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

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‐[FeII(PySBn)2(NCE)2] (1 a–b) and cis‐[FeII(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 Å and Fe−S≈2.5–2.6 Å. 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 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, 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.

High spin iron(ii) complexes based on imidazolyl- and 1,2,3-triazolyl-thione ligands and NCE (E = S, Se or BH3) co-ligands: effect of the S-functional group on the structural and magnetic properties

“…Based on the data shown in Table 2, it is possible to observe that the structural data of the iron(II) complexes agree with the parameters found for related iron(II) complexes, 3,6,12 Tables S6 and S7 (ESI †). The shortest bond type for all complexes comprises the Fe-N NCE bond, as expected due to the anionic nature of the NCE co-ligands, which have average values ranging 2.118-2.145 Å.…”

“…Geometry optimisation of the structurally characterised complexes 1a–b and 2a–c obtained in this work, and of the previously synthesised and structurally related SCO-active complex [Fe II (PySBn) 2 (NCSe) 2 ] ( 3b see below and Fig. S25 and S26, ESI†) and the HS-only complex [Fe II (PySBn) 2 (NCS) 2 ] ( 3a ), 12 were performed on both the high- and low-spin states (see Experimental part for details). To evaluate the most stable spin state the adiabatic energy gap between the HS and LS-states, Δ E HL = E HS − E LS , was evaluated for each metal complex.…”

“…It has been shown that S-containing ligands can be utilised to obtain iron(II) spin crossover (SCO) complexes [1][2][3][4][5][6][7][8][9][10][11][12][13] in some of which the iron(II) centres have a {N 4 S 2 } coordination sphere. SCO materials can reversibly switch between the high spin (HS) and low spin (LS) states by and external stimulus, such as temperature, pressure, applied magnetic field, light irradiation, solvation, etc.…”

“…1, and so far just a few examples of SCO-active octahedral iron(II) complexes comprising {N 4 S 2 } coordination spheres are known. [1][2][3][4][6][7][8]12,13 Moreover, the presence of S-donors in iron(II) metal complexes provides a double advantage to this type of systems. Firstly, a protection of the metallic centre avoiding its oxidation towards iron(III), which has already been observed in systems based on some redox-active sulphur ligands, 22 and secondly, maintaining the magnetic properties of these SCO-active systems.…”

Pyridyl-Thioethers as Capping Ligands for the Design of Heteroleptic Fe(II) Complexes with Spin-Crossover Behavior

“…In recent years, our and other groups have reported SCO in several Fe(II) complexes with the {N4S2} coordination, in which the S-donor atoms are provided by thioether functionalities [3][4][5][6][7][8][9][10][11]. In general, one thinks of thioethers as rather weak ligands, and initially we were quite surprised by the observation of SCO in such coordination environment.…”