Resonance Raman characterization of cationic Co(II) and Co(III) tetrakis(N-methyl-4-pyridinyl)porphyrins in aqueous and non-aqueous media

Abstract

Resonance Raman (RR) spectra of Co(II) and Co(III) complexes of the water-soluble cationic tetrakis(N-methyl-4-pyridinyl)porphyrin [Co IITMpyP(4), CoIIITMpyP(4)] were obtained in aqueous and non-aqueous media, using both B- and Q-band excitation wavelengths. RR spectra of CoIITMpyP(4) are observed for the first time, and they are compared with those of CoIIITMpyP(4). Wavenumbers of the ν 2, ν4, ν8 and ν6 oxidation state-sensitive bands depend on pH in aqueous solution and on the nature of the solvent (methanol, ethanol, DMF, DMSO): they lie in distinct spectral ranges for Co(II) vs Co(III) porphyrins. In addition, wavenumber shifts induced by axial ligation cover regions twofold broader for CoIIITMpyP(4) than for CoIITMpyP(4). This shows that the electronic influence of axial ligands on the π-system of the macrocycle is more effective when cobalt is in a higher oxidation state. This can be due to (i) smaller axial bond lengths of Co(III) porphyrin and, hence, more efficient cobalt-axial ligand interaction as compared with Co(II) complexes, and (ii) CoIIITMpyP(4) being involved in bis-adducts whereas CoIITMpyP(4) is mainly bound with only one axial ligand. The response of the porphyrin ring to the binding of axial ligands having various electron-donating-withdrawing properties was monitored by the behaviour of these oxidation-state sensitive RR bands: their shifts observed in a series of solvents were compared with Gutmann donor and acceptor numbers (DN, AN) of the solvents. A linear positive correlation was observed between Gutmann AN and ν2, ν4, ν 8 and ν6 band wavenumbers for CoIIITMpyP(4), whereas a linear negative correlation was observed for the same bands of CoIITMpyP(4) when DN increases. This means that enhanced electron-accepting properties of axial ligand result in a scarcity of cobalt dπ-orbitals, which in turn leads to a decrease in π back-donation from cobalt to the porphyrin macrocycle. Copyright © 2003 John Wiley & Sons, Ltd.