Electrochemistry and Spectroelectrochemistry of β,β′-Fused Quinoxalinoporphyrins and Related Extended Bis-porphyrins with Co(III), Co(II), and Co(I) Central Metal Ions

A series of cobalt(II) and cobalt(III) porphyrins with fused quinoxaline rings at one or more beta,beta'-pyrrolic units of the macrocycle were synthesized and characterized as to their electrochemical properties in nonaqueous media. Their UV-visible spectra were also measured before and during oxidation or reduction in a thin-layer cell. The investigated quinoxalinoporphyrins are represented as (PQ)Co, (QPQ)CoCl, (PQ(2))CoCl, Co(P)-TA-(P)Co, and Co(PQ)-(QP)Co, where PQ = the dianion of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-quinoxalino[2,3-b']porphyrin, QPQ = the dianion of the corresponding linear bisquinoxalino[2,3-b':12,13-b'']porphyrin, PQ(2) = the dianion of the corresponding corner bisquinoxalino[2,3-b':7,8-b'']porphyrin, and (P)-TA-(P) = the tetraanion of the bis-porphyrin with 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrins fused at opposite ends of tetraazaanthracene. (P)Co and (P)CoCl were also characterized where P = the dianion of 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin. Each compound could be cycled between their Co(III), Co(II), and Co(I) forms under the application of a given oxidizing or reducing potential, although a one-electron reduction of the Co(II) quinoxalinoporphyrins led to a product with mixed Co(I) and porphyrin pi-anion radical character followed by generation of a pure Co(I) pi-anion radical species at more negative potentials. The effect of the position and number of quinoxaline groups on the redox potentials and mechanisms of each electron transfer were elucidated, and comparisons made to structurally similar compounds containing both redox active and redox inactive central metal ions. Surprisingly, the position and number of quinoxaline groups on the macrocycle has little or no effect on the redox potentials for the Co(II) --> Co(III) or Co(III) --> Co(II) processes, but this is not the case for other electron transfer reactions where significant differences are seen between the examined compounds. Significant interactions are also observed between the two porphyrin macrocycles of the laterally bridged dicobalt(II) bis-porphyrin dyad Co(P)-TA-(P)Co in its singly and doubly reduced form, but only weak interactions are seen between the two Co(PQ) units of the single bond biquinolalinyl-bridged dicobalt(II) bis-porphyrin dyad Co(PQ)-(QP)Co.