Gold(III) Porphyrins Containing Two, Three, or Four β,β′-Fused Quinoxalines. Synthesis, Electrochemistry, and Effect of Structure and Acidity on Electroreduction Mechanism

Abstract

Gold­(III) porphyrins containing two, three, or four β,β′-fused quinoxalines were synthesized and examined as to their electrochemical properties in tetrahydrofuran (THF), pyridine, CH₂Cl₂, and CH₂Cl₂ containing added acid in the form of trifluoroacetic acid (TFA). The investigated porphyrins are represented as Au­(PQ₂)­PF₆, Au­(PQ₃)­PF₆, and Au­(PQ₄)­PF₆, where P is the dianion of the 5,10,15,20-tetrakis­(3,5-di-<i>tert</i>-butylphenyl)­porphyrin and Q is a quinoxaline group fused to a β,β′-pyrrolic position of the porphyrin macrocycle. In the absence of added acid, all three gold­(III) porphyrins undergo a reversible one-electron oxidation and several reductions. The first reduction is characterized as a Au^III/Au^II process which is followed by additional porphyrin- and quinoxaline-centered redox reactions at more negative potentials. However, when 3–5 equivalents of acid are added to the CH₂Cl₂ solution, the initial Au^III/Au^II process is followed by a series of internal electron transfers and protonations, leading ultimately to triply reduced and doubly protonated Au^II(PQ₂H₂) in the case of Au^III(PQ₂)⁺, quadruply reduced and triply protonated Au^II(PQ₃H₃) in the case of Au^III(PQ₃)⁺, and Au^II(PQ₄H₄) after addition of five electrons and four protons in the case of Au^III(PQ₄)⁺. Under these solution conditions, the initial Au­(PQ₂)­PF₆ compound is shown to undergo a total of three Au^III/Au^II processes while Au­(PQ₃)­PF₆ and Au­(PQ₄)­PF₆ exhibit four and five metal-centered one-electron reductions, respectively, prior to the occurrence of additional reductions at the conjugated macrocycle and fused quinoxaline rings. Each redox reaction was monitored by cyclic voltammetry and thin-layer spectroelectrochemistry, and an overall mechanism for reduction in nonaqueous media with and without added acid is proposed. The effect of the number of Q groups on half-wave potentials for reduction and UV–visible spectra of the electroreduced species are analyzed using linear free energy relationships