Design and synthesis of a dinucleating ligand system with varying terminal donor functions that provides no bridging donor and its application to the synthesis of a series of Fe(III)-µ-O-Fe(III) complexes.

Based on a rational ligand design for stabilizing high-valent {Fe(µ-O)2Fe} cores, a new family of dinucleating bis(tetradentate) ligands with varying terminal donor functions has been developed: redox-inert biomimetic carboxylates in H4julia, pyridines in susan, and phenolates in H4hilde(Me2). Based on a retrosynthetic analysis, the ligands were synthesized and used for the preparation of their diferric complexes [(julia){Fe(OH2)(µ-O)Fe(OH2)}]·6H2O, [(julia){Fe(OH2)(µ-O)Fe(OH2)}]·7H2O, [(julia){Fe(DMSO)(µ-O)Fe(DMSO)}]·3DMSO, [(hilde(Me2)){Fe(µ-O)Fe}]·CH2Cl2, [(hilde(Me2)){FeCl}2]·2CH2Cl2, [(susan){FeCl(µ-O)FeCl}]Cl2·2H2O, [(susan){FeCl(µ-O)FeCl0.75(OCH3)0.25}](ClO4)2·0.5MeOH, and [(susan){FeCl(µ-O)FeCl}](ClO4)2·0.5EtOH, which were characterized by single-crystal X-ray diffraction, FTIR, UV-Vis-NIR, Mössbauer, magnetic, and electrochemical measurements. The strongly electron-donating phenolates afford five-coordination, while the carboxylates and pyridines lead to six-coordination. The analysis of the ligand conformations demonstrates a strong flexibility of the ligand backbone in the complexes. The different hydrogen-bonding in the secondary coordination sphere of [(julia){Fe(OH2)(µ-O)Fe(OH2)}] influences the C-O, C[double bond, length as m-dash]O, and Fe-O bond lengths and is reflected in the FTIR spectra. The physical properties of the central {Fe(µ-O)Fe} core (d-d, µ-oxo → Fe(III) CT, νas(Fe-O-Fe), J) are governed by the differences in terminal ligands - Fe(III) bonds: strongly covalent π-donation with phenolates, less covalent π-donation with carboxylates, and π-acceptation with pyridines. Thus, [(susan){FeCl(µ-O)FeCl}](2+) is oxidized at 1.48 V vs. Fc(+)/Fc, which is shifted to 1.14 V vs. Fc(+)/Fc by methanolate substitution, while [(julia){Fe(OH2)(µ-O)Fe(OH2)}] is oxidized ≤1 V vs. Fc(+)/Fc. [(hilde(Me2)){Fe(µ-O)Fe}] is oxidized at 0.36 V vs. Fc(+)/Fc to a phenoxyl radical. The catalytic oxidation of cyclohexane with TONs up to 39.5 and 27.0 for [(susan){FeCl(µ-O)FeCl}](2+) and [(hilde(Me2)){Fe(µ-O)Fe}], respectively, indicates the potential to form oxidizing intermediates.

Molecular and electronic structures of dinuclear iron complexes incorporating strongly electron-donating ligands: implications for the generation of the one- and two-electron oxidized forms.

The ligands (L(t-Bu(2)))(2-), (L(Me(2)))(2-), and (L(Cl(2)))(2-) have been employed for the synthesis of the dinuclear Fe(III) complexes [L(t-Bu(2))Fe(µ-O)FeL(t-Bu(2))], [L(Me(2))Fe(µ-O)FeL(Me(2))], and [L(Cl(2))Fe(µ-O)FeL(Cl(2))]. The strongly electron-donating groups (tert-amines and phenolates) were chosen to increase the electron density at the coordinated ferric ions and thus to facilitate the oxidation of the complexes, with the possibility of fine-tuning the electronic structures by variation of the remote substituents. Molecular structures established in the solid (by single-crystal X-ray diffraction) and in solution (by X-ray absorption spectroscopy) show that the Fe ions are five-coordinate in a square-pyramidal coordination environment with the ligand adopting a trans-conformation. Spectroscopic and magnetic characterization establishes the highly covalent nature of the Fe(III)-O(oxo) and Fe(III)-O(Ph) bonds. The variations in the donor capabilities of the phenolates (due to changes in the remote substituents) are compensated for by a flexible electron donation of the Fe(III)-O(oxo) bonding. Spectroelectrochemical characterization demonstrates that [L(t-Bu(2))Fe(µ-O)FeL(t-Bu(2))] can be oxidized reversibly at +0.27 and +0.44 V versus Fc(+)/Fc, whereas [L(Me(2))Fe(µ-O)FeL(Me(2))] and [L(Cl(2))Fe(µ-O)FeL(Cl(2))] exhibit irreversible oxidations at +0.29 and +0.87 V versus Fc(+)/Fc, respectively. UV-vis, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS), and Mössbauer spectroscopy show that the successive oxidations of [L(t-Bu(2))Fe(µ-O)FeL(t-Bu(2))] are ligand-centered leading to the monophenoxyl radical complex [(•)L(t-Bu(2))Fe(III)(µ-O)Fe(III)L(t-Bu(2))](+) (with the oxidation primarily localized on one-half of the molecule) and the diphenoxyl radical complex [(•)L(t-Bu(2))Fe(III)(µ-O)Fe(III•)L(t-Bu(2))](2+). Both products are unstable in solution and decay by cleavage of an Fe(III)-O(oxo) bond. The two-electron oxidized species is more stable because of two equally strong Fe(III)-O(oxo) bonds, whereas in the singly oxidized species the Fe(III)-O(oxo) bond of the non-oxidized half is weakened. The decay of the monocation results in the formation of [L(t-Bu(2))Fe(III)](+) and [L(t-Bu(2))Fe(IV)=O], while the decay of the dication yields [(•)L(t-Bu(2))Fe(III)](2+) and [L(t-Bu(2))Fe(IV)=O]. Follow-up reactions of the oxidized fragments with the counteranion of the oxidant, [SbCl(6)](-), leads to the formation of [Fe(III)Cl(4)](-).