Elucidating the role of the o-methoxy group in the lower rim appended salicylideneamine substituents of calix[4]arene ligands on the molecular and electronic structures of dinuclear Fe(iii)-based diamond-core complexes

Abstract

Two dinuclear double oxygen-bridged iron(III) complexes made of the Fe₂(μ₂-OR)₂ diamond-core cluster resulting from the coordination between iron(III) cations with calix[4]arene ligands and bearing two lower-rim-appended salicylideneamine (4) or o-methoxy-salicylideneamine coordinating sites (5), connected to the macrocyclic platform via three methylene bridges and displaying a salen-type coordination pocket, were synthesized and characterized using single-crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), high-resolution mass spectrometry (HRESI-MS), IR, TG-DSC and EA techniques. Independent of the presence of the o-methoxy group in the salicylideneamine moieties, the as-formed complexes contained two nonequivalent iron(III) atoms, both adopting a distorted octahedral coordination N₂O₄-environment but differing in the cis and trans positions of the nitrogen atoms within the metal coordination sphere, as attested by single-crystal X-ray diffraction and DFT-B3LYP calculations. The possible reason for the observed coordination mode of the macrocyclic ligands is their conformational behavior upon coordination with Fe(III) cations, as confirmed by DFT-B3LYP analysis. ⁵⁷Fe Mössbauer spectroscopy study showed that both Fe(III) atoms were present in high spin states at room temperature and at 80 K in the studied coordination compounds. The methoxy groups located at the ortho position of the salicylideneamine coordinating fragments were observed to influence the energy of the frontier orbitals, resulting in an increase in the bandgap energy from 1.97 eV to 2.07 eV, as experimentally established by solid-state voltammetry study of 4₂-Fe_{2 exp} and 5₂-Fe_{2 exp}, respectively. The interplay between the steric and electronic factors, emerging from the o-methoxy group within coordinating salicylideneamine moieties, seemed to have a negligible effect on the electronic properties of the prepared clusters but significantly impacted the frontier orbital energy distribution, according to both experimental data and theoretical calculations.