Electrochemical dioxygen reduction catalyzed by a (nitro)cobalt(perfluorophthalocyanine) complex and the possibility of a peroxynitro complex intermediate

Abstract

The (nitro)([Formula: see text],[Formula: see text]-dimethyl-4-aminopyridine) complex of perfluorinated cobalt(III) phthalocyanine Co(III)F16Pc(Me2Npy)(NO[Formula: see text] catalyzes the electrochemical oxygen reduction reaction (ORR) in pH 4.0, 7.0, and 10.0 buffer and 0.05 M sulfuric acid solution when deposited on a glassy carbon electrode. Cyclic voltammetry (CV), rotating disk electrode voltammetry (RDE), and rotating ring-disk electrode voltammetry (RRDE) have been used to determine the reduction product as hydrogen peroxide although in concentrations too small to observe by qualitative methods such as oxidation of NaI in solution. The dependence of the values of the peak potentials for the reduction on the pH of the solution and the -log[Me2Npy] are consistent with protonation up to pH 7.6 and pyridine ligand loss during the reduction. The addition of nitrite at 0.1 and 1 M to pH 7.0 solutions in contact with films of CoF16Pc on the glassy carbon electrode decreases the ORR current and shifts the peak potential of the ORR from -0.21 V vs. NHE to -0.19 V vs. NHE. The addition of nitrite at 0.1 and 1 M to films of Co(III)F16Pc(Me2Npy)(NO[Formula: see text] on glassy carbon, however, has no effect on either the current or the potential. While the electrochemical evidence for this proposal is not definitive, modeling has been used to examine the center of reduction in the alternative mechanisms by evaluation of the LUMOs of the hypothetical intermediates in both closed and open shell cases. The formation of five-coordinate Co(II)F16Pc(NO) is proposed to occur initially in the reduction mechanism. It is also possible that O2 reduction takes place at the NO ligand center by way of a nitrogen-bound peroxynitrite intermediate. The [Formula: see text] ligand appears to remain bound during the ORR. Direct coordination of O2 to the metal center requiring a six-coordinate species, Co(III)F16Pc(O[Formula: see text](NO[Formula: see text], Co(II)F16Pc(O[Formula: see text](NO) or [Co(II)F16Pc(O[Formula: see text](NO[Formula: see text]][Formula: see text] and has been considered in DFT modeling studies. The instability of the two-electron reduced, protonation species, [Co(I)F16Pc(NO2OH)][Formula: see text] in its loss of peroxynitrous acid suggests that the reduction of O2 may occur by two one-electron reduction steps rather than a two-electron step.