Syntheses and Characterization of Chiral Zeolitic ... - ACS Publications

Oct 27, 2016 - Anyang Normal University, Anyang, 455000, China. ‡. College of Chemistry, Dalian University of Technology, Dalian 116024, China...
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Syntheses and Characterization of Chiral Zeolitic Silver Halides Based on 3‑Rings Ren-Chun Zhang,† Jun-Jie Wang,† Bai-Qing Yuan,† Jing-Chao Zhang,† Ling Zhou,† Hai-Bin Wang,† Dao-Jun Zhang,*,† and Yong-Lin An*,‡ †

Key Laboratory of New Optoelectronic Functional Materials (Henan Province), College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang, 455000, China ‡ College of Chemistry, Dalian University of Technology, Dalian 116024, China S Supporting Information *

ABSTRACT: The synthesis of chiral zeolites remains a significant challenge because the primary tetrahedral building units are achiral and weak interactions exist between the guest and host frameworks. Here, we present the syntheses and characterization of three new chiral zeolitic halides, [H3(Dabco)2]Ag3X6 (X = Br (1) or I (2), Dabco = 1,4diazabicyclo[2.2.2]octane) and [H2(Dabco)][(Dabco)Ag4I6] (3). Compounds 1 and 2 are isostructural, containing a 4connected zeolitic framework built up from 3-ring units, with high-charge [H3(Dabco)2]3+ located in chiral cages. Compound 3 contains a similar zeolitic [Ag3I6]3− framework to that of 2, but a [Ag(Dabco)]+ unit is incorporated in each 3-ring, with [H2(Dabco)]2+ located in channels. These frameworks are chiral, representing the first examples of chiral zeolitic halides. The chirality transference of the frameworks for 1 and 2 was attributed to the template effect of the chiral [H3(Dabco)2]3+ through strong electrostatic interactions and multiple hydrogenbond interactions. For compound 3, direct coordination interactions play important roles in the chirality transference from the chiral Dabco ligand to the framework.

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INTRODUCTION Zeolites and zeolitic materials with uniform pore architectures are important for industrial processes involving catalysis, separation, and ion exchange.1,2 Chirality has been of great reputation as “a signature of nature”, which is ubiquitous in living systems and plays a crucial role in biological processes.3 Chiral zeolitic materials are of particular interest because of their intriguing structures and promising applications in enantiomerically selective catalysis and separation.4 However, the synthesis of chiral zeolitic structures remains a significant challenge because of the achiral feature of the primary tetrahedral building units.5 Chiral template methods are promising for the synthesis of chiral zeolites, because the chiral center can be introduced during the synthesis to transfer chirality to the inorganic frameworks.6 For example, Wilkinson and co-workers used chiral metal complexes for the synthesis of chiral gallium phosphates.6a However, only limited success has been achieved toward the synthesis of chiral zeolitic frameworks using this approach.4a,6b,f In most cases, chiral molecules or cations do not act as real templates, but instead simply fill space in the structures.4 The transference of chirality is hardly realized because of weak host−guest interactions and a lack of specific interaction sites of the frameworks.5 In 1997, Martin and Zubieta developed a new family of microporous materials, zeolitic metal halides, using reactive CuCl4 and ZnCl4 tetrahedra as primary building uints.7 In © XXXX American Chemical Society

contrast to traditional zeolites, microporous metal halides possess a more flexible way of structural building and have numerous performance advantages including semiconductivity, ionic conductivity, fluorescence, thermochromism, photochromism, and visible-light catalytic activity.8−12 Particularly, the high polarizability of metal-halide building blocks provides the microporous halides with much more negative frameworks that exhibit more structural sensitivity to structure-directing agents (SDAs).9 These features provide new opportunities for developing host−guest chemistry and exploring a range of novel multifunctional materials.10 Among metal halides, Ag−X compounds are of particular interest because of the tetrahedral coordination preference of the Ag+ ion and the highly covalent nature of Ag−X bonds.9,11,12 In principle, the corner-sharing of tetrahedral units can generate novel zeolitic frameworks due to long bonds and flexible bridging angles.4d However, such a linkage undoubtedly results in the high charge density feature of the Ag-X framework because high valent metal cations are absent. One of the possible strategies to compensate the high negative charge of the framework is the introduction of high charge density SDAs.13 In our previous work, corner-sharing linkage between AgI4 tetrahedra was shown to create a 4connected framework in M(en)3Ag2I4 (M = Zn, Ni) (tridymite Received: September 7, 2016

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DOI: 10.1021/acs.inorgchem.6b02121 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details of Compounds 1−3 empirical formula fw temp/K cryst syst space group flack parameter a/Å c/Å α/deg γ/deg V/Å3 Z calcd density/Mg m−3 abs coeff/mm−1 F(000) 2θ (max)/deg index range

no. of collected/unique rflns [R(int)] GOF on F2 final R indices (I > 2σ(I)) wR2 Δρmax, Δρmin/e Å−3

1(L)

1(R)

2(L)

2(R)

3(L)

3(R)

C12H27N4Ag3Br6 1030.45 296 trigonal R32 0.025(16) 12.3313(3) 13.5163(6) 90.0 120.0 1779.9(1) 3 2.884 12.559 1434 51.0 −12 ≤ h ≤ 13 −9 ≤ h ≤ 14 −9 ≤ h ≤ 15 1845/682 0.0236 1.048 0.0245 0.0569 0.588, −0.841

C12H27N4Ag3Br6 1030.45 296 trigonal R32 0.039(14) 12.3368(9) 13.5135(2) 90.0 120.0 1781.2(3) 3 2.882 12.550 1434 51.0 −13 ≤ h ≤ 14 −12 ≤ h ≤ 13 −16 ≤ h ≤ 15 2210/692 0.0236 1.030 0.0201 0.0483 0.712, −0.762

C12H27N4Ag3I6 1312.39 296 trigonal R32 0.07(5) 12.9902(2) 13.8863(4) 90.0 120.0 2029.3 (1) 3 3.222 9.002 1758 52.7 −16 ≤ h ≤ 16 −16 ≤ h ≤ 16 −17 ≤ h ≤ 17 5259/929 0.0335 1.034 0.0189 0.0466 0.963, −0.575

C12H27N4Ag3I6 1312.39 296 trigonal R32 −0.03(6) 13.0088(4) 13.9137(1) 90.0 120.0 2039.1 (2) 3 3.206 8.959 1758 51.4 −15 ≤ h ≤ 15 −11 ≤ h ≤ 13 −16 ≤ h ≤ 13 2559/843 0.0165 1.065 0.0205 0.0509 1.450, −0.865

C12H26N4Ag4I6 1419.25 296 trigonal R32 −0.06(15) 13.0556(2) 14.1761(4) 90.0 120.0 2092.57(7) 3 3.376 9.412 1893 56.7 −17 ≤ h ≤ 16 −17 ≤ h ≤ 16 −18 ≤ h ≤ 18 5735/1163 0.0974 1.013 0.0572 0.1538 1.063, −0.943

C12H26N4Ag4I6 1419.25 296 trigonal R32 0.02(3) 13.0576(3) 14.1692(8) 90.0 120.0 2092.20(14) 3 3.377 9.414 1893 50.0 −14 ≤ h ≤ 14 −14 ≤ h ≤ 15 −16 ≤ h ≤ 16 2780/831 0.0538 1.064 0.0336 0.1250 1.038, −0.900

topology).9a Given the noncompliance of the Loewenstein rule linkages between the AgX4 tetrahedra and flexible bridging Ag− X−Ag angles, there is a great potential for forming zeolitic frameworks through the careful selection of SDAs and precise control of the reaction conditions. In this publication, we present the syntheses, structures, and characterization of three new chiral zeolitic framework halides, [H3(Dabco)2]Ag3X6 (X = Br (1) or I (2)) and [H2(Dabco)][(Dabco)Ag4I6] (3). Compounds 1 and 2 are strictly 4connected zeolitic structures built up from 3-ring units, and charge-balanced by [H3(Dabco)2]3+. Compound 3 consists of a similar zeolitic [Ag3I6]3− framework to that of 2, but chiral [Ag(Dabco)]+ units are incorporated into the framework, which is charge-balanced by [H2(Dabco)]2+. These zeolitic frameworks are chiral and represent the first examples of chrial zeolitic framework halides. The transference of chirality of the frameworks is discussed in detail in this work.

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sphere at 296 K, and a BaSO4 plate was used as reference. The reflectance data collected were converted to the adsorption data using the Kubelka−Munk functions. Synthesis of SDA. (H3O)[H2(Dabco)](NO3)3. Dabco·6H2O (12.0 g) was added to a 250 mL flask, into which HNO3 (69%, 24.0 mL) and distilled water (60.0 mL) were added, and then the solid dissolved to form a colorless solution. The resulting solution was kept undisturbed at room temperature for 5 days; colorless columnar crystals were obtained by filtration. The purity was confirmed by comparison of the experimental and simulated PXRD patterns from the single crystal structure (Figure S1). Preparation of 0.05 M AgBr2− Solution. AgNO3 (1.705 g) was dissolved in 20 mL of N,N-dimethylformamide (DMF); then, the solution was added dropwise to a saturated KBr/DMF solution (100 mL) with excessive KBr settled at the bottom of a flask. The mixture was stirred for 0.5 h, and then filtered. Last, the solution was diluted to 200 mL by the addition of DMF, and then kept away from light. Preparation of 0.1 M AgI2− Solution. AgNO3 (0.865 g) was dissolved in 10 mL of DMF; then, the resulting solution was added dropwise to a saturated KI/DMF solution (20 mL) and stirred until the mixture became a clear solution. Last, the solution was diluted to 50 mL by the addition of DMF. Syntheses of Compounds 1−3. [H3(Dabco)2]Ag3Br6 (1). A 0.100 g portion of (H3O)[H2(Dabco)](NO3)3 was put into a 50 mL flask, into which 0.5 mL of CH3OH and 2.0 mL of DMF were added, then 5.0 mL of 0.05 M AgBr2− solution, respectively. The mixture was kept undisturbed for about 5 days, and then colorless polyhedral crystals can be obtained after washing with DMF, and ethanol, respectively. EDS analysis on several crystals gave an average Ag:Br molar ratio of 1:2, which is in agreement with the results by single-crystal X-ray diffraction analyses. Elemental analysis (%) calcd for 1 (C12H27N4Ag3Br6): C, 13.98; H, 2.62; N, 5.44. Found: C, 14.12; H, 2.58; N, 5.30. [H3(Dabco)2]Ag3I6 (2). The synthesis of 2 is similar to that of 1, except 0.1 M AgI2− solution was used. EDS analysis on several crystals gave an average Ag:I molar ratio of 1:2. Elemental analysis (%) calcd For 2 (C12H27N4Ag3I6): C, 10.98; H, 2.06; N, 4.27. Found: C, 11.06; H, 2.04; N, 4.21.

EXPERIMENTAL SECTION

Materials and General Methods. All reagents were purchased from commercial sources and were used without further purification. Powder X-ray diffraction (PXRD) data were collected on an Ultima III diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature. The step size was 0.02°, and the operating power was 1.6 kW. The simulated PXRD patterns were calculated using the Material Studio program based on single-crystal data of the compounds. Energy-dispersive spectroscopy (EDS) was performed on a Hitachi SU 8010 scanning electronic microscope. Elemental analysis (C, H, N) was carried out on a Vario EL III elemental analyzer. Solid-state circular dichroism (CD) spectra were measured on a JASCO J-820 spectrometer at room temperature with a KBr plate. Thermogravimetric analysis (TGA) was carried out using a Mettler-Toledo Star under a flow of nitrogen (40 mL min−1) from 35 to 650 °C at a heating rate of 10 °C min−1. The UV−vis reflectance spectra were measured using a Shimadazu UV-2550 double-beam, doublemonochromator spectrophotometer, equipped with an integrating B

DOI: 10.1021/acs.inorgchem.6b02121 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry [H2(Dabco)][(Dabco)Ag4I6] (3). AgI (6.0 mg) and NH4I (24.0 mg) were placed in a Pyrex glass tube; then, 120 μL of methanol and 300 μL of acetonitrile as a mixed solvent were added and ultrasonically dispersed; then, Dabco·6H2O (14.0 mg) and 50 μL of distilled water were added into the tube. After the mixture was thoroughly ultrasonically dispersed, the tube was sealed (reagents filled about 5% of the tube) under an air atmosphere, placed in a stainless-steel autoclave, and heated at 105 °C for 7 days, and then cooled to room temperature naturally. White precipitates can be found at the bottom of the tube. Then, the tube was kept undisturbed in the dark for about 2 weeks. White precipitates disappeared and colorless crystals were present in the tube. The products were washed with ethanol several times, and crystals of 3 were obtained as a pure phase in 65% yield based on AgI. EDS analysis on several crystals reveals a Ag:I ratio of 2:3. Elemental analysis (%) calcd for 3 (C12H26N4Ag4I6): C, 10.15; H, 1.83; N, 3.95. Found: C, 10.32; H, 1.75; N, 3.90. Crystal Structure Determination. Crystallographic data collections were performed on a Bruker Smart APEX II diffractometer equipped with graphite-monochromitized Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least-squares on F2 using SHELX-97.14 All non-hydrogen atomic positions were located in Fourier maps and refined anisotropically, while all of the hydrogen atoms were refined with isotropic displacement parameters. For compounds 1 and 2, the hydrogen atoms of C atoms in [H3(Dabco)2]3+ were located at geometrically calculated positions, while the proton hydrogen atoms were located from the difference Fourier maps. For compound 3, part of the proton hydrogen atoms cannot be found from the difference Fourier maps, which were also taken into calculation of the unit cell content. Ag(2) atom has two equivalent crystallographic sites because of the C2 symmetry operation; thus, 0.5 occupancy was assigned to each site. Experimental crystallographic data are summarized in Table 1.

important roles during the crystallization process. Attempts to synthesize the Ag-Br compound that was isostructural to 3 were unsuccessful. Therefore, the resulting Ag-X structures were sensitive to the synthesis parameters. The degree of Dabco protonation was particularly important when it was used as SDAs.9b,11c The synthesis routes for compounds 1−3 allowed us to obtain a pure phase of each compound, as confirmed by PXRD patterns (Figures S3−S5). Crystal Structure. Single-crystal X-ray diffraction reveals that compound 1 crystallizes in trigonal space group R32 and contains a 3D zeolitic anionic [Ag3Br6]3− framework with [H3(Dabco)2]3+ residing in the channels. Interestingly, the zeolitic framework of 1 is constructed exclusively from 3-ring [Ag3Br9]6− units. As shown in Figure 1, the 3-ring unit with D3 symmetry is constructed by the corner-sharing of three tetrahedral AgBr4 units. The bridging Ag−Br−Ag angle in the three-ring unit is 126.5°.

RESULTS AND DISCUSSION Synthesis Chemistry. Ag−X compounds can form anionic frameworks with high charge densities because AgI favors tetrahedral coordination and Ag−X bonds are highly covalent.9a,f The resulting structures are sensitive to the size, charge density, and configuration of the SDAs used and are also affected by the synthesis parameters, such as reaction temperature, solvent, pH value, and the molar ratio of reactants.9−11 Using chiral [H2(Dabco)]2+ (Figure S2), which has a high density, as SDA in the reaction systems at room temperature, two new chiral zeolitic framework halides (compounds 1 and 2) were synthesized, and two enantiomers with different chirality were isolated for each compound. The use of the SDAs with strong acidity and high charge density was critical for the successful syntheses of the compounds. A small amount of methanol was also important for their syntheses. Methanol likely promoted the protons transfer and maintained an acidic reaction environment. The solvothermal treatment of AgI, NH4I, and Dabco·6H2O resulted in the formation of white precipitates, which then underwent transform crystallization to form the chiral hybrid framework compound 3. The addition of an appropriate amount of distilled water was necessary for the synthesis of compound 3. Otherwise, a 1D chain [H(Dabco)][(HDabco)Ag2I4] was obtained.9b The alternative product was stable in the tube and did not show transform crystallization when kept in the dark. Probably, the added appropriate water changed the reaction environment and promoted the crystallization of compound 3, which has a relatively low solubility and high stability at room temperature. In addition, an excess NH4I was used as a mineralizer for the synthesis of 3. This mineralizer significantly increased the solubility of AgI and played

Figure 1. Perspective view (at 30% probability level) of the 3-ring [Ag3Br9]6− unit (a) and the [H3(Dabco)2]3+ cation (b) in compound 1.

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To form a 3-D framework, the 3-rings connect to each other via sharing their terminal Br− ions and generate 7-ring channels with a size of 2.1 × 5.4 Å2 allowing for the van der Waals radii of Br− anions (Figure 2a). The bridging Ag−Br−Ag angle between 3-rings is 140.5°, which is comparable to the typical T−O−T angle found in oxide zeolites. Structural analysis indicates that the zeolitic framework of 1 features an afw topology (Figure 2b), and its framework density (FD) is 5.0 T/ 1000 Å−3. To date, only one type of zeolitic halide has been reported based on 3-ring units. It contains a 3D framework built up from corner-sharing of CuCl4 and ZnCl4 tetrahedra with 8-ring and 11-ring channels.7a In contrast, the zeolitic framework of compound 1 is constructed completely from AgBr4 tetrahedra, resulting in the 3-ring units and the framework with high charge density. The feature of the 3-ring unit is likely originated from the long Ag−Br bond and flexible Ag−Br−Ag angle. As pointed out in Bu’s work,4d long T−O bonds and small T−O−T angles can relieve the strain inherent in a 3-ring structure. Such a trend also reflected in zeolite chalcogenides based on 3-ring units.13a,15 Notably, the framework is chiral (Flack parameter = 0.025). The framework has a chiral cage denoted as a 3276 cage, in which a chiral [H3(Dabco)2]3+ cation with high charge density is encapsulated (Figure 2c,d). Multiple hydrogen bonds are present between the terminal protons of the [H3(Dabco)2]3+ cation and Br− in the 3-ring units. Each [H3(Dabco)2]3+ interacts with the 3276 cage through six hydrogen bonds with N−H···Br distances of 3.23 Å. C

DOI: 10.1021/acs.inorgchem.6b02121 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Perspective view (at 30% probability level) of the structure building unit found in compound 3.

[Ag3I6]3− framework by Ag−I covalent bonds. Worth noting is that the Dabco dangling on the framework played important structure-directing roles in the formation of the framework.9b,c,12a Not only the local C3 symmetry of the N atom was transferred to the framework but also the transference of chirality was realized by a direct Ag−N coordination bond. Chiral Template Effect. The chirality transference of these frameworks can be ascribed to the template effect of organic amines. [H2(Dabco)]2+ is semirigid and can bear some distortion (torsion angle: 16.25°) to show chirality with D3 symmetry (Figure S2). For compounds 1 and 2, chiral [H 2 (Dabco)] 2+ with high charge density was initially introduced into the synthesis systems, while two [H2(Dabco)]2+ cations formed a hydrogen bond along the C3 rotation axis to generate a [H3(Dabco)2]3+ cation (Figure 1b). The [H3(Dabco)2]3+ cation is also chiral with D3 symmetry. Its chirality is originated from two aspects: the internal distortion in each [H2(Dabco)]2+ (torsion angle: 8.25°) and their linear arrangement along the C3 rotation axis through hydrogenbonding interactions. These findings revealed that [H2(Dabco)]2+ maintained its original symmetry (and particularly chirality) during the solution assembly reactions, despite the reduced internal distortion. Interestingly, although there were only slight differences in the structure of the right- and left-handed cations, the chirality of the [H3(Dabco)2]3+ cation determined the chirality of the cage and the framework. As shown in Figure 2c,d, the right- and left-handed 3276 cages were isolated from the compound 1(R) and 1(L), respectively, each of which contained a [H3(Dabco)2]3+ cation with the corresponding handedness. Thus, the chiral [H3(Dabco)2]3+ cation served as a critical template for the construction of the chiral cages and frameworks. It should be noted that electrostatic attractions were dominant between the host framework and the guest [H3(Dabco)2]3+ cation because of their high charge densities. In addition, hydrogen bonds exist between the terminal protons of the [H3(Dabco)2]3+ cation and the framework anions. Each [H3(Dabco)2]3+ formed multiple hydrogen bonds with the framework anions, forming N−H···Br and N−H···I with distances of 3.23 and 3.53 Å, respectively. Both of strong electrostatic attractions and multiple hydrogen bonds between the host framework and the guest template are crucial for the transference of chirality. For compound 3, the chiral organic amine was coordinated to the framework with the Dabco dangling on the framework and formed hydrogen bonds with chiral [H2(Dabco)]2+. Coordination interactions were critical for the transference of structural information, because both the C3 symmetry and the chirality of the template were transferred to the inorganic

Figure 2. (a) The 3D zeolitic [Ag3Br6]3− framework; (b) the afw topology of the 3D framework; (c) the left-handed 3276 cage; (d) the right-handed 3276 cage.

Compound 2 is isostructural to 1, consisting of a zeolitic [Ag3I6]3− framework built up from 3-ring [Ag3I9]6− units and charge balanced by [H3(Dabco)2]3+. The FD of the zeolitic [Ag3I6]3− framework is 4.5 T/1000 Å−3. The FDs of 1 and 2 are comparable to those of zeolite UCR-20 (zeotype RWY) and the CPM-12x series chalcogenides, which have the lowest FD (4.2 T/1000 Å−3).15 These zeolite chalcogenides also contain 3-ring units, but four of which fuse together to form T2 M4Q10 (Q = S or Se) supertetrahedral building units, leading to the formation of decorated zeolitic networks. It should be noted that the zeolitic framework of 2 is built exclusively up from 3ring units; however, it exhibits relatively high FD unexpectedly, comparing with tridymite topologic M(en)3Ag2I4 (M = Zn, Ni, FD = 4.0) and sodalite topologic (N-mepipzH)AgI2, (FD = 3.8).9a,d This finding violates the Brunner−Meier’s prediction that frameworks with low FD can be formed by small rings.16a This departure from expectation can be ascribed to the relatively small size, but high charge density feature, of the [H3(Dabco)2]3+ cation comparing with those of [M(en)3]2+ and [N-mepipzH]+. As calculated by PLATON,16b the guest accessible volume of 2 is 55.76%, while those of M(en)3Ag2I4 (M = Zn, Ni) and [(N-mepipzH)(AgI2)]n are 62.2, 63.1, and 63.7%, respectively. Compound 3 also crystallizes in trigonal space group R32, while it contains a 3D hybrid anionic [(Dabco)Ag4I6]2− framework, with [H2(Dabco)]2+ residing in the channels. The secondary building unit of the framework is a hybrid [(Dabco)Ag4I9]2− cluster, which is similar to the 3-ring [Ag3I9]6− unit in 2, except capped by a [Ag(Dabco)]+ unit. As shown in Figure 3, the Ag(2) atom bonds to three I− anions in the 3-ring unit with the Dabco ligand dangling on the framework, forming a hydrogen bond with the [H2(Dabco)]2+ cation in the channel. The Ag···Ag distance in the [(Dabco)Ag4I9]2− cluster is 3.056 Å, indicating the presence of Ag···Ag interactions.16c Therefore, compound 3 can be viewed as built up from the similar zeolitic [Ag3I6]3− framework to 2, except one terminal proton in each [H3(Dabco)2]3+ cation of 2 was substituted by a Ag+. Then, the resulting chiral [Ag(Dabco)H2(Dabco)]3+ cationic units are incorporated into the zeolitic D

DOI: 10.1021/acs.inorgchem.6b02121 Inorg. Chem. XXXX, XXX, XXX−XXX

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or iodide-based filled valence band to a mainly silver-based empty conduction band.9,11,12 The band gap of ompound 2 is significantly lower than that of its isostructural bromide (compound 1) because of the higher polarizability of Ag−I bonds than Ag−Br bonds. Meanwhile, the band gap of 2 is slightly higher than that of compound 3. Because of the negligible contribution of saturated SDAs in this region, the slight red shift of the band gap of compound 3 compared to that of 2 can be ascribed to the relatively higher Ag/I ratio, because a high M (M = Cu or Ag) content contributes to the low electronic band gap.10b This trend is also reflected in recently reported 3D framework iodoargentates, (DabcoH)2[(Dabco)2Ag14I16] (3.3 eV),9b (N-mepipzH2·2DMSO)Ag4I6 (3.51 eV), (N-mepipzH)AgI2 (3.54 eV),9d and [Cd(phen)3]2Ag13I17 (3.19 eV).9f

framework by direct Ag−N coordination bonds. So far, it remains a significant challenge to realize chirality transference from template to the framework due to the weak host−guest interactions and a lack of specific interaction sites.5 We have shown that strong host−guest interactions and direct coordination bonds can facilitate the transference of structural information from chiral SDAs. This should be useful for design and synthesis of novel frameworks and chiral materials. Circular Dichroism Spectra. Solid-state circular dichroism (CD) spectroscopy was performed on bulk samples of each compound, revealing that they were racemic. There are two enantiomers with different chirality present in the bulk materials, which were characterized by single-crystal X-ray diffraction and CD spectroscopy. Structural analysis reveals that both of the enantiomers crystallize in chiral space group R32 with Flack parameters approaching zero. The chiral framework and template adopted the same handedness in each enantiomer. CD spectra of the single crystals indicated that the right-handed and left-handed samples displayed opposite chiral signals (Figures S6−S8). The cases indicate that the spontaneous resolution occurs during crystallizations of each compound. Attempts to synthesize homochiral bulk materials of compounds 1 and 2 were unsuccessful. This was ascribed to the coexistence of left- and right-handed [H2(Dabco)]2+ in (H3O)[H2(Dabco)](NO3)3 because of glide mirror c in the space group P31c (Figure S2). Thermogravimetric Analyses. Thermogravimetric analyses (TGA) of compounds 1−3 revealed that no obvious weight loss occurred below 240 °C. For compound 1, the TG curve shows one step weight loss of 45.2% between 250 and 480 °C (Figure S9). This was in good agreement with the removal of Dabco and HBr (Calcd: 45.3%). The TG curve (Figure S10) for compound 2 contains a weight loss of 43.2% between 280 and 450 °C, corresponding to the incomplete removal of Dabco and HI (Calcd: 46.0%). The residue was dark gray and contained about 2.2% C, H, and N, as determined by elemental analysis. The TG curve (Figure S11) of compound 3 shows a weight loss of 33.6% between 240 and 480 °C, which can be attributed to the loss of Dabco and HI (Calcd: 33.7%). UV−vis Spectra. UV−vis reflectance spectra confirmed that these compounds were wide-band-gap semiconductors. The optical absorption spectra revealed the band gaps of 4.20, 3.60, and 3.45 eV for 1, 2, and 3 (Figure 4), respectively. Similar to those of other Ag halides, the intense absorption can be ascribed to charge-transfer transition from a primarily bromide

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CONCLUSION Three chiral zeolitic silver halides were synthesized and characterized in this work. Compounds 1 and 2 consist of strictly a 4-connected zeolitic framework built up from 3-ring units, with highly charged [H3(Dabco)2]3+ cations located in their chiral cages. Compound 3 contains a similar zeolitic [Ag3I6]3− framework to that of 2, except for incorporation of chiral [Ag(Dabco)]+ units, with [H2(Dabco)]2+ located in the channels. The syntheses demonstrated the feasibility of the strategy of using high charge-density SDAs to stabilize zeolitic Ag-X frameworks with high negative charge. All of these frameworks are chiral and represent the first examples of chiral zeolitic halides. These results indicate that strong host−guest electrostatic interactions and direct coordination bonding play important roles in the transfer of chirality from the templates to the frameworks. This may provide new ideas in design synthesis of novel frameworks and chiral materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02121. PXRD patterns, CD spectra, and TG curves (PDF) Crystallographic data for 1(L) (CIF) Crystallographic data for 1(R) (CIF) Crystallographic data for 2(L) (CIF) Crystallographic data for 2(R) (CIF) Crystallographic data for 3(L) (CIF) Crystallographic data for 3(R) (CIF) Crystallographic data for (H3O)[H2(Dabco)](NO3)3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-J.Z.). *E-mail: [email protected] (Y.-L.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support of this research by the National Natural Science Foundation of China (21301009, 21405005, 21403006, 21501006, 21603004, 21171028), and the project of the Science and Technology Department of Henan Province (132102210446).

Figure 4. UV−vis spectra of the compounds 1−3. E

DOI: 10.1021/acs.inorgchem.6b02121 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b02121 Inorg. Chem. XXXX, XXX, XXX−XXX