Synthesis, Crystal Structure and I2 Captureof a Cd(II) Coordination Polymer Basedon a Linear Bipyridyl Benzene Ligand①

ZHANG You-Jun LIU Lei-Lei ZHANG Yn WANG Zhi-Peng DONG Xio-Fng LI Qing-Qing CHEN Jing

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Synthesis, Crystal Structure and I2Captureof a Cd(II) Coordination Polymer Basedon a Linear Bipyridyl Benzene Ligand①

ZHANG You-JuanaLIU Lei-LeiaZHANG Yana,bWANG Zhi-PengaDONG Xiao-FangaLI Qing-QingaCHEN Jinga,b②

a(455002)b(450052)

A new complex with formula[Cd(3-pbpmb)2I2]n1(3-= 1,4-bis(pyridine- 3-ylmethoxy)benzene), has been solvothermally synthesized and characterized. The crystal belongs to monoclinic system, space group21/with= 8.2905(17),= 16.078(3),= 13.213(3) ?,= 99.44(3)°,M= 950.86,= 1737.3(6) ?3,= 2,D= 1.818 g/cm3,(000) = 924,= 2.450 mm-1, the final= 0.0198 and= 0.0467 for 3053 observed reflections (> 2()).The Cd(II) ion is coordinated by four N atoms of four 3-ligands and two iodine ions to furnish a distorted octahedral coordination geometry. The Cd(II) ion and its own symmetry-related ions are bridged by N atoms from two 3-ligands to generate a “zigzag” 1-D chain coordination polymer. These 1D chains are further connected into a three-dimensional (3D) framework by C–H×××interactions and intermolecular C–H×××I hydrogen bond. Interestingly, 1 displays efficient sorption of molecular iodine gas (up to 111 wt%), but invalid sorption of molecular water gas.

crystal structure, Cd(II) compound, coordination polymer, I2capture;

1 INTRODUCTION

Nuclear energy is currently consideredasthe best alternative to meet increased worldwide energy demands and reduce greenhouse gas emissions if the challenge problem of nuclear waste management can be well resolved[1-3].129I as an important radioisotope in nuclear waste is dangerous for health because of its volatility and particular long-life (half-life of 1.57′107years)[4]. Therefore, the effective capture and storage of I2has attracted a great deal of attention in recent years. There are many ways to capture iodine, which are mainly divided into solution washing method and solid adsorption method. The solid adsorbents reported in literature are mainly porous materials, such as silver-based zeolites[2], layered double hydroxides(LDHS)[5-7], porous carbon[3], zeoball[8], and so on. However, in most cases, there are some disadvantages for these solid adsorbents. For example, the silver-based zeolites are related tohigh-cost production and complicated fabrication process as well as lacking high adsorption capacity[9]. The deactivation of activated carbon in environments with high humidity casts ashadow on its practical application for the adsorption of iodine[10],since the off-gas of fuel reprocessing plants contains high levels of water vapor[1].Thus, the exploitation of new capturing iodine materials with environment friendly property, high absorption capacity,moisture-stable and cheap is still an important challenge.

The literatures have showed that the compounds with low surface area can also be used as a strong iodine vapor capture agent if there is a strong affinity between iodine and compound. For example, Yao’s group found [Zn2(tptc)(apy)2?(H2O)]·H2O exhibits very high adsorption value (216 wt %) for iodine[4], but BET is only 168 m2×g-1. Such compounds usually possess the conjugated-electron aromatic system, halogen bonds with iodine, or electron-donating of the metal-organic coordinations, which can increase affinity for I2,thus increasing the adsorption amount for iodine[11, 12]. Considering all aspects stated above, our idea in this work was to select a ligand with plentiful phenyl rings (3-) and KI to construct a novel coordination polymer [Cd(3-pbpm)2I2]n1 (3-= 1,4-bis(pyridine-3-ylmethoxy)benzene) under solvothermal conditions. The research shows that 1 displays efficient, competitive sorption of molecular iodine gas from a mixed stream containing iodine and water vapor.

2 EXPERIMENTAL

2. 1 Reagents and measurements

The ligand 3-(1,4-bis(pyridine-3-ylme- thoxy)benzene) was synthesized and purified accor- ding to the reported literature procedure[13]. The commercially purchasedAll other chemicals and reagents were obtained from commercial sources and used as received. Elemental analyses (C, H, and N) were carried out with a Perkin Elmer 240C elemental analyzer. IR spectra were measured on a Varian 800 (Scimitar Series) FT-IR spectrometer in the 4000～400 cm-1region using KBr pellets. The diffraction data were collected on a Bruker APEX-II CCD single-crystal X-ray diffractometer. Powered X-ray diffraction (XRD) patterns of the sample were recorded by an X-ray diffractometer (Rigaku-Ultima III) equipped with a Cu-radiation. The solid UV-vis spectra were recorded at room temperature in reflectance mode using a PerkinElmer Lambda 35 spectrometer with integration sphere. A reference background was registered with BaSO4(white powder, 100% reflectance). Thermal analysis was performed with a Netzsch STA-409 PC thermo- gravimetric analyzer at a heating rate of 10 °C·min?1to 900 °C under an air flow.

2. 2 Synthesis of [Cd(3-pbpmb)2I2]n

Cd(Ac)2·2H2O (0.0213 g, 0.08 mmol), 3-pbpmb (0.0106 g, 0.04 mmol), KI (0.0133 g, 0.08 mmol) and H2O (4 mL) were sealed in a 10 mL Teflon-lined stainless-steel autoclave and heated in an oven to 170 °C for 3000 minutes. After cooling to room temperature, yellow needle crystals were obtained, collected and washed thoroughly withdeionized water and dried in air. Yield: 8 mg (42%, based on 3-).

The powder XRD characterization indicates that the-value of diffraction peaks of the as-synthesized sample is in good agreement with the results simulated on the basis of single-crystal structure, which proves the phase purity of the as-synthesized product (Fig. 1). Anal. Calcd. (%) for C36H32CdI2N4O4: C, 45.5; H, 3.4; N, 5.9. Found: C, 45.2; H, 3.1; N, 5.5. Selected IR (KBr, cm?1): 2360 (w), 1505 (s), 1457 (m), 1430 (m), 1383 (m), 1335 (w), 1283 (w), 1227 (s), 1102 (w), 1030 (s), 927 (w), 804 (s), 703 (s), 644 (m), 525 (m) cm?1.

2. 3 Crystal structure determination

Single-crystal X-ray studies for 1 were performed on a Bruker APEX-II CCD diffractometer at 296(2) K. The determinations of unit cell parameters and data collections were performed with Mo-radiation with radiation wavelength of 0.71073 ? by using thescan mode. In the range of 2.53<<28.30o, a total of 11624 reflections were collected and 3053 were independent withint= 0.0209, of which 2864 were observed with> 2(). Lorentz polarization and absorption corrections were applied by using the multi-scan program[14]. The structure was solved by direct methods with SHELXS-97 program[15]and refined by full-matrix least-squares techniques on2with SHELXL-97[16]. Metal atoms were located from themaps, and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on2. Hydrogen atoms on carbon atoms of ligand were placed in calculated, ideal positions and refined as riding on their respective carbon atoms. The final= 0.0198 and= 0.0467 (= 1/[2(F2) + (0.0203)2+ 0.9274], where= (F2+ 2F2)/3).= 1.112, (Δ)max= 0.311, (Δ)min= –0.580 e/?3and (Δ/)max= 0.001. Selected bond distances and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (o) for 1

Symmetry transformations used to generate the equivalent atoms: A:,1,1; B:1,,1; C:1,,1; D:1,1,

Fig. 1. Powder XRD patterns of as-synthesized polycrystalline powder and of simulated from single-crystal data of 1

2. 4 Iodine vapor capture experiment

No radiological iodine was used in all experiments. Iodine vapor capture experiments were conducted as follows. Two preweighed glass vials containing powder sample (20 mg) and enough crystallites of I2were placed into a large glass vial, and the large glass vial was then capped and wrapped with Teflon tape in order to prevent the leakage of I2vapor. The large vial containing two smaller vials was placed in a convection oven at 75 °C under ambient pressure (typical conditions of fuel reprocessing)[1, 4]. The iodine-loaded samples were taken out after different exposure times and allowed to cool in air, and the mass change was recorded. The iodine uptake was calculated as (m2– m1)/m1× 100 wt%, where m1and m2are the masses of the sample before and after iodine uptake. The above experiment was repeated after replacing crystallites of I2with water or the crystallites of I2and water. The resultant solid sample was washed with cyclohexane (toremove I2adhered to the surface of the sample) until the filtrate became almost colorless and then left in air to dry. The loaded samples are respectively denoted as 1@I2and 1@H2O.

3 RESULTS AND DISCUSSION

3. 1 Description of the structure

1 crystallizes in the monoclinic space group21/, and its asymmetric unit contains one Cd atom, two 3-ligands, and two I atoms. As shown in Fig. 2a, each Cd(1) ion is coordinated by four N atoms (Cd(1)–N(1) = 2.4719(19) ?, Cd(1)–N(2) = 2.366(2) ?) from four 3-pbpmb ligands and two I ions (Cd(1)–I(1) =3.0042(5) ?) to furnish a distorted octahedral coordination geometry. The Cd(1)–N(1) bond length (2.4719(19) ?) is a little longer than that in [Cd2L2(3-pbpmb)3]n(the mean Cd–N bond length 2.365(6) ?, H2L = 2,2?-azodibenzoic acid)[13], but is appreciably smaller than the upper limit for the covalent Cd–N distance (2.54(2)?)[17, 18]. Meanwhile, the Cd–I bond length (3.0042(5) ?) is somewhat longer than the typical Cd–I bond lengths previously reported[18], but is significantly shorter than the corresponding sum of the van der Waals radii for Cd and I (3.56 ?)[18], which may be because iodine ion forms intermolecular hydrogen bond with one hydrogen atom of -CH2- group. The Cd(1)ion and its own one symmetry-related ion are bridged by N atoms from two 3-pbpmb ligands togive a parallelogram-like [Cd2(3-pbpmb)2] unit (5.26× 15.07 ?2,Fig. 2b). Then, such units are interlinked together throughsharing 3-pbpmb ligands to generate a “zigzag” 1-D chain coordination polymer extending along theplane (Fig. 2b).These 1D chains are interconnected with neighbouring ones by C–H×××interactions via edge-to-face orientation between the benzene rings and the pyridine rings of 3-pbpmb (= 2.7802(8) ? and= 118.782(155)°) to form a 2D framework extending along theplane (Fig. 2c). Furthermore, the 2D frameworks are linked via intermolecular C–H×××I hydrogen bond (H(13B)×××I(1A) distance: 3.1271(5) ?, a little shorter than the sum of the van der Waals radii for H and I (ca. 1.2 ? for H, 1.98 ? for I), C(13)–H(13B)···I(1A) angle: 148.752(168)°, symmetry code: A =– 1.5,+ 0.5,+ 0.5; which indicates an intermolecular hydrogen bond between 3-pbpmb and the terminal iodineanion)[18], resulting in a 3D supramolecule (Fig. 2d).

3. 2 Thermal analysis

Measurements of the thermal behaviour of 1 were performed in air from room temperature to 900 °C at 10 °C·min?1. As shown in Fig. 3, there is no obvious weight loss before 250 °C in the TG curve of 1 because there are no solvent molecules in 1. Beyond this temperature, weight loss in two stages is observed. The first weightloss of 25.98% (calcd. 26.71%) in the temperature range of 250～480 °Ccan be assigned to the loss of two iodine ions. Upon further heating, the framework structure decomposes quickly because of the explosive combustion of the 3-pbpmb ligands (obsd. 61.85%; calcd. 61.49%). The ultimate residue for 1(12.05%) is close to thepercentage of cadmium oxide (calcd. 13.50%).

3. 3 Iodine vapor capture

Considering that there are many benzene rings and iodine ions, but no solvent molecules in 1, its capacity toward the molecular I2capture was studied. The amount of I2and water loading was taken at various time intervals and shown in Fig. 4a. As time progressed, the color of the sample changed from beige to red-brown (Fig. 4b).As shown in Fig. 4a, curve A is almost a straight line as time goes on, indicating that under typical conditions of fuel reprocessing, 1 has no adsorption capacity for water. However, curve B goes up rapidly, then becomes slowly after 10 hours, which shows that the capacity toward the molecular I2capture of 1 was very quick originally and the system had reached equilibrium after 47 h. For 1, the amount of I2loading was 111 wt%. In other words, the molecular iodine saturation capacity in 1 was found to be 1110 mg I2per gram of sample. Note that the I2adsorption capacity of 1 in the gas phase surpasses that of what has already been observed for porous complexes, for example {[Co3(BTC)2(TIB)2(H2O)4]·(H2O)4}n(27.92 wt%), {[Cu3(BTC)2(TIB)2(H2O)2]·(H2O)6}n(28.62 wt%)[12]and NiII(pz)[NiII(CN)4] (83 wt%)[19], and is comparable to ZIF-8 (100 wt%)[9]. More importantly, the amount of I2loading of 1 in the mixed stream containing iodine and water vapor can still reach 102 wt%. With respect to industrial online competitive gas sorption, it can be therefore inferred that the water vapor is not a competitive gas concern versus I2in a mixed off-gas stream. More importantly, 1@I2was very stable. 1@I2remained the same color intensity and only 1.9% weight loss was observed after leaving it at room temperature under atmos- pheric pressure for 10 days.

Fig. 2. (a) View of the local coordination environments of Cd(II) ion in 1. Symmetry codes: A = 1–, 1–, –; B = 1+,,-1 (H atoms omitted for clarity). (b) 1D chain of 1 along theplane. (c) 2D supramolecular framework showing the intermolecular C–H···stacking interactions. (d) 3D supramolecular framework showing the intermolecular C–H···I hydrogen bond interactions

Fig. 3. TG analyses of 1 and 1@I2

Fig. 4. (a) Change in the adsorbed amount of I2, relative to time; (b) Photographs of crystals before and after I2adsorption

The thermogravimetric analysis (TG) graph of 1@I2displayed a broad mass loss step from 100 to 250 °C (Fig. 3), and the calculated iodine mass loss was 101.61 wt%, which is close to the saturated adsorption value. The final residue of 5.80% for 1@I2is in agreement with the percentage of cadmium oxide (calcd. 6.67%).

3. 4 UV-vis

Solid state UV-vis absorption spectra of 1 and 1@I2were recorded at room temperature (Fig. 5). A single peak at~260 nm was observed for 1, whereas in the case of 1@ I2, a broad band appeared at the visible region between 230 and 500 nm with four shoulder peaks at~250,~310,~370 and 490 nm. Two absorption bands at~310 and~370 nm may be assigned to the spin and symmetry-allowed?* and?* transitions of I3-[5, 6]. But the spin- forbidden singlet-triplet transitions of I3-, expected at 440 and 560 nm, are not observed. The two absorp- tion bands can be significantly enhanced if I3-is in a low symmetry. The spectroscopic features thus show that the iodine molecules and iodine ions of 1 can form highly symmetric I3-. The absorption band at about 490 nm is assigned to molecular I2[20].

Fig. 5. Solid-state UV-vis spectra for 1 and 1@I2at room temperature

Importantly, no additional peaks at ～800 nm resulting from pristine iodine crystals were observed in the iodine-loaded sample, indicating that there were no unattached iodine molecules[21].

In order to study the interaction of I2and 1, iodine vapor capture experiment was also carried out using single crystal sample of 1. Although the single crystals after capturing I2retained their original external forms, the crystals di?ract poorly, and the resolution of the structure was not possible. Fig. 2d shows that 1 structure contains two types of quadrilateralchannels: A (8.49?′3.83?) and B. According to the literature[22-24], channel A isbig enoughtoaccommodateiodinemolecules(3.35?). The iodide ions in channel A can produce ion-dipole action with I2,electron-rich aromatic networks with conjugated-electrons and strong electron-donor nitrogen atoms can provide a number of possibilities to interact with iodine. The cooperationeffect of three types of interaction largely improves theadsorption ability of I2, so when the temperature rises, iodine molecules move faster, allowing easier access to the A channel.The symmetry-allowed?* and?* transitions of I3-in UV-vis absorption spectra indicate that I2and I-ion in the A channelmay be on a line. Theoretically, the amount of iodine in channel A should be 106.8%, which is very close to the experimental value (111%). Thehigh affinity of 1 makes the adsorption of iodine difficult to lose.

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10 October 2017;

17 January 2018 (CCDC 1810747)

① The work was supported by the National Natural Science Foundation of China (No. 21071006), Henan Province Key Laboratory of New Optoelectronic Functional Materials(No. aynu201703) and Anyang Normal University Breeding Fund

. Tel./Fax: 03722900228. E-mail: chenjinghao2014@163.com

10.14102/j.cnki.0254-5861.2011-1846