Oxidation of hypophosphite and phenylphosphinite ion by perosmate(VIII) ion: Kinetics and mechanism

The oxidation of RHPO2− (hypophosphite, R = H, and phenylphosphinite, R = C6H5), studied under pseudo-first-order conditions (RHPO2− ≫ OsO4), is first-order in OsO4 and the rate dependence on [RHPO2−] is consistent with the Michaelis–Menten kind of kinetics indicating the formation of an intermediate, and the pseudo-first-order rate constant kobs is independent of [OH−]. These kinetics results are explicable by the following mechanism:equation(i)OsO4+OH-⇌K1[OsO4OH]-equation(ii)[OsO4OH]-+OH-⇌K2[OsO4(OH)2]2-equation(iii)[OsO4(OH)]-+RHPO2-⇌β[OsO4(OH)RHPO2]2-equation(iv)[OsO4(OH)RHPO2]2-→kOsO3+RHPO32-+H2OThe proposed mechanism is identical with the mechanism considered in the OsO4-catalysed oxidation with Fe(CN)63− ion except that the rate component dependent on OH− ion is not observed probably because the reaction is slow and, therefore, it is lost on the stopped-flow time scale. The mechanism proposed in the OsO4-catalysed oxidation with Fe(CN)63− ion is suitably modified to rationalise the assumption about the inequality. The values of k (0.153 and 0.0893 s−1 for H2PO2− and C6H5HPO2−, respectively, at 30 °C), and β (28 and 56 dm3 mol−1, respectively, for H2PO2− and C6H5HPO2− at 30 °C) are comparable to those obtained in the OsO4-catalysed oxidation with Fe(CN)63− ion. Reaction with D2PO2− is considerably slower, kH/kD = 4.53, indicating that breaking of P–H bond is the rate determining step. There is a proton transfer from P–H bond to Os–OH bond followed by concerted electron transfers from Os–OH bond to Os and from Os to P–O bond bridging the complex.

Graphical abstractThe oxidation of hypophosphite and phenyl hypophosphinite by OsO4 is represented by the following steps.Figure optionsDownload full-size imageDownload as PowerPoint slide