(Pyrazolato)gold complexes showing room-temperature columnar mesophases. Synthesis, properties, and structural characterization

Mesogenic (pyrazolato)gold complexes of formula [Au(pz)](3) (pz = 3,5-bis(3',4'-di-n-decyloxyphenyl)pyrazole (1), 3-(3',4',5'-tri-n-decyloxyphenyl)-5-(3 ",4 "-di-n-decyloxyphenyl)pyrazole (2), 3-(2',3',4'-tri-n-decyloxyphenyl)-5-(3 ",4 ",5 "-tri-n-decyloxyphenyl)pyrazole (3), 3,5-bis(3',4',5'-tri-n-decyloxyphenyl)pyrazole (4)) have been prepared by reaction of the potassium salts of the nonmesogenic pyrazolate ligands with [AuCl(tht)] (tht = tetrahydrothiophene) in a 1:1 molar ratio. All these compounds show columnar mesophases that remain stable, at room temperature, for long periods of time. The formation of isomers for the nonsymmetrical derivatives-detected by spectroscopic studies-are suggested to be responsible for the subtle changes observed in the transition temperatures. Powder X-ray diffraction measurements show clearly that the supramolecular columnar arrangement appears in the crystalline solids as well as in the mesomorphic phase. An analogue of the mesogenic trinuclear complexes 1-4, having a methoxy group at the phenyl substituents of the pyrazolate ligands, [Au{3,5-(MeOPh)(2)Pz}](3) (5), has been synthesized and characterized by an X-ray single-crystal diffraction experiment. The molecular structure of 5 is based on a nine-membered metallacycle core. The whole molecule exhibits a rough planarity that favors a crystalline structure formed from columnar arrangements of trinuclear complexes. A simple and clear relationship could be established between the solid-state crystal structure of 5 and the X-ray-deduced structure of the mesophases.