; Sutin, J. N. Am. Chern. Soc. 1980, 102, 5798-5809. 56. The part of this curve defined by quenchers 7 - 13 is well within the region where I f1G 23 I << 2i\. which is less than an estimated from an unweighted, For quencher 7, f1G 23 • value of i\ nonlinear that can least-squares, -0.25 V, be calculated three-parameter fit of the data of Tables 3.1 and 3.2 to the logarithmic form of equation (16). Although that is approximately parabolic to data for only half of tion, a V is for the calculated value quenching quenchers 1n there of acetonitrile is of some i\ error 0.97 3 [Ir(~-pz)(COD)] solution. For in 2* fitting an equation the func- physically reasonable by pyridinium comparison, the a value of i\ = 0.74 V is estimated for the quenching of the excited state of [R u(bp y ) ]2 + (bpy 3 = 2,2' -bipyridine) by the bipyridinium quenchers gi ven in Table II of reference 15. 57 . Creutz, C.; Sutin, N. J. Am. Chern. Soc. 1977, 99, 241-243. 58. Miller, J.R.; Beitz, 1984, 106, 5057-5068. J.V.; Huddleston, R.K. J. Am. Chern. Soc. 162 59. Miller, J.R.; Calcaterra, L.T.; Closs, G.L. J. Am. Chern. Soc. G.L.; Miller, J.R. J. Am. Chern. Soc . Gray, H.B., unpublished 1984,' 106, 3047-3049. 60. Calcaterra, L.T.; Closs, 1983, 105, 670-671. 61. Fox, L.S.; Marshall, J.L.; Winkler, J .R.; results. 62. Winkler, J.R.; Marshall, J.L; Netzel, T.L.; Gray, H.B. J. Am. Chern. Soc. 1986, 108, 2263-2266. 63. Caspar, J.V.; Gray, H.B. J. Am. Chern. Soc. 1984, 106, 3029-3030. 64. Marshall, J.L.; Stiegman, A.E.; Gray, H.B. ACS Symp. Ser., 1986, 307, 166-176. 65. Andrieux, C.P.; Merz, A.; Saveant, J.M. J. Am. Chern. Soc. 1985, Kinetics", 2nd ed.; John Wiley and Soc. 1984, 107, 6097-6103. 66. Benson, S.W. "Thermochemical Sons: New York, 1976; p 309. 67. Roundhill, D.M. J. Am. Chern. Soc . 1985, 107, 4354-4356. 68. Roundhill, D.M.; Atherton, S.F. Inorg. Chern. 1986, 25, 4071-4072. 69. Marshall, J.L.; 106, 3027-3029. Stobart, S.R.; Gray, H.B. J. Am. Chern. 163 Chapter 4 Spectroscopic Studies of Tetracarbon yl bis( ~ -pyrazol yl)diiridi urn (I) and Related Complexes 164 Introduction The dimers versatility synthetic provides complexes. 1 ' 2 of the a a general route to In addition to the pyrazolyl-bridged iridium(!) from ligands in carbon monoxide related complexes. 6 ' 7 [Ir(~-pz)(COD)] equivalents of carbonyl ligand 2 pz *)(CO)(PR 3 )] 2 The = large of dimers displacement the binuclear number of that of iridium(!) substituted- can the be pre- cyclooctadiene substituted-pyrazolyl analogues by tetracarbonylbis(~-pyrazolyl)diiridium(I) and reactions two of these complexes rapid displacement (PR 3 ) results in each metal center pyrazole or from (pz *H the and produces phosphine variety cyclooctadiene [Ir(COD)(Cl)] 2 ,3 -s pared iridi um(I) pyrazolyl-bridged give to substituted-pyrazole) with of one trans-[Ir(~- compounds in high yields. 8 ' 9 Our spectroscopic studies su bsti tu ted-pyrazolyl-bridged plexes, presented in interactions iridium(!), and platinum(II) lower-symmetry state is excited states are metal 3B 2 excited localized containing d 8-d 8 (C 2 v) ground and in weakly a several 2, indicate previously the studied D 4h d 8-d 8 be extended may complexes. Specifically, ( 1B2 ) bonding and and triplet ( 3 B2 ) derived 1A 1 by the the pyrazolyl-bridged such as carbon of the rhodi um(I), to these 1A (da ) 2(da *) 2 1 lowest derived from the the states ligands com- description Since ancillary cyclooctadiene the configuration. transitions, and that electronic are pyrazolyl-bridged species 1 0 -l 7 metal-metal singlet of iridium(!) binuclear Chapter metal-metal of electronic from the luminescent predominantly iridium monoxide and metal- complexes phosphines 165 also are predicted to have long-lived fact, luminescence has been observed luminescent for several excited of states. the In tetracarbonyl and dicarbonyl diphosphine binuclear iridium(!) compounds. 1 6 ,l 8 This chapter focuses on [lr(IJ·pz)(C0) 2 ]2 , complexes (CH3)2pz)(C0)2]2 (pzH spectroscopic complexes pyrazole, decreases ligand, and visible absorption and K. states 77 the suggest degree and with of 3-CH 3 pzH that an The metal-metal studies = the the reflected in bridging in the at of emissive l, 3 B2 (da *pa) excited the higher-energy excited understanding of ambient 3,5- spectra the both is separation of of [Ir(IJ-3,5- 3-methylpyrazole, substitution interaction series and metal-metal increasing emission Photophysical of [Ir(IJ-3-CH 3 pz)(C0) 2 ]2 , 3,5-dimeth ylpyrazole ). these studies states is relevant to the photoph ysical properties of these complexes. temperature 166 Experimental Syntheses .M aterials. Acetonitrile Jackson), as and (Burdick n-hexane received. Jackson), and (Baker) were Tetrahydrofuran nitrogen from calcium Carbon monoxide used as hydride (Matheson) received. benzene reagent grade (EM Science) (Aldrich, 95+%, and (Burdick or better was -40 3-methylpyrazole and distilled mesh) used under prior (Aldrich, [Ir(~-pz)(COD)h, [lr(COD)(Cl)h, and to use. 97%) were [lr(~-3,5- and (CH 3 )2 pz)(COD)] 2 were prepared by the methods described in Chapter 2. Inorganic Complexes. Tetracarbonyl bis(~ -pyrazolyl)diiridi um(I), prepared by a method argon atmosphere (0.220g, 0.299 mmol) was a dark red solution. for 45 min at characteristic of vacuum, a dissolved and the product was with stream pz)(C0) 2 ] 2 were (0.050 isolated n-hexane (40 Schlenk techniques, dissolved in temperature mixture was a standard [Ir(~-pz)(C0)2l2· that solution procedures. 6 ,T ' 9 Under an [lr(~-pz)(COD)] (50 mL) to 2 give Carbon monoxide was bubbled through the solution ambient leaving published tetrahydrofuran was 2 ]2 , to related using [lr(~ -pz)(C0) in g) of n-hexane and yellow crystals argon mL). under mL) and additional This bright yellow solution was removed under blue-black residue and solution by slow evaporation to give yellow blue-black dried a solvent (50 and give The recrystallized of to The residue. vacuum. product The residue (0.080 g) was filtered, the hexane of crystals of air-stable was was [Ir(~- crystals redissolved isolated in in the 167 same manner. Yield: 0.130 g (69%). Anal. Calcd for 19.05; N, 19.15; H, 0.96; 8.88 . Found: C, H, 0.98; c 10 H 6 N40 4Iri C, N, 1H 8.91. NMR (acetone-d 6 ): 5 7.93 {d, 2 H, 3,5-H), 6.52 (t, 1 H, 4-H). Tetracar bon yl bis(~- 3-meth ylpyrazol yl)diiridi um(I), (C0 )2 ]2 , was prepared using standard Schlenk techniques under an argon atmosphere. [lr(COD)(Cl)] 2 tetrahydrofuran (30 (0.300 mL) to g, 0.447 orange-red 3-methylpyrazole mL, 1.8 mmol) and 0.994 mmol) were added to this solution argon. The reaction The which solvent product from was counterflow of mixture was stirred at ambient temperature for 20 the solution turned under vacuum benzene (50 removed in triethylammonium dryness. The [Ir(~-3-CH solid for 2 h at against purple and mL). chloride tetrahydrofuran (50 mL), and solution Excess mL, dissolved the solution. in (0.080 time was dissolved an (0.25 during was give triethylamine h mmol) a and 3 solution by-product 3 pz)(COD)] 2 precipitated. [lr(~-3-CH pz)(COD)] the This solid and was was filtered evaporated to dissolved in carbon monoxide was bubbled through ambient temperature. The 2 yellow-orange the solution was evaporated to dryness under vacuum, and the residue was dissolved in acetonitrile (40 CH3pz)(C0)2]2 solvent was with washed Yield: 1.53; N, d ): 6 5 8.51. 7.72 (d, solution was filtered, and the was crystallized by slow evaporation stream of argon . The air-stable, crystalline acetonitrile (2 mL) for C 12H 1oN404Iri 1.55; N, with g This product a 0.210 mL). ice-cold (71%). Anal. Found: C, 1 H, Calcd 22.08; 5-H), 7.71 H, {d, 2.42 (s, 3 H, 3-CH 3 ), 2.41 (s, 3 H, 3-CH 3 ). H, and dried 8.51. 1H 5-H), 6.31 under C, [Ir(~-3- of the product vacuum. 21.88; H, NMR (acetone- (m, H, 2 4-H), 168 Tetracar bon yl bis(~- 3,5-dimeth ylpyrazol yl)diiridi um(I), (CH3)2pz)(C0)2]2, an argon was atmosphere. tetrahydrofuran g, solution had for (100 8 turned using Carbon solution mmol) 1.17 prepared h monoxide mL) at orange standard the Schlenk was vacuum and was filtered, solution of slightly the give large stable crystals temperature. end of the orange were dried with a the product that N, 2 (0.925 initial purple indicative solvent was of removed heated, give a saturated refrigerated for 24 at and evaporated 40-45° C that was to [Ir(~-3,5-(CH ) pz)(C0) ] . crystals of collected, washed argon with h to 3 2 22 The air- ice-cold hexane (5 mL), The remaining solution was slowly evaporated to yield were dried under 1H 8.24. a reaction, a second crop vacuum. of smaller NMR (acetone-d 6 ): crystals Yield: 0.660 g (82%). c 14 H 14 N 4 0 4 Iri C, 24.49; H, 2.05; N, 8.16. for H, 2.09; of through This solution under vacuum. stream The under residue was dissolved in n-hexane. product and Calcd the bubbled 32 The under techniques [lr(~-3,5-(CH ) pz)(COD)] of ambient by [Ir(~ -3,5- B 6.10 (s, I of Anal. Found: C, 24.50; H, 4-H), 2.35 (s, Iif etime measurements 6 H, 3,5-CH 3 ). Physical Measurements Materials. Solvents were for purified, the if absorption, necessary, pump-thaw cycles torr), and bulb-to-bulb flasks equipped Jackson, UV on with Grade) a degassed with high-vacuum distilled Teflon was emission, used as a line into vacuum and glass valves. received, minimum of five freeze- (limiting pressure < 10- round-bottomed storage Acetonitrile degassed, and 3 (Burdick and stored over 169 alumina (Woelm N, Activity Biochemicals) that had (< 10- 3 torr) fluxed over for Grade 1, obtained from ICN Nutritional been activated by heating under dynamic vacuum 24 h. 2-Methylpentane lithium aluminum (Phillips Petroleum) was re- hydride (Alfa, 95o/o), degassed, and stored over lithium aluminum hydride. For molar the measurement extinction moderately of several coefficients air-stable in of of the the solution, absorption iridium n-hexane spectra complexes, (Baker, and the which are "Photrex" Grade) from a freshly opened bottle was used. R u theni urn tris(2,2" -bipyridine) di(hexafl uorophospha te ), [R u(bpy) 3 ](PF 6 ) 2 , was obtained from tallized acetonitrile from a solution of Dr. Michael D. Hopkins and (Burdick and recrys- Jackson) and toluene (Baker) before use. Carbon-13 D, Gold monoxide Label), and (Monsanto, 99%), tetramethylsilane acetone-d 6 (Aldrich, (Aldrich, 99.9+%, 99+atom% NMR Grade) were used as received. [Ir(IJ-pz)( 13 co) 2 h The carbon-13 for the spectroscopic of [Ir(1J-pz)(COD)] 2 situ solution Ia be led experiments in a by complex blanketing spectrophotometric was a cell prepared in 2-methylpentane with carbon- 13 monoxide gas on a high-vacuum line. The exchange of the cycloocta- diene complete ligands for carbon monoxide was upon mixing of the Cary 17 solution. Electronic Absorption Spectroscopy. Electronic absorption or Hewlett-Packard by the methods spectra 8450A given in were measured using spectrophotometer. The Chapter electronic 2. The either a samples were prepa_red absorption spectra 170 at 77 The K were measured using a quartz absorption spectra of complexes the optical dewar in of local design. aerated solutions are emission spectrophoto- described previously. 1 • identical with those in degassed solutions. Electronic Emission Spectroscopy. The meter emission spectra constructed The samples for at Caltech both prepared by the tions of complexes yield measurements.1 9 which has ture,2 a which the ambient were the were measured has methods given in quantum and 77 Chapter 2. K measurements Highly dilute solu- in 2-methylpentane were An acetonitrile solution of [R u(bpy) 3 ](PF 6 )2 , for of 0.062 at ambient indexes of refraction yield ( 1.3 716) been temperature emission ° was used as the standard . meth ylpen tane using an and The acetonitrile used ( 1.3440) for are the very quantum temperaof 2- similar, so no correction (n 1 2 /n 2 2 ) for the difference between them was made. For these line measurements, the excitation wavelength was the 436 nm Hg from a medium-pressure Hg / Xe arc lamp that was filtered with an Oriel 5645 interference samples The emission digitally ment were held For in intensities recording Systems, filter. the Inc.; a 77 K emission liquid-nitrogen-filled were corrected via data Model the 3820 a for computer data measurements, quartz finger dewar. spectrometer response 2 1 interface Line collection (On system) and with Nd:Y AG the by Instru- processing with an internal computer program. Emission Lifetime Measurements. laser Emission lifetime system which citation. The measurements has solutions been for were described the ambient made previously 2 2 temperature, a using 77 532 K, pulsed nm and exvari- 171 able in temperature Chapter order K, 2. All kinetics over the dewar. measurements were samples For the of the at least were held was emission 3 intensity half-lives. in a by the For procedures outlined decays the exhibited liquid-nitrogen-filled monitored with gas dewar a calibrated was used. first- measurements at quartz variable temperature measurements between K, a continous-flow nitrogen ture prepared copper /constantan finger 150 and The sample 77 300 tempera- thermocouple. The laser was opera ted at low power to minimize local sample he a tin g. Additional Measurements. 1H NMR spectra spectrometer. were Chemical recorded shifts are at 400 MHz reported on a JEOL GX-400 in ppm (5) vs. Larry Henling at the tetrameth ylsilane. Elemental analyses were Caltech Analytical Laboratory. obtained by Mr. 172 Results and Discussion The [Ir(~-pz)(C0) h, 2 complex [lr(~-pz)(COD)b and stable solution only in carbon obtained monoxide, in presence 2 has however, [Ir(~-pz)(C0) h tion extended cooling or crystal structure was the Recently, of from the reaction originally reported of excess carbon been isolated by between to monoxide. 9 slow evapora- hexane solutions. 6 ' 7 In addition, of [Ir(~-pz)(C0) h by two determinations 2 be X-ray independ- ent research groups have shown that this complex adopts a conformation similar of to that [lr(~-pz)(COD)] of A.6,7,2S 3.510(1) The (CH 3 )2 pz)(C0) 2 ]2 is determination shown a much has shorter easier distances are influenced shortened with pyrazolyl ligands. and From iridium-iridium and an steric interactions of at 3- and structural angles are [Ir(~-3,5- crystal structure similar conformation A)? the separation complex X-ray (3.245 the C 2 -Ir-N 1 an adopts a distance by substitution isolate, it metal-metal with su bsti tu ted-pyrazolyl to that 2 The the the has iridium-iridium ligands and of the bridging- N 1 -Ir-N 2 , N 2 -Ir-C 1 , 5-positions data, but approximately 90° for are both complexes. (117°) the is steric smaller [Ir(~-pz)(C0) 2 h than that of of the 3,5-dimethylpyrazolyl influence (123°) bridge. 7 and The reflects struc- tures of these compounds are shown in Figure 4.1. Because of our interactions of d8-d 8 binuclear excited states, we energy interest in also systematically studying complexes the prepared in the ground metal-metal and lowest- [Ir(~-3-CH3pz)(C0) 2 h, which is expected to have an iridium-iridium separation between those of 173 Figure 4.1 Structures of [Ir(IJ -pz)(C0) 2 ]2 . (a) [Ir(IJ·3,5-(CH 3 )2 pz)(C0) 2 ]2 and (b) 174 A 175 B I76 [lr(JJ-pz)(C0) 2 ]2 of [Ir(JJ-3,5-(CH 3 ) 2 pz)(C0) 2 ]2 . and [Ir(JJ-3-CH 3 pz)(C0) 2 ]2 diastereomers results from that the this NMR compound · is asymmetry of the a spectrum mixture of 3-methylpyrazol yl In the I H NMR spectrum, doublets at 6 7.72 ppm and 7.7I ppm bridge. are that suggests 1H The assigned the to 5-H resonances of the 3-methylpyrazolyl the two isomers; singlets at 6 2.42 ppm and 2.4I ligand ppm correspond to the The 4-H bridge protons are not 3-CH 3 protons of the two isomers. 6 ppm in re- solved; therefore, a multiplet grated intensities of the The IH NMR spectrum of [Ir(IJ -3-CH pz)(C0) ] is con3 2 2 for the isomers. with sis tent 5-H Stobart's is a I: I CH 3 pz)(C0) 2 ]2 is a mixture based on properties the 6.31 and 3-CH 3 observation CH 3 pz)(COD)] 2 photophysical at of each spectroscopic two isomer studies of observed. signals that diastereomeric of is suggest the mixture. 3 are inte- 1:1 ratio a complex Even isomers, The the likely though [Ir(JJ-3[Ir(JJ-3- spectroscopic to be very and similar, [lr(JJ-3-CH 3 -5-CF 3 pz)(COD)h that are presented in Chapter 2. electronic The [Ir(JJ·3-CH 3 pz)(C0) 2 ]2 , ambient temperature absorption shown [Ir(IJ-3,5-(CH 3 ) 2 pz)(C0) 2 h of some respects, correspond to of Figure resembles the lowest-energy the predominantly • in in and [a(pz),1f (CO)]) separation of [Ir(1J·pz)(C0) 2 ]2 and are spectra the solutions 4.2. The absorption that of [Ir(1J·pz)(COD)] 2 , intense band (3.245 energy of is at spectrum suggested in to localized the Since transition. A), 5 n-hexane metal-metal [lr(IJ·3,5-(CH 3 )2 pz)(C0) 2 ]2 [lr(IJ -pz)(COD )] 2 (3.2I6(1) in iridium-iridium A) is similar the 1A 1 -+ tion is not solely determined by the metal-metal distance but is to that transi- 177 Figure 4.2 absorption Electronic spectra of (a) [Ir(~ -3,5- and 2 2 at 22 :!: 2°C (n-hexane solutions). [Ir(~-pz)(C0) ] (c) 178 = c: 0 0 0 "' 0 0 ~ 0 0 ~ 0 0 ~ 0 0 0 179 ..,$; c: ..: ,.:. 0 0 "<: 0 0 0 C' 0 0 ~ 0 0 ~ 0 0 g 0 0 0 180 u ~ .... E ::: .< 0 c .... 0 g 0 0 0 o- 0 0 ~ 0 0 0 It"\ 0 0 ~ 0 0 0 I8I apparently bands influenced that exhibit by the ligands. 1 6 ancillary vibrational fine structure are high-energy side of the band corresponding to the and on the low-energy side of a band at apparent from a comparison [Ir(IJ-3-CH 3 pz)(C0) 2 h predominantly tion shifts as expected, higher and the electronic a •(dz2) localized energy begins with to shown ~ the shoulders the on This becomes more absorption spectra of in 4.2. The Figure • transi- metal-metal separation, [a(pz),?r (CO)] increasing overlap two I A I ~ 1B 2 transition 336 nm. [Ir(1J-pz)(C0) 2 ]2 , and metal-metal to of Interestingly, lowest-energy vibronically structured in structured band. Upon cooling to absorption spectrum are in shown 77 of Figure at K, each 4.3. each spectrum, and [Ir(IJ-3-CH 3 pz)(C0) 2 ]2 , pz)(C0)2]2, show attributed to I3co shows 398 bands located 410 in the nm progressions ca. of Isotopic labeling decrease in v(Ir-C) in of energy with the increase Cooling in a Nujol mull the a of to I6 500 cm-I of [Ir(IJ- that are with shift ligand. spectrum of the lower in at by to nm and metal-metal transition 365 [lr(IJ-3,5-(CH 3 ) 2 pz)(C0) 2 ]2 the manifested spectra of of is at the spectra enhancement complexes (CH ) pz)(C0) 2 h 3 2 structured considerably; the the expected The pyrazolyl sharpen bands in nm vibrational spectrum. the vibronically complex The v (Ir-C). the the K its ~ in these • [a(pz),?r (CO)] meth yl-su bsti tu tion of sample does absorption K interaction a •(dz2) in 77 not of [Ir(IJ-3,5- result in further for the inte~ac­ resolution of the spectral features. 2 4 In the context of the · molecular orbital diagram tion of two d 8 metal ions complexed with carbon monoxide ligands, 182 Figure 4.3 absorption Electronic spectra of (a) [Ir(~ -3,5- and 2 2 at 77 K (2-methylpentane glasses). [Ir(~-pz)(C0) ] (c) 183 A Q) (.) c: 0 ... .0 0 {/) .0 <I: 300 350 400 A (nm ) 450 500 184 B 300 350 400 >. (nm) 450 500 185 c 300 35 0 .tO O >. (nm ) .45 0 186 shown of Figure 4.4, 2 5 in assignments for these· pyrazolyl-bridged be suggested . 2 6 binuclear From the spectra the major bands iridium(!) recorded at carbonyl nm and 388 nm [Ir(~-pz)(C0) ] 2 2 are assigned -+ [a(pz),1r (CO)] vibronically structured bands, metal separation, proposed are 415 3 to • which bands at [Ir(~-3-CH pz)(C0) ] , 411 a •(dz2) may for [Ir(~-3,5-(CH 3 ) 2 pz)(C0)2h' localized complexes K, the for metal-metal the spectra 77 nm for in the predominantly transition. are less dependent to correspond 22 The on to the two metal- d1r(xz,yz) • [a(pz),1r (CO)] transitions. 2 7 The pz)(C0) 2 ]2 complexes cyclooctadiene from dimers 1B (da • pa) the ambient [Ir( ~- and in Figure for the 4.5, and both the these carbonyl cyclooctadiene in that to the both pyrazolyl-bridged fluorescence 3 B (da • pa) 2 excited in fluid solution. The the corresponding band luminescent ingly, similar and 2 temperature are and states excited states are fluorescence and phosphorescence complexes species, are and listed considerably the emission in phosphorescence are observed spectra are shown and quantum yields Table 4.1. emission maxima iridium(!) quantum higher than yields of at Interestyields those of for the 3 B (da • pa) 2 the excited state of the carbonyl dimers rival that of [Ru(bpy) 3](PF 6 ) 2 .2 8 The states emission and lifetimes of the 3 B (da • pa) 2 excited and of 2 [Ir(~-pz)(C0) h fluorescence energies. intensities are and The temperature-dependent. phosphorescence phosphorescence bands intensity Upon cooling to 77 K, sharpen and shift to higher shows dramatic a with the decrease in temperature. The corrected emission spectra of the enhancement 187 Figure 4.4 Molecular orbital diagram for pyrazolyl-bridged nuclear iridium(!) carbonyl complexes of c 2 v symmetry. bi- 188 Pz ,7T•co ---,r-..---.f\- a 1 ( p cr ) / ,/ / ---~ xz ,yz - - - xy MONOMER / / --- - - al,b2 ___ bl,a2 - - - al,b2 189 Figure 4.5 emission Corrected spectra of [Ir(~-3,5- (I) (3) pz)(C0) 2 ] 2 at ambient temperature (2-methylpentane tions). The spectra are scaled to the same sensitivity. [Ir(~- solu- 190 >. ·---cr. c ~ c c c cr. cr. r I ,.E 450 500 550 600 650 700 WAVELENGTH 750 800 850 900 191 Table 4.1 Emission band maxima and quantum yields for the fluorescent 1B (da • pa) states of and 2 phosphorescent pyrazolyl-bridged 3B 2 (da • pa) excited iridium(!) binuclear carbonyl complexes. Iridium Complexa Emission Band Maxima (nm)b Quantum Yieldsc 1B (da • pa) 2 1B2 3B2 3 2 535 740 0.0051 0.096 3 537 742 0.0035 0.079 540 743 0.0014 0.032 represent [Ir(~-pz)­ (~-3,5-(CH ) pz) (~-3-CH pz) ( ~ -pz) a. The 3 B (da • pa) 2 complex designations are abbreviated and (C0)2]2 and the substituted-pyrazolyl analogues. b. The emission band maxima and quantum yields were measured for 2methylpentane solutions of these complexes. c. An yield acetonitrile for solution emission of of [Ru(bpy) 3 ](PF 6 )2 , 0.062, 2 0 numbers are accurate to :!: 20%. was used which has as standard. the a quantum The 192 these three complexes at 4.6. solution parallel 580 respectively, The ns that the temperature triplet at at 350 in in 2-methylpentane glasses are shown excited-state the luminescence and from variations K 3 B (da *pa) 2 Figure ns, 77 ambient the excited ambient temperature constant state. states of because the ~em tetracarbonyl < 0.1 at ture dependence of T obs 4.7, can to a be fit for to 77 K 10.2 for nonradia ti ve to the is IJS This implies primarily decay study at (knr) of due to of the [Ir(IJ-pz)(COD)h, outlined dimers ambient for lifetimes lifetimes observed K the in the 10.9 IJS at 77 of and 2, 2-methylpentane [lr(IJ-pz)(C0) 2 ]2 . Analogous 3 3 and in for [lr(~..t-3,4,5-(CH ) pz)(COD)]2, Chapter increase to temperature, dependence rate lifetimes in (T obs) are of the 3 B (da *pa) 2 excited equal to (1 /knr), for the tempera- approximately temperature. The these complexes, three modified-Arrhenius data expression shown in Figure the form given of in equation (1 ). In this expression, ti ve decay rate the activation at knr 0 and knr 'exp(-Ea/kb T) represent low and high respectively, energy pathway leading to values of for for deactivation temperatures, the of thermally-accessible the excited the and 1300, 550, and 650 cm- 1, respectively. 2 9 the nonradiaand Ea is nonradia ti ve state. The calculated states of [lr(~..t-3,5- are 193 Figure 4.6 emission Corrected spectra of (a) [Ir(~-.~-3,5- and [Ir(~-.~-pz)(C0) ] 2 2 at 77 K (2-methylpentane glasses). (c) 194 A I t I >- ·.c-, -\) c c: ·-., 0 -~ E ~ 45(1 500 600 650 ?00 I"'AVELENGTH 150 BOO ISO 900 195 B I ~\ /~ I L i I I I \~ / I J \ I f-I Ii l I ->. V'l c: 0 c: c: 0 V'l V'l E (.J.l r ,I ' r r L I ' ( I I ~I I I _.----..-.__.____/ / ' \ \I ·~ / '' ,, i ___,I '- - ~- SCI G 550 600 I I I : I I I I \ ! i I I t I; ~ I ~ \ I I I I I II \ \ ,· I j \ I I ' ~ 650 700 ~JA'vELENGTH 750 81)0 850 I 90 0 196 c I II r- I I I if\ ; .' ( ... ...c:c: ~ I 0 Vl Vl E ~ I lI tII ~ I I I X 25 ~ F 4S C! 500 I ) 550 ---- I I I '~\ \I ~ / I l I J - ~ I \ ' ( u c: \ J >. Vl I ' i ! I \ ~ I l \ \ I \\ I I I \ \ \ \\ 600 650 "'- 700 WA'v'ELEt·iGTH 750 80CJ 85C I S, 0(; 197 Figure 4.7 plots times [lr(~-3,5-(CH ) pz)(C0) h of of the 3 B (da *pa) 2 Arrhenius 3 2 (C0)2]2 (!), and [Ir(~-pz)(C0) 2 J 2 (•). 2 excited-state (•), life- [lr(~-3-CH pz)- 3 198 ... 0 ,..: • 0 <D f'() 0 X ....._ ......... • • . .. • • • • • • • • • .... .. .. .. • .. .. .. .. .. 0 • . •• • I{) • • • • • • •• • 0 .q: 0 ,..; ~ ~ !2 (sqo~/1) Ul ~ = 199 The activation 3 B (da *pa) 2 excited for the for these carbonyl complexes are much cyclooctadiene species discussed in Chapter these iridium carbonyl state than those of Since the singlet-triplet is the considerably possibility of larger both excited and state metal-metal the with increase in for the that proposed the d 8 -d 8 rhodium higher-energy metal Furthermore, energies, is ruled lifetimes of the yields the decrease with predominantly transition shifts metal-metal separation. A 2. dimers fluorescence [a(pz),?r*(CO)] decay of isocyanide d-d This state, separation, quantum lower out. triplet increasing metal-metal higher to the reasonable energy mechanism 3 B (da *pa) excited state, related to 2 the deactivation of the 3 A 2 u(da *pa) excited state of nonradia ti ve for delayed activation ambient-temperature separation. -+ citation. measured emission a *(dz2) a than the localized the in the deactivation thermal splitting thermally-activated In teres tingly, the of energies the complexes, 11 ,l 7 activation to dxz'dyz -+ dx2 -y2 ex- be independent of the pathway for rapid presented in excited state derived whose energy would provides a is thermal from metal- relaxation to the ground state. The of spectroscopic studies this chapter the series and complexes [Ir(~ -pz)(C0) 2] 2 reveal interesting features in the visible and emission spectra at ambient temperature and 77 K. order separation increases in the and degree of metal-metal the predominantly tion. of The metal-metal photophysical The metal-metal ~-3,5-(CH ) pz 3 2 < ~-3-CH pz < interaction is reflected in localized studies absorption of ~-pz, the -+ [a(pz),?r (CO)] • transi- emissive l, 3 B (da • pa) 2 excited da *(dz2) the 3 200 states suggest pathway states. for that a thermal higher-energy deactivation d-d of the excited state 1uminescen.t may • provide (da pa) a excited 201 References and Notes I. Beveridge, S.R.; Stobart, Bushnell, K.A.; Atwood, G.W.; J.L.; Dixon, Zaworotko, Eadie, K.R.; J. MJ. Chern. Am. D.T.; Soc. 1982, 104, 920-922. 2. Eadie, A.W.; Coleman, D.T.; Stobart, S.R.; Zaworotko, MJ.; Zaworotko, MJ.; Atwood, J.L J. Am. Chern. Soc. 1982, 104, 922-923. 3. Bushnell, G.W.; Fjeldsted, D.O.K.; Stobart, S.R.; Knox, S.A.R.; Macpherson, K.A. Organometallics 1985, 4, 1107-1114. 4. Bushnell, G.W.; Fjeldsted, D.O.K.; Stobart, S.R.; Zaworotko, MJ. S.R.; Atwood, J.L.; Chern. Commun. 1986, Can J. Chern. Chim. Acta J. Chern. Soc., Chern. Comrnun. 1983, 580-581. 5. Beveridge, K.A.; Bushnell, G.W.; Stobart, Zaworotko, M.J. Organornetallics 1983, 2, 1447-1451. 6. Harrison, D.G.; Stobart, S.R. S.; Rettig, S.J.; J. Chern. Soc., 285-286. 7. Nussbaum, Storr, A.; Trotter, J. 1985, 63, 692-702. 8. Powell, J.; Kuksis, A.; · Nyburg, S.C.; Wg, W.W. Inorg. 1982, 64, L211-L212. 9. Atwood, D.T.; J.L.; Stobart, Beveridge, S.R.; K.A.; Zaworotko, Bushnell, MJ. G.W.; lnorg. Dixon, Chern. K.R.; Eadie, 23, 4050- Kliger, D.S. 1984, 4057. 10. Mann, K.R.; Gray, H.B. Adv. Chern. Ser. 1979, 17 3, 225-235. 11. Rice, S.F.; Milder, S.J.; Gray, H.B.; Goldbeck, R.A.; Coord. Chern. Rev. 1982, 43, 349-354. 12. Rice, S.F.; Gray, H.B. J. Am. Chern. Soc. 1981, 103, 1593-1595. 202 13. Dallinger, R.F.; Miskowski, V.M; Gray, H.B.; Woodruff, W.H. J. Am. Chern. Soc. 1981, 103, 1595-1596. 14. Rice, S.F.; Gray, H.B. J. Am. Chern. Soc. 1983, 105, 4571-4575. 15. Che, C.-M; Butler, L.G.; Gray, H.B.; Crooks, R.M; Woodruff, Institute of W.H. J. Am. Chern. Soc. 1983, 105, 5492-5494. 16. Smith, T.P. Ph.D. Dissertation, California Technol- ogy, Pasadena, 1982. 17. Rice, S.F. Ph.D. Dissertation, California Institute of Technology, Pasadena, 1982. 18. Marshall, J.L; Fox, L.S.; Gray, H.B., unpublished results. 19. Demas, J.N.; Crosby, G.A. J. Phys. Chern. 1971, 75, 991-1024. 20. Caspar, J.V.; Meyer, T.J. J. Am. Chern. Soc. 1983, 105, 5583-5590. 21. Parker, C.A.; Rees, W.T. Analyst 1960, 85, 587-600. 22. Nocera, D.G.; Winkler, J.R.; Yocom, K.M.; Bordignon, E.; Gray, H.B. J. Am. Chern. Soc. 1984, 106, 5145-5150. 23. The iridium-iridium distance in 2 2 is [lr(~-pz)(C0) ] reported to be H.B., un- 3.51 0(1) A in reference 6 and 3.506 A in reference 7. 24. Hopkins, M.D.; Miskowski, V.M; Marshall, J.L.; Gray, shown Figure 4.4, published results. 25. In the orbital the molecular orbital is in higher difference in diagram energy energy in than the d 1r(xz,yz) between the da and orbitals. d1r the a 1(da) AI though orbitals is not known, the da orbital is likely to be much closer in energy to the d?f orbitals than is suggested from this diagram. 26. For a discussion rhodium(!) and of the iridium(!) absorption spectra of trans-[M(L)(L / ) 2 (CO)] square-planar, d8 complexes see: 203 Brady, R.; Jr.; Vaska, I sci, H.; Flynn, L. B.R.; G.L.; H.B.; Gray, 1485-1488; Chern. 1976, 15, J .; Mason, W.R. Inorg. Chern. and 17 for a discussion of complexes of rhodium(!) and lnorg. Li tren ti, Geoffroy, Peone, J., Geoffroy, G.L.; 16, 1950- 1977, 1955. 27. See references 16 d 8-d 8 square-planar, this transition iridium(!) in with bridging isocyanide ligands. 28. Kalyanasundaram, K. Coord. Chern. Rev. 1982, 46, 159-244. 29. Because the transition regions of the between the high- data for [Ir(~-3,5-(CH ) pz)(C0) ] lifetime and low-temperature 3 2 22 is fairly close to ambient temperature, the number of data points for the thermally-activated limited, so the error the other two complexes. process, in Ea for from this which Ea complex is is determined, greater than is for