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 (St = 2), tetrahedral complex [FeII(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 FeIIN4 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) (SFe = 2). Intramolecular antiferromagnetic coupling between the π radical ligands (Srad = 1/2) and the ferrous ion (SFe = 2) yields the observed St = 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 (St = 2) are shown to contain a neutral α-diimine ligand, a high spin ferrous ion, and two chloride ligands.

本研究通过对四棱柱形（α-二胺）铁二氯化物及其四棱柱形双（α-二胺）铁化合物系列进行莫塞巴谱学、磁化率测量和X射线晶体学分析，深入探讨了其电子结构。此外，还进行了断裂对称密度泛函理论（B3LYP）计算。通过对这些复杂体系电子结构的详尽理解，合成了具有抗磁性（St = 2）的四棱柱形复合物[FeII(4L)2]，其中（4L）1−代表抗磁性单负离子N-叔丁基喹啉酰胺，以此作为包含四棱柱形FeIIN4几何构型和单高自旋二价铁离子的Werner型复合物的基准。与通常将双（α-二胺）铁(0)复杂物的电子结构描述为低价铁(0)物种，并含有两个中性α-二胺配体的说法不同，本研究确立了一个新的观点：这些复杂物实际上含有两个（α-二亚胺基）1−•π自由基单负离子和高自旋二价铁离子（在四棱柱N4几何构型中）（SFe = 2）。π自由基配体（Srad = 1/2）与二价铁离子（SFe = 2）之间的分子内反铁磁耦合导致了观察到的St = 1基态。研究表明，α-二胺是氧化还原非惰性配体，具有能量上低位的反键π*最低未占据分子轨道，能够从过渡金属离子那里接受一个或两个电子。（α-二胺）FeCl2复合物（St = 2）被证明包含一个中性α-二胺配体、一个高自旋二价铁离子和两个氯配体。