Synthesis, characterisation, and electrochemical studies of a dioxo-bridged molybdenum(V) glycinate. Generation of an oxo-bridged MoV–MoVI species

Abstract

The binuclear compound [Mo₂O₄(glyO)₂(H₂O)₂](glyO = NH₂CH₂COO^–), has been prepared and characterized. The aqueous solution of the compound is stable for at least 24 h when the pH of the solution is maintained above 5.5. Below pH 5.5, the compound in aqueous solution undergoes proton-assisted oxidation to a dimeric Mo^V–Mo^VIspecies giving a new electronic absorption band at 850 nm (ε 241 dm³ mol^–1 cm^–1) which is believed to be due to Mo^V–Mo^VI intervalence charge transfer. The kinetics of this oxidation have been studied in aqueous solution in the pH range 5.0–3.7. The oxidation step is preceded by the formation of a protonated species [Mo₂O₄(glyO)₂(H₂O)₂H]⁺ and is first-order dependent on hydrogen ion concentration. The second-order rate constant is 2.52 dm³ mol^–1 s^–1 at 25 °C, I= 0.1 mol dm^–3. The Mo^V–Mo^VI compound could not be isolated in the solid state. Cyclic voltammetric and magnetic studies of the compounds are described.