Catalytic Selective Conversion of Ammonia into Hydrazine by a Ru<sup>II</sup>(trpy) Complex Bearing a Pyridylpyrrole Ligand
Zhou Chen, Xi Zhang, Shan Zhao, Si-Dan Zhong, Xiao-Lv Ding, Shun-Ping Yang, Fuxing Pan, Piao He, Xiao‐Yi Yi
Abstract
Interested by the rapid growth of Ru-based complexes as molecular ammonia oxidation catalysts, this article proposes a Ru II (trpy) complex bearing various pyridylpyrrole ligands as a model complex for the electrocatalytic oxidation of ammonia. Treatment of [Ru(trpy)(Cl) 3 ] (trpy = 2,2′:6′,2″-terpyridine) with deprotonated 2,5-di(pyridin-2-yl)-1 H -pyrrole (HL 1 ), 6-(1 H -pyrrol-2-yl)-2,2′-bipyridine (HL 2 ), and 2-(3,4,5-trimethyl-1 H -pyrrol-2-yl)pyridine (H 2 L 3 ) ligands, followed by anionic metathesis of Cl – by PF 6 –, affords [Ru( K 3 - N, N ′, N ″-L 1 )(trpy)](PF 6 ) ( Ru1 ), [Ru( K 3 - N, N ′, N ″-L 2 )(trpy)](PF 6 ) ( Ru2 ),and [Ru( K 3 - N, N ′, N ″-L 3 )(trpy)] ( Ru3 ), respectively. Ligad L 1 – in Ru1 is hemilabile and readily opens one armed-pyridine for incoming NH 3 coordination to generate [Ru( K 2 - N, N ′-L 1 )(trpy)(NH 3 )](PF 6 ) ( Ru1-NH 3 ). Unlike Ru2 and Ru3, which are entirely devoid of catalysis for ammonia oxidation, complexes Ru1 and Ru1-NH 3 have highly selective electrocatalysis for the conversion of ammonia into hydrazine with 97.8% and 98.1% selectivity, respectively. The TOF max and Faradaic Efficiency (FE) of Ru1 and Ru1-NH 3 reach 316.4 h –1 and 99.2% and 360.8 h –1 and 99.1%, respectively. The cyclic voltammetric measurements combined with density functional theory (DFT) calculations illustrate that Ru1-NH 3 undergoes the bimolecular coupling of Ru III -aminyl and Ru IV -iminyl to form N 2 H 4 . The foot of the wave analysis (FOWA) displays the corresponding apparent rate constant of 4.26 × 10 6 M –1 s –1 .