Polymorphs of spin-crossover iron(II) complex fac-[FeII(HLn-Pr)3]Cl·PF6 (HLn-Pr = 2-methylimidazol-4-yl-methylideneamino-n-propyl): Assembly structures and scan rate dependent spin-crossover properties with thermal hysteresis

Two polymorphs of iron(II) complex fac-[FeII(HLn-Pr)3]Cl·PF6, 1 and 2, were obtained (HLn-Pr=2-methylimidazole-4-yl-methylideneamino-n-propyl ligand). Complex cations, fac-[FeII(HLn-Pr)3]2+, of 1 and 2 had similar octahedral and facial geometries coordinated with three bidentate HLn-Pr ligands. The three imidazole groups in fac-[FeII(HLn-Pr)3]2+ with Δ- and Λ-enantiomorphs are hydrogen-bonded with three Cl− ions, forming one-dimensional (1D) and two-dimensional (2D) network structures in 1 and 2, respectively. Complex 1 had a ladder-like 1D structure formed by imidazole⋯chloride hydrogen bonds, with a circular binuclear net unit of {⋯fac-[FeII(HLn-Pr)3]2+⋯Cl−⋯}2, and showed two successive scan rate dependent spin crossover (SCO) processes with thermal hysteresis. Complex 2 had a 2D network structure formed by imidazole⋯chloride hydrogen bonds with circular binuclear and tetranuclear net units of {⋯fac-[FeII(HLn-Pr)3]2+⋯Cl−⋯}2,4, and showed a one-step scan rate dependent SCO profile centered at 143K with thermal hysteresis. The relaxation property was identified from the scan rate dependence of magnetic susceptibility and temperature dependent Mössbauer spectra and X-ray diffraction studies. The crystal structure of 2 was analyzed at 12 temperatures in a cooling cycle. Upon lowering the temperature, the crystal system transformed from a monoclinic to a triclinic lattice at 210K in a high-spin state, and then transformed to a low-spin state at 150K in the triclinic system.