Nature of Cp*MoO2+ in water and intramolecular proton-transfer mechanism by stopped-flow kinetics and density functional theory calculations

摘要

A stopped-flow study of the Cp*MoO3- protonation at low pH (down to zero) in a mixed H2O-MeOH (80:20) solvent at 25 degrees C allows the simultaneous determination of the first acid dissociation constant of the oxo-dihydroxo complex, [Cp*MoO(OH)(2)](+) (pK(a1) = -0.56), and the rate constant of its isomerization to the more stable dioxo-aqua complex, [Cp*MoO2(H2O)](+) (k(-2) = 28 s(-1)). Variable-temperature (5-25 degrees C) and variable-pressure (10-130 MPa) kinetics studies have yielded the activation parameters for the combined protonation/isomerization process (k(-2)/K-a1) from Cp*MoO2(OH) to [Cp*MoO2(H2O)](+), viz., Delta H = 5.1 +/- 0.1 kcal mol(-1), Delta S = -37 +/- 1 cal mol(-1) K-1, and Delta V = -9.1 +/- 0.2 cm(3) mol(-1). Computational analysis of the two isomers, as well as the [Cp*MoO2](+) complex resulting from the dissociation of water, reveals a crucial solvent effect on both the isomerization and the water dissociation energetics. Introducing a solvent model by the conductor-like polarizable continuum model and especially by explicitly inclusion of up to three water molecules in the calculations led to the stabilization of the dioxo-aqua species relative to the oxo-dihydroxo isomer and to the substantial decrease of the energy cost for the water dissociation process. The presence of a water dissociation equilibrium is invoked to account for the unusually low effective acidity (pK(a1)' = 4.19) of the [Cp*MoO2(H2O)](+) ion. In addition, the computational study reveals the positive role of external water molecules as simultaneous proton donors and acceptors, having the effect of dramatically lowering the isomerization energy barrier.