Deciphering the reaction mechanism and the reactivity of the TCEP species towards reduction of hexachloroiridate(IV)

Despite of wide applications of tris(2-carboxyethyl)phosphine (TCEP) as a reductant, the reductions of single electron oxidants by TCEP are poorly understood mechanistically. Herein the reduction of [IrCl6]2− by TCEP was thus investigated kinetically over a wide pH range at 1.0 M ionic strength. Overall second-order kinetics was proved for the reduction, being first-order with respect to both Ir(IV) and TCEP, respectively. The redox stoichiometry was determined to be Δ[Ir(IV)]:Δ[TCEP] = 2:1 and the corresponding phosphine oxide (TCEPO) was identified as the oxidation product of TCEP by 1H NMR spectroscopy. A reaction mechanism was rationalized in term of parallel reactions of [IrCl6]2− with all the TCEP protolysis species being as the rate-determining steps, in which phosphine radical cations were involved as the transient intermediates. Rate constants of the rate-determining steps were calculated, rendering a reactivity trend for the five TCEP species: P(CH2CH2CO2−)3 ≫ +HP(CH2CH2CO2−)3 ≈ +HP(CH2CH2CO2−)2(CH2CH2CO2H) ≫ +HP(CH2CH2CO2−)(CH2CH2CO2H)2 ≫ +HP(CH2CH2CO2H)3. This is the first time to decipher the reactivity of all the TCEP protolysis species in the redox reactions of TCEP. Additionally, activation parameters were also measured for the reaction of P(CH2CH2CO2−)3 with [IrCl6]2− yielding ΔH5‡ = 26.3 ± 1.4 kJ∙mol−1 and ΔS5‡ = −48 ± 5 J∙K−1∙mol−1; a possible mode of electron transfer is discussed. No doubt, the mechanism and the reactivity derived from the present work can serve as a model for the oxidations of TCEP by other single electron oxidants that are involved in chemically- and biologically-important processes.