Supramolecular Templating Approach for the Solvent-Free Synthesis

Jul 25, 2016 - A series of open-framework metal oxalates (metal = Zn, Co, Mn, Bi, In) were prepared under solvent-free conditions by a supramolecular ...
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Supramolecular Templating Approach for the Solvent-Free Synthesis of Open-Framework Metal Oxalates Furong Guo,†,‡ Cheng Chen,†,‡ Kangcai Wang,§ Qinghua Zhang,*,§ and Zhien Lin*,† †

College of Chemistry, Sichuan University, Chengdu 610064, P. R. China Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, P. R. China

§

S Supporting Information *

columnlike structures were found in these compounds. Their charges vary from positive to neutral and negative. Correspondingly, the host metal oxalates have anionic, neutral, and cationic frameworks. Notably, the cationic framework has rarely been reported in open-framework metal oxalates. In a typical synthesis, a mixture of ZnSO4·7H2O (0.575 g), H2C2O4·2H2O (0.126 g), and 1,4-dimethylpiperazine (135 μL) was sealed in a Teflon-lined stainless steel autoclave and heated at 150 °C for 7 days. The autoclave was subsequently allowed to cool to room temperature. Colorless crystals of compound 1 were obtained (46.1% yield based on zinc). Other metal oxalates were prepared under similar synthetic conditions. The phase purity of compounds 1−6 was confirmed by powder X-ray diffraction (Figures S1−S6 in the Supporting Information, SI). Compounds 1 and 2 are isostructural and crystallize in the orthorhombic space group Pnma (No. 62). As a representative example, the crystal structure of 1 is described here in detail. Compound 1 has a layered structure constructed from two different building blocks: a zinc oxalate chain with the formula Zn(ox)(H2O)2 and a triangular [Zn(H2O)3]2+ complex. It is worth noting that the zinc oxalate chain may serve as a useful building block for the construction of different open-framework structures. For example, the replacement of half of the water molecules in the chainlike building blocks by sulfate groups will create an anionic layered structure with hcb topology.5 In the case of compound 1, the zinc oxalate chains are bridged by [Zn(H2O)3]2+ complexes to form a layered structure with 10 MR windows (Figure 1a). Of particular interest is that the layered structure of 1 is positively charged. Prior to this work, metal oxalates often had neutral and anionic frameworks.6 Hdpa cations and oxalate anions reside in an orderly fashion in the interlayer region. Each H2dmp cation interacts with two sulfate groups through strong N−H···O hydrogen bonds, forming a trimeric SMT (Figure 1b). The N···O distances within the supramolecular anion are between 3.023(4) and 3.179(5) Å. Compound 3 crystallizes in the monoclinic space group P21/c (No. 14). The oxalate ligands in the structure adopt a typical chelating bis-bidentate coordination mode. The linkages between Mn atoms and oxalate ligands generate a layered structure with 12 MR windows (Figure 1c). The metal oxalate layers are packed in an AAAA sequence with an interlayer distance of ca. 13.4 Å. In the interlayer region, there are two different kinds of organic species: Hdpa cations and oxalate

ABSTRACT: A series of open-framework metal oxalates (metal = Zn, Co, Mn, Bi, In) were prepared under solventfree conditions by a supramolecular templating approach. These compounds have cationic, anionic, and neutral frameworks with pore apertures ranging from small 8membered rings (8 MRs) to extra-large 16 and 20 MRs. The zinc oxalate exhibits a proton conductivity of 2.6 × 10−3 S cm−1 at 60 °C under 98% relative humidity.

T

he development of new synthetic methodologies is of great importance to preparing novel crystalline materials, such as chalcogenides and metal−organic frameworks.1 Recently, solvent-free crystallization has attracted considerable attention for its several advantages over the conventional hydrothermal method.2 When the effect of the solvent on the framework structures is eliminated, this synthetic approach opens up the possibility for the creation of new zeotype materials.3 An illustrative example is the solvent-free synthesis of a series of magnesium phosphate−oxalates in the presence of different amines as the templating agents.3c These hybrid-framework solids are normally not accessible by solution-mediated approaches because of the high solvation energy of Mg2+. The utilization of supramolecular templates comprising protonated amines and oxoanions (denoted as SMTs) is promising for the synthesis of new open-framework materials.4 For example, homochiral (10,3)-a metal oxalate frameworks can be created in the presence of the supramolecular cation [(Me2NH2)6(SO4)]4+.4a Compared with amine templates, SMTs possess several advances, such as unique spatial configurations and variable charges that are difficult to achieve by amines. In addition, the hydrogen bonds within SMTs may be exploited for proton conduction application.4c However, the synthesis of crystalline open-framework materials containing SMTs is highly challenging because of the inadequate stability of SMTs under conventional hydrothermal and solvothermal conditions. It is expected that the removal of the solvent in material synthesis will enhance the hydrogen-bonding interactions between amines and oxoanions, which play an important role in stabilizing SMTs. Here we report a supramolecular templating approach for the synthesis of a series of new open-framework metal oxalates under solvent-free conditions (Table 1). These compounds have layered and three-dimensional structures with pore sizes ranging from small 8-membered rings (8 MRs) to extra-large 16 and 20 MRs. Several different SMTs with oligomeric, chainlike, and © XXXX American Chemical Society

Received: June 2, 2016

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DOI: 10.1021/acs.inorgchem.6b01341 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Table 1. Summary of the Crystal Data and Refinement Results compounda

space group

a (Å)

b (Å)

c (Å)

β (deg)

R(F)

SMT charge

SMT structure

[(H2dmp) (SO4)2][Zn3(ox)2(H2O)7] (1) [(H2dmp) (SO4)2][Co3(ox)2(H2O)7] (2) [(Hdpa)4(ox)][Mn2(ox)3] (3) [(H2dap) (Hox)][Bi2(ox)3.5(H2O)]·H2O (4) [(H2epip) (SO4)][In(ox)1.5] (5) [(H2epip) (ox)][In(ox)1.5] (6)

Pnma Pnma P21/c P21/c I41/a I41/a

11.1058(1) 11.1050(4) 13.3554(2) 15.2321(4) 26.0767(6) 25.1335(2)

16.3449(2) 16.4221(6) 15.6931(2) 11.4820(2) 26.0767(6) 25.1335(2)

14.0064(1) 13.9815(5) 10.4539(2) 13.4913(4) 8.9467(3) 9.7454(1)

90 90 102.455(2) 114.520(4) 90 90

0.0441 0.0739 0.0523 0.0301 0.0266 0.0409

negative negative positive positive neutral neutral

trimer trimer pentamer chain column column

a

dmp = 1,4-dimethylpiperazine; ox = oxalate; dpa = diisopropylamine; dap = 1,2-diaminopropane; epip = 1-ethylpiperazine.

Figure 1. View of the structures of various metal oxalates: (a) 1 and 2; (c) 3; (e) 4; (g) 5 and 6. These compounds contain different SMTs: (b) trimer; (d) pentamer; (f) chain; (i) column; (j) column. (h) Left- and right-handed helical channels in 5 and 6.

these small channels are helical and the pitch of the helix is 8.95 Å (Figure 1h). The left- and right-handed helical channels coexist in the structure with a ratio of 50:50. Viewed along the [100] and [010] directions, the compound has elliptical 20 MR channels with a pore size of 6.5 × 17.0 Å2. The SMTs locate within the large channels, which occupy 69.7% of the unit cell volume. Each H2epip cation links three sulfate anions through hydrogen bonds and vice versa, forming a columnlike structure with the shortest N···O distances between 2.709(4) and 2.907(4) Å (Figure 1i). The SMTs attach to the walls through In−O bonds. Interestingly, the sulfate groups in the columnlike SMTs can be replaced by oxalate ligands, and the open-framework structure of 6 will be produced as the resulting product (Figure 1j). The temperature dependence of the magnetic susceptibility of compounds 2 and 3 was measured in the temperature range 2− 300 K (Figures S19 and S20 in the Supporting Information). The magnetic moment (μeff) at 300 K per mole of metal atom is 4.47 μB (for 2) and 5.96 μB (for 3), in agreement with those reported for other cobalt(II) and manganese(II) compounds.10 Thermal evolution of χM obeys the Curie−Weiss rule at temperatures above 50 K (for 2) and 30 K (for 3), with Cm = 2.98 cm3 K mol−1 and θ = −56.6 K for 2 and Cm = 4.89 cm3 K mol−1 and θ = −28.0 K for 3. The negative θ values imply antiferromagnetic interactions between the metal ions. The proton conductivity of compounds 1, 4, and 6 was determined by alternating-current impedance measurements

anions. Each oxalate anion interacts with four Hdpa cations through strong N−H···O hydrogen bonds, forming a supramolecular pentamer (Figure 1d). The N···O distances within the SMT are between 2.717(3) and 2.880(3) Å. Compound 4 crystallizes in the monoclinic space group P21/c (No. 14). It has a three-dimensional structure containing 10-, 8-, and 10-ring channels along the [100], [001], and [011] directions, respectively (Figure 1e). By regarding Bi(1)2O2 dimers as 6-connected nodes and Bi(2) atoms as 4-connected nodes, the structure can be represented as a 4,6-connected net with a point symbol of (44.58.62.8)(55.6)2.7 The SMTs are ordered within the channels, which occupy 35.9% of the unit cell volume (calculated using the program PLATON).8 The hydrogen-bonding interactions between protonated amine molecules and Hox anions in compound 4 form a chainlike structure (Figure 1f). The shortest N···O distances within the supramolecular chain are between 2.776(7) and 2.862(8) Å. Compound 5 crystallizes in the tetragonal space group I41/a (No. 88). Each In atom connects three neighbors through oxalate ligands to form a three-dimensional structure with lig topology.9 Figure 1g shows the framework viewed along the [001] direction. It contains two different types of squarelike channels. The large channel has a 16 MR window with a pore size of 12.0 × 12.0 Å2 (measured from the distance between two O atoms across the window). The small channel appears to have 8 MR windows. Careful analysis of the structure indicates that B

DOI: 10.1021/acs.inorgchem.6b01341 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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under 98.0% relative humidity using a compacted pellet of the crystalline powder sample.11 The bulk conductivity was evaluated by semicircle fittings of the Nyquist plots. As shown in Figure 2a, compound 1 exhibits a temperature-dependent

Communication

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Q.Z.). *E-mail: [email protected] (Z.L.). Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Program for New Century Excellent Talents in University (Grant NCET-12-0375) and the Development Foundation of CAEP (Grant 2015B0302056).

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Figure 2. (a) Nyquist plots of compound 1 at different temperatures. (b) Nyquist plots of compounds 4 and 6 at 60 °C.

behavior of the proton conductivity. The conductivity of the compound at 30 °C is 7.8 × 10−5 S cm−1, much lower than that of Humboldtine (1.3 × 10−3 S cm−1).12 However, when the temperature increases to 45 and 60 °C, the conductivity of 1 increases to 2.9 × 10−4 and 2.6 × 10−3 S cm−1, respectively. In comparison, the conductivity of compounds 4 and 6 at 60 °C was about 2.0 × 10−4 and 6.6 × 10−5 S cm−1, respectively (Figure 2b). In summary, six new metal oxalates with layered and threedimensional structures have been prepared under solvent-free conditions. These compounds contain supramolecular oligomers, chains, and columns as the templating agents. Different from amine templates, the charges of these SMTs can vary from positive to neutral and negative, which offer exciting opportunities to prepare new open-framework metal oxalates with cationic, neutral, and anionic frameworks. Besides metal oxalates, the supramolecular templating approach may be readily extended to prepare other open-framework compounds with interesting physical properties under solvent-free conditions.

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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.6b01341. Experimental details, additional figures, hydrogen-bonding information, IR spectra, TGA curves, powder X-ray diffraction patterns, and magnetic data (PDF) X-ray data in CIF format (CIF) X-ray data in CIF format (CIF) X-ray data in CIF format (CIF) X-ray data in CIF format (CIF) X-ray data in CIF format (CIF) X-ray data in CIF format (CIF) C

DOI: 10.1021/acs.inorgchem.6b01341 Inorg. Chem. XXXX, XXX, XXX−XXX