A 36-Membered Ring Metal Chalcogenide with a Very Low

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A 36-Membered Ring Metal Chalcogenide with a Very Low Framework Density Wei Wang,†,‡ Huajun Yang,‡ Min Luo,‡ Yeshuang Zhong,† Dingguo Xu,† Tao Wu,*,‡ and Zhien Lin*,† †

College of Chemistry, Sichuan University, Chengdu 610064, China College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123, China

‡

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S Supporting Information *

Inspired by the interrupted structures of a family of bimetallic phosphites NTHU-13 with low FDs, we expect that the reduction of the connection number of T2 cluster is favorable for the formation of low FD materials.8 Recently, a (3,4)connected metal chalcogenide framework CSZ-5-InSe with a low FD of 3.9 T/1000 Å3 has be presented, which contains the 3connected In4Se9O T2 cluster and 4-connected In4Se10 T2 cluster as its building blocks.9 Along this line of research, we report here a highly interrupted chalcogenide framework purely based on 3-connected T2 clusters, namely, (Hdmp)4·In3SnS8.5 (denoted as SCU-36; dmp = 3,5-dimethylpiperdine). The inorganic solid has a rare etc topology by regarding each T2 cluster as the structural node. Its interrupted structure features extra-large 36 MR channels and a very low FD of 3.4 T/1000 Å3. To the best of our knowledge, SCU-36 has the lowest FD ever achieved in crystalline three-dimensional inorganic solids. Colorless prismlike crystals of SCU-36 were prepared by the solvothermal reaction of indium powder, tin powder, sulfur powder, 3,5-dimethylpiperdine, and distilled water at 180 °C for 7 days (27% yield based on indium). The powder X-ray diffraction (XRD) pattern of the obtained crystals is in good agreement with the simulated one on the basis of single-crystal data, indicating the phase purity of the as-synthesized compound (Figure S2). Inductively coupled plasma mass spectrometry (ICP-MS) analysis gave an In/Sn ratio of 3.167, which is close to the energy-dispersive spectroscopy (EDS) results (Figure S3). The crystals of SCU-36 remain stable in water and several common organic solvents, such as DMF, methanol, and acetone. Structural analysis reveals that SCU-36 crystallizes in the trigonal space group P31c (No. 159). It has a three-dimensional structure containing supertetrahedral T2 building blocks (Figure 1a). Each T2 cluster consists of three indium atoms, one tin atom, and ten sulfide atoms. All of the metal atoms occupy the tetrahedrally coordinated sites of the cluster. Among the ten sulfide atoms, nine of them are bicoordinated, and the remaining one acts as a terminal ligand. Although it is difficult to distinguish the indium and tin sites in the structure through X-ray diffraction data because of their similar scattering factors, it is believed that the metal sites coordinated by terminal sulfide atoms are inclined to be occupied by tin atoms according to the local charge matching concept.10

ABSTRACT: Reported here is a new open-framework metal chalcogenide containing extra-large 36-ring channels. This compound has a 3-connected etc topology by regarding supertetrahedral T2 clusters as the structural nodes. It has a very low framework density (3.4 tetrahedra per 1000 Å3) with each framework cation participating in three 3-rings. The organic cations within its intersecting channels can be partially exchanged out by Cs+ ions with the preservation of its framework structure.

M

icroporous inorganic materials such as zeolite molecular sieves have been extensively studied because of their commercial applications in catalysis, gas adsorption, and ionexchange processes.1 Of particular interest is the synthesis of new open-framework structures with low framework densities (FDs, calculated from the number of polyhedra per 1000 Å3). According to topological analysis performed by Brunner and Meier, it is believed that low FD zeolites may be achieved by introducing as many 3-membered ring (3 MR) building blocks as possible into the frameworks.2 However, silicates with 3 MRs are rare because of high Si−O−Si stress. The incorporation of other metals such as Ge into open frameworks is therefore desirable for the formation of 3 MR because the Ge−O−Ge angle (∼130°) is smaller than that of Si−O−Si (∼145°).3 Several 3 MR-based building blocks, such as single 3-ring, double 3-ring, and spiro-5 unit (Figure S1), have been observed in a number of germaniumcontaining zeolite materials, such as PKU-9, NUD-1, ITQ-44, and GaGeO-CJ63.4 These compounds have open frameworks with low FDs. For example, the FD of GaGeO-CJ63 (10.5 T/ 1000 Å3) is one of the lowest among 4-connected zeotype oxides.4d An alternative approach to stabilize 3 MR is the replacement of oxygen atoms in zeolite structures with chalcogenide atoms (e.g., S and Se).5 Diverse supertetrahedral building blocks with 3 MR, such as basic-(Tn), panta-(Pn), and capped-(Cn) clusters, have been found in these compositional domains.6 Among them, T2 cluster is a promising candidate for the formation of zeolitic frameworks because all of its sulfur sites in a 4-connected net are bicoordinated. This building block features a high density of 3 MR with each metal center participating three 3-rings. As a result, three-dimensional metal chalcogenides constructed from T2 clusters usually have very low FDs.7 A notable example is the microporous metal chalcogenide UCR-20InSnS with a very low FD of 4.4 T/1000 Å3.7a © 2017 American Chemical Society

Received: August 15, 2017 Published: November 27, 2017 14730

DOI: 10.1021/acs.inorgchem.7b02109 Inorg. Chem. 2017, 56, 14730−14733

Communication

Inorganic Chemistry

Figure 1. (a) Perspective view of the three-dimensional structure of SCU-36 along the [001] direction. (b) Polyhedra view of the extra-large window delimited by 36 InS4 tetrahedra. (c) Topological structure of SCU-36 by considering each supertetrahedral T2 cluster as a 3-connected node. Color code: In/InS4, green; Sn, blue; S, red.

As expected from the coexistence of supertetrahedral building blocks with 3 MRs, extra-large 36 MR channels, and a 3connected net, SCU-36 has a very low FD of 3.4 T/1000 Å3. In comparison, the FDs of 4-connected zeotype oxides are usually larger than 10.0 T/1000 Å3. For non 4-connected oxides with mesoporous channels (e.g., SU-M, NTHU-13), their FDs locate in the region 5−10 T/1000 Å3. For example, the FD of the metal phosphite 72R-NTHU-13 is as low as 5.3 T/1000 Å3. By using supertetrahedral clusters as the building blocks, a number of open-framework chalcogenides with very low FDs have been prepared. An illustrative example is CSZ-5-InSe with a low FD of 3.9 T/1000 Å3. The ratio of 3-connected nodes to 4-connected nodes in this interrupted net is 1:7, which is much lower than that of 1:3 found in SCU-36. It is believed that the presence of a high density of 3-connected metal sites plays an important role in the formation of the low FD structure of SCU-36. Solid-state UV−vis diffuse reflectance spectrum of SCU-36 was recorded at room temperature on a SHIMADZU UV-3600 UV-vis-NIR spectrophotometer. As shown in Figure 2a, the

Adjacent T2 clusters are linked by corner sulfide atoms to form a three-dimensional open-framework structure. Viewed along the [001] direction, the structure has extra-large channels delimited by 36 tetrahedra. The diameter of the 36 MR window is approximately 17.4 Å × 26.9 Å calculated from the distance between the sulfide atoms across the window (Figure 1b). The wall of the 36 MR channels contains extra-large 16 MR windows with pore sizes of 13.3 Å × 15.0 Å. The protonated amine molecules in SCU-36 are disordered within the intersecting extra-large channels. A void space analysis by employing the program PLATON indicates that these extraframework species occupy 73.8% of the unit cell volume, which is comparable to those found in SU-M (74.6%) and 72R-NTHU-13 (75.7%) with extra-large pore openings.11 SCU-36 features an interrupted framework combining both 3and 4-connected metal centers. By regarding tin atoms as 3connected nodes and indium atoms as 4-connected nodes, the structure can be represented as an unprecedented (3,4)connected net with a point symbol of (33.162.17)3(33).12 The framework can be further simplified as an etc net if each T2 cluster acts as a 3-connected node (Figure 1c). Although an etctype chalcogenide framework already appeared in a thiophosphate NaTi2(PS4)3, it has a 2-fold interpenetrating structure, thus precluding the presence of large voids.13 From the view of structural chemistry, an etc net is expected to possess 18 MR channels if each structural node serves as a polyhedral building unit. One way to enlarge its pore size from 18 MR to 36 MR is to insert a 2-connected tetrahedron between two adjacent structural nodes as shown in the structure of NaTi2(PS4)3. Another approach as found in SCU-36 reported here is the use of supertetrahedral T2 clusters as the building blocks (Figure S10). It has been demonstrated that the use of 3-connected building blocks often results in the formation of ladderlike and layered structures. Three-dimensional inorganic solids with 3-connected nets are quite rare in zeotype chemistry. The few examples include the fluorinated borophosphate NH4[BPO4F] with an srs topology, the zinc phosphite SCU-18 with a qzh topology, and the thiophosphate NaTi2(PS4)3 with an etc topology.13,14 The use of tetrahedral clusters as structural building blocks often gives rise to 4-connected nets (e.g., dia, sod, and crb). The formation of (3,4)-connected nets has also been observed when part of tetrahedral clusters adopt 3-connected modes. If all tetrahedral clusters act as 3-connected building blocks, only layered structures with an hcb net have been produced so far.15 The discovery of SCU-36 offers a rare example of a three-dimensional structure with a 3-connected net built from supertetrahedral clusters.

Figure 2. (a) Solid-state UV−vis diffuse reflectance spectrum of SCU36. (b) Calculated band structure of SCU-36. The Fermi level is set at 0 eV.

optical absorption data derived from the reflectance show an optical transition with a band gap of 3.1 eV. To further understand the optical property of SCU-36, we performed firstprinciple calculations using DMol3 code based on density functional theory (DFT).16 The computational model is a pure inorganic skeleton that does not contain the disordered organic cations. The local density approximation (LDA) with Perdew− Wang (PWC) is utilized as the exchange−correlation func14731

DOI: 10.1021/acs.inorgchem.7b02109 Inorg. Chem. 2017, 56, 14730−14733

Communication

Inorganic Chemistry tional.17 Geometry optimizations and electronic properties calculations are performed with the all electron core treatment and double numerical atomic orbital plus polarization (DNP) basis set.18 The k-point was set to 1 × 1 × 2 for geometry optimizations, whereas electronic properties were calculated using a k-point sampling of 2 × 2 × 4. As shown in Figure 2b, SCU-36 is a direct-gap material. The band gap is calculated to be 2.43 eV, which is ∼0.67 eV smaller than the experimental value (the DFT methods always bring down the conduction band levels and underestimate band gaps).19 Detailed atomic orbital contributions to the bands can be further assigned according to the total and partial density of states (DOS) (Figure S6). The valence bands from −5.0 eV to the Fermi level can be mainly assigned to In-5s, In-5p, Sn-5p, and S-3p state mixing of low In4d and Sn-4d states. The conduction bands are mostly formed by In-5s, Sn-5s, and S-3p states. The major reasons to affect the photophysical properties, e.g., the electron transition produced by photoexcitation, can be attributed to those states that constitute around the Fermi level and conduction bands. The valence bands around the Fermi level can be mainly assigned to the S-3p state; the bands around the bottom of the conduction bands are mostly formed by the Sn-5s state. Therefore, we conclude that the photophysical property of SCU-36 is formed mainly due to the positioning of S-3p and Sn-5S states. From the partial DOS, we can also see the In-5s, In-5p and Sn-5s, Sn-5p states completely overlap with S-3s, S-3p states in the region from −13.0 to 0.0 eV, which indicates the covalent interactions of In−S and Sn−S bonds. The ion exchange ability of SCU-36 for the radionuclide Cs was also investigated. Typically, 10 mg of as-synthesized SCU-36 was immersed in 10 mL of 0.1 M CsCl aqueous solution at room temperature for 12 h. The powder XRD patterns of assynthesized SCU-36 and Cs+-exchanged solid show similar peak intensities and relative positions, indicating that the crystalline framework remains intact after the ion-exchange process (Figure 3a). The SEM image shows that the prism crystal

exchanged out by Cs+ ion when the ion-exchange time was extended from 12 to 16 and 32 h, the crystal structure of SCU-36 gradually collapsed on the basis of power XRD patterns (Table S1 and Figure S7). Attempts to increase the Cs+ exchange capacity of SCU-36 by increasing the concentration of CsCl aqueous solution or the ion exchange temperature also caused a breakdown of the framework (Table S2 and Figure S8). In summary, a new three-dimensional open-framework metal chalcogenide was synthesized under solvothermal conditions. This semiconductor features extra-large channels, a low framework density, and a 3-connected inorganic network. The present work illustrates that the use of supertetrahedral T2 clusters as structural building blocks is promising to obtain novel open-framework materials with very low FDs. Different from other 3 MR-containing building blocks, the T2 cluster is readily formed by combining M3+ (e.g., Ga3+, In3+) and M4+ (e.g., Ge4+, Sn4+) cations with sulfide atoms. Further efforts will be devoted to organize T2 clusters into other interrupted nets in the presence of various amines as the structure-directing agents.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02109. X-ray data in CIF format, experimental details, additional figures, IR spectrum, TGA curve, and powder XRD patterns (PDF) Accession Codes

CCDC 1555520 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Huajun Yang: 0000-0002-4664-4042 Min Luo: 0000-0001-8080-0881 Dingguo Xu: 0000-0002-9834-8296 Tao Wu: 0000-0003-4443-1227 Zhien Lin: 0000-0002-5897-9114 Notes

The authors declare no competing financial interest.

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Figure 3. (a) Powder XRD patterns of as-synthesized SCU-36 and Cs+exchanged solid. (b) EDS of Cs+-exchanged SCU-36. Insert: SEM image of the morphology of Cs+-exchanged SCU-36.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 21671142) and Jiangsu Province Natural Science Fund for Distinguished Young Scholars (BK20160006). The authors thank Dr. Daichuan Ma at Analytical and Testing Center, Sichuan University for technical help with the Material Studio calculations.

of SCU-36 could maintain its geometrical shape after Cs+ ion exchange and Cs+ ion could be detected in the crystal by EDS (Figure 3b). The CHN element analysis indicates that ∼70% of the organic cations within the extra-large channels of SCU-36 can be exchanged by Cs+ ions on the basis of the content of N (Table S1). However, gas adsorption experiments show that N2 adsorption at 77 K is negligible for the Cs+-exchanged solid, and the CO2 uptake value is only 6.5 cm3 g−1 at 273 K and 760 Torr (Figure S9). Although more organic cations could be

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