Kinetics and mechanism of the Os(VIII)-catalysed oxidation of hypophosphite and phenylphosphinite ion by alkaline hexacyanoferrate(III) ion

摘要

The stoichiometry Of the OsO4-catalysed oxidation of phosphinite (R = H) and phenylphosphinite ion (R = C6H5) by alkaline hexacyanoferrate(III) is expressed by the equation: 2Fe(CN)(6)(3-) + RHPO2 - 2OH ->(OSO4) 2Fe(CN)(6)(4-) + RHPO3- - H2O The proposed mechanism is based oil the kinetics which has a zero order dependence in Fe(CN)(6)(3), first order in OsO4 and one component of the rate is independent of [OH-]. The Michaelis-Menten kinetics with respect to RHPO2- is supportive of the complex OsO4(OH)RHPO22-, formed from RHPO2- and OsO4(OH)(-) ions, which decomposes with the rate constant k in the rate determining step (Mean: Delta H-k(+) = 19 +/- 6, Delta G(k)(dagger) = 73 +/- 2 and Delta S-k = 182 +/- 18 for H2PO2-, and Delta H-k =18 perpendicular to 3 Delta G(k)(+) =74 +/- 3 kJ mol(-1) and -Delta S-k(+) = 188 +/- 11 JK(-1) mol(-1) for C6H5HPO2-). The formation constant beta for the complex is independent of OH- (Delta H = 29 +/- 1, Delta G = 8 +/- 1 and Delta S = 122 +/- 5 for H2PO2, and Delta H = 8 +/- 1, Delta G = 10 +/- 1 kJ mol(-1) and Delta S = 62 +/- 3 JK(-1) mol(-1) for C6H5HPO2-). However, the formation of the complex [OsO4(OH)(2)RHPO2](3-) is considered unlikely in view of the relatively strong repulsive forces between OsO4(OH)(2)(2-) and RHPO2 ions which react by bimolecular collision with rate constant k(2) (Mean:Delta H-k2K2 = 33 +/- 8, Delta G(k2K2) = 70 +/- 9 and -Delta S-k2K2 = 121 +/- 23 for H2PO2-, and Delta H-k2K2(I) = 30 +/- 4, Delta G(k2K2) = 71 +/- 6 kJ mol(-1) and -Delta S-k2K2(:1) = 140 +/- 14 J K-1 mol(-1) for C6H5HPO2-). The inactive tautomeric form RHPO2- is the considered reactive entity. (C) 2008 Elsevier Ltd. All rights reserved.