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 dioxoaqua complex, [Cp*MoO 2(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*MoO 2(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. © 2007 American Chemical Society.
|Publication status||Published - 14 May 2007|