Syntheses, Crystal Structures and Properties of Two New Zn Based Boron Imidazolate Frameworks①

BI Ming-Yue WEN Yi-HangZHANG Hai-Xia ZHANG Jian

a (Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces,Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China)

b (State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

ABSTRACT Two new boron imidazolate frameworks (BIFs), Zn2[HBH(2-mim)3]2(1,2-PEA)2(EG)2 (BIF-120,EG = ethylene glycol) and Zn[BH(2-mim)3](1,2-HPEA) (BIF-121), were successfully synthesized by mixing the KBH(2-mim)3 ligand and the semirigid aromatic dicarboxylate ligand 1,2-benzenediacetic acid (1,2-H2PEA) under solvothermal conditions. In this paper, the two samples were structurally characterized by single-crystal X-ray diffraction and tested by infrared spectroscopy (IR), UV-visible spectroscopy (UV-Vis), thermogravimetric analysis TGA and X-ray powder diffractions. In addition, the solid-state luminescent properties of these crystals were also investigated.

Keywords: boron imidazolate frameworks, crystal structure, X-ray crystallography, luminescent;

1 INTRODUCTION

Metal-organic frameworks (MOFs)[1-5]have attracted intensive attention because of their fascinating topological structures as well as their potential applications in gas storage/separation, catalysis, dye degradation, sensing, etc.Boron imidazolate frameworks (BIFs) are a sub-class of MOFs constructed by cross-linking various pre-synthesized boron imidazolate ligands and metal cations[6-15]. Due to the tetrahedral configuration of the boron imidazolate ligands, a number of BIF materials have been synthesized by simulating inorganic zeolite AlPO4, such as SOD-type BIF-3[6],RHO-type BIF-9[8], ACO-type BIF-22[12], interrupted-LTA-type BIF-20[11], and interrupted-ATN-type BIF-21[11].These structures possess high density of imidazolium functional groups and tetradentate metal centers on the pore surface. So, BIFs show unique advantages for potential applications in CO2capture and photocatalytic reduction reaction owing to the presence of functional pore surfaces[13-21]. Recent studies have revealed a number of BIFs that exhibit high performance in photocatalytic CO2reduction.In this context, the design and synthesis of new BIFs are quite desirable.

Due to only containing N-donor, the coordination mode of boron imidazolate ligands is limited. In order to construct diverse structures in BIFs, an effective strategy is to co-assemble with carboxylate ligands. To date, a variety of BIFs have been built based on mixing boron imidazolate ligands with long flexible carboxylate ligands or short rigid carboxylate ligands. However, the semirigid carboxylate as an auxiliary ligand used in the BIF system has not been reported.The introduction of semirigid ligand in BIF provides new opportunity for constructing unique structures. Here, two new BIFs were successfully synthesized by employing the BH(2-mim)3-ligand and the semirigid aromatic dicarboxylate ligand 1,2-benzenediacetic acid (1,2-PEA) under solvothermal condition. Single-crystal X-ray diffraction analysis revealed that Zn2[HBH(2-mim)3]2(1,2-PEA)2(EG)2(BIF-120) and Zn[BH(2-mim)3](1,2-HPEA) (BIF-121) possess 2-dimentional layer frameworks. Moreover, the solid-state luminescent properties of these crystals were also investigated.

2 EXPERIMENTAL

2. 1 Materials and instruments

All reagents were purchased commercially and used without further purification. Powder X-ray diffraction analyses were recorded on a Rigaku Dmax2500 diffractometer with CuKα radiation (λ = 1.54056 Å) with a step size of 0.02°. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer at a heating rate of 10 K/min under a N2atmosphere. Photoluminescent spectra were measured using a fluorescent spectrophotometer (Edinburgh,FLS980). IR spectra (KBr pellets) were recorded on an ABB Bomem MB102 spectrometer over a range of 400～4000 cm-1.

2. 2 Syntheses of BIF-120 and BIF-121

KBH (2-mim)3(35 mg, 0.12 mmol), Zn(CH3COO)2·2H2O(26 mg, 0.12 mmol) and 1,2-benzenediacetic acid (29 mg,0.15 mmol) in a mixed solvent of isopropanol (1 mL)/ethylene glycol (2 mL)/water (2 mL) were placed in a 20 mL vial. After that, the sample was heated at 80 ℃ for 3 days and then cooled to room temperature. After filtration, the powder was washed with ethanol and distilled water to obtain colorless crystals (BIF-120) (36 mg, yield about 17%,calculated based on Zn(CH3COO)2·2H2O).

KBH (2-mim)3(35 mg, 0.12 mmol), Zn(CH3COO)2·2H2O(26 mg, 0.12 mmol) and 1,2-benzenediacetic acid (29 mg,0.15 mmol) in a mixed solvent of n-propanol (1 mL)/DMSO(2 mL)/water (2 mL) were placed in a 20 mL vial. The sample was then heated at 80 ℃ for 3 days followed by cooling to room temperature. After filtration, the powder was washed with ethanol and distilled water to obtain colorless crystals(BIF-121) (30 mg, yield about 14%, calculated based on Zn(CH3COO)2·2H2O).

For IR, the KBr tablet method was used to prepare samples,and the data were normalized to find the characteristic peak of 13000～1500 cm-1. The main characteristic peaks of BIF-120 are 3480, 2815, 1401, 1375, 1305, 1118 and 745 cm-1, and those of BIF-121 are 3185, 2560, 1805, 1505, 1480, 1335,1305, 1005 and 750 cm-1.

2. 3 Crystal structure determination

Colorless crystals of BIF-120 and BIF-121 were selected for diffraction data collection on a ROD, Synergy Custom system, HyPix diffractometer. The crystal was kept at 108.33(10) K during data collection. Using Olex2, the structure was solved with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package using Least-Squares minimization. All atoms were refined with anisotropic thermal parameters.

Crystal data for BIF-120 (Zn2[HBH(2-mim)3]2(1,2-PEA)2(EG)2, M = 1150.44 g/mol): monoclinic, space group Cc, a = 19.4341(3), b = 14.2818(2), c = 20.4023(3) Å, β =110.933(2)º, V = 5288.99(15) Å3, Z = 4, T = 108.33(10) K,μ(GaKα) = 1.066 mm-1, Dc= 1.445 g/cm3, 28311 reflections measured (6.846°≤2θ≤113.8°), 9018 unique (Rint= 0.0338,Rsigma= 0.0291) which were used in all calculations. The final R = 0.0402 (I > 2σ(I)) and wR = 0.1103 (all data).

Crystal data for BIF-121 (Zn[BH(2-mim)3](1,2-HPEA), M= 554.73 g/mol): orthorhombic, space group Pbca, a =15.6472(7), b = 15.5892(8), c = 21.1110(9) Å, V = 5149.5(4)Å3, Z = 8, T = 100.00(11) K, μ(CuKα) = 1.686 mm-1, Dc=1.431 g/cm3, 15504 reflections measured (9.038°≤2θ≤133.166°), 4232 unique (Rint= 0.0331, Rsigma= 0.0263) which were used in all calculations. The final R = 0.0319 (I > 2σ(I))and wR = 0.0889 (all data).

3 RESULTS AND DISCUSSION

3. 1 Crystal structure descriptions of BIF-120 and BIF-121

Single-crystal X-ray structure analysis showed that BIF-120 crystalizes in the monoclinic space group Cc. As shown in Fig.1, the asymmetric unit contained two Zn ions,two HBH(2-mim)3ligands and two 1,2-PEA ligands. Both of the Zn ions adopted a ZnN2O2tetrahedral geometry. Each Zn ion was four-coordinated with two N atoms from two different HBH(2-mim)3ligands and two O atoms from two different 1,2-PEA ligands. The tridentate BH(2-mim)3-ligands were protonated during the crystallization process. Two HBH(2-mim)3ligands linked bridged two Zn ions to form a regular quadrilateron with the 2-methylimidazole ring as the four sides (Fig.1). The Zn-N bond lengths range from 1.988 to 2.024 Å and the Zn-O bond lengths vary from 1.948 to 1.964 Å, respectively. Each B atom is covalently bound to three N atoms from 2-methylimidazole ligands with B-N bond lengths from 1.527 to 1.567 Å. Each 1,2-PEA ligand adopted a bis-monodentate coordination mode linking two adjacent regular quadrangles to form a 2-dimensional layer(Fig.2a). As shown in Fig.2b, the 2D layers packed with each other to generate a 3-dimensional supramolecule. Moreover,rich hydrogen bonding interactions exist between them and further stabilize the 3-dimensional framework of BIF-120.

Fig.1. Coordination environment in BIF-120

Fig.2. (a) 2-dimensional layer in the bc plane; (b) 2D layer structure in BIF-120

Single-crystal X-ray structure analysis showed that BIF-121 crystalizes in the orthogonal space group Pbca. As shown in Fig.3, the asymmetric unit contained one crystallographic Zn ion, one BH(2-mim)3-ligand and one 1,2-HPEA ligand. Each Zn ion was four-coordinated with three N atoms from three different BH(2-mim)3-ligands and O atoms from the 1,2-HPEA ligands. The tridentate BH(2-mim)3-ligand worked as μ3-bridge to link three adjacent Zn ions, leading to a 3-connected 2-dimensional layer (Fig.4a).The coordination mode of ligand 1,2-HPEA in BIF-121 was different from that of BIF-120. Only one carboxyl group of 1,2-HPEA ligand deprotonated to coordinate with Zn ion, and the other carboxyl group without deprotonation formed rich hydrogen bonds with the adjacent carboxyl oxygen and nitrogen on the imidazolate ring (Fig.4b and Table 1). By considering the B and Zn atoms as the 3-connected nodes, the framework can be topologically regarded as 3-connected hcb type plane. Furthermore, 1,2-HPEA ligands bridged adjacent 2-dimensional layers by these hydrogen bonds to form a 3-dimensional framework (Fig.5).

Fig.3. Coordination environment in BIF-121

Fig.4. (a) 2-dimensional layer in the ac plane; (b) Hydrogen bonding in BIF-121

Fig.5. Packing motif of BIF-121

Table 1. Intermolecular Hydrogen Bonding Interactions (Å, °) in BIF-121

3. 2 Characterization of BIF-120 and BIF-121

The phase purity of the bulk samples of BIF-120 and BIF-121 was checked by powder X-ray diffraction (PXRD)patterns. As shown in Figs. S1 and S2, the XRD patterns of BIF-120 and BIF-121 were very consistent with the simulated patterns calculated from the single-crystal diffraction data, indicating the successful preparation and phase purity. Thermogravimetric analysis (TGA) in a N2atmosphere was studied to investigate the thermal stability. As indicated by the TGA data of BIF-120 (Fig.S3), there was some weight loss of small guest molecules from room temperature to 100 ℃. It was stable up to 180 ℃, and finally the framework collapsed during heating. BIF-121 (Fig.S5)had almost no weight loss above 250 ℃, which proves that BIF-121 is more stable than BIF-120. But after three days of soaking in ethanol solution, the guest was exchanged. In Fig.S4, it can be seen that BIF-120 is very stable below 180 ℃. The FTIR spectrum in Figs. S6 and S7 shows that the strong peaks of νas(-COO-) at 1500～1490 cm-1and νs(-COO-) at 1350～1490 cm-1are assigned to the ligand 1,2-PEA carboxylic acid groups asymmetric and symmetric stretching vibration[22]. BIF-120 and BIF-121 were immersed in different solvents and different pH solutions for one day(Figs. S8 and S9), and compared with XRD after washing with ethanol. Both of them had better stability.

3. 3 Photoluminescence properties

From the perspective of the interaction between metal and ligand, an inorganic-organic hybrid coordination polymer composed of π-conjugated organic ligands 1,2-PEA and d10metal Zn(II) centers with photoluminescence properties[23,24].The fluorescence properties of complexes BIF-120 and BIF-121 have been tested in the solid state with 365 nm excitation light at room temperature (Fig.6). It can be seen from the emission spectra that BIF-120 shows a broad emission band at 425 nm and BIF-121 shows it at 440 nm (λex= 365 nm), which may be attributed to the coordination interaction between the ligand and the central metal Zn(II)[25].Compared with the free 1,2-PEA and KBH(2-mim)3ligands,BIF-121 has a wide emission range, with a maximum peak at 440 nm, which has a red shift. This emission band can be temporarily attributed to the ligand-to-metal charge transfer(LMCT).

Fig.6. Emission spectra of BIF-120, BIF-121, 1,2-PEA and KBH(2-mim)3 ligand (λex = 365 nm) in the solid state at room temperature

4 CONCLUSION

In summary, two novel 2-dimensional boron imidazolate frameworks (BIF-120 and BIF-121) with BH(mim)3-and 1,2-benzenediacetic acids were successfully prepared and characterized by single-crystal X-ray diffraction analysis as well as spectroscopy. Solid-state photoluminescence studies revealed that BIF-121 shows emission at 440 nm while BIF-120 shows emission at 425 nm.