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