Synthesis, Characterization, and Reactivity of New Hydride Compounds of Tantalum (V)

Abstract

A series of (pentamethylcyclopentadienyl)tantalum bis(phosphine) polyhydride complexes, Cp*TaL2H4 (L = PMe3, PMe2(C6H5), P(OMe)3, and L2 dmpe; Cp* = η5 - C5(CH3)5) and Cp*Ta(PMe3)2H3Cl, have been prepared by high pressure hydrogenation of Cp*TaMe4 or Cp*TaMe3Cl in the presence of L. The hydride ligands are more hydridic than protic in character. All of the compounds react with acetone and methanol to afford isopropoxide and methoxide complexes, respectively. Reactions with carbon monoxide yield carbonyl hydride and dicarbonyl compounds resulting from sequential reductive elimination of dihydrogen. Hydrogenation of ethylene is observed, as well as catalytic dimerization of ethylene to 1-butene. Most reactions of these eighteen-electron polyhydride complexes are thought to involve rate-determining loss of a phosphine ligand. Evidence is presented in support of coordination of acetone to tantalum prior to its reduction to isopropoxide. By contrast, methanol can react qirectly with the coordinatively saturatedtantalum hydride species to generate H2. Low temperature and high field NMR spectroscopy has been used to investigate the coordination geometries of these polyhydride complexes and some niobium analogues. Using symmetry arguments, the spectra indicate a Cs structure with equivalent phosphorus atoms for complexes with monodentate phosphine ligands. This is consistent with an X-ray crystal structure of Cp*Ta(PMe3)2H4, in which the hydride ligands were not located. A different Cs structure, with inequivalent phosphorus atoms, is indicated for compounds with the bidentate dmpe ligand. The reactions of Cp*TaMe3Cl (1) with a variety of alkali metal alkoxide, alkylamide, and alkyl reagents have been examined. Reaction with LiNMe2 produces Cp*Ta(NMe2)Me3, but this decomposes at 25°C to an imine (or metallaazirane) complex, Cp*Ta(CH2NMe)Me2. The decomposition is a first-order, unimolecular process with a large kinetic isotope effect kH/kD = 9.7). Monoalkylamides (LiNHR) react with 1 to form imido complexes Cp*Ta(NR)Me2. Reaction of 1 with lithium diisopropylamide forms a bridging methylene complex, Cp*Me2Ta(µCH2)(µH)2TaMe2Cp*. The alkoxide compounds Cp*Ta(OR)Me3 (R = Me, CHMe2, CMe3) are very stable and decompose only over 100°C. Alkyl complexes are stable only if the alkyl group does not have β-hydrogens. The rates of hydrogen abstraction or elimination processes in this system correlate with the nature of the atom bound to tantalum: for reactions involving a β-hydrogen the order is C > N > O while α-hydrogen abstraction reactions appear to vary in the reverse order, N > C. These rates seem to reflect the thermodynamic preferences in these compounds. Hydrogenation of the imido compounds (Cp*Ta(NR)Me2) in the presence of phosphine ligands yield the first examples of imido-hydride complexes, Cp*Ta(NR)H2(L) (L = PMe3, PMe2(C6H5), R = CMe3, CH2CM3). An alkyl-hydride complex, Cp*Ta(CH2NMe)Me(PMe3)H, has also been prepared. The reaction of Cp*TaMe3(OCMe3) with hydrogen forms an unusual asymmetric dimer, Cp*(Me3CO)2HTa(µH)2 TaH3Cp*, which has been characterized by NMR and IR spectroscopies. The tantalum hetero-olefin complexes Cp*Ta(CH2NMe)Me2 and Cp*Ta(OCMe2)Me2 react readily with olefins, aldehydes, and nitriles. A number of five- and seven-atom metallacycles have been prepared.