Synthesis, characterization and X-ray crystal structure of an iron(III) complex of a tripodal pyridoxal Schiff base ligand: effects of positional disorder on its magnetic properties

Abstract

The synthesis and characterization of fac-{tris[(E)-5-(hydroxymethyl)-2-methyl-4-((methylimino-κN)methyl)pyridin-3-olato-κO]amine}iron(III) 7.5-hydrate is described, together with its crystal structure at 95 K. The FeIII ion is six-coordinated in a trigonally distorted octahedral configuration with the hexadentate tripodal pyridoxal Schiff base ligand. A positional disorder of the pyridoxal hydroxymethyl group is identified, which partly undergoes a cyclization forming a five-membered ring with the imine moiety. The two configurations occur in the ratio 79.5:20.5. In the crystal, the hydrogen bonds and π–π stacking interactions expand the mononuclear units into 2D sheets which are further stabilized by a system of hydrogen bonds to the 3D supramolecular network. The magnetic susceptibility data and EPR measurements indicate that the octahedral Fe(III) centers exist as a mixture of low- and high-spin forms within the temperature range 2–300 K. The presence of a mixture of the majority high-spin and the minority low-spin components showing a spin transition is proposed. The effect of zero-field splitting on magnetic properties is discussed. EPR spectra at X-band display signals of a high-spin Fe(III) complex of the rhombohedral symmetry with the zero-field splitting parameters E/D ratio approximately 1/3, along with the signals of a low-spin Fe(III) complex. Temperature dependence of the EPR spectra revealed a bend temperature T ≈ 88 K attributed to spin transition of the minority Fe(III) centers. The thermal stability of the compound was investigated by (TG, DTG, DTA) techniques. New iron(III) complex with tripodal pyridoxal Schiff base ligand was prepared. Single-crystal XRD data revealed a positional disorder of a side hydroxymethyl group. The magnetic data were analysed using the spin Hamiltonian including the ZFS term. EPR measurements at X-band confirmed magnetic anisotropy of the complex. Thermal stability of the complex was investigated by (TG, DTG, DTA) techniques.