Mechanistic Studies of the Reaction of Ir(III) Porphyrin Hydride with 2,2,6,6-Tetramethylpiperidine-1-oxyl to an Unsupported Ir−Ir Porphyrin Dimer

Abstract

Reaction of hydrido[5,10,15,20-tetrakis(p-tolyl)porphyrinato]iridium(III) (Ir(ttp)H) (1) with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (2) at room temperature gave a 90% yield of the unsupported iridium(II) porphyrin dimer, Ir(II)(2)(ttp)(2) (3). Kinetic measurements revealed that the oxidation followed overall second-order kinetics: rate = k[Ir(ttp)H][TEMPO], k(25 °C) = 6.65 × 10(-4) M(-1). The entropy of activation (ΔS(‡) = -25.3 ± 2.5 cal mol(-1) K(-1)) and the kinetic isotope effect of 7.2 supported a bimolecular associative mechanism in the rate-determining hydrogen atom transfer from Ir(ttp)H to TEMPO.