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Construction of a 3D supramolecular network from 2D coordination layers via anion–π interactions and its catalytic properties
Inorganic Chemistry Communications 24 (2012) 129–133
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Feature article
Construction of a 3D supramolecular network from 2D coordination layers via anion–π interactions and its catalytic properties Pongthipun Phuengphai a, b, d, Sujittra Youngme b,⁎, Ilpo Mutikainen c, Jan Reedijk d, e a
Department of Fundamental Science, Faculty of Science and Technology, Surindra Rajabhat University, Surin 32000, Thailand Materials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55 (A. I. Virtasen aukio 1), 00014 University of Helsinki, Finland d Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands e Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia b c
a r t i c l e
i n f o
Article history: Received 13 March 2012 Accepted 9 August 2012 Available online 21 August 2012 Keywords: Porous coordination polymer Heterogeneous catalyst Dynamic structural transformation Copper
a b s t r a c t A new porous coordination polymer, i.e. the two-dimensional layered network {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2] (BF4)2(H2O)6}n has been synthesized from Cu(BF4)2·6H2O, 4,4′-bpy and with formate as co-ligand. Single-crystal X-ray diffraction analysis reveals a (4,4) 2D coordination network exhibiting voids that contain the counter-ions and water guest molecules. In addition, the title compound is further stabilized by hydrogen bonds and anion–π interactions to form an intricate supramolecular framework. The removal and reintroduction of guest water molecules has been explored to better understand the dynamic structural transformation. The coordination compound shows weak catalytic activity in the cyanosilylation of aldehydes. © 2012 Published by Elsevier B.V. All rights reserved.
Contents
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The design and synthesis of extended frameworks via supramolecular interactions represent a new area of considerable interest [1,2]. In particular, hydrogen bonding, CH–π, π–π and anion–π have been exploited for molecular recognition associated with biological activity, and for engineering of molecular solids [3–5]. In the vast amount of the reported work, a variety of 1D, 2D, and 3D molecular open frameworks have been successfully obtained via the use of coordinating functional groups, such as carboxylates, 4,4′-bpy or its derivatives, and mixtures of both carboxylate and 4,4′-bpy ligands [6–9]. For example, Kitagawa et al., successfully constructed a series of porous Cu(II)/4,4′-bpy coordination polymers via addition of different anions [10]. In coordination networks in general the 4,4′-bpy ligands may act in bidentate bridging, or monodentate terminal modes, resulting in 1D linear, zigzag, ladder, molecular antenna railroads and chains, 2D bilayer, square and rectangular grid networks, or 3D non-interpenetrated and interpenetrated networks. In our previous ⁎ Corresponding author. Fax: +66 43 243 338. E-mail address: [email protected] (S. Youngme). 1387-7003/$ – see front matter © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2012.08.002
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work, we have synthesized a number of porous Zn(II)/4,4′-bpy/ carboxylato coordination polymers, by varying different anions [11]. These compounds exhibited the structural diversity and have been explored for their properties. In the case of copper(II) carboxylato-bridged porous coordination polymers only a few catalytic applications have been reported [12]. In the present study, we report the synthesis and crystal structure of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n [13], and its 3D hydrogen-bonded and anion–π architecture. The dynamic structural transformation and heterogeneous catalytic activities in cyanosilylation have been investigated [14]. In our previous work, we primarily focused on the design and synthesis of copper(II) compounds based on mixed 4,4′-bpy and monocarboxylato ligands [15]. We have discussed the effect of the steric hindrance of different carboxylato ligands (formato and acetato ligands), as well as the influence of the nature of the anion on the solid-state structure of the corresponding coordination materials. The (4,4) 2D structure of title compound is very similar to {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](ClO4)2(H2O)6}n which has been
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reported [15]. The simple replacement of perchlorate to tetrafluoridoborate during the synthetic procedure, however, results in significant structural changes in the anion–π interactions, which probably have an important role in crystal dynamics and structural transformation of the resulting coordination compound {[Cu3(4,4′bpy)3(μ-OOCH)4(H2O)2] (BF4)2(H2O)6}n [16]. As shown in Fig. 1, the new compound contains two different types of copper(II) ions, each having an elongated octahedral N2O4 coordination environment. The basal plane of the octahedron of Cu1 is formed by two oxygen atoms belonging to two formato ligands (O1 and O5a) and two 4,4′-bpy nitrogen atoms (N11 and N21). The octahedral geometry is completed at the axial positions by two oxygen atoms from two formato ligands (O3b and O5), with Cu\O distances of 2.587(2) and 2.361(2) Å, respectively. The Cu\O and Cu\N bond distances [17] can be considered as normal for this type of CuN2O4 coordination environment. As a crystallographic inversion center is located in the center of the four-membered Cu1-O5-Cu1A-O5a ring, Cu1 is bridged to symmetry-related Cu1A ions via two monoatomic bridged μ-O,O′formato ligands (O5 and O5a) and to Cu2 ions via one μ-O,O′,O′formato ligand. This spatial arrangement produces a linear trinuclear [Cu1A, Cu1, Cu2] cluster. These tricopper units are linked via μ-formato-κO bridges, producing a 1D polymeric chain and each chain is connected through 4,4′-bpy ligands to generate a 2D layer in the crystallographic b direction (blue, orange and green 4,4′-bpy ligands in Fig. 2). The coordination environment around the Cu2 ion is best described as an elongated octahedron, whose basal angles, varying from 89.8(1) to 90.1(1)°, are close to the ideal value of 90°. This basal plane contains two water molecules (O4 and O4b) and two nitrogen atoms (N31 and N41) belonging to two 4,4′-bpy ligands. The axial positions are occupied by two oxygen atoms (O3 and O3b) from different μ-O,O′,O′-formato ligands at common distances (Cu2\O distance = 2.297(2) Å). The 1D chains of {Cu2, Cu1, Cu1A} units are connected to each other through 4,4′-bpy ligands, leading to the formation of a (4,4)
Fig. 1. Representation of the linear trinuclear unit of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2] (BF4)2(H2O)6}n. The tetrafluoridoborate anions, the lattice water molecules and the H atoms are not shown for clarity. Selected bond distances (Å) and angles (°):Cu(1)\O(1) 1.988(2), Cu(1)\O(5a) 1.993(2), Cu(1)\N(11) 2.006(3), Cu(1)\N(21) 2.023(3), Cu(1)\O(5) 2.361(2), Cu(1)\O(3b) 2.587(2), Cu(2)\N(31) 2.015(3), Cu(2)\O(4) 2.016(2), Cu(2)\N(41) 2.031(4), Cu(2)\O(3) 2.297(2), O(1)\Cu(1)–O(5a) 178.3(1), O(1)\Cu(1)\N(11) 89.7(1), O(5a)\Cu(1)\N(11) 90.7(1), O(1)\Cu(1)\N(21) 87.9(1), O(5a)\Cu(1)\N(21) 91.7(1), N(11)\Cu(1)\N(21) 175.3(1), O(1)\Cu(1)\ O(5) 99.5(1), O(5a)\Cu(1)\O(5) 78.8(1), N(11)\Cu(1)\O(5) 91.3(1), N(21)\ Cu(1)\O(5) 93.0(1), N(31)\Cu(2)\O(4) 89.8(1), O(4b)\Cu(2)\O(4) 179.7(1), N(31)\Cu(2)\N(41) 180.0(1), O(4)\Cu(2)\N(41) 90.1(1), O(4b)\Cu(2)\O(3b) 91.7(1), O(4)\Cu(2)\O(3b) 88.2(1), N(31)\Cu(2)\O(3) 95.0(1), O(4)\Cu(2)\O(3) 91.7(1), N(41)–Cu(2)–O(3) 84.9(1), O(3b)\Cu(2)\O(3) 169.9(1). Symmetry operations: (a) 1−x, y, 0.5−z; (b) −x, y, 0.5−z.
Fig. 2. 2D coordination network of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n. The 4,4′-bpy ligands connecting the trinuclear chain secondary building units are shown in blue, orange and green.
2D sheets (Figs. 2 and 3). The linear strands containing the metal centers Cu1A and Cu1 interact via π–π contacts between two 4,4′-bpy units (centroid-to-centroid distances ranging from 3.862(2) to 4.130(2) Å; blue and orange 4,4′-bpy ligands in Fig. 2). The two coordinated water molecules (O4 and O4b in Fig. 1) are strongly interacting with an intramolecular formato bridge (O4···O6 = 2.678(2) Å) and are hydrogen-bonded to lattice water molecules (O4···O6W = 2.735(2) Å), which are located between the closely packed 2D layers. The fascinating crystal packing of the solid-state structure is shown in Fig. 4. Indeed, both tetrafluoridoborate ions are involved in weak intermolecular anion–π interactions [18] with two 4,4′-bpy rings from adjacent chains, quite similar to that observed in the analogous perchlorate [15]. The difference is that the tetrafluoridoborates adopt two crystallographic orientations in the crystal lattice with occupancy factors of 0.7 and 0.3, while such a disorder was not observed in the case of perchlorate [15]. The two crystallographically independent BF4− ions are labeled {B1F11–F14} and {B2F21–F24}. As evidenced in Fig. 4, the fluorine atom F12 is quite strongly interacting with an intermolecular pyridine ring resulting the F12···centroid distance of 3.474(6) Å. The fluorine atom F13 is also in contact with the pyridine ring (F13···centroid ring= 3.378(6)Å). Moreover, the second tetrafluoridoborate orientation experiences anion–π contacts as well. The fluorine atom F23 shows significant interactions with the intermolecular pyridine ring (Fig. 4; F23···centroid ring= 3.159(7) Å. In addition, F24 is in contact with the intermolecular pyridine ring (F24···centroid ring= 3.958(8) Å. Furthermore, each
Fig. 3. 2D coordination network of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n, constituted of (4,4) 2D sheets coordination chains that are connected by formato ligands.
P. Phuengphai et al. / Inorganic Chemistry Communications 24 (2012) 129–133
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Table 1 Elemental analyses of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n and of the products after removal and after reintroduction of the guest water molecules. Element Exp.
C H N
Fig. 4. Crystal packing of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n illustrating the anion–π contacts between the tetrafluoridoborate ion and the pyridine rings.
tetrafluoridoborate ion is hydrogen-bonded to lattice water molecules (F12···O8W =2.854(2), F14···O9W= 2.917(2) and F21···O7W = 2.920(2) Å), which are located between the closely packed 2D layers. The occurrences of these non-covalent contacts result in the formation of a three-dimensional supramolecular structure (Fig. 5). Seven related coordination polymers, that contain copper(II) ions, 4,4′-bpy ligands and bridging monocarboxylato co-ligands, have been reported in the literature. In these compounds, the copper(II) ions are connected by carboxylato bridges, yielding the three-dimensional structure [Cu(OOCH)2(4,4′-bpy)]n (I) [19], the two-dimensional framework {[Cu2(4,4′-bpy)(OOCCH3)(OH)(H2O)](H2O)2(SiF6)}n (II) [20], and the one-dimensional polymers {[Cu(4,4′-bpy)(OOCCH3)2] (H2O)2.5}n (III) [21], {[Cu2(4,4′-bpy)2(μ-OOCCH3)2(OOCCH3)2](H2O}n (IV) [22], {[Cu(4,4′-bpy)(OOCCH3)2](H2O)3}n (V) [23,24], {[Cu2(4,4′bpy)(OOCCH3)4](CH3CN)}n (VI) [25], {[Cu2(4,4′-bpy)(OOCCH3)4] (DMF)}n (VII) [26]. Among these seven compounds described earlier, the formato-containing compound I displays a 3D structure, while in the case of our new compound, a 2D network is observed. Only II, containing acetato co-ligands, exhibits a double-stranded 2D sheet architecture, similar to the BF4- compound. The remaining five compounds exhibit double-stranded 1D polymeric chains for III–V, and 1D polymeric chains for VI and VII. It is interesting to note that compounds VI and VII are isostructural. In fact, the sole difference between them lies in the distinct guest solvent molecules, namely CH3CN and DMF. Hence, it appears that the nature of these two solvents does not affect the overall structure of the network. The diffuse reflectance electronic spectrum of the title compound displays a broad peak centered at 15,800 cm −1 and a poorly resolved shoulder at approximately 12,500 cm −1. This feature agrees with an elongated octahedral geometry with an off-z-axis distortion. The
Fig. 5. Packing diagram with hydrogen bond (dash-lines), showing the threedimensional structure of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n, the guest molecules (space-filling mode) filling this empty space and the lattice water molecules are not shown for clarity.
Cal. for After removal of water C34H44B2Cu3 Exp. Cal. for C34H28B2 F8N6O16 Cu3F8N6O8
34.88 35.30 4.25 3.82 7.28 7.26
39.90 2.36 8.22
40.32 2.79 8.30
After water reintroduction (Exp.)
39.17 2.57 7.48
dxz, dyz → dx 2−y2 and dxy, dz2 → dx 2−y2 transitions may be assigned for the broad band and the low-energy shoulder, respectively. The X-band powder EPR spectra of polycrystalline samples have been recorded at room temperature and at low temperature (70 K). The Cu(II)-4,4′-bpy-formato compound shows an axial signal with g// =2.22 and g⊥ =2.09, which is typical for the dx2−y2 ground state of an elongated octahedral geometry. No hyperfine splitting is resolved. The single-crystal X-ray structure also reveals the presence of lattice water molecules, illustrating its porous nature. The possibility of generating a microporous framework by removing the guest molecules has therefore been investigated. Hence, the stability of the framework upon removal/reintroduction of the guest molecules has been monitored in detail, using elemental analysis and XRPD technique (Table 1 and Fig. 6). The weight loss of the title compound involves eight water molecules (two coordinated and six lattice water molecules) when elevating the temperature to 120 °C for 6 h under vacuum. The X-ray powder diffraction reveals the remaining microcrystalline solid with a structure change (Fig. 6c), also in accord with the result of the elemental analysis. Therefore, this compound containing formate and 4,4′-bpy organic ligands is unstable at high temperature. However, the solid-state structure does not collapse when the water molecules are taken out. Unfortunately, the dynamic structural transformation behavior of this compound cannot be achieved after re-immersing in water. The XRPD pattern after rehydrating is different from the original material, indicating a change in the overall structure (Fig. 6d). The title compound has been tested as heterogeneous catalyst for the cyanosilylation of acetaldehyde and benzaldehyde. Thus, the selectivity and activity of the 2D coordination polymer have been examined. Typically, the powdered catalyst is suspended in dichloromethane (CH2Cl2) or tetrahydrofuran (THF). The aldehydic substrate and trimethylsilyl cyanide (1:2 molar ratio) are subsequently added at room temperature and the reaction is carried out for 24 h (Scheme 1) as known for such reactions [27–31]. The course of the reactions has been monitored by gas chromatography (GC).
Fig. 6. XRPD patterns of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n; (a) simulated, (b) as-synthesized, (c) after removal of the guest water molecules, and (d) after reintroduction of the guest water molecules.
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O + R
Me3SiCN
H
Me3SiO
MOF Solvent
R
CN H
R= CH 3, phenyl
acknowledged from the European Cooperation in the Field of Science and Technology (COST) Action D35/0011 and the distinguished scientists fellowship programme of King Saud University, Riyadh, Saudi Arabia. Appendix A. Supplementary material
Scheme 1. Cyanosilylation reaction to test catalytic properties.
Table 2 Cyanosilylation of aldehydes catalyzed by {[Cu3(4,4′‐bpy)3(μ‐OOCH)4(H2O)2](BF4)2(H2O)6}n. Catalyst
Type
Substrate
Solvent
Time (h)
Conversion %
Blank – – –
– 2D 2D 2D
Acetaldehyde Acetaldehyde Acetaldehyde Benzaldehyde
CH2Cl2 CH2Cl2 THF CH2Cl2
24 24 24 24
12 25 20 2
A blank reaction has been performed by performing the cyanosilylation of acetaldehyde without catalyst, at 25 °C. This test reaction gives only conversions of 12% for acetaldehyde, after a reaction time of 24 h. As can be seen, the copper(II)-4,4′-bpy-formato compound is a poorly active catalyst for cyanosilylation of aldehydes. In fact only a conversion of 25% of acetaldehyde is reached after 24 h reaction time in CH2Cl2 (Table 2). When benzaldehyde, a sterically more demanding substrate, is used under similar reaction conditions, a conversion of only 2% is observed. In addition, the conversion of acetaldehyde has been examined in THF for this MOF catalyst. It has been found that the reaction is also poorly efficient in this solvent. Only a yield of 20% is achieved after 24 h, see Table 2. Consequently, the use of THF as solvent is disadvantageous, since it leads to decomposition of the catalyst under this reaction condition, even at room temperature, within days. The decomposition causes a reddish-brown appearance of apparent Cu(I) solid material, with THF as a reducing agent. At room temperature after 24 h the color of the catalyst changes to reddish-brown suggesting full decomposition of the material. Nevertheless, the catalytic property of the title compound for the cyanosilylation of aldehydes has been compared with those of zinc(II) porous coordination frameworks reported earlier [32]. The lower catalytic activities agree with the lower Lewis acidity of the copper(II) compound compared to that of the zinc(II) ones. In summary, we have obtained a new porous coordination polymer, consisting of a so-called (4,4) 2D layered network, by the selfassembly of the coordination moiety [copper(II)‐4,4′-bpy], the formato co-ligand and BF4− anions. The lattice stability of the complex is further enhanced by hydrogen bonds and anion–π interactions to form 3D frameworks. The dynamic structural transformation behavior of the lattice cannot be achieved after dehydration and re-immersing in water. Furthermore, the catalytic activities of copper(II)-4,4′-bpyformato compound in the cyanosilylation of aldehydes have been investigated. The Lewis-acidic, heterogeneous catalysts are capable of generating 2-(methylsiloxy)propionitrile, albeit in low yield after 24 h. Acknowledgments Funding for this work is provided by The Thailand Research Fund, The Royal Golden Jubilee Ph.D. Program of The Thailand Research Fund (PHD/0019/2549), the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission, through the Advanced Functional Materials Cluster of Khon Kaen University and the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education, Thailand. Financial support is
CCDC 804671 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www. ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. References [1] K. Biradha, CrystEngComm. 5 (2003) 374. [2] F.A. Almeida Paz, A.D. Bond, Y.Z. Khimyak, J. Klinowski, New J. Chem. 26 (2002) 381. [3] T.J. Mooibroek, P. Gamez, J. Reedijk, CrystEngComm. 10 (2008) 1501. [4] L. Brunsveld, B.J.B. Folmer, E.W. Meijer, R.P. Sijbesma, Chem. Rev. 101 (2001) 4071. [5] K.T. Holman, A.M. Pivovar, M.D. Ward, Science 294 (2001) 1907. [6] X.J. Li, C. Rong, D.F. Feng, W.H. Bi, Y. Wang, X. Li, M.C. Hong, Cryst. Growth Des. 4 (2004) 775. [7] C. Qin, X.L. Wang, L. Carlucci, M.L. Tong, E.B. Wang, C.W. Hu, L. Xu, Chem. Commun. (2004) 1876. [8] Q.-Y. Liu, L. Xu, CrystEngComm. 7 (2005) 87. [9] Y.-C. Liao, F.-L. Liao, W.-K. Chang, S.-L. Wang, J. Am. Chem. Soc. 126 (2004) 1320. [10] S.-I. Noro, R. Kitaura, M. Kondo, S. Kitagawa, T. Ishii, H. Matsuzaka, M. Yamashita, J. Am. Chem. Soc. 124 (2002) 2568. [11] P. Phuengphai, S. Youngme, P. Kongsaeree, C. Pakawatchai, N. Chaichit, S.J. Teat, P. Gamez, J. Reedijk, CrystEngComm. 11 (2009) 1723. [12] K. Schlichte, T. Kratzke, S. Kaskel, Microporous Mesoporous Mater. 73 (2004) 81. [13] The single-crystal X-ray data were collected at 173(2) K on a Nonius Kappa CCD diffractometer. X-ray crystallography: Crystal and refinement data: C34H44B2Cu3F8N6O16 (MW =1156.99), Monoclinic, P2/c, a=12.226(2), b=11.114(2), c=16.674(3) Å, α=90.00, β=91.00(3), γ=90.00 º, V=2265.31 Å3, Z=2, Dcal=1.696 g cm−3, μ=1.503 (mm−1), F =1174 (000), θ range =2.75–27.53°, GOF=0.976, Final R indices, R1=0.0444, [I>2σ(I)]=wR2=0.0936, R indices (all data)=R1=0.0673, wR2=0.1043, Largest difference peak and hole =0.992, -0.473 eÅ−3, Data collections on a single crystal were performed with a Nonius Kappa CCD diffractometer with graphite-monochromated Mo Kα radiation (λ=0.71073 Å). DENZO-SMN was used for data integration and SCALEPACK corrected data for Lorentzpolarization effects [Z. Otwinowski, W. Minor, Macromol. Crystallogr. A 276 (1997) 307]. The structure was solved by direct methods with the package SIR92 [A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 27 (1994) 435], and the refinement and all further calculations were carried out using the SHELX-TL suite [SHELX-TL, Bruker AXS, Madison, WI, (2001)]. All non-hydrogen atoms were refined anisotropically. The H atoms were introduced at calculated positions and refined with a fixed geometry with respect to their carrier atoms. [14] Catalytic properties. A typical cyanosilylation reaction was performed as follows: 40 mg (0.006 mmol, 0.2 mmol of Cu(II) MOF catalyst was suspended in 5 mL of dry dichloromethane (CH2Cl2) or tetrahydrofuran (THF), followed by the addition of the aldehyde (1.5 mmol) and trimethylsilyl cyanide (3 mmol). The reaction mixtures were stirred at room temperature under argon. The reaction conversions were determined by gas chromatography (GC) analysis. The catalytic recyclability was checked three times with the same batch of catalyst, and no obvious decrease in activity was observed. [15] P. Phuengphai, S. Youngme, I. Mutikainen, P. Gamez, J. Reedijk, Polyhedron 42 (2012) 10. [16] Synthesis of the title compound. A warm solution (20 mL) containing 4,4′-bpy (0.156 g, 1.0 mmol) and NaOOCH (0.136 g, 2.0 mmol) in water–ethanol (10: 10 mL) was added to a hot aqueous solution (10 mL) of Cu(BF4)2•6H2O (0.346 g, 1.0 mmol) under continuous stirring. A few drops of HOOCH 98% were subsequently added to the reaction mixture, yielding a clear blue solution. The resulting blue solution was allowed to stand unperturbed for the slow evaporation of the solvent at room temperature, producing blue crystals after a few days. Yield: 62%. The microcrystalline sample was prepared using the same solvents. The purity and homogeneity were confirmed by X-ray powder diffraction (see Fig. 6b). Elemental analysis (%) for C34H44B2Cu3F8N6O16 (FW 1156.99) calculated (found): C 35.30 (34.88); H 3.82 (4.25); N 7.26 (7.28). IR (cm−1): νas(COO) 1580, νs(COO) 1417, δ(O-C-O) 647, ν(BF4) 1041–1012. [17] S. Youngme, N. Chaichit, K. Damnatara, Polyhedron 21 (2002) 943. [18] R.J. Gotz, A. Robertazzi, I. Mutikainen, U. Turpeinen, P. Gamez, J. Reedijk, Chem. Commun. (2008) 3384. [19] J.L. Manson, J.G. Lecher, J. Gu, U. Geiser, J.A. Schlueter, R. Henning, X. Wang, A.J. Schultz, H.-J. Koo, M.-H. Whangbo, J. Chem. Soc. Dalton Trans. (2003) 2905. [20] L.-G. Zhu, S. Kitagawa, Inorg. Chem. Commun. 5 (2002) 358. [21] B. Conerney, P. Jensen, P.E. Kruger, B. Moubaraki, K.S. Murray, CrystEngComm. 5 (2003) 454. [22] E. Yang, X.-Q. Wang, Y.-Y. Qin, Chin. J. Struct. Chem. 25 (2006) 1365.
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Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Feature article
Construction of a 3D supramolecular network from 2D coordination layers via anion–π interactions and its catalytic properties Pongthipun Phuengphai a, b, d, Sujittra Youngme b,⁎, Ilpo Mutikainen c, Jan Reedijk d, e a
Department of Fundamental Science, Faculty of Science and Technology, Surindra Rajabhat University, Surin 32000, Thailand Materials Chemistry Research Unit, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55 (A. I. Virtasen aukio 1), 00014 University of Helsinki, Finland d Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands e Department of Chemistry, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia b c
a r t i c l e
i n f o
Article history: Received 13 March 2012 Accepted 9 August 2012 Available online 21 August 2012 Keywords: Porous coordination polymer Heterogeneous catalyst Dynamic structural transformation Copper
a b s t r a c t A new porous coordination polymer, i.e. the two-dimensional layered network {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2] (BF4)2(H2O)6}n has been synthesized from Cu(BF4)2·6H2O, 4,4′-bpy and with formate as co-ligand. Single-crystal X-ray diffraction analysis reveals a (4,4) 2D coordination network exhibiting voids that contain the counter-ions and water guest molecules. In addition, the title compound is further stabilized by hydrogen bonds and anion–π interactions to form an intricate supramolecular framework. The removal and reintroduction of guest water molecules has been explored to better understand the dynamic structural transformation. The coordination compound shows weak catalytic activity in the cyanosilylation of aldehydes. © 2012 Published by Elsevier B.V. All rights reserved.
Contents
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. Supplementary material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
The design and synthesis of extended frameworks via supramolecular interactions represent a new area of considerable interest [1,2]. In particular, hydrogen bonding, CH–π, π–π and anion–π have been exploited for molecular recognition associated with biological activity, and for engineering of molecular solids [3–5]. In the vast amount of the reported work, a variety of 1D, 2D, and 3D molecular open frameworks have been successfully obtained via the use of coordinating functional groups, such as carboxylates, 4,4′-bpy or its derivatives, and mixtures of both carboxylate and 4,4′-bpy ligands [6–9]. For example, Kitagawa et al., successfully constructed a series of porous Cu(II)/4,4′-bpy coordination polymers via addition of different anions [10]. In coordination networks in general the 4,4′-bpy ligands may act in bidentate bridging, or monodentate terminal modes, resulting in 1D linear, zigzag, ladder, molecular antenna railroads and chains, 2D bilayer, square and rectangular grid networks, or 3D non-interpenetrated and interpenetrated networks. In our previous ⁎ Corresponding author. Fax: +66 43 243 338. E-mail address: [email protected] (S. Youngme). 1387-7003/$ – see front matter © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.inoche.2012.08.002
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work, we have synthesized a number of porous Zn(II)/4,4′-bpy/ carboxylato coordination polymers, by varying different anions [11]. These compounds exhibited the structural diversity and have been explored for their properties. In the case of copper(II) carboxylato-bridged porous coordination polymers only a few catalytic applications have been reported [12]. In the present study, we report the synthesis and crystal structure of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n [13], and its 3D hydrogen-bonded and anion–π architecture. The dynamic structural transformation and heterogeneous catalytic activities in cyanosilylation have been investigated [14]. In our previous work, we primarily focused on the design and synthesis of copper(II) compounds based on mixed 4,4′-bpy and monocarboxylato ligands [15]. We have discussed the effect of the steric hindrance of different carboxylato ligands (formato and acetato ligands), as well as the influence of the nature of the anion on the solid-state structure of the corresponding coordination materials. The (4,4) 2D structure of title compound is very similar to {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](ClO4)2(H2O)6}n which has been
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reported [15]. The simple replacement of perchlorate to tetrafluoridoborate during the synthetic procedure, however, results in significant structural changes in the anion–π interactions, which probably have an important role in crystal dynamics and structural transformation of the resulting coordination compound {[Cu3(4,4′bpy)3(μ-OOCH)4(H2O)2] (BF4)2(H2O)6}n [16]. As shown in Fig. 1, the new compound contains two different types of copper(II) ions, each having an elongated octahedral N2O4 coordination environment. The basal plane of the octahedron of Cu1 is formed by two oxygen atoms belonging to two formato ligands (O1 and O5a) and two 4,4′-bpy nitrogen atoms (N11 and N21). The octahedral geometry is completed at the axial positions by two oxygen atoms from two formato ligands (O3b and O5), with Cu\O distances of 2.587(2) and 2.361(2) Å, respectively. The Cu\O and Cu\N bond distances [17] can be considered as normal for this type of CuN2O4 coordination environment. As a crystallographic inversion center is located in the center of the four-membered Cu1-O5-Cu1A-O5a ring, Cu1 is bridged to symmetry-related Cu1A ions via two monoatomic bridged μ-O,O′formato ligands (O5 and O5a) and to Cu2 ions via one μ-O,O′,O′formato ligand. This spatial arrangement produces a linear trinuclear [Cu1A, Cu1, Cu2] cluster. These tricopper units are linked via μ-formato-κO bridges, producing a 1D polymeric chain and each chain is connected through 4,4′-bpy ligands to generate a 2D layer in the crystallographic b direction (blue, orange and green 4,4′-bpy ligands in Fig. 2). The coordination environment around the Cu2 ion is best described as an elongated octahedron, whose basal angles, varying from 89.8(1) to 90.1(1)°, are close to the ideal value of 90°. This basal plane contains two water molecules (O4 and O4b) and two nitrogen atoms (N31 and N41) belonging to two 4,4′-bpy ligands. The axial positions are occupied by two oxygen atoms (O3 and O3b) from different μ-O,O′,O′-formato ligands at common distances (Cu2\O distance = 2.297(2) Å). The 1D chains of {Cu2, Cu1, Cu1A} units are connected to each other through 4,4′-bpy ligands, leading to the formation of a (4,4)
Fig. 1. Representation of the linear trinuclear unit of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2] (BF4)2(H2O)6}n. The tetrafluoridoborate anions, the lattice water molecules and the H atoms are not shown for clarity. Selected bond distances (Å) and angles (°):Cu(1)\O(1) 1.988(2), Cu(1)\O(5a) 1.993(2), Cu(1)\N(11) 2.006(3), Cu(1)\N(21) 2.023(3), Cu(1)\O(5) 2.361(2), Cu(1)\O(3b) 2.587(2), Cu(2)\N(31) 2.015(3), Cu(2)\O(4) 2.016(2), Cu(2)\N(41) 2.031(4), Cu(2)\O(3) 2.297(2), O(1)\Cu(1)–O(5a) 178.3(1), O(1)\Cu(1)\N(11) 89.7(1), O(5a)\Cu(1)\N(11) 90.7(1), O(1)\Cu(1)\N(21) 87.9(1), O(5a)\Cu(1)\N(21) 91.7(1), N(11)\Cu(1)\N(21) 175.3(1), O(1)\Cu(1)\ O(5) 99.5(1), O(5a)\Cu(1)\O(5) 78.8(1), N(11)\Cu(1)\O(5) 91.3(1), N(21)\ Cu(1)\O(5) 93.0(1), N(31)\Cu(2)\O(4) 89.8(1), O(4b)\Cu(2)\O(4) 179.7(1), N(31)\Cu(2)\N(41) 180.0(1), O(4)\Cu(2)\N(41) 90.1(1), O(4b)\Cu(2)\O(3b) 91.7(1), O(4)\Cu(2)\O(3b) 88.2(1), N(31)\Cu(2)\O(3) 95.0(1), O(4)\Cu(2)\O(3) 91.7(1), N(41)–Cu(2)–O(3) 84.9(1), O(3b)\Cu(2)\O(3) 169.9(1). Symmetry operations: (a) 1−x, y, 0.5−z; (b) −x, y, 0.5−z.
Fig. 2. 2D coordination network of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n. The 4,4′-bpy ligands connecting the trinuclear chain secondary building units are shown in blue, orange and green.
2D sheets (Figs. 2 and 3). The linear strands containing the metal centers Cu1A and Cu1 interact via π–π contacts between two 4,4′-bpy units (centroid-to-centroid distances ranging from 3.862(2) to 4.130(2) Å; blue and orange 4,4′-bpy ligands in Fig. 2). The two coordinated water molecules (O4 and O4b in Fig. 1) are strongly interacting with an intramolecular formato bridge (O4···O6 = 2.678(2) Å) and are hydrogen-bonded to lattice water molecules (O4···O6W = 2.735(2) Å), which are located between the closely packed 2D layers. The fascinating crystal packing of the solid-state structure is shown in Fig. 4. Indeed, both tetrafluoridoborate ions are involved in weak intermolecular anion–π interactions [18] with two 4,4′-bpy rings from adjacent chains, quite similar to that observed in the analogous perchlorate [15]. The difference is that the tetrafluoridoborates adopt two crystallographic orientations in the crystal lattice with occupancy factors of 0.7 and 0.3, while such a disorder was not observed in the case of perchlorate [15]. The two crystallographically independent BF4− ions are labeled {B1F11–F14} and {B2F21–F24}. As evidenced in Fig. 4, the fluorine atom F12 is quite strongly interacting with an intermolecular pyridine ring resulting the F12···centroid distance of 3.474(6) Å. The fluorine atom F13 is also in contact with the pyridine ring (F13···centroid ring= 3.378(6)Å). Moreover, the second tetrafluoridoborate orientation experiences anion–π contacts as well. The fluorine atom F23 shows significant interactions with the intermolecular pyridine ring (Fig. 4; F23···centroid ring= 3.159(7) Å. In addition, F24 is in contact with the intermolecular pyridine ring (F24···centroid ring= 3.958(8) Å. Furthermore, each
Fig. 3. 2D coordination network of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n, constituted of (4,4) 2D sheets coordination chains that are connected by formato ligands.
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Table 1 Elemental analyses of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n and of the products after removal and after reintroduction of the guest water molecules. Element Exp.
C H N
Fig. 4. Crystal packing of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n illustrating the anion–π contacts between the tetrafluoridoborate ion and the pyridine rings.
tetrafluoridoborate ion is hydrogen-bonded to lattice water molecules (F12···O8W =2.854(2), F14···O9W= 2.917(2) and F21···O7W = 2.920(2) Å), which are located between the closely packed 2D layers. The occurrences of these non-covalent contacts result in the formation of a three-dimensional supramolecular structure (Fig. 5). Seven related coordination polymers, that contain copper(II) ions, 4,4′-bpy ligands and bridging monocarboxylato co-ligands, have been reported in the literature. In these compounds, the copper(II) ions are connected by carboxylato bridges, yielding the three-dimensional structure [Cu(OOCH)2(4,4′-bpy)]n (I) [19], the two-dimensional framework {[Cu2(4,4′-bpy)(OOCCH3)(OH)(H2O)](H2O)2(SiF6)}n (II) [20], and the one-dimensional polymers {[Cu(4,4′-bpy)(OOCCH3)2] (H2O)2.5}n (III) [21], {[Cu2(4,4′-bpy)2(μ-OOCCH3)2(OOCCH3)2](H2O}n (IV) [22], {[Cu(4,4′-bpy)(OOCCH3)2](H2O)3}n (V) [23,24], {[Cu2(4,4′bpy)(OOCCH3)4](CH3CN)}n (VI) [25], {[Cu2(4,4′-bpy)(OOCCH3)4] (DMF)}n (VII) [26]. Among these seven compounds described earlier, the formato-containing compound I displays a 3D structure, while in the case of our new compound, a 2D network is observed. Only II, containing acetato co-ligands, exhibits a double-stranded 2D sheet architecture, similar to the BF4- compound. The remaining five compounds exhibit double-stranded 1D polymeric chains for III–V, and 1D polymeric chains for VI and VII. It is interesting to note that compounds VI and VII are isostructural. In fact, the sole difference between them lies in the distinct guest solvent molecules, namely CH3CN and DMF. Hence, it appears that the nature of these two solvents does not affect the overall structure of the network. The diffuse reflectance electronic spectrum of the title compound displays a broad peak centered at 15,800 cm −1 and a poorly resolved shoulder at approximately 12,500 cm −1. This feature agrees with an elongated octahedral geometry with an off-z-axis distortion. The
Fig. 5. Packing diagram with hydrogen bond (dash-lines), showing the threedimensional structure of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n, the guest molecules (space-filling mode) filling this empty space and the lattice water molecules are not shown for clarity.
Cal. for After removal of water C34H44B2Cu3 Exp. Cal. for C34H28B2 F8N6O16 Cu3F8N6O8
34.88 35.30 4.25 3.82 7.28 7.26
39.90 2.36 8.22
40.32 2.79 8.30
After water reintroduction (Exp.)
39.17 2.57 7.48
dxz, dyz → dx 2−y2 and dxy, dz2 → dx 2−y2 transitions may be assigned for the broad band and the low-energy shoulder, respectively. The X-band powder EPR spectra of polycrystalline samples have been recorded at room temperature and at low temperature (70 K). The Cu(II)-4,4′-bpy-formato compound shows an axial signal with g// =2.22 and g⊥ =2.09, which is typical for the dx2−y2 ground state of an elongated octahedral geometry. No hyperfine splitting is resolved. The single-crystal X-ray structure also reveals the presence of lattice water molecules, illustrating its porous nature. The possibility of generating a microporous framework by removing the guest molecules has therefore been investigated. Hence, the stability of the framework upon removal/reintroduction of the guest molecules has been monitored in detail, using elemental analysis and XRPD technique (Table 1 and Fig. 6). The weight loss of the title compound involves eight water molecules (two coordinated and six lattice water molecules) when elevating the temperature to 120 °C for 6 h under vacuum. The X-ray powder diffraction reveals the remaining microcrystalline solid with a structure change (Fig. 6c), also in accord with the result of the elemental analysis. Therefore, this compound containing formate and 4,4′-bpy organic ligands is unstable at high temperature. However, the solid-state structure does not collapse when the water molecules are taken out. Unfortunately, the dynamic structural transformation behavior of this compound cannot be achieved after re-immersing in water. The XRPD pattern after rehydrating is different from the original material, indicating a change in the overall structure (Fig. 6d). The title compound has been tested as heterogeneous catalyst for the cyanosilylation of acetaldehyde and benzaldehyde. Thus, the selectivity and activity of the 2D coordination polymer have been examined. Typically, the powdered catalyst is suspended in dichloromethane (CH2Cl2) or tetrahydrofuran (THF). The aldehydic substrate and trimethylsilyl cyanide (1:2 molar ratio) are subsequently added at room temperature and the reaction is carried out for 24 h (Scheme 1) as known for such reactions [27–31]. The course of the reactions has been monitored by gas chromatography (GC).
Fig. 6. XRPD patterns of {[Cu3(4,4′-bpy)3(μ-OOCH)4(H2O)2](BF4)2(H2O)6}n; (a) simulated, (b) as-synthesized, (c) after removal of the guest water molecules, and (d) after reintroduction of the guest water molecules.
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O + R
Me3SiCN
H
Me3SiO
MOF Solvent
R
CN H
R= CH 3, phenyl
acknowledged from the European Cooperation in the Field of Science and Technology (COST) Action D35/0011 and the distinguished scientists fellowship programme of King Saud University, Riyadh, Saudi Arabia. Appendix A. Supplementary material
Scheme 1. Cyanosilylation reaction to test catalytic properties.
Table 2 Cyanosilylation of aldehydes catalyzed by {[Cu3(4,4′‐bpy)3(μ‐OOCH)4(H2O)2](BF4)2(H2O)6}n. Catalyst
Type
Substrate
Solvent
Time (h)
Conversion %
Blank – – –
– 2D 2D 2D
Acetaldehyde Acetaldehyde Acetaldehyde Benzaldehyde
CH2Cl2 CH2Cl2 THF CH2Cl2
24 24 24 24
12 25 20 2
A blank reaction has been performed by performing the cyanosilylation of acetaldehyde without catalyst, at 25 °C. This test reaction gives only conversions of 12% for acetaldehyde, after a reaction time of 24 h. As can be seen, the copper(II)-4,4′-bpy-formato compound is a poorly active catalyst for cyanosilylation of aldehydes. In fact only a conversion of 25% of acetaldehyde is reached after 24 h reaction time in CH2Cl2 (Table 2). When benzaldehyde, a sterically more demanding substrate, is used under similar reaction conditions, a conversion of only 2% is observed. In addition, the conversion of acetaldehyde has been examined in THF for this MOF catalyst. It has been found that the reaction is also poorly efficient in this solvent. Only a yield of 20% is achieved after 24 h, see Table 2. Consequently, the use of THF as solvent is disadvantageous, since it leads to decomposition of the catalyst under this reaction condition, even at room temperature, within days. The decomposition causes a reddish-brown appearance of apparent Cu(I) solid material, with THF as a reducing agent. At room temperature after 24 h the color of the catalyst changes to reddish-brown suggesting full decomposition of the material. Nevertheless, the catalytic property of the title compound for the cyanosilylation of aldehydes has been compared with those of zinc(II) porous coordination frameworks reported earlier [32]. The lower catalytic activities agree with the lower Lewis acidity of the copper(II) compound compared to that of the zinc(II) ones. In summary, we have obtained a new porous coordination polymer, consisting of a so-called (4,4) 2D layered network, by the selfassembly of the coordination moiety [copper(II)‐4,4′-bpy], the formato co-ligand and BF4− anions. The lattice stability of the complex is further enhanced by hydrogen bonds and anion–π interactions to form 3D frameworks. The dynamic structural transformation behavior of the lattice cannot be achieved after dehydration and re-immersing in water. Furthermore, the catalytic activities of copper(II)-4,4′-bpyformato compound in the cyanosilylation of aldehydes have been investigated. The Lewis-acidic, heterogeneous catalysts are capable of generating 2-(methylsiloxy)propionitrile, albeit in low yield after 24 h. Acknowledgments Funding for this work is provided by The Thailand Research Fund, The Royal Golden Jubilee Ph.D. Program of The Thailand Research Fund (PHD/0019/2549), the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission, through the Advanced Functional Materials Cluster of Khon Kaen University and the Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education, Thailand. Financial support is
CCDC 804671 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www. ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. References [1] K. Biradha, CrystEngComm. 5 (2003) 374. [2] F.A. Almeida Paz, A.D. Bond, Y.Z. Khimyak, J. Klinowski, New J. Chem. 26 (2002) 381. [3] T.J. Mooibroek, P. Gamez, J. Reedijk, CrystEngComm. 10 (2008) 1501. [4] L. Brunsveld, B.J.B. Folmer, E.W. Meijer, R.P. Sijbesma, Chem. Rev. 101 (2001) 4071. [5] K.T. Holman, A.M. Pivovar, M.D. Ward, Science 294 (2001) 1907. [6] X.J. Li, C. Rong, D.F. Feng, W.H. Bi, Y. Wang, X. Li, M.C. Hong, Cryst. Growth Des. 4 (2004) 775. [7] C. Qin, X.L. Wang, L. Carlucci, M.L. Tong, E.B. Wang, C.W. Hu, L. Xu, Chem. Commun. (2004) 1876. [8] Q.-Y. Liu, L. Xu, CrystEngComm. 7 (2005) 87. [9] Y.-C. Liao, F.-L. Liao, W.-K. Chang, S.-L. Wang, J. Am. Chem. Soc. 126 (2004) 1320. [10] S.-I. Noro, R. Kitaura, M. Kondo, S. Kitagawa, T. Ishii, H. Matsuzaka, M. Yamashita, J. Am. Chem. Soc. 124 (2002) 2568. [11] P. Phuengphai, S. Youngme, P. Kongsaeree, C. Pakawatchai, N. Chaichit, S.J. Teat, P. Gamez, J. Reedijk, CrystEngComm. 11 (2009) 1723. [12] K. Schlichte, T. Kratzke, S. Kaskel, Microporous Mesoporous Mater. 73 (2004) 81. [13] The single-crystal X-ray data were collected at 173(2) K on a Nonius Kappa CCD diffractometer. X-ray crystallography: Crystal and refinement data: C34H44B2Cu3F8N6O16 (MW =1156.99), Monoclinic, P2/c, a=12.226(2), b=11.114(2), c=16.674(3) Å, α=90.00, β=91.00(3), γ=90.00 º, V=2265.31 Å3, Z=2, Dcal=1.696 g cm−3, μ=1.503 (mm−1), F =1174 (000), θ range =2.75–27.53°, GOF=0.976, Final R indices, R1=0.0444, [I>2σ(I)]=wR2=0.0936, R indices (all data)=R1=0.0673, wR2=0.1043, Largest difference peak and hole =0.992, -0.473 eÅ−3, Data collections on a single crystal were performed with a Nonius Kappa CCD diffractometer with graphite-monochromated Mo Kα radiation (λ=0.71073 Å). DENZO-SMN was used for data integration and SCALEPACK corrected data for Lorentzpolarization effects [Z. Otwinowski, W. Minor, Macromol. Crystallogr. A 276 (1997) 307]. The structure was solved by direct methods with the package SIR92 [A. Altomare, G. Cascarano, C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori, M. Camalli, J. Appl. Crystallogr. 27 (1994) 435], and the refinement and all further calculations were carried out using the SHELX-TL suite [SHELX-TL, Bruker AXS, Madison, WI, (2001)]. All non-hydrogen atoms were refined anisotropically. The H atoms were introduced at calculated positions and refined with a fixed geometry with respect to their carrier atoms. [14] Catalytic properties. A typical cyanosilylation reaction was performed as follows: 40 mg (0.006 mmol, 0.2 mmol of Cu(II) MOF catalyst was suspended in 5 mL of dry dichloromethane (CH2Cl2) or tetrahydrofuran (THF), followed by the addition of the aldehyde (1.5 mmol) and trimethylsilyl cyanide (3 mmol). The reaction mixtures were stirred at room temperature under argon. The reaction conversions were determined by gas chromatography (GC) analysis. The catalytic recyclability was checked three times with the same batch of catalyst, and no obvious decrease in activity was observed. [15] P. Phuengphai, S. Youngme, I. Mutikainen, P. Gamez, J. Reedijk, Polyhedron 42 (2012) 10. [16] Synthesis of the title compound. A warm solution (20 mL) containing 4,4′-bpy (0.156 g, 1.0 mmol) and NaOOCH (0.136 g, 2.0 mmol) in water–ethanol (10: 10 mL) was added to a hot aqueous solution (10 mL) of Cu(BF4)2•6H2O (0.346 g, 1.0 mmol) under continuous stirring. A few drops of HOOCH 98% were subsequently added to the reaction mixture, yielding a clear blue solution. The resulting blue solution was allowed to stand unperturbed for the slow evaporation of the solvent at room temperature, producing blue crystals after a few days. Yield: 62%. The microcrystalline sample was prepared using the same solvents. The purity and homogeneity were confirmed by X-ray powder diffraction (see Fig. 6b). Elemental analysis (%) for C34H44B2Cu3F8N6O16 (FW 1156.99) calculated (found): C 35.30 (34.88); H 3.82 (4.25); N 7.26 (7.28). IR (cm−1): νas(COO) 1580, νs(COO) 1417, δ(O-C-O) 647, ν(BF4) 1041–1012. [17] S. Youngme, N. Chaichit, K. Damnatara, Polyhedron 21 (2002) 943. [18] R.J. Gotz, A. Robertazzi, I. Mutikainen, U. Turpeinen, P. Gamez, J. Reedijk, Chem. Commun. (2008) 3384. [19] J.L. Manson, J.G. Lecher, J. Gu, U. Geiser, J.A. Schlueter, R. Henning, X. Wang, A.J. Schultz, H.-J. Koo, M.-H. Whangbo, J. Chem. Soc. Dalton Trans. (2003) 2905. [20] L.-G. Zhu, S. Kitagawa, Inorg. Chem. Commun. 5 (2002) 358. [21] B. Conerney, P. Jensen, P.E. Kruger, B. Moubaraki, K.S. Murray, CrystEngComm. 5 (2003) 454. [22] E. Yang, X.-Q. Wang, Y.-Y. Qin, Chin. J. Struct. Chem. 25 (2006) 1365.
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