Stable paramagnetic half-sandwich Mo(V) and W(V) polyhydride complexes. Structural, spectroscopic, electrochemical, theoretical, and decomposition mechanism studies of [Cp*MH{sub 3}(dppe)]{sup +} (M = Mo, W)

Journal Article · Wed Mar 17 04:00:00 EST 1999 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja9834881· OSTI ID:20013121

Pleune, B; Morales, D; Meunier-Prest, R; Richard, P; Collange, E; Fettinger, J C; Poli, R

Compounds Cp*MH{sub 3}(dppe) (M = Mo, 1; W, 2) are oxidized chemically and electrochemically to the corresponding 17-electron cations 1{sup +} and 2{sup +}. Analogous oxidations of 1-d{sub 3} and 2-d{sub 3} provide 1{sup +}-d{sub 3} and 2{sup +}-d{sub 3}, respectively. Complex 2{sup +} is stable in CH{sub 2}Cl{sub 2}, THF and MeCN at room temperature. A single-crystal X-ray analysis of the PF{sub 6}{sup {minus}} salt of 2{sup +} shows a geometry optimization of the [CpWH{sub 3}(PH{sub 2}CH{sub 2}CH{sub 2}PH{sub 2})]{sup +} model at the B3LYP/LANL2DZ level. Identical calculations on the neutral analogue also reproduce the previously reported trigonal prismatic structure for 1. A blue shift in the M-H stretching vibrations upon oxidation for both Mo and W compounds indicates that a M-H bond strengthening accompanies the oxidation process. The DFT calculations (M-H bond lengths, BDE, and stretching frequencies) are in good agreement with the experimental results. Complex 1{sup +} decomposes in solution at room temperature by one or more of three different mechanisms depending on conditions: H{sub 2} reductive elimination, solvent-assisted disproportionation, or deprotonation. In THF or CH{sub 2}Cl{sub 2}, a reductive elimination of H{sub 2} affords the stable paramagnetic monohydride Cp*MoH(dppe)PF{sub 6} (3), which adds a molecule of solvent in CH{sub 2}Cl{sub 2}, THF, and MeCN. EPR studies show that the CH{sub 2}Cl{sub 2} molecule coordinates in a bidentate model to afford a 19-electron configuration. A solvent dependence of the decomposition rate [k(CH{sub 2}Cl{sub 2}) {approx} 7.8k(THF) at 0 C] and an inverse isotope effect [k{sub H}/k{sub d} = 0.50(3) in CH{sub 2}Cl{sub 2} at 0 C] indicate the nature of 1{sup +} as a classical trihydride and suggest a decomposition mechanism which involves equilibrium conversion to a nonclassical intermediate followed by a rate-determining associate exchange of H{sub 2} with a solvent molecule. In MeCN at 20 C, a solvent-assisted disproportionation (rate = k{sub disp}[1{sup +}]{sup 2}, k{sub disp} = 3.98(9) x 10{sup 3} s{sup {minus}} M{sup {minus}}) and a deprotonation by residual unoxidized 1 (rate = k{sub deprot}[1{sup +}][1], k{sub deprot} = 2.8(2) x 10{sup 2} s{sup {minus}} M{sup {minus}}) take place competitively, as shown by detailed cyclic voltammetric and thin-layer cyclic voltammetric studies. The stoichiometric chemical oxidation of 1 in MeCN leads to a mixture of [Cp*MoH{sub 2}(dppe)(MeCN)]{sup +} and [Cp*MoH(dppe)(MeCN){sub 2}]{sup 2+} by the disproportionation mechanism.

• Research Organization: Univ. de Bourgogne, Dijon (FR)
• Sponsoring Organization: US Department of Energy
• DOE Contract Number: FG05-92ER14230
• OSTI ID: 20013121
• Journal Information: Journal of the American Chemical Society, Journal Name: Journal of the American Chemical Society Journal Issue: 10 Vol. 121; ISSN JACSAT; ISSN 0002-7863
• Country of Publication: United States
• Language: English

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