Water induced spin-crossover behaviour and magneto-structural correlation in octacyanotungstate(iv)-based iron(ii) complexes.

A series of octacyanotungstate(iv)-based iron(ii) complexes with the general formula Fe(L)8[WIV(CN)8]·nH2O [L = (3-pyridyl)methanol (1, 2), 3-methylpyridine (3), (4-pyridyl)methanol (4), and 4-methylpyridine (5); n = 4 for 1, and n = 0 for 2-5] have been synthesized and characterized. Single crystal X-ray diffraction analysis reveals that the FeII ions lie in the centre of the compressed [FeN6] octahedron in all complexes. FeII and WIV ions are alternately bridged by cyano groups forming a three-dimensional (3D) bimetallic framework. Magnetic investigation shows that 1 displays a gradual spin-crossover (SCO) phenomenon with a spin transition temperature (T1/2) of 200 K, and such SCO behaviour is obviously correlated with the lattice water content of the sample. The magnetic measurements of dehydrated samples show that the fractional conversion from the high-spin (HS) to the low-spin (LS) state is reduced with the increasing of dehydration temperature. Complexes 2-5 are in the HS state and do not exhibit SCO properties in the range of 2-300 K. Comparing the octahedral geometry of [FeN6] of five complexes, quantified by using continuous shape measures, the distortion of complex 1 is the highest as a result of the intermolecular hydrogen bonds, which shorten the Fe-N bond distances and thus increase the ligand field strength at the FeII sites. The analysis of correlations between the structural characteristics and magnetic behaviour of 1-5 suggests that the SCO is mainly tuned by the octahedral distortion of the [FeN6] core caused by intermolecular hydrogen bonds. There is an exact correlation between SCO behaviour and the amount of lattice water molecules existing in the crystal. The spin crossover behaviour of these complexes has been computationally studied using the DFT method. The results of the calculations are consistent with the experiments, which prove that complex 1 with severe distortion of the coordination sphere of FeII is prone to exhibit SCO in theory.