Stable Paramagnetic Half-Sandwich Mo(V) and W(V) Polyhydride Complexes. Structural, Spectroscopic, Electrochemical, Theoretical, and Decomposition Mechanism Studies of [Cp*MH3(dppe)]+ (M = Mo, W)

Abstract

Compounds Cp*MH3(dppe) (M = Mo, 1; W, 2) are oxidized chemically and electrochemically to the corresponding 17-electron cations 1+ and 2+. Analogous oxidations of 1-d3 and 2-d3 provide 1+-d3 and 2+-d3, respectively. Complex 2+ is stable in CH2Cl2, THF, and MeCN at room temperature. A single-crystal X-ray analysis of the PF6- salt of 2+ shows a geometry for the cation which is intermediate between octahedral and trigonal prismatic, which is reproduced by geometry optimization of the [CpWH3(PH2CH2CH2PH2)]+ 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+ decomposes in solution at room temperature by one or more of three different mechanisms depending on conditions: H2 reductive elimination, solvent-assisted disproportionation, or deprotonation. In THF or CH2Cl2, a reductive elimination of H2 affords the stable paramagnetic monohydride Cp*MoH(dppe)PF6 (3), which adds a molecule of solvent in CH2Cl2, THF, and MeCN. EPR studies show that the CH2Cl2 molecule coordinates in a bidentate mode to afford a 19-electron configuration. A solvent dependence of the decomposition rate [k(CH2Cl2) ≈ 7.8k(THF) at 0 °C] and an inverse isotope effect [kH/kD = 0.50(3) in CH2Cl2 at 0 °C] indicate the nature of 1+ as a classical trihydride and suggest a decomposition mechanism which involves equilibrium conversion to a nonclassical intermediate followed by a rate-determining associative exchange of H2 with a solvent molecule. In MeCN at 20 °C, a solvent-assisted disproportionation (rate = kdisp[1+]2, kdisp = 3.98(9) × 103 s-1 M-1) and a deprotonation by residual unoxidized 1 (rate = kdeprot[1+][1], kdeprot = 2.8(2) × 102 s-1 M-1) 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*MoH2(dppe)(MeCN)]+ and [Cp*MoH(dppe)(MeCN)2]2+ by the disproportionation mechanism.