Catalytic Ammonia Oxidation to Dinitrogen by a Nickel Complex

Abstract

We report a nickel complex for catalytic oxidation of ammonia to dinitrogen under ambient conditions. Using the aryloxyl radical 2,4,6‐tri‐tert‐butylphenoxyl (tBu3ArO⋅) as a H atom acceptor to cleave the N−H bond of a coordinated NH3 ligand up to 56 equiv of N2 per Ni center can be generated. Employing the N‐oxyl radical 2,2,6,6‐(tetramethylpiperidin‐1‐yl)oxyl (TEMPO⋅) as the H‐atom acceptor, up to 15 equiv of N2 per Ni center are formed. A bridging Ni‐hydrazine product identified by isotopic nitrogen (15N) studies and supported by computational models indicates the N−N bond forming step occurs by bimetallic homocoupling of two paramagnetic [Ni]−NH2 fragments. Ni‐mediated hydrazine disproportionation to N2 and NH3 completes the catalytic cycle.

Electrochemical Ammonia Oxidation with Molecular Catalysts

“…In 2021, Peters et al described stepwise N 2 H 4 formation with a Ni­(II)–NH 2 radical . In 2023, Mock and co-workers developed a catalytic version with a half-sandwich Ni­(II) N-heterocyclic carbene (NHC) complex with hydrogen atom abstraction reagents . In 2021, MacFarlane et al studied AO on heterogeneous Cu(0) metal and ascribed the catalytic activity to an intermediary [Cu II (OH) 4 ] 2– ion .…”

“…39 In 2023, Mock and co-workers developed a catalytic version with a half-sandwich Ni(II) Nheterocyclic carbene (NHC) complex with hydrogen atom abstraction reagents. 40 In 2021, MacFarlane et al studied AO on heterogeneous Cu(0) metal and ascribed the catalytic activity to an intermediary [Cu II (OH) 4 ] 2− ion. 41 Though Cu is nature's choice to oxidize ammonia in biological systems, it was not until recently that it gained wider attention.…”

Synthesis and Reactivity of Fe(II) Complexes Containing Cis Ammonia Ligands

“…We later reported catalytic NH 3 oxidation to N 2 by [Ru­(tetramesitylporphyrin)­(NH 3 ) 2 ] using organic aryloxy compounds as hydrogen atom abstraction (HAA) reagents . These HAA reagents have also been used for oxidation of M–NH 3 complexes with Cr, Mo, Mn, Os, and Ni complexes and have the potential to function as mediators that are regenerated at an electrode . Progress has also been made in homogeneous electrochemical ammonia oxidation using Mn, Fe, Cu, and Ru .…”

Development of a Nickel-Catalyzed N–N Coupling for the Synthesis of Hydrazides

“…Aiming to achieve more consistent results, we explored well-defined Ni complexes and were intrigued by reports of NHC-ligated Ni­(II) half-sandwich catalysts, which have recently been utilized for catalytic oxidative N–N coupling of ammonia to dinitrogen. − These easily synthesized and air-stable complexes have been implicated to undergo hydride-mediated reduction to Ni­(I). , Additionally, the substituent cyclopentadienyl (Cp) ligand has demonstrated surprisingly facile equilibration among η 5 , η 3 , and η 1 binding modes, which we hypothesized could satisfy the previously observed requirement for a coordinatively unsaturated catalyst. , After evaluating a variety of Ni half-sandwich catalysts, we found that the use of 7a afforded a reproducible 61% yield of the desired product (entry 10). Exploration of solvent effects revealed that a 1:4 mixture of CH 2 Cl 2 /THF further improved the yield to 80% (entry 11).…”