Synthesis, structure, and spectroscopic properties of a dinuclear Fe-III(mu(2)-O)Fe-III complex using a strongly electron-donating ligand: Implications for the generation of new high-valent species

The dinuclear complex [LFeIII(mu(2)-O)(FeL)-L-III] (1) (H2L = N,N'-dimethyl-N,N'-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-1,2-diaminoethane) has been synthesized and structurally characterized by elemental analysis, MALDI-TOF MS, FTIR, and single crystal X-ray diffraction. The electronic structure has been investigated by means of electronic absorption, electrochemistry, Mossbauer spectroscopy, and variable-temperature magnetic susceptibility measurements. The iron atoms are five-coordinated in an idealized square-pyramidal coordination environment. The N2O2 donor set of L2- forms the equatorial plane while the bridging oxo-ligand is in the axial position forming a linear Fe-O-Fe unit. The Mossbauer spectrum at 80 K consists of one quadrupole doublet with an isomer shift of 0.45 mm s(-1) and a quadrupole splitting of 1.22 mm s(-1). The low isomer shift is indicative of highly covalent metal-ligand bonds resulting in electron-rich Fe-III high spin centers (S-i = 5/2). Variable-temperature magnetic susceptibility measurement reveals an intramolecular antiferromagnetic coupling (J = -96 cm(-1)). Strong LMCT transitions in the vis-region of the absorption spectrum are in accordance to the highly covalent nature of the Fe-III sites. The electrochemical analysis reveals two oxidative electron-transfer waves at E-1/2 = +0.28 V and E-1/2 = +0.45 V vs Fc(+)/Fc. These experimental data provide insight into the electronic structure of the dinuclear complex and into the ability of the strongly electron-donating ligand L2- to stabilize high-valent dinuclear iron species.