Unusual multi-step sequential AuIII/AuII processes of gold(III) quinoxalinoporphyrins in acidic non-aqueous media

摘要

The electrochemistry of gold(III) mono- and bis-quinoxalinoporphyrins was examined in CH~2Cl~2 or PhCN containing 0.1 M tetra-nbutylammonium perchlorate (TBAP) before and after the addition of trifluoroacetic acid to solution. The investigated porphyrins are represented as Au(PQ)PF_6 and Au(QPQ)PF_6, 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)PF_6 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)PF_6, was also investigated under the same solution conditions. In the absence of acid, all three gold(III) porphyrins undergo a single reversible Au~(III)/ 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 Au ~(III)/Au~(II) processes of Au(PQ)PF_6 and Au(QPQ)PF_6 are followed by an internal electron transfer and protonation to regenerate new Au(III) porphyrins assigned as Au ~(III)(PQH)_+ and AuIII(QPQH)_+. Both protonated gold(III) quinoxalinoporphyrins then undergo a second Au_(III)/Au II 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 Au~(III)(HQPQH)_+ before undergoing a third Au~(III)/Au~(II) process to produce AuII(HQPQH) as a final product. Thus, Au(P)PF_6 undergoes one metal-centered reduction while Au(PQ)PF_6 and Au(QPQ)PF_6 exhibit two and three Au~(III)/Au~(II) processes, respectively. These unusual multistep sequential Au~(III)/AuII processes were monitored by thin-layer spectroelectrochemistry and a reduction/oxidation mechanism for Au(PQ)PF_6 and Au(QPQ)PF_6 in acidic media is proposed.