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

Abstract

A stopped-flow analysis of compound Cp*₂Mo₂O₅ (Cp* = η⁵-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.