Synthesis, structure, magnetic properties and theoretical calculations of methoxy bridged dinuclear iron(iii) complex with hydrazone based O,N,N-donor ligand

Abstract

Biomimetic complexes are artificially engineered molecules that aim to reduce the structural complexity of biological systems in order to unveil the key electronic and structural factors relevant to a protein's function. In this work, a novel coordination compound (L₂2Fe₂2) which mimics non-heme binuclear proteins was synthesized from the Schiff-base ligand HL = (E)-N′-(phenyl(pyridin-2-yl)methylene)isonicotinohydrazide. The crystal structure of L₂2Fe₂2 showed that the intramolecular Fe–Fe distances (3.1–3.2 Å) were analogous to those found in non-heme binuclear ferric proteins. However, in L₂2Fe₂2, two methoxide groups act as bridging units for oxidized iron (Fe³⁺). Such a bridging motif is unprecedented in the biological realm. Magnetic susceptibility measurements demonstrated that L₂2Fe₂2 is characterized by a singlet (S = 0) ground state and a very small magnetic coupling constant J (≪ −1 cm⁻¹). The J value featured by L₂2Fe₂2 differs considerably from the values observed in non-heme binuclear proteins in the oxidized form (−100 cm⁻¹ < J < −10 cm⁻¹), which encompass oxo/hydroxo and carboxylate bridging residues. The singlet ground state of L₂2Fe₂2 as well as the weak magnetic interaction between the two ferric cations was successfully predicted by density functional theory (DFT).