Androgynous Porphyrins. Silver(II) Quinoxalinoporphyrins Act as Both Good Electron Donors and Acceptors

Abstract

The metal-centered and macrocycle-centered electron-transfer oxidations and reductions of silver(II) porphyrins were characterized in nonaqueous media by electrochemistry, UV-vis spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The investigated compounds are {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b']porphyrinato}silver(II), {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':7,8-b'']porphyrinato}silver(II), and {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)bisquinoxalino[2,3-b':12,13-b'']porphyrinato}silver(II). The first one-electron oxidation and first one-electron reduction both occur at the metal center to produce stable compounds with Ag(III) or Ag(I) metal oxidation states, irrespective of the type of porphyrin ligand. The electrochemical HOMO-LUMO gap, determined by the difference in the first oxidation and first reduction potentials, decreases by introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle. This provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors, something not previously observed in other metalloporphyrin complexes. The second one-electron oxidation and second one-electron reduction of the compounds both occur at the porphyrin macrocycle to produce Ag(III) porphyrin pi-radical cations and Ag(I) porphyrin pi-radical anions, respectively. The macrocycle-centered oxidation potentials of each quinoxalinoporphyrin are shifted in a negative direction, while the macrocycle-centered reduction potentials are shifted in a positive direction as compared to the same electrode reactions of the porphyrin without the fused quinoxaline ring(s). Both potential shifts are due to a stabilization of the radical cations and radical anions by pi-extension of the porphyrin macrocycle after fusion of one or two quinoxaline moieties at the beta-pyrrolic positions of the macrocycle. Introduction of quinoxaline groups fused to the Ag(II) porphyrin macrocycle provides a unique androgynous character to Ag(II) quinoxalinoporphyrins that enables them to act as both good electron donors and good electron acceptors.