Unusual Flexibility of Microporous Sulfides during Ion Exchange

Oct 10, 2018 - Synopsis. Three new microporous sulfides were synthesized under solvothermal conditions. They feature thick-walled frameworks but displ...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Unusual Flexibility of Microporous Sulfides during Ion Exchange Ren-Chun Zhang,† Jing-Chao Zhang,† Zhi Cao,† Jun-Jie Wang,† Shuang-Shuang Liang,† Hong-Jing Cong,† He-Jie 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

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/11/18. For personal use only.

S Supporting Information *

ABSTRACT: Open-framework chalcogenides with ion-exchange capacity are promising materials for removing hazardous heavymetal ions and for capturing radioactive Cs+. However, research on the exchange mechanism is limited, especially for the framework chalcogenides that have multiple bridging anions. Generally, openframework chalcogenides that have multiple bridging anions at the window or wall of the channels are rigid during the ion-exchange process. We show here that microporous sulfides with μ3-S2− (where μ3 = triple bridging mode) at the windows exhibit framework flexibility upon ion exchange. Three new microporous sulfides Na4Cu8Ge3S12·2H2O (1), Na3(Hen)Cu8Sn3S12 (where en = ethylenediamine) (2) and (dap)2(Hdap)4Cu8Ge3S18 (where dap = 1,2-diaminopropane) (3) were synthesized under solvothermal conditions. Compounds 1 and 2 contain a copper-rich framework composed of icosahedral [Cu8S12]16− units linked via monomeric GeS44− or SnS44− tetrahedral units, whereas compound 3 features an expanded framework composed of icosahedral [Cu8S12]16‑ units interconnected with dimeric Ge2S64− units. These compounds exhibit unusual ion-exchange properties. Specifically, the frameworks of 1 and 2 (with μ3-S at the small windows) show “breathing action” upon ion exchange of K+ or Rb+, which have relative large sizes, and compound 3 exhibits framework flexibility upon Cs+ ion exchange with both space group and channels changed.

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such as perovskite structures,35 (6,3)-connected pyr and rare (6,4)-connected topologies.10,40 On the other hand, microporous chalcogenides can have more negative frameworks with high polarizability, and this endows the materials with fast-ion conductivity and high ionexchange capacity.7−10 Recently, Kanatzidis et al. reported that open framework chalcogenides with high ion-exchange capacity can remove heavy-metal cations from aqueous solutions, and they are promising materials in the recovery of rare-earth metals.45−49 In particular, crystalline microporous chalcogenides can provide important models for understanding the mechanism of ion-exchange processes.8−10,50 Indeed, some progress has been achieved in framework chalcogenides that contain flexible μ2-S2− (μ2 = double bridging mode) atoms at the windows and wall of the channels. In contrast, thick-wall microporous chalcogenides that contain multiple bridging Q2− (≥3) ions have the advantages of stability and acid or base resistance.51,52 However, their ion-exchange behavior and mechanism still lack in-depth research and understanding. Previously, Ding’s work revealed that thick-wall microporous chalcogenides have rigid frameworks during ion- exchange

INTRODUCTION

Increasing demands for microporous materials in the fields of catalysis, separations, ion exchange, and green chemistry have inspired wide interest in exploring crystalline frameworks of novel chemical compositions, structures, and functions.1,2 In 1989, Berdard et al. proposed an extension of microporous materials from oxides to chalcogenides.3 Since then, microporous chalcogenides have attracted a great deal of research attention, because of their prominent semiconductivity and potential applications in areas ranging from fast-ion conductivity to selective ion exchange, photovoltaic conversion, and visible-light photocatalysis.4−16 Compared to oxides, microporous chalcogenides have more diverse compositions and structures, because of wider chemical and bonding flexibility.17−24 Among the various microporous chalcogenides, most of chalcogenide frameworks are based on large tetrahedral clusters, including Tn, Pn, Cn, and Tp,q series.25−32 These large tetrahedral clusters have variable sizes and tunable compositions and can decorate on 4connected sites to construct zeolitic frameworks with large pores and low framework density. Equally, icosahedral Cu−S/ Se clusters have attracted widespread attention.10,33−44 Unlike tetrahedral clusters, icosahedral Cu−S/Se clusters prefer to decorate on 6-connected sites to form different topologies, © XXXX American Chemical Society

Received: May 6, 2018

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DOI: 10.1021/acs.inorgchem.8b01238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details of Compounds 1−3 Value parameter empirical formula fw temp (K) cryst syst space group a (Å) α (deg) V (Å3) Z Dc (g cm−3) abs coeff (mm−1) F(000) 2θ (max) (deg) index range

no. of collected/unique reflections Rint GOF on F2 R1 (I > 2σ(I)) wR2 Δρmax, Δρmin (e Å−3)

1

2

3

Na4Cu8Ge3S12·2H2O 1203.03 296 cubic Fm3̅c 17.5742(15) 90.0 5427.8(14) 8 2.944 10.380 4510 50.0 −20 ≤ h ≤ 20 −20 ≤ k ≤ 9 −20 ≤ l ≤ 19 3603/214 0.0309 1.041 0.0242 0.0799 0.613, −1.013

Na3(Hen)Cu8Sn3S12 1318.08 296 cubic Fm3̅c 17.9602(3) 90.0 5793.4(3) 8 3.022 9.183 4856 56.4 −22 ≤ h ≤ 22 −23 ≤ k ≤ 18 −23 ≤ l ≤ 23 5627/316 0.0203 1.031 0.0270 0.1191 1.504, −1.352

(dap)2(Hdap)4Cu8Ge6S18 1520.94 296 cubic Im3̅ 22.7448(3) 90.0 11766.5(5) 8 1.717 6.479 5696 50.0 −27 ≤ h ≤ 15 −26 ≤ k ≤ 18 −25 ≤ l ≤ 26 19195/1242 0.1078 1.038 0.0378 0.0994 0.489, −0.494

diethyltriamine (abbreviated as dien) and 200 μL of CH3OH:H2O (VCH3OH:VH2O = 1:1) were added as a mixed solvent and ultrasonically dispersed. The tube was sealed in an air atmosphere, placed in a stainless-steel autoclave, and then heated at 160 °C for 7 days. The products were washed with en and ethanol several times, respectively. Light yellow cubic crystals were obtained in 75% yield based on Cu. The experimental and simulated PXRD patterns are shown in Figure S1 in the Supporting Information. An average composition of Na:Cu:Ge:S = 4:8:3:12 was determined via EDS analysis of several crystals. Elemental analysis (%) calcd. for 1 (H4O2Na4Cu8Ge3S12): C, 0.00; H, 0.32; N, 0.00; S, 31.02. Found: C, 0.02; H, 0.25; N,0.01, S, 30.86. Na3(Hen)Cu8Sn3S12 (2). Synthesis of compound 2 is similar to that of compound 1: Cu powder (0.11 mmol, 7.0 mg), Sn powder (0.04 mmol, 5.0 mg), Na2CO3 (0.1 mmol, 10.6 mg), and S (0.55 mmol, 17 mg) were used as reactants and 300 μL of en and 200 μL of CH3OH were used as a mixed solvent. The products were washed with en, ethanol, and distilled water several times, respectively. Deep red polyhedral crystals of compound 2 were obtained in 68% yield based on Cu. The experimental and simulated PXRD patterns are shown in Figure S2 in the Supporting Information. An average composition of Na:Cu:Sn:S = 3:8:3:12 was determined via EDS analysis of several crystals. Elemental analysis (%) calcd. for 2 (C2H9N2Na3Cu8Sn3S12): C, 1.74; H, 0.65; N, 2.03; S, 27.87. Found: C, 1.92; H, 0.90; N, 1.87; S, 27.62. (dap)2(Hdap)4Cu8Ge6S18 (3). Synthesis of compound 3 is similar to that of compound 1: Cu powder (0.04 mmol, 3.0 mg), GeO2 powder (0.05 mmol, 5.0 mg), and S (0.4 mmol, 13 mg) and 300 μL dap were used as reactants. Pale yellow cubic crystals were obtained in 23% yield based on Cu. The experimental and simulated PXRD patterns are shown in Figures S3 and S4 in the Supporting Information. An average composition of Cu:Ge:S = 4:3:9 was determined via EDS analysis of several crystals. Elemental analysis (%) calcd. for 3 (C18H64N12Cu8Ge6S18): C, 10.97; H, 3.25; N, 8.53; S, 29.26. Found: C, 11.32; H, 3.07; N, 8.45; S, 28.98. Typical Ion-Exchange Experiment. A quantity of 40 mg of polycrystalline compound 1 (2 or 3) was added to 40 mL of MCl aqueous solution (where M = K+, Rb+, or Cs+) (1.0 M); the mixture then was kept at 45 °C and stirred for 12 h. The exchanged products were isolated by filtration, washed several times with deionized water

processes, and can remove heavy-metal cations from aqueous solution.51,52 In this publication, we report three new microporous sulfides: Na4Cu8Ge3S12·2H2O (1), Na3(Hen)Cu8Sn3S12 (2) (where en = ethylenediamine), and (dap)2(Hdap)4Cu8Ge3S18 (3) (where dap = 1,2-diaminopropane). Compounds 1 and 2 contain a copper-rich framework composed of icosahedral [Cu8S12]16− units linked via monomeric MS44− tetrahedral units, whereas compound 3 features an expanded framework composed of icosahedral [Cu8S12]16− units interconnected via dimeric Ge2S64− units. In particular, these framework sulfides contain μ3-S2− at the wall or windows of channels, and they exhibit unusual framework flexibility upon ion exchange.

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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 40 kV and 40 mA. Simulated PXRD patterns were calculated based on single-crystal data of the compounds. Energy-dispersive spectroscopy (EDS) was performed on a scanning electronic microscope (Hitachi, Model SU 8010). Elemental analysis (C, H, N, and S) was performed on a Vario EL III elemental analyzer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analysis were carried out on a Mettler−Toledo Star instrument using an aluminum crucible as a sample container under a flow of nitrogen (40 mL min−1) at a heating rate of 10 °C min−1. The ultraviolet−visible light (UV-vis) reflectance spectra were measured using a double-beam, double-monochromator spectrophotometer that was equipped with an integrating sphere at 296 K (Shimadzu, Model UV-2550); a BaSO4 plate was used as a reference. Collected reflectance data were converted to the absorption data using the Kubelka−Munk functions. Synthesis of Compounds 1−3. Na4Cu8Ge3S12·2H2O (1). Cu powder (0.11 mmol, 7.0 mg), GeO2 powder (0.05 mmol, 4.0 mg), Na2CO3 (0.1 mmol, 10.6 mg), and S (1.0 mmol, 32 mg) were placed in a Pyrex-glass tube (∼10 mL in volume). Then, 300 μL of B

DOI: 10.1021/acs.inorgchem.8b01238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry and ethanol, then dried in air. Single-crystal ion-exchange experiments were performed in a similar route except that several single crystals of each compound were carefully selected via microscopeic examination. 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 the SHELX software package program.53,54 All heavy atomic positions were located in Fourier maps and refined anisotropically. For compound 3, the SQUEEZE program was used to treat the disordered guest species in the pore and channels.55 Crystal data and structure refinement details for 1−3 are given in Table 1.

Crystal Structure Description. Single-crystal X-ray diffraction (XRD) analysis reveals that compound 1 contains a copper-rich [Cu8Ge3S12]4− framework charge-balanced by Na+ ions. It is isostructural to our previously reported A4Cu8Ge3S12 (A = K, Rb) with cubic perovskite structure.35 The anionic [Cu8Ge3S12]4− framework is composed of ideal icosahedral [Cu8S12]16− clusters interconnected with discrete tetrahedral GeS44− units by sharing common S atoms (see Figure 1). Each Cu+ ion adopts planar trigonal geometry and

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RESULTS AND DISCUSSION Synthesis Chemistry. The microporous Cu−Ge/Sn-S system represents one of most complicated systems among open framework chalcogenides, because of the diverse coordination modes of Cu+ ion and the flexible condensation behaviors of/between Cu−S and Ge/Sn−S units.10,34−44,56−58 The final framework structures are sensitive to changes in Cu/ MIV ratio, SDAs (structure-directing agents), and several synthesis parameters, such as reaction temperature, mineralizer, solvent, pH value, and the molar ratio of reactants. To probe the potential ion-exchange properties of copper-rich framework sulfides, small-sized Na+ with high mobility was selected as SDAs for the syntheses. Two copper-rich sulfides (1 and 2) were synthesized via solvothermal reactions of Cu, GeO2/Sn and Na2CO3 in the presence of excess S. Our systematic work reveals that excess sulfur is crucial to synthesize copper-rich framework sulfides and to acquire a pure phase.10 Because Cu−S compounds are insoluble in common solvents, black amorphous precipitate occurs during the syntheses, and this impedes the formation of ternary framework sulfides. In contrast, excess sulfur plays important roles in the syntheses of copper-rich framework sulfides 1 and 2. Excess sulfur generates amount of Sx2− ions under amine alkaline conditions,59−61 and this can effectively increase the solubility of Cu−S compounds and decrease the condensation of Ge/Sn−S units to form copper-rich frameworks, especially in the presence of alkaline-metal cations with high charge density. These effects also were reflected in our previous reported work.10,35,59,62 On the other hand, the amine alkali synthesis system is necessary to produce Na+- templated sulfides. Because of its small size and high charge density, Na+ is apt to form hydration ions under hydrothermal conditions. In 2003, Feng’s group reported the hydrothermal synthesis of zeolitic framework chalcogenides using Na+ hydrate ions as templates.7 However, the hydro effect of Na+ can be remarkably suppressed in organic amine systems. For compound 1, 25% of Na+ ions are hydrated ions, and for compound 2, all of the Na+ ions are free ions. In addition, Na+ shows a relatively weaker structuredirecting effect, compared to organic amine. As reported by Zhang’s group, for the synthesis of (H3O)2(enH2)Cu8Sn3S12, Na+ ions are also present in the synthesis system, although Na+ does not enter the final structure.41 For the syntheses of 1 and 2, Na2CO3 was introduced, and to some extent, this increased the alkalinity of the synthesis system, and decreased the competition effect of protonated organic amine. Therefore, the synthesis of 1 and 2 provides a new method for synthesizing framework sulfides using Na+ ions as a template, and this may be useful for the exploring novel framework chalcogenides with fast-ion conductivity and ion-exchange properties.

Figure 1. View of the three-dimensional (3-D) structure of compound 1.

coordinates to three S2− ions with a Cu−S bond length of 2.250 Å and a S−Cu−S angle of 119.3°; each Ge4+ ion is tetrahedrally coordinated to four S2− ions with the Ge−S bond length of 2.225 Å and the S−Ge−S angle in the range of 108.0°−112.4°. These geometric parameters are in good agreement with reported values.34−36,39 Each S2− ion adopts a μ3-mode and bridges one Ge4+ ion and two Cu+ ions. The 3-D Cu−Ge−S framework exhibits 3-D intersecting channels along the ⟨100⟩ directions with a window size of 1.98 Å × 1.98 Å (between the center of S atoms), and there is a large cavity with a diameter of 7.0 Å at the intersection of the channels. Na+ ions are located in the channels and cavities. Unlike K+ or Rb+, which are distributed over four of six equivalent positions in A4Cu8Ge3S12 (A = K, Rb),35 there are three different crystallographic independent Na+ sites in 1. As shown in Figures 2a and 2b, 75% of Na+ resides near pore openings and on the 4-fold rotation axes: Na(1) atoms are located at the center of the windows with a Na···S distance of 2.831 Å, and Na(3) atoms deviate slightly from the plane of the pore windows with a Na···S distance of 2.977 Å. The remaining 25% of Na+ (Na(2) atoms) is located in large

Figure 2. Different crystallographic Na+ sites in compound 1. C

DOI: 10.1021/acs.inorgchem.8b01238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the linking Ge2S64− units and slight tilting of the [Cu8S12]16− units. As calculated using the program PLATON, dap and Hdap+ occupy 55.4% of the nonframework void space.63 The positions of guest molecules cannot be located, because of the high symmetry and porous nature of the framework, although their presence and quantity can be confirmed via elemental analysis and thermal analysis (see Figure S7 in the Supporting Information). The Cu−Ge−S framework of 3 is comparable to that of [Cu16Ge12S36][Ni(en)3]4(en)xCl1.5 with complex ions as counterions, which is recently reported by Wu’s group.39 These two structures can be viewed as a three-step expansion of the copper-rich [Cu8Ge3S12]4− framework with (6,2) topology.35 The icosahedral [Cu8S12]16‑ cluster can serve as 6-connected building units that interconnect with two bridging GeS4 or Ge2S6 units via sharing a common S−S edge. Interestingly, the framework can maintain the same topology, while the framework can be gradually expanded as partial or all of the GeS4 units are replaced with longer bridging units, such as Ge2S6 units, which can be observed in A4Cu8Ge3S12, [Cu8Ge4S14]4− [Cu8Ge5S16]4−, and [Cu8Ge6S18]4− frameworks (see Figure 4).34−36,39 Structurally, host−guest interactions are important in the formation of a ternary framework. Alkalinemetal ions such as Na+, K+, and Rb+, which have small size and high charge density, can induce the formation of a copper-rich framework with high charge density.35 When more Ge4+ ions are incorporated into the framework, anions on the framework are prone to adopt low bridging modes; also, the framework, which is balanced by counterions with low positive charge, has low charge density. In the meantime, the framework shows a porosity increase from 27% to 55.4%. From this point of view, the Cu−Ge−S frameworks are sensitive to the structure of SDAs and host−guest charge matching is critical for the synthesis. In addition, the framework may be further expanded if SDAs were carefully selected and if longer bridging units, such as linear M3S8 and M4S10, were incorporated as linkers.64 Ion-Exchange Properties. Compounds 1−3 have 3-D open frameworks and exhibit unusual ion-exchange properties. Ion-exchange reactions were performed in aqueous solutions at 45 °C with mild stirring. Our results indicate that the Na+ ion in compounds 1 and 2 can be exchanged with K+ and Rb+ with high capacity (above 90%; see Table S1 in the Supporting Information), and the framework structure remains intact after ion exchange, as confirmed by PXRD data (see Figures S8 and S9 in the Supporting Information). These exchanging results are quite surprising, because the pore opening of the copperrich framework is narrow with a size of 1.98 Å × 1.98 Å, which is significantly smaller than the diameter of K+ (2.74 Å) or Rb+ (3.04 Å).65 How can K+ and Rb+ diffuse through the narrower passage formed by four μ3-S2− ions? Ion exchange is a complex process, and it is relative to thermodynamic and dynamic factors, including temperature, time, stirring, concentration, ion size, solution, and so on. As an important thermodynamic factor, we considered that temperature may be critical to the K+ and Rb+ ion-exchange experiments, because of possible activation and thermal effects of frameworks. Thus, for comparison, K+ and Rb+ ion exchange experiments of 1 were performed at room temperature. Our results reveal that, at room temperature, the ion-exchange ratio is low (∼76%) for a time of 12 h, and 20 h are needed for the exchange ratio to exceed 90%. Thus, temperature is not a decisive factor for the successful ion exchange of K+ or Rb+ in narrow channels.

cavities that are on the 2-fold rotation axis and have a Na···S distance of 3.178 Å (Figure 2c). Two H2O molecules are disordered in each cavity, which can be identified via elemental analysis and thermal analysis (see Figure S5 in the Supporting Information). Compound 2 contains a copper-rich [Cu8Sn3S12]4− framework isostructural with that of 1, and exhibits threedimensional intersection channels.35,41,44 Na+ and Hen+ ions coexist in the channels to compensate for the charge of the framework. In this structure, Na+ cations lie on 4-fold rotation axes and deviate slightly from the plane of the pore windows, adopting tetragonal pyramid geometry coordinated via four S atoms at the window, and the Na···S distance is 2.901(4) Å. Hen+ ions are disordered in the cavities, although their presence can be identified via elemental analysis and thermal analysis (see Figure S6 in the Supporting Information). The small size of and special distribution of Na+ ions in compounds 1 and 2 indicate that these materials have potential ionexchange properties and fast-ion conductivity properties. Compound 3 crystallizes in the cubic space group Im3̅ and is composed of a 3-D anionic [Cu8Ge6S18]4− framework with intersecting 3-D channel systems, in which the counterions reside. As shown in Figure 3, the structure features an

Figure 3. View of the three-dimensional structure of compound 3.

expanded framework composed of icosahedral [Cu8S12]16‑ clusters linked via dimeric Ge2S64− units in 3D directions. Note that the Ge2S64− unit is composed of edge sharing of two GeS44− tetrahedra, which is seriously distorted; specifically, the dihedral angle of the Ge−S(1)S(2)−Ge four-membered ring is 159.3°, the S(4)···S(4) distance is 4.57 Å and that of the S(3)···S(3) is 6.0 Å. Such a feature indicates that linear dimeric Ge2S64− unit can bear some degree of bending and can enable more-flexible linkage. Interestingly, the Cu−Ge−S framework contains a unique 3D channel system formed via the intersection of two different one-dimensional (1-D) channels. A view of the framework along the c-direction shows that two types of 1-D channels alternate mutually and communicate with each other through channels along the a- and b-directions. Although the two types of 1-D channels have the same style of construction, they have different sizes: one is 9.19 Å × 2.88 Å, and the other is 4.98 Å × 5.90 Å. This difference can be attributed to the bending of D

DOI: 10.1021/acs.inorgchem.8b01238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. Step-by-step expansion of icosahedral [Cu8S12]16− cluster-based framework: (a) ball-and-stick mode of icosahedral [Cu8S12]16− cluster; (b) polyhedral mode of [Cu8S12]16− cluster; (c) framework structure of [Cu8Ge3S12]4−; (d) framework structure of [Cu8Ge4S14]4−; (e) framework structure of [Cu8Ge5S16]4−; and (f) framework structure of [Cu8Ge6S18]4−.

Single-crystal XRD was performed on the K+- and Rb+exchanged products to probe these unusual ion exchange properties of 1 and 2. Structural analysis results show that K+ or Rb+ really enter the pores of the frameworks (see Figure 5).

This structure is comparable to A4Cu8Ge3S12 (A = K or Rb), except for the slight increase in unit-cell parameter a and pore size.35 The K···S distances are 3.207 Å for 1(K) and 3.163 Å for 2(K), and the Rb···S distances are 3.266 Å for 1(Rb) and 3.287 Å for 2(Rb), respectively. These distances are in good agreement with those found in A4Cu8Ge3S12 (A = K or Rb),35 while are obviously longer than the Na···S distance previously mentioned. Also, the framework of ion-exchange products expands slightly, compared to the framework of the primitive crystals. As shown in Table 2, the cell parameter a of compound 1 increases from 17.5742 Å to 17.7280 Å for 1(K) and to 17.7351 Å for 1(Rb). For compound 2, a increases from 17.9602 Å to 18.2230 Å for 2(K) and to 18.1861 Å for 2(Rb), respectively. Accordingly, the passage in the framework of 1 expands from 1.98 Å to 2.11 Å for 1(K) and to 2.13 Å for 1(Rb), and that of 2 expands from 2.00 Å to 2.25 Å for 2(K) and to 2.21 Å for 2(Rb). However, the passages for the exchange products are apparently narrower than the diameters of the K+ (2.74 Å) and Rb+ (3.04 Å) ions. Considering the rigid feature of K+ and Rb+, the unusual ionexchange capability can be ascribed to the “breathing” action during the exchange process.8,10 Such a phenomenon involves expansion of the framework windows between cavities to allow K+ or Rb+ cations, which have relatively large sizes, to pass, and then the windows contract after the ion enters. This is a

Figure 5. Structure of K+ ion-exchange product of compound 1.

Table 2. Important Parameters of Compounds 1−3 and Ion-Exchange Products parameter counterion exchange ratio (%) space group a (Å) A+···S (Å) Cu−S−Cu (deg) Cu−S−MIV(deg) ion size (dim) pore size (Å2)

1

1(K)

1(Rb)

Fm3̅c 17.5742 2.831 87.83

Na0.04K3.96 99 Fm3̅c 17.7280 3.207 86.04

Na0.32Rb3.68 92 Fm3̅c 17.7351 3.266 86.22

106.75 1.98(Na+) 1.98 × 1.98

107.79 2.74 (K+) 2.11 × 2.11

108.11 3.04 (Rb+) 2.13 × 2.13

Na4

2

2(K)

Na3

2(Rb)

Fm3̅c 17.9602 2.901 91.29

Na0.14K2.86 94 Fm3̅c 18.2330 3.163 88.3

Na0.15Rb2.85 95 Fm3̅c 18.1861 3.287 89.2

104.07 1.98(Na+) 2.00 × 2.00

106.01 2.74 (K+) 2.25 × 2.25

105.90 3.04 (Rb+) 2.21 × 2.21

E

3

3(Cs)

(Hdap)4 I3̅m 22. 7448 88.07 88.46 100.58−109.10 9.19 × 2.88, 4.98 × 5.90

Cs3.6(Hdap)0.4 90 P3̅m 11.4199 3.773, 3.883 90.9 103.54 3.48(Cs+) 7.72 × 4.32

DOI: 10.1021/acs.inorgchem.8b01238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry dynamic process for ion diffusion through the structure during the ion-exchange experiment. Such an expansion/contraction phenomenon were previously observed in microporous sulfide K6Sn[Zn4Sn4S17] and (H2en)2Cu8Sn3S12.8,10 The Kanatzidis group found that K6Sn[Zn4Sn4S17] displays exceptionally high NH4+ exchange capacity and diffusion of NH4+ passing through narrower windows.8 Our previous work shown that Cs+ ion can enter the pore and channel of (H2en)2Cu8Sn3S12, which has a smaller aperture size than the size of Cs+.10 Such a “breathing” action of the framework is closely related to the flexibility of μ2-S atoms at the windows, and this allows a certain degree of expansion of passage windows. In contrast, S2− at the windows in the framework in 1 and 2 adopts a μ3mode bridging one Ge4+ (or Sn4+) and two Cu+ ions. In most cases, multiple bridging modes (≥3) of S2− are rigid. Our results represent an unusual chalcogenide framework, which has μ3-bridging S2− at the windows, while displaying framework flexibility. In addition, Cs+, Ca2+, Sr2+, and Ba2+ ion-exchange experiments of 1 and 2 were also performed, but the results reveal that only negligible amounts of these ions were exchanged. This can be ascribed to the large size of the Cs+ ion and to the strong hydrate effect of Ca2+, Sr2+, and Ba2+ ions, which is above the tolerance of the elasticity of the copper-rich framework. Compound 3 also exhibits ion-exchange properties. Our results indicate that part of the extra framework cations can be exchanged with K+, Rb+, and Cs+ ion. EDS analysis reveals that ∼15% of the counterions can be exchanged with K+ and Rb+, but 90% can be exchanged with Cs+ ion. The high exchange capacity of Cs+ ion can be attributed to Cs+ having the right size and to its soft acid character, which give it relatively high affinity for the sulfide framework (see Figure S10 in the Supporting Information). Interestingly, unlike the “breathing action” of the copper-rich frameworks of 1 and 2, the framework of 3 shows large flexibility upon Cs+ ion exchange, and this is confirmed via single-crystal XRD. Structural analysis reveals that the space group transforms from I3̅m to P3̅m, while the framework maintains good integrity. Accordingly, the bridging Ge2S6 units straighten as the dihedral angle changes from 159.3° to 180°, and the icosahedral [Cu8S12]16− clusters are regulated with ideal Th symmetry and no tilting, as show in Figure 6. Surprisingly, the channel system formed via the intersection of two different 1-D channels in 3 is uniform in the 3Cs. The alternating channel sizes of 9.19 Å × 2.88 Å and 4.98 Å × 5.90 Å both changed to 7.72 Å × 4.32 Å. Such large degrees of change confirm the flexibility of the framework. Particularly, the μ3-S2− bridging angles varied during the ionexchange experiment. The Ge−S−Cu angles in compound 3 range from 100.6° to 109.1°; however; they are all changed to 103.6° in the final Cs+ exchange products 3Cs. These results prove that the μ3-S2− ions at the windows are flexible and that the bridging angles can bear a certain amount of expansion and contraction. From this perspective, the results also provide an indirect proof of “breathing action” ion-exchange mechanism of compound 1 and 2. In a word, the ion-exchange properties of these compounds provide unusual modes in framework sulfides that contain multiple bridging anions. Optical Properties. The UV-vis reflectance spectra of compounds 1−3 confirm that the compounds are wide-bandgap semiconductors. As shown in Figure 7, the optical absorption spectra revealed that the band gaps are 2.4, 2.2, and 2.65 eV for 1, 2, and 3, respectively, and these are

Figure 6. Framework structure of Cs+ ion-exchange product of compound 3.

Figure 7. UV-vis spectra of compounds 1−3.

consistent with the colors of the compounds. The band gap of 1 is slightly blue-shifted, compared to the isostructural K4Cu8Ge3S12 (Eg = 2.2 eV) and Rb4Cu8Ge3S12 (Eg = 2.3 eV),35 and this can be ascribed to the smaller size of Na+ and to the relatively weaker interaction with the host framework. The band gap of compound 2 is narrower than that of compound 1, because of the incorporation of heavier Sn4+ as linkers, whereas it is comparable to our previously reported (H2en)2Cu8Sn3S12 (Eg = 2.2 eV) with the same Cu/Sn ratio.35 The band gap of compound 3 is wider than those of compounds 1 and 2, as well as some Cu−Ge−S framework sulfides, such as (H2dab)2Cu8Ge4S14·2H2O (Eg = 2.45 eV),37 [Cu8Ge5S16]·x solvent (Eg = 2.5 eV), each of which has a high Cu/Ge ratio;34 however, it is obviously red-shifted, compared to Cu−Ge−S framework sulfides that have low Cu/Ge ratios, such as Tma2Cu2Ge4S10 and Tea2Cu2Ge4S10.56−58 Generally, an increase in the Cu/M ratio of the framework decreases the band gap to some extent, because of the significant contribution of the 3d, 4s, and 4p orbitals of the Cu site to the valence and conduction bands.34

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CONCLUSION In this work, three new microporous sulfides were solvothermally synthesized and characterized. Compounds 1 F

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(4) Feng, P.; Bu, X.; Zheng, N. The Interface Chemistry between Chalcogenide Clusters and Open Framework Chalcogenides. Acc. Chem. Res. 2005, 38, 293−303. (5) Li, H.; Laine, A.; O’Keeffe, M.; Yaghi, O. M. Cavities and Channels Supertetrahedral Sulfide Crystals with Giant. Science 1999, 283, 1145−1147. (6) Zheng, N.; Bu, X.; Wang, B.; Feng, P. Microporous and Photoluminescent Chalcogenide Zeolite Analogs. Science 2002, 298, 2366−2369. (7) Zheng, N.; Bu, X.; Feng, P. Synthetic design of crystalline inorganic chalcogenides exhibiting fast-ion conductivity. Nature 2003, 426, 428−432. (8) Manos, M. J.; Chrissafis, K.; Kanatzidis, M. G. Unique Pore Selectivity for Cs+ and Exceptionally High NH4+ Exchange Capacity of the Chalcogenide Material K6Sn[Zn4Sn4S17]. J. Am. Chem. Soc. 2006, 128, 8875−8883. (9) Ding, N.; Kanatzidis, M. G. Selective incarceration of caesium ions by Venus flytrap action of a flexible framework sulfide. Nat. Chem. 2010, 2, 187−191. (10) Zhang, R.-C.; Yao, H.-G.; Ji, S.-H.; Liu, M.-C.; Ji, M.; An, Y.-L. (H2en)2Cu8Sn3S12: a trigonal CuS3-based open-framework sulfide with interesting ion-exchange properties. Chem. Commun. 2010, 46, 4550−4552. (11) Lin, J.; Dong, Y.; Zhang, Q.; Hu, D.; Li, N.; Wang, L.; Liu, Y.; Wu, T. Interrupted Chalcogenide-Based Zeolite-Analogue Semiconductor: Atomically Precise Doping for Tunable Electro-/Photoelectrochemical Properties. Angew. Chem., Int. Ed. 2015, 54, 5103− 5107. (12) Pienack, N.; Puls, A.; Näther, C.; Bensch, W. The Layered Thiostannate (dienH2)Cu2Sn2S6: a Photoconductive InorganicOrganic Hybrid Compound. Inorg. Chem. 2008, 47, 9606−9611. (13) Lin, Y.; Xie, D.; Massa, W.; Mayrhofer, L.; Lippert, S.; Ewers, B.; Chernikov, A.; Koch, M.; Dehnen, S. Changes in the Structural Dimensionality of Selenidostannates in Ionic Liquids: Formation, Structures, Stability, and Photoconductivity. Chem. - Eur. J. 2013, 19, 8806−8813. (14) Zhao, X.-W.; Qian, L.-W.; Su, H.-C.; Mo, C.-J.; Que, C.-J.; Zhu, Q.-Y.; Dai, J. Co-assembled T4-Cu4In16S35 and Cubic Cu12S8 Clusters: A Crystal Precursor for Near-Infrared Absorption Material. Cryst. Growth Des. 2015, 15, 5749−5753. (15) Zheng, N.; Bu, X.; Vu, H.; Feng, P. Open-Framework Chalcogenides as Visible-Light Photocatalysts for Hydrogen Generation from Water. Angew. Chem., Int. Ed. 2005, 44, 5299−5303. (16) Lin, Q.; Bu, X.; Mao, C.; Zhao, X.; Sasan, K.; Feng, P. Mimicking High-Silica Zeolites: Highly Stable Germanium- and TinRich Zeolite-Type Chalcogenides. J. Am. Chem. Soc. 2015, 137, 6184−6187. (17) Xu, X.; Wang, W.; Liu, D.; Hu, D.; Wu, T.; Bu, X.; Feng, P. Pushing up the Size Limit of Metal Chalcogenide Supertetrahedral Nanocluster. J. Am. Chem. Soc. 2018, 140, 888−891. (18) Baiyin, M.; An, Y.; Liu, X.; Ji, M.; Jia, C.; Ning, G. K2Ag6Sn3S10: A Quaternary Sulfide composed of Silver Sulfide Layers Pillared by Zigzag Chains ∞1[SnS3]2‑. Inorg. Chem. 2004, 43, 3764−3765. (19) Feng, M.-L.; Kong, D.-N.; Xie, Z.-L.; Huang, X.-Y. ThreeDimensional Chiral Microporous Germanium Antimony Sulfide with Ion-Exchange Properties. Angew. Chem., Int. Ed. 2008, 47, 8623− 8626. (20) Vaqueiro, P.; Romero, M. L. Gallium-Sulfide Supertetrahedral Clusters as Building Blocks of Covalent Organic-Inorganic Networks. J. Am. Chem. Soc. 2008, 130, 9630−9631. (21) Wang, W.; Wang, X.; Hu, D.; Yang, H.; Xue, C.; Lin, Z.; Wu, T. An Unusual Metal Chalcogenide Zeolitic Framework Built from the Extended Spiro-5 Units with Supertetrahedral Clusters as Nodes. Inorg. Chem. 2018, 57, 921−925. (22) Wu, T.; Khazhakyan, R.; Wang, L.; Bu, X.; Zheng, S.-T.; Chau, V.; Feng, P. Three-Dimensional Covalent Co-Assembly between Inorganic Supertetrahedral Clusters and Imidazolates. Angew. Chem., Int. Ed. 2011, 50, 2536−2539.

and 2 contain copper-rich frameworks composed of icosahedral [Cu8S12]16− units linked via monomeric MS44− tetrahedral units; and compound 3 features an expanded framework composed of icosahedral [Cu8S12]16− units interconnected via dimeric Ge2S64− units. These microporous sulfides have μ3-S anions at the windows or walls of their 3-D channels, and exhibit unusual ion-exchange properties: framework of 1 and 2 show “breathing” action upon ion exchange of K+ or Rb+, each of which has a relatively large size; compound 3 exhibits framework flexibility upon Cs+ ion exchange with both space group and channels changing. These results change our understanding of ion-exchange properties of microporous sulfides that have thick walls and provide new models for studying ion-exchange mechanisms.

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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.8b01238. Powder X-ray diffraction patterns, and TG and DSC curves of the compounds (PDF) Accession Codes

CCDC 1844864−1844871 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, U.K.; fax: + 44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-J. Zhang). *E-mail: [email protected] (Y.-L. An). ORCID

Ren-Chun Zhang: 0000-0002-9029-0026 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge support of this research by National Natural Science Foundation of China (Nos. 21301009, 21501006, 21171028, 21603004, U1604119, 21403006), Natural Science Foundation of Henan Province (No. 182300410174), the Foundation of the Henan Educational Committee (No. 18B150001), the Program for Innovative Research Team of Science and Technology in the University of Henan Province (No. 18IRTSTHN006).

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