Tuning the Molecular and Cationic Affinity in a Series of Multifunctional Metal–Organic Frameworks Based on Dodecanuclear Zn(II) Carboxylate Wheels

A series of new zinc­(II)–thiophene-2,5-dicarboxylate
(tdc)
MOFs based on novel dodecanuclear wheel-shaped building blocks has
been synthesized in almost quantative yields. Single-crystal X-ray
diffraction analyses reveal 3D porous frameworks with a complex composition
[Zn12(tdc)6(glycolate)6(dabco)3] where glycolate is a deprotonated polyatomic alcohol (ethylene
glycol, EgO2, 1; 1,2-propanediol, PrO2, 2; 1,2-butanediol, BuO2, 3; 1,2-pentanediol, PeO2, 4; glycerol, GlO2, 5) and dabco is 1,4-diazo[2.2.2.]­bicyclooctane.
All compounds 1–5 are isostructural
except for pendant groups of the diols decorating the surface of channels.
The adsorption of small gases (N2, CO2, CH4, C2H2, C2H4,
C2H6) and larger hydrocarbons (benzene, cyclohexane)
both in liquid and vapor phases was thoroughly investigated for all
compounds. The zero-coverage adsorption enthalpies, Henry constants,
and selectivity factors by various models are calculated and discussed.
The versatile adsorption functionality of the title series results
from the variable nature of the diol and could be tailored for a specific
adsorbate system. For example, 1 shows excellent selectivity
of benzene over cyclohexane (20:1 for vapors, 92:1 for liquid phase),
while 4 demonstrates unprecedented adsorption preference
of cyclohexane over benzene (selectivity up to 5:1). The compound 5 demonstrates great adsorption selectivity for CO2/N2 (up to 75.1), CO2/CH4 (up to
7.7), C2H2/CH4 (up to 14.2), and
C2H4/CH4 (up to 9.4). Also, due to
polar nature of the pores, 5 features size-selective
sorption of alkaline metal cations in order Li+ > Na+ > K+ > Cs+ as well as a notable
luminescent
response for cesium­(I) ions and urea.

本研究合成了一系列基于新型十二核轮状结构基元的二价锌-噻吩-2,5-二羧酸根（thiophene-2,5-dicarboxylate, tdc）金属有机框架（Metal-Organic Frameworks, MOFs），产物收率接近定量。单晶X射线衍射分析结果表明，该系列化合物具有三维多孔骨架，其化学组成为[Zn12(tdc)6(glycolate)6(dabco)3]，其中乙醇酸根（glycolate）源自去质子化的多元醇：乙二醇（ethylene glycol, EgO2, 1）、1,2-丙二醇（1,2-propanediol, PrO2, 2）、1,2-丁二醇（1,2-butanediol, BuO2, 3）、1,2-戊二醇（1,2-pentanediol, PeO2, 4）以及甘油（glycerol, GlO2, 5）；而dabco为1,4-二氮杂双环[2.2.2]辛烷（1,4-diazo[2.2.2]bicyclooctane）。除修饰于孔道表面的二醇侧基存在差异外，化合物1~5均为同构体。针对所有化合物，我们系统研究了其对小分子气体（N2、CO2、CH4、C2H2、C2H4、C2H6）以及较大烃类分子（苯、环己烷）在液相与气相中的吸附行为。通过多种模型计算并讨论了零覆盖吸附焓、亨利常数以及选择性因子。该系列标题化合物的多功能吸附性能源于二醇结构的可变性，可针对特定吸附体系进行定制化设计。例如，化合物1对苯相较于环己烷展现出优异的吸附选择性（气相选择性为20:1，液相选择性为92:1）；而化合物4则表现出前所未有的环己烷相较于苯的吸附偏好（选择性最高可达5:1）。化合物5对CO2/N2（最高75.1）、CO2/CH4（最高7.7）、C2H2/CH4（最高14.2）以及C2H4/CH4（最高9.4）均表现出优异的吸附选择性。此外，由于孔道具有极性特性，化合物5还可实现对碱金属阳离子的尺寸选择性吸附，吸附顺序为Li+ > Na+ > K+ > Cs+，同时对铯(I)离子与尿素展现出显著的发光响应。