Mechanism of autoxidation of the chromium(II) porphyrins. In situ infrared spectroscopic detection of the peroxo- intermediate

Abstract

The reaction of Cr(II)(TPP) (TPP is meso-tetraphenylporphyrinato dianion) in solid sublimed layers with dioxygen at 50 K results in formation of a single IR band at 1142 cm−1. The IR spectrum of the sublimated Cr (II)(TPP) exposed to the dioxygen at 77 K exhibits two IR bands at 1142 and 935 cm−1, which are shifted to 1078 and 883 cm−1 respectively by 16O2/18O2 substitution. When heated to 130 K, the bands at 1142 cm−1 (1078 cm−1) are completely transformed into bands at 935 cm−1 (883 cm−1) and this transformation is irreversible. Based on IR frequencies analysis, isotopic dioxygen substitution and DFT calculations the band at 1142 cm−1 is assigned to the end-on type base-free dioxygen adduct, whereas the band at and 935 cm−1 is attributed to the side-on peroxo- intermediate. Following warm-up revealed further changes in IR spectra giving at room temperature well known chromyl O = Cr(IV)(TPP) compound with IR frequencies at 1020 cm−1 (980 cm−1).