A Series of Homochiral Helical Metal–Organic Frameworks Based on

Department of Chemistry, Zunyi Normal College, Zunyi, 563002, P. R. China. §School of Chemical Science and Technology, Yunnan University, Kunming, 65...
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A Series of Homochiral Helical Metal−Organic Frameworks Based on Proline Derivatives Zhong-Xuan Xu,†,‡ Yu-Lu Ma,§ Yu Xiao,† Lei Zhang,*,† and Jian Zhang*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ Department of Chemistry, Zunyi Normal College, Zunyi, 563002, P. R. China § School of Chemical Science and Technology, Yunnan University, Kunming, 650091, P. R. China S Supporting Information *

ABSTRACT: Eight novel three-dimensional homochiral helical metal−organic frameworks (HHMOFs), namely, [Cd3((S)-PIA)2(PPA)(H2O)2]n (1-L), [Cd3((R)-PIA)2(PPA)(H2O)2]n (1-D), [Cd4((R)-PIA)3(DPP)4(H2O)4]n (2-D), [Cd1.5((S)-PIA)(4,4′DIB)2]n (3-L), [Cd1.5((S)-PIA)(DPEE)1.5]n (4-L), [Cd1.5((R)-PIA)(DPEE)1.5 ] n (4-D), [Cd 1.5 ((S)-PIA)(DPEA)1.5 ] n (5-L), and [Cd1.5((S)-PIA)(DPEA)1.5]n (5-D) (H3PIA = 5-(2-carboxypyrrolidine-1-carbonyl) isophthalic acid) have been synthesized using proline-derived ligands ((S)-H3PIA and (R)-H3PIA). Crystallographic analysis indicates that all the complexes contain homochiral left- and/or right-handed helical chains, which are constructed by PIA fragments and Cd(II) ions. Meanwhile, in these complexes nitrogen heterocycle auxiliary ligands play an important role in structural diversity. Some physical characteristics, such as thermal stabilities, solid-state circular dichroism, and photoluminescent properties, are also investigated. Our results highlight an effective method to apply proline ligands to construct interesting HHMOFs.

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INTRODUCTION In the past decade, homochiral metal−organic frameworks (HMOFs) have attracted more and more attention for their distinctive structures and diverse topologies as well as potential applications in enantioselective processes.1 The absolute helicity often goes with chirality in the structures of HMOFs, so the research on the relationship between chirality of molecular building blocks and helicity of polymeric structures will offer a possible way to understand the handedness of compounds.2 In fact, chirality and helicity are also universally bound to each other in nature, such as nucleic acid and proteins.3 Up to now, several synthetic strategies have been successfully applied in the construction of HMOFs, including use of enantiopure ligands, chiral induction, and spontaneous resolution.4 Among them, the application of enantiopure ligands should be the most straightforward and effective method to construct HMOFs.5 As a kind of inexpensive, nontoxic, and readily available nature chiral compounds, amino acids may be the ideal enantiopure linkers for the formation of HMOFs. However, the flexible nature of amino acids has limited their use in HMOFs.6 Recently, to overcome the shortcomings of amino acids, we have used proline and 1.3.5-BTC as starting materials to synthesize a pair of enantiopure ligands, namely, (S)-H3PIA and (R)-H3PIA (Scheme 1a).7 This pair of ligands has several advantages, as follows: (1) the proline unit provides the chiral © XXXX American Chemical Society

Scheme 1. Structures of the Selected Enantionpure Linkers (a) and Auxiliary N-Donor Ligands (b)

Received: September 20, 2015 Revised: October 31, 2015

A

DOI: 10.1021/acs.cgd.5b01359 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Crystal Data and Structure Refinement for Complexes 1−5 compound reference

1-L

1-D

2-D

3-L

chemical formula formula mass crystal system a/Å b/Å c/Å β/° unit cell volume/Å3 temperature/K space group no. of formula units per unit cell, Z radiation type absorption coefficient, μ/mm−1 reflections/unique Rint R1, wR2 (I > 2σ(I)) R1, wR2 (all data) goodness of fit on F2, Flack parameter compound reference

C16H14Cd1.50N2O9 546.89 monoclinic 22.9097(10) 6.8910(3) 14.4132(6) 106.620(4) 2180.38(15) 100.02(13) C2 4 CuKα 12.251 7204/3952 0.0324 0.0426/0.1122 0.0446/0.1144 1.008, −0.005(12) 4-D

C16H10Cd1.5N2O9 542.86 monoclinic 22.8156(11) 6.9955(4) 14.4043(7) 106.694(5) 2202.13(19) 100.12(10) C2 4 CuKα 12.130 3978/2844 0.0317 0.0422/0.1141 0.0458/0.1185 0.974, 0.006(14) 4-L

C80H77Cd3N10O15 1755.72 monoclinic 13.4617(3) 19.9956(3) 17.2734(3) 110.777(2) 4347.19(14) 99.99(18) P21 2 CuKα 6.350 17421/12350 0.0449 0.0470/0.1209 0.0493/0.1239 1.051, −0.023(6) 5-D

C64H48Cd3N10O15 1534.32 monoclinic 15.0810(5) 14.5380(4) 17.9350(5) 95.951(3) 3911.0(2) 100.00(16) P21 2 CuKα 6.984 15964/9506 0.0548 0.0554/0.1465 0.0594/0.1504 1.065, −0.011(9) 5-L

C32H24Cd1.50N4O7 745.15 orthorhombic 15.9125(3) 20.9486(4) 13.9023(2) 4634.26(14) 293(2) P21212 4 MoKα 0.730 58500/8159 0.0378 0.0373, 0.1245 0.0393, 0.1269 1.085, 0.02(3)

C32H25Cd1.50N4O7 746.16 orthorhombic 15.9133(8) 20.9595(9) 13.9143(6) 4640.9(4) 293(2) P21212 4 MoKα 0.729 13314/8415 0.0376 0.0454, 0.1157 0.0574, 0.1205 1.071, −0.11(3)

chemical formula formula mass crystal system a/Å b/Å c/Å unit cell volume/Å3 temperature/K space group no. of formula units per unit cell, Z radiation type absorption coefficient, μ/mm−1 reflections/unique Rint R1, wR2 (I > 2σ(I)) R1, wR2 (all data) goodness of fit on F2, Flack parameter

C32H23Cd1.50N4O7 744.14 orthorhombic 15.8454(12) 20.9947(15) 13.9809(7) 4651.0(5) 293(2) P21212 4 MoKα 0.727 11924/7487 0.0275 0.0451, 0.1267 0.0542, 0.1327 0.883, 0.02(4)

C32H24Cd1.50N4O7 745.15 orthorhombic 15.7264(6) 20.8072(11) 13.9774(6) 4573.7(4) 293.05(10) P21212 4 CuKα 5.944 13282/7576 0.0483 0.0627, 0.1689 0.0717, 0.1781 1.056, −0.020(14)

three-dimensional (3D) open structures with unique helical chains. In this work, we report their synthesis, crystal structures, thermal stabilities, CD spectra, and luminescent properties.

source for construction of HMOFs; (2) the rigid isophthalate unit can efficiently form interesting structures; and (3) the multiple coordination modes of three carboxyl moieties have great superiority in the synthesis of fascinating coordination polymers. For these advantages, the (S)-H3PIA and (R)-H3PIA have provided an available approach to design and synthesize HMOFs.8 Furthermore, the reasonable selection of some Ncontaining auxiliary ligands is also a feasible method in the manipulation of the expected structures of HOMFs.9 With the above consideration, we select five ancillary nitrogen colinkers with different lengths and rigidness/ flexibility, piperazine (PPA), 1,3-di(pyridine-4-yl) propane (DPP), 4,4′-di(1H-inidazole-1-yl)-1,1′-biphenyl (4,4′-DIB), (E)-1,2-di(pyridin-4-yl)ethane (DPEE), and 1,2-di(pyridin-4yl)ethane (DPEA) to assist (S)-H3PIA or (R)-H3PIA to build HMOFs (Scheme 1b). Herein, eight Cd-based HMOFs have been gained by mixed ligands strategy, namely, [Cd3((S)PIA)2(PPA)(H2O)2]n (1-L), [Cd3((R)-PIA)2(PPA)(H2O)2]n (1-D), [Cd4((R)-PIA)3(DPP)4(H2O)4]n (2-D), [Cd1.5((S)PIA)(4,4′-DIB)2]n (3-L), [Cd1.5((S)-PIA)(DPEE)1.5]n (4-L), [Cd 1.5 ((R)-PIA)(DPEE) 1.5 ] n (4-D), [Cd 1.5 ((S)-PIA)(DPEA)1.5]n (5-L), and [Cd1.5((S)-PIA)(DPEA)1.5]n (5-D) (Table 1). It is important to note that all compounds exhibit

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EXPERIMENTAL SECTION

General Procedures. All reagents and solvents employed were purchased from Energy-Chemical and directly used without further purification. The IR spectra were measured as KBr pellets in the range of 400−4000 cm−1 on a Nicolet Magna 750 FT-IR spectrometer. Thermal stability studies were performed using a NETSCHZ STA449C thermoanalyzer in a temperature range between 30 and 800 °C in N2 (20 mL/min flow) and a heating rate of 10 °C/min. To verify the phase purity of all complexes, powder X-ray diffraction (XRD) data were collected using a Miniflex(II) diffractometer with Cu−Kα radiation (λ = 1.54056 Å) in a range of 5.00−50.00°. The solid circular dichroism (CD) spectra were measured on a MOS-450 spectropolarimeter using KCl pellets at room temperature. Solid luminescence spectra data were collected on a Shimadzu RF-5301 spectrophotometer. Synthesis of [Cd3((S)-PIA)2(PPA0)(H2O)2]n (1-L). A mixture of (S)H3PIA (0.031 g, 0.1 mmol), PPA (0.017g, 0.2 mmol), Cd(NO3)2· 4H2O (0.061g, 0.20 mmol), distilled water (2 mL), and DEF (2 mL) was stirred in a screw-capped vial and then heated at 80 °C for 7 days. Colorless block-like crystals of 1-L were obtained in 70% yield based on (S)-H3PIA. Elemental analysis calcd (%) for 1-L: C 35.14, H 2.58, B

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N 5.12; found C 34.57, H 2.82, N 5.44. IR (solid KBr pellet, cm−1): 3334.0s, 3215.1s, 1613.5s, 1570.8s, 1443.4s, 1395.3s, 1085.5w, 1012.6w, 872.8w, 721.4m. Synthesis of [Cd3((R)-PIA)2(PPA)(H2O)2]n (1-D). Complex 1-D was synthesized in a similar way to that described for 1-L except using (R)H3PIA instead of (S)-H3PIA. Colorless block-like crystals of 1-D were obtained in 62% yield based on (R)-H3PIA. Elemental analysis calcd (%) for 1-D: C 35.14, H 2.58, N 5.12; found C 34.76, H 2.72, N 5.98. IR (solid KBr pellet, cm−1): 3421.6s, 3215.1s, 1613.5s,1577.0s, 1443.4s,1389.1s, 1370.5s, 1018.8w, 879.0w, 788.2w, 721.4m. Synthesis of [Cd3((R)-PIA)2(DPP)4(H2O)]n (2-D). (R)-H3PIA(31 mg, 0.1 mmol), DPP (30 mg, 0.15 mmol), Cd(NO3)2·4H2O (62 mg, 0.2 mmol), Na2CO3 (0.15 mmol, 16 mg), ethylene glycol (1 mL), and distilled water (3 mL) in a 23 mL Teflon cup, and then the mixture was stirred for 5 min. After the vessel was sealed and heated at 120 °C for 72 h, the autoclave was subsequently allowed to cool to room temperature. Colorless block-like crystals of 2-D were obtained in 45% yield based on (R)-H3PIA. Elemental analysis calcd (%) for 2-D: C 54.73, H 4.42, N 7.98; found C 52.98, H 5.01, N 7.11. IR (solid KBr pellet, cm−1): 3391.3m, 2930.2w, 1613.5s, 1564.5m, 1419.4m, 1365.0m, 1225.3w, 1012.6w, 788.2w, 733.8w, 508.7w. Synthesis of [Cd3((S)-PIA)(4,4′-DIB)2(H2O)]n (3-L). (S)-H3PIA(31 mg, 0.1 mmol), 4,4′-DIB (43 mg, 0.15 mmol), Cd(NO3)2·4H2O (62 mg, 0.2 mmol), Na2CO3 (0.15 mmol, 16 mg), ethanol (2 mL), and distilled water (2 mL) in 23 mL Teflon cup, and then the mixture was stirred for 5 min. After the vessel was sealed and heated at 120 °C for 72 h, the autoclave was subsequently allowed to cool to room temperature. Colorless block-like crystals of 3-L were obtained in 52% yield based on (S)-H3PIA. Elemental analysis calcd (%) for 3-L: C 50.10, H 3.15, N 9.13; found C 48.02, H 3.91, N 8.53. IR (solid KBr pellet, cm−1): 3425.4m, 3130.5m, 1613.5s, 1559.1s, 1510.2s, 1431.0m, 1370.5s, 1303.7m, 1261.8w, 1128.2w, 1061.5m, 958.2w, 818.5w, 7227.6w, 654.6w, 514.9w. Synthesis of [Cd1.5((S)-PIA)(DPEE)1.5]n (4-L). A mixture of (S)H3PIA (0.031g, 0.1 mmol), DPEE (0.036 g, 0.2 mmol), Cd(NO3)2· 4H2O (0.061g, 0.20 mmol), distilled water (2 mL), and DMF (2 mL) was stirred in a 20 mL screw-capped vial, and then heated at 100 °C for 3 days. Colorless block-like crystals of 4-L were obtained in 60% yield based on (S)-H3PIA. Elemental analysis calcd (%) for 4-L: C 51.65, H 3.12, N 7.53; found C 54.93, H 4.61, N 8.73. IR (solid KBr pellet, cm−1): 3425.6s, 1611.1s, 1572.6s, 1428.4m, 1389.9m, 1362.0m, 1014.0w, 543.8w. Synthesis of [Cd1.5((R)-PIA)(DPEE)1.5]n (4-D). Complex 4-D was synthesized in a similar way to that described for 4-L except using (R)H3PIA instead of (S)-H3PIA. Colorless block-like crystals of 4-D were obtained in 40% yield based on (R)-H3PIA. Elemental analysis calcd (%) for 4-D: C 51.65, H 3.12, N 7.53; found C 54.71, H 4.52, N 8.84. IR (solid KBr pellet, cm−1): 3425.6s, 1605.8s, 1577.9m,1428.4m, 1384.6m, 1362.0, 1014.0w, 538.5w. Synthesis of [Cd1.5((S)-PIA)(DPEA)1.5]n (5-L). Cd(NO3)2·4H2O (0.061g, 0.20 mmol), DPEA (0.036g, 0.2 mmol), (S)-H3PIA (0.031g, 0.1 mmol), distilled water (1 mL), and DEF (3 mL) were placed in a 20 mL screw-capped vial. The sample was heated at 80 °C for 3 days and then cooled to room temperature. Colorless block-like crystals of 5-L were obtained in 90% yield based on (R)-H3PIA. Elemental analysis calcd (%) for 5-L: C 51.51, H 3.38, N 7.51; found C 55.11, H 4.60, N 9.02. IR (solid KBr pellet, cm−1): 3334.0s, 3215.1s, 1613.5s, 1570.8s, 1443.4s, 1395.3s, 1085.5w, 1012.6w, 872.8w, 721.4m. Synthesis of [Cd1.5((R)-PIA)(DPEA)1.5]n (5-D). Complex 5-D was synthesized in a similar way to that described for 5-L except using (R)H3PIA instead of (S)-H3PIA. Colorless block-like crystals of 5-D were obtained in 85% yield based on (R)-H3PIA. Elemental analysis calcd (%) for 5-D: C 51.51, H 3.38, N 7.51; found C 54.46, H 4.50, N 8.98. IR (solid KBr pellet, cm−1): 3421.6s, 3215.1s, 1613.5s, 1577.0s, 1443.4s, 1389.1s, 1370.5s, 1018.8w, 879.0w, 788.2w, 721.4m. Single-Crystal X-ray Studies. Single crystal X-ray data on complexes 1−5 were collected at 100 K or room temperature on a SuperNova CCD diffractometer using graphite monochromated CuKα radiation (λ = 1.5418 Å). The structures were solved by direct methods and refined on F2 full-matrix least-squares using the

SHELXTL-97 program package.10 In complexes 1−5, disordered guest molecules cannot be further identified, so they were subtracted from the reflection data by the SQUEEZE method.11 Furthermore, as for disordered atoms, such as N3A, N3B, N2, C16A, C16B, and C15 in complex 1-D, N3, C15, and C16 in complex 1-L, O1W and C57 in complex 3-L, C30 and C31 in complex 4-D, C4, C24, and C30−33 in complex 5-L, and C4 and C27−34 in complex 5-D have been refined isotropically. The crystal and refinement data are summarized in Table 1.

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RESULTS AND DISCUSSION As a part of our ongoing research efforts, we have tried to prepare some HMOFs by self-assembly of enantiopure organic ligands with metal ions in the present of different N donor coligands. Herein, eight 3D HMOFs were successfully synthesized by hydro(solvo)thermal reactions. All HMOFs are very stable in air and insoluble in common organic solvents. Among them, 1-L and 1-D, 4-L and 4-D, and 5-L and 5-D are enantiomers. Although complex 4-D is formed by rigid coligand DPEE and complex 5-D is formed flexible DPEA, complexes 4D and 5-D are still isostructural for some disorded atoms of the two coligands. For these reasons, we mainly detail the structures of 1-L, 2-D, 3-L, and 5-L. Structures of [Cd3((S)-PIA)2(PPA)(H2O)2]n (1-L) and [Cd3((R)-PIA)2(PPA)(H2O)2]n (1-D). Single-crystal X-ray diffraction analysis has revealed that 1-L and 1-D crystallize in the monoclinic space group C2 with Flack parameters of −0.005(12) and 0.006(14), respectively. The asymmetric unit of 1-L contains one and a half Cd(II) ions, one (S)-PIA ligand, half a PPA ligand, and two coordinated water molecules. In compound 1-L, each (S)-PIA ligand connects five Cd(II) ions and acts as a κ7-linker (Figure 1). For the unique coordination

Figure 1. Coordination environment and trinuclear Cd units in enantiomers 1-L and 1-D. Symmetry codes: (a) 1 − x, y, 1 − z; (b) 0.5 + x, 0.5 + y, z; (c) 1.5 − x, −0.5 + y, 2 − z; (d) 0.5 + x, 0.5 + y, z.

modes of (S)-PIA3− ligand, an almost linear trinuclear Cd (Cd2, Cd1, Cd2) unit Cd3(CO2)6 is formed. In the Cd3(CO2)6 unit, Cd1 has an octahedral coordination geometry, which is coordinated by four carboxylate O atoms from four (S)-PIA ligands, two N atoms from two PPA ligands. Cd2 shows distorted pentagonal bipyramid geometry, which is filled by five O atoms from three carboxylate groups and two water molecules. C

DOI: 10.1021/acs.cgd.5b01359 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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chiral monoclinic P21 space group with a Flack parameter of −0.023(6). The asymmetric unit of 2-D is composed of three Cd centers (Cd1, Cd2, and Cd3), two (R)-PIA3− ligands, four DPP ligands, and a coordinated water molecule. In 2-D, two (R)-PIA3− ligands connect three Cd(II) ions and act as κ6linker and κ4-linker, respectively (Figure 3). Cd1 has distorted

The most outstanding structural feature of 1-L (1-D) is the presence of two types of single-stranded helical chains, regular helical chain, and irregular helical chain (Figure 2). In 1-L, the

Figure 3. Coordination environment in 2-D. Symmetry codes: (a) 1 − x, 0.5 + y, 1 − z; (b) x, y, −1 + z; (c) 1 − x, −0.5 + y, 1 − z; (d) −1 + x, y, z; (e) x, y, 1 + z. Figure 2. (a) The regular left-handed helical chain in 1-L; (b) the regular right-handed helical chain in 1-D; (c) the irregular righthanded helical chain in 1-L; (d) the irregular left-handed helical in 1D; (e) the 3D framework formed from the Cd atoms and the (S)-PIA ligands in 1-L; (f) the 3D framework formed from the Cd atoms and the (R)-PIA ligands in 1-D; (g) the 3D framework of 1-L; (h) the (3,8)-connected topological net of 1-L.

pentagonal bipyramid geometry, and it is coordinated by four O atoms from two carboxylate groups, two N atoms from two DPP ligands, and one water molecule. Cd2 and Cd3 all show octahedral coordination geometries, which are coordinated by three carboxylate O atoms from two carboxylate groups and three N atoms from three PPA ligands, respectively. Considering that the structure of 2-D contains two distinct ligands, (R)-PIA and DPP, we divide the whole framework into two parts to understand the complicated 3D structure better. Interestingly, if only the connectivity between (R)-PIA ligands and Cd(II) ions are considered, a 2D Cd-(R)-PIA layer is generated, which is composed of two type of helical chains (Figure 4a−d). As depicted in Figure 4a, the Cd2 and Cd3 ions are bridged by the (R)-PIA fragments to construct an infinite right-handed helical chain along the b-axis, while the Cd1 and Cd2 ions are bridged by the same fragment to form an infinite left-handed helical chain along the b-axis (Figure 4b). In these helical chains, the (R)-PIA fragment also consists of one proline unit and half an isophthalic acid unit, and the pitch lengths are identical to the length of the b-axis. From the viewpoint of structural topology, the Cd-(R)-PIA layers can be described as a 3-connected hcb net with point symbol of (63) (Figure 4d). Finally, each Cd-(R)-PIA layer is further filled and linked by DPP ligands resulting in a 3D nonclassical pillared-layer framework (Figure 4e,f). In the 3D framework, the Cd1, Cd3, and Cd2 act as 3-, 4-, and 5-connected nodes, respectively. Therefore, the framework of 2-D should be simplified as a (3,3,3,4,5)-connected net with point symbol of (5.6.73.9)(5.6.7)(5.72)(52.6.75.8.9)(6.72) (Figure 4g).12 Structure of [Cd3((S)-PIA)2(4,4′-DIB)2(H2O)]n (3-L). Complex 3-L is also a nonclassical pillared-layer coordination polymer and crystallizes in chiral P21 space group with a Flack

Cd2 ions are bridged by two carboxylate groups from (S)-PIA fragment to form an infinite regular left-handed helical chain running along the b-axis (Figure 2a), and the opposite phenomenon (regular right-handed helical chain) exists in 1D (Figure 2b). As secondary building unit, each regular helical chain is further linked to two adjacent irregular helical chains. Different from the previous regular helical, the irregular helical chain in 1-L is right-handed, consisting of Cd1 ions and two carboxylate groups of PIA fragment (Figure 2c). It is important to note that one proline unit and a half isophthalic acid unit of the PIA ligands are presented in the two helical chains, and the pitch lengths of theirs are identical to the length of b-axis. These left- and right-handed helical chains construct the final 3D PIA-Cd framework along the b-axis in a ratio of 1:1 (Figure 2e,f). Furthermore, as pillars, the PPA ligands provide additional bridges between Cd1 ions to further stabilize the 3D framework of 1-L (1-D) (Figure 2g). In the view of topology, the PIA ligands and Cd3(CO2)6 units act as the 3- and 8-connected nodes, respectively. So the whole framework of 1-L (1-D) can be topologically represented as a (3, 8)-connected net with the point symbol of (4.52)2 (42.510.612.7. 83) (Figure 2h).12 Structure of [Cd3((R)-PIA)2(DPP)4(H2O)]n (2-D). X-ray determination has confirmed that complex 2-D crystallizes in D

DOI: 10.1021/acs.cgd.5b01359 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 4. (a) The right-handed helical chain exists in Cd-(R)-PIA layer; (b) the left-handed helical chain exists in Cd-(R)-PIA layer; (c) the Cd-(R)PIA layer is composed of helical chains in 2-D; (d) the (3,3)-connected topological net of Cd-(R)-PIA layer; (e) the DPP ligand is simplified as a line; (f) the 3D framework of 2-D consisting of Cd-(R)-PIA layers and DPP ligands; (g) the (3,3,3,4,5)-connected topological net of 2-D.

parameter of −0.011(9). The asymmetric unit of 3-L contains three independent Cd(II) ions, two (S)-PIA ligands, two 4,4′DIB ligands, and a coordinated water molecule. As shown in Figure 5, the first (S)-PIA3− ligand acts as κ6-linker to connect five Cd(II) ions, and the second (S)-PIA3− ligand acts as κ7linker to connect six Cd(II) ions. In complex 3-L, Cd1 adopts a distorted pentagonal bipyramid geometry, which is coordinated by five O atoms from three carboxylate groups and two N atom from two 4,4′-DIB ligands. Cd2 is five coordinated by three O

atoms from three carboxylate groups and two N atoms from two 4,4′-DIB ligands. Cd3 is coordinated by six oxygen atoms from three carboxylate groups and a water molecule, showing distorted hepta-coordinate tetrahedron geometry. For the presence of two different ligands, 4,4′-DIB and (S)PIA, the framework of 3-L is also divided into two parts to simplify the complicated structure. In the same way, if only the connectivity between (S)-PIA ligands and Cd(II) ions are considered, a 2D Cd-(R)-PIA layer is generated, containing right-handed helical chains and carboxylate-bridged Cd(II) chains (Figure 6a−c). The Cd3 ions are bridged by the isophthalate unit of (S)-PIA ligand to form a right-handed helical chain along the b-axis. Furthermore, Cd3 is surrounded by three chelating carboxylate groups, and the charge at this site is thus [(−1) × 3]COO− + (+2)Cd = −1. The μ2-bridging carboxylate groups of (S)-PIA link the Cd1 ions and Cd2 ions to form an infinite [Cd2(COO)3]n− chain along the b-axis. Because of the different coordination environment of each Cd(II) center (e.g., for Cd1 site, [(−1 × 1/2) × 3 + (−1 × 1/ 4) × 2]COO− + (+2)Cd = 0),13 the charge distributions in the chain follows the sequences (0, 1, 0, 1···). According to the ratio of 1:1, the helical chain and [Cd2(COO)3]n− chain construct the neutral 2D Cd-(S)-PIA layer. These Cd-(S)-PIA layers are further piled together to yield a 3D framework via Cd1 and Cd2 centers coordinating with N atoms from 4,4′-DIB ligands (Figure 6d,e). Structure of [Cd 1.5 ((S)-PIA)(DPEA) 1.5 ] n (5-L) and [Cd1.5((R)-PIA)(DPEA)1.5]n (5-D). Complexes 5-L and 5-D crystallizes in the orthorhombic space group P21212 with Flack parameters of −0.11(3) and 0.02(3), respectively. The

Figure 5. Coordination environment in 3-L. Symmetry codes: (a) 1 − x, 0.5 + y, 1 − z; (b) x, y, 1 + z; (c) 1 + x, y, 1 + z; (d) −x, −0.5 + y, −z; (e) 1 − x, −0.5 + y, 1 − z. E

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Figure 7. Coordination environment and trinuclea Cd units in enantiomers 4-L and 4-D. Symmetry codes: (a) 1.5 − x, 0.5 + y, 2 − z; (b) −0.5 + x, 3.5 − y, 2 − z; (c) 1 − x, 3 − y, z; (d) 0.5 + x, 3.5 − y, 2 − z; (e) x, y, 1 + z; (f) 1.5 − x, 0.5 + y, 3 − z.

Figure 6. (a) The right-handed helical chain with charge distribution in 3-L; (b) the carboxylate-bridged Cd(II) chain with charge distribution; (c) the 2D Cd-(S)-PIA layer containing right-handed helical chains and carboxylate-bridged Cd(II) chains; (d) 4,4′-DIB ligand simplified as a line; (e) the pillared-layer 3D framework of 3-L.

asymmetric unit of 5-L and 5-D contains one and a half independent Cd(II) ions, one deprotonated PIA ligand, and one and a half DPEA ligands. In complexes 5-L, the (S)-PIA ligand acts as a κ6-linker and connects six Cd(II) ions. Furthermore, an trinuclear Cd (Cd1, Cd2, Cd1a) unit Cd3(CO2)6 exists in 5-L, which is surrounded by six carboxylate groups and four pyridine groups. In this Cd3(CO2)6 unit, Cd1 has a distorted trigonal bipyramid configuration, coordination with four carboxylate oxygen atoms from three (S)-PIA ligands and two N atoms from two DPEA ligands. Cd2 with an octahedral geometry is coordinated by four oxygen atoms from four carboxylate groups and two N atoms from two DPEA ligands (Figure 7). The structural feature in 5-L and 5-D is also the presence of helical chains. As described in Figure 8a, two carboxylate groups from an isophthalic acid unit of (R)-PIA ligand bridge the Cd1 and Cd2 to form an infinite large right-handed single-stranded helical chain along the a-axis, and the opposite left-handed helical chain is found in 5-D (Figure 8b). The above helical chains mainly contain two Cd1 ions, two Cd2 ions, and two isophthalic acid units per turn (Figure 8a,b). Besides, an infinite small left-handed (right-handed) helical chain also exists in 5-L (5-D) along the a-axis, consisting of Cd1, Cd2, and carboxylate groups of the PIA fragment (Figure 8c,d). However, it should be noted that PIA fragments are constructed by a proline unit and a half isophthalic acid unit. According to the ratio of 1:1, the two kinds of helical chains form a Cd-PIA layer with kgd net (Figure 8e,f and Figure S1). Finally, the wavy Cd-(S)-PIA layers are further linked by DPEA ligands, resulting in a 3D typical pillared-layer framework (Figure 8g,h). In the framework, the

Figure 8. (a) The large right-handed helical chain in 5-L; (b) the large left-handed helical chain in 5-D; (c) the small left-handed helical chain in 5-L; (d) the small right-handed helical chain in 5-D; (e) 2D Cd-(S)PIA layer of 5-L consisting of two types of helical chains; (f) 2D Cd(R)-PIA layer of 5-D comprised of two types of helical chains; (g) the DPEA ligand is simplified as a line; (h) the 3D framework of 5-L comprised of wavy Cd-(R)-PIA layers and DPEA ligands; (i) the tfz-d net in 5-L.

(S)-PIA ligands and the tetrameric units can be viewed as the 3and 8-connected nodes, respectively, and thus the whole framework of 5-L is topologically represented as a tfz-d net with a point symbol of (43)2(46.618.84) (Figure 8i).12 F

DOI: 10.1021/acs.cgd.5b01359 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 9. Solid-state CD spectra of 1-D and 1-L (a), 2-D and 3-L (b), 4-D and 4-L (c), and 5-D and 5-L (d).

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DISCUSSION According to the above structural description, different helical chains formed through Cd(II) centers and PIA3− fragments can be found in complexes 1−5, which further prove that the helicity of the intrinsically chiral net can be dictated by enantiopure framework building units. The (S)-PIA3− and (R)PIA3− ligands can adopt a variety of coordination modes, and they are good candidates for the construction of interesting HMOFs with helical chains. By comparison of these HMOFs mentioned above and those ones previously reported based on (S)-PIA3− and (R)-PIA3−,8a−c we maybe get an effective route to better understand the handedness of compounds through the research on the relationship between chirality of molecular building blocks and helicity of polymeric structures. Furthermore, besides the effects of the reaction solvents and temperatures, the significant structural differences among the complexes should be ascribed to the introduction of the auxiliary N-donor ligands. These ancillary coligands have a great influence on the structures of the above complexes for their different structures. PXRD Patterns and Thermal Analysis. The corresponding powder X-ray diffraction (PXRD) experiments have been carried out to check the phase purity of these complexes (Figures S10−S14). The peak positions of the simulated and experimental PXRD patterns are in good agreement with each other, indicating that the crystal structures are truly representative of the bulk crystal products. To investigate their thermal stabilities, the thermal behaviors of 1−5 were also tested in the temperature range of 30−800 °C under a nitrogen atmosphere (Figures S15−S19). For complexes 1-D and 1-L, gradual weight losses between 30 and 200 °C mainly correspond to the release of disorder solvent molecules, and the whole frameworks begin to decompose from 370 °C (Figure S15). Complex 2-D has a weight loss of 10.2% from 30 to 150 °C that is attributed to loss of guest water molecules. After 200 °C, the dehydrate state successively decomposes until 800 °C without stopping (Figure S16). Similarly, a weight loss of 10.8% is observed around 30− 150 °C for 3-L, which also corresponds to the loss of solvent molecule, and the framework is stable up to 360 °C (Figure

S17). For complexes 4-D and 4-L, a gradual weight loss in the range of 30−300 °C should be mainly attributed to the release of guest molecules, and the whole frameworks begin to decompose after 300 °C (Figure S18). As for 5-D and 5-L, weight loss of guest molecules took place between 30 and 200 °C, and then the frameworks begin to decompose (Figure S19). It should be noted that the DPEA is a flexible colinker relative to DPEE, so the frameworks of 5-L and 5-D are much less stable than those in the 4-D and 4-L. Circular Dichroism and Second-Harmonic Generation Efficency. To further demonstrate the homochirality of complexes 1−5, their solid-state circular dichroism (CD) measurements also have been performed (Figure 9). As shown in Figure 9a, the CD spectrum of complex 1-D has a strong positive CD signal at 282 nm, while complex 1-L exhibits a negative CD signal in the same place. The CD spectrum of the bulk sample of 2-D exhibits a weak negative CD signal at 243 nm and a strong positive CD signal at 293 nm. Moreover, the CD spectrum of 3-L shows an obvious Cotton effect (CE) at 370 nm (Figure 9b). The 4-D and 5-D exhibit a similar positive Cotton effect with peaka at 320 and 355 nm, respectively, and their enantiomers 4-L and 5-L also show a negative Cotton effect in the corresponding place (Figure 9c,d). From the above results, complexes 1−5 are all homochiral and crystallize in non-centrosymmetric crystal structures, so their second-order nonlinear optical properties were also carried out on a modified Kurtz-NLO system using 1064 nm at room temperature. Preliminary experimental results revealed that 1-D, 2-D, 3-L, 4-D, and 5-D are second harmonic generation-active with efficiencies approximately 0.25, 0.4, 0.6, 0.3, and 0.5 times that of KDP, respectively. Photoluminescent Properties. Considering the excellent luminescent properties of d10 metal complexes, the solid-state luminescence of compounds 1−5 were investigated in the solid state at room temperature (Figure 10). The photoluminescent spectra have peaks with maxima at 424 nm (λex = 325 nm) for 1-D, 383 nm (λex = 356 nm) for 2-D, 386 nm (λex = 290 nm) for 3-L, 461 nm (λex =385 nm) for 4-D, and 370 nm (λex = 385 nm) for 5-D, respectively. In order to better understand the nature of above emission bands, the luminescence properties of G

DOI: 10.1021/acs.cgd.5b01359 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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the free (S)-H3PIA ligand were also measured, which shows a maximum emission peak at 481 nm under excitation at 387 nm. Cd(II) ions with d10 electron configuration are rather stable and are difficultly oxidized or reduced. Thus, compared to the emission peak of H3PIA ligand, the emissions of 1-D, 2-D, 3-L, 4-D, and 5-D may be assigned to intraligand (n−π* or π−π*) emission.14

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CONCLUSION In summary, we have synthesized a series of novel 3D homochiral MOFs by using a pair of predesigned proline derivative ligands ((R)-PIA and (S)-PIA) to assemble with nitrogen heterocycle auxiliary ligands and Cd(II) ions. All the complexes consist of homochiral left-handed and/or righthanded helical chains, which are constructed by PIA fragments and Cd(II) ions. Meanwhile, the rare charge distribution phenomenon exists in complex 3-L. The phase and enantiomorphism purities of four complexes are proven by PXRD and CD studies, respectively. The success of our work not only affords some new homochiral complexes with interesting structures but also helps us to further understand the relationship between helicity and chirality. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01359. IR spectra, TGA, powder X-ray diffraction patterns (PDF) X-ray crystallographic files (CCDC 1421247−1421250, 1425545, 1425725−1425727) (CIF)

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Figure 10. Photoluminescences of complexes 1−5 and (S)-H3PIA.

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AUTHOR INFORMATION

Corresponding Authors

*(L.Z.) E-mail: [email protected]. *(J.Z.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 program (2012CB821705) and NSFC (21221001, 21425102). H

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DOI: 10.1021/acs.cgd.5b01359 Cryst. Growth Des. XXXX, XXX, XXX−XXX