Iron(II) closo-borate complexes with 1,2,4-triazole derivatives: Spin crossover in the iron(II) closo-borate complexes with tris(pyrazol-1-yl)methane

Methods of synthesis of iron(II) complexes containing cluster closo-borate anions—[Fe(Htrz)3]B10Cl10 (I) (HTrz is 1,2,4-triazole), [Fe(NH2Trz)3]B10Cl10 · 2H2O (II) (NH2Trz is 4-amino-1,2,4-triazole), [Fe{HC(pz)3}2]B10Cl10 (III), [Fe{HC(pz)3}2]B10H10 (IV), and [Fe{HC(pz)3}2]B12H12 · 2H2O (V) (HC(pz)3 is tris(pyrazol-1-yl)methane)—have been developed. The compounds have been studied by the static magnetic susceptibility method (78–500 K) and electronic, IR, and EXAFS spectroscopy. Complexes I and II in the temperature range under consideration remain in the high-spin state. Low-spin complex III shows incomplete spin crossover and decomposes on heating above 440 K. Complexes IV and V are characterized by reversible spin crossover 1 A 1 ⇔ 5 T 2 accompanied by thermochromism (the pink ⇔ white color change). The crossover temperature (T c) for IV and V is 375 and 405 K, respectively.