Dioxygen binding to a cobalt(II) porphycene complex and its auto-oxidized cobalt(III) complex

The reaction between a cobalt porphycene complex, [Co^II(TPrPc)] (TPrPc is a dianion of 2,7,12,17-tetrapropylporphycene), and dioxygen was investigated by means of UV-vis, ESR, and resonance Raman (RR) spectroscopies. These spectra indicate the formation of [Co^III-O₂ ^-.]. In the presence of pyridine as an axial ligand, the formation of a six-coordinated superoxide species was confirmed by its characteristic ESR spectrum. The six-coordinated dioxygen adduct exhibits an RR band at 1144 cm^-1, which is characteristic of O-O stretching of superoxide. ν(Co-O₂) stretching is also observed at 540 cm^-1. The ν(O-O) and ν(Co-O₂) wavenumbers in the complex were assigned by means of ¹⁸O₂ substitution. ν(Co-O₂) of the O₂ adduct shows a higher frequency relative to those of cobalt porphyrin complexes. On standing for one day at room temperature, the compound was auto-oxidized to a Co^III complex, which was structurally determined by X-ray analysis. The complex crystallized in the monoclinic space group P2₁/n, a = 9.6266(4) Å, b = 12.6343(5) Å, c = 15.6566(6) Å, β = 99.1200(10)°, V = 1880.17(13) ų, and Z = 2. The Co^III ion lies on the plane of the porphycenato core, and coordinates four porphycenato nitrogen atoms and two pyridine nitrogen atoms. The complex cation, [Co^III(TPrPc)(Py)2] ⁺, exhibits a slightly distorted octahedral coordination geometry. The average Co-N (porphycenato pyrrole) distance and the Co-N (pyridine) distance are 1.929 and 1.952 Å, respectively.