1D and 2D assembly structures by imidazole⋯chloride hydrogen bonds of iron(ii) complexes [FeII(HLn-Pr)3]Cl·Y (HLn-Pr = 2-methylimidazol-4-yl-methylideneamino-n-propyl; Y = AsF6, BF4) and their spin states

Abstract

Two Fe^II complexes fac-[Fe^II(HL^n-Pr)₃]Cl·Y (Y = AsF₆ (1) and BF₄ (2)) were synthesized, where HL^n-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-[Fe^II(HL^n-Pr)₃]²⁺ with Δ- and Λ-enantiomorphs. Three imidazole groups per Δ- or Λ-fac-[Fe^II(HL^n-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-[Fe^II(HL^n-Pr)₃]²⁺ cations. The 3 : 3 NH⋯Cl⁻hydrogen bonds between Δ- or Λ-fac-[Fe^II(HL^n-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-[Fe^II(HL^n-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-[Fe^II(HL^n-Pr)₃]²⁺⋯Cl⁻}₃ giving a smaller cavity, in which Δ- or Λ-fac-[Fe^II(HL^n-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 T_c↓ = 125.5 K and T_c↑ = 129.5 K and compound 2 showed no spin transition and stayed in the high-spin state over the 5–300 K temperature range.