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-[FeII(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–Nav = 0.186 Å) and an increased unit cell volume (ΔV = 36.08(3) Å3), 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 T1/2 = 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.