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 (H 2 L)(pic) 2 ] 2+ [ 1 , H 2 L = 6,6'-di(1H-pyrazol-3-yl)-2,2'-bipyridine, pic = 4-picoline], where H 2 L is a tetradentate ligand with a bipyridyl unit connected to two pyrazoles. 1 functions as an efficient electrocatalyst for the oxidation of NH 3 to N 2 , 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) 3 N •+ as an oxidant, with a turnover number for N 2 reaching 41. A novel binuclear complex, [Ru III (L)(pic) 2 (N 2 )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 N 2 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 N 2 upon treating with NH 3 . 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 NH 3 on the intermediate.