Nature of (C5Me5)2Mo2O5 in water-methanol at pH 0-14.: On the existence of (C5Me5)MoO2(OH) and (C5Me5)MoO2+:: a stopped-flow kinetic analysis

A stopped-flow analysis of compound Cp* 2Mo2O5 (Cp* = eta(5)-C5Me5) in 20% MeOH-H2O over the pH range 0-14 has provided the speciation of this molecule as well as the rate and mechanism of interconversion between the various species that are present in solution. The compound is a strong electrolyte in this solvent combination, producing the Cp* MoO2+ and Cp* MoO3- ions in equilibrium with a small amount of Cp* MoO2 (OH), the latter attaining ca. 15% relative amount at pH 4. At low pH ( < 2.5) Cp*MoO2+ is essentially the only species present in solution, while the anion Cp* MoO3- is the dominant species at pH > 6. The acid dissociation constant of Cp* MoO2 (OH) has been measured directly (pK = 3.65 +/- 0.02) while the pK for the protonation equilibrium leading to Cp* MoO3H2+ is estimated as < 0. The three trioxygenated species establish rapid proton transfer equilibria among themselves, but transform to the dioxo species Cp* MoO2+ by two slower and independent first-order pathways: loss of H2O from Cp* MoO3H2+ and loss of OH- from Cp* MoO2 (OH). The former pathway is prevalent at pH < 2, where the transformation proceeds to completion, giving rise to kinetics that are first-order in metal and in [H+]. The reverse process is quantitative at pH > 5. The prevalent pathway at high pH is the addition of OH- to Cp*MoO2+, giving rise to kinetics that are first-order in metal and in [OH-]. The kinetics of the equilibration process at intermediate pH are affected by the buffer concentration, indicating a general acid-base catalytic phenomenon. The complete elucidation of the kinetic and thermodynamic scheme was made possible by the combined analyses of the equilibrium in the pH 3-5 range and the kinetics in the extreme pH regions.