ABSTRACT
Three new 3d-3d heterometallic complexes, [CuMII(hmb)6(OH)]ClO4·H2O (M = Zn (1), Ni (2) and Co (3)) (Hhmb = 2-hydroxy-3-methoxy-benzaldehyde), have been synthesized. Structural analysis reveals that the three complexes are isostructural. Three CuII ions are in a perfect trigonal geometrical frustration and form a tetrahedral frustrated system with the introduced fourth ion ZnII/NiII/CoII. Magnetic studies indicate the antiferromagnetic coupling between metal ions in all 1, 2 and 3. Due to the geometrical frustration, spin-frustrated magnetism is also a typical feature of 1-3 and the magnetostructural correlation is far more complicated than their structures. The detailed magnetic investigation found that the ground state spin of 2 cannot be simply determined by the antiferromagnetically linear arrangement of spins, because the strong antiferromagnetic coupling between CuII and NiII ions quenches the spin frustration and ferromagnetically polarizes the spins of the [Cu] unit. Contrarily, the antiferromagnetic coupling between CuII and CoII ions is not strong enough in 3, so there is no similar behaviour compared to 2.
ABSTRACT
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.