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

Abstract

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 Na 2CO 3; using the carbonate competition method, we find the rate constant for the reaction of OH with IrCl 3- 6 to be 4.7 × 10 9 M -1 s -1. The product IrCl 2- 6 disappears rapidly in N 2O-saturated basic solution or in neutral N 2-saturated solution (N 2O absent) but is nearly inert in neutral solution with H 2O 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 × 10 9 M -1 s -1, we suggest that loss of Ir(IV) in neutral deaerated solution without added N 2O 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.