Selective NH3-to-N2H4 Conversion Electrocatalysed by Ruthenium(II)-Cymene Complexes

Abstract

A series of ruthenium(II)-cymene complexes [(η⁶-p-cymene)Ru(pp)Cl] (1 - 4) and corresponding NH₃-ligated complexes [(η⁶-p-cymene)Ru(pp)(NH₃)]PF₆ ([1-NH₃]PF₆ - [4-NH₃]PF₆), where cymene = 4-isopropyltoluene, pp^- = pyridylpyrrole ligand, have been designed and synthesized. The structural modifications of pp^- ligands are accomplished through the attributions of an increasing number of electron-donating methyl group on pyrrole unit. The solid-state structural analysis show that these complexes have a typical piano-stool structure. The electrochemical studies of these complexes illustrate that introduction of methyl group at the pp^- ligand can greatly decrease oxidation potential of Ru^III/II from 0.49 V vs. Cp₂Fe^+/0 for [1-NH₃]PF₆ to 0.16 V vs. Cp₂Fe^+/0 for [4-NH₃]PF₆. The controlled potential coulometry experiments displays these complexes have selective catalysis for oxidation of NH₃ to N₂H₄ with turnover number up to 453.2 at E_app 0.8 V vs. Cp₂Fe^+/0 for [4-NH₃]PF₆ complex. The kinetical and calculated thermodynamical studies show that bimolecular coupling of Ru^II-aminyl pathway and ammonia nucleophilic attack of Ru^IV-imide (generated from disproportionation of Ru^III-amide) pathway are involved in N-N formation.