Kinetics and mechanisms of the oxidation of hypophosphite and phosphite with trans-[RuVI(L)(O)2]2+ (L = 1,12-dimethyl-3,4 : 9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane)

The kinetics of the oxidation of hypophosphite and phosphite by trans-[Ru(L)(O)₂]²⁺ (L = 1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) have been studied in aqueous acidic solutions. The reactions have the following stoichiometry (x = 2 or 3): trans-[Ru^VI(L)(O)₂]²⁺ + H₂PO_x ^- + H₂O → trans-[Ru^IV(L)(O)(OH₂)]²⁺ + H₂PO_x+1 ^-,. The two reactions have the same rate law (P = hypophosphite or phosphite): -d[Ru^VI]/dt = k/(1 + [H⁺]/K)[Ru^VI][P]. For hypophosphite, k = (1.3 ± 0.1).dm³ mol^-1 s^-1 and K= (9.7 ± 0.5) × 10^-2 mol dm^-3 at 298 K and I= 1.0 mol dm^-3. For phosphite, k = (4.8 ± 0.4) × 10^-2 dm³ mol^-1 s^-1 and K= (1.2 ± 0.2) × 10^-2 mol dm^-3 at 298 K and I = 0.2 mol dm^-3. The effects of temperature were studied from 15°C to 40°C. For hypophosphite, ΔH^‡ = (60 ± 2) kJ mol^-1 and ΔS^‡ = (-41 ± 4) J mol^-1 K^-1 at pH = 1.86 and I = 1.0 mol dm^-3. For phosphite, ΔH^‡ = (59 ± 4) kJ mol^-1 and ΔS^‡ = (-75 ± 13) J K^-1 mol^-1 at pH = 2.3 and I = 0.2 mol dm^-3. Deuterium isotope effects have also been investigated. For hypophosphite, the kinetic isotope effect, k(H₂PO₂ ^-)/k(D₂PO₂ ^-) is 4.1 at pH = 1.07 and I = 1.0 mol dm^-3. For phosphite, the kinetic isotopic effect, k(HDPO₃ ^-)/k(D₂PO₃ ^-), is 4.0 at pH = 2.30 at I = 0.2 mol dm^-3. A mechanism involving hydride transfer from P-H to Ru=O is proposed for these two reactions. © The Royal Society of Chemistry 2000.