Assembly of 3D Metal-Organic Frameworks Based on Different Helical

Aug 6, 2009 - In 2, Zn(II) cations are connected by ODPT4− anions and one kind of L2 ligand to form a double helical chain, which is linked by other...
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DOI: 10.1021/cg900371k

Assembly of 3D Metal-Organic Frameworks Based on Different Helical Units: Chiral and Achiral Structures Constructed by Length-Modulated N-Donor Ligands

2009, Vol. 9 4142–4146

Shun-Li Li, Ya-Qian Lan, Jun-Sheng Qin, Jian-Fang Ma,* Jie Liu, and Jin Yang Key Lab of Polyoxometalate Science, Department of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China Received April 4, 2009; Revised Manuscript Received July 17, 2009

ABSTRACT: Two coordination polymers constructed from two structurally related ligands, 4,40 -bis(imidazol-1ylmethyl)benzene (L1) and 4,40 -bis(imidazol-1-ylmethyl)bibenzene (L2) have been synthesized, [Zn2(ODPT)(L1)] 3 3H2O (1) and [Zn2(ODPT)(L2)2] (2), where H4ODPT is 4,40 -oxidiphthalic acid. Their structures have been determined by single crystal X-ray diffraction analyses. In 1, two kinds of L1 ligands coordinate to chiral centers Zn(II) cations to form two types of zigzag chains, which entwine each other to generate the snakelike chains. The phthalic groups from ODPT4- ligands link Zn(II) cations to form a right-handed 21 helical chain. This helical chain and the entwined snakelike chains entangle together to form a chiral subunit by co-ZnII cations. Then, intramolecular oxygen atoms from ODPT4- ligands bridge these subunits to extend a chiral three-dimensional framework with (52 3 84)(53 3 62 3 7)2 topology. In 2, Zn(II) cations are connected by ODPT4- anions and one kind of L2 ligand to form a double helical chain, which is linked by other helical chain constructed by other kind of L2 ligand and Zn(II) cation to generate a 3D framework. Two kinds of networks showing opposite helical units interpenetrate each other giving rise to a 2-fold interpenetrating network with a (3 3 6 3 74)(32 3 72 3 82)(64 3 72) topology. By careful inspections of two structures, we find that Zn(II) cations, the ODPT4- anions, and N-donor ligands show the same coordination modes or analogous configurations. Compound 1 is a chiral structure, and 2 is an achiral framework. The compounds are also characterized by elemental analyses, IR spectra, and thermogravimetric analyses. In addition, the luminescent properties of these compounds are discussed.

Introduction Metal-organic frameworks (MOFs) are of current interest not only for their potential applications in microelectronics, nonlinear optics, zeolite-like materials for molecular selection, ion exchange, and catalysis but also for their intriguing variety of architectures and topologies.1 Chiral MOFs play a key role in biological systems and pharmacy,2 and an increasing amount of effort has been devoted to assemble chiral MOFs. Some recent approaches have been successful in the synthesis of chiral MOFs by using chiral or achiral ligands.3 Thereinto, spontaneous resolution is an important route to construct chiral MOFs by using achiral ligands, which yield a conglomerate.4 Statistically, between 5 and 10% of all racemates form conglomerate crystals,5 indicating that heterochiral interactions are prevalent and more facile than homochiral interactions in the formation of crystalline racemates.6 Therefore, the construction of chiral MOFs is one of the most challenging issues in synthetic chemistry and material science. In addition, chirality and helicity are often closely linked with each other in the same structure because they are intimately associated with the living processes, although they are two distinct concepts.7 The research on the relationship between the chirality of molecular building blocks and the helicity of polymeric structures could ultimately lead to a better understanding of the origin of asymmetry in living systems. However, true rational control in constructing chiral MOFs still remains a distant prospect because of most complexes crystallizing in *To whom correspondence should be addressed. E-mail: jianfangma@ yahoo.com.cn. pubs.acs.org/crystal

Published on Web 08/06/2009

Chart 1. Two Kinds of Neutral Ligands and the Coordination Mode of ODPT4- Anion

centric space groups or interpenetrating structures. Therefore, employing a suitable bridging ligand may be a prerequisite to construct chiral MOFs. Inspired by the aforementioned considerations, the V-shaped aromatic multicarboxylate is selected because it has irregular orientations when it coordinates to metals, which may produce various structural topologies, large pores, and different helices, because of its tetradentate carboxylate arms and its steric bulk. In accord with the literature,8 the ligands 4,40 -bis(imidazol-1ylmethyl)benzene (L1) and 4,40 -bis(imidazol-1-ylmethyl)bibenzene (L2) (Chart 1) as the achiral secondary N-donor ridging ligands are introduced for the following reasons: (i) the neutral bis(imidazole) ligands exhibit the special ability to coordinate to various metal centers in multiform coordination fashions to formulate the compounds, and (ii) the two imidazole rings of L1 and L2 can freely twist around the -CH2- group to meet the requirements of the coordination geometries of metal atoms in the assembly process, which often leads to helical structures, and can more easily produce the new classes of compounds. As accurate prediction of the final structures is impossible, we have r 2009 American Chemical Society

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General Procedures. Chemicals were purchased from commercial sources and used without further purification. Ligands (L1)9 and (L2)10 were prepared according to the literature. Synthesis of [Zn2(ODPT)(L1)] 3 3H2O (1). A mixture of ODPTA (3,30 ,4,40 -oxidiphenone tetracarboxylic dianhydride) (0.16 g, 0.50 mmol), L1 (0.05 g, 0.12 mmol), Zn(OAc)2 3 2H2O (0.11 g, 0.50 mmol), NaOH (0.08 g, 2.00 mmol), and H2O (10 mL) was stirred for 1 h and then sealed in a 25 mL Teflon-lined stainless steel container. The container was heated to 150 °C and held at that temperature for 72 h, then cooled to 100 °C at a rate of 5 °C h-1, and held for 8 h, followed by further cooling to 30 °C at a rate of 3 °C h-1. Colorless crystals of 1 were collected in 76.7% yield based on Zn(OAc)2 3 2H2O. Anal. calcd for C44H40N8O12Zn2 (1003.58): C, 52.65; H, 4.01; N, 11.16. Found: C, 52.63; H, 4.05; N, 11.20. IR (cm-1): 3420 (m), 1598 (s), 1565 (s), 1381 (s), 1298 (m), 1257 (s), 1229 (s), 1140 (w), 1096 (m), 949 (w), 802 (w), 746 (m), 695 (w), 653 (w). Synthesis of [Zn2(ODPT)(L2)2] (2). Compound 2 was prepared similarly to compound 1 by using ODPTA (0.16 g, 0.50 mmol), L2 (0.31 g, 1.00 mmol), Zn(OAc)2 3 2H2O (0.17 g, 0.75 mmol), NaOH (0.08 g, 2.00 mmol), and H2O (10 mL). Colorless crystals of 2 were collected in 64.5% yield based on Zn(OAc)2 3 2H2O. Anal. calcd for C56H42N8O9Zn2 (1101.72): C, 61.05; H, 3.84; N, 10.17. Found: C, 60.99; H, 3.85; N, 10.26. IR (cm-1): 3344 (s), 1620 (s), 1561 (s), 1416 (s), 1381 (s), 1274 (m), 1228 (s), 1155 (m), 1080 (m), 968 (w), 790 (w), 732 (m), 657 (w). Physical Measurements. The C, H, and N elemental analysis was conducted on a Perkin-Elmer 240C elemental analyzer. The Fourier transform infrared spectra were recorded from KBr pellets in the range 4000 to 400 cm-1 on a Mattson Alpha-Centauri spectrometer. Thermogravimetric analysis (TGA) was performed on a Perkin Elmer TG-7 analyzer heated from 35 to 600 °C under nitrogen. The emission/excitation spectra were recorded on a Varian Cary Eclipse spectrometer. X-ray Crystallography. Single crystal X-ray diffraction data for compounds 1 and 2 were recorded on a Bruker Apex CCD diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) at 293 K. Absorption corrections were applied using multiscan technique. All of the structures were solved by Direct Method of SHELXS-9711 and refined by full-matrix least-squares techniques using the SHELXL-97 program12 within WINGX.13 Hydrogen atoms were assigned to geometrically idealized positions. The water H atoms in compound 1 could not be positioned reliably. C44, C440 , C29, C290 O1, O10 , O2, O20 , O8, O80 , O9, O90 O1w, O2w, and O3W atoms in 1 were refined with isotropic temperature parameters. Other nonhydrogen atoms in 1 and 2 were refined with anisotropic temperature parameters. In 1, there are two disordered carboxylate groups. One disordered carboxylate group (C44, O8, O9 and C440 , O80 , O90 ) was refined using O or C atoms split over two equivalent sites, with a total occupancy of 1. The other (C29, O1, O2 and C290 , O10 , O20 ) was refined using O or C atoms split over two sites, with the occupancy of 0.75 and 0.25, respectively. The absolute structure of 1 has been determined with the Flack parameter of 0.009(14).14 The detailed crystallographic data and structure refinement parameters for 1 and 2 are summarized in Table 1. Structure Description of 1. Compound 1 crystallizes in a chiral space group P212121, which has been obtained by spontaneous resolution by using achiral ligand. The structure of 1, shown in Figure 1a, contains two types of ZnII cations and L1 (L1a and L1b) ligands and one type of ODPT4- ligand. Zn1 and Zn2 cations possess typical four-coordinated, tetrahedral geometries {ZnN2O2} with two nitrogen atoms from two L1 ligands and two carboxylic

formula fw crystal system space group a (A˚) b (A˚) c (A˚) V (A˚3) Z Dcalcd (g cm-3) F(000) reflns collected/unique R(int) GOF on F2 R1a [I > 2σ (I)] wR2b for all data largest residuals (e A˚-3) a

1

2

C44H40N8O12Zn2 1003.58 orthorhombic P212121 14.7840(7) 16.8100(8) 18.9970(10) 4721.1(4) 4 1.412 2064 27145/10072 0.039 0.996 0.057 0.159 0.96/-0.32

C56H42N8O9Zn2 1101.72 orthorhombic Pcca 18.6880(11) 13.4170(14) 19.4160(15) 4868.3(7) 4 1.503 2264 23389/4344 0.055 1.016 0.042 0.124 0.53/-0.41

R1 = Σ Fo| - |Fc /Σ|Fo|. b wR2 = |Σw(|Fo|2 - |Fc|2)|/Σ|w(Fo2)2|1/2. )

Experimental Section

Table 1. Crystal Data and Structure Refinements for Compounds 1 and 2

)

tried different synthetic conditions and performed many experiments under different conditions. Fortunately, two novel three-dimensional (3D) chiral and achiral MOFs, namely, [Zn2(ODPT)(L1)] 3 3H2O (1) and [Zn2(ODPT)(L2)2] (2), are successfully isolated by hydrothermal methods. In addition, the infrared spectra and thermogravimetric analyses have been investigated in detail for two compounds, and the luminescent properties have been also investigated.

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oxygen atoms from two carboxyl groups [one is normal, which is presented (O(9)#4 for Zn1 and O(2) for Zn2], and the other is disordered, which is omitted for clarity [O(80 )#4 for Zn1 and O(10 ) for Zn2)] of different ODPT4- ligands [Zn(1)-N(1) = 2.002(4), Zn(1)-N(4)#5 = 2.001(4), Zn(1)-O(3) = 1.935(4), and Zn(1)O(9)#4 = 2.041(9) or Zn(1)-O(8’)#4 = 1.951(10) A˚ for Zn1 and Zn(2)-N(8)#5 = 2.025(4), Zn(2)-N(5) = 2.019(4), Zn(2)O(6)#6 = 1.969(3), and Zn(2)-O(10 ) = 1.99(2) or Zn(2)-O(1) = 1.996(6) A˚ for Zn2, respectively]. The L1a and L1b ligands adopt the same “U-shaped” syn-coordination fashion15 (Figure S1a of the Supporting Information). Each ODPT4- ligand shows a tetra(monodentate) coordination mode (Chart 1). Two types of L1 ligands link all Zn1 and Zn2 cations to generate two kinds of zigzag chains, respectively (Figure 1b,e). It is interesting that two kinds of isolated zigzag chains entwist each other along the crystallographic a-axis.16 The phthalic groups from ODPT4- ligands link Zn1 and Zn2 cations to form a right-handed 21 helical chain with a pitch of 14.78 A˚ (Figure 1c,f). So, this helical chain and entwined snakelike chains entangle together to form a subunit by co-Zn cations (Figure 1d,g and Figure S1b of the Supporting Information). Then, intramolecular oxygen (O5) atoms from ODPT4- ligands bridge these subunits to extend a 3D framework (Figure 1h). If each ODPT4- ligand and ZnII cation is considered as a four-connected node, the structure of 1 possesses a (52 3 84)(53 3 62 3 7)2 topological characteristic (Figure 1i and Figure S1c of the Supporting Information). A more interesting feature is that the chiral structure of 1 is obtained through spontaneous resolution. Spontaneous resolution, known as the segregation of enantiomers upon crystallization, was discovered as early as 1846 by Louis Pasteur,17 and it is still a rare phenomenon that cannot be predicted a priori because the laws of physics determining the processes are not yet fully understood.18 For compound 1, we found that the Zn cations are coordinated by four different groups via covalent linkages, namely, two different O atoms (O3 and O8) and two different N atoms (N1 and N4) for Zn1 and two different O atoms (O2 and O6) and two different N atoms (N5 and N8) for Zn2. The chiral centers (ZnII) are linked by the phthalic groups from achiral ODPT4- ligands to form a righthanded 21 helical chain, and the chiral information may be transferred to the 3D framework by helical units with 2-fold rotation. Structure Description of 2. When the analogous L2 was selected instead of L1 to react with Zn(II) ion and H4ODPT, the naive expectation was that the structural chemistry would be analogous to that of compound 1. In fact, the result is largely unanticipated. As shown in Figure 2a, compound 2 is built by two kinds of ZnII cations, two half L2 ligands (L2a and L2b) and one kind of ODPT4- anion. Each ZnII cation is four-coordinated by two carboxylic oxygen atoms [Zn-O = 2.002(2) and 2.017(2) A˚] from two carboxyl groups of different ODPT4- anions and two nitrogen atoms [Zn-N = 2.020(3) and 2.060(3) A˚] from two L2 ligands. So, the ZnII cation adopts a distorted tetrahedral coordination geometry {ZnN2O2}. The carboxylate ligand and two different L2 ligands are located on 2-fold axes. Two kinds of L2 ligands

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Figure 1. (a) ORTEP diagram showing the coordination environments for ZnII atoms in 1. All H atoms and lattice water molecules and disordered carboxylate groups are omitted for clarity (symmetry codes: #2, -x þ 3/2, -y þ 1, z þ 1/2; #3, -x þ 2, y þ 1/2, -z þ 3/2; #4, -x þ 2, y - 1/2, -z þ 3/2; #5, x - 1/2, -y þ 1/2, -z þ 1; #6, -x þ 3/2, -y þ 1, z - 1/2). (b and e) Ball-stick and schematic representations of two entwined snakelike chains (green, L1a-Zn1; and red, L1b-Zn2). (c and f) Ball-stick and schematic view of the phthalic groups linking ZnII atoms to generate a 21 helical chain along the crystallographic b-axis. (d and g) Ball-stick and schematic view of the subunit formed by three kinds of chains. (h) Polyhedral and ball-stick representation of the 3D structure of 1 [red and green balls represent carboxylic oxygen atoms and intramolecular oxygen atoms (O5) of ODPT4- ligands]. (i) A schematic view of the 3D network of 1 with (52 3 84)(53 3 62 3 7)2 topology (yellow and black balls represent two kinds of six connected nodes, ZnII cations and ODPT4- ligands). exhibit “U-shaped” syn-coordination fashions19 and act as bis(monodentate) ligands (Figure S2a of the Supporting Information). The L2a ligand links Zn1 cation to form a right-handed chain (Figure 2b) with a pitch of 18.69 A˚. Each ODPT4- anion adopts a tetra(monodentate) coordination mode to link four ZnII cations (Chart 1). On the basis of this kind of connection mode, ODPT4anions and all right-handed chains form a double helical channel with a circular window though 3,30 -carboxylate groups coordinating to Zn1 cations (Figure 2c,d). These double helical chians are linked to generate a 2D layer by 4,40 -carboxylate groups coordinating to Zn2 cations (Figure 2e and Figure S2b of the Supporting Information). These layers and all left-handed chains formed by L2b ligand and Zn2 (Figure 2f) with the same pitch of L2a-Zn1 give rise to a 3D network by co-Zn2 cations (Figure 2g).

On further study of the structure, because of the spacious nature of the single network, each double helical chain from one 3D chiral net is entangled by two L2b-Zn2 helical chains from other 3D net with opposite helical units. So, each crystal of 2 is two interpenetrating nets. If the ODPT4- anion, Zn1 and Zn2 cations are considered as four-connected nodes, the structure of 2 can be simplified as a unique four-connected net with a (3 3 6 3 74)(32 3 72 3 82)(64 3 72) topology (Figure 2f). As compared with compound 1, Zn(II) cations are also fourcoordinated by two carboxylate oxygen and two nitrogen atoms from different ligands, but they are in the 2-fold sites and show achiral centers. So, compound 2 shows an achiral structure. In 1 and 2, Zn(II) cations are all four-coordinated by two carboxylate oxygen atoms and two nitrogen atoms from different ligands. The ODPT4- anions with the same coordination mode and

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Figure 3. Solid-state photoluminescent spectra of 1 and 2 at room temperature.

Scheme 1. Schematic View of Two Compounds in This Work

Figure 2. (a) ORTEP diagram showing the coordination environments for ZnII atoms in 2. All H atoms are omitted for clarity (symmetry codes: #1, -x þ 1/2, -y þ 1, z; #2, -x þ 1/2, -y, z; #3, -x, y, -z þ 3/2; #5, -x þ 1, y, -z þ 3/2). (b) Linear and schematic representations of L2a-Zn helical chain along different directions. (c) Linear and schematic representations of ODPT4--Zn helical chain along different directions. (d) Linear and schematic representations of double helical chain along different directions. (e) Polyhedral and ball-stick representation the 2D layer formed by L2a, ODPT4-, and ZnII cations (yellow and green polyhedra represent Zn1 cations and Zn2 cations, respectively). (g) View of the 3D structure of the single network along the a-axis. (h) Ball-stick and schematic representations of entangled double helical chain and L2b-Zn helical chain. (i) A schematic view of the 2-fold interpenetrating network of 2.

L1 and L2 with analogous configurations and different lengths link Zn(II) to form chiral and achiral structures. In 1, L1 ligands coordinate to Zn(II) cations to form entwined snakelike chains and ODPT4- anions connect chiral centers Zn(II) to generate a helical chain, which transfers the chirality to the whole 3D framework. In 2, longer L2 ligands and ODPT4- anions link Zn(II) to construct different helical chains. On the basis of the length and rigidity of the L2 ligands, they coordinate Zn(II) to form a large window, which permits some helical chains from other single net with an opposite helical unit to entangle to each other. Compound 2 is a two-folded interpenetrating structure. From the above-mentioned structural descriptions, we find Zn(II) cations, the ODPT4- anions, and N-donor ligands show the same coordination modes or analogous configurations. Compound 1 is a chiral structure, and 2 is an achiral framework. So, we believe that ligands with different lengths have great influences on the construction of different structures in the self-assembly process (Scheme 1). Luminescent Properties. Luminescent compounds are of great current interest because of their various applications in chemical sensors, photochemistry, and electroluminescent display.20 The luminescent properties of zinc carboxylate compounds have been investigated. The photoluminescent spectra of free neutral ligands L1-L2 and ODPTA measured at room temperature show the emission maxima at 423 (λex =255) for L1, 420 (λex =340) for L2, and 406 (λex = 354) nm for ODPTA, respectively. Compounds 1 and 2 based on L1 and L2 show the emission maxima at 383 (λex = 336) and 402 (λex = 340) nm, respectively. The emission of the ODPTA and L1-L2 ligands may be assigned to π* f n and π* f π transitions of the intraligands. In comparison with the N-donor ligands and multicarboxylate ligand, the emission maxima of compounds 1 and 2 have changed and show blue shifts (Figure 3). It is possible that a combination of several factors together,21 including a change in the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of deprotonated ODPT4- anions and neutral ligands coordinating to metal centers, the charge-transfer transition between N-donor ligands and metal

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centers, N-donor ligands coordinating to the metal centers, which effectively increase the rigidities of the ligands and reduce the loss of energy by radiationless decay, and π* f n and π* f π transitions of the intraligands. In comparison with 1 and 2, the emission differences between them may be caused by different neutral ligands. Thermal Analysis. To characterize the compounds more fully in terms of thermal stability, their thermal behaviors were studied by TGA. The experiments were performed on samples consisting of numerous single crystals of 1 and 2 under N2 atmosphere with a heating rate of 10 °C/min, as shown in Figure S3 of the Supporting Information. For compound 1, the weight loss in the range of 17-165 °C corresponds to the departure of lattice water molecules (observed, 5.6%; calculated, 5.5%), the weight losses in the range of 273512 °C correspond to the removal of the corresponding organic components, and the remaining weight corresponds to the formation of ZnO (observed, 16.8%; calculated, 16.2%). The anhydrous compound 2 begins to decompose at 202 °C and ends above 484 °C. The weight loss (found, 84.5%) corresponds to the loss of organic components (calculated, 85.2%). The remaining weight of 15.5% corresponds to the percentage (14.8%) of ZnO.

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Conclusion

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In summary, two zinc(II) multicarboxylate complexes with length-modulated N-donor ligands based on different helical units have been synthesized under hydrothermal conditions. Compound 1 is a chiral structure, and 2 is an achiral framework. By careful inspections of two structures, Zn(II) cations, the ODPT4- anions, and N-donor ligands show the same coordination modes or analogous configurations. So, we believe that ligands with different lengths have great influences on the construction of different structures in the self-assembly process. Appropriate choices of novel neutral ligands and multicarboxylate anions may offer new opportunities to construct new types of chiral MOFs based on various helical units in the near future.

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Acknowledgment. We thank the Program for Changjiang Scholars and Innovative Research Teams in Chinese University, China Postdoctoral Science Foundation, the Postdoctoral Foundation of Northeast Normal University (NENU), the Training Fund of NENU’s Scientific Innovation Project (NENU-STC08019), the Science Foundation for Young Teachers of NENU (No. 20090407), and the Analysis and Testing Foundation of NENU for support. Supporting Information Available: Diagrams of the structures and TGA curves of compounds 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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