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.
R. Paschke et al., "Synthesis and Mesogenic Properties of Binuclear Copper (II) Complexes Derived from Salicylaldimine Schiff Bases," Inorganic Chemistry, American Chemical Society (ACS), Nov 2003.
The definitive version is available at http://dx.doi.org/10.1021/ic0301021
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