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(HL)(pic)] [, HL = 6,6'-di(1H-pyrazol-3-yl)-2,2'-bipyridine, pic = 4-picoline], where HL is a tetradentate ligand with a bipyridyl unit connected to two pyrazoles. functions as an efficient electrocatalyst for the oxidation of NH to N, with a low overpotential of 0.51 V vs Fc and a Faradaic efficiency of 96%. also undergoes catalytic chemical AO using (4-BrPh)N as an oxidant, with a turnover number for N reaching 41. A novel binuclear complex, [Ru(L)(pic)(N)Ru(L)(pic)] (), was isolated and structurally characterized in the catalytic chemical AO by . Complex possesses a zigzag dianionic μ-hexazene unit where the N derived from ammonia oxidation is bonded to the pyrazoles, with an N-N bond length of 1.3091(70) Å. readily releases N upon treating with NH. 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 ; while in electrochemical AO the N-N bond is formed by nucleophilic attack of NH on the intermediate.