Synthesis, Structure and Properties of a Mixed Mononuclear/Dinuclear Iron(II) Spin-Crossover Compound with the Ligand 4-(p-Tolyl)-1,2,4-triazole

Abstract

A new iron(II) compound with the formula [Fe2(L)5(NCS)4]2[Fe(L)2(NCS)2(H2O)2] (I) [where L = 4-(p-tolyl)-1,2,4-triazole] has been synthesized and subjected to X-ray structure determination. Compound I crystallizes in the triclinic space group P–1 (no. 2) with a = 14.5785(11), b = 16.1253(11), c = 16.1963(8) Å, α = 80.930(5), β = 85.796(5), γ = 78.132(6)°, V = 3676.2(4) Å3. The structure refinement converged to wR2 = 0.172, RF = 0.084. The structure was found to consist of two types of iron-containing structural units, a mononuclear unit and a dinuclear one. The mononuclear unit has a crystallographic inversion centre, and is coordinated by two NCS anions, two triazole N1 nitrogen atoms, and two water molecules, each hydrogen-bonded to one of the two dinuclear units. The dinuclear units consist of two iron(II) ions bridged by three triazole ligands in a 1,2-fashion. The coordination spheres of both iron ions are completed by two NCS anions and one monodentate triazole ligand. The monodentate triazole ligands are connected through the non-coordinating N atom to the mononuclear iron unit by hydrogen bonds from its coordinated water molecule. Magnetic susceptibility measurements indicate a spin transition (ST) only for the iron ions in the dinuclear units, centred at around T1/2 = 111 K. The transition takes place within a relatively narrow T range. The mononuclear iron ion remains in the high-spin (HS) state even at very low temperature, yielding a ratio of 4:1 for the ST and HS iron ions. Temperature-dependent Mössbauer spectroscopy confirms these results.