Pyridyl and pyridiniumyl β-diketones as building blocks for palladium(ii) and allyl–palladium(ii) isomers. Multinuclear NMR structural elucidation and liquid crystal behaviour

Abstract

A series of novel β-diketone ligands bearing alkyloxyphenyl R = C₆H₄OC_nH_2n+1 and pyridyl substituents at 1 and 3 positions of the β-diketone core [HL^R(n)py] have been prepared and structurally characterized. The corresponding β-diketonate complexes [Pd(κ²-L^R(n)py)₂] were isolated as a mixture of cis and trans isomers each of them exhibiting the ligand in a NO- or OO-bidentate coordinative mode which were established by multinuclear NMR studies (¹H, ¹³C and ¹⁵N). However none of those characteristics were determining to achieve liquid-crystalline properties on the new complexes. By contrast reactions of the aforementioned ligands [HL^R(n)py] or those containing the pyridiniumyl substituent [HL^R(n)pyH]⁺ towards the allyl–palladium fragment were successful to afford ionic metallomesogenic materials isolated as PF₆ salts of [Pd(η³-C₃H₅)(κ²-HL^R(n)py)][PF₆] (I) and [Pd(η³-C₃H₅)(κ²-L^R(n)pyH)][PF₆] (II) types. Structural characterization and mesomorphic properties were established by multinuclear NMR and the use of DSC and POM techniques. In complexes I and II the corresponding β-diketone ligands were coordinated as neutral bidentate NO-donor or zwitterionicOO-donor systems, respectively. Complexes of type II exhibit a range of the SmC mesophases significantly greater than that found in the type I.