Facile electroreduction of perrhenate in weakly acidic citrate and oxalate media

Article abstract of DOI:10.1021/ic50224a037

In weakly acidic solution, oxalate and citrate markedly increase the ease of reduction of ReO₄^- through reversible formation of 1:1 complexes. The citrate complex (ReO₄·H₂Cit)^2- is sufficiently stable that it yields a diffusion limited current, whereas the oxalate complex (ReO₄·H₂Ox)^- is formed only as a transient intermediate and it yields a kinetic limited current. Below pH 3 the mechanism of formation of the citrate complex corresponds to ReO₄^- + H₃Cit ? (ReO₄·H₂Cit)^2- + H⁺ while above pH 3 it corresponds to ReO₄^- + H₂Cit^- ? (ReO₄·H₂Cit)^2-. The enhanced ease of reduction of the citrate and oxalate complexes is ascribed to expansion of the Re coordination sphere from 4 to, e.g., 6 through formation of chelated structures by a concerted process in which the incoming ligand transfers protons to coordinated oxo groups. The hydroxyl group of citric acid is necessary for formation of an easily reduced Re(VII) complex, and Dreiding models indicate that citrate is bidentate and occupies one face of the coordination octahedron, with the hydroxyl group H bonded to a coordinated oxo group.