SPECTRAL, MAGNETIC AND ELECTROCHEMICAL PROPERTIES OF METAL OXA-PORPHYRINS AND OXATHIA-PORPHYRINS

Metal derivatives (Cu-II, Ni-II) of monooxa-, dioxa- and oxathia-tetra phenylporphyrins and their one-electron oxidised and reduced species h ave been studied. Electronic spectra of the monooxa and oxathia deriva tives exhibit split Soret bands and a complicated pattern of Q-bands r evealing their lower symmetry. The spectra of one-electron reduced spe cies show only marginal shifts while the one-electron oxidised product of copper monooxaporphyrins show broad, red-shifted bands. Cyclic vol tammetric studies indicated one-electron metal-centered reduction at f airly low potentials forming copper(r) and nickel(r) porphyrins. The r ing-oxidised product exhibits weak antiferromagnetic interaction betwe en the unpaired electrons of copper and the porphyrin ring. The ESR sp ectra of the copper dioxa- and oxathia-porphyrins exhibit rhombic symm etry with unusually small metal hyperfine couplings. A comparison of A (Cu) values and the E-1/2 values for metal reduction suggests that dis tortion towards tetrahedral symmetry and the presence of a soft donor atom like sulfur in porphyrins are required to generate spectral and e lectrochemical properties like those observed for the type I copper ce nter in proteins.