Syntheses, Structures, and Second-Order Nonlinear Optical Properties

Article pubs.acs.org/crystal

Syntheses, Structures, and Second-Order Nonlinear Optical Properties of Chiral Zinc(II) Coordination Polymers Based on (R)‑4-(4(1-Carboxyethoxy)phenoxy)-3-fluorobenzoic Acid and Effect of the Second Ligand with Imidazole Group Han-Tao Ye,†,‡ Chang-Yue Ren,†,‡ Guang-Feng Hou,† Ying-Hui Yu,‡ Xin Xu,†,‡ Jin-Sheng Gao,*,†,‡ Peng-Fei Yan,‡ and Seik-Weng Ng§,∥ †

Engineering Research Center of Pesticide of Heilongjiang University, and ‡School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080, China § Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia ∥ Chemistry Department Faculty of Science, King Abdulaziz University, Jeddah 80203, Saudi Arabia S Supporting Information *

ABSTRACT: To investigate the construction of novel chiral coordination polymers and the effect of N-donor ligands with different lengths as the second ligand, five new chiral zinc(II) coordination polymers based on (R)-4-(4-(1-carboxyethoxy)phenoxy)-3fluorobenzoic acid (H2cpfa), namely, [Zn(cpfa)] (1), [Zn2(cpfa)2(biim-4)2]·2H2O (2), [Zn2(cpfa)2(bix)2]·2H2O (3), [Zn2(cpfa)2(bix)2]·CH3OH·H2O (4), and [Zn2(cpfa)2(bimb)2]·CH3OH·H2O (5) (where biim-4 = 1,1′-(1,4-butanediyl)bis(imidazole), bix = 1,4-bis(imidazole-1-ylmethyl) benzene, bimb = 4,4′-bis((1H-imidazol-1-yl)methyl)biphenyl), have been synthesized under hydro(solvo)thermal conditions. Their structures are determined by single crystal X-ray diffraction and further characterized by infrared spectra (IR), elemental analyses, powder X-ray diffraction (PXRD), circular dichroism spectra (CD), and thermogravimetric (TG) analyses. Complex 1 features a threedimensional (3D) rhombic right-handed structure with a rare topology of 5,5T2. Complexes 2−5 involving N-donor ligands with different lengths display similar twodimensional (2D) sq1 networks. Complex 2 is a 2D 4-connected undulated layer with (H2O)2 dimers filling in the cavities. Similar 2D undulated layers in complex 3 are connected via O−H···O hydrogen-bonds between cpfa2− anions and lattice water molecules to generate a 3D supramolecular structure. In complex 4, 2D (4,4)-sq1 sheets with large cavities form a 2-fold interpenetrated sq1 network, with the two adjacent layers stacking in an ABAB fashion. Complex 5 exhibits a structure very similar to 4, except with double right-handed helical chains available. The structures of above chiral complexes based on H2cpfa could be affected by the combined N-donor ligands’ tuning ability to the coordination number of the Zn(II) center and the coordination configuration of the H2cpfa ligand. Furthermore, the second-order nonlinear optical properties of complexes 1−5 are also studied, and the second-harmonic generation efficiencies of the complexes could be tuned by the introduction of second N-donor ligands.

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INTRODUCTION

(MOFs) have attracted great attention due to their fascinating structural motifs and potential applications in both biological and material science fields.1−4 Among the numerous complexes, chiral coordination polymers (CCPs) evoked considerable interest recently as potent candidates for enantiomorph separation, asymmetric catalysis, nonlinear optics, etc.5−7 Up to now, CCPs can be obtained by two strategies. The first is the spontaneous resolution upon crystallization without any chiral sources, however, which is difficult to use in rational CCPs synthesis due to its unpredictability and uncertain mechanisms of the processes. Moreover, spontaneous resolution could result

In the past decade, the rational design and synthesis of coordination polymers (CPs) and metal−organic frameworks Scheme 1. Synthesis Route of H2cpfa

Received: February 7, 2014 Revised: May 6, 2014

© XXXX American Chemical Society

A

dx.doi.org/10.1021/cg500197v | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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Table 1. Crystal Data and Structure Refinement for Complexes 1−5 crystal parameters

1

2

3

4

5

empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalcd (Mg cm−3) μ (mm−1) collected/unique Rint GOF on F2 R (I > 2σ(I))

C16H11FO6Zn 383.62 orthorhombic P212121 4.9328(10) 10.137(2) 29.718(6) 90 90 90 14860.(5) 24 1.695 1.695 11240/2596 0.0675 1.137 R1 = 0.0989a wR2 = 0.2811b R1 = 0.1122a wR2 = 0.2981b

C52H52F2N8O14Zn2 1181.76 monoclinic P21 10.3254(3) 17.7175(6) 14.3986(4) 90 95.878(2) 90 2620.23(14) 2 1.498 0.997 17770/10137 0.0237 1.017 R1 = 0.0456a wR2 = 0.1006b R1 = 0.0604a wR2 = 0.1092b

C30H27FN4O7Zn 639.93 monoclinic P21 10.4408(8) 18.2171(12) 15.7140(11) 90 98.588(7) 90 2955.3(4) 4 1.438 0.890 10952/8712 0.0219 1.017 R1 = 0.0518a wR2 = 0.1227b R1 = 0.0805a wR2 = 0.1412b

C30.50H28FN4O7Zn 646.94 orthorhombic P212121 10.7914(3) 15.8162(4) 17.8982(6) 90 90 90 3054.85(15) 4 1.407 0.862 23626/5998 0.0369 1.043 R1= 0.0560a wR2 = 0.1445b R1 = 0.0776a wR2 = 0.1599b

C73H62F2N8O13Zn2 1428.05 orthorhombic P212121 13.068(3) 15.168(3) 17.457(4) 90 90 90 3460.0(12) 2 1.371 0.768 33242/7840 0.0525 1.049 R1 = 0.0497a wR2 = 0.1312b R1 = 0.0671a wR2 = 0.1420b

R (all data) a

R1 = (Σ∥Fo| − |Fc|)/Σ|Fo|. bwR2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2.

in racemism in most cases.8 The second is to introduce enantiopure chiral species into the synthetic process. CCPs could be obtained from chiral ligands or from achiral ligands under chiral influence. Adopting chiral ligands has been provn to be the most feasible and effective way for CCPs construction.9 Amino acids, tartaric acids, and their derivatives were reported a lot as chiral sources in chiral complexes synthesis.10 Compared with the other ligands, the carboxylic group could adopt various coordination modes to give high dimensional structures with good thermal stability. Inspired by the aforementioned considerations and the outstanding results made by predecessors, we synthesized a new chiral dicarboxylic acid (R)-4-(4-(1-carboxyethoxy)phenoxy)-3-fluorobenzoic acid (H2cpfa) (Scheme 1), in which two benzene rings linked to each other by a hetero-oxygen atom to give a V-shaped ligand, expected to form chiral frameworks with large cavities.11 The physical properties of CPs, especially those with d10 metal centers, have been widely investigated.12 As a typical d10 metal, zinc(II) was adopted most in previous studies, the CCPs based on which exhibiting distinctive photoluminescent and second-harmonic generation (SHG) properties. In this work, we first obtained a three-dimensional (3D) topological 5,5T2 structure with rhombic cavities based on H2cpfa and zinc(II). Then the second N-donor ligand with different lengths such as biim-4, bix, and bimb were employed respectively to enrich the versatility of the structures and gain a deeper insight into the influence of ligand length on the CCPs’ structures. In the presence or absence of different N-donor ligands, we synthesized five new CCPs based on the H2cpfa ligand under hydro(solvo)thermal conditions, [Zn(cpfa)] (1), [Zn2(cpfa)2(biim-4) 2 ]·2H 2 O (2), [Zn 2 (cpfa) 2 (bix) 2 ]·2H 2 O (3), [Zn 2 (cpfa) 2 (bix) 2 ]·CH 3 OH·H 2 O (4), and [Zn 2 (cpfa) 2 (bimb) 2]·CH3OH·H 2O (5) (where biim-4 = 1,1′-(1,4butanediyl)bis(imidazole), bix = 1,4-bis(imidazole-1-ylmethyl)benzene, bimb = 4,4′-bis((1H-imidazol-1-yl)methyl)biphenyl). Herein, we report their synthesis, crystal structures, infrared spectra (IR), powder X-ray diffraction (PXRD), thermal

stabilities, circular dichroism spectra (CD), and second-order nonlinear optical properties. Furthermore, the effects of Ndonor ligands, pH of the system, and the length of second ligand on the structures of the CCPs have been discussed in detail.

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EXPERIMENTAL SECTION

Materials and Measurements. The biim-4, bix, and bimb were prepared according to the literature methods.13 Other chemicals were of reagent grade and used as received without further purification. C, H, and N analyses were performed on a PerkinElmer 2400 elemental analyzer. Infrared spectra were recorded with a Spectrum one FT-IR spectrometer using KBr pellets in the range of 400−4000 cm−1. The powder X-ray diffractions (PXRD) data of the samples were collected on a Rigaku D/MAX-3B diffractometer using Cu−Kα radiation (λ = 1.5418 Å) and 2θ ranging from 5 to 50°. Thermogravimetric analyses were completed on a PerkinElmer STA 6000 thermal analyzer at a heating rate of 10 °C·min−1. The circular dichroism spectra (CD) of 1−5 were recorded at room temperature with a Jasco J-810(S) spectropolarimeter (KCl pellets). The second-order nonlinear optical intensities were estimated by measuring microcrystalline samples relative to urea by a Spectra Physics Quanta Ray Prolab 170 Nd:YAG laser using the first-harmonics output of 1064 nm with a pulse width of 10 ns and a repetition rate of 10 Hz.14 Synthesis of (R)-4-(4-(1-Carboxyethoxy)phenoxy)-3-fluorobenzoic acid (H2cpfa). Step 1: Anhydrous potassium carbonate (6.91 g, 50 mmol) was added to a DMF solution (100 mL) of (R)(+)-2-(4-hydroxyphenoxy)propionic acid (3.92 g, 20 mmol) and stirred for 3 h at 80 °C, to which 3,4-difluorobenzonitrile (2.79 g, 20 mmol) was then added and stirred at 100 °C for 10 h. The solvent was then removed from the mixture by evaporation, and 100 mL of water was added. The pH value was adjusted to 1−2 by cooled hydrochloric acid (1.0 mol/L) to get 2-(4-(4-cyano-2-fluorophenoxy)phenoxy)propanoic acid. 1H NMR (DMSO): 12.96 (s, 1H), 8.02 (dd, 1H), 7.64 (dd, 1H), 7.17−7.08 (m, 2H), 7.02−6.89 (m, 3H), 4.83 (q, 1H), 1.51 (d, 3H). Step 2: A mixture of 2-(4-(4-cyano-2-fluorophenoxy)phenoxy)propanoic acid (15 g, 50 mmol) and NaOH (10 g, 25 mmol) in water (100 mL) was stirred at 100 °C for 10 h. After the solution was cooled to room temperature, the pH of the mixture was adjusted to 4−5 by cooled hydrochloric acid (1.0 mol/L) to obtain a B



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white solid of H2cpfa after being dried in air. Anal. Calcd for C16H13FO6 (320.07): C, 60.00; H, 4.09%. Found: C, 60.29; H, 4.11%. IR (KBr, cm−1): 3425 (m), 2995 (w), 1726 (s), 1684 (w), 1592 (m), 1507 (s), 1280 (m), 1213 (s), 837 (m). 1H NMR (DMSO): 1.51 (3H, d), 4.83 (1H, d), 6.86 (1H, d), 6.94 (2H, d), 7.00 (2H, d), 7.83 (1H, d), 8.01 (1H, d), 13.08 (2H, s). UV−vis (MeOH, λmax): 227.3, 251.1 nm. Synthesis of [Zn(cpfa)] (1). A mixture of H2cpfa (0.064 g, 0.2 mmol), Zn(OAc)2·2H2O (0.044 g, 0.2 mmol), NaOH (0.016 g, 0.4 mmol), ethanol (4 mL), and H2O (4 mL) was stirred for 2 h in air and then transferred and sealed in a Teflon reactor (15 mL), which was heated at 120 °C for 3 days and then cooled to room temperature at a rate of 10 °C·h−1. Colorless crystals of 1 were obtained in 55% yield (based on Zn(OAc)2·2H2O). Anal. Calcd for C16H11FO6Zn (383.62): C, 50.09; H, 2.89%. Found: C, 50.07; H, 2.91%. IR (KBr, cm−1): 2991 (w), 1680 (s), 1588 (m), 1504 (s), 1209 (s), 1135 (m), 1052 (m), 939 (w), 841 (m). Synthesis of [Zn2(cpfa)2(biim-4)2]·2H2O (2). A mixture of H2cpfa (0.064 g, 0.2 mmol), Zn(OAc)2·2H2O (0.044 g, 0.2 mmol), biim-4 (0.038 g, 0.2 mmol), H2O (6 mL), and ethanol (1 mL) was stirred for 2 h in air, transferred in a Teflon reactor (15 mL), and heated at 120 °C for 3 days, and then cooled to room temperature at a rate of 10 °C·h−1. Colorless crystals were obtained in 70% yield (based on Zn(OAc)2·2H2O). Anal. Calcd for C56H42F2N4O14Zn2 (1181.76): C, 56.86; N, 4.74; H, 3.55%. Found: C, 56.60; N, 4.77; H, 3.60%. IR (KBr, cm−1): 3124 (m), 3055 (w), 2977 (w), 2932 (w), 2932 (w), 2896 (w), 1634 (s), 1559 (m), 1457 (m), 1429 (m), 1288 (m), 1032 (w), 902 (w), 808 (m), 781 (m). Synthesis of [Zn2(cpfa)2(bix)2]·2H2O (3). The preparation of 3 was similar to that of 2 except that bix (0.048 g, 0.2 mmol) was used instead of biim-4. Colorless crystals of 3 were obtained in 56% yield (based on Zn(OAc)2·2H2O). Anal. Calcd for C56H42F2N4O14Zn2 (1279.86): C, 52.51; N, 4.38 H; 3.28%. Found: C, 52.48; N, 4.36; H, 3.30%. IR (KBr, cm−1): 3065 (w), 3031(w), 2982(w), 2935(w), 1662(w), 1447 (m), 1312 (w), 1272 (s), 1237 (s), 1134 (w), 1047 (w), 890 (vw), 831 (w), 787 (m). Synthesis of [Zn2(cpfa)2(bix)2]·CH3OH·H2O (4). The preparation of 4 was similar to that of 3 except that NaOH (0.016 g, 0.4 mmol) was used to adjust pH value. Colorless crystals were obtained in 56% yield (based on Zn(OAc)2·2H2O). Anal. Calcd for C30.50H28FN4O7Zn (639.93): C, 57.19; N, 8.75; H, 4.38%. Found: C, 57.32; N, 8.87; H, 4.33%. IR (KBr, cm−1): 3065 (w), 2976 (w), 2932 (w), 1653 (m), 1428 (m), 1289 (m), 1238 (s), 1134 (w), 1047 (w), 901 (w), 814 (w). Synthesis of [Zn2(cpfa)2(bimb)2]·CH3OH·H2O (5). The preparation of 5 was similar to that of 2 except that bimb (0.063 g, 0.2 mmol) was used instead of biim-4. Colorless crystals of 5 were obtained in 56% yield (based on Zn(OAc) 2 ·2H 2 O). Anal. Calcd for C56H42F2N4O14Zn2 (701.00): C, 62.11; N, 7.99; H, 4.17%. Found: C, 62.08; N, 8.06; H, 4.12%. IR (KBr, cm−1): 3456 (w), 3117 (w), 2974 (w), 1645 (m), 1499 (s), 1431 (m), 1205 (s), 1090 (s), 952 (w), 845 (w). X-ray Crystallography. Single-crystal X-ray diffraction data for complexes 1−5 were collected on a Rigaku R-AXIS RAPID imaging plate diffractometer with graphite-monochromated Mo Kα (λ = 0.71073 Å) at 291 K. Empirical absorption corrections based on equivalent reflections were applied. The structures of 1−5 were solved by direct methods and refined by full-matrix least-squares methods on F2 using SHELXS-97 crystallographic software package.15 The crystal parameters, data collection, and refinement results for 1−5 are summarized in Table 1. Selected bond lengths and angles of 1−5 are listed in Table S1, and the H-bond lengths and angles are shown in Table S2 (see the Supporting Information).

Figure 1. (a) Stick-ellipsoid representation of the asymmetric unit of complex 1 with thermal ellipsoids at 50% probability; the H atom was omitted for clarity. (b) The Zn−O chain with the short distance of Zn···Zn in CPs of complex 1. (c) 3D rhombic framework of complex 1. (d) Rare 5,5T2 topology structure of complex 1 with a point symbol of (43·65·82)(46·64). Symmetric codes: I: −x + 1/2, −y + 2, z + 1/2; II: x − 1/2, −y + 5/2, −z + 2; III: x − 1, y, z; IV: −x + 1, y + 1/2, −z + 3/2.

Zn and O atoms range from 1.844(8) to 2.240(7) Å). The geometry around the Zn(II) center can be best described as a trigonal biyramid. Each Zn(II) atom connects five carboxylic acid molecules, three of which are almost in the same plane, and the other two are perpendicular to this plane with an angle of 81.7(6)°, thus forming a wavelike chain as shown in Figure 1b. In this wavelike chain, the distance of two neighboring Zn(II) ions is 2.801(2) Å, which is relatively small compared with normal metal−metal distances in most coordination polymers. In 1, the two carboxylate groups of cpfa2− anion take trans configuration and exhibit μ2-η1:η1 and μ3-η1:η2 coordination fashions, with a dihedral angle between two benzene rings of 77.3(4)° (Supporting Information, Scheme S1 mode I). In this mode, the V-shaped cpfa2− anions connect Zn(II) ions into a 3D rhombic framework (Figure 1c), and with R-handed helical built by [Zn-(cpca)]4 (Figure S1, Supporting Information). If the cpfa2− anions and Zn(II) ions are considered as five connected nodes, the framework of 1 can be simplified as a 5connected topological 5,5T2 structure with a point symbol of (43·65·82)(46·64), which is rarely reported16 (Figure 1d). Crystal Structure of Complex 2. The biim-4 molecule is introduced as the second ligand to obtain complex 2. As shown in Figure 2a, complex 2 crystallizes in the monoclinic system

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RESULTS AND DISCUSSION Crystal Structure of Complex 1. The single-crystal X-ray crystallographic analysis reveals that complex 1 is in the orthorhombic system with space group in P212121. As shown in Figure 1a, the Zn(II) ion is five-coordinated by five carboxylate oxygen atoms from five different cpfa2− anions (the distances of C



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Figure 2. (a) ORTEP representation of the asymmetric unit of complex 2 (thermal ellipsoids are drawn at the 50% probability level; the H atom and latter water molecule omitted for clarity). (b) 1D Zig-zag chain made by Zn-cpfa in complex 2. (c) Right hand helical chains constructed by Zn(biim-4). (d) sq1 Network formed by biim-4 (blue) connecting Zn-cpfa chains (red) in complex 2. Symmetric codes: I: x, y, z + 1; II: −x + 2, y − 1/ 2, −z + 1; III: −x + 2, y + 1/2, −z + 1.

chains into a wavelike structure along the ab plane (Figure S2b). These hydrogen bonds are in the same direction as that of biim-4 connecting the Zn-cpfa chain, so the space dimension is not increased. Crystal Structure of Complex 3. When the flexible biim-4 ligand in complex 2 was replaced by the semirigid bix ligand, complex 3 with a similar structure was obtained as shown in Figure 3a. The asymmetric unit of 3 contains two crystallographically independent Zn2+ inons, two cpfa2− anions, two bix ligands, and two lattice water molecules. Similar to that in complex 2, the V-shaped cpfa2− anions in complex 3 show cis configuration, and the dihedral angles between two benzene rings are 83.8(3) and 87.8(3) o respectively. The cpfa2− anions connect Zn1 atoms or Zn2 atoms along [0 1 0] directions into 1D zigzag chains respectively (∠Zn−Zn−Zn = 155.9(1)o), with a distance of Zn···Zn 9.313(1) Å. The semirigid μ2-bix ligands connect the Zn1 ions with Zn2 ions into zigzag chains along the [1 2 2] directions with Zn1···Zn2 distances of 13.3179(12) and 14.9829(13) Å, respectively, and the ∠Zn−Zn−Zn is 76.2(5)o (Figure 3b). Interestingly, in complex 2, there are unusual meso-helices built upon Zn-cpfa-Zn-bix chains along [1 0 1] (Figure 3b).9c Furthermore, distorted rectangular (H2O)2(cpfa)2 rings with R44(32) and R44(50) are generated by the hydrogen bonds between two lattice water molecules and cpfa2− anions, eventually obtaining a 3D structure (Figure S3a, Supporting Information). If (H2O)2(cpfa)2 rings and Zn ions are regarded as a 4-connect point, we can simplify the supramolecular

with space group in P21. The asymmetric unit contains two Zn(II) ions, two cpfa2− anions, two biim-4 ligands, and two lattice water molecules. The two Zn(II) ions have a same coordination environment, which can be best described as a tetrahedron defined by two carboxylate oxygen atoms from two cpfa2− anions (the distances between Zn and O range from 1.938(3) to 1.967(3) Å) and two nitrogen atoms from two biim-4 ligands (the distances between Zn and N range from 2.001(3) to 2.029(4) Å). In complex 2, the two carboxylate groups of the cpfa2− anions take cis configuration, adopting μ1-η0:η1 coordination fashion (shown in Scheme S1 mode II). The dihedral angles between two benzene rings are 85.9(2) and 87.5(2) ° respectively. The cpfa2− anions thus connect Zn(II) atoms into a 1D zigzag chain along the [0 0 1] direction as shown in Figure 2b. The distances of Zn···Zn are 7.216(1) and 7.629(1) Å respectively, and ∠Zn−Zn−Zn is 151.8(8)°. Furthermore, the neighboring chains are connected by biim-4 ligands to form a 2D wavelike network. In the Zn-(biim-4) chain, the distance of Zn···Zn is 12.855(1) and 14.139(1) Å, and ∠Zn−Zn−Zn is 81.8(4)o. The flexible Zn-(biim-4) chains show an interesting right-handed helix due to the induction of a chiral cpfa2− anion with a helical distance of 17.718(1) Å. If we consider the Zn2+ions as four-connected nodes, cpfa2− and biim-4 as linkers, the 2D wavelike network can be simplified a (4,4)-sq1 network. In addition, the hydrogen bonds among the water molecules caused the formation of (H2O)2 dimers in complex 2 (Figure S2a, Supporting Information), which interact with O5, O8, O12 of carboxylate ligands to link the one-dimensional (1D) zigzag D



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Figure 3. (a) ORTEP representation of the asymmetric unit of complex 3 (thermal ellipsoids are drawn at the 50% probability level; the H atom and latter water molecule are omitted for clarity). (b) The Zn-cpfa-Zn-bix chain forming meso-helicals along [1 0 1]. (c) 2D (4,4)-sq1 network made by Zn-cpfa chain (red) and Zn-bix chain (blue). Symmetric codes: I: −x + 1, y + 1/2, −z + 1; II: x − 1, y, z − 1.

in Scheme S1 mode III), the dihedral angle between two benzene rings being 88.4(2)o. The cpfa2− anions connect the Zn(II) ions into right-handed helical chains along [1 0 0] directions. Induced by the chiral cpfa2− anions, the bimb ligands also formed right-handed helical chains along the [1 0 0] directions with the pitch of 13.068(3) Å (Figure 5B). The two chains entangle with each other forming a double-helical structure, eventually generating a 2D sheet with a cavity of 17.611 × 15.432 Å2. Such a large cavity caused the occurrence of 2-fold interpenetration and formed double-helicals (Figure 5C). Effects of the Length of N-Donor Ligands. In our studies, we selected five N-donor ligands with different lengths, 4,4′-bipyridine (4,4′-bipy), 1,1′-(1,2-ethyl)bis(imidazole) (biim-2), biim-4, bix, and bimb, to investigate the effect of the mixed ligand length on the self-assembly structures of the complexes. As shown in Table 2, the ligand lengths are described by the distance between Ndonor···Ndonor in a complex, and those of 4,4′-bipy and biim-2 are from the literature.17 As we all know, the carboxylic ligand can adopt various coordination modes, specifically for the ligands with two or more carboxylic groups. However, from the structure descriptions above, we can see that the cpfa2− anions show only two different coordination modes: connect five (complex 1) and connect two (complex 2−5) metals. In 1, one carboxylate bridges the metals in a μ2-η1:η1 mode, while another carboxylate connects the metals in a μ3-η1:η2 mode. In complexes 2−5, all the carboxylate groups possesses the same

architecture as a new topological structure with a point symbol of (62·84) (64·82)2 (Figure S3b). Crystal Structure of Complex 4. Complex 4 was obtained under the same procedures as complex 3, except that NaOH was used to adjust the pH value of the reaction system. As shown in Figure 4a, complex 4 crystallizes in the orthorhombic system with space group in P212121. The asymmetric unit contains one Zn(II) ions, one cpfa2− anion, one bix ligand, one lattice CH3OH molecule, and one lattice water molecule. The central Zn(II) ion in complex 4 is coordinated with two carboxylate oxygen atoms from two cpfa2− anions with Zn---O distances of 1.939(3) and 1.974(4) Å, respectively, to form a left-handed helical chain along the [0 1 0] direction with the pitch of 15.816(1) Å, which is further connected by biim-4 ligand to generate 2D sq1 sheets with cavities of 14.379 × 12.593 Å2 (Figure 4b). The cpfa2− anions show the cis form, and the dihedral angle between two benzene rings is 70.6(3)o. The large cavities provide an appropriate environment for interpenetration occurrence, leading to a 2-fold interpenetrated (4,4)-sq1 network. Furthermore, the two adjacent layers were stacked in an ABAB fashion along the [0 0 1] direction (Figure 4c). Crystal Structure of Complex 5. As a ligand longer than biim-4 and bix, bimb is adopted as N-donor mixed ligand to obtain complex 5. As shown in Figure 5A, complex 5 shows structural characteristics very similar to 4. Nevertheless, there are some differences between the two compounds. The cpfa2− anions in complex 5 take a different trans configuration (shown E



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Figure 4. (a) ORTEP representation of the asymmetric unit of complex 4 (thermal ellipsoids are drawn at the 50% probability level, the H atom and latter CH3OH and water molecule omitted for clarity). (b) 2D 44-sq1 Network (right) made by Zn-cpfa helical (left, red) and Zn-bix chains (middle, blue). (c) The cavity of complex 4 (left), 2-fold interpenetration in complex 4 (right). Symmetric codes: I: −x + 2, y − 1/2, −z + 1/2; II: −x, y + 1/ 2, −z + 1/2.

μ1-η0:η1 coordination mode. Compared with 1, the introduction of mixed N-donor ligand diminishes the coordination number of the Zn(II) center from five to four; however, it seems that the length of mixed N-donor ligand had no influence on the coordination mode of cpfa2− anions. Complex 1 is constructed in the absence of N-donor ligands, and it shows a 3D 5,5T2 topological framework with rhombic cavities. When 4,4-bipy and biim-2 whose Ndonor···Ndonor distance smaller than 8.0 Å were adopted as the second ligand, no products were obtained, although various key factors, such as temperature, solvent, pH value, reactant concentration and ratio, etc., were adjusted and tried. When biim-4, bix, and bimb with longer ligand lengths (from 8.832 to 14.780 Å) were introduced, complexes 2−5 were obtained. The cpfa2− anions in 2−5 first connect the Zn(II) ions into zigzag chains, which are further linked by the mixed N-donor ligand to form final structures with different cavities. In complex 2, a 2D sq1 network is obtained when adopting flexible biim-4 as the mixed ligand, which is induced by chiral cpfa2− ions to form right-handed helical chains. On the basis of semirigid bix ligand, complex 3 exhibits a very similar sql undulated layer structure to 2, in which the bix ligands connect Zn ions to give a zigzag chain instead of right-handed helical

chain in 2 probably due to the larger rigidity of bix than biim-4. Moreover, when a longer ligand bimb instead of bix was introduced, a similar sq1 network was obtained, although the bimb ligand showing right-handed chains was influenced by chiral cpfa2− anions. We also investigated the influence of pH value on the selfassembly process. Complexes 3 and 4 are crystallized in a different space group and show a different crystal system, although they were actually synthesized with the same ligands except that different pH values were employed. Instead of an undulated layer structure exhibited in complex 3, complex 4 shows a 2-fold kite-shaped sq1 network stacked in an ABAB fashion along the c-axis. However, no new structures were achieved in the reaction systems of complexes 1, 2, and 5 although under a different pH environment. In the above five compounds, the H2cpfa ligands in 1 and 5 took a trans configuration, and those in 2−4 took a cis configuration. As shown in Table 2, the distances of Odonor··· Odonor of the cpfa2− anions in 1 and 5 (from 11.951 to 12.418 Å) are remarkably longer than those in 2−4 (from 7.021 to 10.806 Å). It is proposed that when the second ligand with longer length is involved, the H2cpfa ligands are favorable to F



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Figure 5. (A) ORTEP representation of the asymmetric unit of complex 5 (thermal ellipsoids are drawn at the 50% probability level, the H atom and latter CH3OH and water molecule omitted for clarity). (B) 2D (4,4)-sq1 network made by Zn-cpfa right-hand helical (left) and Zn-bimb right-hand helical (right). (C) 2-Fold interpenetration in complex 5 and double helical in 5 (right). Symmetric codes: I: x − 1/2, −y + 1/2, −z; II: x + 1/2, −y + 5/2, −z.

temperature range 50−700 °C, as shown in Figure S6, Supporting Information. For complex 1, the weight loss begins with decomposition starting at 200 °C and ending at 450 °C. The remaining weight of 22.82% corresponds to the percentage (21.21%) of ZnO. Complex 2 has three steps of weight losses. The first weight loss of 3.56% in the range of 120−300 °C is consistent with the removal of two coordination water molecules (calcd. 3.05%). The second step of 320−530 °C can be attributed to the release of the biim-4 ligand (found 37.66%; calcd. 35.74%). Finally, starting at 550 ending over 615 °C, the weight loss can be attributed to the release of the cpfa2−. The remaining weight is assigned to ZnO (found 14.36%; calcd. 13.77%). As same as that in complex 2, complex 3 also contains three weight-loss processes. In complex 3, the weight loss in the range of 110− 300 °C corresponds to the removal of two lattice water molecules (found 2.73%; calcd. 2.81%). At 320−530 °C, the weight loss can be ascribed to the decomposition of the cpfa2− anion (found 50.78%; calcd. 52.67%). The final weight loss of bix molecule happens in the range of 550−620 °C (found 86.22%; calcd. 87.28%). In complex 4, the first weight loss from 60−200 °C corresponds to the removal of methanol molecule (found 4.2%; calcd. 5.14%). At 320−530 °C, the weight loss can be ascribed to the decomposition of the cpfa2− anion (found 49.53%; calcd. 52.10%). Finally, at 550−600 °C, the weight loss can be attributed to the loss of bix molecule (found 88.05%; calcd. 87.42%). Complex 5 loses its first weight in the range of 100−300 °C corresponding to the removal of the half of methanol (found 4.2%; calcd. 5.14%). The anhydrous composition begins to decompose at 330 °C and ends at 500

Table 2. Effect of Mixed N-Donor Ligand with Different Lengths complexes

mixed N-donor ligand

Ndonor···Ndonor

1

2 3 4 5

Odonor···Odonor 11.951, 12.496, 12.557, 13.271

4,4-bipy biim-2 biim-4 bix bix bimb

7.131 7.854 8.832, 9.126 9.320, 10.418 10.953 14.780

7.021, 7.505 10.457, 11.116 10.806 12.418

take trans configuration, by which the H2cpfa molecule is less distorted to match the longer length of second ligand. XRD Patterns and Thermal Analyses. Powder X-ray diffraction (PXRD) and thermogravimetric analysis were carried out to investigate the phase purity and stability of these complexes. The peak positions of the experimental and simulated PXRD patterns are in good agreement with each other, indicating that the crystal structures are truly representative of the bulk crystal products. The differences in intensity may be owing to the preferred orientation of the crystal samples (as shown in Figure S5, Supporting Information). In order to characterize the compounds more fully in terms of thermal stability, their thermal behaviors were studied by TGA. The experiments were performed on samples consisting of numerous single crystals of 1−5 under air atmosphere with a heating rate of 10 °C/min. The TGA measurements of complexes 1−5 were carried out in atmosphere in the G



Article

Figure 6. Schematic representation of the influence of the length of N-donor ligands on the configuration of cpfa2− anions.

°C; the remaining weight can be ascribed to the formation of ZnO (found 88.31%; calcd. 88.39%). The Circular Dichroism Spectra (CD) Analysis. The combination of single-crystal X-ray structure determinations and circular dichroism spectra (CD) is the most common way of gaining as much evidence as possible for the homochirality or enantioenrichment of the bulk material. The bulk crystals are chosen to determine the solid-state CD spectra in a KCl matrix. As seen in Figure 7, all these five CCPs exhibit a strong Cotton effect, proving that they are all single structures. CD spectrum in complexes 1, 2, and 4 exhibit negative CD signals, and those in complexes 3 and 5 show positive CD signals, revealing the presence of enantiomers in complexes 1−5. Second-Harmonic Generation Efficiency. Considering that the homochiral Zn(II) complexes 1−5 are colorless, transparent crystals crystallized in the noncentrosymmetric

Figure 7. Solid-state CD spectra of bulk samples of 1−5.

Table 3. SHG-Active Zn-Based CCPs Built from Chiral Ligand CCPs

chemical formula

dimension

space group

SHG

1 2 3 4 5 6 7 8 9 10 11 12

Zn(cpfa) [Zn2(cpfa)2(biim-4)2]·2H2O [Zn2(cpfa)2(pbib)2]·2H2O [Zn2(cpfa)2(pbib)2]·CH3OH·H2O [Zn2(cpfa)2(bimb)2]·CH3OH·H2O Zn(lac)(inic) Zn(SPA)(H2O)2 Zn(SCMC)(H2O) [Zn(bddp)·(H2O)4]2·H2O (TBC-N4)2Zn(N3)4(H2O) (TBC-N4)2Zn(N3)4(D2O) Zn(tpcc)2(H2O)2

3D 2D 2D 2D 2D 2D 1D 2D 1D 0D 0D 2D

P212121 P21 P21 P212121 P212121 P212121 P212121 P21 P21 P21 P21 C2

0.3× urea 0.6× urea 0.4× urea 0.4× urea 0.3× urea 1.2× urea 1× urea 0.05× urea 0.6× urea 20× KDP 25× KDP 2.8× KDP

H

ref this this this this this 19 20 21 22 23 23 24

paper paper paper paper paper



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chiral space groups, their nonlinear optical properties were investigated. According to the method proposed by Kurtz and Perry, the second-harmonic generation (SHG) efficiency can be measured by using a powder technique. The SHG efficiency is compared with a standard material, such as α-quartz, KH2PO4 (KDP), or urea. KDP has an efficiency of 16× α-quartz, whereas urea has an efficiency of 400× α-quartz. The technologically important LiNbO3 is 600× α-quartz. In this work, the SHG efficiencies of 1−5 and the chiral ligand H2cpfa are measured by using pure microcrystalline samples, and the result is in contrast with urea. The SHG is measured using a pulsed laser at a wavelength of 1064 nm. Upon irradiation, the microcrystalline samples emit a green light at a wavelength of 532 nm. The SHG optical experimental results indicate that both the chiral ligand H2cpfa and the complexes 1−5 are all SHG-active. As listed in Table 3, complexes 1−5 show medium SHG activities compared with several previously reported Zn-based CCPs built by chiral ligands. The SHG efficiency of the chiral ligand H2cpfa is about 0.4 times as much as that of urea; however, after coordination with zinc, complex 1 exhibits a SHG efficiency approximately 0.3 times as much as that of urea. Furthermore, with the introduction of the second N-containing ligands with different lengths, complexes 2−5 with homochiral 2D (4,4)-sq1 networks show SHG efficiencies of 0.6, 0.4, 0.4, and 0.3 times respectively, as much as that of urea. Although the coordination of the chiral ligand H2cpfa with zinc slightly decreased its SHG efficiency, as in complex 1, the introduction of a second ligand with certain length could significantly inhance the SHG efficiencies, as in complexes 2. On the basis of the above results, it seems that the second ligand with shorter length could enhance the SHG efficiency of the complex, which may due to the different effective free volumes in the compounds. The total effective free volumes of complexes 2− 4 calculated by PLATON analysis are 2.4%, 4.2%, 8.5%, and 12.6% of the crystal volume, respectively.18 A ligand with short length may be favorable to form complexes with small effective free volumes, which tend to stack closer to each other. The interactions between the molecules may be strengthened, thus resulting in the enhancement of the SHG efficiencies to a certain extent.

Article

ASSOCIATED CONTENT

S Supporting Information *

Tables S1−S2 and Figures S1−S5. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallography data have been deposited to the Cambridge Crystallography Data Centre with deposition numbers CCDC nos. 978004−978007 and 978003 for complexes 1−5.

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AUTHOR INFORMATION

Corresponding Author

*(J.-S.G.) Tel.: +86 451 86609001; fax: +86 451 86609151; email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (Nos. 21371052 and 21272061), Scientific Research Fund of Heilongjiang Provincial Education Department (12531512).

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REFERENCES

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CONCLUSIONS The purpose of this work is to rationalize the effect of different combined N-donor ligand lengths on chiral coordination polymer construction based on chiral H2cpfa ligands. The introduction of second N-donor ligands has an effect on the coordination modes of H2cpfa and the coordination number of the Zn(II) center. The different lengths of N-donor ligand have no influence on the coordination mode of H2cpfa molecule, based on which four chiral coordination polymers 2−5 are achieved with similar structures. However, different lengths of the second N-donor ligand could affect the configuration of the H2cpfa ligand. It is proposed that when the N-donor ligand with a longer length is favorable for the H2cpfa ligand to take a trans configuration. Moreover, the investigation also indicates that complexes 1−5 have different medium second-order nonlinear optical activities, and the introduction of different ligand could tune the SHG efficiencies of the complexes to a certain extent. I



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Syntheses, Structures, and Second-Order Nonlinear Optical Properties

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