Synthesis, Structure, Water-Induced Reversible Crystal-to-Amorphous

Mar 21, 2007 - Ran Sun, Yi-Zhi Li, Junfeng Bai,* and Yi Pan*. State Key Laboratory of Coordination Chemistry & School of Chemistry and Chemical ...
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CRYSTAL GROWTH & DESIGN

Synthesis, Structure, Water-Induced Reversible Crystal-to-Amorphous Transformation, and Luminescence Properties of Novel Cationic Spacer-Filled 3D Transition Metal Supramolecular Frameworks from N,N′,N′′-Tris(carboxymethyl)-1,3,5-benzenetricarboxamide

2007 VOL. 7, NO. 5 890-894

Ran Sun, Yi-Zhi Li, Junfeng Bai,* and Yi Pan* State Key Laboratory of Coordination Chemistry & School of Chemistry and Chemical Engineering, Nanjing UniVersity, Nanjing 210093, P. R. China ReceiVed September 20, 2006; ReVised Manuscript ReceiVed January 25, 2007

ABSTRACT: Four novel cationic spacer-filled 3D transition metal supramolecular frameworks from N,N′,N′′-tris(carboxymethyl)1,3,5- benzenetricarboxamide (L), {[ML(H2O)3]2[M(H2O)6](H2O)3}n (M ) Zn, 1; Mn, 2; Ni, 3; Co, 4), have been synthesized hydrothermally. X-ray single-crystal diffraction studies confirm that these complexes are isostructural and crystallize in the hexagonal crystal system, with the space group being P3h1c. Each metal ion coordinates to three ligands and each ligand L bridges three metal ions as well, resulting in an infinite 2D anionic (6, 3) layer with large honeycomblike cavities. Interestingly, two adjacent layers are staggered in such a way that non-interepenetrating double-layer structures are formed. Such double layers are separated by the layer of hexaqua-metal cations through O-H‚‚‚O hydrogen bonds between the -CONH- groups and the coordinated aqua molecules. Four complexes could be considered as dynamic molecular solids and exhibit interesting water-induced reversible crystal-to-amorphous transformation properties that are confirmed by TGA and XRD studies. In addition, among them, complex 1 shows a strong blueemitting fluorescence emission band at 457 nm at room temperature and may be a potential candidate for blue luminescence materials. Introduction Recently, metal-organic frameworks (MOFs) have attracted great attention1 because of their intriguing topologies2 such as molecular grids, bricks, herringbones, ladders, rings, boxes, diamondoids, and honeycombs and potential properties such as adsorption, catalysis, guest exchange, magnetism, and optoelectronics.3 In particular, the utilization of highly symmetrical multitopic ligands with N or O donor atoms to construct supramolecular structures is of higher interest.4 Among them, the rigid ligands can lead to predictable architectures of the resulting compounds, the diversity of which has been limited and to some extent, few or even no conformational changes can be observed for these kinds of ligands when reacted with metal ions. By contrast, flexible ligands can adopt different conformations according to the geometric requirements of different metal ions to construct abundant and interesting supramolecular architectures. Explorations on their reactivities and coordination chemistry have been very limited up to now and the investigation of metaln-organic frameworks based on them is still a great challenge.5 We are interested in the utilization of highly symmetric ligands to self-assemble coordination molecular solids with interesting structures and properties. On the basis of [Cp*Fe(η5-P5)],6 we have synthesized inorganic-fullerene-like molecules and 1D or 2D coordination polymers. More recently, we have reported 2D and 3D cadmium(II) complexes with unusual three-, eight-, and ten-connected topologies based on a flexible tripodal ligand containing the -OCH2- group, 1,3,5-tri(carboxymethyl)benzene (TCMB).7 To expand our investigations in this field, we turned our attention to an unexplored tripodal ligand, N,N′,N′′-tris(carboxymethyl)-1,3,5-benzenetricarboxamide (Scheme 1) on the basis of the following considerations: first, in comparison with the most intensively investigated rigid ligand, * Correspondingauthor.E-mail: [email protected](J.B.);[email protected] (Y.P.).

Scheme 1. Structure of N,N′,N′′-tris(carboxymethyl)-1,3,5Benzenetricarboxamide

1,3,5-benzenetricarboxylic acid (BTC), the introduction of the amide groups can increase the flexibility of the entire structure because of its ability to freely rotate; second, the amide groups can lead to the formation of hydrogen bonds and generate dynamic molecular solids.8 Herein, we report four novel complexes with the formula of {[ML(H2O)3]2[M(H2O)6](H2O)3}n (M ) Zn, 1; Mn, 2; Ni, 3; Co, 4) based on the ligand N,N′,N′′-tris(carboxymethyl)-1,3,5benzenetricarboxamide that exhibit interesting 3D compact networks. Such networks are composed of 2D non-interpenetrating double anionic layers with (6, 3) topology that are separated by layers of hexaqua metal cations {[M(H2O)6]2+}n through hydrogen bonds between the carboxyl groups and the coordinated aqua molecules. Furthermore, their water-induced crystalto-amorphous transformation and luminescence properties were also investigated. Experimental Section Materials and Methods. All chemicals were used as purchased without further purification. The ligand L was directly prepared from 1,3,5-benzenetricarbonyl trichloride and glycine. Microelemental analyses were carried out with a PE-2400CHN analyzer. The IR spectra were recorded on a Bruker VECTOR22 FT-IR spectrometer using the KBr pellet technique. Fluores-

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N,N′,N′′-Tris(carboxymethyl)-1,3,5-benzenetricarboxamide

cence measurements were performed on an AMINCO-BOWMAN Series AB2 luminescence spectrometer. The as-synthesized samples were characterized by thermogravimetric analysis (TGA) on a thermoflex analyzer (Rigaku) up to 1073 K using a heating rate of 20 K min-1 for ∼10 mg samples open to air. XRD measurements were performed on a Bruker D8 Advance X-ray diffractometer using Cu KR radiation (0.15418 nm), in which the X-ray tube was operated at 35 kV and 20 mA. Synthesis of {[ML(H2O)3]2[M(H2O)6](H2O)3}n. (M ) Zn, 1; Mn, 2; Ni, 3; Co, 4). The same procedure was employed to prepare all complexes and hence only the zinc compound is described here in detail. A mixture of ligand L (380 mg, 0.1 mmol), Zn(NO3)2‚6H2O (290 mg, 0.1 mmol), and H2O/CH3OH (v/v 10:1; 10 mL) was sealed in a Parr Teflon-lined stainless steel vessel and heated to 100 °C for 2 days and cooled to room temperature in 12 h. Block crystals were isolated by filtration and washed with distilled water and dried in air to give the product. Yield: 75% (based on Zn(NO3)2‚6H2O). Anal. Calcd: C, 29.61; H, 3.94; N, 6.91. Found: C, 29.98; H, 4.16; N, 6.19. Main IR frequencies (KBr pellet, cm-1): 3332 (s, br), 1635 (s), 1534 (s), 1425 (m), 1388 (s), 1285 (s), 1153 (vw), 1047 (w), 1027 (w), 763 (w), 700 (w), 607 (w), 560 (w). For {[MnL(H2O)3]2[Mn(H2O)6](H2O)3}n, yield: 70% (based on Mn(OAc)2‚4H2O). Anal. Calcd: C, 30.39; H, 4.50; N, 7.08. Found: C, 29.50; H, 4.41; N, 6.92. Main IR frequencies (KBr pellet, cm-1): 3364 (s, br), 1650 (m), 1595 (s), 1530 (s), 1432 (m), 1401 (m), 1297 (s), 1263 (m), 1030 (w), 755 (w), 713 (w), 598 (w), 560 (w). For {[NiL(H2O)3]2[Ni(H2O)6](H2O)3}n, yield: 78% (based on Ni(NO3)2‚6H2O). Anal. Calcd: C, 30.10; H, 4.01; N, 7.02. Found: C, 29.35; H, 4.25; N, 6.94. Main IR frequencies (KBr pellet, cm-1): 3369 (s, br), 1652 (m), 1597 (s), 1529 (s), 1431 (m), 1404 (m), 1297 (s), 1263 (m), 1030 (w), 752 (w), 711 (w), 599 (w), 560 (w). For {[CoL(H2O)3]2[Co(H2O)6](H2O)3}n, yield: 70% (based on Co(NO3)2‚6H2O). Anal. Calcd: C, 30.09; H, 4.01; N, 7.02. Found: C, 29.75; H, 4.19; N, 6.90. Main IR frequencies (KBr pellet, cm-1): 3365 (s, br), 1650 (m), 1595 (s), 1531 (s), 1430 (m), 1402 (m), 1297 (s), 1263 (m), 1030 (w), 754 (w), 714 (w), 597 (w), 559 (w). X-ray Data Collection and Structure Determination. Suitable single crystals of 1-4 were mounted on a pyrex fiber with epoxy and affixed to a brass pin for X-ray data collection. The intensity data were collected at room temperature on a Bruker Smart Apex CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). The data collection covered over a hemisphere of reciprocal space by a combination of three sets of exposures, each set had a different φ angle (0, 88, and 180°) for the crystal, and each exposure of 30 s covered 0.3 in ω. The crystal to detector distance was 4 cm and the detector swing angle was -35° to give a data set more than 99% complete. The 30 initial frames were recollected at the end of data collection to monitor crystal decay via analysis of the duplicate reflections, and no significant decay was observed. The raw data were reduced and corrected for Lorentz and polarization effects using the SAINT program and for absorption using the SADABS program. The structures were solved by direct methods using SHELXS 97, and the all nonhydrogen atoms were refined anisotropically by full-matrix leastsquares based on F2 values.9 The largest residual density peak is close to the lanthanide atom. Hydrogen atoms were added theoretically. CCDC 615831(for 1), 615832(for 2), 615833(for 3), and 615834(for 4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from

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Figure 1. Coordination environment of Zn2+ centers in complex 1 with an atom labeling scheme (thermal ellipsoids are drawn at the 30% probability level) in which hydrogen atoms and solvent water molecules are omitted for clarity.

The Cambridge Crystallographic Data Center via http://www.ccdc.cam.ac.uk/data_request/cif. Results and Discussion Synthesis and FT-IR characterization. All compounds are in well-shaped purity, and high-quality single crystals suitable for X-ray diffraction were obtained by the direct reaction of corresponding divalent transition metal salts with ligand L by the hydrothermal method. These crystalline solids are stable in air and insoluble in water or common organic solvents such as methanol and ethanol. The solvent may also influence the process of assembly, and it has been proven that the addition of cosolvent CH3OH helps to the formation of the complexes. The IR spectra of the four compounds show strong absorption bands between 1402 and 1650 cm-1 that can be assigned to the coordinated carboxylate groups,10 and the absence of the strong carboxyl absorption band at 1722 cm-1 indicates the complete deprotonation of L. Structure Description. X-ray diffraction studies reveal that all complexes are isostructural; the crystal and data collection parameters are summarized in Table S1 of the Supporting Information and the selected bond lengths and angles are listed in Table S2. {[ZnL(H2O)3]2[Zn(H2O)6](H2O)3}n (1) is described here representatively. Complex 1 exhibits a three-dimensional supramolecular network containing {[ZnL(H2O)3]2-}n as the host anionic 2D layers and {[Zn(H2O)6]2+}n as the sandwich cation spacer layers. As shown in Figure 1, there are two crystallographically independent Zn(II) ions (Zn1 and Zn2) in the fundamental building unit. Three monodentate carboxylate groups of three different individual ligands L and three oxygen atoms from water molecules (O1w, O1wAi O1wBii) (symmetry code: i, 1 - x + y, 1 - x, z; ii, 1 - y, x - y, z) are bonded to Zn1, resulting in a slightly distorted octahedral coordination geometry. The Zn-O bond distance is 2.082 (2) Å, whereas the Zn1Ow bond length is 2.120 (3) Å. The O-Zn1-O bond angles range from 89.81 to 176.01°. Zn2 is surrounded by six water molecules and hence a hexaqua cation is formed with the Zn2O2w distances being 2.095(2) Å and the O2w-Zn2-O2w bond angles ranging from 89.63 to 179.59°, which are close to those of the ideal octahedron. All these distances and angles are comparable to the reported values.11

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Figure 2. (a) Perspective view of the 2D anionic layer with the honeycomb structure (black lines) in the ab plane. (b) Schematic view of the buckled two layers of the 2D honeycomb network.

Each ligand L adopts a cis,cis,cis-conformation to bridge three Zn1 atoms via three terminal carboxylate groups through C3 symmetry in monodentate coordination mode. The three metal centers are in an equilateral triangle with an edge length (Zn1‚‚‚Zn1 separation) of 8.322(2) Å. Such unit repeats along the ab plane to generate an infinite 2D (6, 3) anionic honeycomb network, which contains large edge-sharing hexagons, with the metal ion and the geometrical center of the phenyl ring of ligand L being the vertex alternately and each arm of L being the edge (Figure 2a).12 The distance of the hexagons located at the opposite metal centers is 13.416 Å. Generally, interpenetrating structures are easily formed for large dimensional cavities.13 However, in our case, two such nets (6, 3) are buckled into each other with the metal atoms of one layer located in the middle of the other, resulting in novel double-layer structures, eschewing interpenetration (Figure 2b).14 The net (6, 3) do not lie in the same plane, and the Zn center and the center of phenyl ring are located “above” and “below” the plane alternately, representing a chair conformation. As expected, different weak interactions may be responsible for this special stacking (Figure 3). First, the independent layers form double layers by the faceto-face π‚‚‚π interactions between the phenyl rings with a distance of 3.711 Å. Second, the amide group is hydrogenbonded to the nonchelating carboxylate oxygen atom of a neighboring layer (dN1‚‚‚O2 ) 2.894(5) Å). Additionally, the 2D double adjacent buckled anionic layers are stacked parallel along the crystallographic c-axis with a distance of 3.73 Å between the aromatic rings of the adjacent layers. It is noteworthy that the rotation angle from one double layer to the next along π-stacking is (30° and the centers of phenyl rings are overlapped. To the best of our knowledge, in most of the reported complexes with such offset manner, a few examples with the rotation angle from one molecule to the next along π-stacking have been found.15 They do not form extended stacks in the double layer offset manner. More interestingly, the layers of {[Zn(H2O)6]2+}n are located among the neighboring bilayers of (6, 3) network through O-H‚‚‚O hydrogen bonding between the coordinated aqua molecules and carboxylate oxygen atoms with the O2w‚‚‚O1 and O2w‚‚‚O3 distance of 2.814(3) and 2.715(3) Å, respectively, making the whole structure into a rare three-dimensional framework16 (Figure 4). Moreover, except the cations, disordered water molecules are also located in the voids between the adjacent anionic double-layer. Details of hydrogen-bonding parameters are summarized in Table S3. Although complexes 1-4 have the same supramolecular structures, the metal-oxygen bond distances and their angles vary slightly corresponding to the radii of the metal ions. Water-Induced Reversible Crystal-to-Amorphous Transformation Properties. Similar to examples reported by Kita-

Figure 3. (a) Formation of the 2D double layers in complex 1 by face-to-face π···π interactions (dotted lines). (b) Two-dimensional double layer for complex 1 showing N-H‚‚‚O hydrogen bonds represented as white dash lines.

gawa’s groups,17 the amide groups in ligand L play an important role in the organization of these interesting supramolecular solids and make them dynamic molecular solids exhibiting a waterinduced reversible crystal-to-amorphous transformation, which were further confirmed by their dehydration and rehydration behaviors using thermogravimetric and X-ray power diffraction analysis. For complex 1, the first weight loss of 20.6% from 25 to 130 °C is attributed to the release of all the water molecules, including the crystallized and coordinated water molecules (Anal. Calcd, 22.2; found, 20.6), in which the gray residue was confirmed with the dehydrated amorphous form of [Zn3L2]n by elemental analysis and the powder X-ray diffraction analysis. The second weight loss covering a temperature range from 280 to 480 °C is supposed to be the release of the coordinated ligand. Inversely, the amorphous powder of [Zn3L2]n obtained from heating complex 1 to 130°C for 2 h can be restored to the crystalline form of {[ZnL(H2O)3]2[Zn(H2O)6](H2O)3}n after being soaked in water for 24 h, which was also confirmed by the elemental analysis (Anal. Calcd: C, 37.82;

N,N′,N′′-Tris(carboxymethyl)-1,3,5-benzenetricarboxamide

Crystal Growth & Design, Vol. 7, No. 5, 2007 893

Figure 6. X-ray powder diffraction of complex 1. (a) Simulated for 1; (b) measured for 1; (c) [Zn2L3]n; (d) restored {[ZnL(H2O)3]2[Zn(H2O)6](H2O)3}n from [Zn2L3]n.

Figure 4. (a) Packing of the 3D framework of complex 1 showing the honeycomb structure stacking viewed from the b-axis. (O, red; C, black; N, dark blue; Zn, green). (b) Packing of the 3D framework of complex 1 in which {ZnO6} octahedrons and [Zn(H2O)6]2+ octahedrons in double layers are specifically represented (anion layers, red and green; cation layers, yellow).

Figure 5. Emission spectra of ligand L (a) and complex 1-4 in solid state at room temperature.

H, 2.54; N, 8.82. Found: C, 37.11; H, 2.83; N, 8.60) and XRD analysis (Figure 6). This indicates that the amorphous [Zn3L2]n may act as a new absorbent for water with the absorptive capacity of 20.6% (w/w). The similar behaviors of dehydration and rehydration were also found in complexes 2-4. The amorphous [Mn3L2]n, [Ni3L2]n, and [Co3L2]n possess absorptive capacities for water of 20.5, 22.5, and 21.7%, respectively. Photoluminescence Property. The luminescence properties of ligand L and all complexes were investigated in the solid

state. Complex 1 exhibits a broad blue fluorescence emission band at 457 nm (λex ) 365 nm) that is red-shifted about 10 nm with an increase in luminescence intensity compared with that of the ligand L (Figure 5). The cause of red-shifted fluorescent emission of complex 1 is neither ligand-to-metal charge transfer (LMCT) nor metal-to-ligand charge transfer (MLCT) in nature and may be assigned to the intraligand fluorescence emission because a broad fluorescence emission band at 467 nm (λex ) 360 nm) for the free ligand was observed.18 The fluorescent intensity of complex 1 is stronger than that of the free ligand, probably because of the enhancement of the rigidity of the ligand in 1 compared with that of the free ligand, thereby reducing the nonradiative decay of the intraligand π-π* excited state. The strong fluorescence emission of complex 1 makes it a potentially useful photoactive material. However, this fluorescence emission has been quenched in complexes 2-4. Conclusion Four novel cationic spacer filled 3D transition metal supramolecular framewoks {[ML(H2O)3]2[M(H2O)6](H2O)3}n (M ) Zn, 1; Mn, 2; Ni, 3; Co, 4) from a new tripodal and amidecontaining ligand, N,N′,N′′-tris(carboxymethyl)-1,3,5-benzenetricarboxamide, have been synthesized under hydrorthermal condition at 100 °C. These supramolecular coordination polymers are isostructural and exhibit rare 3D compact networks composed of 2D noninterpenetrating double anionic layers with a topology of (6, 3) linked by the hexaqua metal cation {[M(H2O)6]2+}n layers through hydrogen bonds. Further investigations with thermogravimetric analysis, elemental analysis, and powder X-ray diffraction methods indicate that the transformation from the crystal form of {[ML(H2O)3]2[M(H2O)6](H2O)3}n to the amorphous powder of [M3L2]n is reversible, and the dried amorphous powder of [M3L2]n may be applied to the absorption of water and water vapor. Moreover, among them, complex 1 exhibits interesting luminescence properties and may be a potential candidate for blue luminescence materials. In summary, our research demonstrates for the first time that ligand L could be as a potential building block to construct novel dynamic supramolecular architectures with interesting properties. The introduction of the amide groups into the high symmetric ligands could facilitate the formation of higher dimensional supramolecular structures. Acknowledgment. This work is supported by National Natural Science Foundation of China (No. 20301010), the Natural Science Foundation of Jiangsu Province, 21st Century Talent Foundation of the Ministry of Education (TCTFME), Foundation for the Returnee of the Ministry of Education (FRME), and Measurement Foundation of Nanjing University.

894 Crystal Growth & Design, Vol. 7, No. 5, 2007 Supporting Information Available: Tables S1-S6; packing diagrams, TGA and DTG curves, X-ray powder diffraction patterns. This material is available free of charge via the Internet at http:// pubs.acs.org.

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