A Novel Supramolecular Plaster Based on An Organic Acid−Base

CRYSTAL GROWTH & DESIGN

A Novel Supramolecular Plaster Based on An Organic Acid-Base Compound: Synthesis, Structure, Mechanical Properties, and Sterilizing Performance

2009 VOL. 9, NO. 2 874–879

Shuo-ping Chen, Yi-xuan Yuan, Ling-ling Pan, Shu-qin Xu, Han Xia, and Liang-jie Yuan* College of Chemistry and Molecular Sciences, Wuhan UniVersity, Wuhan 430072, P. R. China ReceiVed May 25, 2008; ReVised Manuscript ReceiVed October 19, 2008

ABSTRACT: A novel supramolecular plaster, namely (AEDPH3) · (1,2,4-tzH) · (H2O) (1), is synthesized and characterized by infrared spectrum (IR), thermogravimetric analysis (TGA), scanning electron microscope (SEM), elemental analysis (EA), single-crystal X-ray diffraction (SCD), and powder X-ray diffraction (PXRD). The supramolecular plaster is an organic acid-base compound and shows a three-dimensional (3D) sandwich typed supramolecular structure constructed via various hydrogen bonds. The supramolecular plaster has good sterilizing performance as well as excellent mechanical properties similar to the general gypsum plaster widely used in our daily life. The experimental results also prove that the supramolecular material based on hydrogen bonded assembly of small molecules has good mechanical properties. Introduction Gelling materials such as plaster, cement, and ceramic, which are plastic and hardenable, play important roles in human society. These materials mainly consist of inorganic salts or oxides. Sometimes, polymers may be the additives of such materials.1 Thus the interactions between the components in those materials are mostly electrovalent bonds and sometimes may be covalence bonds. Although the hydrogen bond is directional and easy to generate and widely used in designing and obtaining novel structures,2 its ability to construct small molecules to supramolecular gelling materials with applied mechanical performance has not been reported before. On the other hand, crystal engineering based on organic acid-base compounds has been an attractive research area in recent years, not only because of their novel supramolecular architectures3 but also the potential applications in molecular recognition,4 catalysis,5 and biomaterials.6 Among the diverse organic acids, phosphonates contained -PO3H2 functional groups are easy to generate strong hydrogen bonds,7 which can be excellent precursors for the preparation of supramolecular gelling materials. In the investigation of the acid-base compound of 1-aminoethylidenediphosphonic acid ((CH3)C(PO3H2)2(NH2), AEDPH4), we observed that the mixture of AEDPH4 and 1,2,4-triazole could gel formation via hydrogen bonds in water solution. On the basis of this phenomenon, we have successfully synthesized a novel supramolecular material. Though this material is just an organic acid-base compound constructed via hydrogen bonds, it has excellent plastic and hardenable properties as well as good mechanical properties similar to the general gypsum plaster. In additon, compared with normal gypsum plaster, the supramolecular plaster has good sterilizing performance. The novel supramolecular plaster is constructed by AEDPH4, 1,2,4-triazole (1,2,4-tz) and water. The gelation mechanism of the supramolecular plaster is similar to that of general gypsum plaster. The hydrated reaction of AEDPH4 and 1,2,4-tz generates a new organic acid-base compound, namely (AEDPH3) · (1,2,4tzH) · (H2O) (1) (See Figure 1). Because the solubility of compound 1 is much smaller than that of AEDPH4 and 1,2,4* Corresponding author. E-mail: [email protected]. Tel: 86-27-8721-8264. Fax: 86-27-8721-8264.

tz, it would crystallize from the saturated solution and precipitate first. The compound 1 would urge the mixture of AEDPH4 and 1,2,4-tz to continue to dissolve, making the reaction process carry on continuously until the reactors transform into compound 1 completely. The free water molecules would reduce unceasingly while the hydrated response continues because of the hydration and evaporation, which result in the loss of the plasticity and complete coagulation of the supramolecular plaster. Meanwhile, with the growing up of the crystallite of compound 1, the crystallite particles join, stagger, and paragenesize each other gradually, thus generating the mechanical performance, i.e., induration. The hydration and induration processes mentioned above are intercrossing and carried on continuously. Herein, we report the synthesis, characterization, structure, mechanical properties, and sterilizing performance of the supramolecular plaster. Synthesis Materials and Measurements. The AEDPH4 was prepared according to the U.S. Patent 4239695.8 1,2,4-tz (99% purity) was purchased from Alfa Aesar Chemical Reagent Co., Ltd. The elemental analysis data (C, H, N) were obtained from a Perkin-Elmer 240B elemental analyzer. 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. Thermogravimetric analysis (TGA) was carried out with a NETZSCH STA 449C at a heating rate of 10 K/min in air. The SEM images were obtained with a Hitachi SEM X650 scanning electron microscopy. The absorbency data of formazan derivative were recorded in a DG5031-ELISA microplate reader. Synthesis and Process of the Supramolecular Plaster. The supramolecular plaster is simple to synthesize and processing in room temperature and atmospheric pressure. (1) Milling: A mixture of 1-aminoethylidenedisphosphonic acid (AEDPH4) and 1,2,4-triazole (1,2,4-tz) with 1:1 mol ratio was grinded at room temperature, gaining a kind of white powder that was a little sticky. (2) Figuration: With strong agitation, distilled water with 0.43 time amount (mass ratio) to the mixture was added. The mixture began to coagulate after 2 min and became plastic and easily

10.1021/cg800552s CCC: $40.75  2009 American Chemical Society Published on Web 12/22/2008

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Figure 1. Formation and components of the supramolecular plaster: The Oak Ridge thermal ellipsoid plot (ORTEP) of the unsymmetrical unit of the supramolecular plaster with thermal ellipsoids at the 30% probability level.

figurable. It could then be shaped into any forms by a mold (see Figure 2a) or just by handwork (see Figure 2b) without any additional pressure. (3) Induration: After figuration, the supramolecular plaster lost its plasticity and indurated gradually and turned into a smooth, hard, and pure white solid 15 min after the distilled water was added. The appearance of the plaster is almost the same as the commonly used gypsum plaster (calcium sulfate). Special Examples. A mixture of 61.5 g of AEDPH4 (0.3 mol) and 20.73 g of 1,2,4-tz (0.3 mol) was grinded in a muller for 3 min. Then, 35 mL of distilled water was added to the mixture with a strong agitation for 2 min. The mixture was then cast into 0.5 × 1 × 10 cm3 and 2 × 2 × 2 cm3 molds and then compacted by jolting. The samples produced with this method were preserved in the laboratory condition for 2 h. After this period, they were demolded and deposited at room temperature for 7 days, and then tested for flexion and compression. Structure Synthesis of (AEDPH3) · (1,2,4-tzH) · (H2O) (1). To get an accurate structure of this supramolecular plaster, we obtained a colorless single crystal constructed by AEDPH4 and 1,2,4-tz by a low-temperature, low-pressure hydrothermal method:9 A mixture of 0.0256 g of AEDPH4 (0.125 mmol), 0.0172 g 1,2,4tz (0.25 mmol) and 1 mL of distilled water was sealed in a small centrifuge tube and then heated in an oven at 80 °C for 20 days. Then this cetrifuge tube was taken out and the mixture was filtrated immediately as it was hot. Colorless single crystals for X-ray crystallographic analysis were obtianed and dried in air. Yield: 77% (based on 1,2,4-tz). X-ray crystallographic analysis shows that the crystal is a new organic acid-base compound, namely (AEDPH3) · (1,2,4-tzH) · (H2O) (1). Element Analysis: Found, %: C, 16.40; H, 4.81; N, 19.13; Calcd for C4H14N4O7P2: C, 16.44; H, 4.83; N, 19.16. X-ray Crystallography. The powder X-ray diffraction (PXRD) pattern of the supramolecular plaster was obtained with a Shimadzu XRD-6000 diffractometer with Cu KR radiation (λ ) 1.54056 Å) at 40 kV and 30 mA at the scan speed of 4°/min (2θ). Crystallographic measurement of compound 1 was manipulated on a Bruker SMART CCD area-detector diffractometer. The structure was analyzed at room temperature using graphite monochromated Mo KR radiation (λ ) 0.71073) and by direct methods using SHELXS-97 program.10 Non-hydrogen atoms were refined with anisotropic thermal parameters by fullmatrix least-squares calculations on F2 using SHELXL-97. Hydrogen atoms were directly obtained from difference Fourier maps. Crystal data of 1 (C4H14N4O7P2): Mw ) 292.13, T ) 273(2) K, triclinic, space group P1j, a ) 5.5117(4) Å, b ) 9.3252(7) Å, c ) 11.7405(9) Å, R ) 77.762(2)°, β )

79.421(2)°, γ ) 76.283(2)°, V ) 567.23(7) Å3, Z ) 2, Dcacld ) 1.710 g · cm-3, µ ) 0.416 mm-1, R1 ) 0.0308, wR2 ) 0.0855 (all data), GOF ) 1.066. Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Centre as Supplementary Publication No. CCDC 664439. Structure of the Supramolecular Plaster. Combined with the result of X-ray powder diffraction of the supramolecular plaster, it is proved that the structure of the plaster is completely the same as 1 (see Figure S-1 in the Supporting Information). In the supramolecular plaster, i.e., (AEDPH3) · (1,2,4tzH) · (H2O), each AEDPH4 molecule deprotonates one proton and transfers the proton to the amino-nitrogen atom, turning to AEDPH3- zwitterion itself. Meanwhile, each 1,2,4-tz molecule is protonated to form 1,2,4-tzH+. In addition, there is one lattice water molecule in each asymmetric unit of the plaster, making it a stable hydrogen-bonded network. At the begining, two AEDPH3- anions are self-assembled to form a dimer via very strong hydrogen bond O3-H6 · · · O1#5 (2.589(2) Å) with a cyclic motif R22(8). The dimers are then linked together and generate a 1D zigzag chain via R12(7) hydrogen bond motif, which is formed by hydrogen bonds O5-H7 · · · O2#2 (2.569(2) Å) and N6-H6B · · · O2#2 (2.853(2) Å). The amino of AEDPH3- anion remains two H(N6), forming hydrogen bonds close to O1 and O4, generating another R22(8) motif (N6-H6A · · · O4#1 ) 2.816(2) Å, N6-H6C · · · O1#1 ) 2.783(2) Å), extending the 1D chains to a 2D zigzag supramolecular layer. Finally, the 1,2,4-tzH+ cations and water molecules lie in the interlayer, connecting neighboring 2D supramolecular layers to a stable 3D sandwich typed supramolecular network (N1-H1 · · · O6 ) 2.608(2)Å,N3-H3 · · · O1W#1)2.648(2)Å,O1W-H1W1 · · · N4#4 ) 2.925(2) Å, O1W-H2W1 · · · O4#3 ) 2.660(2) Å) (See Figure 3 and Table 1). By now, the AEDPH4 has been successfully crystallized with many metal cations to produce variform novel structures,11 and some supramolecular compounds constructed by AEDPH4 and organic bases are reported by our group,12 such as (AEDPH3) · (2,2′-bipyH2)1/2 · 2H2O (2), (AEDPH3) · (phenH) · 2H2O (3) and (AEDPH3) · (NH4) (4), ((CH3)2NH2) · (AEDPH3) · (H2O) (5). However, these supramolecular compounds can not serve as supramolecular gelling materials with applied plastic and mechanical properties. In compounds 2 and 3, the organic base cations (2,2′-bipyH22+ or phenH+) have large steric hindrance, which weakens the stability of their supramolecular framework, and thus the two compounds has very weak mechanical

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Figure 3. Structure of the supramolecular plaster. (a) 2D supramolecular layers constructed by AEDPH3- anions; (b) 3D supramolecular hydrogen bonded network constructed by AEDPH3- anions, 1,2,4-tzH+ cations, and water molecules. Dashed lines represent hydrogen bonds. Table 1. Hydrogen Bonds of the Supramolecular Plaster (Å and deg)a donor-H · · · acceptor

d(donor · · · acceptor)

∠(donor-H · · · acceptor) (deg)

N(1)-H(1) · · · O(6) N(3)-H(3) · · · O(1W)#1 O(3)-H(6) · · · O(1)#5 O(5)-H(7) · · · O(2)#2 O(1W)-H(1W) · · · N(4)#4 O(1W)-H(2W) · · · O(4)#3 N(6)-H(6A) · · · O(4)#1 N(6)-H(6B) · · · O(2)#2 N(6)-H(6C) · · · O(1)#1

2.608(2) 2.648(2) 2.589(2) 2.569(2) 2.925(2) 2.660(2) 2.816(2) 2.853(2) 2.783(2)

174(3) 178(3) 175(3) 173(2) 160(3) 166(3) 153.8 162.3 149.8

a Symmetry transformations used to generate equivalent atoms: #1 x - 1, y, z; #2 -x + 2, -y + 1, -z; #3 -x + 2, -y + 1, -z + 1; #4 -x + 1, -y, -z + 1; #5 -x + 2, -y + 2, -z.

Figure 2. (a) Some bricks of the supramoleculare plaster which are figurable by mold. (b) Some manual artwares that are made by the supramolecular plaster: left, christmas hat; middle, mortar; right, fungus. (c) The supramolecular plaster served as a chalk.

properties. For compounds 4 and 5, the water-solubility of the two compounds is so good that gelation can not occur. Mechanical Properties The flexural strength and compressive strength of the supramolecular plaster were measured by a TYE-6 Electrically driven testing device. The impact strength was measured by a XJU-22 testing device. The bending modulus was measured by

a WDW-20 universal material testing machine. The test of flexural strength, impact strength and bending modulus were carried out on the samples of dimensions 0.5 × 1 × 10 cm3. The compressive strength test was carried out on the samples of dimensions 2 × 2 × 2 cm2. All tests were carried out in a laboratory condition at 20 ( 2 °C and around 65% relative humidity. Each of the results given in the table represents the average measurements performed on three samples of the same composition. The mechanical properties of the supramolecular plaster are measured and listed in Table 2. For the applied gypsum plaster, the flexural strength should be larger than 1.8 MPa and the compressive strength should be no less than 2.9 MPa (according to the China national standard GB/T 9776-1988).13 From Table 2, it can be observed that the supramolecular plaster can replace the general gypsum plaster which has been widely used in our life. Moreover, its density (1.67 g/cm3) is smaller than that of general gypsum

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Figure 5. IR spectrum of the supramolecular plaster.

Figure 4. SEM image of the surface of the plaster. (a) 2500×; (b)10000×. The SEM images of the surface of the supramolecular plaster represent the samples that are tested for mechanical properties. Table 2. Mechanical Properties and Concreting Time of the Supramolecular Plaster content flexural strength (MPa) compressive strength (MPa) impact strength (KJ/m2) bending modulus (MPa) Mohs scale initial concreting time (min) final concreting time (min)

supramolecular plaster 1.81 4.24 1.18 552.9 2 2 15

plaster (2.60∼2.75 g/cm3). By doping or improving machining method, the mechanical properties of this supramolecular plaster may be enhanced to large extent. SEM, IR, and TGA. The surface of the supramolecular plaster is analyzed by scanning electron microscope (SEM). As shown in Figure 4, the surface of the material is constructed by close accumulated of very thin nanosheets. These nanosheets are compact, which result in the good mechanical properties of the supramolecular plaster. On the other hand, the hydrogenbond interactions among the nanosheets are stronger than that in interlayer. So with scraping via strong force, neighboring nanosheets may be peeled off. Thus, this supramolecular plaster can be used as a chalk. The IR spectrum of the supramolecular plaster is given in Figure 5. The broad bands ranged from 3481 to 3105 cm-1 because of the stretching vibrations of H2O and NH3+ groups. Their rotation vibrations absorptions are located in 1604 cm-1. The peaks ranged from 1540 to 1417 cm-1 are attributed to vibrations of the 1,2,4-tzH+ ions. The symmetrical and unsymmetrical vibration bands of PO2 groups are observed at about 1226-1064 cm-1. The peaks occurred at around 924 cm-1 are assigned to νs (PO3).

Figure 6. TG/DTG/DSC of the supramolecular plaster in air. Green, TG curve; blue, DSC curve; dot, DTG curve.

The TGA of the supramolecular plaster is given in Figure 6. The supramolecular plaster can be stable up to 88 °C in air. Then it decomposes till 130 °C with a weight loss of 6.34% (calcd, 6.43%) attributed to the release of the lattice water molecule. The dehydrated product, (AEDPH3)(1,2,4-tzH), can be stable up to 150 °C. The weight loss occurred between 150 and 200 °C corresponds to the decomposition of the NdN groups of the 1,2,4-tzH+ ions. The observed weight loss (9.97%) is close to the calculated value (9.58%). The residue of the 1,2,4tzH+ ions and AEDPH3- ions can keep stable up to 230 °C and then begin to decompose till 800 °C. The observed total weight loss (67.84%) is slightly smaller than the calculated value (68.51%) if the final product is P2O2. Sterilizing Performance. Compared with normal gypsum plaster, the supramolecular plaster has good sterilizing performance. The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) method was used to determine the sterilizing effect of the supramolecular plaster on Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus, Bacillus megaterium, Bacillus thuringiensis, and Bacillus pumilus).14 The result shows that the supramolecular plaster has very broad antibacterial spectrum: the five kinds of bacteria will be completely killed when the content of the supramolecular plaster is 10 mg/mL or higher. In lower content (5 mg/mL), compared with penicillin, the supramolecular plaster displays better sterilizing performance in controlling the growth of

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has good mechanical properties as well. Though such supramolecular material is just in its beginning, it shows a promising future. Current work is underway to investigate other properties and surface modifications of this supramolecular material as well as to design and synthesize more similar organic supramolecular gelling materials with desirable physical properties and other significant functions. Acknowledgment. This work was supported by grants of the National Nature Science Foundation of China (20671074). We are thankful to Prof. Jin-ping Zhou, Dr. Hua Li, Dr. Xi-xi Shi, Yong Xiao, Qiao-yun Liu, Zhong-jing Li, and Mei-fang Zhang for helpful discussions. Supporting Information Available: PXRD patterns of the supramolecular plaster and compound 1 and table of absorbency of formazan derivative in 0D 570 nm by the MTT method (PDF); CIF file of compound 1. This material is available free of charge via the Internet at http://pubs.acs.org or from the author.

References

Figure 7. (a) Qualitative experiment of the sterilizing effect of the supramolecular plaster and penicillin on Gram-negative bacteria (Escherichia coli, left) and Gram-positive bacteria (Staphylococcus aureus, right). The meaning of the marks in the photos: H2O, negative comparison; A+, penicillin; χ, supramolecular plaster. It is observed that sterilizing rings appear around the supramolecular plaster. (b) The sterilizing effect of the supramolecular plaster and penicillin on Staphylococcus aureus, Bacillus megaterium, Bacillus thuringiensis, Escherichia coli, and Bacillus pumilus. Buff, 5 mg/mL penicillin; carmine, 5 mg/mL supramolecular plaster; blue, 10 mg/mL supramolecular plaster. The concentrations of the cell of five kinds of bacteria: Staphylococcus aureus, 1.362 × 105 CFU/mL; Bacillus megaterium, 1.462 × 105 CFU/mL; Bacillus thuringiensis, 1.327 × 105 CFU/mL; Escherichia coli, 1.200 × 105 CFU/mL; Bacillus pumilus, 1.616 × 105 CFU/mL.

Staphylococcus aureus, Bacillus thuringiensis, Escherichia coli, and Bacillus pumilus, with the exception of a weaker effect on Bacillus megaterium (See Figure 7). Conclusion In conclusion, we have synthesized and characterized the first supramolecular plaster based on an organic acid-base compound, namely (AEDPH3) · (1,2,4-tzH) · (H2O) (1), which shows a 3D sandwich typed supramolecular network constructed via hydrogen bonds. This supramolecular plaster has excellent plastic and hardenable properties, as well as good mechanical properties similar to the general gypsum plaster. In addition, it has good sterilizing performance. Thus, it can be expected that the supramolecular plaster can be used for building, painting, coating and carving, and the coat, brick, or artware constructed by the supramolecular plaster do not need additive antiseptic or sterilization. The result demonstrates that a supramolecular material based on hydrogen bonded assembly of small molecules

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