The role of S⋯S interactions in an Fe(II) spin-crossover complex with thioether tails

The ligand 2-(1-((methylthio)methyl)-1H-1,2,3-triazol-4-yl)pyridine (L) was combined with NCS^- co-ligands to prepare a mononuclear, heteroleptic Fe(II) complex with methyl thioether substituents. X-ray crystallography at 100 K revealed that trans-[Fe^II(L)₂(NCS)₂] (1) crystallised in the triclinic space group P1̄ and hosted a low-spin Fe(II) centre with average Fe-N bond lengths of 1.975 Å. At 273 K the complex exhibits elongated Fe-N bond lengths (ΔFe-N_av = 0.186 Å) and an increased unit cell volume (ΔV = 36.08(3) ų), implying thermal spin-crossover of the Fe(II) centres to the high-spin state. Stabilisation of the low-spin state in 1 at 100 K appears to arise from the formation of strong S⋯S intermolecular interactions between nearby thioether substituents with each other and with the NCS^- ligands of adjacent complexes. After thermal spin-crossover to the high-spin state, these S⋯S contacts are significantly elongated. SQUID magnetometry revealed an abrupt thermally-driven spin transition with T_1/2 = 158 K, aligned with crystallographic data. The role of the Fe-NCS angle on the ligand field splitting energies of 1 was investigated via quantum chemical calculations and compared to analogous trans-[Fe(L)₂(NCS)₂] complexes. Our results suggest that there is unlikely to be a dominant effect of the Fe-NCS angle on the observation of SCO activity, but there may be interplay between the influence of structural distortions on both the ligand field and the long-range elastic lattice interactions. This work highlights the role of S⋯S contacts in stabilisation of the low-spin state in 1 at 100 K and demonstrates their importance in modulating the electronic properties of the Fe(II) centres for switchable molecular magnetic materials and their potential usefulness in the design of new spin-crossover materials.