Effect of Pillar Modules and Their Stoichiometry in 3D Porous Frameworks of Zn(II) with [Fe(CN)6]3–: High CO2/N2 and CO2/CH4 Selectivity

We report the synthesis, single-crystal
structural characterization,
and selective gas adsorption properties of three new 3D metal–organic
frameworks of Zn­(II), {[Zn3­(bipy)3­(H2O)2]­[Fe­(CN)6]2·​2­(bipy)·​3H2O}n (1), {[Zn3­(bipy)]­[Fe­(CN)6]2·​(C2H5­OH)·​H2O}n (2), and {[Zn3­(azpy)2­(H2O)2]­[Fe­(CN)6]2·​4H2O}n (3) (bipy = 4,4′-bipyridyl
and azpy = 4,4′-azobipyridyl), bridged by [Fe­(CN)6]3– and exobidentate pyridyl-based linkers.
Compounds 1–3 have been successfully
isolated by varying the organic linkers (bipy and azpy) and their
ratios during the synthesis at RT. Frameworks 1 and 3 feature a biporous-type network. At 195 K, compounds 1–3 selectively adsorb CO2 and
completely exclude other small molecules, such as N2, Ar,
O2, and CH4. Additionally, we have also tested
the CO2 uptake capacity of 1 and 3 at ambient temperatures. By using the isotherms measured at 273
and 293 K, we have calculated the isosteric heat of CO2 adsorption, which turned out to be 35.84 and 35.53 kJ mol–1 for 1 and 3, respectively. Furthermore,
a reasonably high heat of H2 adsorption (7.97 kJ mol–1 for 1 and 7.73 kJ mol–1 for 3) at low temperatures suggests strong interaction
of H2 molecules with the unsaturated Zn­(II) metal sites
and as well as with the pore surface. Frameworks 1 and 3 show high selectivity to CO2 over N2 and CH4 at 273 K, as calculated based on the IAST model.
The high values of ΔHCO2 and ΔHH2 stem from
the preferential electrostatic interaction of CO2 with
the unsaturated metal sites, pendent nitrogen atoms of [Fe­(CN)6]3–, and π-electron cloud of bipyridine
aromatic rings as understood from first-principles density functional
theory based calculations.

本报告阐述了三种新型三维金属有机框架材料Zn(II) {[Zn3(bipy)3(H2O)2][Fe(CN)6]2·2(bipy)·3H2O}(1)、{[Zn3(bipy)][Fe(CN)6]2·(C2H5OH)·H2O}(2) 和 {[Zn3(azpy)2(H2O)2][Fe(CN)6]2·4H2O}(3)（其中bipy代表4,4'-联吡啶，azpy代表4,4'-偶氮联吡啶）的合成、单晶结构表征及其选择性气体吸附性能的研究。这些材料通过[Fe(CN)6]3-桥连和基于外配位吡啶的连接体连接而成。通过调节合成过程中的有机连接体（bipy和azpy）及其比例，成功分离出化合物1-3。框架1和3呈现双孔型网络结构。在195 K条件下，化合物1-3对CO2具有选择性吸附，并完全排斥其他小分子，如N2、Ar、O2和CH4。此外，我们还测试了1和3在常温下的CO2吸附能力。通过在273和293 K下测得的等温线，计算了CO2吸附的等熵热，分别为1和3的35.84和35.53 kJ mol–1。进一步地，在低温下，1和3对H2的吸附热分别为7.97 kJ mol–1和7.73 kJ mol–1，表明H2分子与不饱和Zn(II)金属位点以及孔隙表面的强相互作用。基于IAST模型，框架1和3在273 K时对CO2相对于N2和CH4表现出高选择性。ΔHCO2和ΔHH2的高值源于CO2与不饱和金属位点、[Fe(CN)6]3-的悬挂氮原子以及联吡啶芳香环的π电子云之间的优先静电相互作用，这一点可从基于第一性原理密度泛函理论计算中获得理解。