Spin Crossover vs. High-Spin Iron(II) Complexes in N4S2 Coordination Sphere Containing Picolyl-Thioether Ligands and NCE (E=S, Se and BH3) Co-Ligands

To study the effect of the chelate ring size on the magnetic properties of thioether-based iron(II) metal complexes, two ligands have been envisaged, synthesised and characterised. The two ligands correspond to the bidentate benzylpicolylthioether (PySBn) and tetradentate 2,3-bis(((2-pyridylmethyl)thio)methyl)quinoxaline (QuinoxS). Five iron(II) complexes have been synthesised, containing either two bidentate ligands or one tetradentate ligand, and two N-bond NCE co-ligands (E=S, Se or BH3): trans-[Fe-II(PySBn)(2)(NCE)(2)] (1 a-b) and cis-[Fe-II(QuinoxS)(NCE)(2)] (2 a-c), a for E=S, b for E=Se and c for E=BH3. The iron(II) complexes have been characterised by standard techniques, X-ray crystallography (except for complex 1 a) and VT-magnetic measurements in the solid state. X-ray crystallography showed that all the complexes are isolated in the high spin (HS) state, based on the relatively long Fe-L bond lengths, Fe-N>2.0 angstrom and Fe-S approximate to 2.5-2.6 angstrom. VT-magnetic measurements demonstrated that complexes 1 a and 2 a-c are stabilised in the HS-state, showing orbital contribution to g and zero field splitting. However, complexes 1 b shows a relatively abrupt, hysteretic, and incomplete at the low-end spin conversion, with T-1/2 down arrow=92, T-1/2 up arrow=98 K and Delta T-1/2=6 K at 5 K min(-1), moreover, the hysteresis loop is scan rate dependent increasing up to 11 K at 10 K min(-1). An analysis of structural and electronic parameters has been performed to rationalise the differing magnetic properties of the metal complexes, such as metallacycle size, bond lengths and angles, and cis- vs. trans-coordination mode. A comparison with the literature-reported spin crossover iron(II) complexes in N4S2 coordination sphere containing NCE co-ligands has been conducted as well, finding that, as previously reported, the Fe-N-C(E) bond angle is diagnostic for determining the spin lability of the metal complexes, and in addition we have found that the N-C(E) bond length is too useful.

Automated summary

The study investigated the effect of chelate ring size on the magnetic properties of thioether-based iron(II) metal complexes by synthesizing and characterizing two types of ligands. The results showed that the complexes were isolated in the high spin state, with orbital contribution to g and zero field splitting. The analysis of structural and electronic parameters helped rationalize the differing magnetic properties of the metal complexes.