Selective NH₃-to-N₂H₄ conversion electrocatalysed by ruthenium(ii)-cymene complexes

A series of ruthenium(ii)-cymene complexes [(η ⁶-p-cymene)Ru(pp)Cl] (1-4) and corresponding NH₃-ligated complexes [(η ⁶-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 Ruⁱⁱ-aminyl species and the pathway of ammonia nucleophilic attack of Ru^iv-imide (generated from the disproportionation of Ruⁱⁱⁱ-amide) are involved in N-N formation.