Organometallic complexes of tungsten and tantalum : synthesis, structure and reactivity

Abstract

W(NAr)(CH-t-Bu)(CH2-t-Bu)2 (Ar = 2,6-i-Pr2C6H3) was synthesized in four steps starting from WCI6. The reaction of W(NAr)(CH-t-Bu)(CH2-t-Bu)2 with various alcohols (1-AdamantylOH, t-BuOH, ArOH, (CF3)2CHOH, (CF3)2MeCOH, CF3Me2COH, (CF3)3COH, C6F5OH) in hydrocarbon solvents gave either W(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) or W(NAr)(CH2-t-Bu)3(OR) depending on the pKa of the alcohol used. W(NAr)(CH2-t-Bu)3(OR) appears to be formed when the alcohol has a relatively low pKa. W(NAr)(CH2-t-Bu)3(OR) evolves neopentane gas to give W(NAr)(CH-t-Bu)(CH2-t-Bu)(OR). The reaction between W(NAr)(CH2-t-Bu)3CI and LiOR similarly afforded W(NAr)(CH-t-Bu)(CH2-t-Bu)(OR). W(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) complexes were found to be active catalysts for various metathesis reactions, with the reactivity varied in the order of OAr - OC(CF3)3 > OCMe(CF3)2 > OCMe2CF3 > OCMe3. Conversion was sometimes limited by the tendency of tungsten to form stable metallacyclobutane in the presence of ethylene, and the bimolecular decomposition of intermediate alkylidenes to yield dimeric species that contain W(IV)/W(IV) double bonds.

(cont.) Chapter 2 Tungsten imido alkylidene complexes were found to undergo bimolecular reaction to give dimers that contain W(IV)/W(IV) double bonds and no bridging ligands. A modest number of W=W species of two general types were obtained and structurally characterized. One type is [W(NR)(CH2-t-Bu)(OR')]2 (NR = N-2,6-Me2C6H3, N-2,6-iPr2C6H3; OR' = OC6F5, OCH(CF3)2) which was found to exist in both heterochiral and homochiral isomeric forms. The other type is [W(NR)(OR')2]2 (NR = N-2,6-Me2C6H3 or N-2,6-iPr2C6H3; OR' = OCMe2CF3, OCMe(CF3)2) which has a virtual C2h symmetry. To the best of our knowledge, these dimers are the first examples of compounds that contain unbridged W(IV)/W(IV) double bonds despite the presence of ligands that would typically be expected to bridge such bonds. These compounds are characterized by W-W distance of 2.4-2.5 A, trans orientation of imido ligands, and -90° angle between the imido ligands and the W=W bond vector. A related unbridged molybdenum dimer, {Mo(NAd)[OCMe(CF3)2]2}2 (NAd = N-l-adamantyl) (Mo=Mo = 2.422 A). was also isolated and crystallographically characterized. Remarkably, a compound with the same stoichiometry was recovered from a sealed NMR tube reaction between Mo(NAd)(CHCMe2Ph)[OCMe(CF3)2]2 and bistrifluoromethylnorbornadiene,

(cont.) and was shown in an X-ray study to be the alternative imidobridged isomer in which a single Mo-Mo bond is present (2.609 A). Conversion of unbridged M=M species to their imido-bridged isomer is yet to be observed. Chapter 3 Reaction of [W(NAr')(OCMe2CF3)2]2 (Ar' = 2,6-Me2C6H3) with carbon tetrachloride gave [W(OCMe2CF3)2CI]2(QL-NAr')2, the structure of which was determined by X-ray diffraction studies. Reaction of [W(NAr')(OCMe2CF3)2]2 with iodine gave mixture of products, two of which were identified to be W(NAr)(OCMe2CF3)31 and W(OCMe2CF3)313. Reaction of [W(NAr')(OCMe2CF3)2]2 with PMe3 gave a stable mono-adduct. Reaction of [W(NAr')(OCMe2CF3)2]2 with CH3CN gave a mixture of two products, both with empirical formula "W(NAr')(OCMe2CF3)2(CH3CN)". The minor product was found by X-ray diffraction studies to be [(CF3)Me2CO]2W(g-NAr')(g-N(Me)C=C(Me)N=)W(NAr')(OCMe2CF3)2, resulting from reductive coupling of two acetonitrile ligands. Reaction of [W(NAr')(OCMe2CF3)2]2 with MeC-CMe gave a mono-adduct, the structure of which shows MeC-CMe bridging the two metal centers in approximately perpendicular manner.

(cont.) Reaction of [W(NAr')(OCMe2CF3)]2 with CO resulted in the formation of W(NAr')2(OCMe2CF3)2 along with other yet unidentified products of the reaction [W(NAr')(OCMe2CF3)2]2 was found to slowly catalyze the ring opening metathesis polymerization of norbomene. However, no alkylidene species was observed by 'H NMR, as only a relatively small amount (<2%) of the dimer species was believed to be "activated" by the olefin. Addition of ethylene to [W(NAr')(OCMeCF3)2]2 gave what is essentially a ditungstacyclobutane complex that contains a bridging alkoxide and a "semi-bridging" ethylene. Upon heating, the ethylene adduct was transformed into a new species that contains an ethyl group as a consequence of activation of an ortho methyl group in the NAr' ligand. Chapter 4 The reaction between H2[F3N2NMe] [F3N2NMe = (3,4,5-F3C6H2NCH2CH2)2NMe] and TaCl5 in the presence of triethylamine in dichloromethane yielded [F3N2NMe]TaCI3, which was used as a precursor compound for the synthesis of various organotantalum complexes. The trimethyl species [F3NNMe]TaMe3 was obtained from the reaction between [F3N2NMe]TaCI3 and methylmagnesium chloride. Treatment of [F3N2NMe]TaCI3 with trimethylsilylmethylmagnesium chloride afforded [F3NNMe]Ta(CHSiMe3)(CH2SiMe3).

(cont.) The monoalkyl complex, [F3N2NMe]Ta(CH2-t-Bu)CI2 was obtained from the reaction between [F3N2NMe]TaCI3 and t-BuCH2MgCl. Further alkylation of [F3N2NMe]TaCI3 with methylmagnesium chloride gave [F3N2NMe]Ta(CH-t-Bu)(CH3). By extending the reaction time in the synthesis of [F3N2NMe]Ta(CH2-t-Bu)C12, [F3N2NMe]Ta(CH-t-Bu)(CH,-t-Bu) could be isolated, albeit in low yield. The reaction between [F3N2NMe]TaCl3 and ethylmagnesium chloride gave the olefin complex, [F3N2NMe]Ta(r2-CH2CH2)(CH2CH3). The alkyl/alkylidene complexes [F3N2NMe]Ta(CHSiMe3)(CH2SiMe3), [F3N2NMe]Ta(CH-t-Bu)(CH3) and [F3N2NMe]Ta(CH-t-Bu)(CH2-t-Bu) have relatively upfield alkylidene H, resonances (0 - 6 ppm) and uniformly low IJCH values. [F3N2NMe]Ta(CHSiMe3)(CH2SiMe3) and [F3N2NMe]Ta(l 2-CH2CH2)(CH2CH3) were crystallographically characterized. The niobium complexes, [F3N2NMe]NbCI3 and [F3N2NMe]NbMe3, were similarly prepared. Tantalum complexes of the triamidoamine ligand [(3,5-CI2C6H3NCH2CH2)3N]3- were synthesized. [(3,5-CI2C6H3NCH2CH2)3N]TaCI2 was obtained from the reaction between H3[(3,5-CI2C6H3NCH2CH2)3N] and TaC15 in the presence of triethylamine.

(cont.) Treatment of [(3,5-Cl2C6H3NCH2CH2)3N]TaCI2 with methylmagnesium chloride gave [(3,5-Cl2C6H3NCH2CH2)3N]TaMe2, the structure of which was determined by X-ray diffraction studies. Ethylene and acetylene complexes, [(3,5-Cl2C6H3NCH2CH2)3N]Ta(tj2-CH2CH,) and [(3,5-CI2C6H3NCH2CH2)3N]Ta(q2-HCCH), were obtained when [(3,5-CI2C6H3NCH2CH2)3N]TaCI2 was reacted with ethylmagnesium chloride and vinyl magnesium chloride, respectively.