Preparation and Physicochemical Properties of Tervalent Cobalt Complexes of Porphyrins

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Abstract
Tervalent cobalt complexes of octaethylporphyrin (OEP) and tetraphenylporphyrin (TPP), [CoIII(OEP)(H2O)2]X (X=ClO4, BF4), [CoIII(OEP)(THF)2]ClO4, [CoIII(TPP)(H2O)2]X (X=ClO4), and [CoIII(TPP)(H2O)Y] (Y=Cl, Br, SCN, N3) were synthesized. Molecular weight measurements indicate that only [CoIII(TPP)(H2O)2]BF4 and [CoIII(TPP)(H2O)2]ClO4 aggregate at higher concentrations in chloroform. In polar solvents such as methanol, all are monomeric. In nonpolar solvents, all the complexes show Soret bands at shorter wavelengths and somewhat paramagnetic 1H NMR spectra; these anomalies are absent in polar solvents. Tetrafluoroborates and perchlorates react with ethyl vinyl ether smoothly to form 2,2-diethoxyethylcobalt(III) complexes which are easily converted to formylmethylcobalt(III) complexes. 1H NMR and absorption spectra of the amine adducts prepared in this study, [CoIII(OEP)(L)2]ClO4 (L=pyridine, 2-methylimidazole) and [CoIII(TPP)(L′)2]ClO4 (L′=pyridine, 4-cyanopyridine, 4-methylpyridine), indicate that the two amine molecules coordinate to the central cobalt atom. As regards redox potentials for the Co(III)/Co(II) couple as measured by cyclic voltammetry, the cobalt(III) porphyrins are categorized into three groups: (i) BF4 and ClO4 salts with redox potentials at about 0.7 V; (ii) halides and pseudohalides with those at 0.2–0.3 V; and (iii) bis(amine) adducts with those at −0.3–+0.1 V. In polar solvents, however, the redox potentials of (i) and (ii) are very similar to one another.

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