Dual Pathways in Catalytic Ammonia Oxidation by a Ruthenium Complex Bearing a Tetradentate Bipyridine-Bipyrazole Ligand : Isolation of a Diruthenium Intermediate with a μ-Hexazene Derivative

We report herein chemical and electrochemical ammonia oxidation (AO) catalyzed by a Ru complex, [Ru^II(H₂L)(pic)₂]²⁺ [1, H₂L = 6,6′-di(1H-pyrazol-3-yl)-2,2′-bipyridine, pic = 4-picoline], where H₂L is a tetradentate ligand with a bipyridyl unit connected to two pyrazoles. 1 functions as an efficient electrocatalyst for the oxidation of NH₃ to N₂, with a low overpotential of 0.51 V vs Fc^+/0 and a Faradaic efficiency of 96%. 1 also undergoes catalytic chemical AO using (4-BrPh)₃N^•+ as an oxidant, with a turnover number for N₂ reaching 41. A novel binuclear complex, [Ru^III(L)(pic)₂(N₂)Ru^III(L)(pic)₂]⁴⁺ (2), was isolated and structurally characterized in the catalytic chemical AO by 1. Complex 2 possesses a zigzag dianionic μ-hexazene unit where the N₂ derived from ammonia oxidation is bonded to the pyrazoles, with an N_N2-N_N2 bond length of 1.3091(70) Å. 2 readily releases N₂ upon treating with NH₃. Based on experimental and DFT studies, in chemical AO the formation of an N-N bond is proposed to occur via bimolecular coupling of a ruthenium pyrazole imido intermediate to give 2; while in electrochemical AO the N-N bond is formed by nucleophilic attack of NH₃ on the intermediate. © 2024 American Chemical Society.`