Catalytic Selective Conversion of Ammonia into Hydrazineby 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 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<i>H</i>-pyrrole (HL₁), 6-(1<i>H</i>-pyrrol-2-yl)-2,2′-bipyridine (HL₂), and 2-(3,4,5-trimethyl-1<i>H</i>-pyrrol-2-yl)pyridine (H₂L₃) ligands, followed by anionic metathesis of Cl^– by PF₆^–, affords [Ru(<i>K</i>³-<i>N</i>,<i>N</i>′,<i>N</i>″-L₁)(trpy)](PF₆) (<b>Ru1</b>), [Ru(<i>K</i>³-<i>N</i>,<i>N</i>′,<i>N</i>″-L₂)(trpy)](PF₆) (<b>Ru2</b>),and [Ru(<i>K</i>³-<i>N</i>,<i>N</i>′,<i>N</i>″-L₃)(trpy)] (<b>Ru3</b>), respectively. Ligad L₁^– in <b>Ru1</b> is hemilabile and readily opens one armed-pyridine for incoming NH₃ coordination to generate [Ru(<i>K</i>²-<i>N</i>,<i>N</i>′-L₁)(trpy)(NH₃)](PF₆) (<b>Ru1-NH</b>_<b>3</b>). Unlike <b>Ru2</b> and <b>Ru3</b>, which are entirely devoid of catalysis for ammonia oxidation, complexes <b>Ru1</b> and <b>Ru1-NH</b>_<b>3</b> 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 <b>Ru1</b> and <b>Ru1-NH</b>_<b>3</b> 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 <b>Ru1-NH</b>_<b>3</b> undergoes the bimolecular coupling of Ru^III-aminyl and Ru^IV-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.