Molybdenum Trihydride Complexes: Computational Model of Oxidatively Induced Reductive Elimination of Dihydrogen

Recent experimental work shows that the 18-electron molybdenum complexes (1,2,4-C₅H₂tBu₃)Mo(PMe₃)₂H₃ (Cp^tBuMoH₃) and (C₅HiPr₄)Mo(PMe₃)₂H₃ (Cp^iPrMoH₃) undergo oxidatively induced reductive elimination of dihydrogen (H₂), slowly forming the 15-electron monohydride species in tetrahydrofuran and acetonitrile. The 17-electron [Cp^tBuMoH₃]⁺ derivative was stable enough to be characterized by X-ray diffraction, while [Cp^iPrMoH₃]⁺ was not. Density functional theory calculations of the H₂ elimination pathways for both complexes in the gas phase and in a continuum solvent model indicate that H₂ elimination from [Cp^iPrMoH₃]⁺ has a lower barrier than that from [Cp^tBuMoH₃]⁺. Further, a specific solvent association, which is stronger for [Cp^tBuMoH₃]⁺ than for [Cp^iPrMoH₃]⁺, contributes to the stability of the former. In agreement with the experimental observations, the calculations predict that [Cp^tBuMoH₃]⁺ would be in a quartet state at room temperature and a doublet state at 4.2 K, while [Cp^iPrMoH₃]⁺ is in a doublet state even at room temperature.