Nature of Cp*MoO 2 + in Water and Intramolecular Proton-Transfer Mechanism by Stopped-Flow Kinetics and Density Functional Theory Calculations
Résumé
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 °C allows the simultaneous determination of the first acid dissociation constant of the oxo−dihydroxo complex, [Cp*MoO(OH)2]+ (pKa1 = −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 °C) and variable-pressure (10−130 MPa) kinetics studies have yielded the activation parameters for the combined protonation/isomerization process (k-2/Ka1) from Cp*MoO2(OH) to [Cp*MoO2(H2O)]+, viz., ΔH⧧ = 5.1 ± 0.1 kcal mol-1, ΔS⧧ = −37 ± 1 cal mol-1 K-1, and ΔV⧧ = −9.1 ± 0.2 cm3 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 (pKa1‘ = 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.
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