Crystal structures and thermodynamics/kinetics of Zn(II) coordination polymers with helical chains

Abstract

Solvothermal reactions of Zn(II) acetates and four V-shaped carboxylates ligands in the presence of 1,4-Bis(2-methyl-imidazol-1-yl)butane afforded four interesting Zn(II) coordination polymers with helical chains, namely, {[Zn(bib)(atibdc)]·2H2O}n (1), {[Zn(bib)(atbip)]·H2O}n (2), [Zn(bib)(2,2′-tda)]}n (3) and {[Zn(bib)(5-tbipa)]·EtOH}n (4), (H2atibdc=5-amino-2,4,6-triiodoisophthalic acid, H2atbip=5-amino-2,4,6-tribromoisophthalic acid, 2,2′-H2tad=2,2′-thiodiacetic acid, 5-H2tbipa=5-tert-butyl-isophthalic acid). 1 reveals a 3D chiral framework with three kinds of helical chains along a, b and c axis. 2 shows a 2D step-type chiral framework with right-handed helical chains. 3 displays a wavelike 2D layer network possessing alternate left- and right-handed helical chains. 4 presents a four-connected 3D framework with zigzag and meso-helical chains. The different spacers and substituent group of carboxylic acid ligands may lead to the diverse network structures of 1–4. The fluorescent properties of complexes 1−4 were studied. In addition, the thermal decompositions properties of 1–4 were investigated by simultaneous TG/DTG–DSC technique. The apparent activation energy E and the pre-exponential factor (A) of skeleton collapse for the complexes 1–4 are calculated by the integral Kissinger's method and Ozawa–Doyle's method. The activation energy E (E1=209.658kJ·mol−1, E2=250.037kJmol−1, E3=225.300kJmol−1, E4=186.529kJ·mol−1) demonstrates that the reaction rate of the melting decomposition is slow. The thermodynamic parameters (ΔH‡, ΔG‡ and ΔS‡) at the peak temperatures of the DTG curves were also calculated. ΔG‡>0 indicates that the skeleton collapse is not spontaneous. ΔHd>0 suggests that the skeleton collapse is endothermic, corresponding to the intense endothermic peak of the DSC curve. The structural stability could be illustrated from the point of thermodynamics and kinetics.