Article pubs.acs.org/crystal
Syntheses, Characterizations, and Properties of Five Interpenetrating Complexes Based on 1,4-Benzenedicarboxylic Acid and a Series of Benzimidazole-Based Linkers Qianqian Guo,† Chunying Xu,‡ Bei Zhao,† Yanyuan Jia,† Hongwei Hou,*,† and Yaoting Fan† †
College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, Henan, 450052, People's Republic of China College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, People's Republic of China
‡
S Supporting Information *
ABSTRACT: Five interesting interpenetrating networks, namely, [Co(p-bdc)(beb)0.5]n (1), {[Co(p-bdc)(bmb)]·H2O}n (2), {[Co(pbdc)(bmp)]·H2O}n (3), {[Zn(p-bdc)(bmp)]·H2O}n (4), and [Zn2(pbdc)2(bmp)(H2O)2]n (5) [p-H2bdc = 1,4-benzenedicarboxylic acid, beb = 1,4-bis(2-ethylbenzimidazol-1-ylmethyl)benzene, bmb = 1,4-bis(2methylbenzimidazol-1-ylmethyl)benzene, and bmp = 1,5-bis(2-methylbenzimidazol) pentane], have been synthesized by employing mixed ligands of various benzimidazole-based ligands with p-H2bdc. Complex 1 possesses a 2-fold interpenetrating 3D framework with (412·63)-pcu topology. Complex 2 exhibits a 3-fold interpenetrating 3D network with 66-dia topology, and complex 3 displays a 4-fold interpenetrating 3D diamond network containing Co/bmp left- and right-handed helical chains. Obviously, with the reducing of the steric hindrance of the Ndonor ligand, complexes 1−3 show interpenetrating networks from 2-fold to 3-fold and 4-fold. Complex 4 is isostructural to 3 and also forms a 4-connected 3D framework with a diamond topology. Complex 5 features a 3D framework generated by 2D → 3D interpenetration and exhibits (82·10)2 topology. Our study shows that the steric hindrance changing of ligands can tune the final interpenetrating networks directly.
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INTRODUCTION
In general, long and low steric hindrance ligands will lead to larger voids that may result in high-fold interpenetrating structures.7 Therefore, the effective strategy for increasing the number of interpenetrating folds is to use long and low steric hindrance ligands.8 A 54-fold interpenetrating 3D framework with 103-srs topology, reported by the Yang group, is the highest level of interpenetration to date, which is a successful example in using this strategy.9 Recently, two ThSi2 networks reported by the Sun group with different degrees of interpenetration, 3-fold and 8-fold, were obtained simply by modulating the length of pyridyl-based organic tectons; that is, the longer ligand favored the higher interpenetration degree and vice versa.10 Accordingly, it is possible to regulate the number of interpenetrating folds by varying the ligands’ length and steric hindrance. We designed and synthesized three Ndonor ligands: beb, bmb, and bmp, which possess different steric hindrance and lengths. Employing them as auxiliary ligands in the self-assembly with organic carboxylic acid pH2bdc and Co(II)/Zn(II) ions, we successfully constructed five interpenetrating 3D MOFs: [Co(p-bdc)(beb)0.5]n (1), {[Co(pbdc)(bmb)]·H2O}n (2), {[Co(p-bdc)(bmp)]·H2O}n (3),
In recent years, metal−organic frameworks (MOFs) with entangled architectures have received remarkable attention, stemming from not only their numerous practical applications and remarkable range of physical properties but also their intriguing variety of architectures and topologies.1 One of the most familiar phenomena related to entanglements is interpenetration. Conceptually, interpenetrating networks can be viewed as a series of independent nets participating in interpenetration mutually.2 Some comprehensive reviews have been published by Robson and Batten, and Ciani et al.3 They have made systematic analysis about interpenetration in metal− organic and inorganic networks as well as the classified topology of a great variety of interpenetrating networks. Up to now, considerable efforts have been invested in interpenetrating coordination polymer frameworks and a large number of interpenetrating networks have been reported by many groups.4 In the vast amount of reported works, the rational synthesis of interpenetrating networks is of particular interest.5 It still remains a long-term challenge, due to the complicated factors that could have influences on the resulting networks, such as versatility of metal ions and ligands, the subtle influences of weak interactions, and other factors, including synthetic conditions.6 © XXXX American Chemical Society
Received: July 20, 2012 Revised: September 17, 2012
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Table 1. Crystallographic Data and Structure Refinement Details for 1−5
a
complex
1
2
3
4
5
formula fw temp (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dc (g·cm−3) μ (mm−1) F(000) GOF on F2 R1[I > 2σ(I)]a wR2 (all data)b
C21H17N2O4Co 420.30 293(2) 0.71073 monoclinic C2/c 21.679(4) 10.889(2) 20.104(4) 120.43(3) 4092.2(14) 8 1.364 0.866 1728 1.175 0.0525 0.1340
C32H28N4O5Co 607.52 293(2) 0.71073 monoclinic C2/c 22.412(4) 8.3970(17) 16.207(3) 100.80(3) 2996.0(10) 4 1.347 0.619 1260 1.112 0.0580 0.1602
C29H30N4O5Co 573.51 293(2) 0.71073 orthorhombic Ibca 16.173(3) 17.917(4) 18.017(4) 90 5220.9(18) 8 1.454 0.705 2376 1.265 0.0520 0.1120
C29H30N4O5Zn 579.98 293(2) 0.71073 orthorhombic Ibca 17.834(4) 18.075(4) 16.151(3) 90 5206.3(18) 8 1.480 0.992 2416 1.147 0.0702 0.1824
C37H36N4O10Zn2 827.52 293(2) 0.71073 monoclinic P21/c 11.622(2) 17.483(4) 17.525(4) 93.95(3) 3552.3(12) 4 1.547 1.416 1704 1.022 0.0909 0.1332
R1 = ∑∥Fo| − |Fc∥/∑|Fo|. bwR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2. Synthesis of {[Co(p-bdc)(bmp)]·H2O}n (3). The same synthetic method as that for 1 was used except that beb was replaced by bmp (33.4 mg, 0.1 mmol). Yield: 62% (based on Co). Anal. Calcd for C29H30N4O5Co (%): C, 60.73; H, 5.27; N, 9.77. Found: C, 60.91; H, 4.95; N, 9.58. IR (KBr, cm−1): 3435 s (−O−H), 2932 m (−C−H), 1606 S (−CO), 1508 m (−CO), 1490 m (−COO−), 1463 w (−COO−), 1380 m (−CN), 1341 m (−CN), 1141 w (−C−N), 817 m (−p−C6H4−), 748 s (−CH2−)5. Synthesis of {[Zn(p-bdc)(bmp)]·H2O}n (4). The same synthetic method as that for 3 was used except that Co(NO3)2·6H2O was replaced by Zn(NO3)2·6H2O (59.4 mg, 0.2 mmol). Yield: 65% (based on Zn). Anal. Calcd for C29H30N4O5Zn (%): C, 60.06; H, 5.21; N, 9.66. Found: C, 60.31; H, 4.87; N, 9.74. IR (KBr, cm−1): 3444 s (−O− H), 3132 m (−C−H), 1613 s (−CO), 1460 m (−COO−), 1399 s (−CN), 1355 s (−CN), 1145 w (−C−N), 817 w (−p−C6H4−), 746 s (−CH2−)5. Synthesis of [Zn2(p-bdc)2(bmp)(H2O)2]n (5). The same synthetic method as that for 4 was used except that the hydrothermal reaction temperature is 130 °C. Yield: 35% (based on Zn). Anal. Calcd for C37H36N4O10Zn2 (%): C, 53.70; H, 4.38; N, 6.77. Found: C, 55.81; H, 4.73; N, 6.25. IR (KBr, cm−1): 3434 s (−O−H), 3124 s (−C−H), 1620 s (−CO), 1499 m (−COO−), 1460 m (−COO−), 1398 s (−CN), 1362 s (−CN), 1148 w (−C−N), 818 m (−p−C6H4−), 744 s (−CH2−)5. Crystal Data Collection and Refinement. The data of the five polymers were collected on a Rigaku Saturn 724 CCD diffractomer (Mo Kα, λ = 0.71073 Å) at a temperature of 20 ± 1 °C. Absorption corrections were applied by using a multiscan program. The data were corrected for Lorentz and polarization effects. The structures were solved by direct methods and refined with a full-matrix least-squares technique based on F2 with the SHELXL-97 crystallographic software package.12 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms of ligands were included in the structure factor calculation at idealized positions by using a riding model and refined isotropically. The hydrogen atoms of coordination water molecules and solvent water molecules were located from the difference Fourier maps, restrained at fixed positions and refined isotropically. Crystallographic crystal data and structure processing parameters for 1−5 are summarized in Table 1. Selected bond lengths and bond angles of 1−5 are listed in Table S1 in the Supporting Information.
{[Zn(p-bdc)(bmp)]·H2O}n (4), and [Zn2(p-bdc)2(bmp)(H2O)2]n (5). The properties of thermogravimetric analysis, X-ray powder diffraction of the complexes, and the chemical stability for boiling water, methanol, ethanol, acetonitrile, and DMF conditions have been discussed. In addition, the photoluminescence properties of 4 and 5 were studied in the solid state at room temperature.
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EXPERIMENTAL SECTION
Materials and Physical Measurements. All reagents and solvents were commercially available except for beb, bmb, and bmp, which were synthesized according to the literature.11 The FT-IR spectra were recorded from KBr pellets in the range of 400−4000 cm−1 on a Bruker Tensor 27 spectrophotometer. Elemental analyses (C, H, and N) were carried out on a FLASH EA 1112 elemental analyzer. PXRD patterns were recorded using Cu Kα1 radiation on a PANalytical X’Pert PRO diffractometer. Thermal analyses were performed on a Netzsch STA 449C thermal analyzer from room temperature at a heating rate of 10 °C min−1 in air. The luminescence spectra for the powdered solid samples were measured at room temperature on a Hitachi 850 fluorescence spectrophotometer. The excitation slit and the emission slit were 2 nm. Synthesis of [Co(p-bdc)(beb)0.5]n (1). A mixture of Co(NO3)2·6H2O (58.2 mg, 0.2 mmol), beb (39.4 mg, 0.1 mmol), pH2bdc (33.2 mg, 0.2 mmol), and NaOH (16.0 mg, 0.4 mmol) in 10 mL of distilled H2O was sealed in a 25 mL Teflon-lined stainless steel container and heated at 160 °C for 3 days. After the mixture cooled to room temperature at a rate of 5 °C/h, dark blue block crystals of 1 were obtained with a yield of 48% (based on Co). Anal. Calcd for C21H17N2O4Co (%): C, 60.01; H, 4.08; N, 6.66. Found: C, 59.74; H, 4.36; N, 6.22. IR (KBr, cm−1): 2971 m (−C−H), 1627 s (−CO), 1505 s (−CO), 1472 m (−CH2−CH3), 1456 m (−COO−), 1391 s (−CN), 1350 m (−CN), 1157 m (−C−N), 821 m (−p− C6H4−), 771 s (−CH2−). Synthesis of {[Co(p-bdc)(bmb)]·H2O}n (2). The same synthetic method as that for 1 was used except that beb was replaced by bmb (36.8 mg, 0.1 mmol). Yield: 68% (based on Co). Anal. Calcd for C32H28N4O5Co (%): C, 63.26; H, 4.64; N, 9.22. Found: C, 63.52; H, 4.41; N, 9.37. IR (KBr, cm−1): 3467 s (−O−H), 2921 w (−C−H), 1602 s (−CO), 1510 m (−CO), 1456 m (−COO−), 1402 w (−COO−), 1384 m (−CN), 1350 m (−CN), 1143 w (−C−N), 819 m (−p−C6H4−), 766 s (−CH2−).
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RESULTS AND DISCUSSION Design and Synthesis of Complexes. We selected a mixed ligand strategy based on the following considerations: (i) B
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Scheme 1. Structures of the Organic Ligands Used in This Work
Figure 1. (a) Coordination environments of the Co(II) ions in 1. Hydrogen atoms are omitted for clarity. (b) Perspective view of the 3D framework in 1. (c) Schematic illustrating the 3D pcu topology in complex 1. Color code: green stick, p-bdc2− linker; yellow stick, beb linker. (d) The 3D 2-fold interpenetrating framework of 1.
links between nodes, which lead to larger free voids. Intending to obtain interpenetrating networks and observe their influences on the final structures, we selected the three bis(benzimidazole) molecules with different substituents and spacers as the secondary ligands. As shown in Scheme 1, the 2position substituent group of beb is ethyl as well as of bmb is methyl, which makes beb possess higher steric hindrance. The flexible nature of −CH2− spacers allows the ligands to bend and rotate freely when coordinating to metal centers. The addition of −CH2− spacers in N-donor ligands makes beb and bmb semirigid as well as bmp absolutely flexible. Complexes 1− 3 were prepared under similar synthetic conditions, by directly assembling the Co(II) ion with organic carboxylic acid p-H2bdc and N-donor ligands in H2O solution under hydrothermal conditions. With the reducing of the steric hindrance of Ndonor ligands, complexes 1−3 show 3D interpenetrating networks from 2-fold to 3-fold and 4-fold.
The effective strategy for obtaining the interpenetrating framework is to use long and low steric hindrance ligands. Some of terephthalic and imidazole derivatives are too short to build an interpenetration structure alone. (ii) As the length of the ligand increases, it is difficult for us to obtain crystalline complexes by using single terephthalic derivatives or imidazole derivatives.9 (iii) According to our recent research, we anticipate that the mix of short organic carboxylic acid and long N-donor ligands would be an effective way in obtaining complexes in crystalline.13 p-H2bdc, with two carboxylic groups extending in the opposite directions, is the longest one among the three benzenedicarboxylic acids (1,2/1,3/1,4-benzenedicarboxylic acid). The coligands, 1,4-bis(2-ethylbenzimidazol-1-ylmethyl)benzene, 1,4-bis(2-methylbenzimidazol-1-ylmethyl)benzene, and 1,5-bis(2-methylbenzimidazol) pentane (beb, bmb, and bmp), possess only two coordination sites in each end. All of these characteristics are propitious to increase the length of C
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Figure 2. (a) Coordination environments of the Co(II) ions in 2. Hydrogen atoms and the free water molecule are omitted for clarity. (b) The pillar-layered 3D framework in 2. (c) Perspective view of the single adamantoid net. (d) Schematic illustrating the 3D dia topology in complex 2. Color code: green stick, p-bdc2− linker; yellow stick, bmb linker. (e) View of the simplified 3D 3-fold interpenetrating array.
axial positions of the paddlewheel are occupied by the N atom. In addition, the b axis is a 2-fold axis that passes the Co2 paddlewheel unit in 1. All of these structural parameters fall into the normal range.14 In 1, beb adopts a symmetrical trans conformation with a Ndonor···N−Csp3···Csp3 torsion angle of 133.25°. The Co(II) ions are coordinated by completely deprotonated p-bdc2− anions, which adopt a bidentate fashion to produce a Co/pbdc2− 2D network with a Co···Co separation of 10.889 and 10.752 Å. beb also acts as a bidentate ligand to bridge these 2D sheets to give rise to a pillar-layered 3D framework (Figure 1b). Owing to the great torsion angle of beb and the long distance of Co···Co (18.339 Å) between adjacent 2D networks, there is a broad unoccupied void space existing in the single 3D framework, which exhibits maximum dimensions (corresponding to the longest intracage Co···Co distances) of 10.889 × 10.752 × 18.339 Å. Of particular interest, the most striking feature of complex 1 is that a pair of identical 3D single nets is interlocked with each other, thus directly leading to the formation of a 2-fold interpenetrated 3D architecture (Figure 1d). This feature can greatly enhance the stability of the whole structure. Similarly, Su and co-workers successfully synthesized a 2-fold interpenetrating framework. In this framework, 2-(1(pyridin-4-ylmethyl)-1H-imidazol-2-yl)pyridine possessing less steric hindrance and length than beb was used as the N-donor ligand.15 To further demonstrate the overall 3D structure of 1, we can consider each paddlewheel [Co2(COO)4] cluster as a six-connecting node, which is linked to six equivalent nodes through four p-bdc2− anions and two beb ligands. Moreover, the bed and p-bdc2− anions are simplified as a linear linker. With further topological analysis by the OLEX program, the whole structure of 1 can be simplified to a 2-fold 3D framework with its topological notation of 412·63 (Figure 1c).
Among the three N-donor liagnds, bmp is the longest one and lowest in steric hindrance. Therefore, it is an excellent constructor for interpenetrating networks. Block-shaped crystals of 4 and 5 were obtained by reacting Zn(NO3)2, bmp, and p-H2bdc in water at different reacting temperatures under hydrothermal conditions. Since bmp is a flexible ligand, the coordination conformation is changeable. In 4, bmp adopts an “Ω” conformation (with a Zn···Zn separation of 14.044 Å), and bmp exhibits an “M” conformation in complex 5 (with a Zn···Zn separation of 14.635 Å), respectively. The dihedral angle between benzimidazole rings in one ligand is 10.68° for 4 and 79.82° for 5. Complex 4 displays a 4-fold interpenetrating 3D network. Different from it, complex 5 was obtained by decreasing the reaction temperature. It was worth noting that temperature played a significant role in synthesizing the two complexes, which induced the 4-fold interpenetrating 3D structure {[Zn(p-bdc)(bmp)]·H2O}n (4) transforming into a 3D interpenetrating framework generated by 2D → 3D interpenetration [Zn2(p-bdc)2(bmp)(H2O)2]n (5). The result reveals a new way to obtain and tune interpenetrating networks. Crystal Structure of [Co(p-bdc)(beb)0.5]n (1). Singlecrystal X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic space group C2/c. The Co(II) ion is coordinated by four oxygen atoms from four p-bdc2− anions [Co−O = 2.024−2.052 Å] and one nitrogen atom [Co− N = 2.068 Å] to furnish a distorted square-pyramidal geometry (Figure 1a). In this square-pyramid, O1, O3, O2A, and O4B lie in the basal plane and N1 inhabits the apex. As shown in Figure 1a, two five-coordination Co(II) ions are engaged by four carboxyate groups (μ2-η1:η1-C(23A)OO/C(14)OO/C(18)OO/C(14B)OO; symmetry codes: A, x, −1 + y, z; B, −x, y, 0.5 − z) from two p-bdc2− anions into the binuclear paddlewheel [Co2(COO)4], with an average Co−O/nonbonding Co···Co distance of 2.045/2.974 Å, respectively. The D
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Figure 3. (a) Coordination environments of the Co(II) ions in 3. Hydrogen atoms and the free water molecule are omitted for clarity. (b) Perspective view of the 3D framework in 3. (c) The Co(II)/bmp left- and right-handed helical chains are connected by p-bdc2− anions along the a axis. (d) Perspective view of the single adamantoid net. (e) Schematic illustrating the 3D dia topology in complex 3. Color code: green stick, p-bdc2− linker; yellow stick, bmp linker. (f) Schematic representation of the 4-fold interpenetrating diamond net in 3.
Crystal Structure of {[Co(p-bdc)(bmb)]·H2O}n (2). As compared to complex 1, the beb coligand is replaced by bmb, and the resulting structure is a 3-fold diamond framework. Single-crystal X-ray diffraction reveals that 2, which crystallizes in the monoclinic space group C2/c, is an infinite 3D coordination framework. Its asymmetric unit contains one Co(II) ion, one beb ligand, one p-bdc2− anion, and one guest water molecule. The coordination environment around Co(II) is shown in Figure 2a. Each Co(II) ion sits in a distorted tetrahedron geometry defined by two nitrogen atoms [Co−N = 2.052 Å] of two beb ligands and two oxygen atoms [Co−O = 1.997 Å] from two separated p-bdc2− anions. The Co−O/N bond lengths are all consistent with corresponding bond lengths found in the literature.14 The two carboxylic groups of p-bdc2− take a uniform coordination mode, all in a μ1-η1:η0 fashion. Co(II) ions are connected by p-bdc2− anions to form a 1D chain. Adjacent 1D chains are further linked by bmb, which adopts a symmetrical trans conformation with Ndonor···N−Csp3 ··· Csp3 torsion angles of 73.79° to form a 2D wavelike layer structure. The extension of the structure into a 3D network is also accomplished by connecting two 2D layers through bmb (Figure 2b). On the basis of the concept of chemical topology, the overall structure is a diamond framework. Notably, there are large adamantanoid
cages that exist, which shows a possibility that 2 may be also displaying interpenetrating structural characteristics. As can be seen, the cages exhibit maximum dimensions (corresponding to the longest intracage Co···Co distances) of 22.412 × 25.191 × 35.787 Å (Figure 2c). The potential voids are filled via mutual interpenetration of two other independent equivalent frameworks, generating a 3-fold interpenetrating 3D architecture (Figure 2e). Similarly, a 3-fold interpenetrating framework, reported by the Zhang group, is based on 1,4-bis(imidazol-l-ylmethyl)-benzene, which possesses a similar length and steric hindrance with bmb.16 From the topological point of view, the Co(II) atom is simplified as a 4-connected node, and p-bdc2− and bmp ligands are considered as linkers. Accordingly, the 3D complex framework of 2 can be simplified to a 4-connected topology with the topological notation of 66 (Figure 2d). Crystal Structure of {[Co(p-bdc)(bmp)]·H2O}n (3) and {[Zn(p-bdc)(bmp)]·H2O}n (4). X-ray crystallography reveals that complexes 3 and 4 have similar structures except that the central metal ion is different (Co(II) for 3, and Zn(II) for 4). Here, we choose 3 to represent the detailed structure. The Xray crystallographic analysis revealed that 3 is a 3D supramolecular architecture with 4-fold interpenetration. The asymmetric unit of 3 consists of a Co(II) ion, a p-bdc2− anion, one molecule of bmp, and one water solvent molecule. E
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Figure 4. (a) Coordination environments of the Zn(II) ions in 5. Hydrogen atoms are omitted for clarity. (b) The two kinds of 1D infinite chains in 5. (c) Schematic illustrating the 2D (82·10)2 topology in complex 5. Color code: green stick, p-bdc2− linker; yellow stick, bmp linker. (d) Schematic representation of the 2D → 3D interpenetrating nets in 5. (e) A perspective view showing the hydrogen-bonding interactions in 5.
Zn(II) ion is four-coordinated by one water molecule, one Ndonor atom of the benzimidazolyl group, and two oxygen atoms derived from two p-bdc2− groups with a slightly distorted tetrahedron geometry, which is completed by two oxygen atoms from two different p-bdc2− anions and one oxygen atom afforded by one water molecule located at the basal plane and one nitrogen atom from one bmp at the apex. The distances of Zn−N are 1.997−2.018 Å, while the Zn−O bond lengths range from 1.925 to 1.978 Å. Both of them are within the normal ranges.17 Compared with complex 4, the bmp ligand in 5 adopts an “M” conformation, and the dihedral angle between two benzimidazole rings in one ligand is 79.82°. Each Zn(II) atom is linked by carboxylate groups of the p-H2bdc ligand in a bis-monodentate coordination mode to form two kinds of 1D infinite chains with Zn···Zn···Zn angles of 111.56 and 104.57°, respectively (Figure 4b). Two kinds of 1D chains are interlinked together by bmp to generate a corrugated 2D structure. From the topological point of view, if the Zn(II) atoms are regarded as a 3-connected node and ligands pH2bdc2−/bmp are considered as linkers, the 2D structure in 5 can be described as a 3-connected net with the point symbol of (82·10)2 (Figure 4c). Interestingly, complex 5 is a new slab topology, according to the testing of the Systre program (Information 1, Supporting Information). The open void space within the 2D network is minimized by the interpenetration of another two identical 2D networks in an inclined fashion, with each layer catenated by two others to produce a 2D → 3D interpenetrating network (Figure 4d). The crystal structure of complex 5 is further strengthened through hydrogen-bonding interactions between the carboxylate oxygen atoms (O1···O9 =
The Co(II) ion is four-coordinated by two carboxylate oxygen atoms from two p-bdc2− anions, and two nitrogen atoms from two bmp ligands, showing a tetrahedral geometry {CoN2O2} (Figure 3a). The Co−O and Co−N distances are quite similar to the normal Co−O and Co−N distances.14 Notably, different from the above complexes, there are leftand right-handed helices constructed by the Co(II) ion and bmp with an “Ω”conformation in 3 (Figure 3c). The helical pitch is 16.173 Å, corresponding to the length of the a axis. The extension of the structure into a 3D network is accomplished by connecting these 1D helices through p-bdc2− anions, which adopt a bis-monodentate mode (Figure 3b). The Co···Co distances across p-bdc2− and bmp are 11.111 and 14.012 Å, respectively. A further analysis indicates that it is a typical diamondoid framework containing large cages like 2 and exhibits dimensions larger than 2 (corresponding to the longest intracage Co···Co distances of 36.034 × 22.745 × 35.034 Å (Figure 3d). The potential voids are filled via mutual interpenetration of the other three independent equivalent diamondoid frameworks in a normal mode, giving rise to a 4fold interpenetrating dia array (Figure 3f). In topology, the analogical simplification strategy for 2 is employed. The 3D complex framework of 3 can be simplified to a 4-connected topology with the topological notation of 66 (Figure 3e). Crystal Structure of [Zn2(p-bdc)2(bmp)(H2O)2]n (5). When the hydrothermal reaction temperature was decreased to 130 °C, a structurally different complex 5 was isolated. In complex 5, the asymmetric unit consists of two Zn(II) ions, one coordinated bmp, two p-bdc2− anions, and two coordinated water molecules. As shown in Figure 4a, there are two unique Zn(II) centers with the same coordination environment. Each F
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intraligand π* → π charge transitions of p-H2bdc due to their similar emission bands. In addition, further investigation indicates that the fluorescent intensities of polymers 4 and 5 dramatically decrease compared with that of p-H2bdc. It may be attributed to the interpenetrating features in complexes 4 and 5, which lower the rigidity of the ligands and increase the loss of energy via vibration motions. In addition, further investigation indicates that the fluorescence intensities of complex 4 heighten relative to complex 5. It may be attributed to the structure difference (4 is a 3D framework; however, 5 is a 3D network generated by 2D → 3D interpenetration), which strengthen the rigidity of the framework and lower the loss of energy.20 The results indicate that the structures of the two complexes have subtle influences on the photoluminescent intensities.
2.586 Å, O9···O3 = 2.588 Å, O6···O10 = 2.608 Å, O7···O10 = 2.581 Å) (Figure 4e). Analyses of Thermal and Chemical Stability. To estimate the stability of the coordination architectures, thermogravimetric analyses (TGA) were carried out. The phase purities of the bulk samples were identified by powder Xray diffraction (Figure S1, Supporting Information). Results of the TGA curves show that the five complexes possess different thermal stabilities (Figure S2, Supporting Information). For complex 1, there is no weight loss until the decomposition of the framework occurs at 350 °C. The mass remnant of 20.1% was consistent with the deposition of Co2O3 (19.7% expected). For 2, a slight decrease in mass at 100 °C (3.0% predicted and 3.5% observed) marks the expulsion of water molecules of crystallization. The TGA curve of 2 shows a one-step weight loss process from 342 to 570 °C, corresponding to the decomposition of organic components. A Co2O3 residue of 13.7% (calcd 13.6%) is observed. For complex 3, the weight loss between 145 and 218 °C corresponds to the release of lattice water molecules (obsd 2.8%; calcd 3.1%). At 580 °C, a Co2O3 residue of 15.2% (calcd 14.5%) is obtained. For complex 4, the weight loss between 110 and 250 °C is attributed to the release of lattice water molecules (obsd 2.9%; calcd 3.1%). The overall framework of 4 begins to collapse at 385 °C, and the ZnO residue of 14.4% (calcd 14.0%) is observed at 610 °C. For complex 5, the weight loss corresponding to the release of coordinated water molecules is observed from 150 to 204 °C (obsd 3.8%; calcd 4.4%). The anhydrous composition begins to decompose at 390 °C. At 580 °C, a ZnO residue of 20.2% (calcd 19.7%) is obtained. Furthermore, the chemical stability of complexes 1−5 was examined by suspending samples in boiling water, methanol, ethanol, acetonitrile, and DMF conditions for 24 h, which imitates the usual solvents of typical industrial chemical processes. During this process, samples were periodically observed under an optical microscope. Complex 1 was found to have kept the original shape for all the usual solvents mentioned above. Complexes 2−5 also showed excellent chemical resistance to these solvents except for boiling DMF. Single-crystal X-ray diffraction showed that the unit cell parameters of them basically did not change. We further characterize the products after treating in boiling DMF by powder X-ray diffraction, which indicates that the crystals of 2− 4 became amorphous after treating in boiling DMF (Figure S3, Supporting Information). The high chemical stability of the five complexes may be ascribed to a homogeneous and dense structure.18 Different with complex 1, there are water molecules that exist in complexes 2−5. Under 154 °C (the boiling point of DMF), complexes 2−5 start to lose their water molecules, which can been seen from the thermal analysis above. That may be the main reason that 2−5 did not keep the original shape for boiling DMF, but 1 did. Photoluminescence Properties. Photoluminescence research of coordination polymers with d10 metal centers and conjugated organic linkers has attracted increasing attention in recent years.19 Hence, we investigated the solid-state photoluminescence properties of Zn(II) polymers 4 and 5, together with the free H2bdc ligand and bmp at room temperature (Figure S4, Supporting Information). The free ligands H2bdc and bmp show intense emissions bands at 369 nm (λex = 331 nm) and 304 nm (λex = 268 nm), respectively. Obviously, the fluorescent emission bands of 4 (λem = 372 nm, λex = 325 nm) and 5 (λem = 354 nm, λex = 319 nm) can be attributed to the
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CONCLUSIONS Five new interpenetrating networks have been successfully isolated under hydrothermal conditions by reaction of organic carboxylic acid and a series of rationally selected N-donor ligands together with Co(II)/Zn(II) salts. Complexes 1−3 exhibit the number of interpenetrating folds changing from 2 to 3 and 4, which may result from the decreasing steric hindrance of N-donor ligands. Complex 4 is isostructural to 3, in which Co(II) ions are replaced by Zn(II) ions. Complex 5 is obtained by using the same materials as with 4, but a different reaction temperature in the self-assembling process, and it possesses a 3D interpenetrated motif generated by 2D → 3D interpenetration. The study of the stability of the complexes demonstrates that they have different thermal stabilities and excellent chemical resistance to boiling water and common organic solvents. In addition, the photoluminescence of the complexes 4 and 5 was studied in the solid state at room temperature. This work may provide a potential route for constructing an interpenetrating network, even regulating the final number of interpenetrating folds by selecting a ligand with the appropriate length and coordination sites.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray crystallographic files in CIF format, the result of 5 tested by the Systre program, selected bond lengths and bond angles, powder X-ray patterns and thermal analyses for 1−5, fluorescence analyses for 4 and 5, powder X-ray patterns of 2−5 after treating them in boiling DMF, and the structural formulas of the free ligands. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax: (86) 0371-67761744. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation (Nos. 20971110 and 91022013). REFERENCES
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