Computational Investigations of the Reactivity of Metalloporphyrins for Ammonia Oxidation

Abstract

Density functional theory and molecular dynamics simulations were used to assess the ability of tetraphenylporphyrin (TPP)M complexes (where M = Cr, Mn, Fe, Co, Ni, Mo, Ru, W, and Os) to coordinate and weaken the N–H bonds of ammonia, as well as their reactivity towards N–N bond formation for N2 generation. Compared to other metalloporphyrins, bis-ammonia complexes (TPP)Mo(NH3)2 and (TPP)W(NH3)2 exhibit low and level N–H BDFEs due to a stabilized (TPP)M(NH3)(NH) intermediate by multiple metal–ligand bonding. These results resemble those previously obtained for polypyridyl metal complexes, suggesting that broad trends in reactivity towards N–H bond cleavage are more metal-dependent rather than ligand-dependent for a metal in a pseudo-octahedral environment with nitrogen-based ligands. We investigated N–N bond formation via NH3 nucleophilic attack on M–NH and M–N intermediates, compared to bimolecular coupling of M-NHx intermediates. We evaluated the reactivity of (TPP)Fe(NH3)2 towards N–N bond formation through a hydrazine pathway, and found amide-amide coupling to form a bridged hydrazido complex to be the most favorable pathway for N–N bond formation. Further investigation of potential N–N bond formation pathways by reaction with NH3 led us to identify a possible FeIII-•NH species with significant aminyl character that bypasses the nucleophilic attack of NH3 and that promotes homolytic N–H bond cleavage of ammonia. This reaction forms a Fe-NH2 moiety and a transient •NH2 radical that subsequently forms an N–N bond with the Fe-NH2 moiety to form a (TPP)Fe(NH3)(N2H4) species. These results indicate the need to evaluate the radical character of imido species and their reactivity towards N–H bond cleavage of ammonia.