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−, CH2 2−, 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.