ELECTROCHEMICAL AND SPECTROELECTROCHEMICAL BEHAVIOR OF COBALT(III), COBALT(II), AND COBALT(I) COMPLEXES OF MESO-TETRAPHENYLPORPHYRINATE BEARING BROMIDES ON THE BETA-PYRROLE POSITIONS

The synthesis and characterization of (meso-tetraphenylporphyrinato)co balt(II) complexes containing six, seven, or eight Br groups at the be ta-pyrrole positions of the macrocycle is reported. Each compound unde rgoes three one-electron oxidations and up to nine one-electron reduct ions depending upon the degree of Br substitution. The first oxidation yields [(TPPBrx)Co(III)]+ while the first reduction gives [(TPPBrx)Co (I)]- where TPPBrx is the dianion of the brominated tetraphenylporphyr in. Cyclic voltammetry studies reveal a positive shift of the metal an d ring-centered redox potentials of the bromo porphyrins as compared t o E1/2 for the reduction and oxidation of unsubstituted (TPP)Co. The o ptical absorption spectra of each electrogenerated Co(I) and Co(III) c omplex were recorded in a thin-layer cell and show that the transition energies for both the Soret and visible bands vary as a function of B r groups on the porphyrin periphery. The electron withdrawing Br subst ituents also produce a red shift in the Soret and visible bands of the porphyrin which follows the order: (TPPBr8)Co > (TPPBr7)Co > (TPPBr6) Co > (TPP)Co. The singly oxidized and singly reduced products are stab le on the cyclic voltammetric and thin-layer time scales, but further reductions beyond [(TPPBrx)CoI]- lead to the stepwise elimination of B r groups to give [(TPP)CoI]-as a final product in solution. Results ob tained by controlled-potential thin-layer spectroelectrochemistry and rotating ring disk electrode voltammetry confirm this experimental obs ervation,