Two Polymorphs of an Organic−Zincophosphate ... - ACS Publications

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Two Polymorphs of an Organic−Zincophosphate Incorporating a Terephthalate Bridging Ligand in an Unusual Bonding Mode Chih-Min Wang,*,† Ming-Feng Pan,† Yen-Chieh Chen,‡ Hsiu-Mei Lin,† Mei-Ying Chung,§ Yuh-Sheng Wen,§ and Kwang-Hwa Lii‡,§ †

Institute of Bioscience and Biotechnology, National Taiwan Ocean University, Keelung, Taiwan 202, Republic of China Department of Chemistry, National Central University, Jhongli, Taiwan 320, Republic of China § Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 115, Republic of China ‡

S Supporting Information *

carboxylatophosphate frameworks difficult.4 Recently, the assembly of both organic and inorganic building blocks to generate a new class of crystalline materials with various compositions and architectures has attracted much attention.5−7 To prepare new hybrid metallophosphates, we introduce the BDC ligand, a well-known landmark as an organic building unit for the formation of MOF compounds, into a system of organically templated metal phosphates under hydro(solvo)thermal reaction conditions. Herein, two polymorphs of a zinc phosphate, (H2DA)Zn2(cis-BDC)(HPO4)2 (1) and (H2DA)Zn2(trans-BDC)(HPO4)2 (2), where DA = 1,7-diaminoheptane, were synthesized and subsequently structurally characterized by single-crystal X-ray diffraction (XRD). This work represents the first example of two organic−zincophosphate polymorphs, detailing the unusual observation of BDC as a bis-monodentate bridging ligand with cis- and trans-coordination models in crystalline materials, resulting in the generation of different inorganic layered structures in 1 and 2. Only five coordination models of BDC as organic building units in crystalline compounds have been previously reported: bis-monodentate; mono-bidentate; and chelating, bridging, and chelating/bridging bis-bidentates.8 Interestingly, in our case, the bis-monodentate BDC ligand showed a very rare modality, with a cis-coordination linkage between the metal centers and carboxyl groups. The synthesis, structural diversity at different synthesis temperatures, and interesting luminescence properties are also discussed. Colorless crystals of two polymorphs were respectively synthesized at 150 and 180 °C for 2 days under similar reaction conditions by heating mixtures of Zn(NO3)2·6H2O (0.5 mmol), 1,4-benzenedicarboxylic acid (0.5 mmol), 1,7-diaminoheptane (2 mmol), aqueous hydrogen fluoride (0.5 mmol, 48% solution), H3PO4 (3 mmol, 85% solution), triethylene glycol (2 mL), and water (7 mL). Attempts to investigate single-crystal-to-singlecrystal (SCSC) transformation from 1 to 2 by heating 1 from room temperature (rt) to 180 °C at 2 °C h−1 in platinum crucibles were unsuccessful. The final product was an unidentified and poorly crystalline sample, which was no longer suitable for crystal structure analysis. The powder XRD patterns of bulk products 1 and 2 are in good agreement with the calculated patterns based on the results of single-crystal XRD analyses (Figures S1 and S2). Elemental analysis results for the

ABSTRACT: Two new polymorphs of a zinc phosphate incorporating the terephthalate organic ligand 1,4benzenedicarboxylate (BDC), (H2DA)Zn2(cis-BDC)(HPO4)2 (1) and (H2DA)Zn2(trans-BDC)(HPO4)2 (2), where DA = 1,7-diaminoheptane, were synthesized via a hydro(solvo)thermal method at different reaction temperatures and structurally characterized by single-crystal X-ray diffraction. Interestingly, the BDC ligands, which adopt the bis-monodentate coordination model with a unusual cis type for compound 1 and with a trans linkage for compound 2, bridge the Zn atoms of the inorganic layers in the generation of two polymorphs with structural diversities (one kind of arrangement of the layered zincophosphate layer in 1; the flat and zigzag sheets of inorganic networks in 2). A simple method for tuning the optical luminescence of the title compound from blue, red, green, yellow, and pink to white emission by stirring powdered samples in lanthanide-cation-containing aqueous ethanol solutions at room temperature for 1−2 h is also presented.

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etal−organic frameworks (MOFs) constructed from metal nodes (or metal clusters) and organic linkers give rise to the generation of various structures, which leads to intensive research of the synthesis and characterization of these materials not only from a basic research perspective but also because of their interesting chemical/physical applications in gas adsorption/desorption, catalysis, and size-selective separation.1 The major advancement of MOFs began with the discovery of the porous MOF-5 framework, which comprises Zn4O(CO2)6 octahedral secondary building units and chelating 1,4-benzenedicarboxylate (BDC) ligands and exhibits a porosity of 61% and a surface area as high as 2320 m2/g.2 Thus far, a large number of MOF compounds containing BDC linkers have been reported. Wang et al. also successfully incorporated the BDC ligand into an inorganic metallophosphate structure to form the first metal carboxylatophosphate framework with bimodal porosity (micropores and mesopores).3 In comparison with MOF compounds consisting of metal nodes coordinated by organic linkers, the more complex coordination linkages between the metal centers, inorganic phosphate groups, and organic ligands make the synthesis of organic−inorganic hybrid materials with metal © 2017 American Chemical Society

Received: March 14, 2017 Published: July 5, 2017 7602

DOI: 10.1021/acs.inorgchem.7b00645 Inorg. Chem. 2017, 56, 7602−7605

Communication

Inorganic Chemistry powder samples are consistent with the formulas presented for the compounds. Anal. Found (calcd) for 1: C, 29.21 (29.10); H, 4.20 (4.23); N, 4.38 (4.52). Anal. Found (calcd) for 2: C, 29.01 (29.10); H, 4.12 (4.23); N, 4.53 (4.52). The yield based on Zn was 69% for 1 (74% for 2). Thermogravimetric analysis (TGA) of the powdered samples of 1 and 2 under a N2 atmosphere over the temperature range from 40 to 900 °C at a heating rate of 5 °C min−1 revealed a number of overlapping weight-loss events, indicating a complex decomposition process for both samples (Figures S3 and S4). The observed overall weight loss of 48.92% for 1 (for 2, it is 49.86%) between 40 and 900 °C is similar to the calculated value of 50.78% for the weight losses of 1·C7H20N2, 1· C8H4O4, and 1·H2O. The frameworks of 1 and 2 are both thermally stable up to 250 °C, as was indicated by the powder XRD patterns of the powdered samples after being heated at 150, 200, 250, 300, and 900 °C for 30 min−1 (Figures S5 and S6). The final residue of both compounds 1 and 2 is Zn2P2O7 (JCPDS 721702). Recently, softer ligands and metal ions were used in the formation of a hybrid zinc phosphate and two zinc imidazolate frameworks, which exhibited high thermal stability up to 500 °C (in air) and 550 °C (under a N2 atmosphere), respectively.7b,9 The low thermal stabilities for 1 and 2 can be attributed to the decomposition of diprotonated amines in the channels of the structures. Suitable crystals were selected for single-crystal XRD analysis, from which their formulas were determined.10 The different reaction temperatures under otherwise identical reaction conditions induce structural diversities in both polymorphs. Compound 1 crystallizes in the orthorhombic Pnma space group, with lattice constants of a = 9.7585(2) Å, b = 27.6003(7) Å, and c = 8.4144(2) Å, while compound 2 is in the monoclinic P21/c space group with constants of a = 26.8117(8) Å, b = 8.9990(3) Å, c = 9.4291(3) Å, and β = 91.806(1)°. Both of their asymmetric units contain the terephthalate ligand, the 1,7-diaminoheptane template, ZnO4 tetrahedra, and phosphate tetrahedra as structural elements. In both compounds, every P atom is bonded to three Zn atoms through three O atoms and remains the fourth coordination site as a terminal P−OH group (with a bondvalence-sum calculation of nearly 1); each Zn atom is tetrahedrally coordinated by three phosphate groups and one BDC ligand. Interestingly, the bis-monodentate BDC ligands, which adopt different coordination models (an unusual cis type for compound 1 and a trans form for compound 2), bridge the Zn atoms of the inorganic layers in the generation of structural diversities between two polymorphs (one kind of arrangement of the layered zincophosphate layer in 1; the flat and zigzag sheets of inorganic networks in 2; Figure 1). To the best of our knowledge, this work represents the first example of two polymorphs of a zinc phosphate incorporating a terephthalate ligand with different structural features not only in the coordination types of their BDC linkers but also in the structures of their inorganic layers. The zincophosphate layers are all constructed from inorganic ZnO4 and HPO4 building units in a 4.82 topology with different distributions of tetrahedra pointing up and down (Figure S7). The diprotonated 1,7-diaminoheptane cations reside as charge-balancing agents in their organic−inorganic hybrid channels (Figure S8). The N atoms of amine molecules form extensive hydrogen bonds with framework O atoms at 2.903− 3.311 Å for compound 1 and at 2.824−3.206 Å for compound 2. The flexible organic molecules not only are widely used as spacefilling counterions or structure-directing agencies in the synthesis of metal phosphites, phosphates, and chalcogenides but also

Figure 1. Perspective view of the 3D zincophosphate frameworks for (a) 1 and (b) 2. The ZnO4 and phosphate tetrahedra are respectively indicated in yellow and green. Black and red circles represent C and O atoms, respectively. The H atoms and organic templates are omitted for clarity.

induce their structural flexibilities in the generation of rich coordination chemistry and diverse physical−chemical properties (such as ion exchange and luminescence).11 A substantial number of luminescent materials have also been demonstrated to exhibit variable emission wavelengths through the doping and/or substitution of lanthanide cations into host solids. Three common synthesis routesthermal decomposition, hydro(solvo)thermal methods, and high-temperature coprecipitationhave been used to prepare these materials.11a Recently, Ln ions were incorporated into a crystalline bio-MOF-1 host through cation substitution in dimethylformamide solutions for a reaction time of 21 days, resulting in a series of luminescent materials.11b However, in the present work, we propose a simple approach for tuning the optical properties of the title compound by stirring the powdered sample (0.03 mmol) in aqueous ethanol solutions (1 mL of water and 4 mL of ethanol) containing lanthanide nitrate salt (0.003 mmol) at rt for 1−2 h. The solid was subsequently separated from the solution by centrifugation. The powder XRD patterns b and e in Figure S9 contain two extra peaks. These two samples were obtained by stirring compound 1 with europium nitrate and a mixture of terbium and europium nitrates in a molar ratio of 1:1, respectively. The powder XRD patterns indicate that the structure of 1 is changed after the incorporation of a significant amount of Eu atoms. To study the details of structural changes for those compounds, several single crystals after being immersed in an aqueous ethanol solution of lanthanide nitrate overnight were indexed on a diffractometer, but their profiles were no longer suitable for crystal structure analysis. Elemental mapping analysis displayed the distribution of Eu cations among this Eu-adsorbed compound (Figure S10). The inductively coupled plasma (ICP) analysis result showed the Eu3+-adsorbed product (1 mg) in the presence of a small amount of Eu3+ cations (0.03 mg; with 3% of Eu3+ in this solid). The results from ICP, energy-dispersive spectrometry, and photoluminescence measurements indicated that the sample contains Eu3+ cations. It is very likely that rare-earth cations are bonded with framework O atoms in the structure. 7603

DOI: 10.1021/acs.inorgchem.7b00645 Inorg. Chem. 2017, 56, 7602−7605

Communication

Inorganic Chemistry

hydro(solvo)thermal method at different reaction temperatures and structurally characterized by single-crystal XRD. Not only the inorganic zincophophate layers but also the coordination models of BDC ligands relied on their structural diversities. The BDC ligands adopted a bis-monodentate model with the different coordination linkages (an unusual cis type for compound 1; a trans form for compound 2) between the carboxyl groups and metal centers of the inorganic layers, resulting in the generation of two polymorphs with different arrangements of layered zincophosphate layers. We also proposed a simple approach for tuning the optical properties of the hybrid metallophosphates from blue, red, green, yellow, and pink to white emission colors. Further research into the synthesis and characterization of new porous frameworks by incorporating terephthalate organic ligands into the structures of inorganic metal phosphates/phosphites is in progress.

As shown in Figure 2, the as-synthesized, Eu-, Tb-, and Eu/Tbincorporated compounds emitted blue, red, green, yellow, pink,

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Figure 2. Luminescence properties of 1 for the as-synthesized sample and for Ln-adsorbed products: emission spectra and their corresponding luminescence from blue, green, red, yellow, pink to white emission for (a) the as-synthesized sample, (b) Tb-adsorbed samples, and (c) Euadsorbed samples and Tb/Eu coadsorbed products where the Tb/Eu molar ratio was (d) 99/1, (e) 1/1, and (f) 99/4.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00645. Powder XRD patterns, TGA curves, and elemental mapping images (PDF)

and white luminescence colors when irradiated with 254 nm UV light using a hand-held UV lamp. To study the nature of their luminescence, the emission spectra for these compounds and the starting materials were all collected in the wavelength range from 350 to 720 nm, with the samples under 325 nm HeCd laser excitation. The BDC ligand and europium nitrate respectively exhibited a broad band centered at 384 nm and characteristic bands of the Eu3+ ion, whereas no luminescence was observed for phosphorous acid and zinc nitrate. The spectral intensity of the Eu-adsorbed compound is higher than that of BDC and europium salt. The spectrum of as-synthesized compound 1 (Figure 2a with a strong broad band) showed a red-shifted emission (by 112 nm) in comparison with the emission of the BDC molecules, which might have originated from the fluorescence of intraligand interactions in the structure (compound 2 emitted a lilac emission color at about 394 nm). The Eu-adsorbed compound (Figure 2c) exhibited the emission bands characteristic of Eu3+ ions for the 5D0 → 7F1 (at ∼591 nm), 5 D0 → 7F2 (∼614 nm), and 5D0 → 7F4 (∼699 nm) transitions, whereas the Tb-adsorbed solid (Figure 2b) showed a band characteristic of Tb3+ ions at about 544 nm for the 5D4 → 7F5 (∼544 nm) transition. Interestingly, the multiple luminescent species in compound 1 displayed complex emission spectra (Figure 2d−f) containing the emission characteristics of BDC ligands, Tb cations, and Eu ions, where the band intensities varied when the molar ratios of Ln ions were changed in the similar reaction conditions,12 resulting in the formation of new luminescent materials with different luminescence properties, including yellow (Tb/Eu: 99/1), pink (Tb/Eu: 1/1), and white (Tb/Eu: 99/4) emissions (CIE chromaticity coordinates of x = 0.290 and y = 0.314). ICP analysis on the white-light-emitting sample shows that 1 mg of the sample contains 0.03044 mg of Tb and 0.00019 mg of Eu (mole ratio Tb/Eu = 153/1). It appears that the sample adsorbs Tb more efficiently than Eu. The complex emission properties may be dominated by energy transfer between lanthanides, intraligand interactions, and the superposition of the luminescence of BDC and Eu and Tb cations.13 In summary, two polymorphs of a zinc phosphate incorporating a terephthalate organic ligand (BDC) were synthesized via a

Accession Codes

CCDC 1529451−1529452 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 Author

*E-mail: [email protected] or [email protected] (C.M.W.). ORCID

Chih-Min Wang: 0000-0002-0891-4523 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Science and Technology of Taiwan for financial support (Grant MOST 105-2113-M-019-002-MY2). REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00645 Inorg. Chem. 2017, 56, 7602−7605