Polymorphs of spin-crossover iron(II) complex fac-[Fe^II(HL^n-Pr)₃]Cl·PF₆ (HL^n-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-[Fe^II(HL^n-Pr)₃]Cl·PF₆, 1 and 2, were obtained (HL^n-Pr = 2-methylimidazole-4-yl-methylideneamino-n-propyl ligand). Complex cations, fac-[Fe^II(HL^n-Pr)₃]²⁺, of 1 and 2 had similar octahedral and facial geometries coordinated with three bidentate HL^n-Pr ligands. The three imidazole groups in fac-[Fe^II(HL^n-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-[Fe^II(HL^n-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-[Fe^II(HL^n-Pr)₃]²⁺⋯Cl⁻⋯}_2,4, and showed a one-step scan rate dependent SCO profile centered at 143 K 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 210 K in a high-spin state, and then transformed to a low-spin state at 150 K in the triclinic system.