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 FeIII–μ-O–FeIII complexes

Abstract

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)FeCl_0.75(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, CO, 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, ν_as(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.