Using Multifunctional Ionic Liquids in the Synthesis of Crystalline Metal

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Using Multifunctional Ionic Liquids in the Synthesis of Crystalline Metal Phosphites and Hybrid Framework Solids Ting Li,† Xiujuan Qi,‡ Jing Li,† Hongmei Zeng,† Guohong Zou,*,† and Zhien Lin*,† †

College of Chemistry, Sichuan University, Chengdu 610064, China School of Materials Science and Engineering, Southwest Univeristy of Science and Technology, Mianyang 621010, China

‡

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 10/27/18. For personal use only.

S Supporting Information *

liquids as reactant−template−solvent systems. One focus of this work is to exploit the anions of ionic liquids as the framework building units and the cations of ionic liquids as the templating agents. A variety of ionic liquids, metal ions, and additional synthetic variants (e.g., F− and organic ligands) can be introduced into the metal phosphite systems, demonstrating the versatility of this synthetic approach. Table 1 summarizes the crystalline metal phosphites and related compounds prepared under ionothermal conditions. Structural analyses reveal that these compounds have threedimensional (for 1 and 5), layered (for 2 and 3), chainlike (for 4), and ribbonlike (for 6) structures. Significantly, compound 3 shows excellent hydrothermal stability and a high proton conductivity (σ = 2.74 × 10−3 S cm−1) at 85 °C and high relative humidity (RH). Moreover, compound 5 represents the first organically templated lanthanide phosphite-oxalate. Single crystals of compound 1 were synthesized by heating a mixture of Zn(OAc)2·2H2O and [Hmpy][H2PO3] at 150 °C for 10 days. This compound has a three-dimensional structure with edge-sharing 4 MR chains bridged by H2PO3 units (Figure 1a). The compound displays circular 12 MR channels along the [001] direction and puckered 16 MR channels along the [110] direction. The pore sizes of the 12 MR window and 16 MR window are 9.2 Å × 11.1 and 5.9 Å × 14.0 Å, respectively. The Hmpy cations reside within the intersecting channels, which occupy 37.0% of the crystal volume. When [Hmpy][H2PO3] was replaced with [Htmg][H2PO3], single crystals of compound 2 were obtained under the same reaction conditions as used for compound 1. This compound has a layered structure decorated with tmg molecules. The zinc phosphite layer has 4 MR and 8 MR windows constructed from strict alternating HPO3 pseudo-pyramids and ZnO4 tetrahedra. Zwitterionic tmg molecules attach to the zinc phosphite layer through covalent Zn−O bonds (Figure 1b). Considering zinc atoms and phosphorus atoms as nodes, the layered structure can be described as a 3-connected fes net with a point symbol of (4.82). Compounds 1 and 2 reported here demonstrate the great potential of multifunctional ionic liquids in the preparation of crystalline metal phosphites. It is well-known that openframework metal phosphites are usually prepared in sealed autoclaves containing a metal source, phosphorous acid, amine, and solvent. A plethora of chemical reactions, equilibria, nucleation, and crystal growth take place throughout the

ABSTRACT: Several crystalline metal phosphites and related hybrid framework solids were prepared under ionothermal conditions using ionic liquids as the reactant, templating agent, and solvent. These compounds display different structures and some appealing properties such as proton conduction. A variety of ionic liquids, metal ions, and additional synthetic variants (e.g., F- and organic ligands) can be introduced into the metal phosphite systems, demonstrating the versatility of the synthetic strategy.

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pen-framework metal phosphites are of current interest because of their appealing properties and potential applications in ion exchange, gas storage, fuel cells, and catalysis.1 Different from 4-connected metal phosphate molecular sieves, metal phosphites often have interrupted structures constructed from metal-centered polyhedra and HPO3 pseudo-pyramids.2 An illustrative example is the (3,4)connected zinc phosphite ZnHPO-CJ1 containing extra-large 24-membered ring (24 MR) channels.3 It is believed that the replacement of PO4 tetrahedra by HPO3 pseudo-pyramids can reduce M−O−P linkages and thus favor the formation of extralarge-pore structures. Wang and co-workers reported a family of gallium zincophosphites NTHU-13 with tunable channel sizes ranging from 24 MR to 72 MR.4 To our knowledge, 72 MR represents the largest pore opening ever achieved in crystalline zeolite-like inorganic solids. Generally, crystalline metal phosphites are prepared by hydrothermal or solvothermal methods under autogenous pressure. Ionothermal synthesis, adopting an ionic liquid as the solvent and sometimes the templating agent, has attracted considerable attention in the burgeoning field of materials science.5 This synthetic approach not only eliminates the reaction pressure but also opens up exciting possibilities for the construction of novel open-framework structures. Notable metal phosphites created under ionothermal conditions include the zinc phosphite NIS-3 with low framework density and the nickel phosphite JIS-3 with 18 MR channels featuring long-range antiferromagnetic ordering at low temperature.6 It is well-known that crystalline metal phosphites have anionic and neutral frameworks. Therefore, the anions (e.g., BF4−, PF6−, Cl−, and Br−) of ionic liquids are generally not incorporated into their free voids. Here, we report the ionothermal synthesis of six new metal phosphites and related hybrid framework solids using ionic © XXXX American Chemical Society

Received: September 4, 2018

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DOI: 10.1021/acs.inorgchem.8b02519 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry Table 1. A Summary of Crystallographic Data for Compounds 1-6 compounda

space group

a (Å)

b (Å)

c (Å)

β (deg)

R(F)

structure

Hmpy·Zn2(HPO3)2(H2PO3) (1) Zn(tmg)(HPO3) (2) 2-Hmim·Ga2F(HPO3)3(H2O)·0.5H2O (3) (Hmim)2·ZrF2(HPO3)2 (4) (4-Hmim)6·Nd4(HPO3)2(H2PO3)2(C2O4)6 (5) Ni(H2PO3)2(mim)(4,4′-bpy)1.5 (6)

P21/n P21/n P21/c P21/n P212121 P21/n

9.3003(2) 10.4197(2) 10.0501(1) 5.3954(1) 12.2278(2) 11.3944(3)

16.5944(3) 9.4451(1) 15.9903(2) 13.4871(4) 13.8480(2) 14.5554(5)

10.00896(2) 10.8195(2) 9.64553(12) 11.3959(3) 33.7650(7) 14.7935(5)

113.568(5) 118.739(3) 103.141(1) 93.339(2) 90 101.461(3)

0.0461 0.0400 0.0477 0.0475 0.0640 0.0569

framework layer layer chain framework ribbon

a mpy = 4-methylpyridine; tmg = N,N,N-trimethylglycine; 2-mim = 2-methylimidazole; mim = N-methylimidazole; 4-mim = 4-methylimidazole; 4,4′-bpy = 4,4′-bipyridine.

the presence of HF has also been investigated. The reaction of Ga2O3, [2-Hmim][H2PO3], and HF (40%) at 150 °C for 10 days gave rise to colorless crystals of compound 3. The asymmetric unit of this compound contains a flouride anion, which locates between two gallium atoms as a bridging ligand. The compound has a layered structure with 3 MR, 4 MR, and 6 MR windows (Figure 1c). One water molecule attaches to a gallium center and protrudes into the center of each 6 MR window. The inorganic layers are stacked in the eclipsed configuration along the [100] direction. The 2-Hmim cations locate within the interlayer region and interact with phosphite oxygen atoms through H bonds. The closest O···N distances are 2.803(5) and 2.926(5) Å, respectively. Fluoride may also act as a terminal ligand to a metal center. For example, the reaction of Zr(OH)4, [Hmim][H2PO3], and HF (40%) at 170 °C for 10 days gave rise to single crystals of compound 4. The asymmetric unit of this compound contains a flouride anion, which bonds to the crystallographically independent zirconium atom as a terminal ligand. The compound has a chainlike structure containing corner-sharing 4 MRs (Figure 1d). The inorganic chains run along the [100] direction and are separated by Hmim cations. The successful synthesis of fluorinated metal phosphites encourages us to incorporate other anions (e.g., oxalate ligand) into metal phosphite frameworks. The reaction of Nd2O3, [4Hmim][H2PO3], and H2C2O4·2H2O at 150 °C for 12 days gave rise to light-violet crystals of compound 5. This compound has a three-dimensional framework templated by 4-Hmim cations. The Nd atoms are linked by oxalate ligands to create honeycomb-like layers parallel to the ac plane. These metal oxalate layers are further connected by HPO3 and H2PO3 anions, generating an open-framework architecture with large 12 MR channels (Figure 1e). Considering Nd atoms as 5-connected nodes, the hybrid framework can be described

Figure 1. Six different metal phosphites and related hybrid framework solids formed under ionothermal conditions: (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, (f) 6. The organic cations in the structures of 1, 4, and 5 are omitted for clarity.

multiple-component systems, making it highly challenging to understand how open frameworks are assembled. The development of simple synthetic systems is therefore highly desirable. In the earlier studies, a three-component approach has been explored to synthesize open-framework metal phosphites in phosphorous acid flux.7 It is worth noting that the utilization of multifunctional ionic liquids as reactant− template−solvent systems could further simplify the reaction systems. For example, compounds 1 and 2 could be synthesized from a mixture of Zn(OAc)2·2H2O and an ionic liquid. The two-component approach suggests that amino phosphites may act as intermediates for open-framework metal phosphites prepared under hydrothermal conditions. It is well-known that fluoride can act as a mineralizing agent to promote the formation of fully condensed zeolite structures. It may also enter inorganic frameworks as bridging or terminal ligands. Thus, the ionothermal synthesis of metal phosphites in

Figure 2. (a) Powder XRD patterns of pristine compound 3 and its hydrothermally treated sample. (b) Arrhenius plot of the proton conductivity of compound 3. (Inset) Nyquist plot of compound 3 at 30 °C under 98% RH. B

DOI: 10.1021/acs.inorgchem.8b02519 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry as a sqp net with a point symbol of (44.66). Prior to this work, considerable efforts have been made in synthesizing metal phosphite-oxalates containing transition metals and group 13 elements.8 Only a neutral lanthanide phosphite-oxalate framework was known.9 As far as we know, compound 5 represents the first lanthanide phosphite-oxalate templated by organic cations. Besides the oxalate ligand, the neutral 4,4′-bpy molecule has also been investigated as a cross-linker. The reaction of NiO, [Hmim][H2PO3], and 4,4′-bpy at 170 °C for 7 days gave rise to green crystals of compound 6. This compound has a ribbonlike structure with nickel atoms connected by 4,4′-bpy ligands (Figure 1f). H2PO3 anions and mim molecules attach to nickel centers as pendants. The width of the ribbon is about 11.384 Å. Interestingly, H2PO3 anions interact with each other through strong H bonds, creating a supramolecular structure with a 2-fold-interpenetrated net. Considering Ni atoms as 5connected nodes, the supramolecular structure can be simplified to a bnn net with a point symbol of (46.64). Among the six phosphite-based compounds, only compound 3 shows exceptionally hydrothermal stability. By immersing its powder sample in boiling water for 8 h, compound 3 could retain its layered structure unchanged. As shown in Figure 2a, the diffraction peaks on the powder X-ray diffraction (XRD) patterns of as-prepared compound 3 and its hydrothermally treated sample correspond well in intensity and position, indicating the superior stability of the layered architecture after hydrothermal treatment. In addition, the presence of protonated 2-methylimidazole cations and water molecules within the robust layers of compound 3 encourage us to evaluate its proton conduction.10 The proton conductivity of this compound was measured by alternating current impedance spectroscopy between 30 and 85 °C under 98% RH. As determined from the Nyquist plots, the proton conductivity of compound 3 has a tendency to increase with the increase of measurement temperature. The proton conductivity at 30 °C is estimated to be 2.20 × 10−4 S cm−1 (Figure 2b). When the measurement temperature increases to 85 °C, it reaches a high value of 2.74 × 10−3 S cm−1. Such a proton conductivity is reminiscent of those of (Him)2· Be3(HPO3)4 (2.03 × 10−3 S cm−1), SCU-2 (5.94 × 10−3 S cm−1), SCU-12 (2.9 × 10−3 S cm−1), and AlPO-CJ72 (3.01 × 10−3 S cm−1) measured under similar conditions.11 The activation energy (Ea) for the proton transfer is 0.19 eV above 45 °C on the basis of the Arrhenius equation σT = σ0 exp(−Ea/kBT). The solid-state photoluminescent spectrum of compound 5 was recorded at room temperature. Upon excitation at 515 nm, compound 5 shows three infrared emission bands with peak maxima at 887, 1052, and 1327 nm. These emissions are similar to those found in Na[Nd3(H2O)4(C2O4)4(CH3PO3)]· 2H2O and assigned to the 4F3/2 → 4I9/2, 4F3/2→ 4I11/2, 4F3/2→ 4 I13/2 transitions, respectively.12 The splitting of the peaks at 887 and 1327 nm may be caused by the two different neodymium coordination environments. The temperature dependence magnetic susceptibility data of compound 6 were recorded in a magnetic field of 1000 Oe between 2 and 300 K. The effective magnetic moment (μeff) at 300 K per mole of nickel atom is 3.41 μB. Such a value is well within the range reported for other Ni(II)-based compounds.13 The χmT value decreases continuously upon cooling and reaches a value of 0.490 cm3 K mol−1 at 2 K. Above 100 K, the susceptibility follows the Curie−Weiss law. The calculated

values of the Curie constant (C) and Weiss constant (θ) are 1.58 cm3 K mol−1 and −30.2 K, respectively. The negative θ value points out the presence of weak antiferromagnetic interactions between the nickel ions. In summary, a series of new metal phosphites and related hybrid framework solids were synthesized under ionothermal conditions adopting multifunctional ionic liquids as reactant− template−solvent systems. The fluorinated gallium phosphite shows exceptionally hydrothermal stability and a high proton conductivity (σ = 2.74 × 10−3 S cm−1) at 85 °C under high humidity conditions. We believe that the high flexibility of ionic liquids will create exciting possibilities in the synthesis of novel crystalline open-framework materials. In particular, the large variety of organic cations indicates that the scope for the synthesis of organically templated lanthanide-based luminescent materials will be very large.

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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.8b02519. Synthesis, characterization, and additional figures (PDF) Accession Codes

CCDC 1863952−1863957 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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AUTHOR INFORMATION

Corresponding Authors

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

Guohong Zou: 0000-0003-4527-0058 Zhien Lin: 0000-0002-5897-9114 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 21875146) for financial support. REFERENCES

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DOI: 10.1021/acs.inorgchem.8b02519 Inorg. Chem. XXXX, XXX, XXX−XXX