Reactions of Oxygen Containing Ligands in the Coordination Sphere of Permethylhafnocene and Permethyltantalocene Citation Asselt, Allan van (1987) Reactions of Oxygen Containing Ligands in the Coordination Sphere of Permethylhafnocene and Permethyltantalocene. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/PF6E-KV88. https://resolver.caltech.edu/CaltechTHESIS:02192014-152435840 Abstract A series of terl-butylperoxide complexes of hafnium, Cp* 2 Hf(R)(OOCMe 3 ) (Cp* = ((η 5 -C 5 Me 5 ); R = Cl, H, CH 3 , CH 2 CH 3 , CH 2 CH 2 CH 3 , CH 2 CH 2 CH 2 CH 3 , CH 2 CHMe 2 , CH=CHCMe 3 , C 6 H 5 , meta-C 6 H 3 (CH 2 )2) and Cp*(η 5 -C 5 (CH 3 ) 4 CH 2 CH 2 CH 2 )Hf(OOCMe 3 ), has been synthesized. One example has been structurally characterized, Cp* 2 Hf(OOCMe 3 )CH 2 CH 3 crystallizes in space group P2 1 /c, with a = 19.890(7)Å, b = 8.746(4)Å, c = 17.532(6)Å, β = 124.987(24)°, V = 2498(2)Å 3 , Z = 4 and R F = 0.054 (2222 reflections, I > 0). Despite the coordinative unsaturation of the hafnium center, the terl-butylperoxide ligand is coordinated in a mono-dentate ligand. The mode of decomposition of these species is highly dependent on the substituent R. For R = H, CH 2 CH 3 , CH 2 CH 2 CH 3 , CH 2 CH 2 CH 2 CH 3 , CH 2 CHMe 2 a clean first order conversion to Cp* 2 Hf(OCMe 3 )(OR) is observed (for R CH 2 CH 3 , ΔHǂ = 19.6 kcal•mol -1 , ΔSǂ = -13 e.u.). These results are discussed in terms of a two step mechanism involving η 2 -coordination of the terl-butylperoxide ligand. Homolytic O-O bond cleavage is observed upon heating of Cp* 2 Hf(OOCMe 3 ) R (R = C 6 H 6 , meta-C 6 H 3 (CH 3 ) 2 ). In the presence of excess 9,10-dihydroanthracene thermolysis of Cp* 2 Hf(OOCMe 3 )C 6 H 6 cleanly affords Cp* 2 Hf(C 6 H 6 )OH and HOCMe 3 (ΔHǂ = 22.6 kcal•mol -1 , ΔSǂ = -9 e.u.). The O-O bond strength in these complexes is thus estimated to be 22 kcal•mol -1 . Cp* 2 Ta(CH 2 )H, Cp* 2 Ta(CHC 6 H 5 )H, Cp* 2 Ta(C 6 H 4 )H, Cp* 2 Ta(CH 2 =CH 2 )H and Cp* 2 Ta(CH 2 =CHMe)H react, presumably through Cp* 2 Ta-R intermediates, with H 2 O to give Cp* 2 Ta(O)H and alkane. Cp* 2 Ta(O)H was structurally characterized: space group P2 1 /n, a= 13.073(3)Å, b = 19.337(4)Å, c = 16.002(3)Å, β = 108.66(2)°, V = 3832(1)Å 3 , Z = 8 and R F = 0.0672 (6730 reflections). Reaction of terlbutylhydroperoxide with these same starting materials ultimately yields Cp* 2 Ta(O)R and HOCMe 3 . Cp* 2 Ta(CH 2 =CHR)OH species are proposed as intermediates in the olefin hydride reactions. Cp* 2 Ta(O 2 )R species can be generated from the reaction of the same starting materials and O 2 . Lewis acids have been shown to promote oxygen insertion in these complexes. 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): Bercaw, John E. Thesis Committee: Collins, Terrence J. (chair) Bercaw, John E. Gray, Harry B. Dougherty, Dennis A. Defense Date: 8 June 1987 Non-Caltech Author Email: vanassea (AT) mcpherson.edu Funders: Funding Agency Grant Number DuPont UNSPECIFIED Shell UNSPECIFIED Record Number: CaltechTHESIS:02192014-152435840 Persistent URL: https://resolver.caltech.edu/CaltechTHESIS:02192014-152435840 DOI: 10.7907/PF6E-KV88 Default Usage Policy: No commercial reproduction, distribution, display or performance rights in this work are provided. ID Code: 8087 Collection: CaltechTHESIS Deposited By: Benjamin Perez Deposited On: 20 Feb 2014 16:44 Last Modified: 21 Dec 2019 03:08 Thesis Files Preview PDF - Final Version See Usage Policy. 12MB Repository Staff Only: item control page

REACTIONS OF OXYGEN CONTAINING LIGANDS IN THE COORDINATION SPHERE OF PERMETHYLHAFNOCENE AND PERMETHYLTANTALOCENE Thesis by Allan van Asselt In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California, U.S.A., Earth 1987 (Submitted June 8, 1987) ii Acknowledgements I guess it is time to finish some twenty odd years of formal education. The past five and a half years at Caltech have been very special and many people have made the experience enjoyable. At the top of the list are members of the Bercaw group, both past and present. Nancy and Jimmy showed me the ropes on Bercaw vacuum lines. DMR, Greg Hillhouse, Larry Fong and Jaun Manriquez exemplified different approaches for entering alternate space. Eric and Paul both played the tuba, and I enjoyed the years playing with them in the Caltech wind ensemble and jazz band. Don, along with the Grubbs group, gave me my first introduction to rock climbing at Caltech. Facilities for water sports on second floor Noyes, were kindly provided by Chris. Tippy Thompson, soon to be a professor at Princeton, has shown me that anything is possible if you catch the right wave. He was also a great housemate and friend. So are Marty and Cindy, who understand my need for rainbows. Lura Street will live on in my heart. Rocco has provided hours and hours and hours of conversation, and has loaned me a few books, although I never seem to finish them. Ray is the only man in the Bercaw group I ever wanted and couldn't have. I thank Barb for the birthday cakes and the wonderful aerobics tights. I pray for Ramsey. It wouldn't do to forget our slimy brit postdocs, Vern, the perfect English gentleman, Dai, proof positive that Britain sent all of her gallant young men overseas, and Ged, the heyokah. I'm glad Trim was part of the lunch crew, otherwise I might never have heard him talk. Emilio, who works over lunch, makes me feel guilty. Pam won't have me to chase into men's restrooms anymore, I suggest it may be time to pick on Ray. I'm sorry to leave LeRoy without a hackysack partner, but I'm happy to leave him with the vacuum line and Janet, before she starts doing sulfur chemistry. And our visiting prof, John Moss, is, after all, John Moss. Claudia, the newest member of the Bercaw group, has been a magic friend. I love you all. While at Caltech, I ended up in a group that not only does fun science, but goes on skiing and backpacking trips together. Back in the old days, we even used to go out drinking together. Thank you, John, for making it all possible. Others at Caltech have been special friends as well. Leon Gelles always defined that sense of cultured bitterness, so neccessary at times. The Space Weapons Study Group has been alot of fun over the years, and I've had the chance to work with some wonderful people on projects other than science. My family is always with me. DuPont and Shell helped support my habits of living under a roof and eating every day. For this, I am grateful. iii Abstract: A series of terl-butylperoxide complexes of hafnium, Cp*2Hf(R)(OOCMe3) (Cp* = (('15-CsMes) ; A = Cl, H, CH3, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3, CH2CHMe2. CH=CHCMe3, C5H5, meta-C5H3(CH3}2} and Cp*('75-cs(CH3}4CH2CH2CH2}Hf(OOCMe3}, has been synthesized. One example has been structurally characterized. Cp*2Hf(OOCMe3}CH2CH3 crystallizes in space group P21/c, with a= 19.890(7)!, b = 8.746(4)A, c = 17.532(6)!, {3 = 124.987(24) 0 , V = 2498(2}A3, Z = 4 and RF = 0.054 (2222 reflections, I > 0}. Despite the coqrdinative unsaturation of the hafnium center, the terl-butylperoxide ligand is coordinated in a mono-dentate ligand. The mode of decomposition of these species is highly dependent on the substituent A. For A = H, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3, CH2CHMe2 a clean first order conversion to Cp*2Hf(OCMe3)(0R} is observed (for A = CH2CH3, t.Hf = 19.6 kcal·mor1, t.Sf = -13 e.u.}. These results are discussed in terms ofa two step mechanism involving '72-coordination of the terl-butylperoxide ligand. Homolytic o-o bond cleavage is observed upon heating of Cp*2Hf(OOCMe3}R (A = CeHe. meta-c6H3(CH3}2}. In the presence of excess 9,1Q-dihydroanthracene thermolysis of Cp*2Hf(OOCMe3}C5H6 cleanly affords Cp*2Hf(C6H6}0H and HOCMe3 (t.Hf = 22.6 kcal·mol-1, t.Sf = -9 e.u.}. The o-o bond strength in these complexes is thus estimated to be 22 kcal mor1. Cp*2Ta(CH2}H, Cp*2Ta(CHCsHs}H, Cp*2Ta(CsH4}H, Cp*2Ta(CH2=CH2}H and Cp*2Ta(CH2=CHMe}H react, presumably through Cp* 2Ta-R intermediates, with H20 to give Cp*2Ta(O}H and alkane. Cp* 2Ta(O)H was structurally characterized: space group P21/n, a= 13.073(3)!, b = 19.337(4)!, c = 16.002(3)A, {3 = 108.66(2} o, V = 3832(1 )A3, Z =8 and RF =0.0672 (6730 reflections}. Reaction of terl- butylhydroperoxide with these same starting materials ultimately yields Cp*2Ta(O}R and HOCMe3. Cp*2Ta(CH 2=CHR}OH species are proposed as intermediates in the olefin hydride reactions. Cp* 2Ta(02}R species can be generated from the reaction of the same starting materials and 0 2. Lewis acids have been shown to promote oxygen insertion in these complexes. tv Table of Contents Acknowledgements ii Abstract iii An Introductory Statement v Chapter 1: Metal Based Oxidation Systems 1 Chapter 2: tert-Butyl Peroxide Complexes of Permethylhafnocene, 21 Cp*2Hf(R)(OOCMe3) Chapter 3: Oxygen Derivatives of Permethyltantalocene 55 v An Introductory Statement When lightning struck the forest, sending flames leaping into the air, an awareness of the awesome power of oxidation chemistry was forced on humankind. After a time, it became possible to generate and preserve fire and apply its power to chemically modify food stuffs. Technology advanced, and torches were developed to give light where none was available before. The exploitation of controlled oxidation processes had begun. The study of combustion has drawn chemists like moths to a flame, and, consequently, the field has evolved quite a bit since the early days. Even so, much remains to be understood concerning the fundamental processes of oxidation. What follows is but a minor contribution to this vast and historic field of enquiry and, in the end, it may not be as Interesting as sitting In front of a campfire In the mountains. But they don't fund too much research in that area, and you do what you can to get_by. 1 CHAPTER 1 Metal Based Oxidation Systems 2 Introduction The selective transfer of oxygen is a process of fundamental importance. Oxygen breathing organisms are literally kinetically controlled oxidation systems, harnessing the exothermicity of oxidation reactions to drive all other processes. On the molecular level, biological systems perform extremely selective oxygen transfer to carry out chemical synthesis. For industry, the oxygenation of petroleum derived feed stocks is often the first step in converting these materials to more useful chemicals. The synthesis of complex organic molecules often requires sources which will deliver oxygen with extreme regio-, stereo- and enantio- selectivity. All of the above processes are complicated by the nature of the dioxygen molecule, which in its ground state contains two unpaired electrons. Most substrates, by contrast, have singlet ground states and no unpaired electrons. Direct reaction of triplet ground state 02 with singlet ground state substrates is a spin forbidden process and generally involves one electron processes with large activation energies. The subsequent radical chain reactions often lead to multiple products. This limits the usefulness of d irect reaction with 0 2 for reactions when selective oxygen transfer is desired. Radical chain reactions are not always nonselective, as can be observed in the controlled autoxidation of isobutane.1 The product, tert-butylhydroperoxide (TBHP), still contains active oxid izing equivalents and is a singlet ground state molecule. radical initiator (CH3 )3CH + ---~1> (CHJ ) 3COOH 80 - 100°C low conversion of RH This suggests, along with the processes observed in biological systems, that one might be able to use derivatives of the 0 2 molecule to circumvent the problems encountered during 3 direct reaction with oxygen. This has, of course, proven to be a viable approach, and most oxidation chemists are occupied with the task of generating clean and selective sources of "preactivated " oxygen and studying their reactivity. These preactivated sources of oxygen have the additional advantage over~ that one can control the electronic and steric environment of the oxygen transfer step. Among these sources are: organic peracids, singlet oxygen, alkylhydroperoxides and hydrogen peroxide in conjunction with transition metal or main group templates, metal complexes which react directly with 02, metal oxo complexes and metal based systems, e.g., the Wacker process, which make indirect use of ~·s oxidizing abilities to reoxidize the metal center. In general, the pursuit of selective oxidation systems has been highly empirical, with little understanding of the actual details of the chemical processes of oxygen transfer. A greater understanding of the mechanisms of oxygen transfer might allow the design of new oxidation systems and the improvement of existing ones. Early Transition Metal Oxidation Systems Our primary focus will be on metal based oxidation systems, most specifically on the reactions of earfy transition metal group N and V complexes with alkylhydroperoxides and ~. Historically, metal based oxidation reactions have been poorfy understood. The active oxygen transfer intermediate was often written as an ill defined metal adduct. More sophisticated speculation concerning metal based oxygen transfer mechanisms has appeared in recent years,1 however, experimental evidence supporting these speculations is still relatively ambiguous. Clean systems for detailed study have been difficult to find. Well characterized model complexes for key proposed intermediates are few and far between. The permethyl- hafnocene and tantalocene systems have provided exceptionally nice examples of proposed intermediates, some new reactivity, and limited mechanistic information. These systems will be discussed in the following chapters. First, it may be 4 useful to examine related oxidation systems, key proposed intermediates and selected models for oxygen transfer. Reactions of Alkvlhydroperoxides w ith Transition Metals: Alkylhydroperoxides, In conjunction with transition metal templates, have proven to be highly successful reagents for selective oxygen transfer to such substrates as olefins, allyl alcohols, sulphides and amines. 1 Transition metal alkylperoxide complexes are often proposed as key intermediates for these reactions. In other systems, metal hydroxide and metal oxo complexes are thought to be the key intermediates in olefin epoxidations and hydroxylation of alkanes, alkanes and arenas. One can classify the most common reactions of alkylhydroperoxides with transition metals into three broad categories. 1) The first involves a formal protiolytic attack of the alkylhydroperoxide (H+ ·ooR) on the metal center (M-X) to give a M-0-0-R complex and HX.2 The oxidation state of the metal does not change, and these reactions only occur if HX is a reasonable leaving group. Pt (OH) + HOOR ------It> Pt(OOR) The photochemical substitutions of cobalt(lll) alkyls with alkylhydroperoxide have the same stoichiometries but probably proceed by a radical mechanism.3 hv Co(III)R' + HOOR The proposed mechanism is listed below. Co(III) (OOR) + R'H 5 Co(III)R' ----1t> Co (II) ----t> R'H + + R' . + HOOR ROO · + Co(III)R' ----t> Co ( III)OOR ROO· + Co(II) ----t> Co(III)OOR · R' · OOR + ·R' 2) · Category two reactions occur when the metal attacks the alkylhydroperoxide 0-0 bond as a .nucleophile4 in a manner similar to the proposed mechanism for olefin epoxidations by peracids.s >0 + RCOOH Hs+ / \. M···0-0 o ~ (TPP)Mn(II) + M=O ROH + R HOOR ---t> (TPP )Mn(IV) (O) + HOR (TPP - meso -tetraphenylporphyrinato) These reactions proceed by H+ transfer to the ·oR group of the alkylhydroperoxide and generate metal oxo complexes and ROH. This reaction mode is favored when the metal can undergo a facile two electron oxidation and R is an electron withdrawing group (peracids, where R =-COR', are an extreme example of this, and often carry out this type of reaction with greater facility than normal alkylperoxides). 6 3) In the third category are reactions which involve electron transfer (analogous to the Haber-Weiss mechanism for Fenton's reagent) from the metal to the alkylhydroperoxide.1 ----1> M(n+1)• OH" + HOOR + M"+ M(n• 1)• + HOOR -----1> Mn+ + ·OOR HOOR 1> ROH + · OOR RO· + 2 ROO · ----1> [ ROOOOR) 1> ·OR + ROOR H+ + ~ 2 HOOR These reactions have been proposed to initially generate metal hydroxide complexes and ·OR. Subsequent reactions can lead to the catalytic decomposition of alkylhydroperoxide 1 (above) or to metal alkylperoxide complexes& (below). In some cases added substrates can be oxidized by the intermediate aikylperoxy radicals. It has also been suggested that the reaction of metal salts with trace amounts of alkylperoxides is the initiation step for metal catalyzed autoxidations. + Co(II) RO· + Co(II) HOOR + -----1> + + ROH -----1> ·OOR Co(III) (OH) 2 Co (II) -----1> HOOR HOOR 3 HOOR Co(III) (OH) + ·OR ·OOR Co(III) (OOR) ---~1> ----1> + Co(III)(OOR) 2 Co (I II) OOR + + ROH + H20 The initially formed metal hydroxide, oxo and alkylperoxide complexes are commonly highly reactive and it is often difficult to determine in which mode the initial reaction of metal complex plus alkylhydroperoxide takes place. In some systems, it is very likely that more than one mechanism is operative and, perhaps, more than one species transfers oxygen to substrate. 7 Metal Alkylperoxide Complexes: The homogeneous molybdenum catalyzed •Halcon process• and the silica supported titanium catalyzed "Shell process• are proposed to operate through metal alkylperoxide species.1 These processes are used commercially to convert propene to propylene oxide on a scale of billions of pounds per year. In the Sharpless titanium based asymmetric epoxidation system, metal alkylperoxide species are the proposed active oxygen transfer intermediates for allyl alcohol oxidation to form epoxides with very high enantiomeric excesses. 7 In addition, alkylperoxide complexes are proposed as transient intermediates in the autoxidation of group IV and other metal alkyl species. a Relatively few examples of M-0-0-R species have been isolated and studied. Successful techniques of synthesis have included photochemical substitution,3 addition of RX to metal peroxide complexes9 and most commonly, metathesis of alkylhydroperoxides with metal hydroxide, alkoxide or carboxylate species.1 0 Although alkylhydroperoxides as oxygen transfer agents have found the greatest use with early transition metals, most of the examples studied, and all but one of the complexes structurally characterized, are found in the group VIII transition metals. These late transition metal complexes will generally oxidize substrates such as carbon monoxide, phosphines and, more rarely, olefins. Relatively minor changes in the ligand environment can have significant effects on the reactivity of these complexes. For example, as one varies the trans ligand of trans-(PPh3)2Pt(R)(OOCMe3) from R =Ph, Ph-o-CN, to CF3 in dichloroethane the yield of oxygenated 1-octene rises from 46 to 55 to 80 percent respectively.2 The cis derivatives will not oxidize olefins at all. The only examples of well characterized early transition metal alkylperoxide complexes, before the work reported in Chapter 2, have been prepared in recent years by Mimoun and coworkers. Again it is interesting to note that seemingly minor changes in ligand 8 environment in these vanadium systems can cause considerable changes in the chemistry of the alkyl peroxide moiety. Two systems have been reported, both with Schiff bases as the primary ligand. With the tridentate Schiff base ligand 2,6-pyridinedicarboxylate (dipic) the complexes (dipic)VO(OOR)(L) (1) (R = t-Bu, CMe2Ph; L = H20, HMPT), which oxidize. ol~fins, alkanes and arenas by radical pathways, can be lsolated. 11 When the base is changed to the N-(2-oxidophenyl)salicylidenaminato tridentate ligand (R'-QPhsai-R") the free ligand L is no longer coordinated and the complex (R'-QPhsai-R")VO(OOR) (2) oxidizes only olefins, probably via a heterolytic pathway.12 Why some complexes react homolytically and others heterolytically remains an open question. This will be discussed a bit further jn connection with the results presented in Chapter 2. •• 2 1 A small number of metal alkylperoxide complexes have been structurally characterized by x-ray diffraction.1 3 Three different binding modes of the alkylperoxide ligand, terminal,13a,b bridging 13c and bidentate,13d are found in these structures. R R I 0 0/ I M 0/ R I 0 /'\ M M 0-0/ ' MI 9 General agreement has been reached that bidentate coordination of the alkylperoxide ligand is a probable step in the heterolytic transfer of oxygen to substrate.14 07 1 Although (dipic)VO(OOt-Bu)(L) (1) appears to react homolytically in solution, the bidentate t-butylperoxide ligand found in the solid state structure 1 1 (above) is the best experimental evidence for this commonly proposed mechanism. Consistent with this mechanism, all of the systems which cleanly transfer oxygen have open coordination sites for binding the alkoxy oxygen of the peroxide ligand. The first kinetic evidence indicating the likelihood of the bidentate binding step is discussed in Chapter 2. 13• Homolytic cleavage of the 0-0 bond will be discussed below. For non-radical olefin epoxidation reactions, the numerous proposals concerning the nature of the actual oxygen transfer step can be encompassed by two different mechanisms.1 The fundamental difference between these two interpretations lies in the still open question of whether it is necessary to coordinate olefin to the metal center before oxygen transfer takes place and what the nature of the alkylperoxide oxygen is (nucleophilic or electrophilic). Mimoun argues that olefin coordination is required to · activate the olefin towards nucleophilic attack by the peroxide (mech A). 15 Sharpless, in his proposed mechanism for early transition metal epoxidation systems, offers the 10 alternative of direct attack of the olefin on an electrophilic peroxide moiety (mech B).1 •16 The Sharpless mechanism is reminiscent of the proposed mechanism for peracid oxidations.5 Mechanism A R 0 R / I 0 1\ M ..···· A '-o 0 A )-Z + MOR Mechanism B 0 A + MOR For the reactions of (R'-QPhsai-R")VO(OOR) with olefins, Mimoun has recently offered the first kinetic evidence for olefin coordination in an a metal alkylperoxide oxidation (mach C).12 He has altered the above mechanism to suggest olefin insertion into an '1 1alkylperoxide followed by (or simultaneous with) '1 2-coordination of the alkoxide oxygen. Several recent reports of olefin insertion into metal oxygen bonds support the possibility of this type of mechanism.17 Mechanism C 0 M I R \ 0 )----\ R I 0 M .·· 'o )----\- A 0 + MOR 11 It seems quite probable that all three of these various mechanisms function in different systems. Olefin coordination in some systems may be necessary. in part, to simply bring the reactants together (metal template effect). For the Sharpless allyl alcohol epoxidations, formation of the metal allyl alkoxide presumably performs the same function and olefin C<?ordination may not be necessary. As mentioned earlier group IV metal alkylperoxide species have been invoked as intermediates in the autoxidation of metal alkyls. Blackburn, Labinger, and Schwartz were the first to explore the interaction of Cp2ZrR(CI) with ~. 1 8 They suggested metal alkylperoxide complexes as intermediates and reported the synthesis of the first isolated Group IV alkylperoxide complex, Cp2Zr(OOt-Bu)CI, from Cp2ZrCI2 and NaOOt-Bu. No characterization of the compound was reported. In 1981, Scotton and Brindley reported the isolation of bisalkoxide species from the radical chain reaction of 0 2 with Cp2MR2 (M Zr, Hf; Cp = =115-csHs; R =Me, benzyl), and suggested the bisalkoxides resulted from the decomposition of alkylperoxide alkyl intermediates.19 Their suggested mechanism for the autoxidation is shown below. Cp2fiR2 + ~ M- Zr, Hf [Cp2M(OOR)R] t> t> Cp2M(OR)2 initiation t> Cp2fiR2 R· + Cp2MR2 ~ + Cp2MR t> ·OOR + ·R · OOR t> Cp2MR(OOR) + ·R Similar transformations have been recently reported by Wolczanski and Lubben for the reaction of (tritox)2MMe2 (tritox (tritox)2M(OMe)2.8 =((CH3)3C)3CO-; M =Ti, Zr) with 02 to yield 12 Metal Qxo Comolexes: Homolytic cleavage of metal alkylperoxides leads to M"•-O · and ·OR. Writing M"•-o. is only reasonable if the metal cannot undergo further oxidation to M(n+1)•=o or M(n+1)•-e-, e.g., when starting from a d 0 metal alkylperoxide complex. As the first step in hydroxylation reactions, Intermediates of the type M-Q · have been invoked to explain hydrogen abstraction from alkanes and the allyl positions of alkanes. A plausible mechanism is shown below.1 + HR. ---t> [M"•-oH 1 ·RJ -----It> M(n-2)+ + ----It> ROH M-Q · has also been proposed to undergo electrophilic attack on arenes as the first step in arene hydroxylations. Metal oxo species can also be generated directly from alkylhydroperoxides without the Intermediate step of forming metal alkylperoxide complexes (vide supra). Alkylhydroperoxides have been used as oxygen sources to generate proposed metal oxo or hydroxo species for cytochrome P-450 model studies.4 They can also be used as oxygen sources to regenerate oxo reagents such as Os04 or Se02 for catalytic oxidations. One of the traditional mechanisms for oxidation with metal oxo reagents involves initial electron transfer from the substrate to the metal center. The substrate radical cation subsequently reacts with the metal oxo or hydroxo species to give products. Another possiblity has been suggested for the reaction of olefins with Os04: an allowed [2+2+2] direct attack of the olefin on the cis metal oxo ligands (below). [2+2+2] 0\.. 0 ~/ L-Os 4\. 0 0 Products 13 In 1977, Sharpless proposed organometallic species as intermediates in the oxidation of olefins by high valent metal oxo species.20 He reasoned that the Lewis acid, d 0 metal center is the likely site of initial olefin coordination and helps explain the apparent electrophilic nature of many of these reactions. Oxygenation takes place with olefin migration to the oxo ligand, possibly forming metallaoxetane intermediates. Cl' //O( Cr-1 ·- - - CI/ \b \ 0 CI,J~ Cr - - - - Products CI/ \'o This mechanism offers a reasonable explanation for the product stereochemistries of low temperature Cr02CI2 oxidation of olefins. It also is consistent with asymmetric induction observed when oxidizing prochiral olefins with Os04 and a chiral amine ligand.21 Nonradical electrophilic attack by metal oxo complexes on C-H bonds has also been proposed for alkane activation. Evidence has accumulated to support the proposition that organometallic species are viable intermediates in these oxidations. However, the number of relevant model compounds, e.g., metal oxo olefin and metal oxo alkyl complexes, remains small and kinetic and stereochemical evidence is still ambiguous. It seems likely that the proposed mechanisms simply represent extremes and that many reactions are intermediate in character or proceed by more than one pathway. The limited evidence in support of. organometallic intermediates is discussed below. 14 Some of the best evidence comes from a recent report by de Boer.22 At short reaction times, Cp*Re03 (Cp* = '7 5-C5Me5) reacts with phosphine in alkyne solution to yield Cp*Re(O)(RCCR). When the reaction is allowed to run longer, species of the stoichiometry Cp*Re(O)((RCCR)20) (3) are isolated. Starting with the phosphine reduced complex [Cp*Re(0)2], De Boer's mechanism for the formation of these species is shown below. These species, when reacted with l2, yield Cp*Re(O)I2 and substituted furans. RCCR (3) + ------~~ RCCR Cp*Re(O)I2 Mayer and coworkers have reported a series of interesting tungsten complexes which contain oxo ligands and coordinated olefin, W(O)(PMePh2) 2(CI) 2(H 2C=CHR) (R = H, Me, CHCH2).2 3 W(O)(PMePh2)2(CI)2(H2C=CH2) can be obtained from the addition of ethylene oxide to W(PMePh2)4CI2,24 providing evidence that the reverse of the olefin oxo complex to epoxide conversion can take place. Earlier Mayer reported aRe oxo alkyne complex.25 Although interesting as model complexes for organometallic intermediates, these compounds as of yet have not shown any tendency to undergo further reactions of the type proposed for oxygen transfer. Collman and coworkers have recently advanced kinetics and shape selectivity arguments for reversibly formed olefin intermediates in olefin epoxidation studies with Mn(TPP)CI (TPP = meso-tetraphenylporphyrinato) or Mn(TMP)CI (TMP = meso-tetramesitylporphyrinato) and lithium hypochlorite. 2 6 In further cytochrome P-450 model studies they have also reported 15 similar results for olefin oxidation with Fe(PFP)CI (PFP =pentaflourophenylporphyrinato) and F5Phlo.27 The proposed intermediates in these systems are metallaoxetanes. Groves and Watanabe have reported spectroscopic evidence for an intermediate in the low temperature epoxidation of olefins with Fe(TMP)CI and m-chloroperbenzoic acid.28 They suggest either an olefin oxo complex or a metallaoxetane species as the intermediate formed. Examples of organometallic species containing both oxo and alkyl ligands on the metal are more prevalent than species with olefin and oxo ligands.29 However, none of the examples yet reported have shown carbon oxygen bond formation, one of the key steps for the oxidation mechanism proposed by Sharpless. Indirect evidence for this type of rearrangement is provided by the V(O)CI3 reaction with Ph2Hg.30 Hydrolytic workup of the product of this reaction gives phenol. Presumably a vanadium oxo phenyl complex is Initially generated and the phenyl group subsequently migrates to the vanadium oxo group. 0 0 II v c1/l'c1 Cl II v Ph/ I 'c1 HOPh Cl A model for the reverse reaction, the migration of alkyl group from an alkoxy ligand to form a metal oxo alkyl species, is found in the rearrangement of Cp*2TaH('7 2-CH20).31 Barbara Burger has shown that the production of Cp*Ta(O)Me upon heating Cp*2TaH('7 2-cH20) probably involves the unobserved intermediate Cp*2Ta(OMe). This intermediate, which is subject to rapid ,8-hydride elimination to return to Cp*2Ta('7 2-cH20)H, occasionally rearranges by migration of the methyl group from oxygen to the metal center. ---1> <11-----'- Cp*2Ta(OCHJ) 16 Metal Dloxygen Complexes: Biological systems are required to use the ~ molecule as their primary source of oxygen. Nature makes use of metal centers in specialized environments to activate oxygen as the first step in canying out the various functions required to keep the system operational; oxygen transport, synthesis, energy conversion, etc. This has prompted a great deal of interest among chemists in modeling these types of reactions. There are a large number of late transition metal model complexes which react cleanly with ~ to form peroxide (or dioxygen) species.32 One finds very few examples of early transition metal complexes which react similarly.1 2 (TPP)TiF + ~ -----11> /0 (TPP)Ti I 'o + (TPP)TiF2 + H20 (TPP)TI(02) can also be generated from H2~· 0 (TPP)Ti-0 + H2~ -----11> (TPP)Ti/ I '0 It is, in fact, much more common with early metals to generate these species from the metathesis reactions of metal complexes with hydrogen peroxide. The late transition metal peroxide complexes in general react as nucleophiles with external substrates. By contrast, most early transition metal peroxo complexes are in their highest oxidation states and tend to react as electrophiles. Others react as radical sources. The electrophilic nature of these reagents may be due again to initially formed organometallic species. Mimoun's proposed mechanism for M(0)(02) 2 oxidation of olefins with a chiral coligand is presented below.33 17 The supporting evidence for organometallic intermediates in these systems is still thin. No early metal complexes containing both peroxo ligands and olefins or alkyls ligands have been reported. The only example of this type of proposed intermediate is lr(PPh3)2(C2H4)(02)CI and there is no evidence to indicate its rearrangement to give epoxide.34 Palladium and platinum peroxide complexes react with carbonyl compounds and activated olefins to give metallacyclic structures of the type proposed by Mimoun.35 These species do not, however, decompose to give epoxides. The first examples of isolated metal peroxide complexes with alkyl ligands will be reported in Chapter 3. 18 1) Sheldon, RA; Kochi, J.K •Metal-Catalyzed Oxidations of Organic Compounds" Academic Press, New York, 1981. 2) Strukul, G.; Michelin, A.A.; Orbell, J.D.; Randaccio, L lnorg. Chem. 1983, 22, 3706. 3) Giannotti, C.; Fontaine, C.; Chiaroni, A.; Riche, C. J.Organomet. Chem. 1976, 113, 57. 4) Yaun, LC.; Bruice, T.C. J.Am. Chem. Soc. 1986, 108, 1643 and references therein. 5) Curci, R.; Edwards, J.O. in •organic Peroxides• Vol. 1, 199, (Ed. Swem, D.) Wileylnterscience, New York, 1970. 6) Saussine, L; Brazi, R.; Rabine, A.; Mimoun, H.; Fischer, J.; Weiss, R. J . Am. Chem. Soc.1985, 10~3534. 7) Sharpless, KB.; Woodard, S.S.; Finn, M.G. Pure Appl. Chem. 1983, 55, 1823. 8) Lubben, T.; Wolczanski, P.T.J.Am. Chem. Soc. 1987, 109, 424. 9) Tatsuno, Y.; Otsuka, S. J. Am. Chem. Soc. 1981, 103,5832. 10) a) ref. 2 b) Strukul, G.; Ros, R.; Michelin, A.A. lnorg. Chem. 1982, 21, 495 c) Mimoun, H.; Charpentier, R.; Mitschler, A.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1980, 102, 1047. 11) Mimoun, H.; Chaumette, P.; Mignard, M.; Saussine, L; Fischer, J.; Weiss, R. Nouv. J. Chim . 1983, 7, 467. 12) Mimoun, H.; Mig nard, M.; Brechot, P.; Saussine, L J . Am. Chem. Soc. 1986, 108, 3711 . 13) a) ref. 2 b) ref. 3 c) ref. 1Oc d) ref. 11 e) van Asselt, A.; Santarsiero, B.; Bercaw, J.E. J . Am. Chem. Soc. 1986, 108, 8291 . 14) Sharpless, KB.; Verhoeven, T.R. Aldrichimica Acta 1979, 12, 63. 15) a) Mimoun, H J. Mol. Cat. 1980, 7, 1 b) ref. 12. 16) Bach, A.D.; Wolber, G.J.; Coddens, B.A. J . Am. Chem. Soc. 1984, 106, 6098. · 19 17) a) Bryndza, H.E. Organometallics 1985, 4, 406. b) Jensen, C.M.; Trogler, W.C. Science 1986, 233, 1069. 18) Blackburn, R.F.; Labinger, JA; Schwartz, J. Tet. Lett. 1975, 34, 3041. 19) Brindley, P.B.; Scotton, M.J.J. Chem. Soc. Perkins //1981 , 419. 20) Sharpless, KB.; Teranishi, A.Y.; Baeckvall, J.-E. J. Am. Chem. Soc. 1977, 99, 3120. 21) Hentges, S.G.; Sharpless, KB. J. Am. Chem. Soc. 1980, 102, 4263. 22 de Boer, E.J.M.; de With, J.; Orpen, A.G. J. Am. Chem. Soc. 1986, 108, 8271 . 23) Su, F.-M.; Cooper, C.; Geib, S.J.; Rheingold, A.L; Mayer, J.M. J. Am. Chem. Soc. 1986, 108, 3545. 24) Mayer,J.M.; Su, F.M.; Bryan, J.C.; Geib, S.J.; Rheingold, A.L Amer. Chern. Soc., 192nd National Meeting, Anaheim, September 1986. 25) Mayer, J.M.; Thorn, D.L ; Tulip, T.H. J. Am. Chem. Soc. 1985, 107, 7454. 26) Collman, J.P.; Brauman, J.l.; Meunier, B.; Hayashi, T.; Kodadek, T.; Raybuck, SA J. Am. Chem. Soc. 1985, 107,2000. 27) Collman, J.P.; Kodadek, T.; Raybuck, SA; Papzian, LM. J. Am. Chem. Soc. 1985, 107,4343. 28) Groves, J.T.; Watanabe, Y. J. Am. Chem. Soc. 1986, 108, 507. 29) a) Herrmann, WA; Serrano, R.; Kusthardt, U.; Ziegler, M.L; Guggolz, E.; Zahn, T., Angew. Chem.lnt. Ed. Engl., 1984,23, 515-517; b) Legzdins, P.; Rettig, S.J.; Sanchez, L, Organometallics, 1985,4, 147Q-1471; c) Feinstein-Jaffe, 1.; Gibson, D.; Uppard, S.J.; Schrock, R.R.; Spool, A., J. Am. Chem. Soc., 1984, 106, 6305; d) Schrauzer, G.N.; Hughes, LA.; Strampach, N.; Robinson, P.R.; Schlemper, E.O., Organometallics 1982, 1, 44; e) Schrauzer, G.N.; Hughes, LA.; Strampach, N.; Ross, F.; Ross, D.; Schlemper, E.O., Organometallics, 1983, 2, 481-485; f) Schrauzer, G.N.; Hughes, LA; Schlemper, E.O.; Ross, F.; Ross, D., Organometallics, 1983, 2, 1163-1166; g) Alves, A.S.; Moore, D.S.; Andersen, A.A.; 20 Wilkinson, G., J. Organomet. Chem., 1969, 19, 387; h) Mowat, W.; Shortland, A.; Yagupsky, G.; Hill, N.J.; Yagupsky, M.; Wilkinson, G., J. Chem. Soc. Dalton Trans., 1972, 533; I)Mertis, K.; Williamson, D. H.; Wilkinson, G., J. Chem. Soc. Dalton Trans., 1975, 607; DMiddleton, A. R.; Wilkinson, G. J. Chem. Soc. Dalton Trans ..1980, 1888; k) Santini.Scampucci, C.; Riess, J.G. J. Chem. Soc. Dalton Trans. 1974, 1433; I) Beattie, I.A.; Jones, P.J., Inorg. Chem. 1979, 18,2318. · 30) a) ref. 19 b) Thiele, K.-H.; Schumann, W.; Wagner, S.; Brueser, W. Z. Anorg. Allg. Chem. 1972, 390, 280 c) Reichle, W.T.; Carrick, W.L J. Organomet. Chem. 1970, 24, 419. 31) van Asselt, A.; Burger, B.J.; Gibson, V.C.; Bercaw, J.E. J. Am. Chem. Soc. 1986, 108,5347. 32) a) Vaska, LAccts. Chem. Res. 1976, 9, 175 b) Valentine, J.S. Chem. Rev. 1973, 73, 235. 33) Mimoun, H.; Seree de Roch, 1.; Sajus, L Tetrahedron 1970, 26, 37 34) a) van der Ent, A.; Onderdelinden, A.L lnorg. Chim. Acta 1973, 7, 203 b)Brown, J.M.; John, A.A.; Lucy, A.R. J. Organomet. Chem. 1985, 279, 245. 35) a) Sheldon, A.A.; van Doom, JA J. Organomet. Chem. 1975, 94, 115 b) Zanderighi, G.M.; Ugo, R.; Fusi, A.; Ben Taarit, Y.lnorg. Nucl. Chem. Lett. 1976, 12, 729. 21 CHAPTER2 terl-Butyl Peroxide Complexes of Permethylhafnocene, Cp*2Hf(R)(OOCMe3) Abstract: A series of terl-butylperoxide complexes of hafnium, Cp*2Hf(R)(OOCMe3) (Cp* = (('7 5-csMes); R = Cl, H, CH3, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3, CH2CHMe2, CH=CHCMe3, C5H5, meta-C5H3(CH3)2) and Cp*('7 5-cs(CH3)4{CH2)s)Hf(OOCMe3), has been synthesized. One example has been structurally characterized. Cp*2Hf(OOCMe3)CH2CH3 crystallizes in space group P2j/C, with a= 19.890(7)A, b = 8.746(4)A, c = 17.532(6)A, p = 124.987(24) o , V = 2498(2)A3, Z = 4 and RF = 0.054 (2222 reflections, I > 0). Despite the coordinative unsaturation of the hafnium center, the terlbutylperoxide ligand is coordinated in a mono-dentate ligand. The mode of decomposition of these species is highly dependent on the substituent R. For R = H, CH2CH3, CH2CH2CH3, CH2CH2CH2CH3, CH2CHMe2 a clean first order conversion to · Cp*2Hf(OCMe3)(0R) is observed (for R = CH2CH3,t1Ht = 19.6 kcal·mor1, !1St= -13 e.u.). These results are discussed in terms of a two step mechanism involving '7 2-coordination of the tert-butylperoxide ligand. Homolytic o-o bond cleavage is observed upon heating of Cp*2Hf(OOCMe3)R {R = C5H5, meta-C5H3(CH3)2). In the presence of excess 9,10-dihydroanthracene thermolysis of Cp*2Hf(OOCMe3)C5H5 cleanly affords Cp*2Hf(C5H5)0H and HOCMe3 (!1H* = 22.6 kcal·mol-1, l1S* = -9 e.u.). The 0-0 bond strength in these complexes is thus estimated to be 22 kcal mor1. 22 Introduction The sterically demanding ligand environment of permethylzirconocene and permethylhafnocene fragments has allowed the isolation of a large number of monomeric hydride complexes. Unlike most late transition metal hydrides, which often react as acids (H•), 1 zirconium and hafnium hydrides consistently react as hydrides (~).2,3 Greg Hillhouse investigated the reaction chemistry of water and ammonia with these complexes (eq 1,2).2 Even these relatively weak acids protonate the metal hydride bond to generate H2. Water reacts cleanly with Cp*2HfH2 (Cp* ='75-csMes) in several steps. Initially, Cp*2Hf(OH)H is formed. In the presence of excess Cp*2HfH2, Cp*2Hf(OH)H reacts to yield the bridging oxo hydride dimer, Cp*2Hf{H)-O-{H)HfCp*2· Additional H20 drives the reaction to Cp*2Hf{OH)2. Ammonia reacts only once to generate Cp*2M{NH2)H and H2. + H20 C> Cp*2Hf(OH) (H) + Cp*2Hf(H)2 (Cp*2Hf(H) }20 + 3 H20 Cp*2HfH2 Cp*2MH2 + (1) Cp*2Hf(OH) (H) ----It> ----t> (Cp*2Hf(H) l20 Cp*2Hf(OH)2 + + H2 2 H2 (2) NHa M- Zr, Hf; Cp* - '7 5 -CsMes The reactivity exhibited by these hydride complexes with water stimulated our interest in synthesizing alkylperoxide derivatives by a similar method (eq 3). Cp*2M (X)(H) + HOOCMe3 (3) M- Zr, Hf This chapter discusses the synthesis and reactivity of a series of hafnium t-butylperoxide complexes. This series provides the first well characterized examples of group IV alkylperoxide complexes. 23 Results and Discussion Svnthesls: The room temperature addition of t-butylhydroperoxide (TBHP) to Cp*2MH23 (M = Zr, Hf) yields Cp*2M(Ot-Bu)(OH) and H2. Low temperature addition (-50°C) of TBHP to Cp*2HfH2 gives a complex which, based on 1H nmr spectroscopic evidence, is Cp*2Hf(OOt-Bu)(H). The decomposition of the zirconium analogue, Cp*2Zr(OOt-Bu)(H), is too fast to allow its observation even at low temperature. The first moderately stable complex we observed was Cp*2Hf(OOt-Bu)CI, synthesized from the reaction of Cp*2Hf(CI)(H) with TBHP. The analogous zirconium derivative is observed at low temperature, but decomposes upon warming to room temperature. The instability of the zirconium species led us to concentrate on preparing hafnium derivatives. A series of hafnium t-butylperoxide complexes were synthesized by protiolytic cleavage of Hf-H bonds with 1.1 equivalents of t-butylhydroperoxide (eq 4,5): Cp*2Hf(R) (OOCMe3) (Cp* = {'1 5-c5Me5); A = Cl, H, CH3, CH2CH3, CH2CH2CH3, CH=CHCMe3, CsH5, metaCsH3(CH3)2 and Cp*(FJ 5,,1_C5(CH3)4(CH2)3)Hf(OOCMe3)). Cp*2Hf(H)(OOCMe3) was not isolated. Cp*2Hf(R)(H) + HOOCMe3 -----~ Cp*2Hf(R)(OOCMe3) + H2 ----1t> (4) (5) 24 The ring opening cleavage of hafnium metallacycles with excess TBHP has provided a second synthesis pathway to give two derivatives not accessible by the first route (R = -----il> Cp*2Hf(CH2CHMe2)(00CMe3) Cp*2Hf~ + HOOCMe3 (6) _ _ __,1> The reactions are carried out at room temperature or 0° C depending on the stability of the product (vide infra) . These compounds are extremely soluble in alkane and arene solvents. Isolation of the compounds was considerably facilitated by the fortuitous discovery, while washing glassware, that these complexes are relatively insoluble in acetone. Reactions are carried out in petroleum ether (many of the starting materials react with acetone) and the products are isolated from acetone. The isolated complexes are stable for months when stored at low temperature. Analytical, 1H nmr, and IR (v(Q-0) =835-850 cm-1) data support the above formulations. We were not able to synthesize several other derivatives of interest. When the alkyl ligand (R') of Cp*2Hf(R') (H) is larger than propyl, the addition of TBHP results in loss of alkane and the formation of Cp*2Hf(OCMe3)(0H) (eq 8). Cp*2Hf(R ' )(H) + HOOCMe3 ---_,1> Cp*2Hf(OCMe3)( 0H) + R'H (8) Several other approaches for synthesizing the benzyl derivative were attempted. Th~ most successful involves the addition of TBHP to the ring-metallated benzyl compound, Cp*('7 5,, 1-C5 Me 4CH 2)Hf(CH 2C6 H5) (eq 9). The reaction yields approximately 40% of the 25 desired product, along with an intractible mixture of other products. No reaction was observed upon addition of TBHP to Cp*2Hf(CH2CsHs)2. + HOOCMe3 - - - - t > Cp*2Hf(CH2CsH5)(00CMe3) + other products (9) We were also unable to synthesize derivatives with alkoxides as the coligand. For example, attempts to generate a species similar to the proposed intermediate of the Sharpless epoxidation system,4 by reacting TBHP with the allyl alkoxide derivatives Cp*2Hf(OCH2CH=CH2)(H), Cp*('7 5,'71-CsMe4CH2)Hf(OCH2CH=CH2)(1) and Cp*2Hf(OCH(CH2CHMe2)C(CH3)=C(CH3))(2), proved unsuccessful. 1 2 Crvstallographic Results: Crystals were grown by slow cooling of an acetone solution of Cp*2Hf(OOt-Bu)(Et). A skeletal view of the molecule with non-hydrogen atoms is given in Figure 1 and depicts the pseudotetrahedral coordination or •bent sandwich" configuration of ligands about the hafnium atom and gives selected bond angles and distances. Also included are the frontier orbitals of a bent metallocene fragment. Figure 2 gives the numbering scheme for the non-hydrogen atoms. The R-Hf-R (R = Cp* ring centroid) angle of 135.0° and range of Hf-C (Cp*) distances, 2.528(14)-2.572(14) A are unexceptional when compared to other bispentamethylcyclopentadienyl-hafnium complexes, cf. , Cp*2Hf(H)('7 3-CH2CHCH2), with average Hf-C(ring) distances of 2.55 A and R-Hf-R angle of 130°.5 The inner-ring carbon atoms of the Cp* ligands are planar, lo1 bz 2o 1 ~ y Lz orbitals for bent metallocenes. Figure 1. Selected bond angles and distances, Cp*2Hf(OOCMe3)CH2CH3. Frontier ~z ~ . eB ·. ~ C24M C11M Cp*2Hf(OOCMe3)CH2CH3. Figure 2. The numbering scheme for non-hydrogen atoms, --..C34 _., N 28 and the methyl groups are displaced from the ring and away from the Hf atom by 0.1 6 A on average; the greatest deviations are by C12M and C13M on one ring and C23M on the other (the shortest CH3 · • · CH3 non-bonding contact is C13M · · • C23M at 3.31 A). In addition, there are intimate contacts between the ring methyl groups and the ethyl group, C11 M · • · C41 3.18 A and C11 M · · · C42 3.39 A. and the ring methyl groups and the t-butylperoxide ligand, C15M · • • 01 2.93 A and C24M · • · 02 2.95 A. The Hf-C41 bond length of 2.280(13) A is normal, cf., 2.295(14) A In (CP2HfMe)20 .6 The Hf-0 bond length of 1.970(8) A indicates considerable w-donation from the lone pair orbitals on 01 into the empty (2a1) 7 orbital on hafnium; the predicted Hf-0 single bond length is approximately 2.19 A. cf., Zr-Q bond length in Zr(acac)4 , 2.20 A and r(Zr) - r(Hf) = 0.01 A. 8 The C41-Hf-01-02 torsion angle is -70.9(7) 0 , a geometry consistent with oxygen lone pair w-donation from 01 into Hf. The 01-02 bond length of 1.489(12) A is like that reported for other transition metal alkylperoxide structures.9 The 02-C31 bond length of 1.437(16) A is normal. The Hf-0102 of 119.6(6) 0 is larger than that reported for similar structures and is possibly a result of unfavorable steric interactions. The sterically demanding t-butoxide group Is found tilted into the middle of the wedge between the two Cp* ligands. The most striking feature of the structure is the monodentate coordination of the t-butylperoxide ligand. As mentioned in the introductory chapter, the related structure of (dipic)VO(OOt-Bu)L contains an '7 2-t-butylperoxide ligand .. For Cp*2 Hf(OOCMe3)(Et), coordination of the alkoxy oxygen to Hf would satisfy the 18 electron rule. This is apparently precluded in the ground state structure by steric interactions of the t-butylperoxide oxygens with the ring methyls. 29 Indirect evidence that the ground state structure in solution is also monodentate is available from the 1H nmr spectra of Cp*2Hf(OOt-Bu)(H). An empirical trend In the hydride shifts of Cp*2Hf(X) (H) has been noted.3b The hydride signals of 16 electron Cp*2HfH2 are found relatively far downfield at 15.6 ppm. The hydride resonances of the closed-shell, 18 electron complexes Cp*2Hf(H)('73 -cH2CHCH2) 3.6 ppm and Cp*2Hf(H)2(CO) 2.7 ppm are shifted significantly upfield of the 16 electron complexes. Table 1 summarizes the chemical shifts for several Cp*2Hf(X)(H) complexes and one can observe an apparent correlation between the hydride shift and the extent of "' donation from X into the empty orbital (2a1) on hafnium. The hydride shift of Cp*2Hf(OOt-Bu)(H) at 10.7 ppm falls in the range of that reported for formally 16 electron, 1r-donating alkoxide derivatives. It is not consistent with the closed-shell, 18 electron configuration of an '12-t-butylperoxide ligand. It is possible, however, that an '12-coordinated intermediate is present in the reactions of these complexes (vide infra). 30 Table 1. 1H NMR Chemical Shifts for the Hydride Resonance of Cp* 2Hf(X)H. X 6 (ppm) H 15.6 15.3 CH2CH(CHa)2 13.6 Br 13.5 CsHs 13.1 CHa 13.0 Cl 12.7 CH 2CHa 12.2 NH(CsHs) 11 .6 N(CHa)2 11 .5 F 11.1 OOCMea 10.3 OH 10.2 OCHa 9.9 17 2-HNNC(CsH4CH3}2 8.2 173 -C3Hs 3.6 H (CO) 2.7 31 Reactivity: At least three modes of decomposition have been observed for the series Cp*2Hf(R)(OOt-Bu). The preferred mode of decomposition is highly dependent on the nature of the R group. When R equals CsHs or m-CsH3(CH3)2 one observes decomposition only upon heating above room temperature. In fact, Cp*2Hf(CsHs)(OOt-Bu), unlike the alkyl derivatives, is stable in the solid state at room temperature for over a year. Upon heating, products consistent with C>-0 bond homolysis and formation of t-butoxide radical are observed. In the presence of 5 - 16 equivalents of radical trap and hydrogen donor 9,1 o-dihydroanthracene, clean first order formation of Cp*2Hf(CsHs) (OH) and t-BuOH is observed (eq 10). Cp*2Hf ( Ct>Hs) (Oot-Bu) Cp*2Hf(C5H5) (OH) 40 - 100° c ----t> + t-BuOH (10) The activation parameters for the above reaction were calculated by measuring the kinetics of the reaction over the temperature range of 40 to 90°C (~H* = 22.6 kcal/mol, ~s* = -9.7 e.u.). Replacing the two meta-hydrogens on the phenyl group with methyl groups, i.e., Cp*2Hf(m--csH3(CH3)2)(00t-Bu), had no measurable effect on the kinetics of decomposition. From the activation parameters one can estimate a value of 22 kcal/ mol for the 0-0 bond strength, considerably lower than those measured for normal dialkylperoxides (37 ± 1 kcal/mol).10 Cp*2Hf{CI)(OOt-Bu) decomposes slowly at room temperature in benzene solution to give an intractable mixture of products. This decomposition likely proceeds by more than one pathway and involves the apparent loss of at least one Cp* ligand. Addition of PMe3, CO, cyclohexene or ethylene has no effect on the decomposition and one observes no oxidation of substrate. 9,1 0-Dihydroanthracene also has no effect on the decomposition. 32 The methyl and neohexenyl derivatives also decompose by more than one pathway to give rise to one major product and an intractible mixture of products similar to those of the chloride derivative's decomposition. For the methyl case we observe approximately 50% conversion to Cp*2Hf(OCH3)(0t-Bu). Observed in about 30% yield, the only identifiable product of the neohexenyl decomposition is Cp*2Hf(CH=CHCMe3)0H. Cs(CH3)4CH2CH2CH2)Hf(OOCMe3) we observe clean first order decomposition at room temperature or below to the corresponding alkoxy t-butoxide derivatives (eq 11, 12; Table 2). Cp*2Hf(R)(OOCMe3) Cp* 2Hf(OR) (OCMe3) (11) benzene-ds Cp*('7 5 , '7 1-Cs(CH3)4CH2CH2CH2)Hf(OOCMe3) 33°C t> Cp*(17 5,'71-Cs(CH3)4CH2CH2CH20)Hf(OCMe3) Table 2. Kinetics of oxygen insertion (benzene-ds. 33 o C). k (X 105, sec-1) Cp*2Hf(CH2CH3)(00CMe3) 9.5 Cp*2Hf(CH2CH2CH2CH3)(00CMe3) 10 Cp*2Hf(CH2CH2CH3)(00CMe3) 15 Cp*('7 5 ,'7 1-Cs(CH3)4CH2CH2CH2)Hf(OOCMe3) 45 Cp*2Hf(CH2CHMe2)(00CMe3) 180 (12) 33 ·>~--~----------------------------~ • • ..... •· ....... .•.. ' (ln[C]) • • • f ·• .... •.. .., ' '• ... .. ·• ' (seconds) 499 b) 1298 899 1698 r-----------------------------~ 9-' .· .· (In (kiT}) .· .· .'0 9-' .· ,' ,' ·' .· .9928 9928 .992 .&92 .8939 .993 .993 .99322 Figure 3. a) Kinetics at 60°C for the transformation of Cp*2Hf(CH2CHa)(OOCMe3) to Cp*2Hf(OCH2CH3)(0CMe3); ln [C] vs. t(sec), k 5.38 X 1o-4. b) Eyring Plot for the rearrangement of Cp*2Hf(CH2CH3)(00CMe3) at 35°,42°, 52° and 60°: ln(k[T) vs. (1fD, ~H* = 19.6 kcal/mol ~st = -13 e.u. = 34 Figure 3 shows a typical example of a kinetics run for Cp*2Hf{CH2CH3)(00CMe3}. The relative rates do not seem to follow any regular pattern and may reflect a multistep pathway for rearrangement (vide infra). Activation parameters for Cp*2Hf{CH2CH3}(00CMe3} {t.H* =19.6 kcal/ mol, t.s* =-13 e.u.; Figure 3) were calculated from kinetics measurements over a temperature range of 35° - 60° C and are consistent with an intramolecular rearrangement. Indirect evidence of the intramolecular nature of the rearrangement was obtained by carrying out the decomposition in the presence of Cp*2HfH2. No formation of Cp*2Hf{H}{OH) or related species was observed.2 No measurable rate change was observed when the kinetics of Cp*2Hf{CH2CH3){00CMe3} decomposition were followed in more polar solvents; CD2CI2, THF-da and acetonitrile-d3fbenzene-ds {40:60}. One observes slower further decomposition of the bis{alkoxides) in coordinating solvents to give Cp*H and unknown metal containing species. Mechanism: As mentioned in Chapter 1, a likely first step in oxygen transfer reactions from metal alkylperoxide complexes is 112-coordination of the alkylperoxide ligand. We have not observed a '72-oOCMe 3 intermediate in this system. The complex rate dependence on the sterics of the alkyl ligand, however, can be interpreted as consistent with a stepwise process as shown in equation 13. ----11> One would expect the preequilibrium step {k1) to respond primarily to steric factors. Particularly noteworthy is the over 10-fold increase in rate observed for the sterically demanding isobutyl complex vs. the n-alkyl derivatives. 35 We are not able to draw many conclusions concerning the actual oxygen transfer step (1<2), which should be sensitive to electronic and steric changes in R. Empirical trends, noted for Cp*2Hf(I)(A) hydrogenations, indicate that bulkier alkyl groups may have weaker hafnium carbon bonds. 11 Halpern has noted similar trends for other transition metal alkyls.12 This factor could contribute to the increased rate of oxygen insertion observed for Cp*2Hf(CH2CHMe2)(00CMe3). Although the range of solvent polarities is severely limited by the high reactivity of these compounds, no significant difference was noted for the rate of rearrangement of Cp*2Hf(CH2CH3)(00CMe3) in more polar solvents, suggesting little polar character for the transition state of step k2. The higher stability of the phenyl derivatives can also be accommodated with this mechanism, since the planar phenyl ring is constrained to lie in the equatorial plane of the bent sandwich structure. Although the crystal structure was disordered, a partial X-ray structure determination for Cp*2Hf(CsHs)(OOCMe3) revealed this arrangement for the phenyl group. The barrier to rotation is sufficiently large that nmr spectra indicate a static structure in solution at ambient temperature. The crowded wedge presumably inhibits the formation of the , 2-00CMe3 intermediate, forcing the complex to find an alternative path for decomposition (0-0 bond homolysis). An additional factor may be the hafnium phenyl bond st rengt h. The hafnium phenyl bond in Cp*2Hf(Ph)(H) is on the order of 20 kcal/mol stronger than the hafnium carbon bond in Cp*(71s,, 1-Cs(CH3)4CH2CH2CH2)HfH.11 No clear conclusions can be d rawn concerning the chloride, methyl and neohexenyl decompositions. A alternative mechanism to initial , 2-t-butylperoxide coordination is suggested by the homolytic 0-0 bond cleavage observed for Cp*2Hf(CsHs)(OOCMe3). In the phenyl case, cleavage of the 0-0 bond is fo llowed by escape of · OCMe3 . When the alkyl group is not 36 phenyl, cleavage of the 0-0 bond to form a caged radical pair is followed by migration of the alkyl group to Hf-0 · to yield Cp*2 Hf(III)OR. Cp*2 Hf(III)OR recombines with the t-butoxide radical to yield the bis(alkoxide) product (Scheme 1). OCMe 3 2" I 0 Cp* 2 Hf(OCMe 3 )(0R) Cp* H f / R II 1 · OCMe 3 2" . 0· ·OCMe 3 Cp* 2 Hf-OR Cp* H f / R Scheme I Conclusions: We have synthesized the first series of group IV metal alkylperoxide complexes and have studied two of the decomposition modes. The success of the synthetic strategy for these reactive complexes rests, In part, on the very fast reaction of the hydride derivatives and moderately fast reaction of the metallacylces with TBHP. With slower reactions, most of these species could never be isolated. Both decomposition pathways involve 0-0 bond cleavage. In the case of the phenyl derivatives, the process appears to involve homolytic cleavage of the 0-0 bond. The 0-0 bond strength for these complexes is estimated to be 22 kcal/ mol. The hydride, ethyl, propyl, butyl, isobutyl and propyl bridged tuck-in derivatives decompose by insertion of an oxygen of the t-butylperoxide ligand into a hafnium carbon bond in a process which has a barrier of approximately 20 kcal/mol. Kinetic data for these reactions is consistent with a multistep decomposition process. 37 The crowded ligand environment surrounding a less reactive, third-row transition metal is very likely responsible for the thermal stability exhibited by members of this series of complexes. These same features seem to dictate the mechanism(s) for rearrangement to the very stable bis(alkoxide) derivatives. 38 tabla J . JIHI!., ' n. and .Analytical J)a'ta. Compound• Anignment c 50.36 (50 . 25) H 6.97 ( 6.87) 13c c,5.c5(CR3>s> 1.96 s C(CR3)3 1 . 26 • c,5.c5(CH3>s> 119.9 c,5.c5<CH3>s> 80.3 C(CH3)3 26.8 C(CH3)3 11.9 Hf·B 10 .28 • (1J5·Cs(CH3)5) 2.00 • C(CH3)3 1.23 • Hf·OB 3.00 • <,5.c5(CH3>s> 1 . 88 • C(CH3)3 1.30 • c 53.82 (54.29) H 7 . 45 ( 7.65) c,5.cs<CH3>5> 1.93 • C(CH3)3 1.19 • v(O·O) - 838 cm·1 • CH3 ·0.25 • 39 Cp*2Bf(OOCMe 3 )CB2 CB3 c 54 . 83 (55.06) 7.72 ( 7.82) c.,5.c5<CR3>5> 1.92 • CH2CH3 1.64 t C(CR3)3 1.20 • CR2CH3 0 .45 q Jmt7.8 CR2CH3 4.26 q Jmt7.2 <., 5 -C5CCR3)5) 1.98 • C(CR3)3 1.38 • CH2CR3 1 . 30 t 5 <'7 -c5CCR3>5> 1.90 • CH2CR2CH3 1 . 63 • CH2CH2CR3 1.33 t C(CH3)3 1 . 19 • CR2CH2CH3 0.39 • c., 5 -c5(CH3>5> 1.91 • CH2 cH2CH 2CH3 1 . 60 II C{CH3 ) 3 1.21 • CH2CH 2CH2cH3 1.17 II CH2CH2CH2CH3 0 . 38 II Jmt7 . 8 H v(O-O) - 845 ea·1 Cp*2Bf(OCMe3 )oCB2 CB3 c 54.75 (55.06) 7 . 75 ( 7.82) H Jmt7 . 2 Cp*2Hf(OOCMe 3 )CB2 CB 2CB3 c 55.68 (55.80) 7 . 92 ( 7.97) H v(O-O) - 838 ea·1 Cp*2Hf(OOCMe 3 )CH2CH2 CH2CH3 c 56 . 25 (56.50) 8 . 01 ( 8 .13) H v(O·O) - 842 cm· l Jmt7 . 1 40 ep•2Bf{OOCMe 3 )CB2 CB{CB3 ) 2d c 56.20 (56 . 51) 7 . 85 ( 8 . 12) c., 5 -c5<CH3>5> 1.95 • Qi2CH(CH3)2 1.27 d C(CH3)3 1.23 • CH2CH( CH3)2 0 . 27 d JHI16 CH2CH(CH3)2 3 . 97 d JHI16 . 8 <.,5.c5(CH3>5> 1 . 99 • C(CH3)3 1.40 • Qi2CH(CH3)2 1 .03 d CH-CHC(CH3 ) 3 6.02 d JHli19 . 5 CH-cHC(CH3 ) 3 5.63 d JH}{"'19 . 5 5 <'7 - c5(CH3 >5> 1.88 • JHI16 H v(O·O) - 842 ea·1 Cp*2Bf{OCMe3 )oCB2CB(CH3 ) 2d JHI16 . 6 Cp*2Bf(OOCHe 3 )CB-cBC{CB 3 ) 3 c 57.82 (58 . 00) 7.88 ( 8 .11) H v(O- O) - 843 em·1 · C(CH3 ) 3 ,CHCHC(CH3 ) 3 1 . 24 • C6R5 7.75. 7.42. 7 . 36. 7 . 18. 7.04. ('I 5 - C5 (CH3 ) 5 ) 1. 79 • C(CH3 ) 3 1 . 30 • Cp*2Bf(OOCMe 3 )C 6H5 c 58.75 (58 . 77) H 7.13 ( 7 . 13) v (O· O) - 848 cm· 1 41 C6R3 (CH3) 2 7 .40. 6 . 80 • 6 . 68 • C6H3(CR3)2 2 . 43 • 2 . 42 • c,5.c5(CR3>5> 1 . 82 • C(CR3)3 1 . 35 • c 60 . 20 (59 . 75) H 7 . 35 ( 7 . 52) Cp*(IJ5·Cs(CB3)4CB2CB2CB2)Bf(OOCHa3)a c 55 . 25 (55.26) H 7 . 49 ( 7 . 49) (IJ 5-c5 (CH3 ) 4CR2 CH2CH2 ) 3 . 08 a 2.69 • 2 . 13 • 1 . 91 • 1 . 80 • 1 . 65 • c,s.cs<CH3>s> 1 . 91 • C(CR3)3 1 . 21 • (IJ 5 -c5 (CH3 ) 4 CH2CR2 CH2 ) 0 . 93 a (IJ5 -c5 (CH3 ) 4CH2 CH2CR2 ) 0 .13 a Cp*(IJ 5 -c5 (CB3) 4 CB2CB2 CB20)Bf(OCHe 3)• (IJ 5 -c5 (CH 3 ) 4 CH2CH2CH20 ) 4 . 18 a 4.03. (IJ 5 -c5 (CH 3 ) 4 CR2CH2 CH20) 2 .41 a 2.10 • 2 . 00 • 1 . 86 • 1 . 84 • Anal yses are recorded vith the calcul ated values in parentheses and vere performed by t he CIT analytical laboratory . IR apactra vera recorded on a Beckman 4240 spectrometer b ) Al l nmr spectra vare recorded on Varian EM390 , JEOL FX90Q, or JEOL CX400 spectrom~ters in ·c6D6 Wlleu otherwise noted . Chemical ahift s a r e reported in ppm relative to TKS . c) Coupling constants are reported in Hertz . d) Isolated and analyzed as a aixture vith the inserted product . ca . 75t ep•2Hf(OOCKe 3 )CH 2CH(CH3 ) 2 . a) Isolated and analyzed as a aixture vith the inserted proauct . ca . SOt Cp* ( IJ 5 c5 ( CH3 ) 4CH2CH2CH2)Hf (OOCKe3) . These are tentative assignments of the aliphat ic protons in the propyl bridge . a) 42 Experimental Section General Considerations: All manipulations were carried out using glove box or high vacuum line techniques unless otherwise noted. Solvents were dried by standard techniques and further purified by vacuum transfer from "titanocene" 13 or benzophenone ketyl. Hydrogen was purified by passage over MnO on vermiculite 14 and activated 4 A sieves. Ethylene, isobutylene, and CO were used directly from the cylinder. Other reagents were degassed and vacuum transferred from 4 A sieves or used directly as received. tert-Butylhydroperoxide was dried by azeotrope distillation and stored cold over 4 A sieves.15 In a typical example 10 mls of 90% t-butylhydroperoxide (5% H2 0, 5% t-BuOH) where mixed in a 50 ml flask with 30 mls of benzene. Boiling chips were added and the H20 , t-BuOH, CsHs azeotrope was distilled off at 67° C until the temperature rose to the boiling point of benzene. Approximately 10 mls of benzene were distilled off of the solution to give 4 M t-butylhydroperoxide in benzene. The concentration of the t-butylhydroperoxide I benzene solution can be increased by allowing more benzene to distill off. Approximate concentration can be determined effectively by nmr integration. 1H nmr spectra were recorded with Varian EM-390, JEOL FX900, and JEOL GX400 spectrometers. 13c spectra were recorded with a JEOL FX900 spectrometer. Infrared spectra were measured on a Beckman 4240 spectrometer as nujol mulls. Elemental analyses were performed by C.I.T. Analytical Laboratories. Molecular weights were determined using a Signer molecular weight apparatus.16 Procedures: Kinetic data were collected by monitoring the reactions in flame sealed nmr tubes with ferrocene as the internal standard. In a typical experiment 25 mg of compound were placed in an nmr tube with 8 mg of ferrocene, solvent was added and the reaction 43 was followed by nmr spectroscopy. Survey reactions were generally carried out in nmr tubes with rubber septa wrapped with parafilm. TBHP solution was added via syringe. The starting hafnium complexes not reported elsewhere were prepared as follows. Cp*2Hf(X}(H) (X= Cl, 1): An improvement over the reported method2 for synthesizing these materials has been developed by A. R. Bulls. 5.594 g (7.97 mmol) of Cp*2Hfl2 were stirred with 0.780 g (10 mmol) of UCH2CMe3 for 24 hours in benzene. The benzene solution was filtered and the benzene was removed in vacuo. 4.34 g (6.71 mmol, 85% yield) of Cp*2Hf(CH2C(Me)3) (I) were isolated at -78°C from petroleum ether. 3.3 g (5.11 mmol) of Cp*2Hf(CH2CMe3)(l) were dissolved in 30 mls of benzene in a appr. 250 ml glass bomb. 1 atmosphere of H2 was admitted at -198° C (3.8 atm at room temperature) and the bomb was heated at 80° C overnight. 2.67 g (4.81 mmol, 94% yield) of Cp*2Hf(I)(H) were isolated from petroleum ether. Cp*2Hf(CsH3(Me)2)(H): 500 mg (0.87 mmol) of Cp*2Hf(I)(H) and 205 mg (1.83 mmol) of UCsH3(Me)2 were stirred together in toluene for 12 hours. Toluene was removed in vacuo and the 310 mg (0.56 mmol, yield 64%) of Cp*2Hf(CsH3(Me)2)(H) were isolated from petroleum ether. Cp•2Hf(R)(OOCMe3) derivatives: The compounds synthesized from hafnium hydride complexes are prepared by addition of a slight excess of dried tert-butylhydroperoxide solution to a petroleum ether solution of the appropriate starting material. The metallacycle ring opening reactions require an excess of TBHP (4- 10 equivalents) in order to insure product formation before decomposition can take place. The additions are carried out at 0°C or room temperature depending on the stability of the product and the solutions are stirred for 5 to 10 minutes and then filtered at that temperature. Although the products can be isolated by condensing and cooling the petroleum ether solutions to -78°C, better 44 yields are obtained if the petroleum ether is vacuum transferred off and acetone is used to isolate the solid. Yields can also be improved by insuring that excess TBHP and benzene are removed from the solution by two or three cycles of adding and removing in vacuo petroleum ether before adding acetone. Yields are generally in the range of 50 to 75%. The solids were stored in a glove box refrigerator. Cp*2Hf(CI)(OOCMe3): 400 mg (0.824 mmol) of Cp*2Hf(CI)(H) were placed in a 25 ml round bottom with septum capped side arm. 7 mls of petroleum ether were vacuum transferred onto the solid at -78° C. 163 pis of 4.8 M TBHP solution were added via syringe. After H2 evolution was complete, the solution was filtered at -78° C, condensed and 275 mg {0.480 mmol, 58% yield) of Cp*2Hf(CI)(OOCMe3) were isolated from petroleum ether at -78° C. Cp*2Hf(CH3)(00CMe3): Cp*2Hf(CH3)(H) (222 mg, 0.48 mmol) was dissolved in 12 mls of petroleum ether and TBHP solution (0.072 mls, 9.3 M) was added. After filtering, petroleum ether was removed in vacuo and 126 mg (0.22 mmol, 48% yield) of Cp*2Hf(CH3)(00CMe3) were isolated from acetone. Cp*2Hf(CH2CH3)(00CMe3): 800 mg (1.67 mmol) of Cp*2Hf(CH2CH3) (H) were placed in a 25 r.b. with septum capped side arm and dissolved in 15 mls of petroleum ether. 1.0 mls of 1.83 M TBHP solution were added via syringe. After H2 evolution the reaction mixture was filtered and petroleum ether was removed in vacuo. 530 mg (0.94 mmol, 56% yield ) of Cp*2Hf(CH2CH3)(00CMe3) were isolated from acetone. Cp*2Hf(CH2CH2CH3)(00CMe3): 507 mg (1.03 mmol) of Cp*2Hf(CH2CH2CH3)(H) were dissolved in 12 mls of petroleum ether at 0° C and 0.274 mls of 4.5 M TBHP solution ~ere added. The solution was filtered and petroleum ether was removed in vacuo. 113 mg 45 (0.19 mmol, 19% yield} of Cp*2Hf(CH2CH2CH3} (OOCMe3} were isolated from acetone. This product was very soluble and proved difficult to crystallize. Cp*2Hf(CH=CHCMe3)(00CMe3): 355 mg (0.66 mmol} of Cp*2Hf(CH=CHCMe3)(H} wer~ dissolved in 12 mls of petroleum ether and 0.86 mls of 9.3 M TBHP solution were added and after H2 evolution was complete the solution was filtered. Petroleum ether was removed in vacuo and 260 mg (0.42 mmol, 63% yield} of Cp*2Hf(CH=CHCMe3)(00CMe3} were isolated from acetone. Cp*2Hf(CsHs)(OOCMe3): 533 mg (1.01 mmol} of Cp*2Hf(CeHs)(H) were dissolved in 15 mls of petroleum ether in a 25 ml r.b with septum capped side arm. 0.9 mls (1.5 eq) of 1.83 M TBHP solution were added via syringe and the mixture was stirred for five minutes before filtering. 367 mg (0.59 mmol, 59% yield) of Cp*2Hf(CeHs)(OOCMe3) were isolated from petroleum ether. . Cp*2Hf(CsH3(Me)2)(00CMe3): 0.095 mls of 9.3 M TBHP solution were added via syringe to a solution of 290 mg (0.52 mmol) of Cp*2Hf(CsH3(Me)2)(H) in 15 mls of petroleum ether. The solution was filtered and petroleum ether removed in vacuo. 224 mg (0.35 mmol, yield 67%) of Cp*2Hf(CsH3(Me)2)(00CMe3) were isolated from acetone. Cp*('7 5 ,f71·CsMe4(CH2)3)Hf(OOCMe3): 409 mg (0.86 mmol) of Cp*(f7 5,f71• CsMe4(CH2)3}Hf(H) were dissolved in 12 mls of petroleum ether. The solution was cooled to 0° C and 0.111 mls of 9.3 M TBHP solution were added. After H2 evolution was complete the petroleum ethe~ s:>lution was filtered at 0° C and petroleum ether was removed in vacuo. The products, 80% Cp*(f7 5 ,J7 1-CsMe4(CH2)3}Hf(OOCMe3} and 20% Cp*(f75 ,f7 1• CsMe4(CH2)30)Hf(OCMe3) were isolated in 26% yield (127 mg, 0.23 mmol) from ace.tone at 0°C. 46 Cp*2Hf{CH2CH{CH3)2)(00CMe3): 400 mg (0.79 mmol) of Cp*2Hf(CH2CH(CH3)CH2) were dissolved in 15 mls of petroleum ether and the solution was cooled to 0° C. 1.5 equivalents of TBHP solution (0.128 mls, 9.3 M) were added and an almost immediate loss of the yellow starting material color was observed. Petroleum ether was removed in vacuo, added, removed, and added again. THe solution was then filtered and petroleum ether was again removed. Acetone at 0° C was used to precipitate 215 mg (0.36 mmol, 46% yield) of the products, 75% Cp*2Hf(CH2CH(CH3)2)(00CMe3) and 25% Cp*2Hf(OCH2CH(CH3)2l(OCMe3). Care was taken remove as much solvent as possible from the solid isolated on the frit by passing argon through it, before warming the solid to room temperature. An earlier attempt at isolating this solid, inadequately dried, resulted in its rapid decomposition upon warming. Cp*2Hf{CH2CH2CH2CH3){00CMe3): Cp*2Hf(CH2CH2CH2CH2) {370 mg. 0.73 mmol) was slurried in 5 mls of petroleum ether and 10 equivalents of TBHP solution were added. Everything went into solution in two minutes and the mixture was stirred for fifty minutes at room temperature. After removing the petroleum ether, benzene and excess TBHP in vacuo petroleum ether was transferred onto the solid again and the solution was filtered. The petroleum ether solution was concentrated to almost an oil and NCCH3 was added to precipitate the solid Cp*2Hf(CH2CH2CH2CH3)(00CMe3) {264 mg, 0.44 mmol, 61% yield). Crystal Structure Determination of Cp*2Hf{CH2CH3)(00CMe3): A single crystal of dimensions 0.22 X .22 X 0.28 mm, bounded by the [1 01 ], (1 01] and (01 0] faces was cooled to 97.5 K on a locally modified Syntex p1 diffractometer equipped with cryocooler, graphite monochromator, and MoKa radiation. The preliminary photographic wor1<up on other crystals indicated a unit cell of monoclinic symmetry, and space group P21/C with Z = 4. The least-squares refinement of the orientation matrix with the setting angles of twelve reflections generated the following cell parameters at 97.5 K: a= 19.890(7) A, b = 8.746(4) 47 A, c = 17.532(6) A. {3 = 124.987(24)0 , V = 2498(2) A3• Intensity data were collected by 6-26 scans of width 2.00° (plus dispersion) at 6°/min. for the hemisphere (±h, ±k, f) to 26 = 40°. The three check reflections indicated no decomposition. The data were averaged over 2/m symmetry (average goodness-of-fit= 1.04 and R = 0.078 for paired reflections), and reduced to F0 2• The form factors were taken from the International Tables for X-Ray Crystallography, IV, Table 2.28, pp. 99-101. The absorption correction appeared to be negligible and was not applied (JJ = 41.46 cm·1, range of transmission factor 0.3-0.5). The coordinates of the hafnium atom were determined form the Patterson map, and the remainder of the structure from subsequent Fourier maps. The hydrogens atoms were placed at idealized positions with B = 3.0 A2 and were not refined. the full-matrix least· squares refinement of atom coordinates and Uij'S of the non-hydrogen atoms, the scale factor, and a term for isotropic secondary extinction (g = 6.6(11) X 10~) minimizing I'..w[(kF0 ) 2 - Fc2] 2 yielded a goodness-of-fit= {I'..w[(kF0 ) 2 - Fc2 ]2 /(n- p)}112 = 1.37 (2349 reflections, 263 parametersO, RF = l: F 0 - Fe /L F 0 = 0.054 (2.222 reflections,/> 0), and R'F = 0.037 (1721 reflections,/> 3o); the largest peak in the llp map was approximately 4 e/A3. The atom coordinates and Ueq's for non-hydrogen atoms are given in Table X1. Table X2 contains the Uij'S for non-hydrogen atoms, Table X3 the atom coordinates for hydrogen atoms and Table X4 the bond lengths and angles, torsion angles, and least-squares plane information. 48 Table Xl. Atom Coordinate• and u.,/• (x104 ) for Cp2Hf( C2Bs) (OOCcB~ ). Atom % II II Hf Cll C12 · C13 C14 CIS CllM CI2M C13M C14M C15M C21 C22 C23 C24 C25 C21M C22M C23M C24M C25M 01 02 C31 C32 C33 C34 C41 C42 2340.2 3) 1720 7 1108 6 853 6 1356 8 1803 7 2007 7 &05 8 70 8 1207 8 2421 7 2512 8 1710 7 1783 7 2638 7 3074 7 2712 7 060 7 1123 7 2050 7 4016 7 3081 4 3863 4 4311 7 6019 7 3709 7 4638 7 3184 7 3836 7 747.4(6) -1076 16 -823 17 737 17 1450 15 250 17 -2652 16 -2082 16 1366 15 3046 18 616 10 430 14 Q08 15 2503 15 2874 14 1612 15 -1061 14 220 15 3685 14 4409 16 1480 15 1022 9) 2534 10 3248 14 4011 17 4482 16 2106 16 -1326 14 -1307 15 2723.3(3) 1320 8 1482 7 1273 7 1011 8 1056 8 1280 8 1660 0 1078 0 727 10) 726 8 4281 7 3641 7 3382 8 3877 8 4410 8 4800 8 3466 8 2887 8 3878 8 6107 8 2538 5 3360 5 3043 8 3043 8 2326 8 2692 8 3331 8 3130 0 u.'~ 120 2} 215 112) 177 42) 171 146) 200 03 257 65 363 46 365 43 306 45 620 56 468 88 201 64 171 87 205 51 102 50 215 35 308 30 353 37 208 41 281 40 335 62 244 35 237 31 210 34 407 47 330 38 328 35 234 41 290 43 49 Table X2. Anlaotroplc Ua;'• (x104 ) for Cp2BI(C2H6)(00C•Hp). Atom Uu U22 Hf 122 3) 201 70 131 69 12 63 283 85 184 81 346 85 410 90 317 87 440 101) 411 87 451 04 165 77 00 78 206 80 272 85 382 83 348 87 231 78 330 78 288 85 181 47 189 50 256 81 227 75 355 81 324 84 274 78 211 81 163 3) 42 3) 355 115) 22 68 256 16 65 317 01 67 go 82 ~8 73 461 121) ~ 74 300 07~ 227 84 259 93 306 89 405 285 85 652 127 204 05 883 134 70 71 163 OS 45 70 312 08 22 65 274 05 100 7.f 161 86 71 71 103 86 188 82 273 103~ 232 78 .f38 106 315 85 261 03 263 81 105 89 215 76 433 99 88 73 348 60 75 47 314 60 09 49 206 86 107 70 483 277 82 304 289 81 372 06 311 85 286 04 106 71 288 05 242 83 Cll C12 C13 C14 C15 CUM C12M C13M C14M C15M C21 C22 C23 C24 C25 C21M C22M C23M C24M C25M 01 02 C31 C32 C33 C34 C41 C42 U" ~l 103l ~~r Uu ulS U2s -6 4) -52 74 -12 80 -26 77 -167 75 -75 81 -31 78 -118 78 -131 76 -201 00 -208 09 -112 76 -86 72 -28 70 68 72 -130 78 -68 74 -83 76 -1 68 -10 76 -1 73 -·7 44 -121 47 61 70 -08 81 -50 70 74 77 -07 66 133 68 12 2) 24 62 10 58 -25 56 -168 67 35 68 38 72 134 77 66 74 33 83 115 70 174 74 45 62 -10 64 -1 65 137 74 154 70 214 74 60 69 104 66 -8 68 -2 41 18 45 130 73 7 69 143 71 106 74 87 66 38 72 -o 4) -57 70 . -35 75 2 78 21 64 ~32 72 -248 77 -08 70 -154 77 -38 90 -208 00 -01 66 -72 70 -so 70 25 68 -200 75 25 75 -166 76 -130 71 -o 76 43 71 -12 45 18 47 73 70 -·3 87 10 80 75 79 -86 65 171 50 Table XS. Hydrogen Atom Coordinate• (x1o•) and B'a for Cp2Hf(C2Hs)(OOC.Hp). Atom % H111 H112 H113 H121 H122 H123 H131 H132 Hl33 B141 Hl42 H143 H151 Hl52 B153 B211 H212 B213 H221 H222 H223 H231 H232 H233 B241 B242 H243 H251 B252 B253 H321 H322 B323 H331 H332 H333 H341 H342 H343 H411 H412 H421 H422 B423 2008 2591 1650 111 690 1027 Q9 -62 -383 1596 1547 716 2179 2530 2977 2312 2711 3268 1021 477 842 1184 1137 &55 2958 3533 2599 4177 4224 4302 6474 5208 4842 3489 3360 4139 4997 4946 4172 2841 3468 4081 4285 3584 " -3359 -2597 -3094 -2340 -1752 -3047 2367 877 1195 3231 3788 3385 1187 -510 927 -1830 -940 -1449 -771 461 667 4259 4445 3214 4502 4569 5237 489 2238 1699 4194 3313 5007 6159 4018 5190 1397 2622 1501 -2293 -1338 -2528 -636 -1124 ~ 1761 1443 636 1037 2146 1896 1178 1489 413 445 1277 259 207 520 1236 4409 5371 5002 3503 2810 3913 2460 3331 2509 3276 4449 3868 6376 5635 4798 3920 4498 4063 2581 1696 2220 3173 2468 2153 3077 4032 3275 3538 2464 B 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 51 Table X4. Bond lengths and angles and leaat-equarea planes. Atom Hf Hf Hf Hf Bf Bf ·m Bf Bf Hf Hf Bf Hf 01 02 C31 C31 C31 Hf C41 en C12 C13 C14 C15 C21 C22 C23 C24 C25 Cn C12 C13 C14 C15 C21 C22 C23 C24 C25 Atom en C12 C13 C14 C15 C21 C22 C23 C24 C25 R1 R2 01 02 C31 C32 C33 C34 en C42 C12 C13 C14 C15 en C22 C23 C24 C25 C21 CUM C12M C13M C14M C15M C21M C22M C23M C24M C25M Dist. ~A) 2.568 14) 2.556~13~ 2.572 13 2.551(14) 2.561(14) 2.568(13) 2.SS6!13j 2.528 13 2.555(13 2.552(14 2.218 2.247 1.070(8) 1.490(12) 1.437(16) l.S4ltl 1.522 20 1.503 20) 2.280(13) l.Sl8(1D! 1.407~19 1.429 19 1.463(19) 1.473~20~ 1.372 20 1 ..(16~19~ 1.426 19 1.436(19 1.385(19 1.444(19 1.512~20 1.510 20 1.482(19 1.456(21) 1.480(21) 1 .508(19~ 1.486(19 1.498(19) 1.479(19) 1.549{20) Atom Hf R1 01 02 02 02 Hf C11 C12 C13 C14 C15 C21 C22 C23 C24 C25 CUM CUM C12M C12M C13M C13M C14M C14M C15M C15M C21M C21M C22M C22M C23M C23M C24M C24M C25M C25M Atom 01 Hf 02 C31 C31 C31 C41 C12 C13 C14 C15 C11 C22 C23 C24 C25 C21 en en C12 C12 C13 C13 C1.f C14 C15 C15 C21 C21 C22 C22 C23 C23 C24 C24 C25 C25 Atom 02 R2 C31 C32 C33 C34 C.f2 C13 C14 C15 en C12 C23 C24 C25 C21 C22 C15 C12 C11 C13 C12 C14 C13 C15 C14 en C25 C22 C21 C23 C22 C24 C23 C25 C24 C21 Angle( 0 ) 110.6(6) 135.0 108 .7~8) 101.5 10) 110.8(U~ 112.4(11 115 .1~0) 108.7 12) 106.6(U) 106.3(12) 107.8t2l no.6 12 107.6 11) 108.5(11) 107.3(12) 109.5~12~ 107.1 12 124.9(13 123.4(12 124.0(12 126.4~12 125.7 12 125.3(12) 125.7~13~ 128.0 13 121.5~13~ 130.2 13 127.7~12) 124.8 12) 122.8(121 128.5~12 127.3 12 122.5(12~ 123.9(12 128.4(12) 126.9~12~ 123.4 12 52 Toralon Angles. Atom C42 C41 Hf 01 01 01 Atom C41 Hf 01 02 02 02 Atom Hf 01 02 C31 C31 C31 Atom Torsion( 0 ) -17.6(10) 01 02 -70.9~7~ C31 176.1 7 C32 171.0(9) C33 56.1{12) -70.1(12) C34 Deviations from Ring Least-.quarea planes•. Atom cu• C12* C13* C14* CIS* Hf CUM Cl2M Cl3M Cl4M Cl5M Dist.(A) -.007 0.010 -.009 0.005 0.001 2.255 -.287 - .182 -.388 -.029 -.151 Atom Dist.(A) C21* 0.001 C22* 0.003 C23* -.006 C24* 0.007 -.005 C25* -2.247 Hf C21M 0.144 C22M 0.259 C23M 0.280 C24M 0.175 C25M 0.078 • Atoms defining plane indicated by asterisk; unit weights used. 53 1) Moore, E.J.; Sullivan, J.M.; Norton, J.R. J. Am. Chem. Soc. 1986, 108, 2257. 2) Hillhouse, G.L; Bercaw, J.E.J. Am. Chem. Soc. 1984, 106, 5472. 3) a) Manriquez, J.M.; McAlister, D.R.; Sanner, R.D.; Bercaw, J.E. J. Am. Chem. Soc. 1976, 98, 6733. b) Roddick, D.M.; Fryzuk, M.D.; Seidler, P.F.; Hillh~use, G.L; Bercaw, J.E. Organometallics 1985, 4, 97, 1694. (c) Cp*2Hf(CH3)H is prepared cleanly by heating Cp*2HfH2 and (CH3)3PCH2 at 80°C for 1.5 hours in toluene. Eric J. Moore, Thesis, Caltech, 1984. (d) Cp*('15·'1 1-cs(CH3)4CH2CH2CH2)HfH is prepared by heating Cp*('15-cs(CH3)4CH2CH2CH3)HfH2 at 80°C for 2 days. (A. Ray Bulls, unpublished results). 4) Sharpless, KB.; Woodard, S.S.; Finn, M.G. Pure Appl. Chem. 1983, 55, 1823. 5) Roddick, D.M. Ph.D Thesis, California Institute ofTechnology,1984. 6) Fronczek, F.R.; Baker, E.C.; Sharp, P.R.; Raymond, KN.; Alt, H.G.; Rausch, M.D. lnorg. Chem. 1976, 15, 2284. 7) For information on the orbitals of bent metallocenes see Lauher,J.W.; Hoffmann,R. J.Am.Chem.Soc. 1976, 98, 1729. 8) a) Silverton,J.V.; Hoard,J.L lnorg.Chem. 1963, 2, 243. b) Zr vs. Hf radii. Pauling,l. "The Nature of the Chemical Bond" Third Edition, Cornell University Press p.256, 1960. 9) Strukul, G.; Michelin, R.A.; Orbell, J.D.; Randaccio, L lnorg. Chem. 1983, 22, 3706. b) Giannotti, C.; Fontaine, C.; Chiaroni, A.; Riche, C. J.Organomet. Chem. 1976, 113, 57. c) Mimoun, H.; Charpentier, R.; Mitschler, A.; Fischer, J.; Weiss, R. J. Am. Chem. Soc. 1980, 102, 1047. d) Mimoun, H.; Chaumette, P.; Mignard, M.; Saussine, L; Fischer, J.; Weiss, R. Nouv. J. Chim. 1983, 7, 467. 10) Baldwin,A.C. in "The Chemistry of Functional Groups, Peroxides" (S. Patai, e.d.) John Wiley & Sons Ltd, p. 97, 1983. 11) Bulls, A.R.; Bercaw, J.E. unpublished results. 54 12) Halpern, J. lnorg. Chim. Acta 1985, 100, 41. 13) Marvich, R.H.; Brintzinger, H.H. J. Am. Chem. Soc. 1971, 98, 2046. 14) Brown, T.L; Dickerhof, D.W.; Bafus, DA; Morgan, G.L Rev. Sci. lnstrum. 1962, 33, 491. 15) Sharpless, KB.; Verhoeven, T.R. Aldrichimica Acta 1979, 12, 63. 16) Clark, E.P. Ind. Eng. Chem., Anal. Ed. 1941, 13, 820. 55 CHAPTER3 Oxygen Derivatives of Permethyltantalocene Abstract: Cp*2Ta(CH2}H, Cp*2Ta(CHCeHs)H, Cp*2Ta(CeH4)H, Cp*2Ta(CH~CH2)H and Cp*2Ta(CH2=CHMe)H react, presumably through Cp*2Ta-R intermediates, with H20 to give Cp*2Ta(O)H and alkane. Cp*2Ta(O)H was structurally characterized: space group P21 /n, a = 13.073(3)A, b = 19.337(4)A, c = 16.002(3}A, fJ = 108.66(2) o, V = 3832(1)A3, Z = 8 and RF = 0.0672 (6730 reflections). Reaction of tert-butylhydroperoxide with these same starting materials ultimately yields Cp*2Ta(O)R and HOCMe3. Cp*2Ta(CH 2=CHR)OH species are proposed as intermediates in the olefin hydride reactions. Cp*2Ta(0:2)R species can be generated from the reaction of the same starting materials and 0 2 . Lewis acids have been shown to promote oxygen insertion in these complexes. 56 Introduction Gibson's reagent, Cp*2TaCI2, reacts cleanly with two equivalents of alkyl lithium, potassium and Grignard reagents (M-R) to afford olefin hydride and alkylidene hydride complexes.1 The tentative mechanism for the formation of these species involves Initial generation of Cp*2Ta(III)-R, which rearranges by o or ,8-H elimination to give product. Cp*2TaH3Is easily prepared from the reaction of Cp*2TaCI2 and excess UAIH4.1 8 Cp*2TaCl2 + 2 LiMe t> Cp*2Ta(-CH2)H " + 2 ClMgCH2Cll3 t> Cp*2Ta(CH~CH2)H " + 2 C1MgCH2CH2Cll3 t> Cp*2Ta(CH~CHMe)H " + 2 BrMgCH-CH2 t> Cp*2Ta(-C-CH2)H " + 2 LiCsHs t> Cp*2Ta(CsH4)H " + 2 KCH2CsHs Cp*2Ta(-CHCsHs)H t> ~ t> Cp*2Ta( '12-o- CH2C5M4)H Key to much of the reactivity of these complexes is the presence of the Ta(lll) alkyl species (Cp*2Ta-R), which in solution is in equilibrium with the ground state structures. These equilibria have been probed by magnetization transfer and chemical trapping studies. 2 Cp*2Ta(-CH2)H + L L - CO, CNMe, CHz-CH2 ----t> Cp*2Ta(CH3) (L) The reactions of Cp*2TaCI2, Cp*2TaH3 and the olefin and alkylidene hydride complexes with H2 0 , HOOCMe3 and ~ are the subject of this chapter. We have isolated and/or observed tantalum oxo hydride, oxo chloride, oxo alkyl, peroxo chloride, peroxo alkyl, and 57 oxo alkoxide complexes. Preliminary results on the reactions of several of these complexes with Lewis acids will also be discussed. Lewis acids have proven useful or essential for a number of transition metal centered transformations, e.g., olefin polymerization,3 olefin metathesis,4 and insertion of CO into metal alkyl bonds.S Lewis acids have been shown to bind to coordinated olefin, carbonyl, halide and hydride ligands as well as to the metal center directly. Recent research on surface catalyzed oxidations suggests that Lewis acid sites next t.o surface oxo sites may play a key role in the activation of substrate molecules.6 Lewis acid interactions with transition metal based oxidation systems have been surprisingly little explored. The best examples come from the studies of (l)Co-N~ {l = tetradentate Jigand).7 (L)Co-N~ 2 (L) Co -NO s + + ----t> ~ ---~t> (L)Co-NO 2 + s-o (L)Co·N~ Lewis acids U+ and BF3 · OEt2 significantly increase the oxidizing ability of Co-N02 complexes. In the absence of Lewis acids (l)Co-N~ plus ~will catalyze only the oxidation of phosphines to phosphine oxides and isonitriles to isocyanates, presumably reactions driven by the formation of strong P=O and RNC=O bonds.7b By the addition of lewis acids, alcohols, organic sulfides, and cyclohe~adiene can be catalytically oxidized. By selecting metal complexes, Pd7c and T17d, which serve as Lewis acids in the sense that they coordinate olefins, one can extend the reactivity of this system to produce ketones or epoxides. The Co-N~ oxidations are illustrative of at least two roles Lewis acids can play in oxidation reactions. In the first case, lewis acids BF3 · OEt2 and u• apparently activate the cobalt complex by complexation to the N02 ligand, increasing its susceptibility to nucleophilic attack by the added reagents {alcohol, sulfides, 58 cyclohexadiene). Alternatively, Lewis acids activate olefins, by coordination, towards nucleophilic attack by the N~ ligand. Results and Discussion Reactions with Water; The olefin and alkylidene hydride complexes react cleanly with H20 at room temperature to yield Cp*2Ta(O)H and alkane. Cp*2TaH3 reacts with water slowly at 140°C to also yield Cp*2Ta(O)H.8 seconds at RT ----it> Cp*2Ta(O)H + C}4 hours at RT -----it> Cp*2Ta(O)H + hours at 140°C ----t> Cp*2Ta(O)H is the first reported example of a transition metal oxo hydride complex with a terminal oxo ligand. Examples with bridging oxo ligands have been reported.9 In the 1H nmr spectrum, the hydride resonance is found relatively far downfield at 7.43 ppm. TheTaH stretch in the infrared spectrum comes at 1808 cm·1 and upon deuteration moves to 1295 cm-1 . TheTa=160 stretch is observed at 850 cm-1 and moves to 807 cm-1 with 180. The relative rates of reaction with H20 correspond, at least qualitatively, with the relative rates of Ta(lll) alkyl formation. A possible mechanism is suggested by the reactions of SiH4 and PH3 apparently add to the metal center in a manner which precludes reductive elimination of methane. 59 In this mechanism (Scheme I) Cp*2Ta(III)-R is trapped by H20 followed by oxidative addition of an Q-H bond. Reductive elimination of methane leaves Cp*2Ta(OH), which proceeds by a-H elimination to give Cp*2Ta(O)H. ~ Ta-H ~~cH, Scheme I Another possibility is that rather than adding the OH bond, the H20 protonates the Ta-R bond directly, to again give Cp*2Ta(OH) and finally product (Scheme II). This mechanism is consistent with one never observing a complex similar to Cp*2Ta(CH3)(X)H when the OH bond adds in such a way as to preclude reductive elimination of methane. 60 • CH4 ~~ Ta ., ... H ~c~, Scheme II Consistent with both of these mechanisms, the addition of D20 to Cp*2Ta(CH2)H gives CH3D and Cp*2Ta(O)D. Attempts to generate Cp*2Ta(OH} from Cp*2Ta(O}H have proven unsuccessful. Upon heating at 11 0°C under CO for several hours one observes slow loss of Cp*H and starting material. The metal containing products of this reaction have not been identified. No reactions were observed with Me3P=CH2 and Me3P=O. It is interesting to note that the reaction of Cp*2Ta(=CH2}CH3 with H20 leads to immediate (seconds) decomposition with loss of Cp*H. This decomposition is not well understood. 61 Crvstalloaraphic Results: From an attempt to grow crystals of Cp*2Ta(CHCsHs)H in petroleum ether, a crystal of Cp*2Ta(O)H (present as an impurity from the reaction of H20 with Cp*2Ta(CHCsHs)H) was isolated by LeRoy Whinnery. Nhi Hua and Richard E. Marsh determined the structure by x-ray diffraction methods. 1 0 The unit cell contains two distinct forms of the molecule. Figure 1 shows the orientations of the two forms of the molecule. Figure 2 shows the pseudotetrahedral orientation of the ligands about Ta(1) and Figure 3 the unit cell. The primary structural difference between the two forms is found in the H-Ta-0 angles: H1-Ta1-01, 71.1(4) 0 ; H2-Ta2-02, 75.5(3) 0 • The Ta1=01 and Ta2=02 bond lengths are the same, 1.76 A. The tantalum Cp* carbon bonds average 2.486(9) A and the ring C-C bonds average 1.411(12) A, with the ring angles averaging 107.96(73)0 • The C-CH3 bonds of the two Cp* rings average 1.503(13) A, with the C-C-CH3 angles averaging 125.75(77)0 • The Cp* centroid-Ta-centroid angles are R1-Ta1-R2, 137.9(2)0 and R3-Ta2-R4, 138.6(2)0 • Figure 1. The two forms of Cp*2Ta(O)H and their relative orientations. 64 Oxo Alkyl Comolexes: The reactions of tert-butylhydroperoxide (TBHP) with permethyltantalocene d0 hydrides appear to be quite different from the reactions observed for permethylhafnocene hydrides (Chapter 2). Instead of metathesis to give H2 and an alkylperoxide complex, tert-butylhydroperoxide serves ultimately as a source of the metal oxo functionality. Accompanied by vigorous H2 evolution, Cp*2TaHs reacts quickly (the reaction is over in seconds) with greater than one equivalent ofTBHP to give Cp*2Ta(O)H. The addition of HOOCMes to Cp*2TaDs gives only Cp*2Ta(O)D (by nmr). Cp*2Ta(O)H undergoes a second much slower (hours) reaction with TBHP to generate Cp*2Ta(O)OH. The Ta=O infrared stretch for Cp*2Ta(O)OH is found at 845 cm·1 • The O.H region contains two resonances, a sharp, weak signal at 3660 cm·1 and a broad signal at 3380 cm·1. Upon deuteration the broad signal moves to 1264 cm·1. Cp*2T~ HOOCMes ---~t> Cp*2Ta(O)H + HOCMes + HOOCMes ---~t> Cp*2Ta(O)OH + HOCMes + Cp*2Ta(O)H + H2 Cp*2Ta(11 2-oCH2)H also reacts cleanly with TBHP, trapping Cp*2Ta(OCHs), to yield Cp*2Ta(O)OCHs and t-butanol. The addition of one equivalent of TBHP to solutions of Cp*2Ta(CH2)H, Cp*2Ta(CHCsHs)H or Cp*2Ta(C6 H4)H results in the immediate formation of the corresponding tantalum oxo alkyl complex. Cp*2Ta(CH2)H + + Cp*2Ta(CHCsHs)H Cp*2Ta(CsJ4)H HOOCMes + Cp*2Ta(O)CHs HOOCMes HOOCMes + + HOCMes Cp*2Ta(O)CH2CsHs ----t> Cp*2Ta(O)CsH5 HOCMes + HOCMes 65 1 H nmr spectra are consistent w ith the above formulations, and the Ta=O infrared stretches are found in the region from 850 to 860 cm-1 • Heating these complexes at 80°C in the presence of TBHP has no further effect. A reasonable mechanism for the formation of these species begins with the trapping of Cp*2Ta-R with HOOCMe3 (Scheme Ill). Oxidative addition of the OH bond would result in an alkylperoxide complex which must decompose to Cp*2Ta(O)(R), possibly by 0-0 bond homolysis and radical abstraction of the tantalum hydride, before reductive elimination of alkane takes place. ~ ~ Ta-R + HOOCMe3 • HOCMe3 -~R Ta ~0 HOOCMe 3 - HOCMe3 -~R Ta-O· --.¢[~H ·O-cMe ~ 3 Scheme Ill An alternative is to consider the trapping step a nucleophilic attack of the d 2 metal complex on the 0-0 bond of the peroxide with concerted transfer of W to -ocMe3 and oxo to metal (Scheme IV). This type of reaction has been invoked to explain the reactions of alkylperoxides w ith cytochrome P-450 model complexes.11 66 -¥ Ta-R ~ HOOCMe,\ + HOOCMe 3 • HOCMe3 -~A ~0 I HOCMe, Schema IV It was mentioned in the introductory chapter that Cp*2Ta(O)(CH3) can also be obtained from the decomposition of Cp*2Ta(172-0CH2)H. 1 b Attempts to regenerate and trap the proposed intermediate Cp*2Ta(OCH3) from Cp*2Ta(O)(CH3) have proven unsuccessful. One observes no reaction upon heating Cp*2Ta(O)(CH3) with CNMe at 140°C for days. This is in contrast to Cp*2Ta(CH2)(CH3) which rearranges cleanly upon heating to give Cp*2Ta(CH2=CH2)H, presumably through Cp*2TaCH2CH3.2 Photolysis of Cp*2Ta(O)(CH3) also does not regenerate Cp*2Ta('7 2-0CH2)H. Ught induced rearrangement has been observed in the conversion of Cp*2Ta(S)CH3 to Cp*2Ta(172-SCH2)H. 12 The reactions of TBHP with Cp*2Ta(CH2=CH2)H and Cp*2Ta(CH2=CHMe)H are more complicated than those of the complexes above. The addition of excess TBHP to the yellow olefin hydride complexes results, over a period of several minutes, in the formation of yellow-orange intermediates (A) and (B) which still contain olefin bound to the metal. One also observes the immediate formation of approximately 5% Cp*2Ta(O)(R) (colorless), which may form through Cp*2Ta-R by a pathway similar to that outlined above for 67 ---~t> (A) Cp*2Ta ( CHz-CHMe )H + HOOCMes - - - - t > Cp*2Ta (CHz-CHR) H + HOOCMes + (B ) HOCMe3 + HOCMes 5% Although as yet it has proven impossible to cleanly isolate (A) or (B), on the basis of subsequent reactivity, color, and nmr spectra, reasonable candidates for these species are (A) • Cp*2Ta(CH2=CH2)(0H) and (B) • Cp*2Ta(CH2=CHMe)(OH). (A) and (B) have been isolated as mixtures with Cp*2Ta(O)(R) and TBHP. Even in the presence of excess TBHP, these complexes are stable for weeks if stored at low temperature in the solid state. The nmr spectra for these complexes show Cp* resonances farther upfield than those observed for any of the other permethyltantalocene complexes. Compare 1.55 ppm for (A) with 1.79 ppm for Cp*2Ta(O)(Et) and 1.70 for Cp*2Ta(CH2=CH2)H. Because of the coordinated propene, (B) has inequivalent Cp*'s at 1.65 and 1.53 ppm. Figures 1 and 2 show representative 1H nmr spectra for (A) and (B) and their decomposition products. (A) and (B) both decompose by at least two pathways to give two products, Cp*2Ta(O)R and Cp*2Ta(O)(H) and olefin. When the reactions were carried out in the presence of excess TBHP, Cp*2Ta(O)(H) was converted to Cp*2Ta(O)(OH). xs TBHP Although the rate of decomposition of (A) varies depending on conditions, the product ratios are relatively constant: 80% Cp* 2Ta(O)(CH2CHs) and 20% Cp*2Ta(O)(OH). Polar solvents considerably increase the rate of decomposition and the percentage of 68 b) ~===c~~~~~ r~· I• i 1 li 1• 7~ 1- .t;·T j---;-..L-1 -,--j I i ·t I --1 - "" J-LL..L- __l • ' I I I ; .L _ Cp*2Ta(O)(Et) -- -~- ~-- -Vt-11-] --i.-.1 _I _ '-- ____] _ _ ~. II _:_ . . ! : ; I ! a -- >-I ! f- ~ io 'lo 'io ~ c) I r!o 1- · a1 ·~ a~ • 1 jo : w H +- ~ v i · :-+'-r · ~-+--r--.1+ ·1 ~~ ( I ;. I Cp*2Ta(O)(OH) I. ~- I 7f- ~ I , I ! ' - I+ r-~ ' ' - - TBHP ~~r- I ~ ~, • . ;---.'--+-~, 1 ; II 10 "- • . r--:- --r . ~--l l -+-- -~' l 1 1 1- I 1. ' l ~- - , . L~-- .. I . --· ' ' j-+- ,_ _j __ ' ~ ·-i. :-~ ~ -i-- ppm scan wt'dh t d - -il I \. • ' • I' ' • . . '. • ·r'-J .., I _L I i-1-t- ~~---1 0! ~- -d--+-r-.L: AJJ-~- W -1--'r:· 1 ' ' 1 I ' --! I TMS ll ·, -· J ' I •!~-~ -r- ,. ..:;.,1 .. -~-I . }\. ' ~- -c--~ - -=-~- ,_...:.- - - ,_t-1~_j --r--rr-r- - l -.,.,J•'•' r t-Bu~H2·f t~- -H-~-iT : -: I I ! ' j ~ . ): ~ - - ~i ;~l I '-j : --: 0 Figure 1. a) Isolated sample of (A), Cp*2Ta(O)(Et), and TBHP. b) Decomposition products upon addition of 0.002 M EtMgCI in ether to (A) . The decomposition is accelerated and yields almost entirely Cp*2Ta(O)(Et). c) Decomposition of (A) without added Grignard. The products are Cp*2Ta(O)(Et) and Cp*2Ta(O)(OH). Expanded spectrum of the region from 0 to 2 ppm. 69 a) -f- ·- i=~+-~:=~ - TBHP ".,.) -r-r-:" 30 - ! . ___.;: .. , I . - ' (B) . I .. - c - r - ' ~ ·- ' t-BuOH ......;.r. r- - ~ t~ -- - -t-r----r- 1- - ,_ H- ~ ro-r I' j,_ I ~ , i ' +- t I -j -~ - - ! ·+~ j ' ' ~-;i I~ l .. r-~ ·+- Figure 2. a) Spectrum of (B) generated in situ from Cp*2Ta(CH2=CHMe)H and excess TBHP. b) Products from the decomposition of (B) without added Grignard, Cp*2Ta(O)(OH) , propene and Cp*2Ta(O)(Pr) . c) Cp*2Ta(O)(Pr), from the decomposition of (B) in the presence of PrMgCI. 70 For (B), both the rate of decomposition and the product ratios are variable. In fact, different batches of Cp*2Ta(CH~CHMe)H (identical by nmr) gave different final product ratios, all after initially forming (B). The cleanest batches of starting material give greater amounts of Cp*2Ta(O)(OH) and propene than Cp*2Ta(O)(CH2CH2CH3). Decomposition of (B) under one atmosphere of propene did not change the product ratios. After wading through numerous nmr experiments to sort out the above reactions, it became clear that the decompositions of (A) and (B) to Cp*2Ta(O)(R) were probably catalyzed by impurities in the starting materials (vide infra). Approximate relative rates, k(50°C), for Cp*2Ta-R formation in the olefin hydride and alkylidene hydride complexes are listed below. Cp*2Ta(CHCsHs)H Cp*2Ta(=CH2)H Cp*2Ta(CsH4)H Cp*2Ta(CH2=CHMe)H Cp*2Ta(CH2=CH2)H sec·1 sec·1 sec·1 sec·1 sec·1 If the equilibrium concentrations of the tantalum(JJI) alkyl species follow the same trends, these rates offer a fairly straightforward explanation for the difference in reactivity between Cp*2Ta(CH2=CHMe)H. For the latter two complexes, the concentration of Cp*2Ta-R is too low for trapping to be the predominant mechanism and a second reaction pathway takes over. Consistent with this, the reaction of Cp*2Ta(CH~CH2)H and Cp*2Ta(CH2=CHMe)H with TBHP is much faster {minutes) than reaction with H20 {hours). For the other complexes reactions with both H20 and TBHP seem to be complete within a minute of addition. Working on the assumption {A) and {B) are the species suggested above, a possible mechanism for their formation involves oxidation of Cp*2Ta(CH2=CHR)H by TBHP to give 71 [Cp*2Ta(CH2=CHR)Hj+OH- and • OCMe3. The t-butoxy radical then abstracts the tantalum hydride hydrogen from [Cp*2Ta(CH2=CHR)Hj+OH- or escapes and generates other radicals by catalyzing the decomposition of TBHP .13 One always observes greater than one equivalent of TBHP consumed in these reactions. Cp*2Ta(CHroCHR)H + HOOCMe3 -----'t> [Cp*2Ta(CH~HR)H]•mr + [Cp*2Ta(CHroCHR)H]•oH· + ·OCMes _ _ __,t> Cp*2Ta(CHroCHR) (OH) · OCMe 3 + HOCMe3 To the best of our knowledge, no examples of olefin hydroxide complexes have been reported, although they have been Invoked as possible intermediates for Wacker-type oxidations 14 and recently, by Trogler and Jensen for platinum catalyzed olefin hydrations. 15 For these late transition metal examples, formation of the olefin hydroxide complex is followed by olefin inserti?n into the metal oxygen bond. HO H LPI_f-H_ 2 I H H + R unprecedented rearrangement. On the basis of the limited experimental evidence one can only speculate as to mechanism. Cp*2Ta(CH2=CHR)(OH) decomposition can be explained by invoking two mechanistic pathways. The first pathway involves loss of olefin to leave Cp*2Ta(OH) (Scheme V). Cp•2Ta(OH) converts rapidly to Cp*2Ta(O)H or is trapped by t-butylhydroperoxide to give Cp*2Ta(O)(OH). Consistent with this being a sterically driven process, the propene derivative loses olefin much more readily than the ethylene complex. The rate of olefin loss from Cp*2Ta(CH2=CHR)(OH) seems to be constant under different reaction 72 conditions. The percentage of final product derived from this pathway is therefore dependent on the rearrangement rate of Cp*2Ta(CH2=CHR)(OH) to Cp*2Ta(O)(CH2CH2R), which can vary considerably. ,H cp· ··... ~o "H Ta cp•/ I TBHP TBHP SchemeV The variable nature of the second decomposition pathway, Cp*2Ta(CH2=CHR)(OH) to Cp*2Ta(O)(CH2CH2R), is difficult to explain. One possibility is that impurity left in the starting material (XMgR) initiates the transformation by electron transfer to Cp*2Ta(CH2=CHR) (OH). [Cp*2Ta(ll) (CH2=CHR) (OH) ]" rearranges quickly to [Cp*2Ta(IV)(O)(CH2CH2R)r which then continues the process by transferring an electron to another molecule of Cp*2Ta(CH2=CHR)(OH). This leads to speculation as to why the tantalum (II) intermediate [Cp*2Ta(II)(CH2=CHR)(OH)r would undergo this rearrangement. The driving force for this proposed rearrangement comes from localizing the negative charge on oxygen. A conceptualization of this process is shown below (Scheme VI): Initially the added electron is localized on the olefin/alkyl ligand. This carbanion pulls the proton off of Ta-OH and leaves the negative charge on oxygen. e 73 ,H - .. - H ,H H ••. c I .·Ta/ y.HR ··.. I ' c-R Cp*. .. Ta ·G Cp*/ \ H OH Cp* ·•• H··· C Cp* / "- o9 H Scheme VI To test the above mechanism, several experiments were carried out. tt was found that stirring the starting olefin complex with HOCMes for several hours,16 to remove residual alkyl grignards, did not influence the rates of (A) and (B) formation, but did significantly slow the rates of decomposition. Conversely, addition of 0.002M of CIMgEt to 0.025 M of (A) resulted in much faster (approximately 30X) 17 decomposition to Cp*2Ta(O)CH2CHs. (B) not only decomposes more quickly with CIMgPr, the only product formed is Cp*2Ta(O)CH2CH2CHs. The addition of both potassium and potassium napthalide increased the rate of decomposition of (A), but the reactions were less clean. Cp*2Ta(=C=CH2)H does not react cleanly with TBHP, although some Cp*2Ta(O)(CH=CH2) is formed. It is possible to generate oxo alkyl complexes using other reagents. Preliminary results with N20 indicate this reagent can also be used to cleanly generate tantalum oxo alkyl species. 74 For Cp*2Ta(CH2=CHMe)H and Cp*2Ta(=C=CH2)H this reaction is much cleaner than the reactions with TBHP. + N2 + H2 Cp*2Tali3 + N20 t> Cp*2Ta(O) (H) Cp*2Ta(-CH2)H + N20 t> Cp*2Ta(O) (Cli3) Cp*2Ta(CH~CHMe)H + N20 t> Cp*2Ta(O)(CH2CH2Cli3) Cp*2Ta(-C-CH2)H + N20 t> Cp*2Ta(O)(CH-CH2) Cp*2Ta(CsH4)H + N20 t> Cp*2Ta(O) (CsH5) Cp*2Ta(CHC61is)H + N20 t> Cp*2Ta(O)(CH2CsHs) + N2 + N2 + + N2 N2 + N2 Me3P=O does not react with Cp*2Ta(CH2)H before decomposition of the tantalum complex to the metalated Cp* species. Cp*2TaCI2 reacts with greater than one equivalent of TBHP and H202 to give Cp*2Ta(O)CI. During the reaction with TBHP one can observe the green color of Cp*2TaCI2 in solution transform to an orange-red color. The orange color fades to give a colorless solution. An intermediate, probably responsible for the orange-red color, Is observed in the nmr with a Cp* resonance at 2.2 ppm, the correct region for a cationic species. This suggests a mechanism for the formation of Cp*2Ta(O)CI: oxidation of Cp*2TaCI2 to [Cp*2TaCI2t oH· followed by loss of HCI. Cp*2TaCl2 + [ Cp*2TaCl2] • OH- HOOCMe3 [Cp*2TaCl2]• OH- ----t> Cp*2Ta(O)Cl + · OCMe3 + HCl To test this mechanism, Cp*2Ta(O)CI was synthesized from [Cp*2TaCI2tBF4- (orange) 18 and excess KOH. In fact, this is a better method of synthesis for Cp*2Ta(O)CI than the TBHP or H202 reactions. 75 Cp*2TaCl2 + AgBF4 (Cp*2TaCl2]•BF4- + ----1t> xs KOH - - - - t > (Cp*2TaCl2]•BF4Cp*2Ta(O)Cl + + Ag (O) KX Lewis Acid Reactions: Initial indications are that Lewis acids (AIR's) react with Cp*2Ta(O)(R) (R =Alkyl, H) to form adducts. Cp*2Ta(O)R + AlRs ----t> Cp*2Ta(O)R·AlRs Cp*2Ta(O)H has two potential sites to bind Lewis acids; the hydride and oxo ligands. Gerard Parkin has isolated Cp*2Ta(O)H · AIEtJ and spectroscopic evidence indicates that AIEtJ is bound to the oxo ligand.19 The hydride resonance in the nmr shifts downfield almost 2 ppm, from 7.43 ppm to 9.10 ppm upon Lewis acid coordination. The Ta-H infrared band shifts from 1808 to 1861 cm·1 and the oxo stretch changes from 850 to 837 cm·1 and becomes broader. Similar observations were made for Cp*2Ta(CH2=CH2)H · AlEl3.20 A crystal structure of this complex confirmed Lewis acid coordination to the olefin ligand rather than the hydride. Adduct formation is the only reaction with Lewis acids yet observed for the oxo alkyl complexes. Cp*2Ta(O)CI undergoes ligand rearrangement with AIMes to yield Cp*2Ta(O)Me · AIMe2CI. Cp*2Ta(O)Cl + AlMe3 ----t> Cp*2Ta(O)Me ·AlMe2Cl Cp*2Ta(O)Cl + AlEt3 ----t> Cp*2Ta(O)Cl·AlEt3 Cp*2Ta(O)Me can be recovered by adding H20 to the reaction to precipitate the aluminum reagents. AIEtJ simply forms an adduct with Cp*2Ta(O)CI and does not appear to alkylate the tantalum. 76 Peroxo Comolexes: Cp*2Ta(O)CI reacts with H2C2 to generate Cp*2Ta(Oi)CI. This reaction is precedented for CP2Nb(O)CI.2 1 Upon heating with PPha. Cp*2Ta(Oi)CI cleanly transfers oxygen to give Cp*2Ta(O)CI and OPPh3 . No other substrates have been successfully oxygenated. Cp*2Ta(O)Me does not react with H202. The other oxo alkyl complexes have not been tested for this reaction. It was mentioned in the introductory chapter that no transition metal peroxo alkyl complexes have been reported. Organometallic intermediates have, however, been suggested for metal peroxide oxidations.22 Cp*2Ta(CH2=CHi)H and Cp*2Ta(CH2=CHMe)H with 02 give the first examples of this type of complex. Cp*2Ta(-CH2)H + ~ - - - - t > 85% Cp*2Ta(~)CH:3 + 15% Cp*2Ta(O)CH:3 Cp*2Ta(~)H + ~ Cp*2Ta(CHrCH2)H + ~ Cp*2Ta(CHrCHMe)H + ~ + ~ Cp*2Ta(-CHC6Hs)H - - - - t > Cp*2Ta(~)C6Hs These reactions were carried out by dissolving the starting materials in petroleum ether, placing one atmosphere of dry 0 2 over the solution at -78°C and allowing the reaction to warm to room temperature. During warming, the solution color changes and white solid precipitates. In general, these compounds are almost insoluble in petroleum ether, moderately soluble in diethyl ether and soluble in aromatic solvents. Molecular weight measurement indicates Cp*2Ta(Oi)CH3 is a monomer. 1 H nmr spectra show Cp* resonances for these complexes always upfield of the corresponding oxo alkyl complexes. The infrared spectra show bands for the 0-0 stretch in the region 860-865 cm·1• 77 The reaction of Cp*2Ta(CH2}H is the least clean of the above reactions. One always observes the formation of at least 10% Cp*2Ta(O)CH3 and occasionally 5 to 10% of an unidentified species. Approximately 3% of Cp*2Ta(O)R is formed in the Cp*2Ta(CH2=CHR')H reactions. At the other extreme, the reaction of Cp*2Ta(=CHCsHs)H with ~ is very clean, even when carried out by shaking an open nmr tube in the air. Cp*2Ta(=C=CH2)H and Cp*2TaHa do not react cleanly with oxygen under the conditions employed. In the latter case one observes the formation of Cp*2Ta(O)H and Cp*2Ta(O)OH. One of the major products in the former reaction is likely Cp*2Ta(02}CH=CH2. Why Cp*2Ta(=C=CH2)H reacts differently than the other complexes with both TBHP and~ is not understood. Trapping of Cp*2Ta-R by~ is a reasonable first step in the formation of Cp*2Ta(02}R (Scheme VII). High spin Cp*2Ta-R would be a biradical with one elect.ron in each of the two non-bonding orbitals.23 A biradical metal center should have no spin imposed barrier for reacting with triplet 02. The proposed initial step Involves end on binding of the ~to give Ta(IV) super oxide species. This complex can then either close to form Cp*2Ta(02}R or react with a second equivalent of Cp*2Ta-R to form Cp*2Ta(R)-QQ-Cp*2Ta(R). Cp*2Ta(R)-Q0-Cp*2Ta(R) decomposes rapidly to Cp*2Ta(O)R. The percentage of Cp*2Ta(O)R formed would then depend on the sterics of the A ligand and the concentration of Cp*2Ta-R. A similar mechanism has been proposed to explain Mo=O formation from 0 2, MoCis and KCN.24 A second possibility, formation of Cp*2Ta(O)R by oxygen transfer from Cp*2Ta(02)R to Cp*2Ta-R was eliminated by the observation that Cp*2Ta(02)CHa does not react with Cp*2Ta(CH2)H. 78 4R R~ -~o-o~5r I 2 -¥. Ta, ~~ Scheme VII -¢ Ta-O ~L~o/ ~ 79 All of the complexes are stable in the solid state if stored at low temperature. Cp*2Ta(QVCH3 is stable at room temperature in benzene for weeks. Cp*2Ta(02}CH2CH3, decomposition after one week in solution. At 80°C Cp*2Ta(QVCH3 will cleanly oxidize PPh3 to OPPh3, leaving Cp*2Ta(O}CH3. The other complexes decompose before transferring oxygen. An attempt to oxidize styrene at 80°C resulted in the very clean transformation of Cp*2Ta(QVCH3 to Cp*2Ta(O}OCH3. Curiously, when Cp*2Ta(QVCHs is heated in the presence of NEts, one observes neither oxidation of the amine nor rearrangement to Cp*2Ta(O)OCH3 before decomposition. When heated alone, one occasionally observes the same transformation as a non-first order process. It seemed unreasonable that styrene was responsible for the observed reactivity. Aldrich styrene, however, contains 1Q-15 ppm 4 tert-butylcatechol as stabilizer and the catechol could serve as an acid catalyst for the rearrangement. To test this possibility both phenot25 and MeS03H were successfully used to catalyze the rearrangement. Heating to 80°C is required to drive the rearrangement at a reasonable rate when trace amounts of acid are used. Heating Cp*2Ta(QVCHs with H20 results in the formation of both Cp*2Ta(O)OCHs and Cp*2Ta(O}CH3.2 6 If one equivalent of CH2CsHs} at room temperature, one observes a slow conversion over a period of three days to R'OH and Cp*2Ta(O)OS~R. H+(trace) ----t> Cp*2Ta(O)OCHs Cp*2Ta(O)OS~R + HOR' A possible mechanism for the acid catalyzed rearrangement is shown below (Scheme VIII). 80 0 Cp*•··ra/ \o Cp* _.... 0 Cp*•·· Ta ~ Cp*_.... 'oR slow " R 11-E' + + fast Scheme VIII These results suggested it might be of interest to test the reactivity of Lewis acids with these complexes. Preliminary results indicate that aluminum alkyls work well to instigate oxygen transfer. AlEl3 reacts immediately and stoichiometrically with Cp*2Ta(~}CH2CH3, ----t> Cp*2Ta(O) (OCH2CH:3) · AlEt3 Over a period of several days at room temperature these adducts convert to Cp*2Ta(O)Et and presumably AIEt20R (bound to the oxo complex). - - - - - t > Cp*2Ta(O)Et · AlEt2(0Pr) Cp*2Ta(02}CH3 reacts much more slowly than the above compounds with AIEi3 and AIMe3. Even in these slower reactions, one never observes a Lewis acid adduct of the peroxo species. Approximately two equivalents of AIMe3 are required to complete the reaction. In 81 the final product mixture one observes AIMex0Me3-x and only 10% of Cp*2Ta(O)(OCH3) · AIMe3, the rest is Cp*2Ta(O)CH3 · A1Me3.27 Initially one might suspect this reaction takes place in a manner similar to the reactions above, i.e., formation of Cp*2Ta(O)(OCHs) · AIMe3 followed by ligand redistribution between aluminum and tantalum. Cp*2Ta(O)(OCH3) · AIMe3, however, does not appear to undergo ligand redistribution on the time scale of the reaction. In addition, Cp*2Ta(~)CsHs reacts with AIMe3 in a similar manner to produce Cp*2Ta(O)CsHs · AIMe3. One does not observe Cp*2Ta(O)CH3 in this reaction. Cp*2Ta(~)CH3 + AlMes ---.....,t> Cp*2Ta(O)Me ·AlMe3 + Cp*2Ta(O) (OMe)·AlMe3 Cp*2Ta (~)CsHs + AlMe3 ----it> Cp*2Ta(O)CsHs · (AlMe3) AIMe3 is a dimer in solution with only a small amount of monomer in equilibrium with the dimer. If one considers attack of the dimer (AIMe3)2 on Cp*2Ta(~)R (R = CHs, CsHs) as the initial step of the above reaction, the coordination of the first AIMe3 activates the peroxo moiety for insertion into the Al-cH3 of the second A1Me3. The difference in reactivity for difference in metal alkyl bond strengths. Metal methyl and metal phenyl bonds tend to be stronger than normal alkyl bonds.28 A recent report by Bryndza, Fong, Paciello, Tam and Bercaw, correlating H-X with M-X bond strengths, would suggest R on the order of 6 kcal/ mol stronger than R kcal/ mol stronger than R =CHs, CsHs bonds are =CH2CH3, CH2CH2CH3 and as much as 18 =CH2C6H5.29 Conclusion Permethyltantalocene alkylidene and olefin hydride complexes have served as starting materials to generate a number of complexes relevant to metal based oxidation systems. The first example of a metal oxo hydride complex was generated by several different 82 routes. New examples have been added to the relatively sparse literature on metal oxo alkyl complexes. If correctly assigned, the proposed intermediates in the reaction of TBHP with Cp*2Ta(CH2=CHR)H are the first observed metal olefin hydroxide complexes. The first organometallic metal peroxo complexes have been isolated from clean reactions with 02. And Lewis acid promotion of oxygen transfer has been shown to take place quite readily. Unifying themes emerge from both the hafnium alkylperoxide (Chapter 2) and tantalum peroxo alkyl studies. Heterolytic transfer of oxygen to a nucleophile (in this case, R") is greatly facilitated by stabilization of the negative charge built up during nucleophilic attack on the 0-0 bond. One can accomplish this in the case of the hafnium alkylperoxide complexes by donating electron density from the alkoxide oxygen Into an empty orbital on the metal. The tantalum peroxo alkyl complexes demonstrate this principle without the ambiguity surrounding the question of 172-alkylperoxide coordination. When the metal no longer contains an accessible empty orbital, as is the case with the tantalum complexes, the addition of an external Lewis acid can perform the same function. All of this is consistent with observations made for peracid oxidations of nucleophiles; the greater the electron withdrawing ability of the acid group, the more the oxidation process is facilitated.30 When the metal does not have the internal capacity or an external reagent for accepting electron density, one observes reactions consistent with homolytic bond cleavage. Relative metal carbon bond strengths also seem to influence the course of reaction. In both the hafnium and tantalum studies, we have observed that the complexes with stronger metal carbon bonds (methyl, phenyl) react more slowly, and often find alternate reaction pathways. It is obvious from the foregoing material that work in the tantalum system is not yet · complete. Work is continu ing in an attempt to study alkyl to oxo migrations. The Lewis acid promoted alkyl to peroxo migrations are also under study. 83 TABLE I. NMR, IR AND ANALYTICAL DATA Compound Assignment 6Cppm) Ta-CH2 10.17(s) ('7 5 -Cs(CHJ)s) 1.95(s) Ta-H 1.75(s ,bd) ('7 5 -Cs(CHJ)s) 12 . 00(q) ( '7 5 -Cs( CH3) s) 107.45(s) CH2 239.47(t) J(Hz) Cp*2Ta(=CH2)H c 54 . 30 H 7.11 (54.08) ( 7 . 08) 13c 126 . 5 1792 v(Ta-H) Cp*2Ta(=CHC5H5)H (I) <J (I) c 59 . 76 H 6.83 (II) t> Cp*2Ta(CH2-o-Ct;H4)H (II) CHCsHs 10.67(s) CHCsHs 7 . 5-6.5(m) Ta-H 2 . 81(s) ('7 5 -Cs(CH3)s) 1. 90(s) CH2CsH4 7 . 5-6.5(m) Ta-H 3.79(s) ('7 5 -Cs(CHJ)s) 1. 60(s) CH2CsH4 0 . 58(s) (59.76) ( 6 . 89) 127.0 84 Cp•2Ta(=CH2)CH3 c 55.92 (55.99) H 7 . 54 ( 7.34) 13c CH2 9.46(s) ('7 5 -Cs (CHJ ) s) 1.77(s) CH3 -0 . 49(s) CH3 9 . 30(q) 120.8 ('7 5 -Cs(CHJ)s) 11 . 59(q) 125 . 7 ('7 5 -Cs(CHJ)s) 108.93(s) CHz 230 . 45(t) Ta-H 7 . 43(s) ( '7 5 - Cs ( CH3 )s) 1.93(s) ( '7 5 - Cs ( CH3)s) 11.43(q) ('7 5 -Cs(CHJ)s) 114.20(s) 129 . 4 Cp•2Ta(O)H c 51.00 (51. 28) H 6 . 59 ( 6 . 67) 1808 v(Ta-H) 1295 v(Ta-D) 850 v(Ta- 160) 807 v (Ta-180) 13c 127.0 Cp•2Ta(O)H · AIEt:3 1861 v(Ta-H) 837 v(Ta-0) Ta-H 9 . 10(s) ('7 5 -Cs(CHJ)s) 1. 77 (s) A1(CH2 CH3)J 1.69(t) 8.0 A1 (CH2 CH3)J 0 . 41(q) 8.0 85 Cp*2Ta(O)OH 3660 v(O-H) 3380 v(O-H) 1264 v(O-D) 845 v(Ta-Q) ('1 5 -Cs(CH3)s) 1.85(s) ('1 5 -Cs(CH3)s) 1. 77 (s) CH3 0 . 25(s) ('1 5 -Cs(CH3)s) 11.65(q) 126.0 CH3 20.34(q) 122 . 0 ('7 5 -Cs(CH3)s) 115 . 26(s) ('1 5 -Cs(CH3)s) 1.88(s) ( '75 - Cs ( CH3)5) 1.77(s) ('7 5 - Cs(CH3)s) 1.68(s) CH3 0.15(s) Cp*2Ta(O)CH3 c 51.89 (52.28) H 6.73 ( 6.89) 13c 855 v(Ta-0) Cp*2Ta(O)CI 850 v(Ta-Q) Cp*2Ta(02)CI 858 v(0-0) Cp*2Ta(02)CH3 c 50.99 (50 . 60) H 6.78 ( 6 . 69) 858 v(0-0) 86 Cp*2Ta(02}CH2CH3 (77 5 -Cs(CH3)s) 1.65(s) 863 v(0-0) CH2CH3 under Cp* CH2CH3 0 . 80(q) ('7 5 -Cs(CH3)s) 1. 64(s) 7.3 Cp*2Ta(02}CH2CH2CH3 c 52.83 (52.47) H 7.00 ( 7.08) 861 v(0-0) CH2CH2CH3 under Cp* CH2CH2CH3 l.3S(t) CH2CH2CH3 0 . 70(m) CH2Cslis 7.75-6.95(m) CH2CsHs 2.10(s) (77 5 -Cs(CH3)s) 1. 67 (s) CH2CH3 1.88(t) (77 5 -Cs(CH3)s) 1.79(s) CH2CH3 0.94(q) 7.2 Cp*2Ta(02}CH2CsHs c 56.34 (56.44) H 6.46 ( 6.49) 863 v(0-0) Compounds not isolated Cp*2Ta(O}CH2CH3 7.6 860 v(Ta-0) 7.6 87 Cp*2Ta(O)CH2CH2CH3 ('7 5 ·Cs(CH3)s) 1. 78(s) CH2CH2CH3 under Cp* CH2CH2CH3 1.28(t) CH2CH2CH3 0 . 84(m) CH2Cslls 7 . 67-7 . 02(m) CH2CsHs 2.29(s) ( '7 5 - Cs ( CH3)s) 1. 78(s) CsHs 7 . 93-6 . 68(m) ( '7 5 - Cs ( CH3 )s) 1. 73(s) ('7 5 -Cs(CH3)s) 1.66(s) CH3 0.38 Al(CH3)3 -0 .27 CsJls 7.92-6.45(m) ( '1 5 - Cs ( CH3 )s) 1.63 ( s) Al(CH3)3 -0.09(s) 7.1 Cp*2Ta(O)CH2CsHs Cp*2Ta(O)CsHs Cp*2Ta(O)CH3 · AIMe3 Cp*2Ta(O)CsHs · AIMe3 88 Cp*2Ta(O)OCH3 OCH3 4.18(s) ('7 5 -Cs(CH3)s) 1.92(s) 89 ExPerimental Section General Considerations: See experimental section, Chapter 2. Oxygen was dried by passage through a trap at -78°C or used directly from the lecture bottle. N20 was freeze-pump-thawed before use. 90% hydrogen peroxide was obtained fonn FMC Corporation adn used as receiced. Procedures: Many of the reactions reported in this chapter were carried out in septa capped nmr tubes. In a typical experiment, 15 mg of starting material were placed in an nmr tube and dissolved in 0.4 mls of benzene-ds. TBHP was added as a benzene solution via syringe. The reactions were then followed by nmr. Cp*2Ta(=CH2)H: 1.114 g (2.14 mmol) of Cp*2TaCI2 were stirred in 20 mls of ether for 20 hours with 4 ml of 1.5M UMe solution. (The end of the reaction is indicated by a lightening of the reaction mixture and loss of the green color of Cp*2TaCI2. The final ether solution is often red.) Ether was removed in vacuo and petrolel:Jm ether was added. To remove residual ether and ensure complete precipitation of UCI, three cycles of adding and removing petroleum ether were perfonned. After filtering, 800 mg (1. 72 mmol, 80% yield) of Cp*2Ta(CH2)H were isolated from petroleum ether at -78°C. One usually observes Cp*2Ta(CH2=CH2)H and Cp*2Ta(CH2)CH3 as minor impurities in this reaction. Cp*2Ta(CD2)D was successfully synthesized from UCD3 generated from CD3Br and U In ether. UCD3 generated from CD3I did not work as well. Cp*2Ta(=CHC6H5)H: 1.028 g (1 .97 mmol) of Cp*2TaCI2 and 644 mg (4.95 mmol, 2.5 eq.) of KCH2CsHs were slurried in 30 ml of ether for 16 hours. Ether was removed in vacuo, petroleum ether was added and removed three times. Petroleum ether was added and the solution filtered. Two crops totaling 734 mg (1.35 mmol, 69% yield) of Cp*2Ta(=CHCsHs)H 90 were isolated from petroleum ether at -78°C. (~HsCH2.}2 was sublimed from the solid before isolation. Cp*2Ta{=CH~Hs)H is In equilibrium in solution with the metallacycle obtained by ortho metallation of the benzyl group. Cp*2Ta(CH2.}CH3: 350 mg (0.75 mmol) ofCp*2Ta(CH2.}H and 98 mg (1.09 mmol) of Me3P=CH2 were stirred for 20 hours In 15 ml of petroleum ether. Petroleum ether and PMe3 were removed in vacuo. Petroleum ether was added, the solution was filtered and 313 mg (0.65 mmol, 87% yield) of Cp*2Ta(CH2)CH3 were isolated at -78°C. Cp*2Ta(O)H: 450 mg (0.91 mmol) of Cp*2Ta(CH2=CHCH3)H were dissolved in 15 ml of benzene. 0.036 ml (> 2eq) of H20 were added to the solution via syringe. The solution was stirred for 18 hours at room temperature until the color had faded to light yellow (the reaction can be heated with a warm water bath to drive It to completion). The benzene solution was filtered and benzene was pulled off in vacuo. Petroleum ether was added, removed, and added again. 265 mg (0.57 mmol, 62% yield) of Cp*2Ta(O)H were isolated at -78°C from petroleum ether. Cp*2Ta(O)OH: 500 mg (1.1 mmol) of Cp*2TaH3 were dissolved in petroleum ether. 0. 7 ml of 9.3 M TBHP were added with the petroleum ether solution at -78°C. The solution was stirred for 32 hours at room temperature. Cycles of pulling off and adding petroleum ether were performed six times to remove excess TBHP and t-butanol. After filtering the petroleum ether solution 365 mg (0.75 mmol, 68% yield) of Cp*2Ta(O)OH were isolated at78°C. The solid was pumped on overnight. Cp*2Ta(O)CH3: 475 mg (1.02 mmol) of Cp*2Ta(CH2)H were dissolved in 15 ml of petroleum ether. 0.250 ml of 4 M TBHP solution were added vis syringe. The solution was stirred for 10 minutes and filtered. Two crops, 354 mg (0.73 mmol, 72% yield), of Cp*2Ta(O)CH3 were isolated from petroleum ether at -78°C. 91 (A) • Cp*2Ta(CH2=CH2)0H (?): 250 mg (0.52 mmol) of Cp*2Ta(CH2=CH2}H were dissolved in 10 ml of petroleum ether. 0.3 ml of 9.3 M TBHP solution were added and the solution was stirred for 25 minutes at room temperature. Petroleum ether was repeatedly pulled off in vacuo, added and pulled off until a solid instead of an oil remained in the flask. 135 mg of solid, containing Cp*2Ta(O}CH2CH3, Cp*2Ta(CH2=CH2}0H and TBHP was isolated from petroleum ether at -78°C. Cp*2Ta(O)CI: 1.09 g (1.8 mmol) of [Cp*2TaCI2tBF4- and 252 mg (2.5 eq) of KOH were slurried together in 15 ml of CH2CI2 for 14 hours. The CH2CI2 was removed in vacuo and toluene was added. After filtering, 363 mg (0.73 mmol, 40% yield} of Cp*2Ta(O)CI were isolated from toluene at -78°C. Cp*2Ta(02)CI: 500 mg (0.99 mmol) of Cp*2TaCI2 were suspended in 15 ml of nitrogen purged toluene. 0.6 ml of 90% H202 were added against an argon flow. The solution Immediately began to change color from dark green to orange. The orange color was located in the H20VH20 phase. The solution became colorless after 4 hours of stirring. 0.200 ml of NEil were added at the end of the reaction and the reaction mixture was stirred for an additional 20 minutes. The toluene layer was separated and washed with H20. The toluene was removed in vacuo and petroleum ether was used to isolate 186mg (0.36 mmol, 36% yield} of Cp*2Ta(02)CI. Cp*2Ta(02)CH3: 474 mg (1 .02 mmol} of Cp*2Ta(CH2}H were dissolved in 30 ml petroleum ether and the solution was cooled to -78°C. 1 atmosphere of dried~ was placed over the solution. The cold bath was removed and with vigorous stirring the solution was allowed to come to room temperature. Petroleum ether was removed and toluene added. The solution was filtered and concentrated. Petroleum ether was added to the toluene solution to precipitate the solid. Two crops totaling 206 mg (0.41 mmol, 40% yield) of 92 Cp*2Ta(QVCH3 were isolated from toluene/petroleum ether at -78°C. The solid contained approximately 10% Cp*2Ta(O)CH3. Cp*2Ta(02)CH2CH3: 460 mg (0.96 mmol) of Cp*2Ta(CH2=CH2)H were dissolved in 25 ml of petroleum ether and the solution was cooled to -78°C. 1 atmosphere of~ was placed over the solution and the solution was allowed to warm to room temperature. The solution was stirred for an additional 25 minutes after reaching room temperature, filtered and 243 mg (0.47 mmol, 49% yield) were isolated at -78°C from petroleum ether. Cp*2Ta(02)CH2CH2CH3: 455 mg (0.92 mmol) of Cp*2Ta(CH2=CHCHa)H were reacted with ~ in similar manner to the above complexes. A benzene/petroleum ether solution was filtered and concentrated to almost an oil. 5 ml of petroleum ether were added and the solid, 290 mg (0.55 mmol, 60% yield) of Cp*2Ta(QVCH2CH2CH3, was isolated at -78°C. Cp*2Ta(02)CH2CsHs: 1 atmosphere of~ was placed over a solution of 350 mg (0.65 mmol) of Cp*2Ta(CHCsHs)H in petroleum ether/toluene at -78°C. After warming to room temperature the solvent was pulled off in vacuo and petroleum ether was added. After filtering, 278 mg (0.48 mmol, 75% yield) of Cp*2Ta(02)CH2CsHs were isolated at -78°C. Lewis Acid Studies: The survey reactions with AIRa were carried out in nmr tubes. AIR3 reagents were generally added as toluene solutions via syringe. The Lewis acid free rearrangement products could be generated by precipitating the aluminum reagents with H20. Crystal Structure Determination of Cp*2Ta(O)H: A single crystal was sealed in a thinwalled glass capillary. The crystal was optically centered on an Enraf Nonius CAD4 diffractometer equipped with MoK a radiation for intensity data collection. Space group P2 1/n, a special setting of #14, was chosen based on the systematic absences of hOI where h + I= 2n + 1, and OkO where k = 2n + 1 in the intensity data. A total of 11187 93 reflections were collected with 1 ~a~ 15 In the quadrant (±h, +k, +f). Crystal data are shown In Table II. Crystal decay was monitored by 3 check reflections which were monitored after every 10000 seconds of exposure. The check reflection Intensities decreased at an average rate of 3.0% over the 79.92 hours of exposure. The Intensities were scaled accordingly. The data were also corrected for absorption with transmission factors varied from 0.061 to 0.286. The position of the tantalum atoms were determined from a Patterson map. From Fourier and Difference Fourier maps, all the remaining atoms were located. The tantalum, oxygen and methyl carbon atoms were allowed to refine with Gaussian parameters. All other atoms were refined with isotropic B's. Several cycles of full-matrix least-squares refinement led to convergence with R = 0.0672 for 6730 reflections and R = 0.042 with F0 > 3oF0 2. The goodness-of-frt =[L..>(F 2-Fc2)2/(n- p)J112 is 1.80 for n =6730 reflections and p =299 0 parameters. Final parameters are contained in Table Ill. Table rv contains the hydrogen coordinates and Table V lists bond lengths. 94 TABLE II. CRYSTAl DATA =13.073(3) A Crystal System : Monoclinic b = 19.337(4) A Space Group : P21/n (#14) c =16.002(3) A F(OOO) :464e p =108.68(2) 0 Radiation : >. Mol<a =0.71073 A Temperature :room a V =3832.35(1 .46) cm3 JJ = 60.41 em -1 =468.42 g/mol Density (calculated) =1.62 g/cm3 Molecular Weight 95 TABLE Ill. ATOM COORDINATES AND Ueq's OR B's Atom Coordinates (X104); Ueq {X104) ~ Atom % Sl z TAl TA2 11C 12C 13C 14C 15C 21C 22C 23C 24C 25C 31C 32C 33C 34C 35C 41C 42C 43C 44C 45C 01 02 C11 C12 C13 C14 C15 C21 C22 C23 C24 C25 C31 C32 C33 C34 C35 C41 C42 C43 C44 C45 2133 .3~ 2508 .3 4467 4695 2670 1201 10) 2360 2275 3341 10~ 1604 10 -416 7 -12 7 3891 8 2232 8 2562 8 4514 7 5319 7 2259 8 1108 7 -109 7 88 8 1594 8 2602 12) 2468 8 3622 7 3748 7 2873 7 2209 7 2688 6 1829 6 2285 7 1491 7 632 7 818 6 3692 6 2935 6 3133 6 4007 6 4350 6 1650 6 1186 6 624 6 687 7 1325 7 1278 .2~ 1392 .2 11095 729 6 -224 5 -375 6 343 5 2106 5 2864 6 2865 5 2031 6 1474 6 1068 5 -121 5 -328 6 637 6 1450 5 2801 5 1435 5 754 6 1798 6 3036 5 1497 6 1391 6 734 4 530 5 117 5 45 5 414 4 2099 4 2456 5 2490 5 2104 5 1851 4 820 4 323 4 197 4 607 4 1005 4 2288 4 1685 4 1378 4 1868 5 2410 5 6183 .2~ 1384 .2 8123 6 6266 7 5242 6 6429 6 8270 5 8323 5 7065 7 5130 6 5179 6 7116 6 3694 5 2672 6 819 6 739 6 2539 6 2745 6 3088 5 1206 5 -272 5 658 6 5300 6 290 5 7404 5 6567 5 6119 5 6657 5 7448 5 7326 5 6764 5 5903 5 5915 5 6809 5 2778 5 2310 5 1498 5 1485 5 2244 5 2057 5 2208 5 1379 5 717 5 1129 5 il :~ Ue 9 orB ~~ 491 400 859 32 1054 36 965 36 1149 39 857 32 836 33 1201 42 1048 38 952 36 963 33 761 28 782 29 963 36 892 30 916 32 823 31 744 26 823 30 981 37 871 31 2640 63 1778 46 4.8 2 5.0 2 5.1 2 4.9 2 4.4 2 4.6 2 5.2 2 5.0 2 5.0 2 4.3 2 3.9 2 4.0 2 4.3 2 4.1 2 4.4 2 4.1 2 4.1 2 4.4 2 4.8 2 4.7 2 96 TABLE IV. HYDROGEN COORDINATES AND B'a Atom Coordinates (X1 04) Atom H1 H2 HUa HUb HUe H12a H12b H12c H13a Hl3b H13c H14a H14b H14c H15a H15b H15c H21a H21b H21c H22a H22b H22c H23a H23b H23c H24a H24b H24c H25a H25b H25c H31a H31b H31c H32a H32b H32c H33a H33b H33c H34a H34b H34c H35a H35b H35c H41a % ~ z B 1253 3339 5100 4U9 4731 997 2052 776 1247 1537 1221 375 813 -277 88 -698 -887 -286 -223 -173 615 575 2013 1771 2623 2848 2614 3338 3280 2547 3088 2181 2341 1515 1249 1113 1853 981 808 1613 -504 -349 200 -743 -85 -480 777 1006 138 1167 1878 1578 2508 5250 1315 8394 8590 7893 6598 6497 5634 5U8 4762 5254 6522 5763 6807 8412 8758 8122 8486 8551 8523 7735 6746 6809 5336 4735 4773 4581 5276 5U5 6579 7583 7396 4036 3947 3648 3091 2177 3068 843 196 960 1043 298 495 2971 2828 1969 3338 2.0 2.0 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.7 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.9 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.4 7.2 5009 5289 4468 3365 2103 2302 1404 743 740 2338 2852 1552 3083 1866 2175 3686 3893 3221 2137 1865 839 -289 -973 -649 -556 397 -407 4712 3398 3737 2725 1644 1908 3074 2344 1887 5341 4121 4488 5942 5136 5527 2677 97 TABLE IV. (Cont.) Atom % Sl % B H41b H41c H42a H42b H42c H43a H43b H43c H44a H44b H44c H45a H45b H45c 2755 1687 1714 1150 364 140 -30 -870 318 300 -732 2073 1915 892 3069 3112 1631 891 1587 419 516 909 2201 1335 1814 2911 3436 3211 2520 2905 3577 3099 3135 790 1775 860 -605 -480 -371 353 1165 200 7.2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 7.2 98 TABLE V. BOND LENGTHS Distance(!) TAI-Cll TA1-C12 TA1-C13 TA1-C14 TAl-CIS TA1-C21 TA1-C22 TA1-C23 TA1-C24 TA1-C2S TAI-01 TA1-H1 TA1-R1 TA1-R2 Cll -C12 Cll -CIS en -uc C12 -C13 C12 -12C C13 -C14 C13 -13C C14 -C1S C14 -14C C15 -1SC C21 -C22 C21 -C2S C21 - 21C C22 -C23 C22 - 22C C23 -C24 C23 -23C C24 - C2S C24 -24C C2S - 2SC Distance( A) 2.S06 8 2.469 9 2.4S8 9 2.493 9 2.S4S 8 2.S47 8 2.445 9 2.482 9 2.460 9 2.S08 8 1.762 12) 1.6S7 2.186 2.180 1.456 12 1.390 12 1.502 13 1.390 12 l.S16 14 1.411 12 1.497 14 1.412 12 1.492 14 1.S1S 13 1.410 12 1.401 12 1.513 12 1.437 13 l.S26 1S 1.3S3 12 1.482 14 1.456 12 1.501 13 l.S1S 13 gj TA2 -C31 TA2 -C32 TA2 -C33 TA2 -C34 TA2 -C3S TA2 -C41 TA2 -C42 TA2 -C43 TA2 -C44 TA2 -C4S TA2-02 TA2-H2 TA2-R3 TA2-R4 C31 -C32 C31 -C3S C31 -31C C32 -C33 C32 -32C C33 -C34 C33 -33C C34 -C35 C34 -34C C35 -35C C41 -C42 C41 -C4S C41 -41C C42 -C43 C42 -42C C43 -C44 C43 -43C C44 -C4S C44 -44C C4S -45C 2.S27 8 2.SOO 8 2.439 8 2.442 8 2.476 8 2.488 8 2.SS2 8 2.461 8 2.4S6 9 2.4S6 9 1.73S 10) 1.701 2.167 2.176 1.410 11 1.438 11 1.483 12 1.42S 11 l.SOS 12 1.396 11 l.SOO 13 1.386 11 1.S40 13 1.478 13 1.371 11 1.428 12 l.S07 13 1.42S 11 1.S22 12 1.444 12 l.Sll 12 1.370 12 l.S31 13 l.S24 13 gj 99 1) a) Gibson, V.C.; Bercaw, J.E.; Bruton, W.J.; Sanner, R.D. Organometallics 1986, 5, 976. b) van Asselt, A.; Burger, B.J.; Gibson, V.C.; Bercaw, J.E. J. Am. Chem. Soc. 1986, 108, 5347. 2) Pari<in, G.; Bunel, E.; Burger, B.J.; Trimmer, M.S.; van Asselt, A.; Bercaw, J.E. J. Mol. Cat. 1987, in press. 3) Parshall, G.W. •Homogeneous Catalysis• John Wiley & Sons, New Yori<, 1980. 4) Grubbs, R.H. in "Comprehensive Organometallic Chemistry" Vol. 8, p. 499 (ed. G. Wilkinson, F.GA Stone, E.W. Abel) Pergamon Press, Oxford, 1982. 5) a) Richmond, T.G.; Basolo, F.; Shriver, D.F. lnorg. Cham. 1982, 21, 1272 b) Butts, S.S.; Strauss, S.H.; Holt, E.H.; Stimson, R.E.; Alcock, M.W.; Shriver, D.F. J. Am. Chem. Soc. 1980, 102, 5093. 6) Madix,R.J. Science 1986, 1159. 7) a) Tovrog, B.S.; Diamond, S.E.; Mares, F.; Szalkiewicz, A. J. Am. Chem. Soc. 1981, 103, 3522. b) Tovrog, B.S.; Diamond, S.E.; Mares, F. ibid. 1980, 102,270. c) Tovrog, B.S.; Mares, F.; Diamond, S.E. ibid. 1980, 102, 6616. d) Diamond, S.E.; Mares, F.; Szalkiewicz, A., Muccigrosso, DA: Solar, J.P. ibid. 1982, 104, 4266. 8) Olefins also react slowly at 140°C with Cp*2TaH3 to give the olefin hydride complexes. Ref. 1a. 9) Hillhouse, G.L; Bercaw, J.E. J. Am. Chem. Soc. 1984, 106, 5472. 1O) LeRoy L Whinnery, Nhi G. Hua, Richard E. Marsh, Caltech. 11) Yaun, LC.; Bruice, T.C. J. Am. Chem. Soc. 1986, 108, 1643 and references therein. 12) Pari<in, G.; Bercaw, J.E. unpublished results. 13) Chapter 1. 14) Collman, J.P.; Hegedus, LS. •Principles and Applications of Organotransitio!" Metal Chemistry" University Science Books, Mill Valley, 1980. 15) Jensen, C.M.; Trogler, W.C. J. Am. Chem. Soc. 1986, 108, 723. 100 16) Cp*2Ta(CH2CH2)H and HOCMe3 can be heated to 80°C with no reaction. Cp*2Ta(CH2)H activates Cp* ring methyl groups to give tuck-in complexes before reacting with HOMe3. 17) Because (A) and (B) have not been isolated cleanly the rate comparisons are very approximate. 18) Trimmer, M.S.; Bercaw, J.E. unpublished results. 19) Parkin, G.; Bercaw, J.E. unpublished results. 20) McDade, C; Gibson, V.C.; Santarsiero, B.D.; Bercaw, J .E. J. Am. Chem. Soc. accepted for publication. 21) a) Sala-Pala, J.; Roue, J.; Guerchais, J.E. J. Mol. Cat. 1980, 7, 141 . b) BkoucheWaksman, 1.; Bois, C.; Sala-Pala, J.; Guerchais, J.E. J. Organomet. Chem. 1980, 195,307. 22) a) Mimoun, H.; Seree de Roch, 1.; Sajus, L Tetrahedron 1970, 26, 37. b) See Chapter 1. 23) Cp*2TaCI is probably a paramagnetic complex. Parkin, G. unpublished results. For a discussion of bent metallocene frontier orbitals see Lauher, J; Hoffmann, R. J. Am. Chem. Soc. 1976,98, 1729. 24) Arzoumanian, H.; Alvarez, R.L; Kowalak, A.D.; Metzger, J. J. Am. Chem. Soc. 1977, 99, 5175. 25) Phenol contains H3P0 4 as stabilizer and this may be the actual catalyst. 26) This compound is probably formed by the reaction of H20 with Cp*2Ta(02)CH3 to give H202 and Cp*2Ta(O)CH3. It was mentioned earlier that the reverse reaction does not go. 27) The analysis of this reaction by nmr was complicated by the fact that the Cp~ resonances of Cp*2Ta(02)CH3 and Cp*2Ta(O)CH3 · AIMe3 are degenerate at 90MHz. 101 28) Bruno, J.W.; Marks, T.J.; Morss, LR. J. Am. Chem. Soc. 1983, 105, 6824. 29) a) Bryndza, H.E.; Fong, LK.; Paciello, R.A.; Tam, W.; Bercaw, J.E. J. Am. Chem. Soc. 1987, 109, 1444. b) For relative R-H bonds strengths see Lowry, T.H.; Richardson, K.S. •Mechanism and Theory in Organic Chemistry• Harper & Row, New York, 1981. 30) Curci, R.; Edwards, J.O. in •organic Peroxides• Vol. 1, 199, (Ed. Swern, D.) Wileylnterscience, New York, 1970.