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)(NH(3))][PF(6)](2) (1) bearing a terpyridine ligand, but different from AO catalyst [Ru(dpp)(bipy)(NH(3))] (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 N(2)H(4) formation selectivity of 84.7% at E (app) of 1.0 V. The DFT calculations reveal that N(2)H(4) as a dominant product is generated via an ammonia nucleophilic attack of ruthenium(iv)-imide to form N(2)H(4) followed by N(2)H(4)-by-NH(3) substitution.