CRYSTAL GROWTH & DESIGN
Synthesis of Chiral Coordination Polymers by Spontaneous Resolution Feng
Li,†,‡
Taohai
Li,†,‡
Xiaoju
Li,†,‡
Xing
Li,†,‡
Yuling
Wang,†,‡
and Rong
Cao*,†
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou, 350002, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
2006 VOL. 6, NO. 6 1458-1462
ReceiVed February 19, 2006; ReVised Manuscript ReceiVed April 8, 2006
ABSTRACT: Three complexes have been synthesized from the reactions of 3,3-tetramethyleneglutaric acid (H2TMG), Cd(NO3)2, and secondary ligands: [Cd(TMG)]n (1), {[Cd(TMG)(1,10-phen)‚H2O]‚2H2O}n (2), and [Cd(TMG)(4,4′-bipy)]n (3) (phen ) 1,10phenanthroline, 4,4′-bipy ) 4,4′-bipyridine). By adopting potential chiral ligands, metal-organic helical chains as a transmitter for chiral information, and coordination bonds as chiral discriminative interactions, the chiral coordination polymer 3 with an interesting three-dimensional architecture has been synthesized successfully by spontaneous resolution. Introduction Chirality has been of great importance in chemistry, pharmacy, biochemistry, and materials science.1-3 Recently, chiral coordination polymers have become a topic of intense interest due to their intriguing potential applications in enantioselective synthesis, asymmetric catalysis, porous materials, nonlinear optical materials, and magnetic materials.4-6 In general, chiral coordination polymers can be obtained by two routes. The first is stereoselective synthesis using enantiopure chiral species. To date, different chiral species have been employed, such as chiral reactants, solvents, or chiral physical environments.7,8 Recently, chiral templates (chiral metal complexes) have been used to form chiral metal phosphates.9 The second is spontaneous resolution upon crystallization without any chiral sources.10,11 The synthesis of chiral species from achiral or racemic ligands is the key issue in studying the genesis of chirality in biological systems. Discovered as early as in 1846 by Louis Pasteur, spontaneous resolution is still a relatively scarce phenomenon and cannot be predicted a priori because the laws of physics determining the processes are not fully understood yet. To synthesize chiral coordination polymers without chiral auxiliary, two main difficulties must be solved: (i) how to generate chiral units from achiral components and (ii) how to induce the chiral information of the chiral units into higher dimensional chiral coordination polymers.12 The chiral units can be obtained from potential chiral ligands or inherently chiral octahedral metal centers.13,14 The bottleneck of this strategy hinges on how to extend the chirality of the building units by chirally discriminative interactions to realize homochiral assembly. There are only a few examples of spontaneous resolution in which the chiral information in enantiomeric forms of the building blocks can be transmitted into higher dimensionality to generate chiral coordination polymers.15,16 As described previously, the transformation was achieved by chirality direction and chirality transfer. Each metal center of the metal-organic helical coordination polymers has the same chirality, and a helix may be a good model to transmit chiral information. Taking this into consideration, we try to obtain * To whom correspondence should be addressed. E-mail: rco@ ms.fjirsm.ac.cn. † Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences. ‡ Graduate School of the Chinese Academy of Sciences.
chiral coordination polymers through synthesizing helices from achiral ligands. Carboxylate ligands have been already proven to be efficient for the generation of a helical coordination polymer,17,18 so we chose achiral carboxylate (3,3-tetramethyleneglutaric acid) as ligands, exploring the isolation of chiral coordination polymers. With achiral ligands, we obtained three complexes: [Cd(TMG)]n (1), {[Cd(TMG)(1,10-phen)‚H2O]‚ 2H2O}n (2), and [Cd(TMG)(4,4′-bipy)]n (3) (H2TMG ) 3,3tetramethyleneglutaric acid, phen ) 1,10-phenanthroline, 4,4′bipy ) 4,4′-bipyridine), in which 3 is a chiral coordination polymer with homochiral helical chains. Experimental Section Materials and General Methods. H2TMG was purchased from Aldrich and used without further purification. All the other reagents were commercially available and used as purchased. Elemental analyses were carried out on an Elementar Vario EL III analyzer. Infrared (IR) spectra were recorded with PerkinElmer Spectrum One as KBr pellets in the range 4000-400 cm-1. X-ray powder diffraction (XRPD) measurements were recorded on a RIGAKU DMAX2500PC diffractometer using Cu KR radiation. The sample used in the measurement for the chiral complex 3 was conglomerate. Synthesis of [Cd(TMG)]n 1. A mixture of H2TMG (0.024 g, 0.1 mmol) and Cd(NO3)2‚4H2O (0.031 g, 0.1 mmol) in 15 mL of distilled water was sealed in a Teflon-lined stainless autoclave and heated at 160 °C for 3 days under autogenous pressure and then cooled to room temperature during 2 days. Colorless prism crystals were obtained. Yield: 30%. Element analysis (%): Calcd. for C9H12O4Cd: C 36.45, H 4.08; found: C 36.44, H 4.10. IR (KBr, cm-1): 2952(s), 2871(m), 1571(vs), 1518(vs), 1407(vs), 1386(vs), 1315(s), 1234(s), 1185(m), 1097(w), 736(m), 628(m). Synthesis of {[Cd(TMG)(1,10-phen)‚H2O]‚2H2O}n 2. A mixture of H2TMG (0.024 g, 0.1 mmol), 1,10-phen (0.020 g, 0.1 mmol), and Cd(NO3)2‚4H2O (0.031 g, 0.1 mmol) in 15 mL of distilled water was sealed in a Teflon-lined stainless autoclave and heated at 160 °C for 3 days under autogenous pressure and then cooled to room temperature during 2 days. Colorless needlelike crystals were obtained. Yield: 51%. Element analysis (%): Calcd. for C21H26N2O6.5Cd: C 48.24, H 5.01, N 5.36; found: C 48.23, H 5.03, N 5.34. IR (KBr, cm-1): 3454(m), 3270(m), 2959(m), 2866(m), 1622(m), 1538(s), 1408(s), 1313(w), 1142(w), 1101(w), 845(s), 769(w), 725(s), 685(w). Synthesis of [Cd(TMG)(4,4′-bipy)]n 3. A mixture of H2TMG (0.024 g, 0.1 mmol), 4,4′-bipy (0.016 g, 0.1 mmol), and Cd(NO3)2‚4H2O (0.031 g, 0.1 mmol) in 15 mL of distilled water was sealed in a Teflon-lined stainless autoclave and heated at 160 °C for 3 days under autogenous pressure and then cooled to room temperature during 2 days. Colorless needlelike crystals were obtained. Yield: 46%. Element analysis (%): Calcd. for C19H20N2O4Cd: C 50.40, H 4.45, N 6.19; found: C 50.41,
10.1021/cg0600905 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/19/2006
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Table 1. Crystallographic Data for Complexes 1-3a complex
1
2
3
empirical formula molecular mass crystal system space group a (Å) b (Å) c (Å)
C9H12CdO4 296.59 monoclinic P2(1)/c 14.059(2) 6.4810(10) 10.2610(14) 90 104.821(2) 90 903.8(2) 4 2.180 2.399 584 5.09-27.53 0.0254 0.0310 1.004
C21H26CdN2O6.5 522.84 monoclinic P2(1)/c 8.5786(12) 10.4460(13) 24.323(4) 90 98.270(8) 90 2157.0(5) 4 1.610 1.055 1064 3.09-27.44 0.0648 0.0793 1.121
C19H20CdN2O4 452.77 trigonal P3(1)2(1) 11.9079(8) 11.9079(8) 21.104(2) 90 90 120 2591.6(4) 6 1.741 1.292 1368 3.42-24.70 0.0419 0.0428 1.034
β (°) (Å3)
V Z Dc (g cm-3) µ (mm-1) F(000) 2θ range (°) R1 [I > 2σ(I)] R1 (all data) S (F2)
R ) ∑||Fo| - |Fc||/∑|Fo|. ) [∑w(Fo - Fc2)2/∑w(Fo2)2]1/2. ) 1/[s2(Fo2) + (0.0320P)2 + 0.8502P], where P ) (Fo2 + 2Fc2)/3 for 1. ) 1/[s2(Fo2) + (0.0850P)2 + 2.3803P], where P ) (Fo2 + 2Fc2)/3 for 2. ) 1/[s2(Fo2) + (0.0630P)2 + 4.3006P], where P ) (Fo2 + 2Fc2)/3 for 3. a
w w w
wR(F2)
2
H 4.44, N 6.18. IR (KBr, cm-1): 2949(m), 1600(s), 1538(s), 1439(m), 1407(s), 1327(m), 1220(s), 1073(m), 1006(m), 812(s), 780(m), 677(m), 628(s), 499(m). The samples of 1-3 were characterized by XRPD at room temperature (Figure S1, Supporting Information). The patterns calculated from the single-crystal X-ray data of 1-3 are in agreement with the observed one, confirming the phase purity of the three compounds. X-ray Crystallographic Studies. The intensity data of 1-3 were collected on a Rigaku CCD diffractometer with graphite-monochromatized Mo KR (λ ) 0.71073 Å) radiation at 173(2) K. All absorption corrections were performed using the CrystalClear program.19 The structures were solved by direct methods20 and refined on F2 by fullmatrix least-squares using the SHELXTL-97 program package.21 All non-hydrogen atoms were refined anisotropically. The organic hydrogen atoms were positioned geometrically and refined using a riding model. The hydrogen atoms bonded to water oxygen atoms were located from Fourier difference maps and refined isotropically. The crystallographic data for complexes 1-3 are listed in Table 1, and selected bonds and angles are in Table 2. CCDC reference numbers for 1-3 are 294567, 294568, and 294569, respectively.
Table 2. Selected Bond Lengths (Å) and Angles (°) for Complexes 1-3a Cd(1)-O(1) Cd(1)-O(2)#1 Cd(1)-O(2)#2 Cd(1)-O(4)#3 O(1)-Cd(1)-O(2)#2 O(2)#1-Cd(1)-O(4)#3 O(2)#1-Cd(1)-O(3)#1 Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(3)#5 Cd(1)-O(4)#5 O(4)#5-Cd(1)-N(1) O(2)-Cd(1)-O(1) N(2)-Cd(1)-O(2) O(7)-Cd(1)-N(2) Cd(1)-O(1) Cd(1)-O(2) Cd(1)-O(3)#6 Cd(1)-O(3)#7 N(1)-Cd(1)-N(2)#8 O(2)-Cd(1)-O(1) O(2)-Cd(1)-O(3)#6
Complex 1 2.276(2) Cd(1)-O(4)#4 2.314(2) Cd(1)-O(3)#1 2.337(2) Cd(1)-O(3)#4 2.347(2) 167.68(7) O(4)#4-Cd(1)-O(3)#1 93.78(7) O(4)#4-Cd(1)-O(3)#4 76.58(7) O(4)#3-Cd(1)-O(3)#4 Complex 2 2.413(4) Cd(1)-O(7) 2.408(5) Cd(1)-N(1) 2.531(4) Cd(1)-N(2) 2.322(4) 161.16(16) O(7)-Cd(1)-O(1) 53.68(14) O(4)#5-Cd(1)-O(3)#5 81.99(17) N(1)-Cd(1)-O(3)#5 141.72(16) Complex 3 2.487(5) Cd(1)-O(4)#7 2.313(5) Cd(1)-N(1) 2.362(4) Cd(1)-N(2)#8 2.431(4) 176.37(18) O(3)#6-Cd(1)-O(3)#7 54.53(17) O(4)#7-Cd(1)-O(3)#7 87.64(17) O(4)#7-Cd(1)-O(1)
2.356(2) 2.475(2) 2.481(2) 74.91(7) 53.88(7) 74.97(7) 2.310(4) 2.386(4) 2.372(5) 82.87(14) 53.54(14) 109.58(15)
2.388(5) 2.366(6) 2.437(5) 71.61(17) 53.17(15) 93.26(16)
a Symmetry transformations used to generate equivalent atoms: #1 x, -y + 3/2, z - 1/2; #2 -x - 1, y - 1/2, -z - 3/2; #3 x, -y + 1/2, z - 1/2; #4 -x - 1, -y + 1, -z - 1; #5 x + 1, y, z; #6 -y + 1, x - y + 1, z + 1/3; #7 y - 1, x, -z; #8 x, y - 1, z.
Results and Discussion Syntheses. Our aim is to construct high-dimensional chiral coordination polymers through synthesizing helices from achiral ligands. To generate chiral molecular units, we are interested in alicyclic carboxylate ligands. Lying between aromatic and aliphatic carboxylate ligands, alicyclic carboxylate ligands are “moderately” flexible, which make them potential sources of conformational chirality. Upon coordination to metal centers, the freedom to rotate of all the C-C bonds of such a ligand is restrained, and the ligands can be locked in a twist chiral conformation. In this paper, we adopt H2TMG as ligand, exploring the reactions of H2TMG with cadmium(II). We choose cadmium(II) due to its d10 configuration that permits a wide variety of geometries and coordination numbers, which provides more possibility to isolate chiral coordination polymers. Crystal Structure of [Cd(TMG)]n 1. First, we studied the reaction of H2TMG and Cd(NO3)2 at the ratio of 1:1:1 without any secondary ligands. An X-ray diffraction study reveals that complex 1 is a two-dimensional (2D) network. As shown in Figure 1, each central cadmium ion is coordinated by seven oxygen atoms from six carboxylate groups in a distorted pentagonal pyramidal geometry. Two TMG2- molecules bridge two cadmium ions to form a metal-organic ring as the basic
Figure 1. The coordination environment of cadmium ions in 1 with the thermal ellipsoids at 50% probability level (all H atoms were omitted for clarity). Symmetry codes: #A -x - 1, y - 1/2, -z - 3/2; #B x, -y + 3/2, z - 1/2; #C -x - 1, -y + 1, -z - 1; #D x, -y + 1/2, z - 1/2.
Scheme 1
structural unit of the 2D network. Through the bridging of carboxylate groups with different modes (Scheme 1a,b), adjacent metal-organic rings are connected and extended into a 2D network in the [011] plane (Figure 2). Crystal Structure of {[Cd(TMG)(1,10-phen)‚H2O]‚2H2O}n 2. Without any secondary ligand, complex 1 obtained from the reaction of H2TMG and Cd(NO3)2 is not chiral and even does
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Figure 2. The 2D network of 1 viewed along [100].
Figure 3. The coordination environment of cadmium ions in 2 with the thermal ellipsoids at 50% probability level (all H atoms were omitted for clarity). Symmetry code: #A x + 1, y, z.
not consist of our expected helices models. To construct helices, we introduced a secondary ligand into the reaction system. First, we selected the terminal ligand of 1,10-phen as the secondary ligand. The introduction of a terminal ligand can occupy two coordinated sites of the cadmium ion, which may result in a one-dimensional (1D) helical chain. The π-π interactions between 1,10-phen may play the role of discriminative interactions to realize homochiral assembly. From the reaction of H2TMG, 1,10-phen, and Cd(NO3)2 at the ratio of 1:1:1, we obtain complex 2. An X-ray diffraction study shows that complex 2 is a 1D helical chain. Similar to 1, the central cadmium ions have distorted pentagonal bipyramidal coordination geometry in 2. Four oxygen atoms from two carboxylate groups, one oxygen atom from the coordinated water, and two nitrogen atoms from the 1,10-phen occupy seven coordination sites, respectively (Figure 3). Each TMG2- with chelating mode bridges two cadmium ions and extends into an infinite 21 helical chain along the [100] direction. The coordination of 1,10-phen and water molecules prevents the 1D chain from being extended into a higher dimensional network. It is interesting that all 1,10-
Figure 4. The “helices pairs” formed by hydrogen bonds and π-π interactions (the right- and left-handed helical chains are labeled in red and green, respectively).
Figure 5. The coordination environment of cadmium ions in 3 with the thermal ellipsoids at 50% probability level (all H atoms were omitted for clarity). Symmetry codes: #A y - 1, x, -z; #B -y + 1, x - y + 1, z + 1/3; #C x, y - 1, z.
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Figure 6. The 21 helical chain and linear chain along the [100], [010], or [110] direction.
phen are situated in the same side of the chain, and all coordinated water molecules are in the other side. As a hydrogen-bonding donor, each coordinated water of the lefthanded helical chain forms two hydrogen bonds with two carboxylate groups of the adjacent right-handed helical chain (O7W-H7WA‚‚‚O3 (2.729 Å), O7W-H7WB‚‚‚O1 (2.739 Å)) to yield a “helices pair”. All “helices pairs” are extended into a three-dimensional (3D) architecture by the π-π interactions between 1,10-phen (3.52 Å). Similar to the hydrogen bonding, the π-π interactions also exist between helical chains with different chirality (left- and right-handed), respectively, so the left- and right-handed helical chains arrange alternately. The supramolecular interactions (hydrogen-bonding and π-π interactions) play the role of achiral discriminative interactions, and the helices of 2 are formed as mesomerate (Figure 4). Crystal Structure of [Cd(TMG)(4,4′-bipy)]n 3. From complex 2, we find that the supramolecular interactions are difficult to predict and control. Compared with the supramolecular interactions, the coordination bonds have predictable directivity, which may be more suitable to act as chiral discriminative interactions in the synthesis of chiral coordination polymers. Thus, we replaced 1,10-phen by 4,4′-bipy as a secondary ligand. We choose 4,4′-bipy for three reasons: (1) The coordination of two 4,4′-bipy occupy two coordinated sites of each cadmium ion, which will play a role similar to 1,10-phen and result in helical chains. (2) Through bridging two metal centers, the coordination of 4,4′-bipy will replace the supramolecular interactions as discriminative interactions. (3) The C-C bond between two pyridine rings of 4,4′-bipy can rotate in certain degree, which makes it easier to accommodate the requirement of chiral discriminative interactions. From the reactions of H2TMG, 4,4′-bipy with Cd(NO3)2, complex 3 was obtained. An X-ray diffraction study reveals that complex 3 is a 3D architecture containing homochiral helices. As shown in Figure 5, the central cadmium ion in 3 is coordinated by five oxygen atoms from three carboxylate groups and two nitrogen atoms from two 4,4′-bipy in a distorted pentagonal bipyramidal geometry. Each TMG2- bridges two cadmium ions through different coordination modes (Scheme 1c,d) to form infinite 21 helical chains [Cd2(TMG)2]n, while each 4,4′-bipy bridges two cadmium ions to form infinite linear chains [Cd(4,4′-bipy)]n. These two kinds of chains are basic structural units of the 3D architecture of 3, and they are connected into “chain pair” through sharing metal centers (Figure 6). All of the 21 helices in 3 are right-handed with the same formula, and 3 is a chiral coordination polymer. The helical chains, parallel to [100], [010], and [110] directions, respectively, are not independent; they are interlinked by the µ2-oxygen atoms of carboxylate groups into metal-organic rings vertical to [100], [010], and [110] directions (Figure 7a). By sharing a metal ion and TMG2-, each of six such metal-organic rings (two in [100], two in [010],
Figure 7. (a) The metal-organic ring composed of three kinds of helical chains extending along the [100], [010], and [001] directions (the TMG ligand molecules belonging to three kinds of helical chains are labeled in three colors). (b) The hexagonal nanounit.
and two in [110] plane) constitute a hexagonal nanounit with a diameter of 1.19 × 1.19 nm based on the distances between the centers of opposite atoms (Figure 7b). The nanounits are extended into nanochannels along the [001] direction via the bridging of µ2-oxygen atoms, and all nanochannels are connected into a 3D architecture by sharing metal-organic rings. The void of each nanochannel is occupied by a linear [Cd(4,4′bipy)]n chain to form the final interpenetration structure (Figure 8). Vibrational Spectra. The asymmetric stretching bands of 1 appear as a quartet around 1560 cm-1, and the symmetric stretching vibrations around 1413 cm-1 appear as a trio, which is different from the usual single or double bands. The IR spectrum of 2 shows typical chelating carboxylate antisymmetric and symmetric stretching bands at 1538 and 1408 cm-1.22 Because of the asymmetric and symmetric carboxylate stretches, the IR spectrum of 3 shows trio bands around 1565 cm-1 and double bands 1386 cm-1, respectively. The differences in the IR spectra of 1-3 may arise from the different coordination modes of the carboxyl groups, which is in agreement with the structural data (Scheme 1).
1462 Crystal Growth & Design, Vol. 6, No. 6, 2006
Figure 8. The 3D architecture of 3 along the [001] direction.
Conclusions Three complexes, [Cd(TMG)]n (1), {[Cd(TMG)(1,10-phen)‚ H2O]‚2H2O}n (2), and [Cd(TMG)(4,4′-bipy)]n (3), have been prepared from the reaction system of H2TMG, Cd(NO3)2, and secondary ligands. By adopting potential chiral ligands, a metal-organic helical chain as a transmitter for chiral information, and coordination bonds as chiral discriminative interactions, the chiral coordination polymer 3 with an interesting 3D architecture has been synthesized successfully by spontaneous resolution. This study may provide useful insight into the synthesis of chiral coordination polymer by spontaneous resolution. Acknowledgment. The authors acknowledge the financial support from the National Natural Science Foundation of China (90206040, 20325106, 20521101, 20333070), the Natural Science Foundation of Fujian Province (2005HZ01-1, E0520003), the Distinguished Oversea Scholar Project and the “One-hundred Talent” Project from CAS. Supporting Information Available: XRD patterns and crystallographic information files (CIF) of three compounds are available free of charge via the Internet at http://pubs.acs.org.
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CG0600905
