Substituent Effects on the Site of Electron Transfer during the First Reduction for Gold(III) Porphyrins

Abstract

Gold(III) porphyrins of the type (P-R)AuPF(6), where P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and R is equal to H (1), NO(2) (2), or NH(2) (3) which is substituted at one of the eight beta-pyrrolic positions of the macrocycle, were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each compound undergoes three reductions, the first of which involves the central metal ion to give a Au(II) porphyrin or a Au(III) porphyrin pi-anion radical depending upon the nature of the porphyrin ring substituent. A similar metal-centered reduction also occurs for compounds 1, 3, and Au(III) quinoxalinoporphyrin, (PQ)AuPF(6) (4), where PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin, and these results on the three Au(III) porphyrins overturn the long held assumption that reductions of such complexes only occur at the macrocycle. In contrast, when a NO(2) group is introduced on the porphyrin ring to give (P-NO(2))AuPF(6) (2), the site of electron transfer is changed from the gold metal to the macrocycle to give a porphyrin pi-anion radical in the first reduction step. This change in the site of electron transfer was examined by electrochemistry combined with thin-layer UV-vis spectroelectrochemistry and ESR spectroscopy of the singly reduced compound produced by chemical reduction. The reorganization energy (lambda) of the metal-centered electron transfer reduction for (P-H)AuPF(6) (1) in benzonitrile was determined as lambda = 1.23 eV by analyzing the rates of photoinduced electron transfer from the triplet excited states of an organic electron donor to 1 in light of the Marcus theory of electron transfer. The lambda value of the metal-centered electron transfer of gold porphyrin (1) is significantly larger than lambda values of ligand-centered electron transfer reactions of metalloporphyrins.

Evidence of the withdrawing effect of a pyridinium N-cation on the electrochemical reduction of gold (III) tetraphenylporphyrin

“…6). These observations are consistent with earlier reports that the ESR signals induced by the added electron on a gold (III) porphyrin were particularly temperature and substituent dependent [16,17]. Here, it can be reasonably postulated that the electrochemically generated species after the first reduction step can be attributed to the Au(III) porphyrin p-anion radical.…”

“…Similar shaped UVVis spectra were also observed during the electrochemical reduction of gold porphyrins [16,17]. The initial spectrum was totally recovered after reverse electrolysis at 0 V, in agreement with the reversibility observed in the cyclic voltammetry experiments.…”

“…In addition, these authors claimed that this electronic transfer occurred on the porphyrin ring and that the gold (III) cation was electrochemically inactive in this reduction step. More recent papers [16,17] proposed that this first reduction actually involves either the central metal ion, thus generating an Au(II) porphyrin, or the Au(III) porphyrin ligand to give p-anion radical, depending on the periphery of the porphyrin ligand. Studies of binary systems in aqueous media, in which a gold porphyrin and a pyridinium cation are linked via a C-C bond, have been reported [18,19], where the porphyrin is reduced before the pyridinium group.…”

Gold(III) phthalocyanine chloride: Optical and structural characterization of thin films

“…They can be chemically or electrochemically reduced to gold(II) species in M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT solutions, however, isolating them was unsuccessful [7,8]. Au(III) porphyrins have been shown to exhibit cytotoxic properties and anticancer activity [9].…”

Electrochemical monitoring of the oxidative coupling of alkynes catalyzed by triphenylphosphine gold complexes

“…Similar responses were obtained for complexes 2 and 3. This electrochemistry separates from those reported for gold complexes with dithiolate and dithiolene ligands [21,22] and porphyrins [23,24], among others [25].…”