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

Citation

Mayer, James Moers (1983) Synthesis, Characterization, and Reactivity of New Hydride Compounds of Tantalum (V). Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:03212012-094858041

Abstract

A series of (pentamethylcyclopentadienyl)tantalum bis(phosphine) polyhydride complexes, Cp*TaL_2H_4 (L = PMe_3, PMe_2(C_6H_5), P(OMe)_3, and L_2 dmpe; Cp* = η^5-C_5(CH_3)_5) and Cp*Ta(PMe_3)_2H_3Cl, have been prepared by high pressure hydrogenation of Cp*TaMe_4 or Cp*TaMe_3Cl 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 H_2.

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 C_s structure with equivalent phosphorus atoms for complexes with monodentate phosphine ligands. This is consistent with an X-ray crystal structure of Cp*Ta(PMe_3)_2H_4, in which the hydride ligands were not located. A different C_s structure, with inequivalent phosphorus atoms, is indicated for compounds with the bidentate dmpe ligand.

The reactions of Cp*TaMe_3Cl (1) with a variety of alkali metal alkoxide, alkylamide, and alkyl reagents have been examined. Reaction with LiNMe_2 produces Cp*Ta(NMe_2)Me_3, but this decomposes at 25°C to an imine (or metallaazirane) complex, Cp*Ta(CH_2NMe)Me_2. The decomposition is a first-order, unimolecular process with a large kinetic isotope effect k_H/k_D= 9.7). Monoalkylamides (LiNHR) react with 1 to form imido complexes Cp*Ta(NR)Me_2. Reaction of 1 with lithium diisopropylamide forms a bridging methylene complex, Cp*Me_2Ta(µCH_2)(µH)_2TaMe_2Cp*. The alkoxide compounds Cp*Ta(OR)Me_3 (R = Me, CHMe_2, CMe_3) 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)Me_2) in the presence of phosphine ligands yield the first examples of imido-hydride complexes, Cp*Ta(NR)H_2(L) (L = PMe_3, PMe_2(C_6H_5), R = CMe_3, CH_2CMe_3). An alkyl-hydride complex, Cp*Ta(CH_2NMe)Me(PMe_3)H, has also been prepared. The reaction of Cp*TaMe_3(OCMe_3) with hydrogen forms an unusual asymmetric dimer, Cp*(Me_3CO)_2HTa(µH)_2 TaH_3Cp*, which has been characterized by NMR and IR spectroscopies.

The tantalum hetero-olefin complexes Cp*Ta(CH_2NMe)Me_2 and Cp*Ta(OCMe_2)Me_2 react readily with olefins, aldehydes, and nitriles. A number of five- and seven-atom metallacycles have been prepared.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry, tantalum, hydride, polyhydride, organometallic, insertion, amide, imido, imine, phosphine
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:
  • Gray, Harry B.
  • Grubbs, Robert H.
  • Weinberg, William Henry
Defense Date:6 July 1982
Record Number:CaltechTHESIS:03212012-094858041
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:03212012-094858041
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6860
Collection:CaltechTHESIS
Deposited By: John Wade
Deposited On:21 Mar 2012 22:08
Last Modified:26 Dec 2012 04:40

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