Reduced nitrogen hydride complexes of a cofacial metallodiporphyrin and their oxidative interconversion. an analysis of ammonia oxidation and prospects for a dinitrogen electroreduction catalyst based on cofacial metallodiporphyrins

Article abstract of DOI:10.1021/ja00047a016

Oxidation of a bis(ammine) complex of a cofacial metallodiporphyrin, (NH₃)₂Ru ₂DPB(1-tert-butyl-5-phenylimidazole)₂, leads to N-N bond formation and ultimately yields a bridged dinitrogen complex via bound hydrazine and diazene complexes. Each of the nitrogen hydrides involved in the successive two-electron/two-proton oxidative interconversions - the μ-diazene, μ-hydrazine, and bis(ammine) complexes - have been isolated and characterized. The electrochemical and acid-base properties of each of these complexes are reported. Mechanisms of ammonia oxidation are suggested. Stepwise two-electron/two-proton oxidations from the bis(ammine) complex to the hydrazine complex, from the hydrazine complex to the diazene complex, and from the diazene complex to the dinitrogen complex can be accomplished by tert-butyl hydroperoxide or sequential reaction of ferricinium hexafluorophosphate and a base. The prospects for electrocatalytic six-electron ammonia oxidation and electrocatalytic four-electron reduction of dinitrogen to hydrazine are discussed. On the basis of this analysis, reduction of hydrazine to ammonia by these systems is unlikely due to the energetically unfavorable thermolysis of the N-N bond of the bound hydrazine.