title: Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging Diiron Dinitrogen Complexes.
abstract: 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.
authors.affiliation: Department of Chemistry, Yale University , 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Sean F
authors.initials: SF
authors.lastname: McWilliams
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University , Fargo, North Dakota 58105, United States.
authors.forename: Kenton R
authors.initials: KR
authors.lastname: Rodgers
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University , Fargo, North Dakota 58105, United States.
authors.forename: Gudrun
authors.initials: G
authors.lastname: Lukat-Rodgers
authors.affiliation: Department of Chemistry, Yale University , 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Brandon Q
authors.initials: BQ
authors.lastname: Mercado
authors.affiliation: Department of Chemistry, Yale University , 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Katarzyna
authors.initials: K
authors.lastname: Grubel
authors.affiliation: Department of Chemistry, Yale University , 225 Prospect Street, New Haven, Connecticut 06520, United States.
authors.forename: Patrick L
authors.initials: PL
authors.lastname: Holland
mesh_terms: D008672:Metals, Alkali / Q000737:chemistry*
chemical_list: D008672:Metals, Alkali