The development of efficient electrocatalysts for ammonia oxidation is crucial for utilizing ammonia as a fuel. Herein, by employing a hybrid bipyridine-carboxylate-sulfonate ligand, we successfully optimized the primary and secondary coordination spheres of a Ru-based ammonia oxidation catalyst. The resulting Ru(bcs-κ3O,N,N)(Py)2(NH3) (Ru-bcs-NH3; bcs = 2,2'-bipyridine-6-carboxylate-6'-sulfonate; Py = pyridine) catalyst achieves a record turnover number (TON = 1.2 × 104, after heterogenization) and catalytic rate (kobs = 230 s-1) among reported molecular systems in organic phase. Compared with the analogues Ru-bds-NH3 (bds = 2,2'-bipyridine-6,6'-disulfonate) and Ru-bda-NH3 (bda = 2,2'-bipyridine-6,6'-dicarboxylate), we revealed the division of labor between carboxylate and sulfonate groups during ammonia oxidation by Ru-bcs-NH3: the coordinated carboxylate provides strong electron donation to lower the redox potential and the energy barrier of the rate-determining step, while the dangling sulfonate group enables efficient proton shuttling without competing with the key active site. This work establishes ruthenium complexes with bifunctional ligand environments as promising molecular platforms for the efficient conversion of ammonia to nitrogen.
Xiong et al. (Thu,) studied this question.