Noncentrosymmetric and Homochiral Metal−Organic Frameworks of

Apr 9, 2010 - Received January 28, 2010; Revised Manuscript Received March 5, 2010. ABSTRACT: Four ... Compound 1 crystallizes in the space group Pna2...
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DOI: 10.1021/cg1001368

Noncentrosymmetric and Homochiral Metal-Organic Frameworks of (S)-2-(1H-Imidazole1-yl) Propionic Acid

2010, Vol. 10 2355–2359

Gao Zhang, Shi-Yan Yao, Dong-Wei Guo, and Yun-Qi Tian* Institute of Chemistry for Functionalized Materials, College of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, P. R. China Received January 28, 2010; Revised Manuscript Received March 5, 2010

ABSTRACT: Four noncentrosymmetric coordination polymers of [Mn((S)-imp)((R)-imp)] (1), [Cu((S)-imp)2]¥ (2), [Zn((S)imp)2]¥ (3), and [Co((S)-imp)2]¥ (4) [(S)-imp = (S)-2-(1H-imidazole1-yl) propionate] have been prepared and structurally characterized by X-ray single crystal diffraction, X-ray powder diffraction (XRPD), infrared (IR) spectroscopy, elemental analysis, and thermogravimetric analysis (TGA). Compound 1 crystallizes in the space group Pna21, exhibiting a twodimensional structure; 2 crystallizes in the space group of P21, showing a chiral layered structure of (4, 4)-net, whereas 3 and 4 are isostructural, and they crystallize in the space group P212121, exhibiting a 3-fold interpenetrating diamondoid net. Because the ligand used for the synthesis is an enantiopure chiral compound, the obtained metal-organic frameworks show the homochirality in bulk except for compound 1, in which the ligand configuration was racemized during the solvothermal reaction.

Introduction Noncentrosymmetric coordination polymers have currently attracted increasing attention of scientists in material chemistry owing to their interesting architectures as well as potential applications in second-order nonlinear optical (NLO) materials.1-8 It is well-known that the absence of a center of symmetry is one of the essential and all-important aspects for the materials. Although a great deal of progress has been made in the synthesis of noncentrosymmetric crystalline solids, the most challenging task still remains in the design and preparation of such materials.1a Generally, noncentrosymmetric solids based on infinite networks can be rationally designed by taking advantage of well-defined metal coordination geometries in combination with carefully chosen bridging ligands.1 Several approaches toward the achievement of noncentrosymmetric crystalline solids with promising NLO properties have appeared in recent years,6-9 of which the employment of chiral organic linkers to generate noncentrosymmetric or chiral metal-organic frameworks (MOFs) is the most convenient and effective strategy. The achiral ligand of 2-(1H-imidazole-1-yl) acetic acid (Hima) has been recently reported to form one- and two-dimensional (1D, 2D) centrosymmetric MOFs of [M(ima)n]¥, (M = Ag (I), n = 1; M = Mn(II), Cu(II), Ni(II), and Cd(II), n=2).12a,13 However, we noticed that its structure-similar chiral compound of (S)2-(1H-imidazole1-yl) propionic acid (H-(S)-imp) (Scheme 1) has been, so far, merely prepared as an intermediate compound for chiral ionic liquids.10 Thus, using the ligand for construction of noncentrosymmetric or chiral MOFs of Mn(II), Cu(II), Zn(II), and Co(II) compounds should be a good choice because the ligand H-(S)-imp contains a chiral -CH(CH3)- spacer between imidazolato and carboxyl groups and functions as the asymmetric bridge between metal centers to enhance the formation of noncentrosymmetric MOFs. In this paper, we report four of noncentrosymmetric MOFs:

Scheme 1

[Mn((S)-imp)((R)-imp)]¥ (1), [Cu((S)-imp)2]¥ (2), [Zn((S)imp)2]¥ (3), and [Co((S)-imp)2]¥ (4) based on this enantiopure chiral ligand, of which 2, 3, and 4 are the MOFs with homochirality. Experimental Section

*Corresponding author. E-mail: [email protected]. Fax: 86-41182156858. Tel: 86-411-82159141.

Materials and Measurements. Sodium (S)-2-(1H-imidazole1-yl) propionate (Na-(S)-imp) was prepared according to a literature method.10 All other chemicals and solvents used in the syntheses were commercially available and used without further purification. The elemental analyses were carried out on a Perkin-Elmer 2400 elemental analyzer. The IR spectra were recorded (400-4000 cm-1) on a FT-IR spectrometer TENSOR 27. X-ray powder diffraction (XRPD) analyses were carried out on a Bruker D8 Advance and TGA was performed under static air atmosphere with a heating rate of 5 C/min by using a Perkin-Elmer Diamond thermogravimetric analyzer. X-ray Single-Crystal Structure Determination. Crystallographic measurements for 1-4 were carried out on a Bruker SMART-ApexII CCD diffractometer with graphite-monochromatized MoKa radiation (λ=0.71073 A˚) at 293(2) K. The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2 using the SHELX program package.11 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms on carbon atoms were generated geometrically and refined isotropically. Details of the crystal parameters, data collection, and refinements for 1-4 are summarized in Table 1. Selected bond lengths and angles for compounds 1-4 can be seen in Supporting Information. Synthesis of [Mn((S)-imp)((R)-imp)]¥ (1). 32.4 mg (0.2 mmol) of Na-(S)-imp was dissolved in 1.0 mL of distilled water that was neutralized with HCl (1.0 M) to pH=6. After 19.8 mg (0.10 mmol) of MnCl2 3 4H2O (dissolved in 6.0 mL of ethanol) was added into the above-mentioned aqueous solution, the reaction mixture was stirred at room temperature for half an hour and then placed into a Teflonlined autoclave (10 mL). The autoclave was sealed and heated at

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Table 1. Crystallographic Data and Structure Refinement Summary for Complexes 1-4a 1

2

3

C12H14CuN4O4 empirical formula C12H14MnN4O4 formula weight 333.21 341.81 space group orthorhombic monoclinic P21 crystal system Pna21 a (A˚) 15.215(2) 7.6566(12) b (A˚) 5.0455(7) 10.0243(16) c (A˚) 16.937(2) 9.2447(14) β () 90 92.735(2) 1300.2(3) 708.74(19) V (A˚3) Z 4 2 -1 1.038 1.562 μ [mm ] 1.702 1.602 Dc (kg m-3) T (K) 293(2) 293(2) theta range for 2.41-25.00 2.21-25.00 data collection () 1.044 1.026 goodness-of-fit on F2 R1=0.0354, wR2=0.0757 final R indices R1=0.0261, wR2=0.0660 [I > 2σ(I)] R1=0.0429, wR2=0.0795 R indices (all data) R1=0.0297, wR2=0.0683 Flack parameter 0.61(3) 0.01(2) P P P P a R1= (|F0| - |Fc|)/ |F0|; wR2=| w(|F0| - |Fc|)2/ wF02]1/2.

120 C for 72 h. After the autoclave was cooled to room temperature, colorless crystals of 1 were separated by filtration, washed with 3  10 mL of ethanol, and dried in air (yield: ca. 0.006 g, 17.5% based on MnCl2 3 4H2O). Elementary analysis for 1 = C12H14MnN4O4: calcd. C, 43.26; H, 4.24; N, 16.81; found: C, 42.23; H, 4.19, N, 16.80%. IR (KBr, cm-1): 3434(s), 3124(m), 2989(w), 1602(vs), 1505(m), 1382(s), 1266(s), 1112(m), 1073(m), 983(w), 931(w), 879(w), 834(w), 744(m), 686(m), 447(w). Synthesis of [Cu((S)-imp)2]¥ (2). 32.4 mg (0.2 mmol) of Na-(S)imp was dissolved in 1.0 mL of distilled water and the solution was neutralized with HCl (1.0 M) to pH=6. After adding 29.1 mg (0.10 mmol) of CuNO3 3 3H2O (dissolved in 6.0 mL of N,N-dimethylformamide (DMF)) into the above-mentioned aqueous solution, the reaction mixture was stirred at room temperature for a half an hour and then placed into a Teflon-lined autoclave (25 mL). The autoclave was sealed and heated at 100 C for 48 h. As the autoclave was cooled to room temperature, blue crystals of 2 were separated by filtration, washed with 3  10 mL of ethanol, and dried in air (yield: ca. 0.01 g, 21.4% based on CuNO3 3 3H2O). Elementary analysis for 2=C12H14CuN4O4: calcd. C, 42.17; H, 4.13; N, 16.39; Found: C, 42.14; H, 4.08, N, 16.38%. IR (KBr, cm-1): 3434(s), 3137(s), 1621(vs), 1512(m), 1545(w), 1389(s), 1305(w), 1273(m), 1228(w), 1105(s), 983(w), 840(w), 763(m), 699(m), 653(w), 589(w). Synthesis of [Zn((S)-imp)2]¥ (3) and [Co((S)-imp)2]¥ (4). The procedures were similar to that for the synthesis of 2 where the salt Cu(NO3) 3 3H2O was replaced by 29.7 mg (0.1 mmol) of Zn(NO3)2 3 6H2O for 3 and 29.1 mg (0.1 mmol) of Co(NO3)2 3 6H2O for 4 (yield for 3: ca. 0.008 g, 23.3% based on Zn(NO3)2 3 6H2O, while yield for 4: 0.04 g, 39.4% based on Co(NO3)2 3 6H2O). Elementary analysis for 3=C12H14ZnN4O4: Anal. Calcd. C, 41.94; H, 4.11; N, 16.30; Found: C, 41.97; H, 4.14, N, 16.26%. IR (KBr, cm-1): 3434(s), 3143(s), 1641(vs), 1518(s), 1389(s), 1350(vs), 1305(s), 1260(s), 1105(vs), 1034(w), 983(w), 950(w), 847(m), 782(m), 737(s), 667(s), 512(w); elementary analysis for 4 = C12H14CoN4O4: calcd. C, 42.74; H, 4.18; N, 16.62; found: C, 42.77; H, 4.12, N, 16.61%. IR (KBr, cm-1): 3438(vs), 3144(s), 2997(w), 2927(w), 1640(vs), 1517(s), 1460(w), 1389(s), 1351(s), 1306(m), 1261(s), 1107(s), 839(m), 781(m), 736(m), 666(s), 518(m).

Results and Discussion Preparation of Compounds 1-4. The preparation for compounds 1-4 were performed under solvothermal conditions where the solvent used for compound 1 was ethanol, whereas for compounds 2-4 the solvent was N,N-dimethylamide (DMF) or N,N-dimethylacetamide (DMA)). The reaction temperature was available in the range of 100120 C. Since the ligand used for the preparation is enantiopure

4

C12H14ZnN4O4 343.64 orthorhombic P212121 8.4470(12) 12.5254(17) 13.3182(18) 90 1409.1(3) 4 1.764 1.620 293(2) 2.23-24.99

C12H14CoN4O4 337.20 orthorhombic P212121 8.3992(10) 12.6546(16) 13.3111(16) 90 1414.8(3) 4 1.234 1.583 293(2) 2.22-24.99

1.040 R1=0.0208, wR2=0.0483

1.035 R1=0.0333, wR2=0.0647

R1=0.0228, wR2=0.0490 0.006(10)

R1=0.0426, wR2=0.0681 0.006(19)

Figure 1. The ORTEP drawing of asymmetric unit and the coordination geometry of the Mn(II) in compound 1 (30% probability level thermal ellipsoids).

Na-(S)-imp, the observation for both (S)-imp and (R)-imp in the achiral framework of 1 reveals that the ligand racemization has taken place during the solvothermal reaction. On the basis of the fact that the compounds 2-4 were obtained as homochiral products under the same solvothermal temperature, but different solvents, we ascribe the ligand racemization in compound 1 to the autogenerated high pressure from the low-boiling-point ethanol. Crystal Structure Descriptions. [Mn((S)-imp)((R)-imp)]¥ (1). Single crystal X-ray diffraction study reveals that compound 1 crystallizes in space group Pna21. As shown in Figure 1, each Mn(II) atom in compound 1 shows octahedral coordination geometry with the equatorial positions coordinated by four oxygen atoms from four carboxyl groups and the apical sites coordinated by two nitrogen atoms from two imidazolato groups, displaying an almost regular octahedron (Mn-O distances in the range from 2.182(2) to 2.236(3) A˚ and Mn-N distances from 2.248(2) to 2.250(3) A˚ , all N-Mn-O and four O-Mn-O angles ca. 90, see Table 1 in

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Supporting Information). In the crystal of 1, each of the imp ligands provides all of its two carboxyl oxygen and one nitrogen atoms to bridge the octahedral Mn atoms along the b-axis by carboxyl groups in syn-anti mode and along the a-axis by imidazolato groups, forming an infinite 2D layer (Figure 2a) that is similar to the centrosymmetric MOF

Figure 2. (a) The single-layer of 1 parallel to the ab plane viewed along the c-axis (top) and the multilayers stacked in ABAB sequence along the b-axis (bottom) in crystal 1; (b) a pair of the enantiomers of left-handed and right-handed helices built up of (R)-imp and (S)imp ligands, respectively.

Figure 3. ORTEP drawing of the asymmetric unit and the coordination geometry of Cu in compound 2 (30% probability level thermal ellipsoids).

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of Mn(ima)2 reported by Wang et al.12 However, with the achiral structure in space group of Pna21, compound 1 exhibits containing both of (S)-imp and (R)-imp in equal population, revealing that the enantiopure Na-(S)-imp used in the synthesis has been racemized. They build up the enantiomers of left-handed and right-handed helical chains (Figure 2b) that furnish the noncentrosymmetric 2D sheets of compound 1. And these sheets are stacked in ABAB sequence and interacted by hydrogen-bonds between methyl and carbonyl groups in completing the whole crystalline structure. [Cu((S)-imp)2]¥ (2). Compound 2 crystallizes in the space group of P21. The asymmetric unit of 2 contains one Cu(II) and two (S)-imp ligands (Figure 3), in which Cu(II) shows a square-planar geometry that is coordinated by two oxygen and two nitrogen atoms from four individual (S)-imp ligands (the bond distances for Cu-O, Cu-N and angles for O-Cu-N, see Table 1 in Supporting Information). Unlike in compound 1, each (S)-imp ligand in compound 2 provides only one of its carboxyl oxygen atoms to coordinate with one Cu(II). Thus, the (S)-imp ligands function as the spacers between the Cu atoms with Cu 3 3 3 Cu distances of 7.837(5) and 7.587(5) A˚, respectively, forming a 2D wave-like sheet of (4, 4)-net (Figure 4a). It should be noted that although the left-handed and righthanded helical chains (Figure 4b) are distinguishable in the (4,4)-network of compound 2, the net chirality of the compound cannot be killed because these helical chains are not the real enantiomers owing to the enantipure chiral building block of (S)-imp ligand involved. Thus, the single crystals of compound 2 are not only chiral individually, but also homochiral in bulk. It should also be noted that if we neglect the chirality of compound 2, its structure is quite similar to that of Cu(ima)2:12a they both have 2D wavelike sheets stacked with the ABAB sequence in the crystal structure (Figure 5). [Zn((S)-imp)2]¥ (3) (and [Co((S)-imp)2]¥ (4)). Compounds 3 and 4 are isostructural. They have almost the same unit cell parameters (Table 1) with the space group of P212121. As shown in Figure 6, the asymmetric unit of 3 (or 4) contains one metal (M=Zn, Co) atom and two (S)-imp ligands where the metal atom is tetrahedrally coordinated by two oxygen and two nitrogen atoms from four individual (S)-imp ligands. The bond lengths of M-N and M-O are in the range of 2.01-2.02 (1) and 1.93-1.96 (1) A˚, respectively. The angles

Figure 4. (a) The single-layer structure of (4,4)-net constructed from left- and right-handed helices and (b) a pair of left- and right-handed helices which are actually not related as the enantiomers.

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of O-M-O, N-M-O, and N-M-N are in the range of 96.20(7)-122.49(8) (see Table 1 in Supporting Information). Thus, the metals in the frameworks of 3 and 4 have a

Figure 5. (a) The chiral single-layer structure of compound 2 viewing the alternate left-handed (L) and right-handed (R) helical chains along the b axis; (b) the multilayers stacked in ABAB sequence in compound 2 viewed along the [101] direction.

Figure 6. ORTEP drawing of the asymmetric unit of 3 (30% probability level thermal ellipsoids).

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tetrahedral geometry and they are linked by the ligands into a 3D diamondoid net with M 3 3 3 M distances in the range of ca. 9.04-9.11 A˚ (Figure 7a). Three folds of such independent diamondnoid nets mutually interpenetrate (Figure 8) to furnish the complete crystal structures of compounds 3 and 4. Like the other chiral diamondnoid MOFs9a,14 and metal inorganic frameworks (MIFs)15 with the space group of P212121, the 21 helices along a-, b-, and c-axis can be isolated from compounds 3 and 4. As shown in Figure 7b, the a1- and a2-helices are distinct left- and right-handed helical chains parallel to the a-axis, while the b- and c-helices are also different left- and right-handed chains along the b- and caxis, respectively. It is just these helices that have made such diamondnoid MOFs (and MIF) chirality no matter whether or not the bridge ligands are chiral. However, the homochirality can only be ensured in the cases that they contain enantiopure chiral building blocks,16-19 such as the enantiopure chiral (S)-imp ligand in compound 3 (and 4). Otherwise, the homochirality must be confirmed by CD spectroscopy. Thermogravimetric, X-ray Powder Diffraction and SHG Analysis. The thermal stability and the purity of the compounds 1-4 in bulk were determined by thermogravimetric analysis (TGA) and X-ray powder diffraction (XRPD) (see

Figure 8. The ball-and-stick diagram to display the 3-fold interpenetrating nets of compound 3 (or 4) (balls: metals, sticks: (S)-imp ligands).

Figure 7. (a) The single-set of chiral diamondoid framework of compound 3 (and 4) constructed by distinct 21- helical chains along a-, b-, and c-axis, respectively. (b) The 21-helical chains of a1-, a2-, b-, and c-helices isolated from the framework of compound 3 (or 4) to show their structural distinctness.

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Supporting Information). The compounds 1-4 demonstrate thermal stability up to 275, 230, 330, and 300 C, respectively. And their thermal decomposition residuals of metal oxides are observed for compound 1 as MnO with a weight percentage of 21.68% (caled 21.29%), compound 2 as CuO with 22.84% (caled 23.27%), compound 3 as ZnO with 22.82% (caled 22.68%), and compound 4 as CoO with 21.86% (caled 22.22%), correspondingly. The XRPD patterns of the bulk products of compounds 1-4 are agreeable to those simulated from their single-crystal data that reveals the compounds are in single phase. The preliminary Kurtz powder second-harmonic generation (SHG) measurements on the colorless 1 and 3 showed that they both have no observable SHG intensity versus that of potassium dihydrogen phosphate (KDP). This result reveals the complexes of (S)-imp ligand have poor NLO properties because ideal NLO chromophores typically contain a good electron donor and acceptor connected through a conjugated bridge so that the noncentrosymmetry requirement for second-order NLO processes can be fulfilled on a macroscopic level.1a,20

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Conclusion By use of the enantiopure chiral ligand of (S)-imp, four noncentrosymmetric MOFs of [Mn((S)-imp)((R)-imp)]¥ (1), [Cu((S)-imp)2]¥ (2), [Zn((S)-imp)2]¥ (3), and [Co((S)-imp)2]¥ (4) have been synthesized. Although the coordination modes and the framework topologies of the above compounds are the same as those of the structure-similar achiral ligand of Hima, the framework symmetry of the compounds are all lower than those derived from the ligand of Hima: Compared the centrosymmetric MOFs of the achiral ligand of Hima, the frameworks synthesized from the H-(S)-imp are all unexceptionally noncentrosymmetric, of which compounds 2-4 are even homochiral. The only achiral MOF 1 resulted from the configuration racemization during the solvothermal reaction that was attributed to the high pressure autogenerated from the low-boiling-point ethanol. Although the noncentrosymmetry of the MOFs suggests that these compounds may serve as NLO materials; however, their very weak SHG intensities versus KDP indicated that these MOFs are not the right choice for such materials. Acknowledgment. We thank the National Science Foundation of P. R. China (NSFC) (No. 20571013), The Foundation for Author of National Excellent Doctoral Disserttion of P. R. China (FANEDD) (No. 200733), and the Education Foundation of Liaoning Province of P. R. China (EFLPC) (No. 20060470) for the finance support.

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Supporting Information Available: The XRPD patterns, IR, TGA, and table (PDF); CIF files of the compounds 1-4. This information is available free of charge via the Internet at http:// pubs.acs.org/.

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