PNNL

Abstract

Weakening and cleaving N-H bonds is crucial for improving molecular ammonia oxidation catalysts. We report the synthesis and hydrogen atom abstraction chemistry of bis(ammonia) Cr 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 hydrogen atom abstraction reaction results in the formation of CrV(por)(ºN), with the nitride deriving from ammonia, as indicated by UV-Vis and FT-IR spectroscopy, and a single crystal structural determination of Cr(TPP)(ºN). Subsequent oxidation of this CrV-nitrido complex results in the formation of CrIII(por), with scission of the CrºN bond. A computational analysis illustrates the progression from CrII to CrV and evaluates the energetics of abstracting hydrogen atoms from CrII-NH3 to generate CrVºN. The formation and isolation of CrV(por)(ºN) illustrates the stability of these species and the need to chemically activate the nitride moiety for atom transfer or N-N coupling reactivity.