Mesomorphism of Four-Coordinated Four-Chained Metal Complexes Based on Pyrazolylpyridine Derivatives

Abstract

A homologous series of four-chained silver(I) and zinc(II) metal complexes based on the coordination of two N N′-bidentate ligands pz^R2py 2-[3,5-bis(4-alkyloxy)phenyl]pyrazol-1-yl]pyridine (R = C₆H₄OC_nH_2n+1, n = 16, 18) have been synthesised. All compounds [M(pz^R2py)₂][A]_m (M = Ag, m = 1, n = 16, 18, (1,2), (3,4), CF₃ (OTf^–) (5,6), (7,8); M = Zn, m = 2, n = 16, 18, (9,10)) exhibit liquid crystal properties showing enantiotropic SmA phases, independently of the starting pz^R2py ligands (non-mesomorphism for n = 18 or monotropic liquid crystal for n = 16), as well as the coordination geometry.

The silver(I) derivatives [Ag(pz^R2py)₂]⁺ containing and as counteranions (1–4) are four-coordinated cationic complexes, but those containing OTf^– and (5–8) exhibit a five-coordinated metal environment with distorted base-square pyramidal/trigonal bipyramidal geometry involving the counteranion. For the zinc(II) related compounds [Zn(pz^R2py)₂][NO₃]₂ (9–10), the tetrahedral metal environment is also distorted by the presence of coordinative interactions with the groups.

Neutral tetrahedral compounds [ZnCl₂(pz^R2py)] (R = C₆H₄OC_nH_2n+1, n = 16 (11), 18 (12), 1 (13)) have also been investigated. Crystal structure of the first member of this series with n = 1, [ZnCl₂(pz^An2py)] (13), has been characterised by single crystal X-ray diffraction. The molecules adopt the tetrahedral coordination environment with a planar ZnNNCN core. That structure for the related compounds containing longer alkyloxy chains (11–12) suggests a rod-like molecular shape in agreement with their observed liquid crystal lamellar phases.

Keywords:

• four-chained complexes
• four-coordinated silver and zinc complexes
• pyrazolylpyridine ligands
• smectic mesophases
• XRD studies

Acknowledgments

Financial support from the DGI of the Ministerio de Educación y Ciencia (Project No. CTQ2006-13344) and Universidad Complutense/Comunidad Autónoma de Madrid (UCM2006-910300) is gratefully acknowledged.

Notes

^a Ref. [12]

^b Refs. [13 14]

*Temperature observed by POM.

(a) At this temperature the texture of the SmA mesophase is maintained but no mobility is observed.

^a Temperatures on cooling.

^b Halo of the molten alkyloxy chains.

^c See discussion.

*Temperature after mesophase is observed.