A Porous 4‑Fold-Interpenetrated Chiral Framework Exhibiting Vapochromism, Single-Crystal-to-Single-Crystal Solvent Exchange, Gas Sorption, and a Poisoning Effect

The synthesis and characterization of a 4-fold-interpenetrated
pseudodiamond
metal–organic framework (MOF), CoII(pybz)2·2DMF [pybz = 4-(4-pyridyl)­benzoate], are reported. N,N-Dimethylformamide
(DMF) of the channels can be removed to give the porous framework,
and it can also be exchanged for methanol, ethanol, benzene, and cyclohexane.
It is a rare example of a stable MOF based on a single octahedral
building unit. The single-crystal structures of CoII(pybz)2·2DMF, CoII(pybz)2, CoII(pybz)2·4MeOH, and CoII(pybz)2·2.5EtOH have been successfully determined. In all of them,
the framework is marginally modified and contains a highly distorted
and strained octahedral node of cobalt with two pyridine nitrogen
atoms and two chelate carboxylate groups. In air, the crystals of
CoII(pybz)2·2DMF readily change color from
claret red to light pink. Thermogravimetric analysis and Raman spectroscopy
indicate a change in coordination, where the carboxylate becomes monodentate
and an additional two water molecules are coordinated to each cobalt
atom. In a dry solvent, this transformation does not take place. Tests
show that CoII(pybz)2 may be a more efficient
drying agent than silica gel and anhydrous CuSO4. The desolvated
CoII(pybz)2 can absorb several gases such as
CO2, N2, H2, and CH4 and
also vapors of methanol, ethanol, benzene, and cyclohexane. If CoII(pybz)2 is exposed to air and followed by reactivation,
its sorption capacity is considerably reduced, which we associate
with a poisoning effect. Because of the long distance between the
cobalt atoms in the structure, the magnetic properties are those of
a paramagnet.

本文报道了一种四重互穿伪金刚石型金属有机框架（metal–organic framework, MOF）CoII(pybz)₂·2DMF的合成与表征，其中pybz为4-(4-吡啶基)苯甲酸根。该框架孔道内的N,N-二甲基甲酰胺（N,N-Dimethylformamide, DMF）可被脱除以得到多孔骨架，同时该溶剂也可被甲醇、乙醇、苯与环己烷置换。这是一例基于单一八面体构筑单元的稳定MOF，较为罕见。研究团队已成功解析出CoII(pybz)₂·2DMF、CoII(pybz)₂、CoII(pybz)₂·4MeOH以及CoII(pybz)₂·2.5EtOH的单晶结构。在所有上述结构中，框架均发生小幅修饰，且均包含一个高度畸变且受张力的钴八面体结点，该结点结合有两个吡啶氮原子与两个螯合羧酸根配体。在空气中，CoII(pybz)₂·2DMF晶体极易从酒红色转变为浅粉色。热重分析与拉曼光谱结果表明，其配位模式发生变化：羧酸根变为单齿配体，且每个钴原子额外配位两个水分子。在无水溶剂中，该转变不会发生。实验测试表明，CoII(pybz)₂或许是比硅胶与无水硫酸铜更高效的干燥剂。脱溶剂后的CoII(pybz)₂可吸附CO₂、N₂、H₂、CH₄等多种气体，以及甲醇、乙醇、苯与环己烷的蒸气。若将CoII(pybz)₂暴露于空气中并进行再活化处理，其吸附容量会显著下降，我们将这一现象归因于中毒效应。由于结构中钴原子间距离较长，该材料的磁学性质表现为顺磁性。