Spin Crossover vs. High‐Spin Iron(II) Complexes in N4S2Coordination Sphere Containing Picolyl‐Thioether Ligands and NCE (E=S, Se and BH3) Co‐Ligands

Abstract

AbstractTo 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‐[FeII(PySBn)2(NCE)2] (1 a–b) andcis‐[FeII(QuinoxS)(NCE)2] (2 a–c),afor E=S,bfor E=Se andcfor E=BH3. The iron(II) complexes have been characterised by standard techniques, X‐ray crystallography (except for complex1 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 Å and Fe−S≈2.5–2.6 Å. VT‐magnetic measurements demonstrated that complexes1 aand2 a‐care stabilised in the HS‐state, showing orbital contribution togand zero field splitting. However, complexes1 bshows a relatively abrupt, hysteretic, and incomplete at the low‐end spin conversion, with T1/2↓=92, T1/2↑=98 K and ΔT1/2=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, andcis‐vs. trans‐coordination mode. A comparison with the literature‐reported spin crossover iron(II) complexes in N4S2coordination 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.