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)cobalt(II) complexes containing six, seven, or eight Br groups at the beta-pyrrole positions of the macrocycle is reported. Each compound undergoes three one-electron oxidations and up to nine one-electron reductions 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 tetraphenylporphyrin. Cyclic voltammetry studies reveal a positive shift of the metal and ring-centered redox potentials of the bromo porphyrins as compared to E1/2 for the reduction and oxidation of unsubstituted (TPP)Co. The optical absorption spectra of each electrogenerated Co(I) and Co(III) complex 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 Br groups on the porphyrin periphery. The electron withdrawing Br substituents 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 stable on the cyclic voltammetric and thin-layer time scales, but further reductions beyond [(TPPBrx)CoI]- lead to the stepwise elimination of Br groups to give [(TPP)CoI]-as a final product in solution. Results obtained by controlled-potential thin-layer spectroelectrochemistry and rotating ring disk electrode voltammetry confirm this experimental observation,