Iron(II) Metallomesogens Exhibiting Coupled Spin State and Liquid Crystal Phase Transitions near Room Temperature

Abstract Reaction of the ligand 2,2,2‐tris(2‐aza‐3‐((5‐akloxy)(6‐methyl)(2‐pyridyl))prop‐2‐enyl)ethane (C_n‐tameMe) with Fe(X)₂ · sH₂O salts afforded a series of spin crossover metallomesogens with general formula [Fe(C_n‐tame)](X)₂ · sH₂O (s ≥ 0), with n = 6, 10, 12, 14, 16, 17, 18, or 20 and X = ClO₄⁻ or BF₄⁻. Single crystal X‐ray measurements have been performed on the [Fe(C₆‐tameMe)](ClO₄)₂ (C₆‐1) derivative at 100 K. The complex C₆‐1 crystallizes in the triclinic system and adopts the Pbca space group. The iron(II) ion is in a distorted octahedral environment shaped by three imine and three pyridine nitrogen atoms of the C_n‐tameMe ligand. The average Fe–N^im and Fe–N^py bond distance is 1.918(2) Å and 2.084(2) Å, respectively, which are characteristic for the Fe(II) ion in the LS state. Neighbor molecules are packed in a head‐to‐head fashion forming a layered microsegregated structure. Ionic layer is composed of cationic spin crossover head‐groups and perchlorate anions, whereas alkyl tails are arranged into a non‐polar hydrocarbon layer. The complex [Fe(C₁₈‐tameMe)](ClO₄)₂ (C₁₈‐1) and the series [Fe(C_n‐tameMe)](BF₄)₂ (n = 10, 12, 14, 16, 17, 18, 20) (C_n‐2) series show thermally driven spin crossover and mesomorphism above 300 K. The thermotropic behavior of compounds C_n‐2 depends on the chain length defined by n. A common structural feature among all homologues of the series is a similar layered structure and adoption on melting of a smectic mesophase at ca. 350–375 K. Formation of the mesophase is due to the melting of the alkyl chains rather than a rearrangement of the ionic bilayers composed of polar SCO cations and BF₄⁻ or ClO₄⁻ anions. This gives rise to a mesophase with molten alkyl chains but with restricted movement through the ionic layer. Compounds C_n‐2 show thermochromic properties, they are dark violet in the LS state (T < 275 K, Cr) and become red in the HS state (T > 275 K, Cr, S_X). These bi‐functional materials combine spin state change and mesomorphic behavior in a narrow interval of temperatures, 300–375 K.