Weakening the N-H Bonds of NH 3 Ligands: Triple Hydrogen-Atom Abstraction to Form a Chromium(V) Nitride.

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 = 5,10,15,20-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 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.