Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging Diiron Dinitrogen Complexes

Alkali
metal cations
can interact with Fe–N2 complexes, potentially enhancing
back-bonding or influencing the geometry of the iron atom. These influences
are relevant to large-scale N2 reduction by iron, such
as in the FeMoco of nitrogenase and the alkali-promoted Haber–Bosch
process. However, to our knowledge there have been no systematic studies
of a large range of alkali metals regarding their influence on transition
metal–dinitrogen complexes. In this work, we varied the alkali
metal in [alkali cation]2[LFeNNFeL] complexes (L = bulky
β-diketiminate ligand) through the size range from Na+ to K+, Rb+, and Cs+. The FeNNFe
cores have similar Fe–N and N–N distances and N–N
stretching frequencies despite the drastic change in alkali metal
cation size. The two diketiminates twist relative to one another,
with larger dihedral angles accommodating the larger cations. In order
to explain why the twisting has so little influence on the core, we
performed density functional theory calculations on a simplified LFeNNFeL
model, which show that the two metals surprisingly do not compete
for back-bonding to the same π* orbital of N2, even
when the ligand planes are parallel. This diiron system can tolerate
distortion of the ligand planes through compensating orbital energy
changes, and thus, a range of ligand orientations can give very similar
energies.