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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. more...

The design and first application of a new dinucleating ligand system to mimic high-valent oxidation states of O(2)-dependent diiron enzymes is presented.

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)](-). more...

Oxidation of the diferric complex [LFe(III)OFe(III)L] to the monoradical complex [LFe(III)OFe(III)L ](+) and the diradical complex [L Fe(III)OFe(III)L ](2+) is followed by a decay into monomeric complexes including a highly reactive putative [LFe(IV)[double bond, length as m-dash]O] complex. more...

The ligand L (2-) (H 2L = N, N'-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)-NO 3)] and [LFeCl]. L (2-) 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)-NO 3)] 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)-NO 3)]. 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) bisphenoxyl radical complex. These studies imply that the ligand L (2-) is capable of providing a flexible coordination geometry with two binding sites for substrates and the allocation of two oxidation equivalents on the ligand. more...

A series of trinuclear Cu(II) complexes with the tris(tetradentate) triplesalen ligands H(6)talen, H(6)talen(tBu(2) ), and H(6)talen(NO(2) ), namely [(talen)Cu(II) (3)] (1), [(talen(tBu(2) ))Cu(II) (3)] (2), and [(talen(NO(2) ))Cu(II) (3)] (3), were synthesized and their molecular and electronic structures determined. These triplesalen ligands provide three salen-like coordination environments bridged in a meta-phenylene arrangement by a phloroglucinol backbone. The structure of [(talen)Cu(II) (3)] (1) was communicated recently. The structure of the tert-butyl derivative [(talen(tBu(2) ))Cu(II) (3)] (2) was established in three different solvates. The molecular structures of these trinuclear complexes show notable differences, the most important of which is the degree of ligand folding around the central Cu(II)-phenolate bonds. This folding is symmetric with regard to the central phloroglucinol backbone in two structures, where it gives rise to bowl-shaped overall geometries. For one solvate two trinuclear triplesalen complexes form a supramolecular disk-like arrangement, hosting two dichloromethane molecules like two pearls in an oyster. The FTIR spectra of these complexes indicate the higher effective nuclear charge of Cu(II) in comparison to the trinuclear Ni(II) complexes by the lower C--O and higher C=N stretching frequencies. The UV/Vis/NIR spectra of 1-3 reflect the stronger ligand folding in the tert-butyl complex 2 by an intense phenolate-to-Cu(II) LMCT. This absorption is absent in 1 and is obscured by the nitro chromophore in 3. The more planar molecular structures cause orthogonality of the Cu(II) d(x(2)-y(2) ) orbital and the phenolate O p(z) orbital, which leads to small LMCT dipole strengths. Whereas 1 and 3 exhibit only irreversible oxidations, 2 exhibits a reversible one-electron oxidation at +0.26 V, a reversible two-electron oxidation at +0.59 V, and a reversible one-electron oxidation at +0.81 V versus Fc(+)/Fc. The one-electron oxidized form 2(+) is strongly stabilized with respect to reference mononuclear salen-like Cu complexes. Chemical one-electron oxidation of 2 to 2(+) allows the determination of its UV/Vis/NIR spectrum, which indicates a ligand-centered oxidation that can be assigned to the central phloroglucinol unit by analogy with the trinuclear Ni triplesalen series. Delocalization of this oxidation over three Cu(II)-phenolate subunits causes the observed energetic stabilization of 2(+). Temperature-dependent magnetic susceptibility measurements reveal ferromagnetic couplings for all three trinuclear Cu(II) triplesalen complexes. The trend of the coupling constants can be rationalized by two opposing effects: 1) electron-withdrawing terminal substituents stabilize the central Cu(II)-phenolate bond, which results in a stronger coupling, and 2) ligand folding around the central Cu(II)-phenolate bond opens a bonding pathway between the magnetic Cu(II) d(x(2)-y(2) ) orbital and the phenolate O p(z) orbital, which results in a stronger coupling. Density functional calculations indicate that both spin-polarization and spin-delocalization are operative and that slight geometric variations alter their relative magnitudes. more...