Estimations of Fe–N₂ 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

Abstract

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.

Stability and bonding of carbon(0)‐iron−N2 complexes relevant to nitrogenase co‐factor: EDA‐NOCV analyses

“…The nature and strength of the FeN N 2 bonds in the ground state hypothetical [LP 2 S 3 Fe‐N 2 ] n− (n = 1, 2, 3) complexes ( 1‐T , 2‐D , 3‐T ) (Table 4) are comparable to those of the previously reported C‐Fe(0/I/II)N 2 complexes of type A experimentally reported by Peters et al 3,7,13 (Figures 1 and 12) and corresponding theoretical (EDA‐NOCV) analysis from our group 21 . The corresponding value for (C π ‐ ring )Fe(0)N 2 bond of complex of type B reported by Holland et al 9,15a is significantly higher by 10–20 kcal/mol 20 …”

“…21 The corresponding value for (C πring )Fe(0) N 2 bond of complex of type B reported by Holland et al 9,15a is significantly higher by 10-20 kcal/mol. 20 The pairwise interactions resulting from the breakdown of the total orbital interactions (ΔE orb ) indicates three major contributions (Tables 4 and 5). The deformation densities and the associated frag- S5) as interacting fragments.…”

“…The EDA‐NOCV method 17–19 is more appropriate in explaining the nature of the bond as one of the major strengths of the method is its ability to provide the best bonding model to represent the bonding situation in the equilibrium geometry 20,21 . However, the EDA‐NOCV analysis of paramagnetic species is quite challenging 16,20,21 . The N 2 has been considered in singlet and ligand‐Fe in different spin states for our EDA‐NOCV fragmentations and analyses 17,18 .…”

“…20,21 However, the EDA-NOCV analysis of paramagnetic species is quite challenging. 16,20,21 The N 2 has been considered in singlet and ligand-Fe in different spin states for our EDA-NOCV fragmentations and analyses. 17,18 Charge flow has been shown by deformation densities, red!blue.…”

“…The C‐atoms of model Fe‐complexes developed by both the research groups of Peters and Holland are not as electron rich as it is in FeMoco (C 4− ). Recently, the EDA‐NOCV analyses 17–19 of a few model complexes showed 20,21 that the light element (C) does not play a significant role during N 2 binding rather, the role of phosphine or sulfur atoms is higher towards the binding of N 2 via π‐backdonation. Notably, the central carbon atoms of those complexes accept a little electron density from Fe‐centre during N 2 binding, which has been concluded from the deformation of electron densities.…”