Novel Two-Dimensional Network Constructed from Polyoxomolybdate Chains Linked through Copper-Organonitrogen Coordination Polymer Chains: Hydrothermal Synthesis and Structure of [H2bpy][Cu(4,4′-bpy)]2[HPCuMo11O39]
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 1 257-260
Ying Lu,† Yan Xu,‡ Enbo Wang,*,† Jian Lu¨,† Changwen Hu,† and Lin Xu† Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, People’s Republic of China, and Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received March 22, 2004
ABSTRACT: A novel compound, [H2bpy][Cu(4,4′-bpy)]2[HPCuMo11O39] (1), has been prepared under mild hydrothermal conditions and has been structurally characterized by single-crystal X-ray diffraction. Compound 1 exhibits a novel two-dimensional network constructed from the interconnecting of polyoxomolybdate chains, {HPCuMo11O39}n4n-, and transition metal coordination polymer chains, {Cu(4,4′-bpy)}nn+. It is the first example of a two-dimensional structure formed by a polyoxomolybdate chain through the linkage of a transition metal coordination polymer chain. Introduction The significant interest in crystal engineering of organic-inorganic hybrid solid state materials reflects their structure diversity and vast range of properties with applications to catalysis, sorption, clathration, electrical conductivity, magnetism, and photochemistry.1 The evolution of organic-inorganic hybrid materials is dependent upon the synthesis of new solids possessing unique structures and properties, although the synthetic design of such materials remains a challenge in solid state chemistry. One approach for the design of organic-inorganic hybrid materials is to introduce some interesting transition metal complexes, which serve as organic-inorganic bridging ligands, into the covalent backbone of inorganic oxides. The transition metal complexes can dramatically influence the inorganic oxide microstructure, and a series of novel organic-inorganic hybrid materials have been found.2 Polyoxometalates (POMs), as one kind of significant metal oxide cluster with nanosizes and abundant topologies, have recently been employed as inorganic building blocks for the construction of organic-inorganic hybrid materials with various transition metal complexes as the bridging ligands. The assemblies possess interesting one-dimensional (1D),3 two-dimensional (2D),4 and three-dimensional (3D)5 structures and exhibit potential applications in catalysis, medicine, and electrical conductive and magnetic materials. Although they are capable of acting as inorganic building units to form dimer, trimer, and 1D chains through assembly as shown in some of the literature,6 POMs exist as discrete clusters in all reported transition metal complex bridged structures. As a continuation of the hydrothermal synthesis of various POM derivatives, we are trying to construct higher dimensional organic-inorganic hybrid * To whom correspondence should
[email protected]. † Northeast Normal University. ‡ The University of Tokyo.
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addressed.
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materials from POM building blocks. More recently, we have prepared two compounds containing 1D chains of transition metal-substituted Keggin heteropolymolybdate [H2bpy]2[Hbpy][PCuMo11O39]‚H2O and [H2bpy]2[Hbpy][PZnMo11O39]‚2.75H2O (bpy ) 4, 4′-bpyridine).7 It will be of great interest to investigate whether or not this kind of POM chain can be introduced into transition metal coordination complex bridged structures under similar hydrothermal conditions. On the basis of further studies on this system, we have demonstrated, for the first time, that such a POM chain can be exploited to create novel organic-inorganic hybrid solid materials by means of the linkage of transition metal complexes. Herein, we report a significant compound, [H2bpy][Cu(4,4′-bpy)]2[HPCuMo11O39] (1), which shows a 2D structure constructed from the interconnecting of polyoxomolybdate chains, {HPCuMo11O39}n4n-, and transition metal coordination polymer chains, {Cu(4,4′-bpy)}nn+. Experimental Section General Methods and Materials. The reagents were purchased commercially and used without further purification. Elemental analyses (C, H, and N) were performed on a PerkinElmer 2400 CHN Elemental Analyzer. Mo, Cu, and P were determined by a Leeman inductively coupled plasma (ICP) spectrometer. The infrared spectrum was obtained on an Alpha Centaurt FT/IR spectrometer with a pressed KBr pellet in the 4000-400 cm-1 regions. The electron paramagnetic resonance (EPR) spectrum was recorded on a Brucker ER 200D spectrometer at room temperature. X-ray powder diffraction (XRPD) patterns were recorded on a Siemens D5005 diffractometer with Cu KR (λ ) 1.5418 Å) radiation. Synthesis of [H2bpy][Cu(4,4′-bpy)]2[HPCuMo11O39] (1). A mixture of Na2MoO4‚2H2O (0.27 g), H3PO4 (0.243 g), CuCl2‚ 2H2O (0.095 g), 4,4′-bpy (0.08 g), and H2O (10 mL) was stirred for 20 min in air. The mixture was then transferred to a Teflonlined autoclave (20 mL) and kept at 160 °C for 6 days. After the mixture was slow cooled to room temperature, red crystals were filtered off, washed with distilled water, and dried in a desiccator at room temperature to give a yield of 50% based on Mo. The ICP analysis showed that compound 1 contained 45.8% Mo, 8.6% Cu, and 1.5% P (calcd: Mo, 44.5; Cu, 8.0; P,
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Table 1. Crystal Data and Structure Refinement for 1 empirical formula Fw crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Dcalcd (g/cm3) µ (mm-1) T (K) λ (Å) final R1,a wR2b [I > 2σ(I)] final R1,a wR2b (all data) a
C30H27Cu3Mo11N6O39P 2372.50 triclinic P1 10.781(2) 10.885(2) 12.781(3) 96.91(3) 99.55(3) 103.73(3) 1332.0(5) 1 2.956 3.806 293(2) 0.71073 0.0580, 0.1606 0.0643, 0.1714
R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) ∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]1/2.
1.3%). The elemental analysis was found as follows: C, 15.6; H, 1.4; N, 3.6 (calcd: C, 15.2; H, 1.2; N, 3.5%). IR (KBr pellet, cm-1): 3445(m), 3082(m), 1627(w), 1607(m), 1488(m), 1415(m), 1363(w), 1220(w), 1200(w), 1087(m), 1076(m), 1050(s), 952(vs), 939(vs), 852(s), 802(vs), 765(vs), 730(s), 699(s). Single-Crystal X-ray Diffraction (XRD). A red single crystal of 1 was carefully selected under a polarizing microscope and glued at the tip of a thin glass fiber with cyanoacrylate (super glue) adhesive, respectively. Single-crystal structure determination by XRD was performed on a R-axis RAPID IP diffractometer equipped with a normal focus, 18 kW sealed tube X-ray source (Mo KR radiation, λ ) 0.71073 Å) operating at 50 kV and 200 mA. Data processing was accomplished with the RAXWISH processing program. The empirical absorption correction was applied. On the basis of systematic absences, the space groups were determined to be P1 h or P1. The examination of the statistics intensity distribution shows that the compound would be compatible with a noncentric space group. The structure was solved in the space group P1 by the direct method and refined by full-matrix least squares on F2 using SHELXL 97 software,8 and the topology of the final structure confirmed the noncentric character. All of the nonhydrogen atoms were refined anisotropically. The hydrogen atoms of the 4,4′-bpy molecules were placed at calculated positions. The hydrogen atom attached to the [PMo11CuO39] cluster could not be located. The oxidation state bound values of Cu(2) and Cu(3) can be unambiguously assigned as +1 on the basis of the valence sum calculations, the crystal color, and the coordination environments, which may be reduced by the 4,4′-bpy ligand and are similar to previous publications.9 Further details of the X-ray structural analysis are given in Table 1. Selected bond lengths are listed in Table 2.
Results and Discussions Synthesis. Hydrothermal synthesis has recently been proven to be a useful technique in the preparation of solid state oxides and organic-inorganic hybrid materials. In a specific hydrothermal process, many factors can affect the formation and crystal growth of products, such as the type of initial reactants, starting concentrations, pH values, reaction time, and temperature. In our case, the pH value of the reaction system was of crucial importance for the crystallization of products. Compound 1 and previously reported compound [H2bpy]2[Hbpy][PCuMo11O39]‚H2O7 were both separated from the hydrothermal reactions of Na2MoO4‚2H2O, H3PO4, CuCl2‚ 2H2O, and 4,4′-bpy at 160 °C for 6 days with a slight difference in pH values of the initial reaction solutions. At low pH (2.5-3.0), the Cu(I) and Cu(II) coexist in the hydrothermal system and compound 1 was formed. At
Lu et al. Table 2. Selected Bond Lengths (Å) for 1a Mo(1)-O(33) Mo(1)-O(32) Mo(1)-O(34) Mo(1)-O(37) Mo(2)-O(30) Mo(2)-O(31) Mo(2)-O(38) Mo(3)-O(27) Mo(3)-O(31) Mo(3)-O(26) Mo(4)-O(2) Mo(4)-O(26) Mo(4)-O(30) Mo(4)-O(39) Mo(5)-O(23) Mo(5)-O(21) Mo(5)-O(41) Mo(6)-O(29) Mo(6)-O(18) Mo(6)-O(38) Mo(7)-O(16) Mo(7)-O(34) Mo(7)-O(14) Mo(7)-O(40) Mo(8)-O(1) Mo(8)-O(35) Mo(8)-O(43) Mo(9)-O(9) Mo(9)-O(10) Mo(9)-O(43) Mo(10)-O(8) Mo(10)-O(25) Mo(10)-O(3) Mo(11)-O(6)#1 Mo(11)-O(4) Mo(11)-O(3) Mo(11)-O(7) Cu(1)-O(9) Cu(1)-O(1) Cu(1)-O(42) Cu(2)-N(2) Cu(3)-N(4) P(1)-O(37) P(1)-O(38) P(1)-O(41) P(1)-O(42)
1.667(6) 1.893(7) 2.076(8) 2.485(9) 1.740(8) 1.931(6) 2.401(8) 1.647(8) 1.820(5) 2.013(9) 1.681(8) 1.903(9) 1.973(7) 2.480(10) 1.849(7) 2.042(9) 2.495(8) 1.722(6) 1.883(8) 2.482(11) 1.651(8) 1.975(6) 2.066(7) 2.468(12) 1.793(8) 1.952(6) 2.371(11) 1.670(6) 1.817(7) 2.373(10) 1.667(7) 2.032(7) 2.110(9) 1.661(8) 1.929(7) 1.961(7) 2.457(10) 1.926(5) 2.069(8) 2.431(11) 1.900(7) 1.942(8) 1.422(10) 1.524(9) 1.532(10) 1.616(12)
Mo(1)-O(36) Mo(1)-O(35) Mo(1)-O(40) Mo(2)-O(28) Mo(2)-O(10) Mo(2)-O(29) Mo(2)-O(39) Mo(3)-O(4) Mo(3)-O(25) Mo(3)-O(7) Mo(4)-O(24) Mo(4)-O(5) Mo(4)-O(42) Mo(5)-O(20) Mo(5)-O(22) Mo(5)-O(36) Mo(6)-O(19) Mo(6)-O(17) Mo(6)-O(22) Mo(6)-O(41) Mo(7)-O(15) Mo(7)-O(24) Mo(7)-O(42) Mo(8)-O(13) Mo(8)-O(12) Mo(8)-O(23) Mo(9)-O(11) Mo(9)-O(12) Mo(9)-O(17) Mo(9)-O(38) Mo(10)-O(14) Mo(10)-O(32) Mo(10)-O(7) Mo(11)-O(18) Mo(11)-O(21) Mo(11)-O(41) Cu(1)-O(5) Cu(1)-O(15) Cu(1)-O(6) Cu(2)-N(1) Cu(2)-O(3) Cu(3)-N(3) P(1)-O(39) P(1)-O(40) P(1)-O(43) P(1)-O(7)
1.845(8) 1.936(7) 2.428(10) 1.686(8) 1.863(7) 1.993(7) 2.460(8) 1.786(8) 1.996(6) 2.332(9) 1.762(7) 1.910(7) 2.334(10) 1.633(8) 1.882(7) 2.042(7) 1.699(7) 1.879(7) 2.075(6) 2.519(9) 1.797(7) 1.988(7) 2.381(11) 1.652(8) 1.922(7) 2.084(8) 1.651(6) 1.767(7) 1.949(8) 2.517(9) 1.925(8) 2.036(7) 2.452(8) 1.884(8) 1.931(8) 2.377(9) 1.814(7) 1.959(7) 2.177(8) 1.897(8) 2.426(7) 1.953(9) 1.488(9) 1.531(10) 1.533(11) 1.637(10)
a Symmetry transformations used to generate equivalent atoms: 1, x + 1, y, z.
higher pH (3.3-3.7), the Cu element only exists as Cu(II) and compound [H2bpy]2[Hbpy][PCuMo11O39]‚H2O7 was obtained. This phenomenon, the different acidity/ basicity value of the reaction under hydrothermal reaction media might influence the transformation of different oxidation states of second transition metal elements, such as copper elements (I or II), also was found in the syntheses of transition metal coordination complex bridged molybdenum oxides.9a Crystal Structure. The structure of 1 consists of 1D chains of Keggin anions [HPCuMo11O39]4-, {Cu(bpy)}nn+ linear cationic chains, and H2bpy2+ cations. The polyoxoanion [HPCuMo11O39]4- (Figure 1) is based on the well-known R-Keggin structure of PMo1210 with one Mo atom substituted by a Cu atom. In compound 1, all Mo centers exhibit a {MoO6} octahedral environment. Mo-O distances are 1.663(7) Å for terminal oxygen (Mo-Ot), 1.918(7) Å for O bonded to two Mo atoms (corner sharing Mo-Oc and edge sharing Mo-Oe), 2.434(10) Å for O bonded to three Mo atoms and one P atom (Mo-Oi), and 1.661(8) Å for O bonded to one Cu atom and one Mo atom. The central P atom is surround by a cube of eight
[H2bpy][Cu(4,4′-bpy)]2[HPCuMo11O39]
Figure 1. ORTEP drawing of 1 showing the labeling of atoms with thermal ellipsoids at 50% probability. The hydrogen atoms are omitted for clarity.
Figure 2. View of the 2D network in 1. All C and H atoms are omitted for clarity, and the 4,4′-bpy molecular rods are shown as single bold lines.
oxygen atoms with each site of them half-occupied. The P-O distances varied between 1.422(10) and 1.637(10) Å, average 1.538(10) Å. The bond lengths of monosubstituted Cu(1) are quite different from those of the Mo atoms. Five of the Cu(1)-O lengths are in the range of 1.814(7)-2.177(8) Å. The sixth corresponding to MoOi is quite long [2.431(11) Å]. According to the charge balance, the polyoxoanion should be monoprotonated. Bond valence calculations indicate that the possible protonation site is at the doubly bridged O34. The Keggin anions in compound 1 are connected by a common oxygen atom forming chains running along the a-axis (Figure 2). This bridging oxygen atom connects two opposite positions of the Keggin unit occupied by Cu(II) and Mo(VI), rather than two interval positions as reported in compounds [H2bpy]2[Hbpy][PCuMo11O39]‚ H2O and [H2bpy]2[Hbpy][PZnMo11O39]‚2.75H2O.7 Because of the difference in connecting site, the straight chain is formed in compound 1, while a zigzag chain is shown in the previously reported compounds [H2bpy]2[Hbpy][PCuMo11O39]‚H2O and [H2bpy]2[Hbpy][PZnMo11O39]‚2.75H2O. However, this straight chain is similar to the polyoxotungstate chains in compounds (ET)8-
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Figure 3. Packing of the 2D network showing the formation of 1D channelin 1. All H atoms are omitted for clarity, and the 4,4′-bpy molecular rods in the Cu-bpy chains are shown as single bold lines.
[PMnW11O39]‚2H2O,6c [NEt3H]5[XCoW11O39]‚3H2O (X ) P, As),6d and [Co(dpa)2 (OH2)2]2[Hdpa][PCoW11O39].6e It is interesting that there are three distinct copper coordination environments in compound 1: one [Cu1(II)] is located in the Keggin anion and displays an octahedral CuO6 coordination geometry as mentioned above, while the remaining two [Cu2(I) and Cu3(I)] exhibit trigonal CuN2O and linear CuN2 coordination geometry, respectively. As shown in Figure 1, Cu2 and Cu3 form two 1D Cu-bpy chains along the b-axis by the connection of 4,4′-bpy, respectively. The dihedral angles of the two pyridine rings of bpy are 39.4 and 44.1° for the Cu2 and Cu3 chains, respectively. Furthermore, Cu2 chains bridge the polyoxoanion chains arranged in parallel along the b-axis to form a novel 2D network with the Cu-O distance being 2.426(7) Å, which is comparable with that of 2.442(8) Å in compound [{Cu(bpe)}4(Mo8O26)]‚4H2O.11 The 2D layered structure of 1 is an example of a new topology in polyoxomolybdates. We believe that the bridging of the Cu2 chain plays a critical role in the formation of the straight POM chain in compound 1, which constrained the tendency of the formation of the zigzag POM chain as in previously reported compounds [H2bpy]2[Hbpy][PCuMo11O39]‚H2O and [H2bpy]2[Hbpy][PZnMo11O39]‚2.75H2O. As shown in Figure 3, the 2D networks in compound 1 are arranged in parallel along the c-axis and further connect each other through hydrogen bonding of the H2bpy2+ cations with N-H(bpy)‚‚‚O (polyoxoanion) distances of 2.888-3.045 Å to generate an interesting 3D supramolecular structure. It is noteworthy that such a 3D framework possesses 1D channels along the b-axis, which are filled with the discrete Cu3-bpy chains. The EPR spectrum of 1 at room temperature shows a Cu2+ signal with g| ) 2.379 and g⊥ ) 2.094 and hyperfine constant A| ) 100.3 G (Figure 4), in good accordance with the valence sum calculation. The experimental and simulated XRPD patterns of compound 1 are shown in Figure 5. Their peak positions are in good agreement with each other, indicating the phase purity of the products. The differences in intensity may be due to the preferred orientation of the powder samples.
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Lu et al. Supporting Information Available: Three-dimensional framework structure of 1 and IR spectrum for 1. X-ray crystallographic information files (CIF) for compound 1. This material is available free of charge via the Internet at http:// pubs.acs.org.
References
Figure 4. EPR spectrum of 1.
Figure 5. (a) Simulated and (b) experimental XRPD patterns of 1.
Conclusion Compound 1 provides a novel 2D network formed by the interconnecting of POM and coordination polymer chains and confirms the utility of hydrothermal methods for the synthesis of novel structural organic-inorganic hybrid materials. Furthermore, the title compound demonstrates that not only the vast class of polyoxoanion clusters but also the POM chains formed by polyoxoanion clusters may be used as building units in the design of organic-inorganic hybrid materials with higher dimensionalities through the linkage of transition metal coordination complexes. Because the variations in the identity of the POM chain and transition metal coordination complex linker may be introduced, a vast chemistry of solids is accessible and may provide a rational route to the modification of the electronic, magnetic, and optical properties of such materials. Acknowledgment. This work was supported by the National Science Foundation of China (20171010).
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