Spin crossover behavior in a family of iron(ii) zigzag chain coordination polymers

Abstract

The paper reports the synthesis and detailed characterization of two new Fe(II) compounds: [Fe(pyim)₂(bpen)](ClO₄)₂·2C₂H₅OH (2) and [Fe(pyim)₂(bpe)](ClO₄)₂·C₂H₅OH (3) (pyim = 2-(2-pyridyl)imidazole, bpen = 1,2-bis(4-pyridyl)ethane, and bpe = 1,2-bis(4-pyridyl)ethene). Both compounds and the earlier synthesized [Fe(pyim)₂(bpy)](ClO₄)₂·2C₂H₅OH (1) (bpy = 4,4′-bipyridine) form a family of one-dimensional spin crossover coordination polymers. Variable-temperature magnetic susceptibility measurements and Mössbauer spectroscopy have revealed rather gradual spin transitions centered at 176 and 198 K for 2 and 3, respectively. The fitting of magnetic properties with the regular solution model leads to the enthalpy and entropy of spin transitions and the cooperativity parameter equal to ΔH = 12.3 kJ mol⁻¹, ΔS = 68.5 J mol⁻¹ K⁻¹, Γ = 1.80 kJ mol⁻¹ for 2 and ΔH = 13.6 kJ mol⁻¹, ΔS = 68.1 J mol⁻¹ K⁻¹, Γ = 2.05 kJ mol⁻¹ for 3. The crystal structures of 2 and 3, resolved by X-ray diffraction at 293 K, belong to the monoclinic space group C2/c (Z = 4). Both compounds display a one-dimensional infinite zigzag-chain structure. The polymer chains are stacked into two-dimensional sheets through intermolecular π-interactions. The crystal packing of both compounds encloses two kinds of channels in which the counter ions and ethanol molecules are inserted. The DFT calculations of binuclear fragments extracted from three polymers resulted in the energy gaps between the LS and HS states being ordered as the observed transition temperatures. The influence of bridging ligands in the studied family of compounds was found in the modulation of the energy gap between the LS and HS states, leading to different transition temperatures.