Selenidostannates and a Silver Selenidostannate Synthesized in

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Selenidostannates and a Silver Selenidostannate Synthesized in Deep Eutectic Solvents: Crystal Structures and Thermochromic Study Kai-Yao Wang,*,† Hua-Wei Liu,† Shu Zhang,† Dong Ding,† Lin Cheng,‡ and Cheng Wang*,† †

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Institute for New Energy Materials and Low-Carbon Technologies, School of Materials Science and Engineering, Tianjin Key Laboratory of Advanced Functional Porous Materials, Tianjin University of Technology, Tianjin 300384, People’s Republic of China ‡ College of Chemistry, Tianjin Normal University, Tianjin 300387, People’s Republic of China S Supporting Information *

ABSTRACT: Deep eutectic solvents (DESs) have been adopted as reaction media for solvothermal synthesis of crystal materials. In the present work, we extended the scope of DESs in chalcogenidometalate preparation by including dimethylamine, ethylamine, and trimethylamine hydrochlorides and synthesized a series of novel SnSe and Ag-Sn-Se compounds: i.e., [NH2(CH3)2]2Sn3Se7·0.5NH(CH3)2 (1), [NH4]2Sn4Se9 (2), [NH3C2H5]2Sn3Se7 (3), and [NH4]3AgSn3Se8 (4). Compounds 1 and 3 possess honeycomb lamellar [Sn3Se7]n2n− structures featuring large hexagonal windows, while compound 2 features a rare [Sn4Se9]n2n− anionic layer consisting of tetrameric {Sn4Se10} clusters as secondary building units (SBUs). Compound 4 comprises infinite [AgSn3Se8]n3n− chains built by {Sn3Se8} units with Ag+ linkers, and it represents the first heterometallic chalcogenide synthesized in DESs. The organic ammonium cations of halide salts or in situ formed ammonium cations from the decomposition of urea act as templating agents for the formation of the inorganic frameworks. Compound 4 exhibits a marked thermochromic performance in the visible light range owing to the negative temperature dependence of its band gap (Eg = 2.305−2.119 eV in the range of 100−450 K). The gold−dark red−gold color change is highly reversible in five rounds of heating and cooling, without any phase transition of the material, shedding light on the consequent device innovations.

1. INTRODUCTION Solvothermal techniques have been extensively developed in inorganic synthesis owing to the great experimental maneuverability and allowance for trapping kinetically metastable phases that may not be accessible through high-temperature routes.1 Since the pioneering work by Morris and co-workers,2 solvothermal synthesis involving predominantly ILs as media has been developed for the preparation of crystalline inorganic materials owing to the negligible vapor pressure and particularly to the ionic environment.3−5 The dual role of ILs as both solvents and templates precludes the common competition between the solvent molecule and the template for interaction with the growing solid in traditional solvothermal procedures, in particular favoring the targeted construction of porous architectures. Nevertheless, the high price of ILs as well as their hazardous toxicity and poor biodegradability hamper their prospects in industrial applications and thus new candidates need to be explored to utilize the synthetic technique in a more rational way.6−8 Deep eutectic solvents (DESs), first proposed by Abbott and co-workers in 2003, are types of eutectic mixtures with a © XXXX American Chemical Society

freezing point much lower than those of either of the individual components.9 In addition to their excellent solvating properties, negligible vapor pressure, and high thermal stability, DESs possess many advantages in comparison to traditional imidazolium-based ILs. (1) DESs can be easily prepared by mixing organic ammonium halides with hydrogen bond donors (HBDs), e.g. amides, carboxylic acids, and polyols, avoiding all the problems of purification and waste disposal encountered with ILs.10 (2) The price of DESs is much lower than that of ILs because of their cheap components such as ChCl and urea. (3) Many of the DESs have been proven to be nontoxic and biodegradable, reinforcing their greenness as media for inorganic synthesis.11 Notably, the DESs are not entirely composed of ionic species, and the presence of neutral HBDs can provide novel and complementary environments from common ILs. Hitherto, DESs have attracted considerable attention for the synthesis of crystalline materials, such as Received: September 13, 2018

A

DOI: 10.1021/acs.inorgchem.8b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data for 1−4 empirical formula formula wt cryst syst space group T/K λ/Å a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z Dc/Mg m−3 μ/mm−1 F(000) no. of measd rflns no. of indep rflns Rint no. of params GOF R1,a wR2 (I > 2σ(I)) R1, wR2 (all data)

1

2

3

4

C5H19.5N2.5Se7Sn3 1023.52 monoclinic C2/c 293(2) 0.71073 23.5939(8) 13.6409(4) 15.0372(7) 90 102.177(4) 90 4730.7(3) 8 2.874 13.904 3640 26736 4824 0.0466 143 1.043 0.0293, 0.0741 0.0365, 0.0768

H8N2Se9Sn4 1221.48 orthorhombic Pnma 293(2) 0.71073 13.2274(6) 12.4270(5) 12.6102(5) 90 90 90 2072.83(15) 4 3.914 20.547 2112 12382 2213 0.0699 85 1.037 0.0455, 0.1183 0.0601, 0.1429

C4H16N2Se7Sn3 1000.98 orthorhombic Cmca 293(2) 1.54178 13.5561(3) 24.0194(6) 27.0184(6) 90 90 90 8797.4(4) 16 3.023 40.015 7072 15866 4653 0.0913 147 1.079 0.0846, 0.2302 0.0923, 0.2360

AgH12N3Se8Sn3 1149.75 tetragonal P4/nbm 100(2) 0.71073 8.2155(3) 8.2155(3) 13.4134(10) 90 90 90 905.33(9) 2 4.218 21.216 1004 3076 518 0.0342 26 1.041 0.0267,0.0652 0.0325,0.0674

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑w[(Fo)2 − (Fc)2]2/∑w[(Fo)2]2}1/2.

a

zeolite analogues,2,12,13 metal−organic frameworks,14−16 organic−inorganic hybrids,17,18 and nanoparticles.19,20 Chalcogenidometalates represent a unique class of solidstate materials with a remarkable structural variety21−23 and manifold properties rendering them of interest in ion exchange,24,25 photocatalysis,26−28 energy storage,29,30 nonlinear optics,31,32 and thermoelectrics,33,34 to name several. Very recently, our group applied the deep eutectic solvothermal method in the preparation of choline and trimethylpropylammonium cation templated selenidostannates, i.e. [(CH 3 ) 3 N(CH 2 ) 2 OH] 2 [Sn 3 Se 7 ]·H 2 O and [(CH3)3N(CH2)2CH3]2[Sn3Se7], which display remarkable thermochromic performance.35 This study put forward guidelines not only for the structural design through the organic template adjustment but also for the performance improvement on desirable chalcogenide materials. Herein, we extend the scope of urea-based DESs by including dimethylamine, ethylamine, and trimethylamine hydrochlorides, which lead to a family of Sn-Se and Ag-Sn-Se compounds: namely, [NH2(CH3)2]2Sn3Se7·0.5NH(CH3)2 (1), [NH4]2Sn4Se9 (2), [NH3C2H5]2Sn3Se7 (3), and [NH4]3AgSn3Se8 (4). The synthetic optimization and structural features as well as crucial roles of templates are analyzed in detail for these solid materials. Moreover, compound 4 represents the first heterometallic chalcogenide isolated in DESs and displays a marked, reversible thermochromic property in the range of 100−450 K.

200−2000 nm using a PerkinElmer Lambda 750 UV/vis spectrophotometer, and a BaSO4 plate was used as a standard (100% reflectance). Temperature-dependent reflectance spectra of the smooth polycrystalline sample were collected in the wavelength range of 370−1050 nm on an Ideaoptics PG2000L spectrometer equipped with a HL2000 tungsten halogen light source (color temperature 2915 K) and a FIB-Y-600-DUV fiber reflection probe placed at a 45° orientation, and the STD-WS was used as a standard (100% reflectance). The absorption F(R) data were calculated from reflectance spectra by using the Kubelka−Munk function, F(R) = (1 − R)2/2R,36 where R is the reflectance. Thermogravimetric analyses were performed on a Netzsch TG 209 F3 device at a heating rate of 10 °C min−1 in nitrogen. 13C NMR spectra of the compounds dissolved in mixed N2H4·H2O (98%)/D2O were recorded on a Bruker Avance III 400 instrument at room temperature by using 5 mm tubes. Room-temperature powder X-ray diffraction (XRD) patterns were collected in the angular range of 2θ = 5−80° on a Rigaku SmartLab 9KW diffractometer using Cu Kα radiation. Temperature-dependent powder XRD patterns were recorded on the Rigaku XtaLab PRO single-crystal X-ray diffractometer using the powder power tool (Cu Kα radiation). Elemental analyses on H, C, and N were performed on an Elementar Vario EL cube instrument. 2.2. Syntheses. 2.2.1. [NH2(CH3)2]2Sn3Se7·0.5NH(CH3)2 (1). A mixture of Sn (0.119 g, 1.0 mmol), Se (0.211 g, 2.67 mmol), dimethylamine hydrochloride (0.58 g, 7.1 mmol), urea (0.64 g, 10.67 mmol), and 0.3 mL of N2H4·H2O (98%) (∼6.17 mmol) was sealed in a stainless steel reactor with a 20 mL Teflon liner and heated at 160 °C for 24 h. After it was cooled to room temperature by natural ventilation, the product was washed with distilled water and then dried in the air. A 0.110 g portion of pure platelike orange crystals of 1 were collected in a yield of 32.2% based on Sn. Anal. Calcd for C5H19.5N2.5Se7Sn3 (1023.52): C, 5.86; H, 1.97; N, 3.42. Found: C, 5.80; H, 2.10; N, 3.39. 2.2.2. [NH4]2Sn4Se9 (2). A procedure similar to that used for 1 was adopted, except that ethylamine hydrochloride (0.58 g, 7.1 mmol) was used instead of dimethylamine hydrochloride, and the amount of N2H4·H2O (98%) was adjusted to 0.4 mL (∼8.23 mmol). A 0.141 g portion of pure block dark red crystals of 2 were collected in a yield of

2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All reagents and chemicals were purchased from commercial sources and were used without further purification. FTIR spectra (KBr pellets) were recorded on a PerkinElmer Frontier Mid-IR FTIR spectrometer. Solid-state UV/ vis reflectance spectra were measured in the wavelength range of B

DOI: 10.1021/acs.inorgchem.8b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Schematic Illustration of Synthetic Procedures for 1−4 and Photographs of the Products

honeycomb-like porous [Sn3Se7]n2n‑ layer identical with that in 1, while compound 2 features a totally different layered structure with a stoichiometry of [Sn4Se9]n2n−. The structural difference indicates a predominant structure-directing role of ammonium in comparison to ethylammonium in the synthesis of 2. A DES composed of trimethylamine hydrochloride and urea (molar ratio 1:2) was also explored for preparing selenidostannate, but instead only an unknown amorphous black powder was obtained. In this context, we selected the silver cation Ag+ as an additional metallic source to increase the structural diversity due to its versatile coordination geometries and good reactivity with selenium.38−42 The reaction resulted in red crystals of 4 in a high yield up to 71.3%. This heterometallic compound consists of infinite Ag-Sn-Se chains, which are templated by the in-situ formed ammonium cations as in the case of 2. Four synthetic parameters need to be finely tuned for obtaining optimal products of 1−4. (1) Different from the stoichiometric ratio in the formula, an excess of selenium was applied in the reaction system to enhance the yield and quality of the crystals. (2) The optimal reaction temperature for all four compounds is 160 ± 5 °C (Figures S3, S9, and S15). A temperature lower than 150 °C could not promote the reaction but gave an unknown white powder and unreacted tin and selenium. A temperature up to 170 °C or higher could reduce the crystalline quality and yield of the target products. (3) The reaction time must be longer than 24 h for 1−3 and 3 days for 4, respectively. A shorter time is not enough for the reaction between tin and selenium, whose residues remain the main phases of the solid-state products. (4) The addition of N2H4· H2O (98%) as auxiliary solvent as well as its volume proportion are crucial for the isolation of 1, 2, and 4 (Figures S2, S7, and S13).26,35,43,44 Reactions without N2H4·H2O resulted in a black powder of unreacted Se as the main product for the syntheses of 1 and 2, and a mixture of Se and SnSe2 for the synthesis of 4. The target compounds appeared and raised in yield as N2H4·H2O was increased, accompanied by the gradual disappearance of Se. Besides, white powder phases were occasionally obtained as byproducts, whose structures cannot be identified due to the poor crystalline quality of the tiny particle. The optimal molar ratios of N2H4· H2O to urea were finally tuned to 0.58:1, 0.77:1, and 0.77:1 to obtain pure phases of 1, 2, and 4, respectively. Further increasing the proportion of N2H4·H2O over urea would

46.1% based on Sn. Anal. Calcd for H8N2Se9Sn4 (1221.48): H, 0.66; N, 2.29. Found: H, 1.05; N, 2.03. 2.2.3. [NH3C2H5]2Sn3Se7 (3). A procedure similar to that used for 2 was adopted except that urea was removed and N2H4·H2O (98%) was adjusted to 1.0 mL (∼20.6 mmol). A 0.152 g portion of pure platelike red crystals of 3 were collected in a yield of 45.4% based on Sn. Anal. Calcd for C4H16N2Se7Sn3 (1000.98): C, 4.80; H, 1.61; N 2.80. Found: C, 4.73; H, 1.58; N, 2.67. 2.2.4. [NH4]3AgSn3Se8 (4). A mixture of Sn (0.119 g, 1.0 mmol), AgOAc (0.056g, 0.33 mmol), Se (0.211 g, 2.67 mmol), trimethylamine hydrochloride (0.76 g, 8.0 mmol), urea (0.96 g, 16.0 mmol), and 0.6 mL of N2H4·H2O (98%) (∼12.36 mmol) was sealed in a stainless steel reactor with a 20 mL Teflon liner and heated at 160 °C for 24 h. After it was cooled to room temperature by natural ventilation, the product was washed with distilled water and then dried in the air. A 0.274 g portion of pure platelike red crystals of 4 were collected in a yield of 71.3% based on Sn. Anal. Calcd for AgH12N3Se8Sn3 (1149.75): H, 1.05; N, 3.65. Found: H, 1.22; N, 3.70. 2.3. Crystallography. Indexing and data collections of single crystals 1−4 were performed on a Rigaku XtaLab PRO diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54178 Å) at 293 K (for 1−3) or 100 K (for 4). The absorption corrections were applied using a multiscan technique. Direct methods (SHELXS97) successfully located the Sn, Ag, and Se atoms, and successive Fourier syntheses (SHELXL2014) revealed the remaining atoms.37 Refinements were conducted by fullmatrix least squares against |F|2 using all data. All of the Sn, Ag, and Se atoms and most C and N atoms were refined with anisotropic displacement parameters, while some highly disordered C and N atoms were refined isotropically. The hydrogen atoms bonded to C and N atoms were positioned with idealized geometry. The relevant crystallographic data and structure refinement details are shown in Table 1.

3. RESULTS AND DISCUSSION 3.1. Synthesis. The reaction of tin and selenium in DESs composed of dimethylamine and ethylamine hydrochlorides and urea (molar ratio 1:1.5) offered pure orange plates of 1 and dark red blocks of 2, respectively (Scheme 1). The dimethylammonium cations were present as templates in 1, in good line with the elemental analysis result of C:N = 2:1 and 13 C NMR spectroscopy (Figure S23). Nevertheless, only ammonium cations were detected in 2, instead of ethylammonium from the chloride salts, suggesting the partial decomposition of the urea during the reaction. To confirm this, the urea was removed from the media, and the corresponding product proved to be a different compound, 3, exclusively templated by ethylammonium. Compound 3 comprises a C

DOI: 10.1021/acs.inorgchem.8b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Sn4+ ion is surrounded by five Se2− ions, forming a distorted {SnSe5} trigonal-bipyramidal coordination geometry with Sn− Se bond lengths ranging from 2.5153(8) to 2.8481(7) Å. Three such units connect with each other through sharing edges to form a trigonal {Sn3Se10} cluster with a semicubanelike {Sn3Se4} core (Figure 2). They are further linked via Sn(μSe)2Sn bridges to create a honeycomb [Sn3Se7]n2n− layer featuring large hexagonal windows. These nanoporous [Sn3Se7]n2n− layers are stacked in an ABAB sequence along the c axis, and the windows are arranged in a staggered fashion (Figure 3a).

prohibit the formation of these compounds, instead leading to SnSe, SnSe2, or another unknown amorphous black powder. Removal of urea, namely using solely N2H4·H2O as the solvent, could not produce 1, 2, and 4. In addition, reactions performed in a mixture of organic amine hydrochloride and a volatile solvent such as water or alcohol gave only an amorphous black or red powder. These results suggest the irreplaceability of the DES as the media in the syntheses of 1, 2, and 4. 3.2. Structure Description. 3.2.1. [NH2(CH3)2]2Sn3Se7· 0.5NH(CH3)2 (1) and [NH3C2H5]2Sn3Se7 (3). Single-crystal XRD reveals that both 1 and 3 feature the honeycomb inorganic layers of [Sn3Se7]n2n−. Compound 1 crystallizes in the C2/c space group, and the asymmetric unit contains three Sn4+ ions, seven Se2− ions, two dimethylammonium cations, and half a neutral dimethylamine molecule (Figure 1a). Each

Figure 3. Stack fashion of the [Sn3Se7]n2n− layers in (a, c) 1 and (b, d) 3. Nonidentical layers in the periodic arrangement are shown in different colors.

Figure 1. Asymmetric units of (a) 1, (b) 2, (c) 3, and (d) 4. Color code: Sn (dark blue), Ag (tan), Se (light orange), N (blue), C (gray), H (white).

Figure 2. (a−c) Interlinkages of the central {Sn3Se10} cluster with the three adjacent clusters and (d−f) the layered structures of 1, 3, and [(CH3)3N(CH2)2OH]2[Sn3Se7]·H2O,35 respectively. The moieties shown in (a)−(c) are highlighted in blue in (d)−(f). Color code: Sn (dark blue), Se (light orange). D

DOI: 10.1021/acs.inorgchem.8b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. (a) [Sn4Se9]n2n− layer in 2 and the stack fashion of the [Sn4Se9]n2n− layers along the (b) a axis and (c) b axis. Color code: Sn (dark blue), Se (light orange).

Single-crystal XRD and 13C NMR measurements on 1 (Figure S23) indicate the presence of dimethylammine species, which play significant structure-directing roles in the crystal growth. A larger proportion (∼80%) of these molecules are protonated to balance the negative charge of the inorganic framework, while the rest (∼20%) maintain the neutral state and contribute to the final solvate product. All of these species are located in the interlamellar spaces or the voids of the windows and form extensive N−H···Se and C−H···Se hydrogen bonds with the inorganic layers, resulting in a 3D supramolecular framework (Figure S18). Compound 3 crystallizes in the Cmca space group and features a [Sn3Se7]n2n− layer similar to that described above. The asymmetric unit of 3 consists of three Sn4+ ions, seven Se2− ions, and two ethylammonium cations. Different from 1, the [Sn3Se7]n2n− layers in 3 stack in an ABCDABCD sequence along the c axis, preventing the extension of nanotunnels through the parallel layers. The interlayer distance of 3 was determined to be ∼6.75 Å (using Diamond software), slightly shorter than that of 1 (∼6.96 Å). Considering the isomeric nature of ethylammonium and dimethylammonium, this space reduction is possibly caused by the absence of neutral molecular species in the crystal lattice of 3. These inorganic layers are interconnected by the ethylammonium cations to form a 3D supramolecule through N−H···Se and C−H···Se hydrogen bonds (Figure S20). Compounds containing anionic [Sn3Se7]n2n− layers have been synthesized using different templates, including alkalimetal cations,45 organic ammonium,46−53 metal−amine complexes,54−58 imidazolium derivatives,40 etc. (see Table S1). As shown in Figure 2, these [Sn3Se7]n2n− layers can be briefly classified into three different types, which differ in aspects of window shape, size, and linkage fashion between clusters. (I) Layers in this type feature compressed hexagonal windows caused by the bent linkage, in coincident clockwise or counterclockwise sequence, of the central {Sn3Se10} cluster with all three adjacent clusters. In 1 for instance, the dihedral angle of the bent Sn···(μ-Se)2···Sn plane lies in the range of 158−160.7°, contributing to a pseudo-C3 symmetry of the unit built of four clusters (Figure 2a,d). Only a limited number of compounds templated by Cs+, organic ammonium, or polymer fall into this category.45,49,53 (II) For layers in this type, adjacent {Sn3Se10} clusters are interconnected in a linear manner, where all the Sn···(μ-Se)2···Sn dihedral angles are close to 180°, as in the case of 3 (Figure 2b,e). This linkage fashion endows a pseudo-C3v symmetry on the tetrameric moiety and results in a [Sn3Se7]n2n− layer with regular hexagonal micropores.40,50−52,54,58 (III) Layers in this type incorporate the above two linkage modes: i.e., one linkage out

of the three keep the linear fashion, while the other two are forced to bend in coincident sequence (Figure 2c,f). As a result, the tetramer loses its 3-fold symmetry, leading to the formation of elliptical windows perforated on the [Sn3Se7]n2n− layer.35,46−48,55−58 Ultimately, the great structural diversity comes down to the crucial structure-directing roles of the templates, which interact with the framework by means of electrostatic attraction, hydrogen bonds, and van der Waals forces. Both space filling and charge compensation appear to have a great effect on the connectivity of individual inorganic building units, thereby generating a rich isomerism for the anionic structures. 3.2.2. [NH4]2Sn4Se9 (2). Compound 2 crystallizes in the Pnma space group, featuring a layered [Sn4Se9]n2n− network templated by ammonium cations. The asymmetric unit of 2 contains two Sn4+ ions, four and a half Se2− ions, and one ammonium cation (Figure 1b). One Sn4+ ion (Sn1) is surrounded by five Se2− ions, leading to a distorted {SnSe5} trigonal bipyramid similar to that in 1. Another Sn4+ ion (Sn2) coordinates with four Se2− ions to generate a distorted {SnSe4} tetrahedron with Sn−Se bond lengths ranging from 2.4768(17) to 2.6003(15) Å. Either {SnSe5} or {SnSe4} can undergo aggregation through edge and corner sharing to form {Sn2Se8} and {Sn2Se7} dimers, respectively. The interconnection between one {Sn2Se8} and one {Sn2Se7} leads to a tetrameric {Sn4Se10} cluster with a square configuration. Adjacent clusters, organized in alternating up−down orientations, are further linked via Sn(μ2-Se)2Sn bridges to create the [Sn4Se9]n2n− layer with compressed-rectangular {Sn8Se8} windows (Figure 4a). The [Sn4Se9]n2n− layers are stacked in an ABAB sequence along the a axis, resulting in an staggered arrangement of the windows (Figure 4b). Ammonium cations are located in the interlamellar space (∼6.6137 Å) as templates, and they form extensive N−H···Se hydrogen bonds with the inorganic layers to generate a 3D supramolecular framework (Figure S19). The stoichiometric ratio Sn:Se = 4:9 was primarily identified for open-framework selenidostannates, including 3D-[Fe(bipy) 3 ]Sn 4 Se 9 ·2H 2 O (bipy = 2,2′-bipyridyl), 59 3D(Bmmim)2[Ni(1,2-pda)3]Sn8Se18 (1,2-pda = 1,2-diaminopropane),60 and 3D-(Bmmim)1.5(dienH)0.5Ni(dien)2[Sn4Se9]2 (dien = diethylenetriamine).61 These porous compounds feature different interpenetrating channel systems, where the voids are occupied by organic ammoniums, metal complexes, or imidazole derivatives as cationic templates. In comparison, the layered structure of [Sn4Se9]n2n− in 2 represents a rare case that was only observed in the alkali metal cation templated compounds A2Sn4Se9·H2O (A = Rb, Cs).62 The structural feature of the [Sn4Se9]n2n− layer is also different from that of E

DOI: 10.1021/acs.inorgchem.8b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry the known thiostannates(IV) of the same formula type,63−65 where the semicubane-like {Sn3S4} cores are interconnected through tetrahedral {μ-S2SnS2} and double-sulfide {μ-S}2 bridges to form an infinite layer with elliptic 32-memberedring windows. Note that the [Sn4Se9]n2n− layer in 2 contains both tetrahedral {SnSe4} and trigonal-bipyramidal {SnSe5} units, and this is different from those in 1 and 3 where the tin centers are exclusively five-coordinate. In 1 and 3, a trigonal {Sn3Se10} cluster consisting of three identical edge-shared {SnSe5} units can be viewed as the secondary building unit (SBU) for the [Sn3Se7]n2n− layer (Figure 5a). In 1 for example, the equatorial

Figure 6. [AgSn3Se8]n3n− chain viewed from different directions. Color code: Sn (dark blue), Ag (tan), Se (light orange).

has four equivalent Sn−Se bonds with a bond length of 2.5232(6) Å, while the side two {SnSe4} tetrahedra are slightly distorted with Sn−Se bond lengths ranging from 2.4880(7) to 2.6158(7) Å, similar to the structure parameters of the previously reported discrete [Sn3Se8]4− anion.66 The {Sn3Se8} clusters are further connected by four-coordinate Ag+ cations, with an Ag−Se bond length of 2.7651(6) Å, to form an infinite chain of [AgSn3Se8]n3n−. Ammonium cations are located around the chains, acting simultaneously as templating and charge-balancing agents. The wider chemical and bonding flexibility of Ag+ ion as well as its specific affinity toward the heavy Se2− have contributed to a group of heterometallic silver selenidostannates such as A3AgSn3Se8 (A = Rb, K),38 [(Me)2NH2]0.75[Ag1.25SnSe3],39 [bmmim]7[AgSn12Se28],40 (NH4)4Ag12Sn7Se22,41 and [CH3NH3]2[H3O]Ag5Sn4Se12· C2H5OH.42 Recently, compound 4 has also been isolated by another synthetic route, but in a relatively low yield coupled with the presence of a black impurity.42 Instead, the utilization of DESs in this work leads to an optimal product with a significantly enhanced yield (>70%) and purity. 3.3. Powder XRD, Thermogravimetric (TG) Analysis, and Optical Properties. Powder XRD patterns of 1−4 are shown in Figure 7. The 2θ peaks match well with the corresponding simulated patterns from the single-crystal structures, indicating the high purity for all four compounds. Owing to the preferred orientations of the polycrystalline powder, some peak intensities are inconsistent with the simulated values, especially for 1 and 3. TG measurements were performed on 1−4 under a N2 atmosphere from 40 to 800 °C (Figure 8). The TG curve of 1 displays a weight loss of 12.49% (calcd 11.01%) in the range of 90−230 °C, corresponding to the loss of two and a half dimethylamine molecules per formula. A following weight loss of 29.54% (calcd 31.05%) up to 640 °C can be assigned to the release of one H2Se molecule and three Se atoms. For compound 2, a weight loss of 9.77% (calcd 9.42%) occurs in the range of 50−240 °C, and this can be attributed to the removal of two ammonia molecules and one H2Se molecule. Another weight loss of 24.70% (calcd 25.86%) from 500 to 650 °C might be caused by the removal of four Se atoms. Compound 3 features a two-step weight loss with an overall value of 39.63% (calcd 40.76%) in the range of 190−640 °C, and this can be assigned to the removal of two ethylamine molecules, one H2Se group, and three Se atoms. The TG curve of 4 displays the first weight loss of 13.67% (calcd 15.01%) in the range of 190−265 °C, corresponding to the release of three ammonia and one and a half H2Se molecules. A second weight loss of 19.88% (calcd 20.60%) occurs from 280 to 640 °C, and this should be attributed to the removal of three Se atoms. The

Figure 5. Structures of the (a) {Sn3Se10} and (b) {Sn4Se13} clusters in [Sn3Se7]n2n− and [Sn4Se9]n2n− layers and (c) the structural derivation from the former to the latter clusters. Color code: Sn (dark blue), Se (light orange).

Sn1−Se bond lengths for each {SnSe5} range from 2.5153(8) to 2.5727(7) Å and are slightly shorter than those in the axial positions (2.6917(7)−2.7877(7) Å). In comparison, a tetrameric {Sn4Se13} cluster is identified as the SBU for the [Sn4Se9]n2n− layer in 2 (Figure 5b). Although the Sn1 atom maintains its five-coordinate sphere similar to that in {Sn3Se10}, the coordination geometry of the Sn2 atom is significantly distorted toward a tetrahedron. The associated weaker Sn2−Se4 bond (2.6003(15) Å) compared to the remaining three Sn2−Se bonds (2.4768(17)−2.5412(14) Å) is caused by the presence of a long subordinate bond of Sn2−Se5 (3.3665(16) Å) in the opposite position. Assuming the edgeshared dimeric {Sn4Se8} moiety remains intact (brown in Figure 5c), the structural variation from the {Sn3Se10} to {Sn4Se13} cluster can be clarified as the substitution of the {SnSe2} by {Sn2Se5} fragment (blue-gray in Figure 5c). 3.2.3. [NH4]3AgSn3Se8 (4). Compound 4 crystallizes in the space group P4/nbm, and its asymmetric unit contains 3/8 Sn4+ ion, 1 Se2− ion, 1/8 Ag+ ion, and 3/8 ammonium cation. Its structure features an infinite anionic [AgSn3Se8]n3n− chain composed of {Sn3Se8} clusters connected by Ag+ linkers (Figure 6). Each Sn4+ ion is tetrahedrally coordinated by four Se2− ions, and three such {SnSe4} units aggregate together through sharing edges to form the trimeric {Sn3Se8} cluster with a linear configuration. The middle {SnSe4} tetrahedron F

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Figure 7. Experimental and simulated powder XRD patterns of 1−4.

Figure 8. Thermogravimetric curves of 1−4.

linear part of [F(R)E]2 plots are determined to be 2.18 (for 1), 1.82 (for 2), 2.02 (for 3), and 2.20 eV (for 4), consistent with their observed colors. The band gaps of 1 and 3 are comparable to those of other selenidostannates containing anionic [Sn3Se7]n2n− layers (Table S1). In comparison, 2 displays a relatively narrower band gap owing to the unique electronic structure of the [Sn4Se9]n2n− layer. The band gap of 4 is comparable to that of [bmmim]7[AgSn12Se28] (2.2 eV),40 but it exhibits a blue shift in comparison to those of other

powder XRD patterns of the TG residues of 1−4 indicate the presence of SnSe as the primary products, which is in accordance with the TG analysis results (Figure S25). There should be some amorphous AgI-containing selenides in the TG residue of 4, as no removal of Ag is detected during the TG measurement. The Kubelka−Munk spectra (Figure 9) of 1−4 display steep absorption edges, confirming the expected semiconductor nature. The optical band gaps obtained by extrapolation of the G

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Figure 9. Kubelka−Munk spectra of 1−4.

Figure 10. (a) Color change of a single crystal of 4 from 100 to 450 K. (b) Temperature-dependent UV−vis reflectance spectra and (c) the corresponding plots derived from Kubelka−Munk equations from 100 to 450 K with an interval 20 K (not taking 450 K into account). (d) Variation of the band gap with temperature for sample 4 and the nonlinear fitting using the Varshni equation Eg = E0 − αT2/(T + β).

function of temperature.68 In our previous study, we reported a remarkable thermochromic behavior of [Sn3Se7]n2n− layer containing compounds, and managed to improve the thermochromic reversibility by means of alkyl substitution on the organic ammonium templates.35 The success underlined the significant roles of the employed DESs on the crystal

reported silver selenidostannates, such as K2Ag2SnSe4 (1.8 eV), 6 7 A 3 AgSn 3 Se 8 (1.8 eV; A = K, Rb), 3 8 and [(Me)2NH2]0.75[Ag1.25SnSe3] (1.7 eV).39 3.4. Thermochromic Property of 4. Continuous thermochromism in semiconductors is a type of stimuli− response phenomenon featuring a gradual color change as a H

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Figure 11. (a) Temperature-dependent powder XRD patterns of 4 recorded in the range of 100−450 K. (b) Powder XRD patterns of 4 in five rounds of heating and cooling. (c) Varied 2θ values of the 122 peak as a function of temperature in the five-round test for 4. Inset: alternating color change of the polycrystalline sample 4 at 100 and 450 K.

2.324 eV, and the constants α and β are 6.98 × 10−4 and 249.9, respectively (Figure 10d). The thermochromism of silver selenidostannates have rarely been reported and can be neglected due to their intrinsically darker color (usually Eg < 1.9 eV at room temperature) and thus poorer chromatic recognition toward temperature changes (Table S2). In comparison, the E0 value of 4 is located in a suitable visible range and is remarkably higher than that of black SnSe2 (1.4 eV), a typical compact phase which exhibits temperaturedependent optical absorption edges.76 The [AgSn3Se8]n3n− chain in 4 can be viewed as an Ag+-bridged infinite analogue of the discrete trimeric [Sn3Se8]4− anion66 that features similar structural parameters, including bond lengths and angles. Therefore, the blue-shifted band gap of 4 in comparison to the dense SnSe2 can be explained by the quantum confinement as well as the lower connectivity or lower coordination number of the Sn and Se atoms in the former. This effect resembles the construction of a [Sn3Se7]n2n− layer in our previous study,35 where the perforation on the layer results in the formation of quantum “antidot” lattices with blue-shifted band gaps and accentuates the thermochromic performance. Even though the band gap variation of 4 toward temperature is not as marked as for [(CH3)3N(CH2)2CH3]2[Sn3Se7] (Eg = 2.494−2.229 eV in the range of 130−410 K; E0 = 2.551 eV),35 it contributes to a different color change interval at lower wavelengths for compensation. A good color change reversibility is also of significance for an ideal chromatic material to fulfill. The cycling temperaturedependent powder XRD measurement confirms the persistence of the thermal stability for at least five rounds of heating and cooling in the range of 100−450 K (Figure 11b), except for slight shift of the peaks to lower/higher 2θ values owing to the thermal expansion/contraction of the lattice. Correspondingly, a reversible color change of gold−dark red−gold is

engineering science and functionalization of materials. We are interested in the thermochromic behaviors of compounds with novel anionic structures different from the [Sn3Se7]n2n− layer. Experimental observations revealed an intrinsic dark color of compound 2 (Eg = 1.82 eV) and thus a poor chromatic recognition in the cooling and heating. Therefore, we investigated emphatically the thermochromic behavior of the silver selenidostannate 4 via spectroscopic and XRD techniques. The thermochromic performance of a single crystal of 4 is shown in Figure 10a, where a remarkable color change from gold to dark red can be observed when the temperature increases from 100 to 450 K. Temperature-dependent UV−vis reflectance spectra on a polycrystalline sample of 4 indicate a continuous red shift of the absorption band in the temperature range of 100−450 K (Figure 10b), with a decrease of band gap from 2.305 to 2.119 eV calculated from the Kubelka−Munk function (Figure 10c). Upon cooling back to 100 K, the band gap of 4 increased again and the color changed back to gold as in the original state. Note that there is no phase transition or decomposition during the heating and cooling process according to the temperature-dependent powder XRD patterns (Figure 11a), which is consistent with the aforementioned TG behavior of 4. According to the literature, the negative temperature dependence of the band gap arises from a shift in the relative position of the conduction and valence bands due to not only the temperature-dependent lattice dilation69 but also the dominant temperature-dependent electron−lattice interaction.70,71 This is different from the mechanism of most discontinuous thermochromic compounds, such as VO2,72 M2HgI4 (M = Ag, Cu),73 and CuW1−xMoxO4,74 that involves a first- or second-order structural phase transition. Nonlinear fitting of the band gap energies of 4 by the Varshni equation Eg = E0 − αT2/(T + β)75 results in a E0 (band gap at 0 K) value of I

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Notes

The authors declare no competing financial interest.

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4. CONCLUSION We have extended the scope of the deep eutectic solvothermal synthesis for chalcogenidometalates by including dimethylamine, ethylamine, and trimethylamine hydrochlorides, resulting in a series of novel Sn-Se and Ag-Sn-Se compounds: i.e., [NH2(CH3)2]2Sn3Se7·0.5NH(CH3)2 (1), [NH4]2Sn4Se9 (2), [NH3C2H5]2Sn3Se7 (3), and [NH4]3AgSn3Se8 (4). Compounds 1 and 3 possess 2D honeycomb-like anionic [Sn3Se7]n2n− layers with large hexagonal windows. Compound 2 features a rare [Sn4Se9]n2n− layered structure consisting of tetrameric {Sn4Se10} clusters as SBUs. Compound 4 consists of infinite [AgSn3Se8]n3n− chains built by {Sn3Se8} units and Ag+, and it represents the first heterometallic chalcogenide synthesized in DESs. The organic ammonium cations of halide salts or in situ formed ammonium cations from the decomposition of urea act as templates for the crystal growth of 1−4. Crucial synthetic parameters, e.g. reactant ratio, temperature, reaction time, and auxiliary solvent, have been properly tuned for the isolation of optimal products. Compound 4 exhibits a marked, reversible gold−dark red thermochromic behavior due to the negative temperature dependence of its band gap (Eg = 2.305−2.119 eV in the range of 100−450 K), which would be beneficial for future device innovations.

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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.8b02610. Synthetic details and FTIR, PXRD, 13C NMR, and UV− vis reflectance spectra (PDF) Accession Codes

CCDC 1866367−1866370 contain 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 [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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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for K.-Y.W.: [email protected]. *E-mail for C.W.: [email protected]. ORCID

Kai-Yao Wang: 0000-0002-6358-9911 Lin Cheng: 0000-0003-0891-7350 Cheng Wang: 0000-0002-2085-7090 Funding

This work was financially supported by the National Natural Science Foundation of China (21701123, 21501133, and 21571170), National Key R&D Program of China (2017YFA0700104), and Tianjin Municipal Science and Technology Commission (18JCQNJC06200 and 17JCZDJC38000). J

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DOI: 10.1021/acs.inorgchem.8b02610 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02610 Inorg. Chem. XXXX, XXX, XXX−XXX