Magnetic properties and pH-controlled reversible interconversion of µ-oxido into µ-hydroxido in oxo-carboxylato bridged iron(III) dimers: Theoretical and experimental insights

Three dinuclear complexes of formula [FeIII2(µ-O)(µ-OOCR)(L)2](ClO4)·G, with R/G=H/(EtOAc)2 (1·2EtOAc), CH3/(Et2O)2 (2·2Et2O) and C6H5/(CH3OH)2(CH3CN)2 (3·2CH3OH·2CH3CN), [where HL = 2-((((1-methylbenzimidazol-2-yl)methyl)(pyridin-2-ylmethyl)amino)methyl)phenol] have been prepared. The ligand provides a N,N,N,O donor set in its deprotonated form. X-ray diffraction analyses revealed that the complexes have a similar oxido-carboxylato-bridged structure where each metal ion has a tertiary amine, one imidazolyl N atom, one pyridyl N atom and a phenolate O atom as terminal coordination. Thus, each metal centre assumes a FeIIIN3O3 coordination in an octahedral environment. Addition of acid lead to the generation of similar dinuclear complexes formulated as [FeIII2(µ-OH)(µ-OOCR)(L)2](ClO4)2, with R=H (1H), CH3 (2H) and C6H5 (3H) in solution. This protonation of the µ-O bridge is reversible and addition of base regenerates the original complexes (1–3). Magnetic measurements showed intramolecular antiferromagnetic Fe···Fe interactions in compounds 1–3 with J = ‒276(1), ‒197(1) and ‒190(1) cm−1, respectively. DFT/B3LYP calculation helped us to explicitly state the sign and magnitude of the magnetic coupling constants along with an approaching to the spin density distribution.