CRYSTAL GROWTH & DESIGN
A Novel Supramolecular Resin Based On an Organic Acid-Base Compound
2009 VOL. 9, NO. 6 2668–2673
Shuo-ping Chen,† Ling-ling Pan,† Yi-xuan Yuan,† Xi-xi Shi,‡ and Liang-jie Yuan*,† College of Chemistry and Molecular Sciences, Wuhan UniVersity, 430072 Wuhan, P. R. China, and Key Laboratory for Biomedical Informatics and Health Engineering, Institute of Biomedical and Health Engineering, Shenzhen Institutes of AdVanced Technology, Chinese Academy of Science, 518054 Shenzhen, P. R. China ReceiVed NoVember 4, 2008; ReVised Manuscript ReceiVed March 12, 2009
ABSTRACT: A novel supramolecular resin, namely, R-(AEDPH2) · (enH2) · 3H2O (1) (AEDPH2 ) 1-aminoethylidenediphosphonic acid and en ) ethylenediamine), is synthesized and characterized by infrared (IR) spectroscopy, thermogravimetric analysis (TGA), elemental analysis (EA), single crystal X-ray diffraction (SCD), and powder X-ray diffraction (PXRD). The supramolecular resin is an organic acid-base compound and shows a three-dimensional (3D) tessellate-type supramolecular structure constructed via various hydrogen bonds, which contains a kind of D3 water cluster. It has good mechanical properties as well as excellent flame retardant performance. In addition, the supramolecular resin can form a single crystal, namely, β-(AEDPH2) · (enH2) · 3H2O (2), by an interesting gel-to-crystal transformation. Compound 2 is the isomer of 1 which illustrates a sandwich-type supramolecular architecture contained an R4 water cluster. Introduction The hydrogen bond is widely used in designing and obtaining novel structures.1 In the past two decades, supramolecular chemistry based on hydrogen bonded assembly has attracted considerable attention due to exquisite architectures and potential interesting functions.2 Among diverse supramolecular materials, organic acid-base compounds are given close attention because of their novel supramolecular architectures3 and potential applications in molecular recognition, optics, biomaterials, etc.4 We focused on the construction of small molecular organic acid-base compounds via a hydrogen bond to novel supramolecular materials with applied mechanical performance. Among diverse organic acids, 1-aminoethylidenediphosphonic acid ((CH3)C(PO3H2)2(NH2), AEDPH4) can generate various strong hydrogen bonds,5 which can be excellent precursors for the preparation of supramolecular gelling materials that have good mechanical performance. Recently, we reported the first supramolecular plaster on the basis of an organic acid-base compound, namely, (AEDPH3) · (1,2,4-tzH) · H2O (2) (1,2,4-tz ) 1,2,4-triazole).6 The supramolecular plaster has good sterilizing performance as well as outstanding mechanical properties similar to the general gypsum plaster. In order to design more similar organic supramolecular gelling materials with the help of crystal engineering, a flexible molecule, i.e., ethylenediamine (en) was chosen as the base component to replace the rigid ligand 1,2,4tz. A novel supramolecular gelling material similar to resin is successfully synthesized and reported herein. It is a kind of hotsetting resin with good mechanical properties. Meanwhile, because of its special organic components, the supramolecular resin has excellent flame retardant performance. In addition, the supramolecular resin can generate an interesting gel-to-crystal transformation to form single crystals of its isomer. Synthesis The supramolecular resin is simple to synthesize and processing it at room temperature and atmospheric pressure can be selfexothermic forming: * To whom correspondence should be addressed. Tel: +86-27-8721-8264. Fax: +86-27-8721-8264. E-mail:
[email protected]. † Wuhan University. ‡ Chinese Academy of Science.
(1) AEDPH4 and distilled water with 1:4.4 mol ratios were mixed together. (2) en with the same mole amount of AEDPH4 was added to the mixture with strong agitation. The mixture was selfexothermic immediately, and its temperature rose to 90 °C. At this time, the supramolecular resin was a kind of clear mucilaginous liquid. The mucilaginous liquid could be poured in a mold to shape into any form (see Figure 2a and the Experimental Section). It could also be a mucilage since it adhibited to its own surface (see Figure 2b) or coated the surface of other substances like plastic, wood, paper, etc (see Figure 5). (3) After molding, the supramolecular resin would gradually cool down and become turbid and more viscous. When its temperature decreased to about 40 °C, the supramolecular resin eventually turned into a smooth and hard solid. The color of the solid may be white (if the en was new) or yellow (if the en was laid for some time). Note that the resin is hot setting for it cannot melt when it is reheated. Structure In order to get the precise structure of this supramolecular resin, a colorless single crystal was obtained by slow evaporation (see the Experimental Section). Results of X-ray crystallographic analysis show that the crystal is a new organic acid-base compound, namely, R-(AEDPH2) · (enH2) · 3H2O (1). Combined with the results of X-ray powder diffraction of the supramolecular resin, it is proven that the structure of the supramolecular resin is the same as that of 1 (see Figure 1 and Figure S-1 in the Supporting Information). In the supramolecular resin, i.e., R-(AEDPH2) · (enH2) · 3H2O (1), each AEDPH4 molecule deprotonates two protons and transfers one proton to the amino-nitrogen atom, turning to the AEDPH22- anion itself. Meanwhile, each en molecule is protonated to form the enH22+ cation. In addition, there are three lattice water molecules in each asymmetric unit of the resin. This resin displays a very stable hydrogen bonded network. First, a one-dimensional (1D) zigzag supramolecular chain of AEDPH22- anion is formed by strong hydrogen bonds among
10.1021/cg801233x CCC: $40.75 2009 American Chemical Society Published on Web 04/02/2009
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Figure 1. Formation and components of the supramolecular resin: the Oak Ridge thermal ellipsoid plot (ORTEP) of the unsymmetrical unit of the supramolecular resin with thermal ellipsoids at the 30% probability level.
) 2.994(3) Å) (see Figure 3a). Then, each enH22+ cation can generate five N-H · · · O hydrogen bonds to the neighboring AEDPH22- anion, connecting the 1D supramolecular chains to a three-dimensional (3D) tessellate-type supramolecular network. Meanwhile, the three water molecules in the interspaces of the 3D network are linked together to form a V-shaped D3 water cluster (O3W-H2W3 · · · O1W#1 ) 2.741(4) Å, O3W-H1W3 · · · O2W#2 ) 2.793(5) Å). This water cluster connects with the host via various strong O-H · · · O and N-H · · · O hydrogen bonds, making the 3D supramolecular network more stable (see Figure 3b). Mechanical Properties The mechanical properties of the supramolecular resin are tested and listed in Table 1. It is observed that the mechanical properties of the supramolecular resin are similar to that of gypsum plaster,7 which could meet the full needs for coating, mucilage, and brick. Moreover, by doping or improving the machining method, the mechanical properties of this supramolecular resin can be enhanced to a large extent. Thermogravimetric Analysis (TGA) The TG curve of the supramolecular resin is shown in Figure 4. The supramolecular resin can be stable up to 89 °C in the air. Then, it decomposes until 148 °C with a weight loss of 16.39% (Calcd 16.93%), attributed to the release of the three lattice water molecules. The dehydrated product, (AEDPH2)(enH2), can be stable up to 267 °C. The weight loss occurring between 267 and 700 °C corresponds to the decomposition of the AEDPH22- and enH22+ ions. The final residue at 900 °C is maybe 0.5 P2O5, with the observed total weight loss of 77.43% (Calc. 77.77%). Flame Retardant Performance
Figure 2. (a) Bricks of the supramolecular resin which are shaped by a mold. (b) The supramolecular resin serves as mucilage.
adjacent phosphonate groups and amino groups (O3-H3 · · · O(6)#6 ) 2.440(3) Å, N1-H1B · · · O4#6 ) 2.836(3) Å, N1-H1C · · · O6#5
Nitrogen-phosphorus compounds play an important role as flame retardant reagents.8 The supramolecular resin has a high content of phosphorus and nitrogen (Calc. for C4H23N3O9P2: P 19.41%, N 13.16%). Also, from the TG curve, it can be observed that this supramolecular resin can be stable up to a relatively high temperature (267 °C) after dehydration. Thus, the supramolecular resin is expected to have excellent flame retardant performance. The oxygen-index method was used to find the flame retardant effect of the supramolecular resin, and the result shows that the limiting oxygen index (LOI %) of supramolecular resin is 100%, i.e., the supramolecular resin is completely noncombustible even in 100% pure oxygen atmosphere. The resin is a typical intumescent flame retardant, which is suitable
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Figure 3. Structure of the supramolecular resin. (a) 1D supramolecular chain constructed by AEDPH22- anions. (b) 3D tessellate-type supramolecular network of the supramolecular resin. Green: AEDPH22- anions. Blue: enH22+ cations. Table 1. Mechanical Properties of the Supramolecular Resina content
supramolecular resin
tensile strength (MPa) bending strength (MPa) compressive strength (MPa) tensile modulus (MPa) bending modulus (MPa)
0.79 2.24 3.81 25.69 244.27
a The measurement conditions of the mechanical properties are listed in the Experimental Section.
for flame retardant coating for plastic, wood, paper, etc. When the coating of supramolecular resin is exposed to the flame, it expands and forms charry layers gradually to prevent the combustible from contacting the flame (see Figure 5). Gel-to-Crystal Transformation It is known that gel diffusion is one of the most important methods of crystal growth,9 and gel is commonly used as a substrate in traditional crystal growth. However, herein it is the first time to report that gel can transform into a single crystal directly. The supramolecular resin can generate an interesting gel-to-crystal transformation; that is, by putting the supramolecular resin in a suitable amount of distilled water, it can transform into a single crystal automatically which can be used for single crystal diffraction analysis (see the Experimental Section). Proved by single crystal diffraction, this kind of single crystal is another new acid-base supramolecular compound, namely, β-(AEDPH2) · (enH2) · 3H2O (2), which is the isomer of the supramolecular resin (see Figure 6a).
Figure 4. TG curve of the supramolecular resin under nitrogen.
Compound 2 displays a 3D supramolecular network different from that of the supramolecular resin. First, two AEDPH3anions are self-assembled to form a stable dimer via four hydrogen bonds: two are O6-H6 · · · O1#6 (2.4948(19) Å) and the other two are N1-H1B · · · O1#6 (2.825(2) Å). Second, neighboring dimers are linked together by hydrogen bond N1-H1C · · · O4#3 (3.017(2) Å) and N1-H1A · · · O3#3 (2.840(2) Å) with an R22(8) hydrogen bonded motif, forming a 1D linear double supramolecular chain along the b axis (see Figure 6b). Meanwhile, four water molecules (two O2W, two O3W) form
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Figure 5. Qualitative experiment of the flame retardant performance of the supramolecular resin. (a) The polypropylene bar is flammable. (b) The polypropylene bar is still burning even when the alcohol burner is taken away. (c) The polypropylene bar coated with supramolecular resin is nonflammable. (d) The supramolecular resin expands and gradually forms charry layers to prevent the polypropylene bar from contacting the blaze.
an R4 cyclic water cluster (O3W-H1W3 · · · O2W ) 2.878(2) Å, O2W-H2W2 · · · O3W#4 ) 2.826(2) Å), which links the neighboring 1D double chains together to generate a twodimensional (2D) pillar layer (O3W-H2W3 · · · O4 ) 2.781(2) Å, O2W-H1W2 · · · O3 ) 2.743(2) Å). Finally, each enH22+ cation generates six N-H · · · O hydrogen bonds to the neighboring AEDPH22- anion, connecting the 2D pillar layers to a 3D sandwich-type supramolecular network (see Figure 6c). The single crystal of compound 2 can only be obtained by this method. Conclusion In conclusion, we have synthesized and characterized a novel supramolecular gelling material similar to resin on the basis of an organic acid-base compound, namely, R-(AEDPH2) · (enH2) · 3H2O (1). The supramolecular resin shows a 3D tessellate-type supramolecular network constructed via hydrogen bonds. It can be self-exothermic forming and has good mechanical properties, as well as excellent flame retardant performance. Thus, it is expected that the supramolecular resin can be used for adhering, filling, and repairing. Meanwhile, it can be used in flame retardant coating. In addition, the supramolecular resin could generate an interesting gel-to-crystal transformation, forming its isomer single crystal of, i.e., β-(AEDPH2) · (enH2) · 3H2O (2). Compound 2 shows a 3D sandwich-type supramolecular network different from that of 1. These results prove that a supramolecular material based on hydrogen bonded assembly of small molecules can also have good mechanical properties. Meanwhile, such material may have other interesting functions (such as flame retardant performance) and can be designed and synthesized via crystal engineering. Current work is underway to design and synthesize more similar organic supramolecular gelling materials with desirable physical
properties and other significant functions, as well as to further investigate the direct transformation from a gel to a single crystal with the existence of water. Experimental Section General Materials and Measurements. The AEDPH4 was prepared according to the U.S. Patent 4239695.10 en (99% purity) was purchased from Sinopharm Chemical Reagent Co., Ltd. The elemental analysis data (C, H, N) were obtained with a Perkin-Elmer 240B elemental analyzer. Infrared (IR) spectra were recorded as KBr pellets at a range of 400-4000 cm-1 on a Nicolet 5700 FT-IR spectrometer with a spectral resolution of 4.00 cm-1. TGA was carried out with a NETZSCH STA 449C at a heating rate of 10 K/min under nitrogen. The limiting oxygen index (LOI %) of the supramolecular resin was tested by a JF-3 oxygen index testing machine on the samples of dimensions 12 × 0.6 × 0.3 cm. The powder X-ray diffraction (PXRD) pattern of the supramolecular resin was obtained with a Shimadzu XRD-6000 diffractometer with Cu KR radiation (λ ) 1.54056 Å) at 40 kV and 40 mA at the scan speed of 4°/min (2θ). Special Example for the Synthesis of the Supramolecular Resin. A mixture of 15.375 g of AEDPH4 (0.075 mol) and 6 mL of distilled water was placed in a cup. Then, 4.5 g of en (0.075 mol) was added to the mixture with strong agitation for 2 min. The mixture was self-exothermic immediately and became a kind of mucilaginous liquid. Then, it was cast into 0.5 × 1 × 10 cm and 2 × 2 × 2 cm molds with agitation, respectively. The samples produced in this way were cooled down in the laboratory condition for 24 h. At the end of this period, they were demolded and tested for elasticity, flexion, and compression. Synthesis of r-(AEDPH2) · (enH2) · 3H2O (1). A mixture of 0.1025 g of AEDPH4 (0.5 mmol), 0.03 g of en (0.5 mmol), and 10 mL of distilled water was vaporized in a beaker for 20 days at room temperature. Colorless crystals for single crystal diffraction analysis were obtained. Yield: 75% (based on AEDPH4 powder). Elemental Analysis. Found, %: C, 15.04; H, 7.23; N, 13.11. Calcd for C4H23N3O9P2: C, 15.05; H, 7.26; N, 13.16. IR (KBr pellet, ν in cm-1, intensity): 3108 s, 1630 m, 1549 m, 1384 m, 1089 s, 957 m, 904 m, 782 m, 518 m.
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Figure 6. Structure of compound 2. (a) the Oak Ridge thermal ellipsoid plot (ORTEP) of the unsymmetrical unit of compound 2 with thermal ellipsoids at the 30% probability level. (b) 1D supramolecular chain constructed by AEDPH22- anions. (c) 3D sandwich-type supramolecular network of compound 2. Green: AEDPH22- anions. Blue: enH22+ cations. Measurement Conditions of the Mechanical Properties. All mechanical properties of the supramolecular resin were tested by a WDW-20 universal material testing machine. The tests of flexural strength, bending strength, flexural modulus, and bending modulus were carried out on the samples of dimensions 0.5 × 1 × 10 cm. The compressive strength test was carried out on the samples of dimensions 2 × 2 × 2 cm. All tests were carried out in a laboratory condition at 20 ( 2 °C, and each of them shown in Table 1 represents an average figure of three samples with the same composition.
Special Example for the Gel-to-Crystal Transformation (Synthesis of β-(AEDPH2) · (enH2) · 3H2O (2)). 0.5 g of supramolecular resin and 3 mL of H2O were placed in a glass vial and sealed at room temperature. Colorless crystals for single crystal diffraction analysis gradually grew from the supramolecular resin. The supramolecular resin was completely transformed to single crystals within 2 days. Yield: 82% (based on supramolecular resin). Elemental Analysis. Found, %: C, 15.02; H, 7.25; N, 13.13. Calcd for C4H23N3O9P2: C, 15.05; H, 7.26; N, 13.16. IR (KBr pellet, ν in
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Table 2. Crystallographic Data and Structure Correction Parameters of Compounds 1 and 2a compound
1
2
empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Dcacl (g · cm-3) µ (mm-1) F (000) crystal size (mm) data/restraints/ parameters completeness (%) goodness-of-fit on F2 final R indices [I > 2σ (I)] R indices (all data)
C4H23N3O9P2 319.19 orthorhombic P212121 7.1271(7) 10.278(2) 18.006(2) 90 90 90 1319.0(2) 4 1.607 0.374 680 0.20 × 0.10 × 0.08 3106/0/196
C4H23N3O9P2 319.19 triclinic P1j 5.6469(16) 9.008(3) 13.229(4) 97.626(4) 99.857(5) 101.154(4) 640.8(3) 2 1.654 0.319 340 0.51 × 0.29 × 0.27 2443/9/196
99.0 1.069 R1 ) 0.0448, wR2 ) 0.1193 R1 ) 0.0485, wR2 ) 0.1242 0.837, -0.489
97.4 1.164 R1 ) 0.0284, wR2 ) 0.0836 R1 ) 0.0293, wR2 ) 0.0847 0.426, -0.328
(∆F)max,(∆F)min (e/Å3)
a R1 ) [Σ(|Fo| - |Fc|)/Σ|Fo|; wR2 ) [Σ[w(|Fo|2 - |Fc|2)2]/Σ[w(|Fo|2)2]1/2, w ) 1/[σ2|Fo|2 + (xp)2 + yp]; where p ) [|Fo|2 + 2|Fc|2]/3.
cm-1, intensity): 3417 s, 1614 m, 1514 m, 1365 m, 1140 s, 1095 s, 1043 s, 982 m, 939 m, 779 m, 545 m. X-ray Crystallographic Analysis. Crystallographic measurements were manipulated on a Bruker SMART CCD area-detector diffractometer. Structures of compounds 1 and 2 were analyzed using graphite monochromated Mo KR radiation (λ ) 0.71073 Å) at 273 K. All data were corrected for absorption using the SADABS program. The structures were processed by direct methods using the SHELXS-97 program.11 All of the non-hydrogen atoms were corrected with anisotropic thermal parameters by full-matrix least-squares calculations on F2 using SHELXL-97. Hydrogen atoms were directly obtained from Difference Fourier Maps, and several DFIX commands were applied on hydrogen atoms in compound 2. Drawings were finished by Mercury 1.4.1 software. Crystallographic data and structural correction parameters are listed in Table 2. Hydrogen bond distances and angles are listed in Table S-2 (Supporting Information). Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Centre as Supplementary Publication No. CCDC 687465 (1) and 685827 (2). Copies of the data may be obtained free of charge on application to The Director, CCDC, 12, Union Road, Cambridge CB21EZ, UK (Fax: (+44) (1223) 336033. E-mail:
[email protected]).
Acknowledgment. This work was supported by grants from the National Nature Science Foundation of China (No. 20671074). We are thankful to Dr. Nian Hua Huang, Dr. Xiang Gao Meng, Ms. Shu-qin Xu, Mr. Yong Zhou, and Prof. Pin-jing Zhao for helpful discussions. Supporting Information Available: Figure S-1: PXRD patterns of the supramolecular resin and compound 1. Table S-2: hydrogen bond
distances and angles of compounds 1 and 2. S-3: cif file of compound 1. S-4: cif file of compound 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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