Ruthenium(III)–aminopolycarboxylato complexes active for the reduction of the N–N bond of hydrazine and phenylhydrazine in aqueous acidic media

Abstract

Interactions of hydrazines N₂H₄X⁺ (X = H or Ph) with tri-, tetra- and penta-chelated ruthenium(III)–aminopolycarboxylic acid complexes giving the respective monomeric hydrazinium (Ru^III–N₂H₄X⁺) adducts have been investigated by potentiometry, spectrophotometry and voltammetry in aqueous acidic solution at 25 °C. The deprotonation and metal hydrolysis constants of the complexes and their N₂H₄X⁺ adducts in 0.1 M Na₂SO₄ solution were determined. At pH 2.8, the complexes exhibited a quasi-reversible one-electron reduction wave of Ru^III → Ru^II in sampled dc in the potential range between −0.16 and −0.37 V vs. SCE, while their hydrazinium adducts obtained in situ by adding an excess of N₂H₄X⁺ showed an additional two-electron reduction wave assigned to Ru^III–N₂H₄X⁺ → Ru^I–N₂H₄X⁺ in the potential range of −0.02 to −0.35 V vs. SCE. The species Ru^I–N₂H₄X⁺ on successive decomposition and hydrolysis give one mole of each of NH₃, NH₂X and ruthenium(III) species. Further, the Ru^III–N₂H₄X⁺ complexes have been used as electro-catalysts for the reduction of N₂H₄X⁺ to NH₃ and NH₂X at a mercury pool cathode in acidic solutions of pH 1.9 and 2.8. The quantity of ammonia produced in all cases is linear with time. The E_1/2 of Ru^III–N₂H₄X⁺ → Ru^I–N₂H₄X⁺ and the turnover number are correlated with the sigma basicity (ΣpK_a) of the aminopolycarboxylic acids and the results are discussed in terms of the hydrolytic tendency of the metal, the number of co-ordinating groups and the steric repulsion caused by the increase in size of the aminopolycarboxylic acid.