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(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 Eapp 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.