Estimations of Fe-N 2 Intrinsic Interaction Energies of Iron-Sulfur/Nitrogen-Carbon Sites: A Deeper Bonding Insight by EDA-NOCV Analysis of a Model Complex of the Nitrogenase Cofactor.

The MoFe 7 S 9 C 1- unit of the nitrogenase cofactor (FeMoco) attracts chemists and biochemists due to its unusual ability to bind aerial dinitrogen (N 2 ) at ambient condition and catalytically convert it into ammonia (NH 3 ). The mode of N 2 binding and its reaction pathways are yet not clear. An important conclusion has been made based on the very recent synthesis and isolation of model Fe(I/0)-complexes with sulfur-donor ligands under the cleavage of one Fe-S bond followed by binding of N 2 at the Fe(0) center. These complexes are structurally relevant to the nitrogenase cofactor (MoFe 7 S 9 C 1- ). Herein, we report the EDA-NOCV analyses and NICS calculations of the dinitrogen-bonded dianionic complex Fe 0 -N 2 ( 1 ) (having a C Ar ← Fe π-bond) and monoanionic complex Fe I -N 2 ( 2 ) (having a C Ar -Fe σ-bond) to provide a deeper insight into the Fe-N 2 interacting orbitals and corresponding pairwise interaction energies (EDA-NOCV = energy decomposition analysis coupled with natural orbital for chemical valence; NICS = nucleus-independent chemical shifts). The orbital interaction in the Fe-N 2 bond is significantly larger than Coulombic interactions, with major pairwise contributions coming from d(Fe) orbitals to the empty π* orbitals of N 2 (three Fe → N 2 ). Δ E int values are in the range of -61 to -77 kcal mol -1 . Very interestingly, NICS calculations have been carried out for the fragments before and after binding of the N 2 molecule. The computed σ- and π-aromaticity values are attributed to the position of the Fe atoms, oxidation states of Fe centers, and Fe-C bond lengths of these two complexes.