Metal-organic frameworks (MOFs), assembled from organic ligands and metal ions/clusters[1–3], have been attracted researcher’s attention over the last few years by virtue of their fascinating architectures and topologies[4] as well as broad potential applications such as gas adsorption/separation[5, 6], luminescence[7, 8], catalysis[9, 10], magnetic[11, 12], sensing[13, 14], and so on. Many chemists are working hard to explore new strategies in order to synthesize target materials, but it is very challenging to synthesize MOFs owning expected structure and performance[15, 16]. Compared with traditional porous materials, the greatest advantage of MOFs is that it can control the metal nodes and organic linkers in the synthesis process, thus resulting in a variety of MOFs materials possessing different surface areas, pore environments and chemical functions[17–19]. Therefore, judicious selection of ligands with different size, shape and functional group together with metal ions can effectively build novel and functional MOFs[20–22]. Moreover, the highly crystalline nature of MOFs enables its structure can be precisely determined from single crystal X-ray diffraction and molecular level information can be obtained, which make the research of mechanism is clearer[23].
2-Nitroimidazole (nIm), a nitro-functionalized imidazole connector, has been extensively employed to prepare Zeolitic Imidazolate Frameworks (ZIFs)[24–27]. On one hand, the nitro group has the higher dipole moment, which is conducive to formation of dipole-quadrupole interactions with CO2 molecules[28]. On the other hand, imidazole N atoms and nitro O atoms can provide robust coordination abilities in bridging metal ions[29]. In contrast to widely-used transition metal ions, main group Pb2+ ion features a flexible coordination preference and 6s25d10 electronic configuration, giving rise to more opportunities for the construction of unique frameworks and excellent luminescent properties[30]. However, to the best of our knowledge, MOFs building from Pb2+ ions and 2-nitroimidazole have never been reported.
Based on the above considerations, a MOF comprising Pb2+ ion and nIm, [Pb5(OAc)7(nIm)3]n (1), has been successfully prepared. 1 is based on the ribbon-shaped Pb-O secondary building unit (SBU) and shows a 2D porous framework with a 1D tubular channel. Moreover, 1 reveals moderate CO2 adsorption capability, and nitro groups together with metal ions play an important role in adsorption process which is confirmed by GCMC simulations. We also explored the electronic structures and luminescence properties, and 1 presents a broad emission band originated from intraligand charge transfer.