Molecular and electronic structures of mononuclear iron complexes using strongly electron-donating ligands and their oxidized forms

The ligand L2- (H2L = NAr-dimethyl-N,N'-bis(3,5-di-t-butyl-2-hydroxybenzyl)-1,2-diaminoethane) has been employed for the synthesis of two mononuclear Fe-III complexes, namely, [LFe(eta(2)-NO3)] and [LFeCl]. L2- is comprised of four strongly electron-donating groups (two tert-amines and two phenolates) that increase the electron density at the coordinated ferric ions. This property should facilitate oxidation of the complexes, that is, stabilization of the oxidized species. The molecular structures in the solid state have been established by X-ray diffraction studies. [LFeCl] is five-coordinate in a square-pyramidal coordination environment with the ligand adopting a trans-conformation, while [LFe(eta(2)-NO3)] is six-coordinate in a distorted octahedral environment with the ligand in a beta-cis conformation. The electronic structures have been studied using magnetization, EPR, Mossbauer (with and without applied field), UV-vis-NIR, and X-ray absorption spectroscopies, which demonstrate highly anisotropic covalency from the strong sigma- and pi-donating phenolates. This analysis is supported by DFT calculations on [LFeCl]. The variations of the well-understood spectroscopic data in the solid state to the spectroscopic data in solution have been used to obtain insight in the molecular structure of the two complexes in solution. While the molecular structures of the solid states are retained in solutions of nonpolar aprotic solvents, there is, however, one common molecular structure in all protic polar solvents. The analysis of the LMCT transitions and the rhombicity E/D clearly establish that both compounds exhibit a beta-cis conformation in these protic polar solvents. These two open coordination sites, cis to each other, allow access for two potential ligands in close proximity. Electrochemical analysis establishes two reversible oxidation waves for [LFeCl] at +0.55 V and +0.93 V vs Fc(+)/Fc and one reversible oxidation wave at +0.59 V with an irreversible oxidation at +1.07 V vs Fc+/Fc for [LFe(eta(2)-NO3)]. The one- and the two-electron oxidations of [LFeCl] by chronoamperometry have been followed spectroscopically. The increase of a strong band centered at 420 nm indicates the formulation of [LFeCl](+) as a Fe-III monophenoxyl radical complex and of [LFeCl](2+) as a Fe-III bisphenoxyll radical complex. These studies imply that the ligand L2- is capable of providing a flexible coordination geometry with two binding sites for substrates and the allocation of two oxidation equivalents on the ligand.