Reactivity of tris(3-hydroxypropyl)phosphine species towards reduction of hexachloroiridate(IV): a comparison with tris(2-carboxyethyl)phosphine

Tris(3-hydroxypropyl)phosphine (THPP) was shown to be a more powerful reductant than tris(2-carboxyethyl)phosphine (TCEP), but its oxidation kinetics and mechanisms were scarcely investigated. The reduction of single electron oxidant [IrCl6]2− by THPP was thus investigated comparatively and shown to follow overall second-order kinetics. The established logk′ versus pH profiles for THPP and TCEP were different, where k′ is the observed second-order rate constant. THPP was oxidized to (HOCH2CH2CH2)3PO, confirmed by 1H NMR spectroscopy, which is in line with the determined stoichiometry. On the other hand, the proposed reaction mechanism is similar to that suggested for the TCEP reaction. The reactivity of THPP was derived, enabling a reactivity comparison with TCEP at physiological pH. Surprisingly, the measured activation parameters for the reduction of [IrCl6]2− by P(CH2CH2CH2OH)3 and P(CH2CH2CO2−)3 were essentially identical, providing information on the transition state and on the electron transfer mode. This work also defines the pKa value of THPP under well-defined conditions (pKa = 6.97 ± 0.05 at 25.0 °C and 1.0 M ionic strength) as derived by the kinetic approach.