Kinetics of chromium(V)-oxo and chromium(IV)-oxo porphyrins: Reactivity and mechanism for sulfoxidation reactions

Abstract

In this work, chromium(IV)-oxo porphyrins [CrIV(Por)(O)] (2) (Por = porphyrin) were produced either by oxidation of [CrIII(Por)Cl] (1) with iodobenzene diacetate or visible light photolysis of porphyrin‑chromium(III) chlorates. Subsequent oxidation of 2 with silver perchlorate gave chromium(V)-oxo porphyrins [CrV(Por)(O)](ClO4) (3) in three porphyrin ligands, including 5,10,15,20-tetramesitylporphyrin(TMP, a), 5,10,15,20-tetrakis(2,6-difluorophenyl)porphyrin(TDFPP, b), and 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (TPFPP, c). Complexes 2 and 3 reacted with thioanisoles to produce the corresponding sulfoxides, and their kinetics of sulfoxidation reactions with a series of aryl methyl sulfides(thioanisoles) were studied in organic solutions. Chromium(V)-oxo porphyrins are several orders of magnitudes more reactive than chromium(IV)-oxo species, and representative second-order rate constants (kox) for the oxidation of thioansole are (0.40 ± 0.01) M−1 s−1 (3a), and (2.82 ± 0.20) × 102 M−1 s−1 (3b), and (2.20 ± 0.01) × 103 M−1 s−1 (3c). The order of reactivity for 2 and 3 follows TPFPP > TDFPP > TMP, in agreement with the electrophilic nature of metal-oxo complexes. Hammett analyses indicate significant charge transfer in the transition states for oxidation of para-substituted thioanisoles by [CrV(Por)(O)]+. The ρ+ constants are −1.69 for 3a, −2.63 for 3b, and − 2.89 for 3c, respectively, mirror values found previously for related metal-oxo species. A mechanism involving the electrophilic attack of the CrV-oxo at sulfides to form a sulfur cation intermediate in the rate-determining step is suggested. Competition studies with chromium(III) porphyrin chloride and PhI(OAc)2 gave relative rate constants for oxidations of competing thioanisoles that closely match ratios of absolute rate constants from chromium(V)-oxo species, which are true oxidants under catalytic conditions.