Iron(II) metallomesogens exhibiting coupled spin state and liquid crystal phase transitions near room temperature

Reaction of the ligand 2,2,2-tris(2-aza-3-((5-aktoxy)(6-methyl)(2-pyridyl))prop-2-enyi)ethane (C,,-tameMe) with Fe(X)2 - sH20 salts afforded a series of spin crossover metallornesogens with general formula [Fe(C,,-tame)] (X)2 - sH20 (s > 0), with n = 6, 10, 12, 14, 16, 17, 18, or 20 and X=CI04- or BF4_- Single crystal X-ray measurements have been performed on the [Fe(C6tameMe)](CI04)2 (CO) derivative at 100K. 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,-tameMe ligand. The average Fe-N and Fe-NPY bond distance is 1.918(2) A and 2.084(2) A, 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)](Cl04)2 (Cis-1) and the series [Fe(C,tameMe)](BF4)2 (n = 10, 12,14,16,17,18,20) (C,-2) series show thermally driven spin crossover and mesomorphism above 300 K. The thermotropic behavior of compounds C,-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 C104- anions. This gives rise to a mesophase with molten alkyl chains but with restricted movement through the ionic layer. Compounds C,-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, Sx). These bi-functional materials combine spin state change and mesomorphic behavior in a narrow interval of temperatures, 300-375 K.