Weakening the N–H Bonds of NH₃ Ligands: Triple Hydrogen-Atom Abstraction to Form a Chromium(V) Nitride

Abstract

Weakening and cleaving N−H bonds is crucial for improving molecular ammonia (NH 3 ) oxidation catalysts. We report the synthesis and H-atom-abstraction reaction of bis(ammonia)chromium porphyrin complexes Cr(TPP)(NH 3 ) 2 and Cr(TMP)(NH 3 ) 2 (TPP = 5,10,15,10,15, using bulky aryloxyl radicals. The triple H-atom-abstraction reaction results in the formation of Cr V (por)(�N), with the nitride derived from NH 3 , as indicated by UV−vis and IR and single-crystal structural determination of Cr(TPP)(�N). Subsequent oxidation of this chromium(V) nitrido complex results in the formation of Cr III (por), with scission of the Cr�N bond. Computational analysis illustrates the progression from Cr II to Cr V and evaluates the energetics of abstracting H atoms from Cr II -NH 3 to generate Cr V �N. The formation and isolation of Cr V (por)(�N) illustrates the stability of these species and the need to chemically activate the nitride ligand for atom transfer or N−N coupling reactivity.

Magnetic Relaxations of Chromium Nitride Porphyrinato Complexes Driven by the Anisotropic g‐Factor

“…Single‐crystals of 1 and 2 suitable for single‐crystal X‐ray diffraction (SCXRD) analyses were prepared by the solvent diffusion method using chloroform and hexane as good and poor solvents, respectively. The crystal packing and space group of 1 are different from those of previously reported [CrN(TPP)] crystallized using fluorobenzene and hexane [32] . 2 is an isostructural with those reported by Drew et al .…”

“…The crystal packing and space group of 1 are different from those of previously reported [CrN(TPP)] crystallized using fluorobenzene and hexane. [32] 2 is an isostructural with those reported by Drew et al in 1984. [33] In the present study, 1 and 2 crystallized in the space group I4/m, and no solvent molecule was accommodated in the crystal packings.…”

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

“…The solid was then dissolved in H 2 O (50 mL) and slowly added to a stirring solution of NH 4 PF 6 (3.168 g, 19.44 mmol, 2.1 equiv) in H 2 O (100 mL) to give a flocculant pale yellow solid. The mixture was stirred for 15 min before the solid was collected on a glass frit and rinsed with H 2 O (2 × 50 mL) to remove excess NH 4 [ NMe2 (NCCN)][PF 6 ] 2 . In a N 2 -filled glovebox, a 50 mL Schlenk flask was charged with 2-(1H-imidazole-1-yl)-N,N-dimethylpyridin-4amine (1.26 g, 6.69 mmol), 1,4-dioxane (35 mL), and a PTFEcoated stir bar.…”

“…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 .…”

Oxidation-induced ambiphilicity triggers N–N bond formation and dinitrogen release in octahedral terminal molybdenum(v) nitrido complexes

“…For N-H bond cleavage, coordination-induced bond weakening 10 of the N-H bond(s) of NH 3 upon ligation to a metal can be substantial and the factors that inuence the extent to which bond weakening occurs are a subject of current study. 5,[11][12][13][14][15] When weakened below a threshold of about 52 kcal mol −1 , elimination of H 2 (BDFE = 104 kcal mol −1 ) becomes thermodynamically feasible and kinetic pathways involving metal hydrides 11 or bimolecular H 2 elimination in coordinatively saturated systems 15 can lead to "spontaneous" loss of H 2 from any of the M-NH n (n = 3, 2, or 1) intermediates formed via loss of hydrogen atoms. Alternatively, use of hydrogen atom abstracting reagents like the 2,4,6-tri-tertbutylphenoxy radical (ArOc) can be employed if bond weakening of N-H to values below the strength of the ArO-H bond (74.8 kcal mol −1 (ref.…”