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 (H 3 PO 2 ) by a ruthenium(VI) nitrido complex, [(L)Ru VI (N)(OH 2 )] + ( 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)Ru VI (N)(OH 2 )] + + 3H 3 PO 2 + H 2 O → 2[(L)Ru III (NH 2 P(OH) 2 )(OH 2 )] + + H 3 PO 3 . The pseudo-first-order rate constant, k obs , depends linearly on [H 3 PO 2 ], 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 H 3 PO 2 molecule and K a is the dissociation constant of H 3 PO 2 . 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 D 3 PO 2 at pH 1.16, suggesting P-H bond cleavage in the rate-determining step. On the other hand, when the kinetics were determined in D 2 O, an inverse KIE of 0.21 ± 0.03 (H 3 PO 2 in H 2 O vs H 3 PO 2 in D 2 O) was found. On the basis of experimental results and DFT calculations, the proposed mechanism involves an acid-catalyzed tautomerization of H 2 P(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.