Insensitivity of vanadyl-oxygen bond strengths to radical type ((2)A(1u) vs (2)A(2u)) in vanadyl porphyrin cation radicals

Resonance Raman (RR) spectra are reported for vanadyl octaethylporphyrin, O V(OEP), tetramesityltetramethylporphyrin, OV(TMTMP), and tetramesitylporphy rin, OV(TMP), and their corresponding pi-cation radicals obtained by chemic al and electrochemical oxidation. The behavior of the nu(2) RR porphyrin "m arker band", which moves to higher frequency upon oxidation of the OV(OEP) and OV(TMTMP) and to lower frequency for OV(TMP), shows that the resultant cation radicals have predominantly (2)A(1u) and (2)A(2u) ground states, res pectively. In contrast to earlier work (Macor, K. A.; Czernuszewicz, R. S.; Spiro, T. G. Inorg. Chem. 1990, 29, 1996), it is demonstrated here that th e shift of the nu(V=O) is insensitive to radical type, behavior which is in agreement with similar studies of the ferryl analogues (Czarnecki, K.; et al. J. Am. Chem. Soc. 1996, 116, 2929 and 4680). It is suggested that the o bserved downshifts of the nu(V=O) previously reported for RR spectra of van adyl porphyrin pi-cation radicals, relative to their neutral parents, are m ost reasonably ascribed to trans oxo ligand coordination (most probably a w ater molecule) during low-temperature electrochemical oxidation of the neut ral species.