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

Abstract

Precise regulation of the active site of molecular catalysts is appealing because it could provide insights into 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 a pincer ligand, was designed and synthesized. The CSU-3 complex featured a Cl⁻ ligand at the axial position as the active site for ammonia oxidation (AO), and is structurally analogous to AO catalyst [Ru(trpy)(dmabpy)(NH₃)][PF₆]₂ (1) bearing a terpyridine ligand, but different from AO catalyst [Ru(dpp)(bipy)(NH₃)] (CSU-2) containing unsubstituted dipyridylpyrrole as a hemilabile ligand with the active site at an equatorial position. To gain insight into the role of active-site and ligand regulation in the AO reaction, the structure and electrochemical properties of CSU-3 and its catalytic performance and mechanism for the AO reaction were comparably studied. Complex CSU-3 has good selective catalytic performance for the oxidation of ammonia to hydrazine with a turnover frequency (TOF) of 258.8 h⁻¹ and N₂H₄ formation selectivity of 84.7% at E_app of 1.0 V. The DFT calculations reveal that 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.