Understanding the factors governing the ammonia oxidation reaction by mononuclear ruthenium complex

Abstract

Precise regulation of the active site of molecular catalysts is appealing because it could catch a glimpse of the catalytic mechanism and possibly provide a new strategy for catalyst design. A ruthenium complex [Ru(dpp_{Me, COMe})(bipy)(Cl)] (CSU-3) containing -Me and -COMe substituted dipyridylpyrrole as pincer ligand was designed and synthesized. Complex CSU-3 featured the Cl- ligand at axial position as active site for ammonia oxidation (AO), which is structurally analogous with AO catalyst [Ru(trpy)(dmabpy)(NH3)][PF6]2 (1) bearing terpyridine ligand, but is different from AO catalyst [Ru(dpp)(bipy)(NH₃)] (CSU-2) containing unsubstituted dipyridylpyrrole as hemilabile ligand with active site at equatorial position. To insight into the role of active-site and ligand regulation in AO reaction, structure, electrochemical properties of CSU-3 and its catalytic performance and mechanism for AO reaction are comparably studied. Complex CSU-3 has good selective catalytic perforamnce for oxidation of ammonia to hydrazine with turnover frequency (TOF) of 258.8 h^-1 and N₂H₄ formation selectivity of 84.7% at Eapp of 1.0 V . The DFT calculation reveal N₂H₄ as a dominant product is generated via an ammonia nucleophilic attack of Ruthenium(IV)-imide to form N₂H₄ followed by N₂H₄-by-NH₃ substitution