Design of Novel Three-Dimensional Coordination Polymers Based on

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Design of Novel Three-Dimensional Coordination Polymers Based on Triangular Trinuclear Copper 1,2,4-Triazolate Units Quan-Guo Zhai, Can-Zhong Lu,* Shu-Mei Chen, Xin-Jiang Xu, and Wen-Bin Yang State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1393-1398

ReceiVed January 9, 2006; ReVised Manuscript ReceiVed March 8, 2006

ABSTRACT: By controlling the molar ratios of copper dichloride dihydrate and 1,2,4-triazole (trz), we generated a series of novel three-dimensional coordination polymers constructed from µ3-oxo or µ3-Cl triangular copper units, exhibiting six-connected porous self-penetrating net, graphite layer, and inorganic-organic hybrid network structures, respectively. Introduction The rational design of coordination polymers constructed from metals and bridging organic ligands is now of interest, due to their intriguing architectures1 and potential application as new materials.2 The utilization of polynuclear metal clusters as building blocks has proved to be a versatile strategy to construct supramolecular coordination frameworks, especially highly connected structures. To date, a large number of coordination polymers that are assembled from polynuclear metal clusters have been explored;3 however, most of the organic components involved are polycarboxylates, which are not ideal candidates for the predictable assembly of coordination polymers from the crystal engineering and crystal structure prediction point of view.4 In fact, the best targets should be in simple topologies by employment of simple ligands and fewer components.2f,5 1,2,4-Triazole and its derivatives, which are usually studied as precursors of compounds with importance in medicine, biology, and industry, have gained more and more attention as ligands to metals due to their interesting bridging modes. Numerous papers on their coordination chemistry have shown that they can lead to a wide variety of compounds, depending upon the nature of the substituents; thus, a large number of polynuclear complexes and coordination polymers containing substituted triazole ligands can be found in the literature.6 However, only a few structures with unsubstituted triazole have been reported, which is most likely due to the fact that this ligand almost always immediately produces insoluble precipitates with nearly all transition-metal ions.6a Hydrothermal techniques, which are widely used in the synthesis of inorganic materials and coordination polymers, can effectively surmount the solubility problem. On the basis of this, taking into account the diverse bridging modes (N1,N2, N2,N4, and N1,N2,N4), we speculate that interesting structures can be obtained with unsubstituted 1,2,4-triazole under hydrothermal conditions. Encouraged by novel 3D hybrid materials based on triazole and polyoxometalates,7 we designed and tuned different conditions to attain a series of copper triazolates using hydrothermal techniques: [Cu3(µ3-O)(µ3-trz)3]2[OH]2‚15H2O (1), Cu[Cu3(µ3O)(µ3-trz)3Cl3] (2), and [Cu2(µ3-trz)(µ3-Cl)2] (3). To our knowledge, no extended framework based on µ3-oxo trinuclear copper 1,2,4-triazolate units has been reported to date. Experimental Section Materials and General Procedures. All of the chemicals were obtained from commercial sources and were used without further * To whom correspondence should be addressed. Tel: 86-591-83705794. Fax: 86-591-83714946. E-mail: [email protected].

purification. The IR spectra (KBr pellets) were recorded on a Magna 750 FT-IR spectrophotometer. C, H, and N elemental analyses were determined on an Elementar Vario EL III elemental analyzer. Thermal stability studies were carried out on a NETSCHZ STA-449C thermoanalyzer under air (30-800 °C range) at a heating rate of 10 °C min-1. X-ray powder diffraction data were recorded on a Rigaku MultiFlex diffractometer with a scan speed of 0.05-0.2° min-1. Magnetic measurements for powder examples of 1-3 were carried out with a Quantum Design PPMS Model 6000 magnetometer. Syntheses. (a) [Cu3(µ3-O)(µ3-trz)3]2[OH]2‚15H2O (1). A mixture of CuCl2‚2H2O (0.17 g, 1.0 mmol) and 1,2,4-triazole (0.069 g, 1.0 mmol) in 10 mL of H2O was introduced into a Parr Teflon-lined stainless steel vessel (25 cm3), after which the vessel was sealed and heated at 180 °C for 5 days under autogenous pressure. The mixture was cooled to room temperature at a rate of 0.5 °C min-1. The resulting blue octahedral crystals of 1 were filtered, washed with H2O, and airdried (yield 45% based on Cu). When CuSO4‚5H2O was used instead of CuCl2‚2H2O under the same hydrothermal conditions, the yield of 1 was increased to 85% (based on Cu). However, the use of Cu(NO3)2‚ 3H2O and Cu(OAC)2‚H2O in our experiments did not lead to the formation of 1. Anal. Calcd for C12H44Cu6N18O19: C, 12.80; H, 3.94; N, 22.39. Found: C, 12.77; H, 3.90; N, 22.41. IR (KBr pellets, λ/cm-1): 3434.64 (s), 1635.00 (s), 1514.69 (s), 1384.48 (m), 1296.08 (m), 1172.64 (m), 1101.68 (s), 1000.82 (m), 882.70 (w), 667.40 (m), 617.52 (w). (b) Cu[Cu3(µ3-O)(µ3-trz)3Cl3] (2). A mixture of CuCl2‚2H2O (0.20 g, 1.2 mmol) and 1,2,4-triazole (0.062 g, 0.9 mmol) in 10 mL of H2O was introduced into a Parr Teflon-lined stainless steel vessel (25 cm3), and then the vessel was sealed and heated at 180 °C for 5 days under autogenous pressure. The mixture was cooled to room temperature at a rate of 0.5 °C min-1. Almost phase-pure dark blue dodecahedral crystals of 2 were collected, washed with H2O, and air-dried (low yield: 20% based on Cu). Anal. Calcd for C6H6Cu4N9OCl3: C, 12.41; H, 1.04; N, 21.71. Found: C, 12.47; H, 0.99; N, 22.72. IR (KBr pellets, λ/cm-1): 3425.34 (s), 3148.10 (w), 1633.86 (m), 1507.90 (vs), 1384.81 (w), 1303.86 (vs), 1206.94 (vs), 1164.22 (m), 1087.40 (vs), 1041.90 (w), 996.65 (m), 908.81 (w), 839.47 (m), 672.83 (m), 432.42 (m). (c) [Cu2(µ3-trz)(µ3-Cl)2] (3). A mixture of CuCl2‚2H2O (0.34 g, 1.0 mmol) and 1,2,4-triazole (0.069 g, 2.0 mmol) in 10 mL of H2O was introduced into a Parr Teflon-lined stainless steel vessel (25 cm3), after which the vessel was sealed and heated at 180 °C for 5 days under autogenous pressure. The mixture was cooled to room temperature at a rate of 0.5 °C min-1. Black prismatic crystals of 3 were collected, washed with H2O, and air-dried (low yield: 25% based on Cu). Anal. Calcd for C2H2Cu2N3Cl2: C, 9.03; H, 0.758; N, 15.79. Found: C, 9.10; H, 0.79; N, 15.72. IR (KBr pellets, λ/cm-1): 3426.00 (s), 3138.59 (w), 1631.21 (m), 1512.19 (s), 1384.55 (w), 1290.02 (s), 11.98.25 (w), 1181.60 (w), 1108.68 (s), 997.85 (m), 868.63 (m), 652.48 (s). X-ray Crystallography. Suitable single crystals of 1-3 were carefully selected under an optical microscope and glued to thin glass fibers. Crystallographic data for all compounds were collected with a Siemens Smart CCD diffractometer with graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å) at T ) 293(2) K. Absorption corrections were made using the SADABS program.8 The structures were solved

10.1021/cg0600142 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/21/2006

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Table 1. Crystal Data and Structure Refinements for 1-3

empirical formula formula wt cryst size (mm 3) cryst descripn cryst syst space group a (Å) b (Å) c (Å) V (Å3) Z F(000) F (Mg/m3) T (K) abs coeff (mm-1) θ for data collecn (deg) reflections collected no. of unique rflns (R(int)) no. of data/params goodness of fit on F2 R1, wR2 (I > 2σ(I))a R1, wR2 (all data) a

1

2

3

C12H44Cu6N18O19 1125.89 0.13 × 0.12 × 0.10 blue octahedron cubic Fd3hc 24.8200(5) 24.8200(5) 24.8200(5) 15 289.9(5) 16 9088 1.956 293 3.371 3.28-27.44 25 813 740 (R(int) ) 0.0619) 740/44 1.010 0.0557, 0.1410 0.0557, 0.1410

C6H6Cu4N9OCl3 580.71 0.10 × 0.08 × 0.04 black dodecahedron hexagonal P63/mmc 11.6208(7) 11.6208(7) 5.9640(8) 697.49(11) 2 560 2.765 293 6.605 3.51-27.46 5216 333 (R(int) ) 0.0552) 333/30 1.085 0.0432, 0.0823 0.0451, 0.0831

C2H2Cu2N3Cl2 266.05 0.20 × 0.10 × 0.10 black prism orthorhombic Imma 6.764(4) 6.981(4) 12.338(7) 582.6(6) 4 508 3.033 293 8.101 3.30-27.47 2224 397 (R(int) ) 0.0187) 397/32 1.087 0.0268, 0.0636 0.0272, 0.0638

R1 ) ∑(|Fo| - |Fc|)/∑|Fo|; wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]0.5.

Figure 1. Trinuclear copper units in 1-3. using direct methods and refined by full-matrix least-squares methods on F2 by using the Shelx-97 program package.9 All non-hydrogen atoms were refined anistropically. Crystal data as well as details of data collection and refinements for 1-3 are summarized in Table 1.

Results and Discussion Preparations of the Complexes. To solve the problem that unsubstituted 1,2,4-triazole immediately produces an insoluble precipitate with transition-metal ions, hydrothermal techniques were used in our experiments. The hydrothermal reaction of CuCl2‚2H2O with the ligand 1,2,4-triazole in a molar ratio of 1:1 at 180 °C produced bright blue octahedral crystals of 1 in about 45% yield. When the amount of CuCl2‚2H2O was gradually increased, a decrease in the yield of 1 was observed and then dark blue dodecahedral crystals of 2 were obtained. A further increase in the amounts of salts led to the formation of black prismatic crystals of 3. The almost phase-pure products were obtained under certain stoichiometric conditions (Cu:trz ) 1:1 for 1, 4:3 for 2, and 2:1 for 3), whereas mixtures were obtained under intermediate conditions. Attempts to increase the amounts of organic ligands only led to microcrystals or precipitates. Structure Description. X-ray single-crystal diffraction analysis reveals that 1 features an extended 3D six-connected selfpenetrating porous structure constructed of triangular CuII subunits (Figure 1). [Cu3(µ3-O)(µ3-trz)3]+ has a Cu3 triangle unit linked by one central µ3-O group; each edge is bridged by a trz

ligand in the N1,N2 bridging mode, and each copper atom is coordinated by a N4 atom of the trz ligand from the adjacent trinuclear unit. The coordination geometry around each CuII ion can be described as ideal square planar with O-Cu-N and N-Cu-N angles of 180°. The Cu-O bond length is 1.9850(9) Å. Cu-N bond lengths are 1.992(5) and 2.009(6) Å. Each Cu atom forms short contacts (Cu-O ) 2.605(4) Å) with two guest water molecules, which lie symmetrically at either side of the trinuclear plane. The µ3-O atom is in a standard planar triangular coordination sphere, with Cu-O-Cu angles of 120°, which is indicative of an sp2 configuration for the oxygen atom. This arrangement holds the Cu atoms at average distance of 3.439(0) Å, and three triazole rings are located in the Cu3 plane to form a rigid 3-fold plane. In most of the reported triangular trinuclear copper structures, the µ3 ligand is OH, which is located above the Cu3 plane in certain distances.6a,10 Each rigid trinuclear copper cluster is interlinked by the other six identical units to generate a turbo-shaped structure (Figure 2a), with an angle between two adjacent triangular planes of 60°. The resulting unique 3D self-penetrating network of a cubic topology is built upon six-connected nodes (Figure 2b). Although a few previously reported six-connected coordination polymers display selfpenetration,11 the structure of 1 is unprecedented because of the novel rigid triangular oxo-centered building blocks and the rare connection modes among them. Just like the metal-organic frameworks reported by Lin and co-workers,11c even after interpenetration, complex 1 still

Coordination Polymers Based on Trinuclear Cu Units

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Figure 2. (a) Building block unit including the asymmetric units present in 1. (b) Schematic illustrating the six-connected self-penetrating net. The trinuclear building blocks are shown as orange balls. (c) Stick representation (left) and space-filling diagram (right) of the tubular channels formed in 1 along the a, b, or c axis. (d) Tubular structure of each channel. A yellow cylinder indicates the tube interior. Color scheme in (a), (c), and (d): Cu, cyan; O, red; N, blue; C, white. Hydrogen atoms and guest H2O or HO- units are omitted for clarity.

presents a tubular framework as viewed from the a, b, or c direction (Figure 2c). The wall of each tube consists of a unit that is formed by four trinuclear clusters connected with each other as shown in Figure 2d. Taking the van der Waals radii into account, the aperture may only admit the passage of a sphere with a 4.5 Å diameter. If this highly open structure is viewed from the [-1,0,1] axis, a larger channel can be seen with cross dimensions of ca. 11.1 × 5.9 Å (Figure S1; Supporting Information). The channels are fully occupied by the severely disordered HO- or H2O guests. The total solventaccessible volume of the channel is approximately 6458.4 Å3, which accounts for 42.2% of the total cell volume. In comparison with the only two reported porous metal-organic frameworks constructed from 1,2,4-triazole ligands, [ZnF(atrz)]‚ guest12 and [Ag6Cl(atrz)4]‚OH‚6H2O13 (atrz ) 3-amino-1,2,4-

triazole), this solvent-accessible volume is the largest. In contrast to a vast number of porous coordination polymers constructed by tetrameric Zn4O clusters,1a only a few examples based on triangular oxo-centered building units have been reported.2c,14 Further, it should be pointed out that examples of triangular trinuclear complexes obtained with 1,2,4-triazole or its derivatives are very rare, though quite a few of them were obtained with pyrazole. To our knowledge, only a few discrete trinuclear complexes with substituted 1,2,4-triazole have been investigated,10 and no extended framework has been reported to date. The crystal structure of 2 shows that it is also constructed from a triangular trinuclear unit (Figure 1), which is similar to that of complex 1 except that each copper atom is coordinated by one terminal Cl atom (Cu-Cl ) 2.347(2) Å, N-Cu-Cl ) 92.94(14)°) instead of the N4 atom; thus, the six-connected unit

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Figure 3. (a) Local coordination geometry of ligands and metals in the structure of 2. (b) The 2D 63 net viewed along the c axis. The trinuclear building blocks are shown as orange balls. (c) 3D graphite structure in 2 linked by Cu‚‚‚Cu short contacts. Color scheme: Cu, cyan; O, red; N, blue; C, white; Cl, green. Hydrogen atoms are omitted for clarity.

becomes a three-connected one (Figure 3a). However, in [Cu3(µ3-O)(µ-pz)3Cl3]2-,15 which has a similar structure, one pyrazole rotates out of the Cu3O plane and the other two pyrazoles are bent out on either side of this plane by 33.0°. The second crystallographically unique copper atom (Cu(2)) coordinates to three N4 atoms from three adjacent trinuclear units, assuming an ideal trigonal-planar coordination geometry (Cu-N ) 1.966(7) Å). The rule of charge balance indicates the valence of Cu(2) to be +2. Although there have been many reports on trigonally coordinated CuI complexes with cyanide ligands or other mixed systems, three-coordinated CuII is very rare. The whole network of 2 exhibits three-connected 2D 63 topology 16 with three-coordinated copper atoms and cyclic trinuclear units as hexagonal nodes (Figure 3b). When Cu‚‚‚ Cu short contacts (2.9820(4) Å) are taken into account as linkers and the Cu(2) atoms act as five-connected nodes, the 2D networks are stacked in an ABAB... fashion to form the layered graphite structure (Figure 3c). As in Cu2(im)3 or Cu3(im)4,17

there exist infinite linear copper chains (Cu-Cu-Cu ) 180°) along the c axis (Figure S2; Supporting Information). For 3, µ3-Cl triangular mixed-valence copper units (Figure 1) form novel 2D inorganic layers (Figure 4a), which are linked by µ3-trz ligands to generate a 3D organic-inorganic hybrid framework (Figure 4b and Figure S4 (Supporting Information)). The 2D inorganic layer can be viewed as two wavy sheets penetrated together along the a direction (Figure S3; Supporting Information). Cu(1) has a trigonal-planar geometry (Cu-N ) 1.931(5) Å; Cu-Cl ) 2.2849(15) Å; N-Cu-Cl ) 128.62(4)°; Cl-Cu-Cl ) 102.76(8)°), which can be assigned a charge of +1 on the base of charge neutrality. The bivalent Cu(2) is a shortened octahedron, N(CuCl4)N, in which two apical positions are occupied by nitrogen atoms (Cu-N ) 1.935(3) Å and CuCl ) 2.5789(13) Å). Although hybrid coordination polymers based on copper halides and aromatic nitrogen-donor ligands are well-known, compound 3 is of interest with respect to the following aspects: (i) it exhibits unique 2D inorganic layers

Coordination Polymers Based on Trinuclear Cu Units

Figure 4. (a) 2D network in 3 constructed from µ3-Cl trinuclear mixedvalance copper units. (b) 3D structure formed by the 2D network and µ3-trz. Color scheme: Cu, cyan; N, blue; C, white; Cl, green. Hydrogen atoms are omitted for clarity.

which are different from those of basic copper halide skeletons18 such as square dimers, cubane tetramers, zigzag chains, doublestranded ladders, hexagonal grid chains, et al.; (ii) it has mixedvalence copper atoms; (iii) it presents the first example of 1,2,4triazoles as organic components in the filed of inorganicorganic hybrid copper halides. TG Analysis, Heating-Cooling and Dehydration-Hydration Experiments. TGA of 1 (Figure S6; Supporting Information) first shows a weight loss (found 24.02%) from 130 to 300 °C, which is attributed to the loss of guest water molecules (calcd 24.48%). The weight loss observed between 300 and 450 °C was slight; after that a sharp weight loss started and ended at 625 °C. The total weight loss in the temperature range 300625 °C was 37.03%, which corresponds to the loss of triazole groups (calcd 36.27%). For 2 and 3, similar weight losses were observed in the temperature range 300-750 °C, which were due to the decomposition of organic components and chlorine (Figure S6). The N2 sorption experiments for 1 showed that only surface adsorption had occurred, indicating that nitrogen molecules could not diffuse into the channels at this temperature. On the other hand, heating-cooling and dehydration-hydration experiments were carried out according to the TGA results and

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monitored by X-ray powder diffraction techniques (XRPD, Figure S7 (Supporting Information)). In comparison to the original crystals, the dehydrated solid obtained by heating 1 at 300 °C for 30 min showed an almost identical XRPD (curves b and e in Figure S7). The loss of water caused 1 to change color from bright blue to cyan. When 1 was heated to 350 °C, some of the characteristic peaks became weak or disappeared, but the intensity of the lines could be recovered after this dehydrated solid was immersed in water (curve i in Figure S7), which suggests that the dehydration of 1 is reversible. Further heating at 400 and 450 °C led to a severe disappearance of characteristic peaks, indicating that the decomposition had begun. When this finding is combined with the slight weight loss between 300 and 450 °C, it can be concluded that the [Cu3(µ3-O)(µ3-trz)3]+ framework is rather stable, which may be attributed to the whole rigid building blocks and the selfpenetrating nets. It should be pointed out that, though a high thermal stability of porous metal-organic frameworks is the most desirable quality for application purposes, it has been rarely achieved to date.2,13,14 ESR and Magnetic Properties. The ESR spectra of 1-3 (Figure S8a; Supporting Information) were recorded on polycrystalline samples at room temperature. The following parameters were obtained: g ) 2.1519 for 1, g ) 2.1002 for 2, and g ) 2.0060 for 3. The temperature-dependent molar magnetic susceptibility for three complexes is measured at 5 kOe in the temperature range 2-300 K (Figure S8). The χMT values for 1 and 2 at room temperature are 0.508 and 0.571 cm3 K mol-1, which are much lower than the valued expected for three (for 1) or four (for 2) uncoupled copper(II) ions. With decreasing temperature, the gradually decreasing χMT value indicates the presence of antiferromagnetic interactions. The magnetic data obey the Curie-Weiss law in the temperature region (20-300 K for 1 and 2-300 K for 2), and fitting in the range gives values of C ) 0.5133 cm3 K mol-1, Θ ) -57.65 K and C ) 0.4373 cm3 K mol-1, Θ ) -6.373 K, respectively. It should be noted that there exists a small “plateau” in the χM versus T curve of 1 at 0.0053 cm3 mol-1 in the range 12-17 K. A similar result has been founded in the magnetic behavior of Na3[Co6(OH)(C8H4O4)6]‚H2O,19 which contains µ3-O triangular cobalt units. The “plateau” indicates that there exists a residual moment, which may be due to the stabilization of an uncompensated moment by the frustration effect in the triangular motifs.19 For 3, antiferromagnetic behavior is observed down to 5 K, and a further decrease of the temperature causes a sharp increase of χMT, suggesting the presence of ferromagnetic interactions. The magnetic data in the range of 5-300 K were well fitted to the Curie-Weiss law with C ) 0.3692 cm3 K mol-1 and Θ ) -9.066 K, indicating antiferromagnetic interactions between Cu2+ ions. Further study on the magnetic properties is underway. Conclusions Three novel 3D coordination polymers exhibiting different topological structures have been successfully synthesized and characterized, which are all constructed by triangular trinuclear copper units. This is the first time that trinuclear µ3-O subunits have been obtained with unsubstituted triazole ligands. The isolation of these complexes demonstrates that using hydrothermal techniques can effectively surmount the solubility problem of metal-1,2,4-triazolate systems, as expected. On the basis of this work, further synthesis and structural studies of other transition metals with unsubstituted 1,2,4-triazole are under way in our laboratory.

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Acknowledgment. This work was supported by the 973 program of the MOST (Grant No. 001CB108906), the National Science Foudation of China (Grant Nos. 20425313, 90206040, 20521101, 20333070, and 20303021), the Natural Science Foundation of Fujian Province (Grant No. 2005HZ01-1), and the Chinese Academy of Science. Supporting Information Available: X-ray crystallographic data as a CIF file, tables of bond lengths and angles, and figures giving additional plots of the structures, infrared spectra, thermogravimetric analysis, XRPD patterns for 1, ESR spectra, and magnetic results for 1-3. This material is available free of charge via the Internet at http:// pubs.acs.org.

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