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

Abstract

The oxidation of hypophosphorous acid (H3PO2) by a ruthenium­(VI) nitrido complex, [(L)­RuVI(N)­(OH2)]+ (Ru VI N; 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)­RuVI(N)­(OH2)]+ + 3H3PO2 + H2O → 2­[(L)­RuIII(NH2P­(OH)2)­(OH2)]+ + H3PO3. The pseudo-first-order rate constant, k obs, depends linearly on [H3PO2], and the second-order rate constant k 2 depends on [H+] according to the relationship k 2 = 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 VI N via a proton-coupled N-atom transfer pathway.