An iron-free nitrogenase? Computational estimation of the ability to bind/activate N 2 in manganese and cobalt complexes ...

Activation of N 2 to ammonia occurs in nitrogenases at an unusual carbide-iron-sulfur active site. To deconvolute the factors that allow the nitrogenase Fe to bind N 2 , we have recently reported a density functional theory (DFT) study on a series of Fe(H 2 O) n R(N 2 ) complexes, Fe(0)/Fe(I)/Fe(II)/Fe(III), and R = H 2 O, HO − , CH 2 2− , or H 2 S. 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 N 2 . None of the Co(II) and Co(III) models form clear bonds with N 2 ; 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 N 2 ; however, neither are known in biological systems. Mn(II), especially with a methylene ligand, appears able to form a weak metal-dinitrogen bond. However, ...