Thermally Stable Mesomorphic Palladium(II)−Carbene Complexes

The first thermally stable liquid crystalline Pd(II)−carbene complexes, trans-Pd((CnH2n+1)2−bimy)2Cl2 and Pd((CnH2n+1)2−imy)2Cl2 (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)2Cl2 shows that the two bimy planes are coplanar and are tilted ∼72° from the square plane around the Pd atom. In addition, the four alkyl chains are outstretched ∼28° 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.