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 to [4-NH₃]PF₆), where cymene = 4-isopropyltoluene and pp⁻ = pyridylpyrrole ligand, have been designed and synthesized. Structural modifications of pp⁻ ligands are accomplished through the use of an increasing number of electron-donating methyl groups on the pyrrole unit. Solid-state structural analysis shows that these complexes have a typical piano-stool structure. Electrochemical studies of these complexes illustrate that the introduction of a methyl group to the pp⁻ ligand can greatly decrease the 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. Controlled potential coulometry experiments show that these complexes exhibit selective catalysis for the oxidation of NH₃ to N₂H₄ with a turnover number of up to 453.2 at E_app 0.8 V vs. Cp₂Fe^+/0 for the [4-NH₃]PF₆ complex. Kinetic and theoretical thermodynamic studies show that the pathway of bimolecular coupling of Ru^II-aminyl species and the pathway of ammonia nucleophilic attack of Ru^IV-imide (generated from the disproportionation of Ru^III-amide) are involved in N–N formation.