Solvatomorphism and structural-spin crossover property relationship in bis[hydrotris(1,2,4-triazol-1-yl)borate]iron(II)

Two solvatomorphs of the mononuclear bis[hydrotris(1,2,4-triazol-1-yl) borate] iron(II) complex, [Fe(HB(tz)(3))(2)] (1) and [Fe(HB(tz)(3))(2)]center dot 6H(2)O (1 center dot 6H(2)O), were obtained by modifying the nature of the crystallization solvent. The crystal structure, thermal stability and spin crossover properties of the crystals and associated bulk powder samples were analysed using variable-temperature single-crystal and powder X-ray diffraction, thermogravimetry, calorimetry, and Raman and Fe-57 Mossbauer spectroscopy as well as by means of magnetic susceptibility and optical microscopy measurements. The orthorhombic (Cmca) solvatomorph 1 center dot 6H(2)O loses water between ca. 323-353 K, leading to the disintegration of the crystals into the polycrystalline sample 1. The solvent-free crystals of 1 crystallize in the orthorhombic space group Pbca with half a complex molecule in the asymmetric unit. They exhibit a remarkably abrupt phase transition around 334 K between the high spin and low spin states. This (isostructural) spin transition is accompanied by a nearly isotropic change of the Fe-II -N bond lengths (8.3 perpendicular to 0.5%) and a highly anisotropic unit cell volume change (4.6 +/- 0.1%). The very high cooperativity of the spin transition in 1 (Gamma = 5700 +/- 50 J mol(-1)) is rather unusual in mononuclear SCO compounds. This property can be related to the relatively high stiffness of the lattice (Debye temperature theta(D) = 198 +/- 2 K), which involves numerous C-H center dot center dot center dot N hydrogen contacts between each molecule with fourteen neighboring molecules.