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

Abstract

To study the effect of the thione functional group embedded in two different types of heterocycles, imidazolyl and 1,2,3-triazolyl, on the magnetic properties of iron(II) octahedral complexes, two bidentate thione-containing ligands have been synthesised: 3-methyl-1-(pyridyl)imdazolyl-2-thione (ImPyS) and the novel 3-methyl-4-phenyl-1-(2-pyridyl)-1,2,3-triazolyl-5-thione (TrzPyS). Six iron(II) complexes have been synthesised, containing two bidentate ligands and two N-bond NCE co-ligands (E = S, Se or BH₃): trans-[Fe^II(ImPyS)₂(NCE)₂] (1a–c) and trans-[Fe^II(TrzPyS)₂(NCE)₂] (2a–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 1c) and VT-magnetic measurements in the solid state (except for complex 2b). 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 within 2.118–2.268 Å and Fe–S ≈ 2.5–2.6 Å. VT-magnetic measurements demonstrated that complexes 1a–c and 2a–c are stabilised in the HS-state, showing orbital contribution to g and zero field splitting. DFT calculations performed on both the high- and low-spin states for complexes 1a–b and 2a–c confirm that in the gas phase the HS-state is the most stable. 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 σ-donation vs. π-acceptor ability of the ligands. The structural parameters Fe^II–N–C(E) bond angle and N–C(E) bond length observed in 1a–c and 2a–c complexes were compared with the literature-reported spin crossover iron(II) complexes in {N₄S₂} coordination sphere containing (NCE)⁻ co-ligands.