An iron-free nitrogenase? Computational estimation of the ability to bind/activate N₂ in manganese and cobalt complexes

Abstract

Activation of N₂ to ammonia occurs in nitrogenases at an unusual carbide-iron-sulfur active site. To deconvolute the factors that allow the nitrogenase Fe to bind N₂, we have recently reported a density functional theory (DFT) study on a series of Fe(H₂O)_nR(N₂) complexes, Fe(0)/Fe(I)/Fe(II)/Fe(III), and R = H₂O, HO⁻, CH₂²⁻, or H₂S. Reported here are computations on Mn and Co complexes in settings and with methodology identical to those previously employed for the related Fe systems in order to compare the ability of these metals to activate N₂. None of the Co(II) and Co(III) models form clear bonds with N₂; Co(0) and Co(I) do form such bonds but are not known to be possible (at least not in settings with available coordination positions) in biological systems. Mn(0) and Mn(I) are also found to efficiently bind and activate N₂; however, neither are known in biological systems. Mn(II), especially with a methylene ligand, appears able to form a weak metal-dinitrogen bond. However, considering that Fe(I) systems but not Mn(I) systems are known in biological inorganic centers, and considering that Fe(I)/Mn(I) do bind/activate N₂ more efficiently than Fe(II)/Mn(II), iron may be argued to be the better choice for a nitrogenase active site.

Keywords:

• Nitrogenase
• Manganese
• Cobalt
• Dinitrogen
• DFT

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Funding from the Romanian Ministry of Education and Research (projects PN-III-P4-ID-PCCF-2016-0142, PN-III-P1-1.1-PD-2021-0279 and PFE-550-STAR-UBB-N) is gratefully acknowledged.