Vanadium-Schiff Base Complexes as Catalysts for the Four-Electron Reduction of Dioxygen

Citation

Liu, Zenghe (2002) Vanadium-Schiff Base Complexes as Catalysts for the Four-Electron Reduction of Dioxygen. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BV5W-DK24. https://resolver.caltech.edu/CaltechTHESIS:02022012-100419178

Abstract

The coordination chemistry and electrochemistry of complexes of vanadium(III, IV, V) with salen (H_2salen = N,N'-ethylenebis(salicylideneamine)) were first examined. The origin of the puzzling results reported in previous reports in the literature was identified using microelectrode voltammetry and the modifications required are specified. VO(salen) was found to disproportionate in acidified acetonitrile. The equilibrium constant for this disproportionation reaction was measured. The stoichiometry and kinetics of the reaction between O_2 and the V(III)salen complex were examined and a possible mechanism for this four-electron reduction of O_2 is suggested.

The study was then expanded to vanadium complexes with fifteen Schiff base (SB) ligands analogous to salen. Electrochemical and spectral characteristics of the complexes were evaluated and compared. With several of the ligands the V(III) complexes are much more stable in the presence of acid than is the complex with salen. Equilibrium constants for the disproportionation of the oxovanadium (IV) complexes were evaluated. The vanadium(III) complexes reduce dioxygen to form two oxo ligands. In the presence of acid some of the complexes investigated participate in a catalytic electroreduction of dioxygen.

The otherwise slow reaction between O_2 and decamethylferrocene or V^(III)(SB)^+ is significantly accelerated if these three reactants are present. In the absence of acid the reduction of O_2 proceeds stoichiometrically to yield the two oxo groups in (SB)V^(IV)O. In the presence of acid the reaction becomes catalytic and the O_2 is reduced to H_2O. Preliminary results on the kinetics of the O_2 reduction was presented.

Electrochemical study on vanadium-naphophen (naphophen = N, N'-1, 2-phenylenebis-(2-hydroxy-l-naphthylideneiminate)) complexes in acetonitrile containing excess acetic anhydride was also conducted. V^(IV)O(naphophen) was found to undergo deoxygenation by two equivalents of acid to give V^(IV)(naphophen)^(2+), which had not persisted in previous study. Several reactions proposed previously to describe the chemistry of vanadium-Schiff base complexes in acetonitrile were directly demonstrated with this species.

The oxovanadium-naphophen complexes were found to spontaneously adsorb on EPG electrodes. They also exhibited remarkable stability towards acid after adsorption. Their surface electrochemical behavior was studied and characterized using cyclic voltammetry.

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