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.