Resonance Raman characterization of five-coordinate dioxygen adducts of porphyrinatocobalt(II) complexes formed in low temperature O2 matrices

Abstract

Resonance Raman (RR) spectra of five-coordinate dioxygen adducts of cobalt(tetramesitylporphine) and cobalt(tetramesopropylporphine) formed in low temperature O₂ matrices are reported. For the first time, all three vibrations expected for the Co–O₂ fragment, ν(O–O), ν(Co–O₂) and δ(Co–O–O), were observed for ¹⁶O₂ and ¹⁸O₂ isotopomers. These bands have been assigned based on oxygen isotope shift data as well as normal coordinate analysis of the Co–O₂ moiety. This work presents also the first observation of two distinct ν(O–O) bands due to the Co–¹⁶O–¹⁸O and Co–¹⁸O–¹⁶O isomers in ‘end-on’ type dioxygen adducts. The mechanism of resonance enhancement, the effect of the axial ligand on the ν(O–O) and ν(Co–O₂) frequencies and the differences in metal–O₂ bonding between cobalt and iron porphyrins are discussed.