Homochiral Layered Coordination Polymers from Chiral N

Article pubs.acs.org/crystal

Homochiral Layered Coordination Polymers from Chiral N‑Carbamylglutamate and Achiral Flexible Bis(pyridine) Ligands: Syntheses, Crystal Structures, and Properties Yuehong Wen, Tianlu Sheng, Zhenzhen Xue, Zhihao Sun, Yanlong Wang, Shengmin Hu, Yihui Huang, Jie Li, and Xintao Wu* State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China S Supporting Information *

ABSTRACT: Four new homochiral two-dimensional (2D) coordination polymers, namely, [Zn2L(NCG)2(H2O)2·4H2O]n (1), [Zn(bpe)(NCG)· 3H2O]n (2), [Zn(bpe)(NCG)(NaNO3)0.5H2O]n (3), and [Zn(bpp)(NCG)·3.5H2O]n (4) (NCG = N-carbamylglutamate, L = 1,2-bis(4′pyridylmethylamino)ethane, bpe = 1,2-bis(4-pyridyl)ethane, bpp = 1,3bis(4-pyridyl)propane), have been prepared under mild conditions. The structures of CPs 1−4 were determined by single-crystal X-ray diffraction analysis. Complexes 1 and 2 display novel homochiral wavelike networks, with adjacent layers stacked in an AA type arrangement. Complex 3 shows an interesting 2D double-layered framework, which is further connected to form a 3D supermolecular structure through hydrogen bonding and aromatic π−π stacking interactions. The 2D structure of complex 4 contains two uncommon helical chains of bpp and NCG, with two flexures in each single strand. Different layers are stacked in an offset ABAB fashion. The mixed-ligand strategy, cooperation of a chiral and an achiral ligand, is applied to prepare the homochiral neutral layered CPs. Complexes 1−4 display second harmonic generation efficiencies, which are approximately 0.7, 0.9, 0.4, and 0.6 times as much as that of urea powder. In addition, the photoluminescence of all complexes has also been investigated in the solid state.

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INTRODUCTION Chirality is a widespread phenomenon in nature, playing a very important role in many areas of society and science.1 Therefore, lots of scientists focus on the topic of preparation of chiral materials.2 In the past few years, chiral metal−organic frameworks (MOFs) or coordination polymers (CPs) are of great interest, because of their intriguing potential applications in optical resolution, enantioselective synthesis, asymmetric catalysis, and so on.3 Various approaches have been developed to prepare the chiral CPs, such as the usage of chiral building blocks, spontaneous resolution, the influence of a chiral physical environment, etc.4−6 The most direct route is using the enantiopure organic compounds as ligands to prepare the bulk homochiral MOFs materials.7 Alternatively, the mixedligand approach,8,9 cooperation of a chiral ligand with an achiral ligand, has been developed, which makes the structures of homochiral CPs more diverse. Nonetheless, it is a challenge to prepare pure CPs applying the mixed-ligand method, as each ligand tends to form their respective CPs. Hence, the choice of two matched ligands is crucial. Ligands with two different functionalities, such as carboxylate combined with an N-donor ligand, have been proven successful in construction of various achiral CPs. 10 However, the cooperation of a chiral dicarboxylate and a flexible N-donor ligand to prepare homochiral CPs has been less explored.11 On the other hand, © 2014 American Chemical Society

a series of achiral ionic layered CPs have been prepared and applied in the construction of homochiral layered intercalation compounds or 3D MOFs in our recent work.12,4f,5g In order to extend such studies, we aim to synthesize some homochiral neutral 2D layers, which might be used as good layered host candidates as well. Utilizing the mixed-ligand approach, homochiral neutral layers should be assembled easily from a chiral dicarboxylate and various achiral N-donor ligands in the mild conditions. L-Glutamic acid is a natural amino acid with two carboxylic acid groups and one amine. Every functional group can coordinate to metals, and the amine can react with remote acid to form pyroglutamic acid, causing the final metal−organic frameworks assembled from L-glutamic acid very unpredictable.13 To make the coordination mode simple and avoid the side reaction, an efficient method is to protect the free amine group. N-Carbamylglutamic acid (NCGA), formed by adding a carbamyl group to the amine of glutamic acid, is a good example. It is worth mentioning that NCG (N-carbamylglutamate) is a metabolically stable analogue of N-acetylglutamate, which activates carbamylphosphate synthase-1 Received: June 4, 2014 Revised: October 13, 2014 Published: October 27, 2014 6230

dx.doi.org/10.1021/cg501398c | Cryst. Growth Des. 2014, 14, 6230−6238

Crystal Growth & Design

Article

(CPS-1, a key enzyme in arginine synthesis in enterocytes).14 NCG can be used as a good chiral ligand candidate; however, it has not been used in the field of coordination chemistry. When NCG reacts with metals, it can function as a simple chiral dicarboxylate linker. In addition, the carbamido group in NCG may contribute to the hydrogen bonding interactions to some degree. Therefore, NCG was chosen as the chiral ligand, cooperating with various flexible bis(pyridine) ligands (Scheme 1), to self-assemble with Zn(II) ions. We report herein the

with a Vario MICRO CHNOS Elemental Analyzer. The infrared spectra with KBr pellets were recorded in the range of 4000−400 cm−1 on a PerkinElmer Spectrum One FT-IR Spectrometer. The solid-state luminescence emission/excitation spectra were recorded on an FLS920 fluorescence spectrophotometer. Powder X-ray diffraction (PXRD) data were collected on a DMAX-2500 diffractometer with Cu-Kα. Circular dichroism (CD) spectra were conducted on a Jasco J810 spectrodichrometer. SHG measurements on powder samples of CPs 1−4 and urea were carried out by the Kurtz and Perry method using a Nd:YAG laser (1064 nm). Synthesis of [Zn2L(NCG)2(H2O)2·4H2O]n (1). A solution of L (48 mg, 0.2 mmol) and Zn(NO3)2·6H2O (60 mg, 0.2 mmol) in H2O/ EtOH (6 mL/3 mL) was stirred for 10 min. Then, a solution of NCGA (38 mg, 0.2 mmol) and 1 N NaOH (0.4 mL) in H2O (6 mL) was added. After 20 min, the mixture was filtered and the filtrate was slowly evaporated for 3 days to yield colorless crystals. Yield: 71 mg, 83%. Elemental analysis calcd (%) for C26H46N8O16Zn2: C 36.42, H 5.41, N 13.07; found: C 36.35, H 5.38, N 12.86. IR (solid KBr pellet, ν/cm−1) 3418 (S), 3278 (vw), 2948 (w), 2345 (vw), 1653 (s), 1625 (m), 1561 (s), 1436 (m), 1412 (s), 1388 (m), 1354 (w), 1327 (w), 1305 (w), 1284 (w), 1229 (m), 1199 (w), 1150 (w), 1034 (m), 982 (m), 927 (w), 884(), 738 (w), 677 (w), 616 (w), 491 (w). Synthesis of [Zn(bpe)(NCG)·3H2O]n (2). Method 1: A solution of bpe (74 mg, 0.4 mmol) and Zn(NO3)2·6H2O (60 mg, 0.2 mmol) in H2O/EtOH/DMF (6 mL/3 mL/3 mL) was stirred for 10 min. Then, a solution of NCGA (38 mg, 0.2 mmol) and 1 N NaOH (0.4 mL) in H2O (6 mL) was added. After 20 min, the mixture was filtered and the filtrate was slowly evaporated for 5 days to yield colorless crystals. Method 2: A solution of bpe (37 mg, 0.2 mmol) and Zn(NO3)2·6H2O (60 mg, 0.2 mmol) in H2O/EtOH/DMF (6 mL/3 mL/3 mL) was stirred for 10 min. Then, a solution of NCGA (38 mg, 0.2 mmol) and 1 N NaOH (0.45 mL) in H2O (6 mL) was added. After 20 min, the mixture was filtered and the filtrate was slowly evaporated for 5 days to yield colorless crystals. Yield: 41 mg, 42%. Elemental analysis calcd (%) for C18H26N4O8Zn: C 43.96, H 5.33, N 11.39; found: C 43.98, H 5.27, N 11.41. IR (solid KBr pellet, ν/cm−1) 3413 (S), 2962 (w), 2341 (vw), 1658 (s), 1620 (s), 1577 (s), 1527 (vw), 1434 (m), 1392 (m), 1354 (vw), 1303 (w), 1269 (vw), 1223 (w), 1184 (w), 1139 (w), 1067(m), 1025 (m), 835 (m), 704 (vw), 548 (m).

Scheme 1. Structures of the Chiral N-Carbamylglutamate Ligand and Flexible Bis(pyridine) Ligands

syntheses, structural characterization, chirality, photoluminescence, and nonlinear optical properties of four homochiral layered metal−organic frameworks.

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EXPERIMENTAL SECTION

Materials and Methods. NCGA, bpe, bpp, Zn(NO3)2·6H2O, and solvents were purchased commercially and used directly. The hydrogenated Schiff base L was synthesized according to the literature procedure.15 All manipulations were carried out under aerobic and mild conditions. Elemental analyses (C, H, and N) were performed

Table 1. Crystal Data and Refinement Results for Complexes 1−4 empirical formula M crystal system space group flack parameter a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V/Å3 Z Dc/g cm−3 μ/mm−1 θrange (deg) h, k, l, ranges

F(000) R1,a wR2b [I > 2σ(I)] GOF on F2 a

1

2

3

4

C52H92N16O32Zn4 857.45 monoclinic P21 −0.02(1) 9.38(3) 16.81(4) 11.48(3) 90.00 100.426(16) 90.00 1781(9) 2 1.599 1.429 2.1736−27.4855 −12 to 12, −21 to 21, −14 to 11 892 0.0434, 0.1379 1.049

C36H52N8O16Zn2 491.80 monoclinic P21 0.01(1) 7.584(3) 15.434(6) 9.559(4) 90.00 98.887(6) 90.00 1105.5(8) 2 1.477 1.162 3.0215−27.5322 −9 to 9, −19 to 20, −11 to 12 512 0.0418, 0.0933 0.999

C72H88N18O30Zn4Na2 498.27 monoclinic P21 0.01(1) 9.483(3) 17.454(6) 13.003(4) 90.00 99.291(5) 90.00 2124.0(12) 4 1.558 1.219 2.4691−27.4895 −12 to 12, −22 to 22, −16 to 16 1028 0.0478, 0.1089 0.998

C38H58N8O17Zn2 514.83 triclinic P1 0.01(1) 9.756(2) 10.346(3) 11.822(3) 86.418(5) 86.785(6) 87.184(5) 1187.9(5) 2 1.439 1.086 3.1781−27.4972 −12 to 12, −13 to 13, −15 to 15 538 0.0397, 0.1079 1.047

R = ∑(∥Fo| − |Fc∥)/∑|Fo|. bRw = {∑w[(Fo2 − Fc2)2]/∑w[(Fo2)2]}1/2. 6231



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Figure 1. (a) Coordination configurations of Zn(II) atom in crystal 1 (hydrogen atoms and disordered oxygen atoms have been omitted for clarity). Symmetry code: a) x + 1, y, z; b) −x + 2, y − 1/2, −z + 2. (b) 2D layer of 1 and the zigzag chain composed of L and Zn(II). (c) 2D layer of 1 viewed down the a axis and four helical chains of NCG. (d) 3D structure of 1 viewed down the a axis (red dashed lines represent the hydrogen bonds). Synthesis of [Zn(bpe)(NCG)(NaNO3)0.5H2O]n (3). A solution of bpe (55 mg, 0.3 mmol) and Zn(NO3)2·6H2O (90 mg, 0.3 mmol) in H2O/EtOH/DMF (6 mL/3 mL/3 mL) was stirred for 10 min. Then, a solution of NCGA (57 mg, 0.3 mmol) and 1 N NaOH (0.6 mL) in H2O (6 mL) was added. After 20 min, the mixture was filtered and the filtrate was slowly evaporated for 5 days to yield colorless crystals. Yield: 78 mg, 52%. Elemental analysis calcd (%) for C18H22N4.5O7.5Na0.5Zn: C 43.39, H 4.45, N 12.65; found: C 43.36, H 4.69, N 12.75. IR (solid KBr pellet, ν/cm−1) 3405 (s), 3342 (s), 3228 (vw), 3097 (vw), 2920 (m), 2869 (w), 2489 (vw), 1957 (w), 1683 (m), 1611 (s), 1561 (s), 1505 (vw), 1434 (m), 1400 (s), 1320 (m), 1294 (m), 1274 (vw), 1227 (m), 1202 (vw), 1147 (w), 1139 (w), 1067(m), 1033 (m), 835 (m), 628 (vw), 539 (m). Synthesis of [Zn(bpp)(NCG)·3.5H2O]n (4). A solution of bpp (59 mg, 0.3 mmol) and Zn(NO3)2·6H2O (90 mg, 0.3 mmol) in H2O/ EtOH/DMF (6 mL/3 mL/3 mL) was stirred for 10 min. Then, a solution of NCGA (57 mg, 0.3 mmol) and 1 N NaOH (0.6 mL) in H2O (6 mL) was added. After 20 min, the mixture was filtered and the filtrate was slowly evaporated for 5 days to yield colorless crystals. Yield: 89 mg, 58%. Elemental analysis calcd (%) for C19H29N4O8.5Zn: C 44.32, H 5.68, N 10.88; found: C 44.28, H 5.63, N 10.91. IR (solid KBr pellet, ν/cm−1) 3439 (s), 3375 (s), 3059 (vw), 2949 (w), 2932 (w), 286 (w), 2485 (vw), 2346 (vw), 1949 (vw), 1645 (m), 1611 (s), 1620 (s), 1573 (m), 1535 (w), 1430 (m), 1404 (s), 1350 (w), 1315 (w), 1282 (vw), 1231 (m), 1185 (w), 1134 (vw), 1071 (m), 1029 (m), 936 (w), 818 (w), 716 (w), 619 (w), 581 (vw), 518 (w). Single-Crystal Structure Determination. The X-ray singlecrystal structure analyses for the compounds were performed on a Rigaku SCXmini diffractometer (Mo-Kα radiation, λ = 0.71073 Å, graphite monochromator) at 293(2) K. Empirical absorption corrections were applied to the data using the SADABS program.16 The structures were solved by the direct method and refined by the full-matrix least-squares on F2 using the SHELXL-97 program.17 All of the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms attached to carbon were located at their ideal

positions. Crystal data for 1−4 are presented in Table 1. Selected bond lengths and angles of 1−4 are listed in Tables S1.1−S1.4 of the Supporting Information.

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RESULTS AND DISCUSSION Description of Crystal Structures. Crystal Structure of [Zn2L(NCG)2(H2O)2·4H2O]n (1). Complex 1 crystallizes in the chiral monoclinic space group P21 with flack parameters of −0.02(1); its asymmetric unit consists of two Zn(II), one L, two NCG, two coordinated water molecules, and four free water molecules. Zn1 is four-coordinated by two N (N3 and N4b) atoms of two different L ligands and two O (O1 and O5a) atoms of carboxylate groups from two different NCG, forming a distorted tetrahedral coordination geometry. Different from Zn1, Zn2 adopts a distorted trigonal-bipyramidal coordination geometry via coordinating with two N (N1 and N2) atoms from L, three O atoms from one water (O11), and two carboxylate groups from two different NCG (O6 and O10a) (Figure 1a). In complex 1, the Zn−N and Zn−O bond lengths range from 2.037(5) to 2.177(5) Å, and from 1.973(7) to 2.223(5) Å, respectively. The Zn···Zn distances spanned by half an L or one NCG are 7.09 (1) and 9.38(3) Å, respectively. The N−Zn−N angles are 83.2(2)−101.3(2)°, while the N− Zn−O and O−Zn−O angles range from 89.3(2)° to 172.2(1)°, and from 87.8(2)° to 101.4(3)°, respectively. The ligand L adopts a “linear” shape18,12 and functions as a bridging linker, connecting different Zn(II) to form big zigzag chains (Figure 1b). Different chains are linked together by the NCG ligand through sharing the Zn(II) ions with the L ligand, resulting in a novel 44-sql 2D wavelike layer. The layer contains four different helixes (A, B, C, and D) composed of Zn(II) and NCG (Figure 1c). Helixes A and C are left-handed, whereas B and D are right-handed along the a axis. The helical pitches are all 9.38 Å 6232



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Figure 2. (a) Coordination configurations of Zn(II) atom in crystal 2 (hydrogen atoms and disordered oxygen atoms have been omitted for clarity). Symmetry code: a) x, y, z + 1; b) −x − 3, y − 1/2, −z + 4. (b) 2D layer of 2 and the right-handed helical chain composed of bpe and Zn(II). (c) 2D layer of 2 viewed down the b axis and two left-handed helical chains of NCG. (d) 3D structure of 2 viewed down the c axis (red dashed lines represent the hydrogen bonds and π−π stacking interactions).

according to the length of the a axis. Adjacent layers are stacked in an AA type arrangement, with free water intercalated between the layers. There exist various hydrogen bonds (N− H···O and O−H···O), and the distances of O···O and O···N vary from 2.82 to 3.16 Å (Figure S1, Supporting Information). As a result, the 3D architecture of complex 1 was constructed, as shown in Figure 1d. Crystal Structure of [Zn(bpe)(NCG)·3H2O]n (2). Similar to complex 1, complex 2 crystallizes in the chiral monoclinic space group P21 as well. The flack parameters is 0.01(1), and its asymmetric unit consists of one Zn(II), one bpe, one NCG, and three free water molecules (Figure 2a). All the Zn(II) ions in complex 2 are four-coordinated by two nitrogen atoms of two different bpe ligands and two oxygen atoms from two separate NCG ligands to afford the distorted tetrahedral geometry ZnN2O2. The bond lengths of Zn−O are 1.947(2) and 1.982(2) Å, and the two Zn−N distances are 2.050(3) and 2.067(3) Å, respectively. The Zn···Zn distances spanned by one bpe or one NCG are 13.23 (1) and 9.55(1) Å, respectively. The bond angles around each Zn(II) ion vary from 93.9(1)° to 126.4(1)°. The flexible bpe ligands are linked by Zn(II) ions to form a 1D right-handed helical chain with the pitch of 15.43(1) Å along the b axis (Figure 2b). The NCG ligand displays a bi(monodentate) coordinated mode, which connected those different chains via Zn−O coordination bonds to form a 2D wavelike layer framework of 44-sql topology. Meanwhile, two similar left-handed helixes composed of NCG and Zn(II) were observed (Figure 2c). The helical pitches are 9.55(1) Å along the c axis. Adjacent layers are packed in an AA type arrangement through aromatic π−π stacking interactions between pyridyl rings of bpe ligands (face-to-face distance is about 3.66 Å) and intermolecular hydrogen bonding

interactions (the lengths of hydrogen bonds vary from 2.52 to 2.99 Å) (Figure 2d and Figure S2, Supporting Information). Crystal Structure of [Zn(bpe)(NCG)(NaNO3)0.5H2O]n (3). A single-crystal X-ray diffraction analysis reveals that complex 3 has an interesting 2D double-layered structure. Complex 3 crystallizes in the chiral monoclinic space group P21 as well, with flack parameters of 0.01(1). The asymmetric unit consists of one Zn(II), one bpe, one NCG, half an NaNO3, and one coordinated water molecule (Figure 3a). Both Zn1 and Zn2 adopt a distorted tetrahedral coordination geometry, which is completed by two N (N1, N4b and N2, N3, respectively) atoms from two bpe ligands and two O (O1, O3a and O7, O9a, respectively) atoms from two NCG ligands. The bonds lengths are Zn−N = 2.036(4)−2.067(4) Å and Zn−O = 1.925(3)− 1.992(3) Å. The Zn···Zn distances spanned by one bpe or one NCG are 13.19 (1) and 9.48(1) Å, respectively. The coordination angles around the Zn(II) ion vary from 95.0(2)° to 132.7(2)°. Na is five-coordinated by two O (O 2f and O 4f) of two carboxyl from one NCG, one O (O10) of the carbamido group from another NCG, one O (O12) of NO3, and one O (O14) of water. The bonds lengths are Na−O = 2.323(5)−2.540(6) Å. The bpe ligands are linked by Zn(II) ions to form a 1D left-handed helical chain with the pitch of 17.45(1) Å along the b axis (Figure 3b). The NCG ligands display a bi(monodentate) and a tetradentate coordinated mode, which connected those different chains to form a 44-sql 2D wavelike layer framework. As illustrated in Figure 3c, two similar right-handed helixes composed of NCG and Zn(II) are formed. The helical pitches are 9.48(1) Å along the a axis. Adjacent layers are connected by the Na−O bonds [2.323(5) Å], assisted with π−π stacking interactions (3.90 Å) and intramolecular hydrogen bonds (vary from 2.24 to 2.89 Å) 6233



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Figure 3. (a) Coordination configurations of Zn(II) atom in crystal 3 (hydrogen atoms and disordered oxygen atoms have been omitted for clarity). Symmetry code: a) x − 1, y, z; b) x, y − 1, z; c) x, y + 1, z; d) −x − 1, y − 1/2, −z − 2; e) x + 1, y, z; f) −x − 1, y + 1/2, −z − 2. (b) 2D layer of 3 and the left-handed helical chain composed of bpe and Zn(II). (c) 2D layer of 3 viewed down the a axis and two right-handed helical chains of NCG. (d) 2D double-layered structure of 3 viewed down the a axis (red dashed lines represent the hydrogen bonds and π−π stacking interactions). (e) 3D structure of 3 viewed down the a axis (red dashed lines represent the intermolecular hydrogen bonds).

Crystal Structure of [Zn(bpp)(NCG)·3.5H2O]n (4). Complex 4 crystallizes in the chiral triclinic space group P1 with flack parameters of 0.01(1). The asymmetric unit consists of one Zn(II), one bpp, one NCG, and three and a half uncoordinated water molecules (Figure 4a). The Zn(II) centers are fourcoordinated, showing a ZnN2O2 environment composed of two N atoms from two bpp ligands and two O from two NCG ligands. In complex 4, the Zn−N and Zn−O bond lengths

(Figure S3, Supporting Information), to form a novel 2D double-layered framework. The two layers are arranged in an AB type (Figure 3d). Neighboring 2D double-layers are further packed in an AA type arrangement through intermolecular hydrogen bonding interactions (Figure 3e). The lengths of hydrogen bonds are 2.13 Å (N7−H7C···O11), 2.21 Å (N8− H8b···O12), and 2.72 Å (C2−H2A···O5). 6234



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Figure 4. (a) Coordination configurations of Zn(II) atom in crystal 4 (hydrogen atoms and disordered oxygen atoms have been omitted for clarity). Symmetry code: a) x + 1, y, z; b) x, y, z − 1. (b) 2D layer of 4 and the helical chains of bpe and NCG. (c) Adjacent layers of 4 viewed down the a axis and c axis (red dashed lines represent the hydrogen bonds). (d) 3D structure of 4 viewed down the a axis (red dashed lines represent the hydrogen bonds).

range from 2.038(4) to 2.060(4) Å, and from 1.935(4) to 1.980(3) Å, respectively. The N−Zn−N and O−Zn−O angles are 105.9(2)° and 106.2 (1)°, 107.0(1)° and 111.6(1)°, respectively, while the N−Zn−O angles range from 97.0(1)° to 119.1(1)°, indicative of a distorted tetrahedral coordination geometry around the Zn(II) ion. The Zn···Zn distances spanned by the bpp ligand and NCG ligand are 11.82(1) and 9.75(1) Å, respectively. The bpp ligands are linked by Zn(II) ions to form an uncommon helical chain {Zn-bpp-Zn-bpp-Zn}n with two flexures in a single strand,19 which is different from meso-helical.20 The pitches are 11.82 (1) Å along the c axis. The NCG ligand displays a bi(monodentate) coordinated mode, which is connected by Zn(II) ions to form a similar helical chain {Zn-NCG-Zn-NCG-Zn}n with the pitch of 9.75 (1) Å along the a axis as well (Figure 4b). A 44-sql 2D layer framework is constructed by those different helical chains through sharing the Zn(II). The adjacent layers are stacked offset with each other in an ABAB fashion, via intermolecular hydrogen bonding interaction between the neighboring and the alternate layers (Figure 4c,d). The lengths of hydrogen bonds vary from 2.09 to 2.96 Å (Figure S4, Supporting Information). Synthesis of the Homochiral CPs and Comparison of the Structures. Complexes 1−4 were prepared by selfassembly of a chiral dicarboxylate and various achiral flexible bis(pyridine) ligands under mild conditions. It is noteworthy that different crystals (complexes 2 and 3) were obtained,

although the same N-donor ligand bpe was applied. In complex 3, NaNO3 takes part in coordination, which does not appear in complex 2. The difference of 2 and 3 may come from the different pH values during the preparation, as complex 2 was synthesized with an excess of bpe or NaOH. Nonetheless, this phenomenon was not observed when ligand L and bpp were used. The NCG ligand is a semirigid dicarboxylate ligand possessing two flexible −CH2− groups; thus, it may display various conformations. However, it maintains the same conformation in complexes 1−4, except for the different orientation of the carbamido group. In complexes 1, 2, and 4, NCG acts as a simple bidentate bridge to link Zn(II) ions via two monodentate carboxyl groups, whereas, in complex 3, one NCG acts as a simple bidentate bridge and another NCG is a tetradentate ligand to bridge one Zn(II) ion and one Na (I) via the syn-anti-COO− mode. As a result, four helixes in complex 1, two left-handed helixes in 2, two right-handed helixes in 3, and an unusual helical chain in 4, composed of NCG and Zn(II), are formed. On the contrary, achiral ligand L, bpe, and bpp adopt diverse conformations with respect to the relative orientations of the −CH2− groups (Figure 5). The dihedral angles of two pyridyl rings are 22.1° for 1, 1.8° for 2, 44.2° for 3, and 64.5° for 4, respectively. At last, a 1D big zigzag chain in 1, a right-handed helical chain in 2, a left-handed helical chain 6235



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As shown in Figure 6, the CD spectrum of complex 1 exhibits a positive Cotton effect with peaks around 263 and 302 nm, and

Figure 5. Conformations of bis(pyridine) ligands in CPs 1 (a), 2 (b), 3 (c), and 4 (d).

in 3, and a helical chain with two flexures in 4, are constructed by those bis(pyridine) ligands and Zn(II), respectively. The two types of chains are waved together by sharing the Zn(II) ions, to form four interesting 2D wavelike networks. The adjacent layers are further arranged by the hydrogen bonding and aromatic π−π stacking interactions. The carbamido groups played a very import role as the donors and acceptors of the hydrogen bonds. In complex 1, neighboring layers are stacked in an AA type arrangement through the hydrogen bonding interaction. The N−H and O of the carbamido group function as the donors and acceptors, respectively. In complex 2, different layers are also packed in an AA type arrangement through aromatic π−π stacking interactions between pyridyl rings of bpe ligands and intermolecular hydrogen bonding interactions. The carbamido groups act as the acceptors of the hydrogen bonds (aromatic H···O/N). Different from complex 2, NaNO3 linked together two layers of complex 3 through the coordination bonds to form a new 2D double-layered framework. The new 2D doublelayers are further packed in an AA mode arrangement through intermolecular hydrogen bonding and aromatic π−π stacking interactions. The NO3− and adjacent urea are fixed together by strong intermolecular hydrogen bonding (N−H···O). In complex 4, adjacent layers are stacked offset with each other in an ABAB fashion. The carbamido groups of each layer can form various hydrogen bonds with three neighboring layers. Thermal Stabilities and Powder X-ray Diffraction of the Complexes. In order to confirm the phase purity of these complexes, PXRD patterns were recorded for complexes 1−4. The experimental PXRD patterns of complexes 1−4 are in good agreement with the simulated ones calculated from the single-crystal diffraction data, indicating that they are in a pure phase (Figures S5−S8, Supporting Information). To examine the thermal stability of the four complexes, thermogravimetric (TG) measurements were performed in a N2 atmosphere. TG curves of 1−4 are shown in Figure S9 in the Supporting Information. Complexes 1−4 remained stable up to ∼210, ∼185, ∼190, and ∼ 215 °C, respectively. The TGA curve of complex 1 shows that the loss of H2O molecules occurs between 56 and 147 °C (found 12.50%, calc. 12.61%). In complex 2, a weight loss of 9.95% (calc. 10.99%) was found with respect to the loss of free water molecules, while a weight loss of 3.75% (calc. 3.62%) and 12.11% (calc. 12.25%) were observed in complexes 3 and 4, respectively. Circular Dichroism. Considering that the homochiral complexes 1−4 crystallize in the chiral space groups, their solid-state circular dichroism spectra (CD) were investigated.

Figure 6. Solid-state CD spectra of samples 1 (a), 2 (b), 3 (c), and 4 (d).

a negative Cotton effect around 248 and 279 nm (Figure 6a); Similarly, complex 2 shows positive CD signals at 264 and 300 nm, and negative CD signals at 250 and 273 nm (Figure 6b). Complex 3 exhibits a positive Cotton effect with peaks at 230 and 300 nm, and a negative Cotton effect around 282 nm (Figure 6c). Complex 4 exhibits a positive Cotton effect with peaks at 232 and 299 nm, and a negative Cotton effect around 283 nm (Figure 6d). The results confirm their homochiral nature, which is in good accord with the structures obtained by single-crystal X-ray diffraction. Nonlinear Optical Properties. Complexes 1−4 are chiral coordination polymers; therefore, their second-order NLO properties were studied by using a Nd:YAG laser (1064 nm). Second harmonic generation (SHG) on microcrystalline samples of the four compounds was carried out by using the Kurtz−Perry method21 at room temperature, in order to evaluate their potential application as second-order NLO materials. The results show that CPs 1−4 display SHG efficiencies, which are approximately 0.7, 0.9, 0.4, and 0.6 times that of urea powder in the particle size of 150−212 μm (Figure 7 and Figure S10, Supporting Information), indicating the obvious effects of the chirality on the SHG efficiency in CPs 1− 4.

Figure 7. Comparison of the measured SHG efficiencies of complexes 1−4 with that of urea. 6236



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Luminescence Property. Considering the excellent photoluminescent properties of Zn(II) complexes, the solid-state luminescent properties of CPs 1−4 together with free ligands L, bpe, and bpp were investigated at room temperature. From Figure 8, it can be seen that broad emission bands are centered

Article

ASSOCIATED CONTENT

S Supporting Information *

Additional tables, PXRD, TGA, and additional figures. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+) 86-591-83719238 (X.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support of the Major State Basic Research Development Program of China (973 Program, 2012CB821702 and 2014CB845603), the National Science Foundation of China (21173223, 21203194, and 21233009), and Fujian Province (2013J05039).

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Figure 8. (a) Solid-state emission spectra of L, bpp, and complexes 1 and 4. (b) Solid-state emission spectra of bpe and complexes 2 and 3.

at 450 nm for CPs 1 (λex = 367 nm) and 4 (λex = 365 nm), and 473 and 581 nm for CPs 2 and 3 (λex = 356 nm), respectively. Complexes 1 and 4 have similar emission spectra, consistent with the free ligand L and bpp (Figure 8a), while complexes 2, 3, and bpe have similar emission spectra (Figure 8b). Therefore, their luminescent mechanism possibly originates from the ligand-centered emission. The observation above suggests that they may serve as candidates for hybrid inorganic−organic photoactive materials.

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REFERENCES

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CONCLUSION

In summary, the derivative of L-glutamic acid, N-carbamylglutamate (NCG), was used first as a chiral ligand. The transformation of the free amino to the carbamido group simplifies the possible coordination mode of L-glutamic acid, making their MOFs much more predictable. Through the cooperation of NCG and achiral flexible bis(pyridine) ligands, four novel layered neutral homochiral coordination polymers containing different helical chains were prepared. In construction of these Zn(II) complexes, NCG acted as a simple chiral dicarboxylate linker. The carbamido groups of NCG made a significant contribution to the diverse hydrogen bonds. This mixed-chiral dicarboxylate and achiral N-donor ligand strategy provides an efficient way to prepare homochiral neutral layered materials. Although their second-order NLO and luminescence properties are moderate, they may be enhanced greatly if the carbamyl group is replaced by other functional groups. Other bis(pyridine) ligands and derivatives of Lglutamic acid and the applications of these homochiral layered CPs are being investigated in our group. 6237



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