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

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.