Catalytic Selective Conversion of Ammonia into Hydrazine by a Ru^II(trpy) Complex Bearing a Pyridylpyrrole Ligand

Interested by the rapid growth of Ru-based complexes as molecular ammonia oxidation catalysts, this article proposes a RuII(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)-1H-pyrrole (HL1), 6-(1H-pyrrol-2-yl)-2,2′-bipyridine (HL2), and 2-(3,4,5-trimethyl-1H-pyrrol-2-yl)pyridine (H2L3) ligands, followed by anionic metathesis of Cl– by PF6–, affords [Ru(K3-N,N′,N″-L1)(trpy)](PF6) (Ru1), [Ru(K3-N,N′,N″-L2)(trpy)](PF6) (Ru2),and [Ru(K3-N,N′,N″-L3)(trpy)] (Ru3), respectively. Ligad L1– in Ru1 is hemilabile and readily opens one armed-pyridine for incoming NH3 coordination to generate [Ru(K2-N,N′-L1)(trpy)(NH3)](PF6) (Ru1-NH3). Unlike Ru2 and Ru3, which are entirely devoid of catalysis for ammonia oxidation, complexes Ru1 and Ru1-NH3 have highly selective electrocatalysis for the conversion of ammonia into hydrazine with 97.8% and 98.1% selectivity, respectively. The TOFmax and Faradaic Efficiency (FE) of Ru1 and Ru1-NH3 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-NH3 undergoes the bimolecular coupling of RuIII-aminyl and RuIV-iminyl to form N2H4. The foot of the wave analysis (FOWA) displays the corresponding apparent rate constant of 4.26 × 106 M–1 s–1.