Neutral Transition Metal Hydrides as Acids in Hydrogen Bonding and Proton Transfer: Media Polarity and Specific Solvation Effects

Structural, spectroscopic, and electronic features of weak hydrogen-bonded complexes of CpM(CO)₃H (M = Mo (1a), W (1b)) hydrides with organic bases (phosphine oxides R₃PO (R = n-C₈H₁₇, NMe₂), amines NMe₃, NEt₃, and pyridine) are determined experimentally (variable temperature IR) and computationally (DFT/M05). The intermediacy of these complexes in reversible proton transfer is shown, and the thermodynamic parameters (ΔH°, ΔS°) of each reaction step are determined in hexane. Assignment of the product ion pair structure is made with the help of the frequency calculations. The solvent effects were studied experimentally using IR spectroscopy in CH₂Cl₂, THF, and CH₃CN and computationally using conductor-like polarizable continuum model (CPCM) calculations. This complementary approach reveals the particular importance of specific solvation for the hydrogen-bond formation step. The strength of the hydrogen bond between hydrides 1 and the model bases is similar to that of the M−H···X hydrogen bond between 1 and THF (X = O) or CH₃CN (X = N) or between CH₂Cl₂ and the same bases. The latter competitive weak interactions lower the activities of both the hydrides and the bases in the proton transfer reaction. In this way, these secondary effects shift the proton transfer equilibrium and lead to the counterintuitive hampering of proton transfer upon solvent change from hexane to moderately polar CH₂Cl₂ or THF.