Spectral, magnetic and electrochemical properties of metal oxa- and oxathia-porphyrins

Abstract

Metal derivatives (Cu^II, Ni^II) of monooxa-, dioxa- and oxathia-tetraphenylporphyrins and their one-electron oxidised and reduced species have been studied. Electronic spectra of the monooxa and oxathia derivatives exhibit split Soret bands and a complicated pattern of Q-bands revealing their lower symmetry. The spectra of one-electron reduced species show only marginal shifts while the one-electron oxidised product of copper monooxaporphyrins show broad, red-shifted bands. Cyclic voltammetric studies indicated one-electron metalcentered reduction at fairly low potentials forming copper(I) and nickel(I) porphyrins. The ring-oxidised product exhibits weak antiferromagnetic interaction between the unpaired electrons of copper and the porphyrin ring. The ESR spectra of the copper dioxa- and oxathia-porphyrins exhibit rhombic symmetry with unusually small metal hyperfine couplings. A comparison of A^Cu values and the E_½ values for metal reduction suggests that distortion towards tetrahedral symmetry and the presence of a soft donor atom like sulfur in porphyrins are required to generate spectral and electrochemical properties like those observed for the type I copper center in proteins.