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 ]B 10 Cl 10 ( I ) (HTrz is 1,2,4-triazole), [Fe(NH 2 Trz) 3 ]B 10 Cl 10 · 2H 2 O ( II ) (NH 2 Trz is 4-amino-1,2,4-triazole), [Fe{HC(pz) 3 } 2 ]B 10 Cl 10 ( III ), [Fe{HC(pz) 3 } 2 ]B 10 H 10 ( IV ), and [Fe{HC(pz) 3 } 2 ]B 12 H 12 · 2H 2 O ( 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.