Hydrigen-Evolving Systems. 4. The Reduction of Molecular Nitrogen and of Other Substrates in the Vanadium(II)-Pyrocatechol System

Article abstract of DOI:10.1021/ja00400a029

Reactions of molecular nitrogen and of other substrates with vanadium(II)-pyrocatechol complexes occur in two-electron steps at monomeric V(II) centers rather than by collective electron-transfer processes involving polynuclear clusters of vanadium(II)-pyrocatechol complexes.The reduction of acetylene, for example, obeys the stoichiometic equation V^IIPC + C2H2 + 2H⁺ -> V^IVPC + C2H4 at high substrate concentrations.At low concentrations of substrate acetylene or with less reactive substrates S, the overall reaction stoichiometries are expressed more closely by the equation 2V^IIPC + S + 2H⁺ -> 2V^IIIPC + H2S, primarily because of the concurrent side reaction V^IIPC + V^IVPC -> 2V^IIIPC.In the absence of added reducible substrates, H2 is evolved spontaneously according to the equation 2V^IIPC + 2H⁺ -> 2V^IIIPC + H2.The reduction of N2 to NH3 occurs in a stepwise fashion with N2H2 and N2H4 as the intermediates.The high reduction potential of N2 to N2H2 and the tendency of N2H2 to decompose into the elements are mainly responsible for the extreme dependence of the yields of NH3 on pH and solvent.