Bridged 3,5-disubstituted pyrazolate ligands as support of metallomesogens containing [Pd(η3-C3H5)]+ fragments X-ray crystal structure of [Pd(η3-C3H5)(μ-pzR2)]2•CH2Cl2 (R = C6H4OC12H25).: Part III

The mesomorphic properties of two new long-chain 3,5-(4-n-alkoxyphenyl)pyrazoles Hpz(R2) (R = C6H4OCnH2n+1, n = 16, 18) have been studied and compared with those of related compounds containing shorter chains (n = 4, 6, 8, 10, 12, 14). For liquid crystal purposes, the derivative with 14 carbon atoms in the chain behaves as the best compound on the basis of the lowest melting point and the widest stability range of the mesophases. New Pd(II) complexes of formula [Pd(eta(3)-C3H5)(mu-pz(R2))](2) (R = C6H4OCnH2n+1, n = 8, 10, 12, 14, 16, 18) (1-6) containing the mesomorphic ligands as pyrazolate-bridging groups have been prepared and characterised. In solution, three conformational isomers (two symmetric and one asymmetric corresponding to different orientations of the allyl groups) have been found. In the solid state, the X-ray crystal structure of 3 (n = 12) evidenced the asymmetric isomer. The structure shows a bowlic core drawn on the basis of the boat-like conformation of the six-membered ring Pd(NN)(2)Pd. However, a rod-like shape could also be considered by taking the molecular dimensionality into account (47.80 Angstrom in length x 4.75 Angstrom in wide). The Pd-complexes 2-6 were found to have liquid crystal properties exhibiting enantiotropic smectic phases consistent with the molecular characteristics. Moreover, the molecular packing of 3 is described as the layer-like type in which the molecules are parallel to the c-axis but tilted off the layer, this fact appearing similar to the molecular ordering in the smectic fluid phases. (C) 2003 Elsevier B.V. All rights reserved.