CRYSTAL GROWTH & DESIGN
Polythreading of Infinite 1D Chains into Different Structural Motifs: Two Poly(pseudo-rotaxane) Architectures Constructed by Concomitant Coordinative and Hydrogen Bonds
2006 VOL. 6, NO. 2 439-443
Xin-Long Wang, Chao Qin, and En-Bo Wang* Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal UniVersity, Changchun 130024, P. R. China ReceiVed August 3, 2005; ReVised Manuscript ReceiVed October 12, 2005
ABSTRACT: Two new compounds, [Co(4,4′-bipy)(H2O)4]‚(H2bptc)‚2H2O (1) and [Ni(4,4′-bipy)(H2O)4]‚0.5(btc)‚H2O (2) (4,4′bipy ) 4,4′-bipyridine, H4bptc ) 3,3′,4,4′-biphenyltetracarboxylate acid, H4btc ) 1,2,4,5-benzenetetracarboxylate acid), have been synthesized under hydrothermal conditions and characterized by elemental analysis, IR, TG analysis, and single-crystal X-ray diffraction. X-ray structural analysis revealed that two different structural motifs (1D covalent zigzag chain and 1D hydrogen-bonded ladder for 1 and 1D linear chain and 2D hydrogen-bonded sheet for 2) can be found within their crystal structures, and most intriguingly, they coexist in such a way that 1D covalently bonded chains of composition [M(4,4′-bipy)(H2O)4]2+ (M ) Co or Ni) thread into the hydrogen-bonded 1D ladders or 2D sheets formed by uncoordinated carboxylate anions and lattice water molecules, giving rise to overall entangled architectures with unusual poly(pseudo-rotaxane)-type arrangements. Thermal stability of the two compounds was also studied in this paper. Introduction The crystal engineering of supramolecular architectures based on metal and organic building blocks has been rapidly expanding in recent years owing to their novel and diverse topologies and potential applications in host-guest chemistry, catalysis, electrical conductivity, and magnetism.1,2 Generally, extended highdimensional networks can be obtained by the assembly of lower dimensional coordination polymers via noncovalent intermolecular forces such as hydrogen-bonding and π-π stacking interactions.3 4,4′-Bipyridine (4,4′-bipy), as a rigid rodlike bifunctional ligand, has been extensively employed to construct one-dimensional linear polymeric chains4 and therefore is an excellent candidate for the construction of supramolecular architectures. In addition, an important target in the formation of supramolecular assembly is to establish the possible connections between units. Fortunately, multicarboxylate ligands have been proven to be good candidates because they can be regarded not only as hydrogen-bonding acceptors but also as hydrogen-bonding donors, depending upon the number of deprotonated carboxylic groups. Although many noteworthy covalent networks have been constructed by the contemporary use of 4,4′-bipyridine and multicarboxylate ligands,5 reports on the combination of them for assembly of supramolecular compounds remain few.6 In view of the excellent coordination capability of 4,4′bipyridine and the good H-donor/acceptor nature of multicarboxylate anions, we employed 4,4′-bipyridine and two analogous tetracarboxylate ligands (see Scheme 1), 1,2,4,5-benzenetetracarboxylate acid (H4btc) or 3,3′,4,4′-biphenyltetracarboxylate acid (H4bptc), as mixed organic building units with the aim to construct high-dimensional supramolecular networks in expectation that these groups may generate covalent or hydrogenbonding interactions with transition metal ions in the assembly process. Herein, we report the syntheses and structures of two novel compounds, [Co(4,4′-bipy)(H2O)4]‚(H2bptc)‚2H2O (1) and [Ni(4,4′-bipy)(H2O)4]‚0.5(btc)‚H2O (2). Both 1 and 2 possess unusual entangled frameworks in which infinite 1D covalently * Tel: +86-431-5098787. Fax: +86-431-5098787. E-mail: wangenbo@ public.cc.jl.cn.
Scheme 1
bonded chains thread into the infinite ladders or 2D sheets generated from hydrogen-bonding interactions, showing the unusual poly(pseudo-rotaxane) arrangements. Experimental Section Materials. All chemicals purchased were of reagent grade and used without further purification. All syntheses were carried out in 20 mL Teflon-lined autoclaves under autogenous pressure. The reaction vessels were filled to approximately 60% volume capacity. Water used in the reactions is distilled water. Synthesis of [Co(4,4′-bipy)(H2O)4]‚(H2bptc)‚2H2O (1). A mixture of Co(NO3)2‚6H2O (146 mg, 0.5 mmol), H4bptc (165 mg, 0.5 mmol), 4,4′-bipy (78 mg, 0.5 mmol), and water (10 mL) was adjusted to pH 5.5 with 1 M NaOH solution and then heated at 160 °C for 5 days. Red crystals of 1 were obtained when the mixture was cooled to room temperature at 10 °C/h. Yield: 169 mg, 52% based on Co. Anal. Calcd (%) for C26H28CoN2O14: C, 47.93; H, 4.33; N, 4.30. Found: C, 47.84; H, 4.26; N, 4.47. IR data (KBr, cm-1): 3440s, 3215m, 1710s, 1628s, 1619m, 1562s, 1460s, 1401s, 1369s, 1286m, 1130w, 1039w, 940w, 831m, 786m, 750s, 649m, 568m, 534s, 428w. Synthesis of [Ni(4,4′-bipy)(H2O)4]‚0.5(btc)‚H2O (2). A mixture of Ni(NO3)2‚6H2O (145 mg, 0.5 mmol), H4btc (127 mg, 0.5 mmol), 4,4′bipy (78 mg, 0.5 mmol), and water (10 mL) was adjusted to pH 5.0 with 1 M NaOH solution and then heated at 160 °C for 5 days. Green crystals of 2 were obtained when the mixture was cooled to room temperature at 10 °C/h. Yield: 92 mg, 43% based on Ni. Anal. Calcd (%) for C15H19N2NiO9: C, 41.89; H, 4.45; N, 6.51. Found: C, 41.66; H, 4.28; N, 6.64. IR data (KBr, cm-1): 3454s, 3200m, 1645m, 1624w, 1580w, 1548s, 1490s, 1428s, 1363s, 1300w, 1288w, 1160w, 1045m, 904w, 824m, 779s, 767w, 686s, 635m, 549w, 454w, 416w. X-ray Crystallography. Single crystals of compounds 1 and 2 were glued on a glass fiber. Data were collected on a Rigaku R-AXIS RAPID IP diffractometer with Mo KR monochromated radiation (λ ) 0.710 73Å). Empirical absorption correction was applied. The structures
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Table 1. Crystal Data and Structure Refinement of 1 and 2
empirical formula M T, K λ, Å crystal system space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z µ, mm-1 R1a [I > 2σ(I)] wR2b [I > 2σ(I)] a
1
2
C26H28CoN2O14 651.43 293(2) 0.71073 triclinic P1h 10.556(2) 10.704(2) 13.184(3) 103.50(3) 108.82(3) 100.97(3) 1312.8(4) 2 0.734 0.0475 0.1083
C15H19N2NiO9 430.03 173(2) 0.71073 monoclinic C2/c 19.920(4) 11.256(2) 15.487(3) 90 94.77(3) 90 3460.5(12) 8 1.176 0.0546 0.1318
R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) ∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]1/2.
were solved by the direct method and refined by the full-matrix leastsquares method on F2 using the SHELXTL crystallographic software package.7 Anisotropic displacement parameters were applied to all nonhydrogen atoms. The organic hydrogen atoms were generated geometrically; the aqua hydrogen atoms were located from difference maps. The crystal data and structure refinement of compounds 1 and 2 are summarized in Table 1. Selected bond lengths and angles for 1 and 2 are listed in Supporting Information Table S1. Physical Measurements. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 CHN elemental analyzer. FTIR spectra were recorded in the range 400-4000 cm-1 on an Alpha Centaurt FTIR spectrophotometer using a KBr pellet. TG analyses were performed on a Perkin-Elmer TGA7 instrument in flowing N2 with a heating rate of 10 °C min-1.
Results and Discussion Syntheses and General Characterization. Because the mixing of metal salts and carboxylate solution usually results in precipitation in traditional aqueous reactions, making it difficult to grow crystals of complexes, hydrothermal methods were adopted in our reaction, which cause a reaction that shifts from the kinetic to the thermodynamic domain as compared to traditional aqueous reactions.1d By plenty of parallel experiments, it was found that the pH values of the reactions play an important role in the isolations of the two compounds. If the pH values of the reactions were lower or higher than those listed in the Experimental Section, no products could be obtained. However, we found that the above reactions were not sensitive to the reaction temperature. If the reaction temperature was set 10 °C above or below 160 °C, the intended crystals remained to be found. The IR spectra of 1 and 2 both show the characteristic vibration peaks of 4,4′-bipy and carboxylic acids. Compound 1 shows broad OH stretching bands at 3215-3440 cm-1 (32003454 cm-1 for 2), showing the presence of water molecules in the compound. The well-resolved peaks of aromatic rings span over the region 1286-1628 cm-1 for 1 (1363-1645 cm-1 for 2). The presence of the characteristic bands at around 16901730 cm-1 in compound 1 attributed to the protonated carboxylic group indicates that deprotonation of the H4bptc ligand is incomplete, while the absence of the corresponding peaks in 2 shows the complete deprotonaton of the H4btc ligand.8 The analyses of IR spectra of 1 and 2 are in good agreement with crystal structures and charge balance considerations. Crystal Structures. X-ray crystallography shows that compounds 1 and 2 both contain one-dimensional covalently bonded chains formed by 4,4′-bipy ligands connecting metal Co or Ni
Figure 1. ORTEP drawings of 1 (a) and 2 (b) showing the labeling of atoms with thermal ellipsoids at 50% probability.
atoms. In 1, the equatorial positions of the distorted [CoN2O4] octahedral coordinated geometry are taken up by two nitrogen atoms of 4,4′-bipy ligands (Co(1)-N(1) ) 2.141(2) Å, Co(1)-N(2) 2.136(2) Å) and two coordinated water molecules (Co(1)-O(2W) ) 2.088(2) Å, Co(1)-O(4W) ) 2.080(2) Å), and the axial positions are occupied by the other two coordinated water molecules (Co(1)-O(1W) ) 2.132(2) Å, Co(1)-O(3W) ) 2.120(2) Å), shown in Figure 1a. Compound 2 contains two independent Ni(II) atoms with 50% occupancy, one 4,4′-bipy ligand, 0.5 btc ligand, four coordinated water molecules, and one lattice water molecule in the asymmetric unit. Each Ni(II) center exhibits a slightly distorted octahedral coordination geometry, defined by four aqua ligands (Ni-O(W) ) 2.027(3)2.091(3) Å) in the equatorial positions and two pyridyl nitrogen donors (Ni-N ) 2.096(5)-2.160(4) Å) from two different 4,4′bipy ligands in the apical positions (see Figure 1b). The 4,4′-bipy groups serve as bis-monodentate ligands and bridge adjacent Co or Ni centers (Co‚‚‚Co ) 11.341(4) Å, average Ni‚‚‚Ni ) 11.348(2) Å) into infinite chains. In compound 1, though the two pyridyl rings of a 4,4′-bipy group are coplanar, the two 4,4′-bipy ligands bound to a cobalt atom are almost perpendicular to each other with the angle of N-Co-N about 86.75(4)°, thus leading to the formation of a 1D zigzag [Co(4,4′-bipy)(H2O)4]2+ chain (A) shown in Figure 2, top. In 2, two independent Ni atoms are each bridged by 4,4′-bipy ligands to generate two linear polymeric chains (see Figure 3a), namely, -Ni(1)-bipy-Ni(1)-bipy- (A) and -Ni(2)-bipy-Ni(2)-bipy- (B), which extend in two different directions (rotated by 120° about the c axis). The two pyridyl rings of a 4,4′-bipy group in B-type chain are essentially coplanar, whereas those in A-type chain are not coplanar with the dihedral angle of 31°.
Polythreading of Infinite 1D Chains
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Figure 2. Views of the two structural motifs in 1: 1D covalently bonded chain (top) and the hydrogen-bonded 1D ladder (bottom). Figure 4. Polythreading of 1D chains (A) into the ladders (B) in 1.
Figure 3. Views of the two structural motifs in 2: 1D covalently boned chains (a) and the hydrogen-bonded 2D layer (b).
A fascinating and peculiar structural feature of 1 and 2 is that another different structural motif can be found within their respective crystal structures when considering the hydrogen bond bridges involving lattice water molecules and carboxylate anions, and what is most important is that the two different structural motifs in each compound coexist in the rare poly(pseudorotaxane) arrangement. In compound 1, the uncoordinated H2bptc2- anions and lattice water molecules form a 1D hydrogen-bonded anionic ladder (B) of composition [(H2bptc)(H2O)2]2-, shown in Figure 2, bottom. That is, lattice water molecules O(5W) and O(6W) serve as H-donor and interact with the carboxylate oxygen atoms O(1), O(2), O(5), and O(8) (H-acceptor) of H2bptc2- anions via hydrogen bonds [O(5W)‚‚‚O(2) 2.886(4) Å, O(5W)‚‚‚O(5) 2.769(4) Å, O(6W)‚‚‚O(1) 2.850(4) Å, O(6W)‚‚‚O(8) 2.703(4) Å]. The ladders run in a parallel direction and stack on each other with a large separation of 14.9 Å in a superposed fashion, therefore generating two types of cavities with sizes of ca. 5.5 Å × 6.7 Å and 6.7 Å × 10.3 Å. The two motifs (A and B) are entangled in such a way that the larger cavities of the B-ladders are threaded by A-chains, thus giving an unprecedented poly(pseudo-rotaxane)-type architecture, as illustrated in Figure 4. The multipoint hydrogen bonds, associated with the coordinated water molecules of 1D zigzag chain and carboxylate oxygen atoms and lattice water molecules of 1D ladder [O(1W)‚‚‚O(6) 2.846(4) Å, O(1W)‚‚‚O(1) 2.852(3) Å, O(2W)‚‚‚O(6W) 2.701(3) Å, O(3W)‚‚‚O(4) 2.750(3) Å, O(4W)‚‚‚O(5W) 2.687(3) Å, O(4W)‚‚‚O(3) 2.862(4) Å], further stabilize the whole entangled array. Compared with the entangled network of 1, compound 2 displays a different yet interesting structural array. That is, in the bc plane, the btc anions together with the lattice water
molecule O(1W) are connected into a 2D (4,4) sheet (C) with rectangular windows via hydrogen-bonding interactions [O(1W)‚‚‚O(1) 2.804(6) Å, O(1W)‚‚‚O(4) 2.906(6) Å] (see Figure 3b). Similar to 1, adjacent layers are stacked in a parallel fashion with an interlayer separation of 9.96 Å so that large rectangular channels of ca. 9.6 Å × 8.4 Å are formed. It is noteworthy that B-type chains derive other chains noted as B′ via symmetry operation [-x, y, z - 0.5]. More remarkably, B and B′ chains pass through the C sheets inclined in such a way that the threading components within adjacent rows of windows run on the two decks rotated by 120° about the c axis on passing from one level to the successive one, thus resulting in an interesting poly(pseudo-rotaxane) array (see Figure 5). The linear A-chains are packed between the 2D layers, making an angle of 120° with chains B and B′. Thus, another peculiar structural feature of this compound consists of the presence of the three directional polymeric chains. To the best of our knowledge, the known cases that contain 1D chains spanning three different directions are quite rare.9 Likewise, the covalent linear chains located in the channels are stabilized by hydrogen bonding to the btc4- groups [O(5)‚‚‚O(3) 2.642(5) Å, O(6)‚‚‚O(4) 2.791(5) Å, O(6)‚‚‚O(1) 2.807(5) Å, O(7)‚‚‚O(2) 2.660(5) Å, O(7)‚‚‚O(3) 2.716(4) Å, O(8)‚‚‚O(1) 2.720(5) Å, O(8)‚‚‚O(3) 2.847(4) Å] and to the lattice water molecule O(1W) [O(5)‚‚‚O(1W) 2.720(5) Å]. It should be noteworthy that a Co-complex isostructural with 2 has been reported by Lu et al. that is also obtained via hydrothermal methods.6c In this compound, two crystallographically independent Co(II) centers, similar to 2, are connected into one-dimensional chains through intervening 4,4′-bipy ligands with Co‚‚‚Co separation of ca. 11.44 Å. The Co-N (2.136(4)-2.207(3) Å) and Co-O (2.043(3)-2.154(3) Å) bond lengths are longer than those of 2. Just as observed in 2, these chains are further extended into a three-dimensional framework through complex hydrogen bonds among the solvent water molecules, aqua ligands, and carboxylate anions. The number and intensity of hydrogen bonds are comparable with that of 2. Although it is silent about the presence of this unusual polythreaded array, a complex hydrogenbonding scheme in the packing has been envisaged by authors. Entangled structures containing different structural motifs are at present rare,10 and even less common are structures in which 1D polymers thread into the rings belonging to other motifs of a different nature.11-15 Such extended architectures, according to Ciani and co-workers, can be considered as infinite poly(pseudo-rotaxanes) in that the different motifs can, in principle, be disentangled without breaking links.16 Given that the threading components in the previously reported examples exclusively extend in the same direction, the two directional threading units
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the range 120-220 °C (observed 20.75%, calcd 20.93%). The release of the 4,4′-bipy and btc ligands in 2 occurs between 240 and 410 °C (observed 61.82%, calcd 61.70%). The remaining weight of 17.43% indicates that the final product is NiO (calcd 17.37%). Conclusions In summary, two interesting supramolecular networks constructed by concomitant coordinative and hydrogen bonds, [Co(4,4′-bipy)(H2O)4]‚(H2bptc)‚2H2O (1) and [Ni(4,4′-bipy)(H2O)4]‚ 0.5(btc)‚H2O (2), that show unusual poly(pseudo-rotaxane) arrays have been synthesized and characterized. The new types of entangled networks here reported represent another important branch in the realm of entanglement and open interesting perspectives in the study of these materials. The successful isolation of the two species not only provides intriguing examples of chemical topology but also demonstrates that the introduction of multicarboxylate ligands may provide multiple hydrogen-bonding interactions, which may endow enormous potential for assembling novel supramolecular architectures. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant No. 20371011). Supporting Information Available: TGA curves, selected bond length and angles, and X-ray crystallographic information files (CIF) for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.
References
Figure 5. (a) Crystal packing of 2, showing the 3D poly(pseudorotaxane) array and (b) space-filling plot of the two threading chains with different propagating directions in the windows of the layers.
observed in 2 represent a completely new type of threading mode within polythreaded systems. Furthermore, in almost all the entangled arrays containing “separable” motifs, linear polymeric chains are the most popular threading elements, whereas poly(pseudo-rotaxane)-type structures involving zigzag chains, as shown in 1, are less common.13 Therefore, the two entangled networks here reported, besides enriching the supramolecular field, provide new examples for polythreaded systems. Thermal Stability Analyses. To examine the thermal stability of the two compounds, thermal gravimetric (TG) analyses were carried out for 1 and 2 between 30 and 800 °C (Figure S1). Probably due to the existence of multiple hydrogen bonds associated with the lattice water molecules in 1 and 2, the initial loss temperature of lattice water molecules is somewhat higher than those without any hydrogen-bonding interactions (generally starts from ca. 50 °C).17 For 1, a total weight loss of 16.34% occurred over the temperature range 100-240 °C, corresponding to the loss of six water molecules (calcd 16.58%), followed by two-step continuous weight losses of 24.23% and 48.04% in the ranges 240-350 and 350-440 °C consistent with the removal of 4,4′-bipy (calcd 23.96%) and bptc ligands (calcd 47.96%), respectively. The remaining weight of 11.39% corresponds to the percentage (11.50%) of Co and O components, indicating that the final product is CoO. The thermal decomposition behavior of 2 is much like that observed for 1. The compound loses five water molecules in
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CG050376U
