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 pzR2py 2-[3,5-bis(4-alkyloxy)phenyl]pyrazol-1-yl]pyridine (R = C6H4OCnH2n+1, n = 16, 18) have been synthesised. All compounds [M(pzR2py)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 pzR2py ligands (non-mesomorphism for n = 18 or monotropic liquid crystal for n = 16), as well as the coordination geometry. The silver(I) derivatives [Ag(pzR2py)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(pzR2py)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(pzR2py)] (R = C6H4OCnH2n+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(pzAn2py)] (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.