Achiral Coordination Polymers Based on 2

Article pubs.acs.org/crystal

Assembly of Chiral/Achiral Coordination Polymers Based on 2-(Pyridine-3-yl)-1H-4,5-imidazoledicarboxylic Acid: Chirality Transfer between Chiral Two-Dimensional Networks Containing Helical Chains Song-Liang Cai, Sheng-Run Zheng,* Zhen-Zhen Wen, Jun Fan, and Wei-Guang Zhang* School of Chemistry and Environment, South China Normal University, Guangzhou, 510006, China S Supporting Information *

ABSTRACT: Three kinds of new coordination polymers, [Mn(μ3-HPyIDC)(H2O)]n (1a and 1b), {[Cd3(μ4-PyIDC)2(H2O)4]·H2O}n (2), and [Ca(μ4-HPyIDC)(H2O)]n (3a and 3b), were synthesized from the solvothermal reactions of 2-(pyridine-3-yl)-1H-4,5-imidazoledicarboxylic acid (H3PyIDC) with the corresponding metal salts. Crystal structure analysis showed that all coordination polymers were based on an analogical chiral twodimensional (2D) secondary building unit (SBU) containing helical chains. The chirality of the 2D SBU was transferred to neighboring 2D SBUs via hydrogen bonds and coordination bonds in 1 and 3, respectively, resulting in two homochiral frameworks. When opposite chirality was transferred between neighboring 2D SBUs via Cd(II) ions, an achiral framework was generated, resulting in 2D nets packed in alternating P and M chirality in compound 2. Furthermore, compounds 1, 2, and 3 feature 2D (82·10), 3D (62·82)(62·82·102), and cds network topologies, respectively. Circular dichroism (CD) measurements also confirmed that the resulting crystals of 1 and 3 are racemic mixtures. Compounds 2 and 3 showed strong fluorescent emissions at room temperature.

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INTRODUCTION Chiral metal−organic frameworks (MOFs) are an emerging class of materials with various potential applications in medicine, nonlinear optical devices, asymmetric catalysis, separation, and chiral magnets.1,2 One of the most challenging tasks in the application of chiral MOFs is the design and assembly of chiral MOFs from achiral ligands. In this case, building blocks with opposite chirality often crystallize in the same crystal, leading to an achiral framework. Occasionally, spontaneous resolution, where two enantiomers crystallize out separately and generate a conglomerate, occurs.3 Although more and more reports about spontaneous resolution have been documented, the intrinsic mechanisms to explain the detailed processes involved have yet to be fully understood. The concept of “chirality transfer” was recently employed to explain the formation of a homochiral framework from a structural standpoint.4 The overall framework can be decomposed into lower dimensional chiral secondary building units (SBUs) with intra- or intermolecular interactions between them. If the same chiral message of a chiral SBU is transferred to adjacent SBUs and then to the whole framework (in a homochiral manner), the product will be homochiral. The interactions between chiral SBUs are denoted as “homochiral interactions.” However, products will be achiral if the transfer follows a heterochiral manner, and the corresponding interactions can be denoted as “heterochiral interactions.” Even if the concept of “chirality transfer” does not actually lead to the directional © 2012 American Chemical Society

synthesis of chiral compounds, it can greatly increase the possibility of obtaining homochiral coordination polymers with appropriate metal ions and ligands. The success of “chirality transfer” depends on two factors: the formation of chiral building blocks and a mechanism with which to transfer chiral messages homochirally. Higher dimensional chiral SBUs are speculated to be suitable for the construction of a chiral framework because fewer interactions must be considered in this case. Hence, to obtain chiral MOFs, an easily accessible chiral building block that is as highly dimensional as possible must first be designed and then strong discriminative interactions must be introduced to initiate the homochirality transfer process. Helical structures, which represent a common chiral motif, have been found in many MOFs.5 However, the homochiral interactions between helical chains are often stopped in twodimensional (2D) layers and cannot be easily extended into three-dimensional (3D) coordination frameworks. In the present study, we report the construction of three MOFs by selfassembling 2-(pyridine-3-yl)-1H-imidazole-4,5-dicarboxylate (H3PyIDC) and different metal ions into compounds [Mn(μ3-HPyIDC)(H2O)]n (1a and 1b), {[Cd3(μ4-PyIDC)2(H2O)4]·H2O}n (2), and [Ca(μ4-HPyIDC)(H2O)]n (3a and 3b). The transfer of stereochemical information between neighboring chiral chains Received: January 6, 2012 Revised: March 22, 2012 Published: March 29, 2012 2355

dx.doi.org/10.1021/cg3000203 | Cryst. Growth Des. 2012, 12, 2355−2361

Crystal Growth & Design

Article

Table 1. Crystal Data and Structure Refinement of Compounds 1−3 chemical formula M crystal system space group a (Å) b (Å) c (Å) α (°) β (°) γ (°) V (Å3) Z T (K) F(000) Dcalcd (g cm−3) μ (mm−1) λ (Å) Rint data/restraint/parm GOF R1 [I = 2σ(I)]a wR2 [I = 2σ(I)]b Flack a

1a

1b

2

3a

3b

C10H7MnN3O5 304.13 orthorhombic P2(1) 2(1) 2(1) 6.6833(15) 8.6783(19) 17.820(4) 90 90 90 1033.5(4) 4 298 (2) 612 1.955 1.301 0.71073 0.0891 2244/0/178 0.982 0.0516 0.0756 0.01(3)

C10H7MnN3O5 304.13 orthorhombic P2(1) 2(1) 2(1) 6.690(3) 8.682(4) 17.825(8) 90 90 90 1035.5(8) 4 298(2) 612 1.951 1.298 0.71073 0.0799 2399/0/172 1.008 0.0511 0.0668 0.04(3)

C20H18Cd3N6O13 887.60 monoclinic P2(1)/n 6.7085(7) 8.6142(9) 22.1516(19) 90 102.106(3) 90 1251.6(2) 2 298(2) 856 2.355 2.608 0.71073 0.0312 2456/3/199 1.045 0.0303 0.0608

C10H7CaN3O5 289.27 orthorhombic P2(1) 2(1) 2(1) 6.7642(10) 9.5430(14) 16.524(2) 90 90 90 1066.6(3) 4 298(2) 592 1.801 0.612 0.71073 0.0370 2312/0/172 1.048 0.0336 0.0787 0.03(4)

C10H7CaN3O5 289.27 orthorhombic P2(1) 2(1) 2(1) 6.7491(14) 9.5380(19) 16.526(3) 90 90 90 1063.8(4) 4 298(2) 592 1.806 0.613 0.71073 0.0290 2316/0/172 1.048 0.0264 0.0653 0.04(3)

R1 = Σ||Fo| − |Fc||/|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2, where w = 1/[σ2(Fo2) + (aP)2 + bP]. P = (Fo2 + 2Fc2)/3. 1119(w), 1031(w), 988(m), 870(w), 820(m), 789(w), 708(w), 683(w), 638(w), 584(w), 480(w), 420(w). {[Cd3(μ4-PyIDC)2(H2O)4]·H2O}n (2). Compound 2 was synthesized in a procedure similar to that described for 1, except that MnCl2·6H2O was replaced by CdCl2·2.5H2O (0.20 mmol). Yield 52% (based on the Cd). Elemental analysis calcd (%) for C20H14Cd3N6O13: C, 27.19, H, 1.60, N, 9.51. Found: C, 27.22, H, 1.58, N, 9.53%. IR (KBr, ν/cm−1): 3343(s), 2971(w), 2926(w), 1664(w), 1580(s), 1532(w), 1519(w), 1436(m), 1399(s), 1352(w), 1324(w), 1279(m), 1250(w), 1158(w), 1130(w), 1108(w), 1031(w), 1000(w), 862(w), 805(w), 737(w), 702(w), 666(w), 550(w), 497(w). [Ca(μ4-HPyIDC)(H2O)]n (3). An identical procedure as that for 3 was followed to prepare 1, except that MnCl2·6H2O was replaced by CaCl2 (0.20 mmol). Yield 72% (based on the Ca). Elemental analysis calcd (%) for C10H7CaN3O5: C, 41.52, H, 2.44, N, 13.86. Found: C, 41.47, H, 2.46, N, 13.90%. IR (KBr, ν/cm−1): 3697(s), 2923(w), 2853(w), 1664(w), 1634(w), 1580(s), 1531(w), 1456(m), 1391(s), 1352(m), 1271(w), 1237(w), 1198(w), 1129(w), 1027(w), 978(m), 894(w), 851(w), 822(w), 788(m), 727(w), 707(w), 635(w), 544(w), 515(w), 459(w). X-ray Data Collection and Structure Refinement. Data collections were performed at 298 K on a Bruker Smart Apex II diffractometer with graphite-monochromated Mo KR radiation (λ = 0.71073 Å) for six compounds 1−3. Absorption corrections were applied by using the multiscan program SADABS.8 Structural solutions and full-matrix least-squares refinements based on F2 were performed with the SHELXS-979 and SHELXL-979 program packages, respectively. All the non-hydrogen atoms were refined anisotropically. All the hydrogen atoms were placed at calculated position except two water hydrogen atoms on O(2w) in compound 2 and on O1W in compound 1a were located from difference maps and refined with isotropic temperature factors. Details of the crystal parameters, data collections, and refinements for compounds 1−3 are summarized in Table 1. Selected bond lengths and angles are shown in Table S1. Further details are provided in Supporting Information. CCDC 96098, 96099, 859599, 859600, 859601, and 859601 are for compounds 1−3, respectively.

results in chiral layers, and then further chirality transfer between layers results in the formation of achiral or homochiral 3D MOFs. Imidazole-based dicarboxylate ligands have been proven to be excellent multidentate ligands.6 However, the coordination chemistry of H3PyIDC requires further investigation because only several mononuclear complexes,7a 2D Ln(III) networks,7b a 3D Pb(II) high-connected framework,7c and a 3D Nd(III) framework7d have been reported thus far. In addition, compound 3 represent the first coordination polymer based on H3PyIDC and s-block metal ions.

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EXPERIMENTAL SECTION

Materials and Measurements. The ligand H3PyIDC is synthesized according to the literature.7b Other materials were reagent grade obtained from commercial sources and used without further purification, and solvents were dried by standard procedures. Elemental analyses for C, H, N were performed on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FTIR-170SX spectrophotometer in KBr pellets. Thermogravimetric analyses were performed on Perkin-Elmer TGA7 analyzer with a heating rate of 10 °C/min in flowing air atmosphere. The luminescent spectra for the solid state were recorded at room temperature on Hitachi F-2500 and Edinburgh-FLS-920 with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5.0 nm. The solid (KBr pellets) circular dichroism (CD) spectra were recorded on a JASCO J-810 spectropolarimeter. [Mn(μ3-HPyIDC)(H2O)]n (1). A mixture of MnCl2·6H2O (0.20 mmol), H3PyIDC (0.20 mmol), Et3N (0.1 mL), and EtOH/H2O (1:1, 8 mL) was sealed in a 10 mL Teflon-lined stainless-steel reactor, heated at 170 °C for 72 h under autogenous pressure, and then slowly cooled to room temperature at a rate of 5 °C/h. Pale-yellow block crystals of 1 were collected by filtration and washed with distilled water and ethanol several times. Yield 69% (based on the Mn). Elemental analysis calcd (%) for C10H7MnN3O5: C, 39.49, H, 2.32, N, 13.82. Found: C, 39.22, H, 2.51, N, 13.62%. IR (KBr, ν/cm−1): 3380(s), 3217(m), 3059(w), 1659(s), 1614(m), 1580(w), 1531(m), 1475(w), 1460(m), 1420(w), 1400(s), 1358(m), 1335(w), 1277(w), 1233(w), 1198(m), 1136(w), 2356



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RESULTS AND DISCUSSION Formation of the Chiral 2D Net in Compounds 1−3. The ligand H3PyIDC contains a 1H-imidazole-4,5-dicarboxylate motif, which is suitable for constructing helical chains, as seen in the study done by Lu.6g In Lu’s research, some helixbased structures from 1H-imidazole-4,5-dicarboxylate ligand were constructed, where various secondary ligands connected to the helix to generate chiral/achiral 2D networks. The present study aims to construct helical chains on H3PyIDC from a 1Himidazole-4,5-dicarboxylate motif and connect helical chains into a 2D net using an additional pyridyl group. Indeed, this type of chiral 2D SBU is obtained in compounds 1−3. As shown in Scheme 1, the HPyIDC2− anions alternately bridge

Figure 1. Top: View of the coordination environment of M (II) in net I in M (left) and P (right) chirality (H atoms are omitted for clarity). Color code: M(II), cyan; O, red; N, blue; C, black. Bottom: The axially chiral conformations of H3PyIDC in net I in M (left) and P (right) chirality.

Scheme 1. Formation of Chiral 2D Network in Compounds 1−3a

Scheme 2. Coordination Modes in (a) the Chiral 2D Network I, (b) Compound 1, (c) Compound 2 and (d) Compound 3

a

Only 2D network based on M helix is shown.

and ligand centers are important in the successive connection of subunits to form compounds 1−3. Chirality Transfers between Homochiral 2D Networks in Compounds 1−3. The chiral 2D net I is arranged uniquely, as shown in Scheme 3. The structural diversity depends

the metal ions [M(II)] to form a 1D infinite helical chain of [MHIDC]∞ around the 21 axis (helix I in P or M chirality). Pyridyl groups are alternately located on both sides of the chains. In helix I, only four coordination sites of the M(II) center are occupied by the 1H-imidazole-4,5-dicarboxylate group. Therefore, the coordination sphere of M(II) center in helix I will be unsaturated if metal ions with a coordination number greater than 4 are chosen. In compounds 1−3, one of the remaining coordination sites on the unsaturated metal center of helix I is filled by the pyridyl group on HPyIDC2− from the adjacent helix I, which self-interconnects helices I to a 2D network (Scheme 1). The resulting 2D network is homochiral (net I in P or M chirality) because the H3PyIDC group that connects the two helices I does not introduce a mirror plane or symmetry center into the net. New helical chains are formed via the connection of pyridyl groups and M(II) centers (helix II in P or M chirality). Detailed analyses reveal that chirality comes from the configuration of metal geometry and the conformation of the H3PyIDC ligand. For example, the 2D chiral network based on an M helix has all of its metal centers displaying Λ-configurations and all of its H3PyIDC ligands taking on a λ-conformation (Figure 1). The chiral conformation originates from C−C bond rotation between two rings on the achiral ligand. Bond rotation is “locked” and separated in the crystal, although this phenomenon is fairly unlikely to occur.10 The homochirality of stereogenic metal centers may help form helix I, whereas the axially chiral conformations of the H3PyIDC ligand could contribute to the homochiral interaction of helix I. Only five coordination sites around the metal center are used in a 2D chiral net, which can be considered “unsaturated” when metal ions with a coordination number greater than 5 are used. The HPyIDC2− anions in this 2D network are also “unsaturated” because they leave some donors that can act as coordination or hydrogen bonding donors (Scheme 2a). Both unsaturated metal

Scheme 3. Chirality Transfer of 2D Chiral Networks in Compounds 1−3

on the metal ions used under the experimental conditions. As shown in Scheme 3, Mn(II) ions form a 3D supramolecular framework via 2D coordination networks plus hydrogen bonds, Cd(II) ions form a 3D coordination framework via analogical 2D coordination networks plus a Cd(II) ion as connector, and Ca(II) compounds form a 3D framework via the selfinterconnection of 2D coordination networks. Different connection combinations lead to the formation of various chiral or achiral frameworks, indicating that the interactions between chiral SBUs are very important for spontaneous resolution (discussed in detail below). 2357



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The pitch of both helices I and II is 8.6142(9) Å, the adjacent Cd···Cd distances in helices I and II are 5.756 and 5.831 Å, respectively, and the angle between the imidazole ring and pyridyl group is 25.70(13)o in 2. All these data are similar to those in 1, which indicates that the conformation of the 2D chiral network here is similar to that in 1. As shown in Figure 3,

When MnCl2 is reacted with H3PyIDC, compounds 1a and 1b are obtained. Crystallographic analysis reveals that compounds 1a and 1b crystallize in the same chiral space group, P212121, which exhibits the 2D coordination network shown in Scheme 1 and Figure 1. The flack parameters of 1a and 1b is 0.01(3) and 0.04(3), respectively, indicating enantiomeric purity of the single crystals. In the present study, however, only the structure of 1a is discussed in detail. The unsaturated metal center in net I (as discussed above) is saturated by a coordinated water molecule. Thus, the Mn(II) ion has a coordination number of 6 and exhibits octahedral geometry. The μ3-HPyIDC2− ligand adopts a μ3-kN,O:kO,O′:kN coordination mode to bridge three Mn(II) ions in N,O-chelating, O,O′-chelating, and monodentate fashions (Scheme 2b). In the 2D network, the helical chains extend along the b direction with a pitch of 8.6783(19) Å. The adjacent Mn···Mn distance in helices I and II are 5.778 and 6.070 Å, respectively. The uncoordinated N and O atoms on the ligand form NH···O and OH···O hydrogen bonds between two layers, which further connect the 2D networks into a 3D framework. The interactions transfer the chiral message from net to net because the configuration does not introduce a symmetric center or mirror, which leads to the spontaneous resolution of compound 1 (Figure 2).

Figure 3. Top: View of the coordination environment of Cd(II) in 2 (H atoms are omitted for clarity). Color code: Mn(II), cyan; O, red; N, blue; C, black. Middle: The chiral 2D network in 2. Bottom: The 3D supramolecular framework in 2.

there are two crystal independent Cd(II) ions in the asymmetric unit. The coordination environment of Cd(1) ions is similar to that of Mn(II) in 1, whereas the Cd(2) ions are coordinated by two H3PyIDC anions via N,O-chelation and two water molecules in the apical position. The coordination mode of H3PyIDC in 2 can be seen as potential N and O coordination sites in net I (Scheme 2a) become coordinate to another M(II) center (Scheme 2c). The 2D nets are linked by Cd(II) ions, which generate a 3D framework. However, the connection brings about an inverted center to Cd(II) ions. Therefore, P and M networks are arranged alternately along the c direction, resulting in an achiral 3D MOF. Small channels are formed along the a direction and occupied by uncoordinated water molecules. If only the water molecules are removed, complex 2 has 59.0 Å3 potential solvent volume (4.7%) estimated by PLATON.11 The transfer mechanism in 2 cannot allow for the transfer of chiral messages between 2D networks. From the formation of chiral net I, chirality is maintained if the chiral building blocks are self-interconnected by the SBUs themselves (without the aid of other building blocks). Similarly, if the M(II) centers in 2D SBUs can link themselves via coordination bonds between

Figure 2. Top: View of the coordination environment of Mn(II) in 1a (left) and 1b (right) (H atoms are omitted for clarity). Color code: Mn(II), cyan; O, red; N, blue; C, black. Middle: The chiral 2D network in 1a (left) and 1b (right). Bottom: The 3D supramolecular framework in 1a (left) and 1b (right).

As discussed above, the “unsaturated” coordination sites in complex 1 is essential in the chirality formation and transfer between helical chains and layers. To further prove this, the ratio of Mn(II)/H3PyIDC or pH value is increased, and the 3D Mn(II) MOF of complex 1 can not be obtained. When Cd(II) salt is used instead of Mn(II) salt, a 3D MOF based on the analogical chiral 2D network is successfully obtained (complex 2). 2358



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networks can help in understanding the structural diversity and rational design of a network to tailor it for specific needs.13 From the structural analyses, strong relationships in the structures of compounds 1−3 are found. In the present study, topological analysis is used to elucidate such relationships (Figure 5).

M(II) ions in one SBU and the donor in an adjacent SBU, a chiral network could be deduced to form. Successful results are not obtained when metal ions with a coordination number of 6 are selected because the remaining coordination sites are easily occupied by coordination water molecules under the experimental conditions. In this case, an alternative strategy can be used, specifically, introduction of metal ions with a higher coordination number (higher than 6). Ca(II) ions, which have a geometry corresponding to metal ions with a coordination number of 7 and can bind to both O and N atoms,12 is employed to accomplish this goal. The resulting complexes 3a and 3b crystallize in the same chiral space group, P212121. In the present work, only complex 3a is described in detail. As shown in Figure 4,

Figure 5. Left: the topology of 1; middle: top view and side view of the topology of 2; right: the topology of 3. (Red balls represent the metal centers, and yellow balls represent the ligand centers.)

Both H3PyIDC and M(II) can be seen as topological points. In compound 1, both H3PyIDC and M(II) can be considered as three-connected nodes and yield a (82·10) network topology. This type of topology is an uncommon three-connected 2D layer compared with (6, 3) and (4·82) nets. In compound 2, Cd(1) is three-connected nodes while H3PyIDC is four-connection nodes. The point symbols of H3PyIDC and Cd(1) are (62·82·102) and (62·82), respectively. Thus, the point symbol for the framework is (62·82)(62·82·102), as calculated by TOPOS.14 In compound 3a, both H3PyIDC and M(II) are four-connected nodes and display a point symbol of (65·8). The resulting 3D net has a cds network topology. From structural analyses, both the 3D (62·82)(62·82·102) and cds networks are found to contain an (82·10) subnet. Utilizing the methods employed by Wells, the 3D network can be suitably decomposed into the connection of the 2D layer.13c In the present study, as shown in Scheme 3, if all three connection nodes in an (82·10) layer connect to adjacent layers, a cds network is formed. However, if only half of the three-connection nodes in a (82·10) layer connect to adjacent layers (Figure 5), a (62·82)(62·82·102) network is formed. All of the networks are topologically achiral, which ensures that the chirality of compounds 1 and 3 is due to the conformation of the metal center and H3PyIDC rather than the connectivity of M(II) and the ligand. X-ray Powder Diffraction, Thermal Analyses, and Photoluminescent Properties. X-ray powder diffraction (XRD) was used to check the purity of three compounds. As shown in Figures S1−S3, Supporting Information, all the peaks displayed in the measured patterns closely match those in the simulated patterns generated from single-crystal diffraction data, indicating single phases of the three compounds are formed. The results of thermogravimetric (TG) analyses (Figure 6) indicate that both compounds 1 and 3 were stable to about 340 οC, and then it began to decompose with a continuous weight loss up to 670 and 760 οC, respectively. Whereas compound 2 lost its water guest molecules below 180 οC, the weight loss found of 1.6% was consistent with that calculated (2.0%). After the loss of guest water molecules, the coordination water molecules were lost (found 9.4%, calculated 10.0%) from 180 to 260 οC, and then the 3D framework was stable up to 430 οC. The results indicate that the Cd(II) frameworks of 2 can be thermally stable at high temperature, while the MOF of 1 and 3 is less thermal stable. The relative higher stability of 2 may be due to its higher dimension framework (compared to 1) and stronger Cd−X (X = N or O) bonds (compared to 3).

Figure 4. Top: View of the coordination environment of Ca(II) in 3a (left) and 3b (right) (H atoms are omitted for clarity). Color code: Mn(II), cyan; O, red; N, blue; C, black. Middle: The chiral 2D network in 3a (left) and 3b (right). Bottom: The 3D supramolecular framework in 3a (left) and 3b (right).

six of the seven coordination atoms around Ca(II) are similar to those of Mn(II) ions in 1 and Cd(II) ions in 2. The seventh coordination site is occupied by an O atom from H3PyIDC. The coordination mode of H3PyIDC can be seen as a potential O donor in Scheme 2a becomes coordinate to a metal ion (Scheme 2d). Similar 2D SBUs were also formed in 3a. The pitch of helices I and II is 9.5430(14) Å, longer than that in 1 and 2. The adjacent Ca···Ca distances in helices I and II are 6.365 and 6.586 Å, respectively, also slightly longer than those in 1 and 2. In addition, the angle between the imidazole ring and the pyridyl group is 28.76(7)o in 3. These parameters indicate that the 2D SBU is more extended in 3 than in 1 and 2 because of the coordination geometry and long bonds around Ca(II) ions. These 2D networks are self-interconnected by coordination bonds between Ca(II) ions and O atoms from the adjacent 2D SBUs. Similar to 1, homochirality transfer is found when 2D networks are joined in this manner, thus yielding a homochiral 3D MOF (Figure 4). Topological Relationships between Compounds 1−3. Clarification of the topological relationships between different 2359



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Figure 6. TG curve of compounds 1−3.

Figure 8. CD spectroscopy of compound 1.

Previous studies have indicated that metal coordination polymers containing d10 ions or s-block ions exhibit photoluminescent properties.15 Thus, we also investigated the photoluminescent property of compounds 2 and 3 in the solid state at room temperature. As shown in Figure 7, compounds 2 and

Figure 9. CD spectroscopy of compound 3.

dependent on interlayer connections. If the 2D nets are connected by hydrogen bonds or self-interconnected, the chirality of the originally formed chiral net is uniformly transferred to neighboring nets, leading to the formation of a homochiral 3D framework. If the 2D nets are bridged by another metal ion that acts as an inversion center, the opposite chirality of the originally formed chiral net will be transferred to nearby nets, resulting in an achiral 3D framework. The results indicate that chiral coordination polymers could be easily obtained via spontaneous resolution using the “chiral transfer” strategy with appropriate metal ions and ligands.

Figure 7. Emission spectra of compounds 2 and 3 in the solid state at room temperature.

3 exhibit strong photoluminescence with a maximum emission band observed at 415 and 450 nm, respectively. The free H3PyIDC ligand displays fluorescent emission band at 469 nm (λex = 380 nm) based on the literature.16 The observed blueshifted (19 nm) of compound 2 may be assigned to the intraligand transitions,17 whereas the blue-shifted (54 nm) of compound 3 may be assigned to the ligand-to-metal charge-transfer (LMCT).18 CD Spectroscopy. The solid-state CD spectra of compounds 1 and 3 were recorded on single crystals (the crystal sizes are about 0.22 × 0.20 × 0.15 and 0.40 × 0.38 × 0.35 for crystal 1 and 3, respectively) with KBr pellets between 200 and 400 nm at room temperature (Figures 8 and 9). The CD spectra present an obvious Cotton effect. The crystals of 1a and 1b exhibit opposite Cotton effects at the same wavelengths (five peaks at 215, 240, 265, 290, and 320 nm); this is also observed in crystals of compounds 3a and 3b. The CD spectra indicate the coexistence of left-handed and right-handed enantiomers in one pot, which confirms spontaneous resolution during the course of the crystallization. To further investigate whether the enantiomeric excess exists in these chiral crystals, the bulk samples of 1 and 3 were measured by CD spectrum; however, bulk samples of 1 and 3 showed a silent CD spectrum (Figures S4−S5, Supporting Information). All the results confirmed that the resulting crystals of both 1 and 3 are racemic mixtures.3e In conclusion, three new compounds assembled from H3PyIDC were synthesized. All compounds contain similar chiral 2D layers with (82·10) topology. The chirality transfer between 2D nets was

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Additional structural figures for the related compounds, Tables of selected bond lengths and angles, PXRD, as well as X-ray crystallographic files in CIF format for compounds 1−3 are available. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*Phone: +86-20-39310383. Fax: +86-20-39310187. E-mail: [email protected] (S.-R.Z.); [email protected] (W.-G.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was fnancially supported by the National Natural Science Foundation of P. R. China (Grant Nos. 21003053 and 2360



Article

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21171059), the Natural Science Foundation of Guangdong Province (Grant No. 10451063101004667).

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