Thermally stable mesomorphic palladium(II)-carbene complexes

The first thermally stable liquid crystalline Pd(II)-carbene complexes, trans-Pd((CnH2n+1)(2)-bimy)(2)Cl-2 and Pd((CnH2n+1)(2)-imy)(2)Cl-2 (n = 10, 12, 14, 16; bimy = benzimidazol-2-ylidene, imy = imidazol-2-ylidene), have been prepared and characterized. The crystal structure of trans-Pd((C10H21)(2)-bimy)(2)Cl-2 shows that the two bimy planes are coplanar and are tilted similar to 72 degrees from the square plane around the Pd atom. In addition, the four alkyl chains are outstretched similar to 28 degrees off the bimy ring planes and are almost perpendicular to the square plane. The arrangement of this type of rodlike molecules results preferentially in the formation of a monolayered lamellar structure in the solid state, and these Pd(II)-carbene complexes appear to arrange in a similar lamellar structure in the mesophase. Phase-transition temperatures determined by differential scanning calorimetry show that the carbon lengths of alkyl chains have a greater influence on the clearing temperatures than on the melting temperatures. The Pd-imy complexes also have a wider temperature range of the mesophases than the Pd-bimy analogues.