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-μ-O-Fe complexes

Based on a rational ligand design for stabilizing high-valent {Fe(μ-O) 2 Fe} cores, a new family of dinucleating bis(tetradentate) ligands with varying terminal donor functions has been developed: redox-inert biomimetic carboxylates in H 4 julia, pyridines in susan, and phenolates in H 4 hilde Me 2 . Based on a retrosynthetic analysis, the ligands were synthesized and used for the preparation of their diferric complexes [(julia){Fe(OH 2 )(μ-O)Fe(OH 2 )}]·6H 2 O, [(julia){Fe(OH 2 )(μ-O)Fe(OH 2 )}]·7H 2 O, [(julia){Fe(DMSO)(μ-O)Fe(DMSO)}]·3DMSO, [(hilde Me 2 ){Fe(μ-O)Fe}]·CH 2 Cl 2 , [(hilde Me 2 ){FeCl} 2 ]·2CH 2 Cl 2 , [(susan){FeCl(μ-O)FeCl}]Cl 2 ·2H 2 O, [(susan){FeCl(μ-O)FeCl 0.75 (OCH 3 ) 0.25 }](ClO 4 ) 2 ·0.5MeOH, and [(susan){FeCl(μ-O)FeCl}](ClO 4 ) 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(OH 2 )(μ-O)Fe(OH 2 )}] influences the C-O, C&z.dbd;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, 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(OH 2 )(μ-O)Fe(OH 2 )}] is oxidized ≤1 V vs. Fc + /Fc. [(hilde Me 2 ){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 Me 2 ){Fe(μ-O)Fe}], respectively, indicates the potential to form oxidizing intermediates. We have developed the dinucleating ligands H 4 julia, susan, and H 4 hilde Me 2 and present their μ-oxo diferric complexes.