A fast-kinetic investigation of the redox chemistry of iridium chloride complexes using pulse radiolysis

Reactions initiated by OH radicals or e^-_aq in aqueous IrCl^3-₆ solutions were studied by electron pulse radiolysis using a 600 keV Febetron electron accelerator. Solutions of IrCl^3-₆ were made basic by adding Na₂CO₃; using the carbonate competition method, we find the rate constant for the reaction of OH with IrCl^3-₆ to be 4.7 × 10⁹ M^-1 s^-1. The product IrCl^2-₆ disappears rapidly in N₂O-saturated basic solution or in neutral N₂-saturated solution (N₂O absent) but is nearly inert in neutral solution with H₂O present. We find that IrCl^2-₆ reacts rapidly with hydrogen peroxide in basic media, as confirmed on the benchtop and by stopped-flow kinetics. We therefore infer that reaction with HO^-₂ may account for the loss of IrCl^2-₆ under basic conditions. Since e^-_aq reduces Ir(III) chloride to the Ir(II) state with a rate constant of 6.1 × 10⁹ M^-1 s^-1, we suggest that loss of Ir(IV) in neutral deaerated solution without added N₂O may involve electron transfer from Ir(II). Loss of Ir(IV) in aerated solution is attributed to reduction by the superoxide ion, O^-₂. Kinetic simulation of the system on the model described gives good agreement with our experimental results.