Pyridylpyrazole derivatives. A new type of mesogenic bidentate ligands inducing mesomorphism on their related PdX 2 complexes M.J. Mayoral, M.C. Torralba, M. Cano * , J.A. Campo, J.V. Heras Departamento de Quımica Inorg anica I, Facultad de Ciencias Quımicas, Universidad Complutense, Madrid E-28040, Spain Received 4 December 2002; accepted 27 January 2003 Abstract A new family of 3,5-bis(4-n-alkyloxyphenyl)-1-(pyridin-2-yl)-1H-pyrazole ligands (pypz R2 ) and their complexes of formula c- [PdCl 2 ðpypz R2 Þ] have been synthesised and their mesomorphism was investigated . The effectiveness of the asymmetric molecular design in promoting thermotropic mesophases is greater for complexes than for the related ligands, although the latter exhibit mesophases at lower temperatures. These complexes are the first example of metallomesogens containing N,N 0 -bidentate ligands co- ordinated to PdCl 2 . Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Long-chain 3,5-disubstituted-1-(pyridin-2-yl)pyrazoles; Pd(II) complexes; Mesomorphism 1. Introduction In the liquid crystal research metallomesogens are a continuous expansion area [1–3]. However, so far cala- mitic metallomesogens containing N,N 0 -bidentate li- gands are very scarce and the few examples described, such as Cu(II) or Ni(II) derivatives with 2-substituted pyrrole ligands, mainly exhibited nematic phases at temperatures higher than 200 °C [4]. By contrast, a va- riety of metal derivatives with Schiff base ligands have been reported as calamitic mesogens [1] and, among them, the square-planar Pd(II) derivatives exhibited a higher stability of the mesophases. The square-planar geometry of metal centre has been associated to the mesomorphism of a wide variety of Pd(II) complexes. It appears to favour the molecular interactions and hence the stability of the mesophases. In this context, mesogenic PdX 2 derivatives based on bidentate ligands have been centred exclusively on or- thopalladated azobenzenes and related species. These are dimers of the type di-(l-halobridged)orthopalla- dated or more scarcely mononuclear derivatives [1,2,5,6], the latter usually exhibiting nematic and smectic phases at lower temperatures than the related symmetrical binuclear compounds. In a previous work we reported the mesomorphic properties of the pyrazoles Hpz R2 (R ¼ C 6 H 4 OC n H 2nþ1 ; n ¼ 4, 6, 8, 10, 12, 14), containing alkyloxyphenyl sub- stituents at third and fifth positions of the heterocyclic ring, as well as the chemistry of their complexes [RhClðCOÞ 2 ðHpz R2 Þ] which were found to be non-liquid crystal materials [7]. Similar results were obtained using the non-mesogenic 3-substituted pyrazoles Hpz R (R ¼ C 6 H 4 OC n H 2nþ1 ; n ¼ 6, 8, 10) [8]. By contrast, we found that t-[PdCl 2 ðHpz R Þ 2 ] (R ¼ C 6 H 4 OC n H 2nþ1 ; n ¼ 6, 8, 10, 12, 14, 16, 18) were calamitic, their liquid crystal properties being associated to the strategic molecular skeletal design [9]. Following these precedents we are now implicated in the study of a new type of ligands and their Pd-com- plexes based on the mesogenic pyrazoles Hpz R2 . The ligands were pyridylpyrazole type asymmetric deriva- tives designed by incorporation of a pyridyl group at first position of the parent pyrazole Hpz R2 . They should be appropriate candidates to be co-ordinated to a PdX 2 group in a N,N 0 -bidentate form giving rise to a cis Inorganic Chemistry Communications 6 (2003) 626–629 www.elsevier.com/locate/inoche * Corresponding author. Fax: +34-91-3944352. E-mail address: mmcano@quim.ucm.es (M. Cano). 1387-7003/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S1387-7003(03)00053-4