PNNL

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 ammo-nia, 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 nitrogen pseudo-octahedral environment. We investigated N-N bond formation via NH3 nucleophilic attack on M-NH and M-N intermediates, compared to bimolecular coupling of M-NHx in-termediates. We evaluated the reactivity of (TPP)Fe(NH3)2 towards N-N bond formation via a hydrazine pathway, and found amide-amide coupling to form a bridged hydrazido complex to be the most favorable pathway for N-N bond for-mation. Further investigation of possible 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 homo-lytic 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.