Kinetics of the oxidation of hypophosphite,deuteriohypophosphite, phosphite and neutralized phenyl phosphonous acid by tris(pyridine-2-carboxylato)manganese(III) in picolinate-picolinic acid buffer medium

Hypophosphite, deuteriohypophosphite, phosphite and neutralized phenyl phosphonous acid are oxidized by tris(pyridine-2-carboxylato)manganese(III) in Na(pic)–picH where Na(pic) = sodium salt of picolinic acid and picH = picolinic acid] buffer in the 4.63–5.45 pH range. All the reactions appear to follow the same mechanistic pathway in which the substrate initially forms a 1:1 intermediate complex with the oxidant. The complex then breaks down in the rate-determining step to give a free radical intermediate which subsequently reacts with another molecule of oxidant to give the products. A deuterium isotope effect of k _H/k _D = 4.26 at 313 K, was observed in the oxidation of hypophosphite indicating that proton removal from phosphorus takes place in the rate-determining step. A mechanism consistent with the experimental observations has been proposed.