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 (H₃PO₂) by a ruthenium(VI) nitrido complex, [(L)Ru^VI(N)(OH₂)]⁺ (Ru^VIN; L = N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion), has been studied in aqueous acidic solutions at pH 0-2.50. The reaction has the following stoichiometry: 2[(L)Ru^VI(N)(OH₂)]⁺ + 3H₃PO₂ + H₂O → 2[(L)Ru^III(NH₂P(OH)₂)(OH₂)]⁺ + H₃PO₃. The pseudo-first-order rate constant, k_obs, depends linearly on [H₃PO₂], and the second-order rate constant k₂ depends on [H⁺] according to the relationship k₂ = k[H⁺]/([H⁺] + K_a), where k is the rate constant for the oxidation of H₃PO₂ molecule and K_a is the dissociation constant of H₃PO₂. At 298.0 K and I = 1.0 M, k = (2.04 ± 0.19) × 10^-2 M^-1 s^-1 and K_a = (6.38 ± 0.63) × 10^-2 M. A kinetic isotope effect (KIE) of 2.9 ± 0.1 was obtained when kinetic studies were carried out with D₃PO₂ at pH 1.16, suggesting P-H bond cleavage in the rate-determining step. On the other hand, when the kinetics were determined in D₂O, an inverse KIE of 0.21 ± 0.03 (H₃PO₂ in H₂O vs H₃PO₂ in D₂O) was found. On the basis of experimental results and DFT calculations, the proposed mechanism involves an acid-catalyzed tautomerization of H₂P(O)(OH) to HP(OH)₂; the latter molecule is the reacting species which reacts with Ru^VIN via a proton-coupled N-atom transfer pathway.