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)

Abstract

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 RuO is proposed for these two reactions.