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, [RuII(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, [RuIII(L)(pic)2(N2)RuIII(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 NN2-NN2 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.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

Nitrogen oxyanion reduction by Co(ii) augmented by a proton responsive ligand: recruiting multiple metals

Deoxygenation of nitrite oxygen with divalent cobalt was achieved using (PNNH)CoCl2, carrying a pyridyl pincer ligand with one P(t-Bu)2 arm and one pyrazole arm. Reaction of (PNNH)CoCl2 with NaNO2 at a 2 : 5 mole ratio promptly forms equimolar (PNNH)Co(NO2)3 and (PNN)Co(NO2)(NO), {CoNO}8 with the lost ligand proton combined with removed oxo as hydroxide. These two CoIII products are characterized, showing a bent CoNO unit as the fate of the reduced nitrogen. DFT calculations are consistent with two one-electron CoII reductants binding to one NO2- bridge, then proton transfer being needed for facile N/O bond scission. A species detected by low temperature execution of this reaction contains cobalt in two oxidation states with an N,O bridging nitro group and pincer ligands that have been deprotonated, showing the active participation of the proton responsive ligand.

Metallo-supramolecular assembly of protic pincer-type complexes: encapsulation of dinitrogen and carbon disulfide into a multiproton-responsive diruthenium cage

A protic pincer complex and rigid diphosphine linker formed a cage, which incorporated N₂ and CS₂ into the multiproton-responsive cavity.