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Title: Computational Investigations of the Reactivity of Metalloporphyrins for Ammonia Oxidation

Journal Article · · Topics in Catalysis
ORCiD logo [1]; ORCiD logo [1];  [2]; ORCiD logo [1]; ORCiD logo [1]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
  2. Xavier Univ. of Louisiana, New Orleans, LA (United States)
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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 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 intermediates. 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 formation. 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 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. Furthermore, these results indicate the need to evaluate the radical character of imido species and their reactivity towards N-H bond cleavage of ammonia.

Research Organization:
Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES)
Grant/Contract Number:
AC05-76RL01830
OSTI ID:
1846384
Report Number(s):
PNNL-SA-165238
Journal Information:
Topics in Catalysis, Journal Name: Topics in Catalysis Journal Issue: 1-4 Vol. 65; ISSN 1022-5528
Publisher:
SpringerCopyright Statement
Country of Publication:
United States
Language:
English

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  • Marenich, Aleksandr V.; Cramer, Christopher J.; Truhlar, Donald G.
  • The Journal of Physical Chemistry B, Vol. 113, Issue 18, p. 6378-6396 https://doi.org/10.1021/jp810292n
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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
N-H bond cleavage
N-N bond formation
ammonia oxidation
density functional theory
electrocatalysis
electrochemistry
metalloporphyrins