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

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journal contribution
posted on 2022-07-13, 15:14 authored by Brian J. Cook, Melissa Barona, Samantha I. Johnson, Simone Raugei, R. Morris Bullock
Weakening and cleaving N–H bonds is crucial for improving molecular ammonia (NH3) oxidation catalysts. We report the synthesis and H-atom-abstraction reaction of bis­(ammonia)­chromium porphyrin complexes Cr­(TPP)­(NH3)2 and Cr­(TMP)­(NH3)2 (TPP = 5,10,15,20-tetraphenyl-meso-porphyrin and TMP = 5,10,15,20-tetramesityl-meso-porphyrin) using bulky aryloxyl radicals. The triple H-atom-abstraction reaction results in the formation of CrV(por)­(N), with the nitride derived from NH3, 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 CrIII(por), with scission of the CrN bond. Computational analysis illustrates the progression from CrII to CrV and evaluates the energetics of abstracting H atoms from CrII-NH3 to generate CrVN. The formation and isolation of CrV(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.

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