SPECTROSCOPIC AND ELECTROCHEMICAL CHARACTERIZATION OF NICKEL BETA-OXOPORPHYRINS - IDENTIFICATION OF NICKEL(III) OXIDATION-PRODUCTS

Seven nickel di- and tri-beta-oxoporphyrins ([3,3,7,8,12,13,17,17-octaethyl-2,18(3H,17H)-porphinedionato(2-)]nickel, [3,3,8,8,12,13,17,18-octaethyl-2,7(3H,8H)-porphinedionato(2-)]nickel (NiDP-II), [3,3,7,7,12,13,17,18-octaethyl-2,8(3H,7H)-porphinedionato(2-)]nickel, [3,3,7,8,12,12,17,18-octaethyl-2,13(3H,12H)-porphinedionato(2-)]nickel, [3,3,7,8,13,13,17,18-octaethyl-2,12(3H,13H)-porphinedionato(2-)]nickel, [3,3,7,8,12,12,18,18-octaethyl-2,13,17(3H,12H,18H)-porphinetrionato(2-)]nickel (NiTP-A), and [3,3,8,8,13,13,17,18-octaethyl-2,7,12(3H,8H,13H)-porphinetrionato(2-)]nickel) were prepared by hydrogen peroxide oxidation of octaethylporphyrin followed by metalation. These compounds and the previously synthesized nickel mono-beta-oxoporphyrin [3,3,7,8,12,13,17,18-octaethyl-2(3H)-porphinonato(2-)]nickel (NiMP) (Stolzenberg, A. M.; Glazer, P. A.; Foxman, B. M. Inorg. Chem. 1986, 25, 983-991) were characterized by electronic absorption spectroscopy, H-1 NMR spectroscopy, resonance Raman (RR) spectroscopy, and cyclic voltammetry. EPR spectra of the one-electron-oxidation products of NiMP, NiDP-II, and NiTP-A in methylene chloride and acetonitrile have also been obtained. A solvent-induced switch in the site of one-electron abstraction occurs in NiDP-II, where a Ni(III)DP-II series is produced in acetonitrile solution and a Ni(II)DP-II cation radical is generated in methylene chloride solution. In both solvents one-electron oxidation of NiMP yields a Ni(II)MP cation radical while one-electron oxidation of NiTP-A yields Ni(III)TP-A species. EPR signals from the Ni(III) centers (g(av) = 2.24) are observed at liquid-nitrogen temperature. This is the first report of the Ni(III) state in a porphyrinic derivative at room temperature. The Ni3+/2+ potential for NiTP-A is 0.34 V (vs SCE), which is one of the lowest reported for this redox couple. We suggest that stabilization of Ni(III) by the beta-oxoporphyrins is based on a balance of pi-conjugation, ring ruffling, and axial ligation. The nickel beta-oxoporphyrins exhibit a greater number of Raman bands than nickel octaethylporphyrin (NiOEP) due to their lower symmetry. Vibrational modes that are only IR-active in NiOEP (D4h symmetry are observed with strong intensities in the Raman spectra of the nickel beta-oxoporphyrins, which are best described as having C2 symmetry. A carbonyl stretching mode in the 1708-1716-cm-1 region is observed as one of the strongest bands in the RR spectrum of each nickel beta-oxoporphyrin, and is an identifying feature of the beta-oxoporphyrin spectra.