Two FeII complexes fac-[FeII(HLn-Pr)3]Cl·Y (Y = AsF6 (1) and BF4 (2)) were synthesized, where HLn-Pr is 2-methylimidazole-4-yl-methylideneamino-n-propyl. Each complex-cation has the same octahedral N6 geometry coordinated by three bidentate ligands and assumes facial-isomerism, fac-[FeII(HLn-Pr)3]2+ with Δ- and Λ-enantiomorphs. Three imidazole groups per Δ- or Λ-fac-[FeII(HLn-Pr)3]2+ are hydrogen-bonded to three Cl− ions or, from the viewpoint of the Cl− ion, one Cl− ion is hydrogen-bonded to three neighbouring fac-[FeII(HLn-Pr)3]2+ cations. The 3 : 3 NH⋯Cl−hydrogen bonds between Δ- or Λ-fac-[FeII(HLn-Pr)3]2+ and Cl− generate two kinds of assembly structures. The directions of the 3 : 3 NH⋯Cl−hydrogen bonds and hence the resulting assembly structures are determined by the size of the anion Y, though Y is not involved into the network structure and just accommodated in the cavity. Compound 1 has a 1D ladder structure giving a larger cavity, in which the Δ- and Λ-fac-[FeII(HLn-Pr)3]2+ enantiomorphs are bridged by two NH⋯Cl−hydrogen bonds. Compound 2 has a 2D network structure with a net unit of a cyclic trimer of {fac-[FeII(HLn-Pr)3]2+⋯Cl−}3 giving a smaller cavity, in which Δ- or Λ-fac-[FeII(HLn-Pr)3]2+ species with the same chirality are linked by NH⋯Cl−hydrogen bonds to give a homochiral 2D network structure. Magnetic susceptibility and Mössbauer spectral measurements demonstrated that compound 1 showed an abrupt one-step spin crossover with 4.0 K thermal hysteresis of Tc↓ = 125.5 K and Tc↑ = 129.5 K and compound 2 showed no spin transition and stayed in the high-spin state over the 5–300 K temperature range.
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