Layered iron(II) spin crossover complex constructed by NH⋯Br− hydrogen bonds with 2 K wide thermal hysteresis, [FeIIH3LMe]Br·CF3SO3 (H3LMe = tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine)

Abstract

A series of compounds [FeIIH3LMe]Br·Y·nMeOH (Y=PF6- (1), AsF6- (2), SbF6- (3), CF3SO3- (4); n=0 or 1) were synthesized, where H3LMe is a hexadentate N6 tripodal ligand of the neutral form, tris[2-(((2-methylimidazol-4-yl)methylidene)amino)ethyl]amine, and their structures and magnetic properties were investigated. The compounds 1−3 with counter anions Y=PF6-, AsF6-, and SbF6- contain methanol as a crystal solvent, and show no SCO behaviors, while the corresponding Cl− compounds have no crystal solvent and show a variety of SCO behaviors. The compound [FeIIH3LMe]Br·CF3SO3 (4) has no crystal solvent and has isomorphous structure to the Cl− compounds, and shows an abrupt spin transition between the HS (S=2) and LS (S=0) states with a hysteresis about 2K and large frozen-in effect below 72K. The T1/2↑ and T1/2↓ values are 98 and 96K, whose values are higher than those of corresponding Cl− compound about 15K and the width of hysteresis is narrower than that of corresponding Cl− compound about 2K. The crystal structures of 4 were determined at 296 and 93K, where the crystal system and space group showed no change between these temperatures. The structures at both temperatures have a same 2D layered structure, which is composed of NH⋯Br− hydrogen bonds between the Br− ion and the imidazole NH groups of three neighboring cations [FeIIH3LMe]2+. This network structure is the same as that of corresponding Cl− compound. The 600nm light irradiation at 5K induced the LIESST effect.