Substituent Effects on the Site of Electron Transfer during the First Reduction for Gold(III) Porphyrins

Gold(III) porphyrins of the type (P-R)AuPF₆, where P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and R is equal to H (1), NO₂ (2), or NH₂ (3) which is substituted at one of the eight β-pyrrolic positions of the macrocycle, were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each compound undergoes three reductions, the first of which involves the central metal ion to give a Au(II) porphyrin or a Au(III) porphyrin π-anion radical depending upon the nature of the porphyrin ring substituent. A similar metal-centered reduction also occurs for compounds 1, 3, and Au(III) quinoxalinoporphyrin, (PQ)AuPF₆ (4), where PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin, and these results on the three Au(III) porphyrins overturn the long held assumption that reductions of such complexes only occur at the macrocycle. In contrast, when a NO₂ group is introduced on the porphyrin ring to give (P-NO₂)AuPF₆ (2), the site of electron transfer is changed from the gold metal to the macrocycle to give a porphyrin π-anion radical in the first reduction step. This change in the site of electron transfer was examined by electrochemistry combined with thin-layer UV-vis spectroelectrochemistry and ESR spectroscopy of the singly reduced compound produced by chemical reduction. The reorganization energy (λ) of the metal-centered electron transfer reduction for (P-H)AuPF₆ (1) in benzonitrile was determined as λ = 1.23 eV by analyzing the rates of photoinduced electron transfer from the triplet excited states of an organic electron donor to 1 in light of the Marcus theory of electron transfer. The λ value of the metal-centered electron transfer of gold porphyrin (1) is significantly larger than λ values of ligand-centered electron transfer reactions of metalloporphyrins.