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

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)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.

Variation of the Molecular and Electronic Structures of μ‐Oxo Diferric Complexes with the Bridging Motive

“…The comparison of the FT‐IR spectral ranges (Figure a) of the acetato and carbonato complexes to [(susan){Fe III Cl( μ ‐O)Fe III Cl}](ClO 4 ) 2 , where the terminal chloro ligands are IR‐silent, allows the assignment of some characteristic vibrations (Table ). The ν as (FeOFe) stretch at 814 cm –1 in [(susan){Fe III (OAc)( μ ‐O)Fe III (OAc)}] 2+ is almost identical with 816 cm –1 found in [(susan){Fe III Cl( μ ‐O)Fe III Cl}] 2+ . In the doubly‐bridged complexes [(susan){Fe III ( μ ‐O)( μ ‐OAc)Fe III }] 3+ and [(susan){Fe III ( μ ‐O)( μ ‐CO 3 )Fe III }] 2+ , ν as (FeOFe) shifts to 764 and 745 cm –1 , respectively.…”

“…Sections of the FT‐IR spectra measured on (a) KBr pellets and (b) CH 3 CN solutions. The spectrum of [(susan){Fe III Cl( μ ‐O)Fe III Cl}] 2+ is added for comparison …”

“…Solvents and starting materials were of the highest commercially available purity and used as received. The ligand susan [4,7‐dimethyl‐1,1,10,10‐tetra(2‐pyridylmethyl)‐1,4,7,10‐tetraazadecane] was synthesized according to the published procedure . The syntheses of the complexes [(susan){Fe III (OAc)( μ ‐O)Fe III (OAc)}](ClO 4 ) 2 and [(susan){Fe III ( μ ‐O)( μ ‐OAc)Fe III }](ClO 4 ) 3 have been communicated recently …”

“…Although many biomimetic diiron complexes show interesting reactivities, the optimization of functional model complexes is still the focus of current research. In this respect, we have developed a dinucleating ligand family (Scheme ) providing two tetradentate ligand compartments covalently bridged by a flexible ethylene linker with a variation of the terminal donors from phenolates, carboxylates to pyridines . Based on a systematic investigation of a large series of structurally related dinuclear complexes varying in the terminal donors, the bridging donors, and the metal ions, we want to establish a correlation of the molecular and electronic structure to function.…”

“…In a recent study, we have shown the variation of the electronic structures and reactivities in catalytic C–H hydroxylation with the terminal donors using the ligands (julia) 4– , (hilde Me ) 4– , and susan (Scheme ) . The complexes [(susan){FeCl( μ ‐O)FeCl}](ClO 4 ) 2 and [(hilde Me2 ){Fe( μ ‐O)Fe}] show a catalytic reactivity for the oxidation of cyclohexane with peroxides.…”