A 2D porous Pb-MOF based on 2-nitroimidazole: CO2 adsorption, electronic structure and luminescence

A new porous metal-organic framework, [Pb 5 (Ac) 7 (nIm) 3 ] n (1), has been successfully synthesized by employing 2-nitroimidazole ligand and Pb 2+ ion. 1 contains novel the ribbon-shaped Pb-O SBU and reveals a 2D porous framework with a 1D tubular channel. Moreover, 1 shows moderate adsorption uptake towards CO 2 and luminescence properties from intraligand charge transfer. We further conrmed nitro group and metal ion are important adsorption sites by GCMC simulations, and the electronic structures of 1 was investigated.


Introduction
Metal-organic frameworks (MOFs), assembled from organic ligands and metal ions/clusters [1][2][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][18][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][21][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][25][26][27]. On one hand, the nitro group has the higher dipole moment, which is conducive to formation of dipole-quadrupole interactions with CO 2 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 Pb 2+ ion features a exible coordination preference and 6s 2 5d 10 electronic con guration, 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 Pb 2+ ions and 2-nitroimidazole have never been reported.
Based on the above considerations, a MOF comprising Pb 2+ ion and nIm, [Pb 5 (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 CO 2 adsorption capability, and nitro groups together with metal ions play an important role in adsorption process which is con rmed by GCMC simulations. We also explored the electronic structures and luminescence properties, and 1 presents a broad emission band originated from intraligand charge transfer.

Materials and general methods
All reagents were commercially available. Fourier transform infrared spectrum (FTIR) was obtained with a Nicolet FTIR 170 SX spectrophotometer in the range of 4000 − 400 cm − 1 . The uorescent spectra for the solid samples were measured with a Hitachi F-4500 uorescence spectrophotometer at room temperature. Elemental analyses for C, H, and N were performed with a Perkin-Elmer 2400C Elemental analyzer. Thermogravimetric analysis (TGA) was measured with a NETZSCH TG 209 thermal analyzer under a nitrogen atmosphere with a heating rate of 10°C min − 1 . Powder X-ray diffraction (PXRD) pattern was recorded on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα, 1.5418 Å). Gas sorption was tested with a Micrometrics ASAP 2020M apparatus.

Crystallography
Crystal structure was determined at 296(2) K by a Bruker SMART APEX II CCD diffractometer with a Mo Kα radiation source. The structure was solved by direct methods and re ned on F 2 by full-matrix leastsquares methods using the Olex2 program [31]. The non-H atoms were re ned anisotropically, while the H atoms xed to their geometrically ideal positions were re ned isotropically. It was failed to determine the right model of solvent molecules from the difference Fourier map, thereby the SQUEEZE routine of PLATON program was used in structural re nement [32]. The re nement result and selected bond distances/angles are given in Tables 1 and S1, respectively. CCDC Number: 2041796.

PXRD and TGA
The phase purity of sample of 1 was con rmed by coincident PXRD patterns between the simulated and experimental (Fig. 2). TGA of 1 was obtained in a N 2 atmosphere (Fig. 3). 1 is stable up to 150 ℃, beyond this temperature, the framework begins to collapse.

Luminescence properties
As shown in Fig. 4, luminescent spectra of free ligand and 1 in solid state were recorded at room temperature. The ligand exhibits a broad emission band with two emission peaks at 470 and 485 nm when excited at 300 nm, which may be attributed to the π → π* or n → π* transitions of the intraligands.
1 also shows two uorescence maximum emissions centered at 470 and 485 nm upon excitation at 423 nm, which is similar to that of free ligand. Therefore, the emission band of 1 can probably be ascribed to the intraligand charge transfer of nlm ligand [33].

Frontier molecular orbital analysis
In order to further explore the electronic structures of MOFs, density functional theoretical (DFT) calculations were performed. As shown in Fig. 5, the frontier molecular orbital (FMO) includes the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in which HOMO value determines a molecule's ability to provide electron whereas LUMO value determines the electronaccepting nature.
The HOMO-LUMO energy plot reveals that HOMO of 1 is primarily contributed from the coordinated carboxylate groups (donor). In LUMO, the electron density is mainly focused on the nitro groups. The energy of HOMO is -6.489 eV whereas of LUMO is -4.343 eV. The energy gap ∆E of HOMO-LUMO is 2.146 eV which is slightly high, indicating that the complex is stable [34].

Sorption properties
Due to the existence of channels in 1, sorption properties were assessed by gas adsorption experiment.
The adsorption isotherms of N 2 at 77 K reveal no uptake, but 1 reveals a CO 2 loading of 25.8 cm 3 cm − 1 at 298 K and 760 mmHg (Fig. 6a), which is higher than that of 1-Eu [35] and {[Cd(bdc)(4-bpmh)]} n ·2 n(H 2 O) [36], but lower than that of {[Cd(2-NH 2 bdc)(4-bpmh)]} n ·2 n(H 2 O) [36]. In order to further understanding of the interaction details of 1 with CO 2 , GCMC simulations have been carried out at 298 K and 100 kPa. The obtained density contours uncovered the mostly populated sites are located in the vicinity of O atoms of -NO 2 groups and Pb atoms in pore (Fig. 6d). Two preferential CO 2 binding sites in 1 were found. For CO 2 -I, two electronegative O atoms of two -NO 2 groups simultaneously attract one electropositive C atom of one

Conclusions
In summary, a porous Pb-MOF based on ribbon-shaped SBUs and 2-nitroimidazole has been constructed.
In addition, 1 shows not only moderate CO 2 adsorption capability which is con rmed by GCMC simulations and experiment, but also luminescence emission band with two emission peaks resulting from intraligand charge transfer. And the frontier molecular orbital of 1 was analyzed.