Unusual Multi-Step Sequential AuIII/AuII Processes of Gold(III) Quinoxalinoporphyrins in Acidic Non-Aqueous Media

The electrochemistry of gold­(III) mono- and bis-quinoxalinoporphyrins was examined in CH2Cl2 or PhCN containing 0.1 M tetra-n-butylammonium perchlorate (TBAP) before and after the addition of trifluoroacetic acid to solution. The investigated porphyrins are represented as Au­(PQ)­PF6 and Au­(QPQ)­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 Au­(QPQ)­PF6 there is a linear arrangement where the quinoxalines are fused to pyrrolic positions that are opposite each other. The porphyrin without the fused quinoxaline groups, Au­(P)­PF6, was also investigated under the same solution conditions. In the absence of acid, all three gold­(III) porphyrins undergo a single reversible AuIII/AuII process leading to the formation of a Au­(II) porphyrin which can be further reduced at more negative potentials to give stepwise the Au­(II) porphyrin π-anion radical and dianion, respectively. However, in the presence of acid, the initial AuIII/AuII processes of Au­(PQ)­PF6 and Au­(QPQ)­PF6 are followed by an internal electron transfer and protonation to regenerate new Au­(III) porphyrins assigned as AuIII(PQH)+ and AuIII(QPQH)+. Both protonated gold­(III) quinoxalinoporphyrins then undergo a second AuIII/AuII process at more negative potentials. The electrogenerated monoprotonated monoquinoxalinoporphyrin, AuII(PQH), is then further reduced to its π-anion radical and dianion forms, but this is not the case for the monoprotonated bis-quinoxalinoporphyrin, AuII(QPQH), which accepts a second proton and is rapidly converted to AuIII(HQPQH)+ before undergoing a third AuIII/AuII process to produce AuII(HQPQH) as a final product. Thus, Au­(P)­PF6 undergoes one metal-centered reduction while Au­(PQ)­PF6 and Au­(QPQ)­PF6 exhibit two and three AuIII/AuII processes, respectively. These unusual multistep sequential AuIII/AuII processes were monitored by thin-layer spectroelectrochemistry and a reduction/oxidation mechanism for Au­(PQ)­PF6 and Au­(QPQ)­PF6 in acidic media is proposed.