Supramolecular Organization of [TeCl6]2– with Ionic Liquid Cations

2 days ago - However, compared with other main-group metal halides, the supramolecular organizations of tellurium(IV) halides with IL cations have bee...
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Supramolecular Organization of [TeCl6]2− with Ionic Liquid Cations: Studies on the Electrical Conductivity and Luminescent Properties Nan-Nan Shen,†,‡ Min-Lan Cai,†,§ Ying Song,† Ze-Ping Wang,† Fu-Quan Huang,∥ Jian-Rong Li,*,† and Xiao-Ying Huang*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P.R. China ‡ University of Chinese Academy of Sciences, Beijing, 100049, P.R. China § Fujian Normal University, Fuzhou, Fujian 350007, P.R. China ∥ Fujian Agriculture and Forestry University, Fuzhou, Fujian 350002, P.R. China S Supporting Information *

ABSTRACT: A series of hybrid tellurium chlorides based on ionic liquids (ILs), namely, α-[Bmim]2TeCl6 (1, Bmim = 1butyl-3-methyl imidazolium), β-[Bmim]2TeCl6 (2), [HOOCMim]2TeCl6 (3, HOOCMim = 1-carboxymethyl-3-methyl imidazolium), [Bzmim]2TeCl6 (4, Bzmim = 1-benzyl-3-methyl imidazolium), [EPy]2TeCl6 (5, EPy = 1-ethylpyridinium), [Bmmim]2TeCl6 (6, Bmmim = 1-butyl-2,3-dimethyl imidazolium), have been synthesized and characterized. Different kinds of supramolecular networks have been obtained via selfassemblies of isolated [TeCl6]2− anions and various ionic liquid cations. Interestingly, all the title compounds exhibit semiconducting behaviors: their optical absorption edges calculated from reflectance spectra are in the range of 2.54−2.68 eV; their electrical conductivities measured by using two-probe direct current (DC) method indicate values from 2.06 × 10−9 to 4.65 × 10−6 S/cm, which are typical for semiconductors and comparable to the reported crystalline hybrid metal halides. The luminescent property studies reveal that only compounds 3 and 6 exhibit intense emissions both at 77 and 298 K, probably owing to the minimum distortion of the TeCl6 octahedra in 3 and 6.

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INTRODUCTION Organic−inorganic hybrid metal halides have drawn evergrowing attention in multifarious fields, not only owing to their abundant structural versatility including discrete clusters, chains, layers, and frameworks,1 but also due to their semiconducting behaviors such as light harvesting2 and electrical conductivity.3 Of special interest are the hybrid lead halide perovskites, risen as highly promising candidate materials for solar cells with rapidly increasing power conversion efficiency exceeding 23%.4 Recently, other mercury-like ions Sn2+, Sb3+, and Bi3+ with the same external electrons shell of s2 as the Pb2+ but low toxicity have been widely investigated as environmentally friendly alternatives for lead(II).5 Tellurium, a kind of semimetallic element with semiconducting behavior and narrow bandgap, has been widely studied in diverse branches of chemistry. The metallic and nonmetallic characteristics of Te lead to the production of versatility in terms of oxidation states, as well as applications. Particularly, the chemistry of Te4+ has gained considerable interest. Similar to Sn2+, Pb2+, Sb3+, Bi3+, the Te4+ ion also possesses s2 configuration of external electron shell and exhibits luminescent properties.6 In fact, Te4+ has been widely used in © XXXX American Chemical Society

the construction of hybrid halides during the past few decades, and the common inorganic moieties are [TeX5]−,7 [TeX5]n−,8 [TeX6]2−,6b−f [Te2X9]−,9 [Te2X10]2−,10 and [Te3X13]−.7b,11 Interestingly, the configuration of the Te4+ ion in most of the anions (except for [TeX5]−) exhibits six-coordinated octahedra with halogens in which the lone pair of Te4+ is not active, similar to that in perovskite halides. Compared with that of the widely studied hybrid perovskites of other metal ions of the main group (e.g., Pb2+, Sn2+, Sb3+, Bi3+); however, the semiconductive behaviors of hybrid tellurium(IV) halides have been less studied. Research on ionic liquids (ILs) has been developed into a tremendously popular field of chemistry due to their inherent “green” features, such as low volatility, large liquid ranges, nonflammability, high stabilities, considerable ionic conductivity, tunable polarity, and wide electrochemical window.12 Compared with traditional molecular solvents, ILs possess special ionic reaction conditions, which may promote the formation of compounds with novel structures.13 Recently, ILs Received: February 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b00297 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for the Title Compounds CCDC empirical formula formula mass crystal system space group a/Å b/Å c/Å β/° V/Å3 Z λ T/K ρcalcd /g cm−3 μ/mm−1 F(000) measured refls. independent refls. no. of parameters Flack parameter Rint R1a (I > 2σ(I)) wRb (F2) (I > 2σ (I)) GOF a

1

2

3

4

5

6

1821453 C16H30N4TeCl6 618.74 orthorhombic P212121 17.7466(4) 17.7466(4) 8.7874(3) 90 2767.52(13) 4 0.71073 295(2) 1.485 1.663 1232 9096 5688 249 −0.565(19) 0.0319 0.0462 0.0597 1.033

1821454 C16H30N4TeCl6 618.74 monoclinic P21 20.6455(7) 9.4619(3) 27.9901 102.135(4) 5345.6(3) 8 0.71073 297(2) 1.538 1.722 2464 27634 18240 1213 −0.010(13) 0.0290 0.0499 0.0819 1.022

1821455 C12H18N4O4TeCl6 622.60 monoclinic P21/n 9.9973(15) 9.3938(10) 12.4418(18) 103.625(15) 1135.6(3) 2 0.71073 297(2) 1.821 2.041 608 4748 2289 129

1821456 C22H26N4TeCl6 686.77 tetragonal I4̅ 17.8941(4) 17.8941(4) 8.6372(4) 90 2765.62(18) 4 0.71073 100(2) 1.649 1.674 1360 3569 2426 152 −0.03(6) 0.0428 0.0581 0.0891 1.013

1821457 C14H20N2TeCl6 556.62 monoclinic P21/n 9.2120(5) 11.7683(5) 10.1842(6) 102.240(5) 1078.97(10) 2 0.71073 297(2) 1.713 2.121 544 5587 2288 108

1821458 C18H34N4TeCl6 646.79 monoclinic P21/c 10.5934(2) 13.0952(2) 10.6533(2) 112.138(3) 1368.91(5) 2 0.71073 295(2) 1.569 1.685 648 16226 3103 137

0.0292 0.0307 0.0668 1.010

0.0254 0.0215 0.0527 1.004

0.0270 0.0317 0.0598 1.009

R1 = ∑||Fo| −|Fc||/∑|Fo|. bwR2 = [ ∑w(Fo2−Fc2)2/∑w(Fo2)2]1/2.

materials. Only compounds 3 and 6 exhibit intense luminescence both at 77 and 298 K due to the minimum distortion of [TeCl6]2− anions. This work shows the first instance of conductance and luminescence studies in hybrid tellurium halides assembled by ILs.

have been widely investigated in the synthesis of novel hybrid metal halides with the formulas of [A]m[MXn] (A = ionic liquid cations; M = metal ions; X = Cl, Br, I).14 In such ionic compounds, the interactions between cations and anions, such as ionic interactions, hydrogen bonds,15 anion-π contacts,16 together with other factors, for instance, the length of the alkyl chains, alkyl−alkyl interactions,17 nature of anion, polymorphism,18 and structural disorder, have crucial influences on the assembly of various supramolecular structures.19 Even more interesting is that the introduction of metal ions allows ILs to be developed into different kinds of functional materials. However, compared with other main-group metal halides, the supramolecular organizations of tellurium(IV) halides with IL cations have been rarely reported. To our best knowledge, thus far only Laitinen et al. reported the identification of mixed halogens-containing anion in [Bmmim]2[TeX2Y4] (X, Y = Cl, Br) by theoretical calculations and Raman spectroscopy.20 Herein, we report six hybrid tellurium(IV) chlorides assembled by [TeCl6]2− anions and different IL cations, namely, α-[Bmim]2TeCl6 (1, Bmim = 1-butyl-3-methyl imidazolium), β-[Bmim]2TeCl6 (2), [HOOCMim]2TeCl6 (3, HOOCMim = 1-carboxymethyl-3-methyl imidazolium), [Bzmim]2TeCl6 (4, Bzmim = 1-benzyl-3-methyl imidazolium), [EPy]2TeCl6 (5, EPy = 1-ethylpyridinium) [Bmmim]2TeCl6 (6, Bmmim = 1-butyl-2,3-dimethylimidazolium). Single crystal structural analyses indicate that the individual [TeCl6]2− ions are completely isolated from each other and situate in the cavities organized by large IL cations, final leading to the formation of three-dimensional (3D) (for 1−5) or twodimensional (2D) (for 6) supramolecular networks. The semiconductive behaviors and the luminescent properties have been studied. Interestingly, all the title compounds display excellent electrical conductivities with a maximum value of 4.65 × 10−6 S/cm, which fall in the ranges of semiconducting

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EXPERIMENTAL SECTION

Electrical Conductivity Measurements. The electrical conductivity data were measured on a KEITHLEY 4200-SCS semiconductor characterization system using two probes direct current (DC) method. The samples were fully ground, and the powder was pressed into pellets with a diameter of 2.5 mm. The thickness of the pellets for compounds 1−6 is 0.55, 0.44, 0.56, 0.53, 0.61, and 0.45 mm, respectively. The pellets were connected to the meter by platinum wires with conductive silver past. The current−voltage (I−V) curves of 1−6 were scanned under various voltage ranges. The electrical conductivity can be calculated by plots of current density (J) versus electric field strength (E): σ = J/E, J = I/S, E = V/L. σ is the conductivity; S and L refer to the cross area and the thickness of the pellets, respectively. Syntheses. α-[Bmim]2TeCl6 (1). A mixture of TeCl4 (0.270 g, 1.0 mmol), [Bmim]Cl (0.349 g, 2.0 mmol), and acetonitrile (99%, 4 mL) was loaded in a 20 mL Teflon-lined stainless-steel autoclave and heated at 100 °C for 4 days, followed by cooling to room temperature (RT) naturally. A homogeneous yellow solution was produced. A pure phase of yellow columnar-like crystals was obtained upon evaporating from the resultant solution at RT (Yield: 0.564 g, 91.1% based on tellurium). Elemental analysis, calcd. (%) for C16H30N4TeCl6: C, 31.06; H, 4.89; N, 9.05. Found: C, 31.51; H, 5.16; N, 9.15. β-[Bmim]2TeCl6 (2). A similar method used in the synthesis of compound 1 was applied, except the heating temperature was changed from 100 to 140 °C. A pure phase of yellow block-like crystals was obtained upon evaporating from the resultant solution at RT (Yield: 0.380 g, 60.9% based on tellurium). Elemental analysis: calcd. (%) for C16H30N4TeCl6: C, 31.06; H, 4.89; N, 9.05. Found: C, 31.08; H, 5.10; N, 8.99. B

DOI: 10.1021/acs.inorgchem.8b00297 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (a−e) Crystal structure packing diagrams for compounds 1−6 assembled by TeCl4 and different ionic liquid cations. Te, rose, pink, plum, violet; Cl, turquoise; C, gray; N, blue; O, red, H, light gray. [HOOCMim] 2 TeCl 6 (3). TeCl 4 (0.270 g, 1.0 mmol), [HOOCMmim]Cl (0.353 g, 2.0 mmol), and acetonitrile (99%, 4 mL) were loaded in a 20 mL Teflon-lined stainless-steel autoclave, heated for 4 days at 100 °C, followed by cooling to room temperature naturally. Yellow block-like crystals were isolated after washing the product with ethanol three times (Yield: 0.325 g, 52% based on tellurium). Elemental analysis, calcd. (%) for C12H18N4O4TeCl6 C, 23.15; H, 2.91; N, 9.00. Found: C, 23.37; H, 2.87; N, 8.96. [Bzmim]2TeCl6 (4). TeCl4 (0.270 g, 1.0 mmol), [Bzmim]Cl (0.417 g, 2.0 mmol), and acetonitrile (99%, 4 mL) were loaded in a 20 mL Teflon-lined stainless-steel autoclave and heated at 120 °C for 4 days. A single phase of yellow columnar-like crystals could be obtained after cooling to room temperature naturally (Yield: 0.384 g, 56% based on tellurium). Elemental analysis: calcd. (%) for C22H26N4TeCl6: C, 38.47, H, 3.81, N 8.16; found: C, 38.50, H 3.76, N, 8.13. [EPy]2TeCl6 (5). A similar method in the synthesis of compound 4 was used, except that the [Bzmim]Cl (0.417 g, 2.0 mmol) was replaced by [EPy]Cl (0.287 g, 2.0 mmol). The obtained yellow columnar-like crystals were collected, washed with ethanol several times, and dried in air (Yield: 0.315 g, 57% based on tellurium). Elemental analysis: calcd. (%) for C14H20N2TeCl6: C, 30.21, H, 3.62, N 5.03; found: C, 30.26, H, 3.80, N, 5.08.

[Bmmim]2TeCl6 (6). A similar method in the synthesis of compound 1 was used, except that the [Bmim]Cl (0.349 g, 2 mmol) was replaced by [Bmmim]Cl (0.377 g, 2 mmol). A pure phase of orange block-like crystals was obtained upon evaporating from the resultant solution at RT (Yield: 0.550 g, 87% based on tellurium). Elemental analysis, calcd. (%) for C17H32N4TeCl6: C, 33.42; H, 5.30; N, 8.66; found: C, 33.58; H, 5.32; N, 8.65.

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RESULTS AND DISCUSSION Crystal Structure Descriptions. Detailed crystallographic data and structure-refinement parameters of the title compounds are listed in Table 1. The early studies suggested that solid state TeCl4 features a discrete tetrameric cubane-like structure,21 as shown in Figure 1. Since the terminal Te−Cl bonds are distinctly shorter than those containing μ3-Cl atoms, the structure can be regarded either as a covalent molecule (Te4Cl16) or ionic formula [(TeCl3+Cl−)4].21b,22 Owing to the amphoteric behavior, TeCl4 can react with both Lewis bases and Lewis acids. According to the previous research, degradation in the presence of stoichiometric amounts of C

DOI: 10.1021/acs.inorgchem.8b00297 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry chlorides occurred progressively in the transformation between (Te4Cl16) and different kinds of anions (Scheme 1).10b Hence, Scheme 1. Transformation of (Te4Cl16) into Different Kinds of Anions

we are curious about the reactivity trends of TeCl4 with Cl− containing ILs. In this work, the reactions of tellurium tetrahalide and IL in the molar ratio of 1:2 have been carried out affording six different supramolecular structures built up by discrete octahedral [TeCl6]2− anions and different IL cations. Figure 1 shows the crystal structure packing diagrams of compounds 1−6. As depicted, the IL cations in all the title compounds work as walls. Every discrete [TeCl6]2− anion is situated in the cavities and hydrogen bonded to different numbers of surrounding cations forming supramolecular capsules (Figure 2). In compounds 1−4, such capsules are further prolonged into one-dimensional (1D) supramolecular channel-like structures through the bridging function of IL cations. In addition, the anion-π interactions are also present in 2 and 3, while in compound 5, the anion-π interactions play crucial roles in the generation of 1D channels. Such supramolecular channels are arranged in parallel, resulting in the formation of 3D supramolecular networks in 1−5. Differently, the supramolecular capsules in compound 6 are extended into 2D supramolecular layers. The single 1D channels in 1−5 and 2D supramolecular layer in 6 are shown in Figure S2. It is worthy to note that asymmetric units and the hydrogen bonding environments of [TeCl6]2− anions in 1−6 are totally different. The mentioned structural characteristics of the title compounds will be illustrated in detail as follows. Compound 1 belongs to the non-centrosymmetric P212121 space group. The asymmetric unit is composed of two crystallographically independent [Bmim]+ cations and one [TeCl6]2− anion, as depicted in Figure S1. The Te−Cl bond lengths range from 2.503(3) Å to 2.540(3) Å (Table S1). As shown in Figure 1a, the [Bmim]+ cations are organized into square-shaped channels along the c axis. Each [TeCl6]2− anion, residing in the cavities, interacts with eight surrounding cations through Cl···H−C bonds (Figure 2a). Such capsules are extended along the c axis through the connection of the common cations forming 1D supramolecular chains (Figures S2a and S3a) which are parallel stacked along the a and b axes. The distances of Cl···H−C are in the range of 2.76−2.99 Å (Figure S3b and Table S2). Compound 2 is also built up by [Bmim]+ cation and [TeCl6]2− anion. Owing to the different conformations of the

Figure 2. (a−f) View of a single H-bonded “capsule” with the [TeCl6]2− unit in the title compounds and their H-bonding interactions with the surrounding cations. Te, rose, pink, plum, violet; Cl, turquoise; C, gray; N, blue; O, red, H, light gray.

n-butyl chains on the [Bmim]+ cations, the crystal structure of 2 is totally different from that of 1. Compound 2 crystallizes in the non-centrosymmetric space group of P21 and the asymmetric unit contains four crystallographically distinct octahedral [TeCl6]2− ions and eight [Bmim]+ cations (Figure S1). The bond lengths of Te−Cl vary from 2.490(3) to 2.573(3) Å (Table S1). In the structure of 2, a 3D supramolecular network is fabricated by anion-π interactions and hydrogen bonds between [TeCl6]2− anions and [Bmim]+ cations. The [Bmim]+ cations of each kind are stacked along the b axis, forming four different irregularly shaped channels (Figure 1). The inside [TeCl6]2− anions connect the surrounding [Bmim]+ cations through hydrogen bonds, generating four different kinds of 1D supramolecular chains, as depicted in Figures S2b and S4. The hydrogen bonding D

DOI: 10.1021/acs.inorgchem.8b00297 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry environments of four unique [TeCl6]2− anions are depicted in Figure 2b. The [Te(1)Cl6]2− gets hydrogen bonded to seven adjacent [Bmim]+ cations, whereas for the other three species of anions, only six [Bmim]+ cations connect them via hydrogen bonds (Figure S5a). The anion-π interactions created by Cl(15), Cl(18), Cl(23) atoms are also present in the supramolecular chains fabricated by [Te(3)Cl6]2− and [Te(4)Cl6]2− anions (Figure S5b). The bond lengths of Cl···H−C are in the range of 2.65−2.97 Å, and the distances of the anion-π contacts range from 3.724(6) Å to 3.746(8) Å (Table S3). Compounds 1 and 2 are supramolecular isomeric to each other. The conformation of the n-butyl chains on the [Bmim]+ cation have great influence on the construction of the supramolecular networks in 1 and 2. According to the references, polymorphism based on the conformational flexible [Bmim]+ cations have been reported.18 Generally, two factors, the torsion angles of the Cα−Cβ and Cβ−Cγ, have been involved in the conformational studies of the [Bmim]+ cations. The angle is close to 180° for trans (T) conformation and near 60° for the gauche (G) conformation, roughly exhibiting the twisting directions of the n-butyl chains. In compound 1, two crystallographically independent [Bmim]+ cations both possess GT conformation, and the torsion angles are listed in Table S4, whereas, in compound 2, eight crystallographically unique cations feature three conformations of GT, TT, and TG (Table S4). Such conformations greatly influence the cation−anion interactions as well as alkyl−alkyl, alkyl−imidazolium/pyridinium interactions.23 In compound 2, four kinds of hydrogen bonding environments of [TeCl6]2− have been constructed by eight conformational flexible [Bmim]+ cations, and additionally, the anion-π interactions have been created by Cl and electrondeficient imidazolium rings, whereas in compound 1, there is only one kind of H-bonded capsules and the anion-π interactions are negligible. Given all that, the 3D supramolecular networks in compounds 1 and 2 are totally different, as shown in Figure 1a,b. Compound 3 belongs to the monoclinic crystal system with the space group of P21/n. The asymmetric unit consists of half of the formula of [HOOCMim]2TeCl6, as shown in Figure S1. The Te atom is situated at the position of an inversion center and is coordinated by six Cl− ions forming a nearly perfect octahedra with the Te−Cl bond lengths ranging from 2.5243(9) to 2.5413(10) Å (Table S1). Every [TeCl6]2− anion exhibits hydrogen bonds with the eight surrounding [HOOCMim]+ cations (Figure 2c). Such supramolecular units are extended into 1D chains through the bridging function of IL cations and O···H−O bonds (Figures S2c and S6a). In addition, anion-π interactions between Cl(3) atoms and the imidazole rings with the distance of 3.8376(19) Å (Table S5) also exist in the supramolecular networks. The 1D channels are packed parallelly and linked each other by common cations, finally generating 3D supramolecular structures. The detailed Cl···H−C and O···H−C bonds as well as the anion-π interactions are shown in Figures S6b−d. The lengths of Cl··· H−C bonds are ranged from 2.72 to 3.14 Å, and the distance of O2···H1A-O1 is 1.89(4) Å (Table S2). Compound 4 belongs to the tetragonal noncentrosymmetric space of I4̅ and the asymmetric unit contains two 0.25 Te4+ ions, two integrated and two half Cl− anions and one [Bzmim]+ cation (Figure S1). Two kinds of Te atoms are located at the 4fold rotation-inversion axis and six coordinated by Cl− ions in the form of octahedral geometry with the Te−Cl bond lengths varying in the range of 2.525(4)−2.539(4) Å (Table S1). As

shown in Figure 1d, the [Bzmim]+ cations are parallel stacked along the c axis, forming two types of square-shaped channels, which are further arranged alternately along the b and a axes by sharing common edges. The [Te(1)Cl6]2− and [Te(2)Cl6]2− anions are situated in these two kinds of channels respectively and exhibit Cl···H−C hydrogen bonds with the surrounding [Bmim]+ cations, resulting in the formation of a 3D supramolecular network. The hydrogen bonding environments of [Te(1)Cl6]2− and [Te(2)Cl6]2− are different. As depicted in Figure 2d, there are eight [Bzmim]+ cations interacting with [Te(1)Cl6]2− anion via hydrogen bonds, extending into 1D supramolecular chains along the c axis, while the [Te(2)Cl6]2− is surrounded by only four [Bzmim]+ cations through the linkage of hydrogen bonds, forming a 0D supramolecular unit (Figures S2d and S7). The distances of Cl···H−C bonds range from 2.62 to 2.94 Å (Figure S7 and Table S2). Compound 5 crystallizes in the monoclinic crystal system with the space group of P21/n, and its structure features a 3D supramolecular network fabricated by hydrogen bonds and anion-π interactions between the anions and cations (Figure 1e). The asymmetric unit consists of one-half Te4+ ions, three Cl− ions, and one [EPy]+ cation (Figure S1). The Te atom is six coordinated by Cl− ions in the form of octahedron. The distances of Te−Cl bonds are in the range of 2.5305(8)− 2.5335(8) Å (Table S1). Every [TeCl6]2− anion interacts with the six surrounding [EPy]+ cations which are arranged in the square-like shape via hydrogen bonds (Figure 2e). Different from the above compounds, the H-bonded “capsules” in 5 are prolonged into 1D supramolecular chains (Figure S2e and Figure S8a) only by the connection of anion-π interactions between Cl(1) ion and the electron-deficient pyridine ring with the distance of 3.4928(19) Å (Figure S8c and Table S6). The detailed Cl···H−C bonds are depicted in Figure S8b, and the lengths range from 2.89 to 2.95 Å (Table S2). Compound 6 crystallizes in the monoclinic space group of P21/c. One half Te4+ ions, three Cl− ions, and one [Bmmim]+ cations constitute the asymmetric unit (Figure S1). An inversion center completes the coordination geometry of Te to form octahedral [TeCl6]2− with the Te−Cl bond lengths ranging from 2.5178(5) to 2.5512(6) Å (Table S1). Note that the structure of 6 has been reported by Laitinen et al.,20 and herein we investigate its structure detail and conductance and luminescence properties which are not involved before. The structure of 6 features 2D supramolecular layers (Figures 1f and S2f). As depicted in Figure 2f, each anion is surrounded by six [Bmmim]+ cations via the connection of hydrogen bonds and extends along the bc plane, finally resulting in the formation of 2D infinite layers which are aligned eclipsing one another along the a axis (Figure S2f). There is no other interactions between adjacent layers owing to the large interlayer distances. The hydrogen bonds are listed in Figure S9 and the lengths are in the range of 2.74−2.99 Å (Table S2). Compared with the hybrid tellurium(IV) halides fabricated by traditional protonated organic amine cations, the title compounds feature totally different structure types. The main reason may be the unique template effects of the bulky ionic liquid cations, which are responsible for the abundant supramolecular interactions with anions. As mentioned above, as well as previous research, the ionic liquid cations tend to be organized into channels where the anions situate, thus resulting in different packing diagrams of the [TeCl6]2− anions in 1−6. Taking every isolated [TeCl6]2− as a node and linking the adjacent nodes, different six-connected topological nets of title E

DOI: 10.1021/acs.inorgchem.8b00297 Inorg. Chem. XXXX, XXX, XXX−XXX

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imidazolium-based ionic liquids. Compound 5 undergoes onestep weight reduction in the range of 260−400 °C, while the decomposition processes of the other five compounds were similar to two-step weight losses during the temperature range of 190−650 °C. Optical Property and Electrical Conductivity. Inspired by the reported hybrid metal halides as optoelectronic materials, the semiconducting behaviors of the title compounds have been studied. As depicted in Figure 4, the optical

compounds can be formed. As shown in Figure 3, the discrete anions in compounds 1 and 4 are arranged in a bsn

Figure 3. (a−f) Topological nets for compounds 1−6 by connecting the Te atoms of adjacent [TeCl6]2− anion.

superlattice; in compound 2, the packing diagram of anions can be simplified into a vby type, while those in compounds 3, 5, and 6 are pcu nets. The closest Te···Te distances are in the range of 8.211−10.593 Å. Additionally, different kinds of weak interactions such as hydrogen bonds and anion-π between cations and anions are formed and help to stabilize the supramolecular networks. While owing to the different structures of the cations, the final supramolecular networks and the supramolecular interactions are diverse. For example, in compounds 3, the introduction of carboxyl functionalized ionic liquids enrich the types of intermolecular interactions. Different from the compounds 1, 2, and 4, in which the 1D supramolecular chains are packed continuously and connected by sharing common walls, the O··· H hydrogen bonds in 3 have vital influence on the fabrication of 3D supramolecular structures. The anion-π interactions play great roles on the fabrication of 3D architectures in compound 5. Compound 6 features 2D supramolecular layers, while the remaining six exhibit 3D supramolecular networks. Furthermore, the conformations of the cations play great roles on the formation of intermolecular interactions as well as the final structures. In compound 1, there are two kinds of [Bmim]+ cations in the asymmetric unit with the butyl chain featuring GT conformation. By contrast, in compound 2, eight kinds of [Bmim]+ cations with GT, TT, and TG conformations exist in the asymmetric unit. As a result, in compound 1, the [Bmim]+ cations are organized into square-like channels, while in compound 2, the cations are arranged into irregular-shaped channels. PXRD and Thermal Stability Analyses. The comparative patterns of six compounds, as depicted in Figure S10, suggest the experimental patterns of the title compounds joint well with the simulated ones, indicating phase purity of the samples (Figure S10). Thermogravimetric analyses under the N2 atmosphere have been shown in Figure S11. Compounds 3 was stable up to 190 °C, while the remaining five compounds were stable up to 260 °C, possessing better thermal stability. As depicted in Figure S11, the decomposition processes of the compound 5 fabricated by [EPy]+ cations are different from those of the remaining compounds which are constructed from

Figure 4. Solid-state optical absorption spectra of the title compounds.

absorption edges for 1−6 were estimated to be 2.63, 2.67, 2.68, 2.66, 2.60, and 2.54 eV, respectively. The comparable optical absorption edges in compounds 1−6, which are in consistent with their crystal colors, could be attributed to their similar isolated inorganic units. These values are also comparable with those of the reported [TeCl6]2− containing compounds.6c,f Electrical conductivities of the title compounds were recorded using a two-point direct current (DC) method with pressed pellets of the powder samples at 298 K (Figure 5). The conductivities of 1−6 are 6.50 × 10−7, 2.44 × 10−7, 5.20 × 10−9, 2.06 × 10−9, 4.65 × 10−6 S/cm, and 5.60 × 10−7 S/cm, respectively. Since compounds 1−6 possess similar band gaps and anionic structures, the differences in magnitude of the electrical conductivities may be caused by the cations with

Figure 5. Plots of current density (J) versus electric field strength (E) for 1−6 at 298 K. F

DOI: 10.1021/acs.inorgchem.8b00297 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Excitation and emission spectra of compound 3 (a) and 6 (b) at 77 and 298 K; for 3 λex = 365 nm, λem (77 K) = 630 nm, λem (298 K) = 610 nm; for 6 λex = 465 nm, λem (77 K) = 680 nm, λem (298 K) = 660 nm. Inset: photographs of powders of compounds 3 and 6 under ultraviolet light irradiation. (c) The configuration coordinate diagram of s2 complexes.

luminescent properties, making them possible for potential applications.6a,14b,25 In this work, the luminescent properties of the title compounds have been measured, and only compounds 3 and 6 emitted intense luminescence both at 77 K and ambient temperature. As shown in Figure 6, the luminescent spectra of 3 and 6 both display broad emission bands centered at 610 and 660 nm with a full-width at half-maximum (fwhm) of 130 and 154 nm at ambient temperature, respectively, corresponding to a quite wide excitation ranging from 250 to 500 nm. Two compounds show large Stokes shift of 245 and 195 nm, and the quantum yields were determined to be 2.49% and 0.79% at room temperature, respectively. As the temperature was reduced to 77 K, the shapes of the spectrum remained unchanged, while the emission intensities of two compounds improved by 3 orders of magnitude with a decrease of fwhm. The luminescence decay curves of two compounds at 77 K are shown in Figures S12 and S13, giving lifetimes of 4.222 μs for 3 and 2.956 μs for 6 at 77 K. While at room temperature, the lifetimes were too short to be detected, probably because of the increase of the nonradiative processes as temperature rising. According to the previous research, the luminescence of mercury-like metal ions is mainly attributed to the electrons transitions from Te 6s to Te 6p and Cl 4p orbits,6a,26 and the structural distortion of the MX6 octahedron has great influence on the broad band emission and large Stokes shifts. Molecular structural reorganization on the excited state is well-known for a number of hybrid halide emitters of mercurylike metal ions.25 It has been demonstrated that the MX6 octahedron of s2 complexes exhibit a distorted configuration with the metal ion situated in the off-center positions in the ground state, while the excited states of s2 ions are characterized as higher symmetrical octahedron geometry.6a,26 The excited processes for s2 complexes can be illustrated by the configuration coordinate diagram (Figure 6c).25f Upon light absorption, the MX6 octahedra are excited into excited states with higher energy and then undergo ultrafast structural variation to lower energy excited states, thus resulting in the large Stokes shift and broad band emissions. The MX6 octahedron featuring minor distortion in the ground state needs a smaller amount of transitional energy to expend on the structure reconstruction of the excited states. While more energies need to be consumed on the structural reorganization processes for the MX6 octahedra featuring greater distortion at ground states. In addition, the strong lattice vibration caused by structural reorganization may lead to the dissipation of excited

different sizes. It has been demonstrated that the ILs that feature larger cations exhibit low ionic conductivity because the larger size contributes to the increase of ion association, mainly resulting from the abundant supramolecular interactions between cations and anions.24 We anticipated that the extensive supramolecular interactions can further influence the electron transport of the inorganic moieties. In this work, the cations in the compounds 1, 2, and 6 possess similar sizes, hydrogen bonds environments, and their electrical conductivities are basically in the same magnitude. While the introduction of the function groups enhances the interactions between cations and anions in compounds 3 and 4, thus resulting in the two magnitudes lower electrical conductivities. Compound 5 with smaller cations display higher electrical conductivities. So far, the electrical conductivities of several kinds of hybrid metal halides have been studied. For instance, in 2015, Karunadasa et al. demonstrated the pressure-induced changes in the electronic conductivities of layered Cu−Cl hybrid perovskite. The conductivity increases from 1.8 × 10−9 S/cm at 7 GPa to 2.9 × 10−4 S/cm at 51.4 GPa.3b In 2016, Xu et al. reported the anisotropic electrical conductivities of [Pb18I54(I2)9] wheel cluster;3c at room temperature, the values are 0.9 × 10−10 S/cm and 0.8 × 10−9 S/cm along the a and c axis, respectively; when heating to 180 °C, the conductivities increase to 1.8 × 10−9 S/cm and 0.9 × 10−6 S/cm. In 2017, four organic−inorganic bismuth halides with 1D [BiCl5 ]2−, [BiBr5]2−, [BiI5]2−, and 2D [Bi2Cl2I7]3− inorganic anions were synthesized by Zheng et al., and the electrical conductivities are 3.98 × 10−11, 2.64 × 10−9, 1.5 × 10−7, and 1.5 × 10−6 S/cm, respectively, indicating that both the kinds of halogens and the structural connectivity have significant influences on the electrical conductivities.3a By contrast, the electrical conductivities of compounds 1−6 fall the ranges of semiconductive materials and are comparable to the reported hybrid metal halides. It is well-known that inorganic−organic hybrid perovskite-like structures of mercury-like metal ions (Pb2+, Sn2+, Sb3+, Bi3+) have been widely investigated in the field of solar cells. Since the heavy halogens and high dimensional inorganic anions are beneficial to enhance the charge transfer of hybrid metal halides, it is anticipated that the hybrid tellurium halides could be developed into novel kinds of light harvesting materials by introducing heavier halogens or improving the structural connectivity. Fluorescence Measurements. It has been demonstrated that the mercury-like ions (e.g., Pb2+, Sn2+, Sb3+, Bi3+) with the configuration of the external electrons s2 shell possess special G

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ing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

energy via nonradiative transition between the excited and the ground states, (E point in Figure 6c). Hence, it can be concluded that the discrete MX6 with minor distortion are responsible for the luminescence. In this work, all the title compounds feature isolated TeCl62− anion, while only the emissions of compound 3 and 6 could be recognizable by the naked eyes both at 77 K and ambient temperature. The distortion of the [TeCl6]2− anion in the title compounds have been compared. As listed in Table S7, the variation of Te−Cl lengths in 3 and 6 does not exceed 0.017 and 0.0334 Å, and the maximum deviation of the value of Cl− Te−Cl bond angles from the ideal values (90°) does not exceed 0.83° and 1.39°, respectively, while in the remaining compounds, the TeCl6 octahedrons exhibit greater deformation and the maximum deviation of the Te−Cl bond lengths and the Cl−Te−Cl angles reach 0.107 Å and 2.89°, respectively. Taking these two factors in consideration comprehensively, the [TeCl6]2− anions in compounds 3 and 6 display minimum distortions, thus favoring the emission at 77 K and ambient temperature. The narrower band emissions of 3 and 6 at 77 K (Figure 6a,b) could be attributed to reduced thermally populated vibrational states.

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Corresponding Authors

*E-mail: [email protected]. Fax: (+86)591-63173145 (J.-R.L.). *E-mail: [email protected]. Fax: (+86)591-63173145 (X.Y.H.). ORCID

Xiao-Ying Huang: 0000-0002-3514-216X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NNSF of China (Nos. 21671187 and 21521061). The authors thank Prof. Gang Xu for the help on the electrical conductivity measurements.

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REFERENCES

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CONCLUSIONS In summary, we reported six hybrid tellurium chlorides assembled by IL cations and [TeCl6]2− anions. In these compounds, the IL cations are organized into channel structures. The [TeCl6]2− anions residing in the cavities exhibit hydrogen bonds and anion-π interactions with the surrounding cations, resulting in the formation of 3D (for 1−5) and 2D (for 6) supramolecular architectures. It is worthwhile pointing out that the hydrogen bonding environments of the inorganic anions has been mostly affected by the IL cations of different kinds, and we anticipate that the introduction of ILs give access to novel and diverse hybrid metal halides. The observed optical and electrical behaviors confirmed that compounds 1−6 are semiconductors with the band gaps ranging from 2.54 to 2.68 eV and the maximum electrical conductivity of 4.65 × 10−6 S/ cm, which are comparable to the reported hybrid metal halides. It is anticipated that such kind of hybrid tellurium halides can be developed into novel light harvesting materials. Additionally, compounds 3 and 6 exhibit intense emission at 77 and 298 K owing to the s2 configuration of Te4+ and the minimum distortion of TeCl6 octahedron. Future research will be focused on the rational design of novel hybrid tellurium halides with heavier halogens and extended structural connectivity by using ILs and studies on their semiconducting properties will be further carried out.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00297. Materials and methods, X-ray crystal structure determination more structural details and figures, powder X-ray diffraction patterns and TGA, additional fluorescence results (PDF) Accession Codes

CCDC 1821453−1821458 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 emailH

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