Bis(α-diimine)iron Complexes: Electronic Structure Determination by Spectroscopy and Broken Symmetry Density Functional Theoretical Calculations

The electronic structure of a family comprising tetrahedral (α-diimine)iron dichloride, and tetrahedral bis(α-diimine)iron compounds has been investigated by Mössbauer spectroscopy, magnetic susceptibility measurements, and X-ray crystallography. In addition, broken-symmetry density functional theoretical (B3LYP) calculations have been performed. A detailed understanding of the electronic structure of these complexes has been obtained. A paramagnetic (S_t = 2), tetrahedral complex [Fe^II(⁴L)₂], where (⁴L)¹⁻ represents the diamagnetic monoanion N-tert-butylquinolinylamide, has been synthesized and characterized to serve as a benchmark for a Werner-type complex containing a tetrahedral Fe^IIN₄ geometry and a single high-spin ferrous ion. In contrast to the most commonly used description of the electronic structure of bis(α-diimine)iron(0) complexes as low-valent iron(0) species with two neutral α-diimine ligands, it is established here that they are, in fact, complexes containing two (α-diiminato)^1−• π radical monoanions and a high-spin ferrous ion (in tetrahedral N₄ geometry) (S_Fe = 2). Intramolecular antiferromagnetic coupling between the π radical ligands (S_rad = ¹/₂) and the ferrous ion (S_Fe = 2) yields the observed S_t = 1 ground state. The study confirms that α-diimines are redox noninnocent ligands with an energetically low-lying antibonding π* lowest unoccupied molecular orbital which can accept one or two electrons from a transition metal ion. The (α-diimine)FeCl₂ complexes (S_t = 2) are shown to contain a neutral α-diimine ligand, a high spin ferrous ion, and two chloride ligands.