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

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(dppMe, 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.