Synthesis and Mesogenic Properties of Binuclear Copper (II) Complexes Derived from Salicylaldimine Schiff Bases


The synthesis and mesomorphic (liquid crystal) properties of new binuclear dihalocopper(II) complexes derived from N- and ring-substituted salicylaldimine Schiff bases are reported, together with the mesomorphic properties of their monomeric precursor complexes. with just N-substituents both the dichlorodicopper(II) binuclear complexes and their mononuclear analogues are waxy solids with melting points that increase with their N-chain length. However, with both N- and ring-substituents in the 4-positions, the mononuclear and binuclear complexes are each liquid crystalline or mesogenic, except in case of the mononuclear complexes where the N-substituent is straight chain alkyl. The other mononuclear complexes exhibit a variety of liquid crystal phases: smectic A, C, and E (SA, SC, and SE, respectively). The liquid crystal phase SA is observed in the binuclears with shorter chain N-substituents p-R−O−C6H4− and shorter chain ring-substituents. The chain lengths were increased until the phase behavior expanded to a further form SC in the case of an N-substituent p-C14H29O−C6H4− and a −OC12H25 ring substituent. This points the way toward achieving multiphase behavior with these binuclear systems. The Cu−Br analogues of the binuclear complexes behave similarly but with significant qualitative differences, specifically lower mesophase stability and higher melting temperatures. The structures of the nonmesogenic binuclears ([Cu(N-dodecylSal)X]2, X = Cl, Br) were determined with the aid of X-ray crystallography. These are prototypes for the structures of the binuclear complexes and especially for the shape of the central Cu2O2 X2 core in the binuclears: distorted planar coordination about the copper with distortion toward tetrahedral measured by a characteristic twist angle τ (0° planar; 90° tetrahedral). The binuclear complexes also show magnetic coupling which can be used to estimate the geometry. For [Cu(N-dodecylSal)X]2 τ > 36°, which corresponds to weaker coupling than observed in the mesogenic binuclears where a stronger magnetic coupling indicates a geometry closer to planar (τ = 25°). The mesophases were characterized by differential scanning calorimetry (DSC) analysis and optical polarized microscopy.



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