The oxidation of 2-thiopyrimidine and 2-thiouracil by Ir(IV) complexes

The kinetics of the oxidation of 2-thiopyrimidine (TP) by IrCl^2-₆ and IrBr^2-₆, and 2-thiouracil (TU) by IrCl^2-₆ follow a second-order rate law, first-order with respect to both the concentration of the oxidant and reductant. The second-order rate constants at 25.0 °C and at μ = 0.10 M are: 3.30 M^-1 sec^-1 for TP+IrCl^3-₆ at pH 2.10, 0.187 M^-1 sec^-1 for TU + IrCl^2-₆ at pH 2.10, and 344 M^-1 sec^-1 for TP + IrBr^2-₆ at pH 5.3. A pH-rate profile is observed to have nearly a constant rate at pH 2.1 to 3.5 and a rapid increase in rate from pH 3.5 to 5.5. This is accounted for by the equilibrium pKa of the thiols and by the higher reactivity of the mercaptide anions The activation parameters calculated from the temperature dependence studies are: ΔH^≠ = 7.5 kcal/ mole, Δs^≠ = -31.0 e.u. for the TP + IrCl^2-₆, and ΔH≠ = 10.1 kcal/mole, ΔS≠ = -27.8 e.u. for the TU + IrCl^2-₆. A free radical mechanism is postulated to account for the above observed kinetics.