Oxidation of Hypophosphorous Acid by a Ruthenium(VI) Nitrido Complex in Aqueous Acidic Solution. Evidence for a Proton-Coupled N-Atom Transfer Mechanism.

The oxidation of hypophosphorous acid (HPO) by a ruthenium(VI) nitrido complex, [(L)Ru(N)(OH)] (; L = ,'-bis(salicylidene)--cyclohexyldiamine dianion), has been studied in aqueous acidic solutions at pH 0-2.50. The reaction has the following stoichiometry: 2[(L)Ru(N)(OH)] + 3HPO + HO → 2[(L)Ru(NHP(OH))(OH)] + HPO. The pseudo-first-order rate constant, , depends linearly on [HPO], and the second-order rate constant depends on [H] according to the relationship = [H]/([H] + ), where is the rate constant for the oxidation of HPO molecule and is the dissociation constant of HPO. At 298.0 K and = 1.0 M, = (2.04 ± 0.19) × 10 M s and = (6.38 ± 0.63) × 10 M. A kinetic isotope effect (KIE) of 2.9 ± 0.1 was obtained when kinetic studies were carried out with DPO at pH 1.16, suggesting P-H bond cleavage in the rate-determining step. On the other hand, when the kinetics were determined in DO, an inverse KIE of 0.21 ± 0.03 (HPO in HO vs HPO in DO) was found. On the basis of experimental results and DFT calculations, the proposed mechanism involves an acid-catalyzed tautomerization of HP(O)(OH) to HP(OH); the latter molecule is the reacting species which reacts with via a proton-coupled N-atom transfer pathway.