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

摘要

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 LFeNNFeLmodel, which show that the two metals surprisingly do not competefor back-bonding to the same π* orbital of N2, evenwhen the ligand planes are parallel. This diiron system can toleratedistortion of the ligand planes through compensating orbital energychanges, and thus, a range of ligand orientations can give very similarenergies.