Homochiral Metal–Organic Frameworks Embedding Helicity Based on

Nov 20, 2017 - Novel homochiral metal−organic frameworks (HMOFs) embedding distinctive helical chains have been created by a combination of semirigi...
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Homochiral Metal−Organic Frameworks Embedding Helicity Based on a Semirigid Alanine Derivative Guo-Xiu Guan, Xu Liu, Qi Yue,* and En-Qing Gao School of Chemistry and Molecular Engineering, Shanghai Key Laboratory of Green Chemistry and Chemical Processes, East China Normal University, Shanghai 200241, P. R. China S Supporting Information *

ABSTRACT: Five novel homochiral metal−organic frameworks (HMOFs) have been synthesized using a semirigid alanine-derived ligand and varied auxiliary N-donor ligands, namely, [Cu2(TMAla)2(bipy)2]·10H2O (1) and [Cu(TMAla)(bpea)0.5(H2O)]·H2O (2), Ln(TMAla)(HTMAla)(phen) (Ln = Eu, 3; Tb, 4; Gd, 5), where H2TMAla = terephthaloyl-mono(L-alanine), bipy = 4,4′-bipyridine, bpea = 1,2bis(4-pyridyl)ethane, phen = 1,10-phenanthroline. Crystallographic analysis indicates that all the complexes contain homochiral left- and/or right-handed helical chains, which are constructed by TMAla fragments and metal ions. Meanwhile, the introduction of auxiliary N-donor ligands gives rise to the formation of HMOFs with different structural topologies. Complex 1 shows a three-dimensional (3D) quartzdual net with a 4-connected (75·9) topology in the presence of bipy. Complex 2 possesses a novel 3D pillared-layer framework with a (3,4,4)-connected (6·104· 12)(63)2(64·102) topology by using bpea instead of bipy. Complexes 3−5 are isomorphous and feature a 2-fold interpenetrating 3D quartz net with a 4-connected (64·82) topology due to the participation of phen. Complexes 3 and 4 exhibit characteristic Eu(III) and Tb(III) emissions in the red and green light regions, respectively. The solid-state circular dichroism (CD) spectra of all complexes exhibit strong CD signals. Our results highlight that the semirigid chiral linker is successful for the construction of interesting HMOFs with unique helical chains.

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INTRODUCTION Homochiral metal−organic frameworks (HMOFs) have gained considerable research interest for their potential applications such as chiral separation, enantioselective sorption, asymmetric catalysis, etc.1−16 Currently, the selection of chiral ligand as the primary linker to impart homochirality is the most straightforward and effective strategy for preparation of homochiral HMOFs. A series of the natural chiral amino acids have been widely utilized as organic linkers to construct homochiral coordination polymers, which are inexpensive, nontoxic, and readily available.17−23 However, natural amino acids are not ideal chiral ligands in the synthesis of HMOFs owing to their structure flexibility. In order to overcome this shortcoming, the functional groups (−NH2 or −COOH) of amino acids modified by the suitable rigid benzoate units have been considered as an effective synthetic strategy to construct HMOFs.14,24−30 As a special form of one-dimensional chirality, helicity often closely linked with local chirality in the same structure. Various fascinating helical chains have been found in a number of HMOFs.8,26,28,29,31−36 Nonetheless, it is a great challenge to design and construct HMOFs with fascinating helical structures predictably because of the uncontrollability of the assembly process. Thus, rational design and judicious choice of chiral ligands are crucially important for the creation of intriguing structural and multifunctional HMOFs with helical motifs. © XXXX American Chemical Society

According to above-mentioned strategy, a chiral alanine derivative H2TMAla (Scheme 1) has been synthesized by modifying the -NH2 group of L-alanine with terephthalic acid in monosubstitution. Containing L-alanine and benzoate units, the Scheme 1. Structures of the Chiral Alanine Derivative Linker (a) and Auxiliary N-Donor Ligands (b)

Received: September 26, 2017 Revised: November 9, 2017 Published: November 20, 2017 A

DOI: 10.1021/acs.cgd.7b01365 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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min. The obtained suspending solution was transferred into a Teflonlined stainless steel container (20 mL) and heated at 120 °C for 2 days. Colorless block crystals were isolated until the container was slowly cooled to room temperature. Yield: 35% (based on H2TMAla). Elemental analysis found (calcd) % for C34H27N4O10Eu (3): C, 50.17 (50.82); H, 3.56 (3.39); N, 7.72 (6.97). IR (KBr, cm−1): 3331(m), 3055(w), 2978(w), 2641(w), 2512(w), 1666(s), 1588(m), 1554(s), 1459(w), 1424(s), 1278(m), 1166(w), 1020(w), 933(w), 864(m), 734(m), 579(w), 510(w). Synthesis of Tb(TMAla)(HTMAla)(phen) (4). The preparation method of 4 was similar to that of 3 except that Eu(NO3)2·6H2O (22.3 mg, 0.05 mmol) and H2O (5 mL) were replaced by Tb(NO3)2·6H2O (22.7 mg, 0.05 mmol) and H2O (6 mL), respectively. Meanwhile, 0.5 mL EtOH was removed. Colorless block crystals of 4 were isolated. Yield: 40% (based on H2TMAla). Elemental analysis found (Calcd) % for C34H27N4O10Tb for (4): C, 51.22 (50.38); H, 3.32 (3.36); N, 6.95 (6.91). IR (KBr, cm−1): 3322(m), 3063(w), 2986(w), 2641(w), 2512(w), 1666(s), 1597(m), 1554(s), 1459(w), 1416(s), 1278(m), 1166(w), 1020(w), 933(w), 864(m), 726(m), 579(w), 510(w). Synthesis of Gd(TMAla)(HTMAla)(phen) (5). 5 was synthesized in a similar procedure to that of 3 by using Gd(NO3)2·6H2O (21.7 mg, 0.05 mmol) in place of Eu(NO3)2·6H2O (22.3 mg, 0.05 mmol). Colorless block crystals of 5 were isolated. Yield: 40% (based on H2TMAla). Elemental analysis found (calcd) % for C34H27N4O10Gd (5): C, 49.89 (50.49); H, 3.66 (3.36); N, 7.15 (6.93). IR (KBr, cm−1): 3331(m), 3055(w), 2978(w), 2641(w), 2512(w), 1666(s), 1588(m), 1554(s), 1459(w), 1424(s), 1278(m), 1166(w), 1020(w), 933(w), 864(m), 734(m), 579(w), 510(w). X-ray Crystallography. Single-crystal X-ray diffraction data of 1−5 were collected at 298 K on a Bruker APEX II-CCD diffractometer equipped with a graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å). All structures were solved using the direct method and refined by the full-matrix least-squares techniques on F2 using SHELXTL-2014 program.37 All non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles, and the hydrogen atoms attached to the ligands were located in calculated positions and refined using a riding model. The hydrogen atoms attached to oxygen atoms were located from difference Fourier maps. The highly disordered water molecules found could not be well located and modeled during the final refinement, so the SQUEEZE option of PLATON program was used to move them.38 Structure refinement after modification of the data with the SQUEEZE led to better refinement and data convergence. The final chemical formulas of complexes were calculated on the crystallographic data combined with TGA and elemental analysis. Structure refinement parameters and crystallographic data for 1−5 are listed in Table 1. Main bond lengths and angles for 1−5 are listed in Tables S1−S5.

H2TMAla is a semirigid chiral ligand and has been used in the fabrication of related homochiral coordination polymers.26,27 This distinctive chiral ligand could furnish a new and feasible approach to design and construct HMOFs with fascinating helical architectures and outstanding properties. Additionally, to enhance the structural dimension and stability of HMOFs, the introduction of auxiliary N-donor ligands into metal-chiral coordination polymer systems would be an efficient method. The N-donor ligands with different shapes, spacer lengths, and rigidness/flexibility should have an important influence on the final structure of HMOFs. In order to obtain characteristic HMOFs of amino acids, we selected auxiliary bipy, bpea, and phen to assist H2TMAla to build HMOFs. Herein, two Cu-based HMOFs, namely, [Cu2(TMAla)2(bipy)2]·10H2O (1) and [Cu(TMAla)(bpea)0.5(H2O)]·H2O (2), and three isostructural Ln-based HMOFs, namely, Ln(TMAla)(HTMAla)(phen) (Ln = Eu, 3; Tb, 4; Gd, 5) have been successfully synthesized. It is important that all complexes display unique helical chains. The photoluminescence properties of complexes 3 and 4 are investigated. The homochiral structures of these complexes are confirmed by the combining single-crystal X-ray diffraction and solid-state CD spectra.

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EXPERIMENTAL SECTION

Materials and General Measurements. All chemical reagents were commercially purchased. Powder X-ray diffraction (PXRD) data were recorded using a Bruker D8 ADVANCE. Thermogravimetric (TG) analyses were carried out on TGA Q500 V20.10 Build 36 thermogravimetric analyzer from room temperature to 800 °C under N2 atmosphere with a heating rate of 10 °C/min. The IR spectra were obtained on a Bruker NicoletiS50 spectrometer in the range of 4000− 400 cm−1. Solid-state photoluminescence spectra and lifetimes were performed on an FLS980 fluorescence spectrometer. Absolute quantum yields were determined on a QM/TM/IM steady-state and time-resolved fluorescence spectrofluorometer in the solid state at room temperature. Solid-state circular dichroism (CD) spectra were determined on a JASCO J-815 spectrometer. Synthesis of [Cu2(TMAla)2(bipy)2]·10H2O (1). Cu(NO3)2·3H2O (12.1 mg, 0.05 mmol), H2TMAla (12 mg, 0.05 mmol), and bipy (7.8 mg, 0.05 mmol) were suspended in a 4.5 mL mixed solvent of H2O and isopropanol (2:1, v/v) and stirred for 30 min. The final suspending solution was put into a Teflon-lined stainless steel container (20 mL) and heated at 120 °C for 2 days. Blue rod-like crystals were obtained, while the container was cooled to 25 °C in 5 h. Yield: 10% (based on H2TMAla). Elemental analysis found (calcd) % for C21H17N3O5Cu (1): C, 46.52 (46.28); H, 4.82 (4.99); N 7.55 (7.71). IR (KBr, cm−1): 3417(m), 3067(w), 1649(m), 1607(s), 1549(m), 1382(s), 1290(w), 1215(m), 1073(w), 815(m), 731(w), 648(w). Synthesis of [Cu(TMAla)(bpea)0.5(H2O)]·H2O (2). Cu(OAc)2· H2O (10 mg, 0.05 mmol), H2TMAla (12 mg, 0.05 mmol), and bpea (9.2 mg, 0.05 mmol) were suspended in 3 mL of H2O and stirred for 30 min. And then, the resulting suspending solution was put into a Teflon-lined stainless steel container (20 mL) and heated at 120 °C for 2 days. Blue-purple block crystals were collected when the container was slowly cooled to room temperature. Yield: 40% (based on H2TMAla). Elemental analysis found (calcd) % for C17H17N2O6Cu (2): C, 46.12 (47.83); H, 4.61 (4.49); N, 6.43 (6.56). IR (KBr, cm−1): 3481(m), 3305(s), 3062(w), 2986(w), 2928(w), 1648(s), 1623(m), 1581(s), 1531(s), 1405(s), 1305(m), 1280(w), 1221(w), 1155(w), 1096(w), 1012(m), 879(w), 829(s), 745(s), 711(m), 561(m), 527(m). Synthesis of Eu(TMAla)(HTMAla)(phen) (3). Eu(NO3)2·6H2O (22.3 mg,0.05 mmol), H2TMAla (17.8 mg, 0.075 mmol), 1,10-phen (9 mg,0.05 mmol), and NaOAc (2 mg, 0.025 mmol) were suspended in a 5.5 mL mixed solvent of H2O and EtOH (10:1, v/v) and stirred for 30

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RESULTS AND DISCUSSION Crystal Structure of [Cu2(TMAla)2(bipy)2]·10H2O (1). Complex 1 crystallizes in the chiral hexagonal space group P61. There are 2 Cu(II) ions, 2 TMAla anions, 2 bipy ligands, and 10 lattice water molecules in the asymmetric unit of 1. The coordination geometries of both Cu(II) ions are characterized as distorted trigonal bipyramid and are furnished by three carboxylate oxygen atoms from three different TMAla anions and two nitrogen atoms from two bipy ligands. The equatorial plane was formed by three carboxylate oxygen atoms, while two nitrogen atoms occupy the axial positions with the N3−Cu1− N4B and N5−Cu2−N6C angles of 175.701(4) and 175.366(4)°, respectively, as shown in Figure 1. Two completely deprotonated TMAla anions act as the same μ3linker and possess two kinds of carboxylate groups based on a distinct coordination fashion (Scheme S1a): (a) the carboxylate group from the benzoate unit adopts monodentate bridging mode to link a Cu(II) ion (Cu1 or Cu2); (b) the carboxylate group from the alanine unit adopts bidentate bridging to B

DOI: 10.1021/acs.cgd.7b01365 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Structure Refinement Parameters and Crystallographic Data of 1−5 complex

1

2

formula C21H17N3O5Cu molecular weight 454.93 crystal system hexagonal space group P61 a (Å) 11.0773(6) b (Å) 11.0773(6) c (Å) 77.251(6) α (deg) 90 β (deg) 90 γ (deg) 120 Z 12 V(Å3) 8209.2(9) ρcalcd (g cm−3) 1.104 μ (Mo Kα) 0.826 (mm−1) F(000) 2796 reflections 114422/13708 collected/ unique Rint 0.0921 data/restraints/ 13708/31/542 parameters goodness-of-fit 1.031 on F 2 R1/wR2 0.0620, 0.1522 [I > 2σ(I)] R1/wR2 (all data) 0.0842, 0.1597 Flack parameter 0.083(7) largest residues 0.414 and (e A−3) −0.397 compound formula molecular weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z V (Å3) ρcalcd (g cm−3) μ (Mo Kα) (mm−1) F(000) reflections collected/unique Rint data/restraints/parameters goodness-of-fit on F 2 R1/wR2 [I > 2σ(I)] R1/wR2 (all data) Flack parameter largest residues (e A−3)

3

C17H17N2O6Cu 408.88 orthorhombic P21212 13.5719(3) 10.4878(2) 13.4116(3) 90 90 90 4 1909.00(7) 1.423 1.178

C34H27N4O10Eu 803.60 hexagonal P6522 10.2677(2) 10.2677(2) 52.8732(16) 90 90 120 6 4827.4(2) 1.658 2.015

840 26226/4604

2412 66520/4031

0.0455 4604/9/283

0.0430 4031/29/231

0.998

1.047

0.0339, 0.0751

0.0545, 0.1083

0.0470, 0.0794 0.026(9) 0.354 and −0.243 4

0.0578, 0.1101 0.013(6) 1.189 and −2.575

C34H27N4O10Tb 810.50 hexagonal P6522 10.2453(3) 10.2453(3) 52.719(2) 90 90 120 6 4792.4(3) 1.685 2.280 2424 66650/4045 0.0425 4045/30/231 1.099 0.0554, 0.1081 0.0591, 0.1103 0.013(5) 1.404 and −2.887

Figure 1. Coordination environment of the Cu(II) ions in 1 (symmetry codes: A = 1 + x − y, 1 + x, 0.16667 + z; B = −1 + x, y, z; C = 1 + x, y, z). All hydrogen atoms are omitted for clarity.

5 C34H27N4O10Gd 808.90 hexagonal P6522 10.2595(2) 10.2595(2) 52.8106(15) 90 90 120 6 4814.0(2) 1.674 2.133 2418 66641/4033 0.0625 4033/25/231 1.062 0.0532, 0.1135 0.0626, 0.1202 0.010(7) 1.307 and −2.665

Figure 2. Infinite double-stranded left-handed helical chain of 1 running along the c-axis (a). The hexagonal left-handed helical channel formed by helical chain viewed along the c-axis (b). The homochiral porous 3D framework of 1 formed by left-handed helical chains and bipy ligands (c). The irregular cage-type substructures of 1 viewed along the c-axis (the yellow balls represent the cages) (d) and the baxis (e).

axis. Such a helical chain is composed of six binuclear [Cu2(COO)2] units and six pairs of TMAla anions per turn. It should be noted that the helical chain produces a hexagonal left-handed helical channel in 1 (Figure 2b). All the helical channels lie parallel to each other exhibiting perpendicular directions in the ab plane. The Cu1 and Cu2 ions from each chain are connected to symmetry-related Cu1 and Cu2, respectively, from other six left-handed helical chains through bipy ligands, generating a homochiral porous three-dimensional (3D) framework (Figure 2c).

connect Cu1 and Cu2, leading to a [Cu2(COO)2] binuclear unit with the Cu···Cu distance of 4.1906(3) Å. The binuclear units are connected by pairwise the TMAla anions to develop an infinite double-stranded left-handed helical chain with a 61screw axis running along the c-axis (Figure 2a). The helical pitch is 78.0409(54) Å, which is equal to the length of the c C

DOI: 10.1021/acs.cgd.7b01365 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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carboxylate oxygen atoms from two TMAla anions and two nitrogen atoms from two bpea ligands. Cu2 adopts distorted {CuO6} octahedral coordination sphere, which is defined by two carboxylate oxygen atoms from two TMAla anions, two carbonyl oxygen atoms of the amide groups from two different TMAla anions, and two coordinated water molecules. The TMAla anion also acts as a μ3-linker in 2 (Scheme S1b), which is different with the coordination mode of complex 1: (a) the carboxylate group from the alanine unit serves as monodentate bridging mode to bind Cu1; (b) the carboxylate group from the benzoate unit serves as monodentate bridging mode to bond Cu2; (c) the carbonyl oxygen atom from the amide group acts as monodentate bridging mode to bond another Cu2. For the presence of two different ligands, TMAla anion and bpea ligand, the framework of 2 is divided into two parts to simplify the complicated structure. If only the connectivity between TMAla anions and Cu(II) ions is considered, a highly undulated 2D {Cu(TMAla)}n layered motif running along the ab plane is fabricated (Figure 5a). In this layer, each TMAla

Consequently, each left-handed helical channel is separated by the bipy ligands into the same six irregular cage-type substructures per turn with an inner diameter of about 3.5 Å, consisting of four [Cu2(COO)2] binuclear units, two pairs of bipy, and two pairs of TMAla anions (Figure 2d,e). Although the hexagonal windows are blocked by the bipy ligands along the [001] direction, the square windows are still remaining along the [100] and [010] directions (Figure S1). PLATON analysis gave a total effective free volume of 2968.0 Å3 per cell (36.2% of the crystal volume). The voids are filled by 10 lattice water molecules per formula. The topological analysis is applied to simplify intricate 3D framework though using the program TOPOS.39 The dinuclear [Cu2(COO)2] units can be considered as 4-connected nodes, while both TMAla anions and bipy ligands can be taken as linkers. Therefore, the whole network can be rationalized as a 4-connected quartz-dual net with the Schläfli symbol of (75·9) (Figure 3).

Figure 3. Four-connected quartz-dual topological net of 1.

Crystal Structure of [Cu(TMAla)(bpea)0.5(H2O)]·H2O (2). Complex 2 is solved in the chiral orthorhombic space group P21212. Its asymmetric unit is comprised of a half Cu1(II) ion, a half Cu2(II) ion, one TMAla anion, a half bpea ligand, one coordinated and one lattice water molecules. As illustrated in Figure 4, Cu1 has a {CuO2N2} square-planar coordination environment, which is completed by two

Figure 5. Undulated 2D {Cu(TMAla)}n layered motif of 2 running along the ab plane (a) and viewed along the b-axis (b). Purple lines highlight the right-helical chains with (−Cu2−O3−C4−C5−C6−C8− C10−C11−O5−)n as a repeating unit running along the b-axis (c). Blue lines highlight the left-helical chains with (−Cu2−O3−C4−N1− C2−C1−O1−Cu1−O1−C1−C2−N1−C4−O3−)n as a repeating unit running along the b-axis (d).

anion connects two Cu2 ions to afford a (4,4)-connected 2D grid network with a diagonal separation of 10.4878(2) × 13.5719 (3) Å measured between two nearest Cu2 ions. It is interesting that the {Cu1(OC2N)2} coordination units cross over the rhombic windows on the opposite sides of the layer in the alternating up and down mode and locate in the peaks and valleys of the undulated layer, serving to reinforce the {Cu(TMAla)}n layer via Cu1 ions connected the carboxylate groups of the alanine units in the layer. The thickness of the 2D layer is 14.9604(2) Å, corresponding to the distance of two Cu1 ions between the peak and valley of the layer (Figure 5b). The most striking structural feature of such an undulated layer is the presence of two types of single-stranded helical chains along the b-axis. The Cu2 ions are bridged by the TMAla fragments to constitute an infinite right-handed helix, namely, (−Cu2−O3−C4−C5−C6−C8−C10−C11−O5−)n repeating

Figure 4. Coordination environment of the Cu(II) ions in 2 (symmetry codes: A = −x, −y, z; B = x, y, 1 + z; C = −x, 1 − y, z; D = −0.5 − x, 0.5 + y, −z; E = 0.5 + x, 0.5 − y, −z). All hydrogen atoms are omitted for clarity. D

DOI: 10.1021/acs.cgd.7b01365 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 6. (2,3,4)-Connected topological net of 2D {Cu(TMAla)}n layer in 2 (a). The homochiral 3D pillared-layer framework of 2 viewed down the b-axis (b). The (3,4,4)-connected topological net of 2 (c).

units (Figure 5c). The TMAla fragment is composed of a half benzoate unit and the carbonyl oxygen atom from the amide group. Interestingly, the Cu1 and Cu2 ions are also connected by the TMAla fragments to give a left-handed helical chain (Figure 5d). However, this TMAla fragment is different from that in the right-handed helix, consisting of a carboxylate group of the alanine unit and an amide group. The left-handed helical chain contains two Cu(II) ions, two carboxylate groups from the alanine units, and two amide groups per turn, namely, (−Cu2−O3−C4−N1−C2−C1−O1−Cu1−O1−C1−C2− N1−C4−O3−)n repeating units. These right- and left-handed helical chains are alternately joined together through sharing (−Cu2−O3−C4−) units and are parallelly arranged in columns to give rise to a homochiral 2D layer along the ab plane. All helical chains have a 21-screw axis and a pitch of 10.4878(2) Å, according to the length of the b axis. By considering Cu1 as a 2-connected node, Cu2 as a 4connected node, and TMAla anion as a 3-connected node, the 2D layer can be described as a (2,3,4)-connected net with Schläfli symbol of (63)2(64·102)(6) (Figure 6a). The adjacent 2D layers are further extended to a homochiral 3D pillared-layer framework by the bpea ligands bridging Cu1 and Cu2 ions between adjacent layers (Figure 6b). From the viewpoint of structural topology, both Cu1 and Cu2 ions can be regarded as a 4-connected node, the TMAla anion can be treated as 3-connected node, the bpea ligand can be taken as linker, and final 3D pillared-layer framework of 2 can be thus represented as a (3,4,4)-connected net with the Schläfli symbol of (6·104·12)(63)2(64·102) (Figure 6c). Crystal Structure of Ln(TMAla)(HTMAla)(phen) (Ln = Eu, 3; Tb, 4; Gd, 5). Single-crystal X-ray diffraction analysis confirms that complexes 3, 4, and 5 are isostructural 3D frameworks. As a representative example, only the structure of 3 was analyzed here in detail. Complex 3 belongs to the chiral hexagonal space group P6522. The asymmetric unit includes a half Eu(III) ion, a half TMAla anion, a half HTMAla anion, and a half phen ligand. As depicted in Figure 7, eight-coordinated Eu1 ion resides on a 2-fold axis and is regarded a distorted triangle-dodecahedral geometry. The coordination sphere of Eu(III) ion is occupied by six carboxylate oxygen atoms (O1, O2, O1A, O2A, O4B, and O4C) from four TMAla anions and

Figure 7. Coordination environment of the Eu(III) ions in 3 (symmetry codes: A = 1 − y, 1 − x, 0.16667 − z; B = 1 + y, −x + y, 0.16667 + z; C = 1 + x − y, −y, −z). All hydrogen atoms are omitted for clarity.

two nitrogen atoms (N2 and N2A) from one phen ligand. The Eu−O bond lengths fall between 2.39(7) and 2.58(8) Å, while the Eu−N length is measured as 2.51(5) Å. Different from the coordination patterns of complexes 1 and 2, the TMAla anion acts as a μ2-linker in 3 (Scheme S1c): (a) the carboxylate group from the alanine unit employs a chelating mode to ligate a Eu(III) ion; (b) the carboxylate group from the benzoate unit employs a monodentate bridging mode to bind a Eu(III) ion. So, each TMAla anion connects two Eu(III) ions, and each Eu(III) ion links to four surrounding Eu(III) ions through four TMAla anions to generate a 3D homochiral framework (Figure 8a). The first prominent structural feature of 3 is the existence of both single-stranded and double-stranded helical chains. As illustrated in Figure 8b, two carboxylate groups of the TMAla anions bridge the Eu1 ions to constitute a small right-handed single-stranded helical chain running along the c-axis, which contains six TMAla anions and six Eu(III) ions per turn. These Eu(III) ions are alternately encircled by the two carboxylate groups of benzoate units and two carboxylate groups of alanine units. Each small helical chain is further linked to three adjacent large helical chains. Different from the small helical chain, the large helical chain is left-handed double-stranded, which is comprised of six TMAla anions and six Eu(III) ions per turn E

DOI: 10.1021/acs.cgd.7b01365 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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active locations for host−guest interactions. Topologically, each Eu(III) ion can be seen as a 4-connected node, and the TMAla anion can be taken as a linker, and then the overall 3D architecture of 3 can be envisioned as a 4-connected quartz net with a Schläfli symbol of (64·82) (Figure 9a). The single 3D framework of 3 possesses a larger interconnected void space. The hexagonal window has an average diagonal separation of about 20.53 Å, measured between two nearest Eu(III) ions from diagonal locations, and an average edge-to-edge separation of about 11.3953(2) Å, measured between two nearest carbonyl oxygen atoms (O3) of the amide groups from opposite edges viewed down the [001] direction (Figure S2a). The two kinds of the irregular windows have an average diagonal separation of about 26.47 × 20.53 and 19.88 × 20.53 Å, respectively, measured between two nearest Eu(III) ions from diagonal locations viewed along the [100] and [010] directions (Figure S2b). All dimensions of channels include the van der Waals radius. To occupy the large space and stabilize the overall framework, a pair of independent equivalent frameworks is mutually interpenetrated with each other, triggering the formation of a 2-fold interpenetrated network (Figure 9b). As result of the interpenetration, the hexagonal and irregular windows from the two networks are blocked along the [001] and [100] directions, respectively (Figure S3a,b). The irregular windows of two networks are overlapped together along the [010] direction (Figure S3c). Therefore, there are remaining very small irregular open channel spaces after the 2-fold interpenetration. PLATON analysis gave a total effective free volume of only 247.1 Å3 per cell (5.1% of the crystal volume).

Figure 8. 3D homochiral framework of 3 viewed down the c-axis (a). The right-handed single-stranded helical chain running along the c-axis (b). The left-handed double-stranded helical chain running along the c-axis (c).

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running along the c-axis (Figure 8c). In this large helical chain, each Eu(III) ion is surrounded by one carboxylate group of the benzoate unit and one carboxylate group of the alanine unit from the TMAla anion. The pitch lengths of both helical chains are identical to the length of the c-axis. It is worth noting that the large helical chain forms a hexagonal left-handed helical channel running along the c-axis, which is the second remarkable structural feature of 3. Moreover, each large lefthanded double-stranded helical chain is surrounded with six adjacent small right-handed single-stranded helical chains. The above right- and left-handed helical chains are combined together through sharing Eu(III) ions to create the final homochiral porous 3D architecture. Additionally, the third outstanding structural feature of 3 is that surface of the hexagonal left-handed helical channel is decorated by the carbonyl oxygen atoms (O3) of the amide groups pointing toward the centers of channels, which may assign the potential

DISCUSSION The chiral alanine-derived ligands with semirigid conformation manifest diverse coordination modes and play an indispensable role in the creation of helical chains in the complexes 1−5. The bipy and bpea show linear conformations and act as the linker between the helical chains in 1 and 2. The phen with chelating coordination conformation only functions as a garniture in 3− 5. These results illustrate that the integration of L-alanine and rigid benzoate unit can provide a new and feasible approach to design and construct multifunctional HMOFs with intriguing helical motifs. Circular Dichroism. To further demonstrate homochirality of complexes 1−5, their circular dichroism (CD) spectra have been produced in the solid state, because all complexes crystallize in the chiral space groups. The CD spectra of all complexes exhibit the obvious positive and/or negative Cotton

Figure 9. Single 4-connected quartz topological net of 3 (a). A pair of identical topologies that mutually penetrate each other (b). F

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effect with different peaks varying from 210 to 327 nm (Figure S4), better validating the homochiral nature of complexes. The results are consistent with the structures identified by singlecrystal structural analysis. Photoluminescence Properties. The solid-state photoluminescent spectra of complexes 3 and 4 together with the free H2TMAla and phen ligands were investigated at ambient temperature (Figure 10). Under excitation at 249 nm with UV

Figure 11. Fluorescent excitation (red) and emission (black, λex = 358 nm) spectra of 3 in the solid state.

an inversion center,48−50 which is confirmed by the 5D0 → F2/5D0 → 7F1 intensity ratio of about 5.65. These results are good accordance with the single crystal structures. When excited at 358 nm, 4 manifests four typical emission bands at 491, 544, 585, and 620 nm, which are associated with the 5D4 → 7FJ (J = 6, 5, 4, 3) transitions of Tb(III) ion, respectively (Figure 12). The highest intensity 5D4 → 7F5 transition arising 7

Figure 10. Fluorescent excitation (dotted lines) and emission (solid lines) spectra of free H2TMAla (red, λex = 249 nm) and free phen ligands (black, λex = 402 nm) in the solid state.

light, the free H2TMAla ligand shows an emission at 405 nm. The free phen ligand shows emission at 449 nm (λex = 402 nm). Usually, the emission peaks for these organic ligands are attributed to ligand-centered (LC) π* → π electronic transitions.40−44 When the ligands bond with the lanthanide ions, the photoluminescent spectra of the complexes 3 and 4 display characteristic emission bands corresponding to lanthanide ions, whereas the ligand-based emissions derived from the free H2TMAla and phen are not present in the 400−480 nm region. Obviously, the efficient energy transfer from the ligands excited states to the central metal ions f-excited states occurs in the course of photoluminescence for complexes 3 and 4.45 Under excitation at 358 nm, complex 3 displays the characteristic luminescent bands at 580, (592, 596), 616, 650, and (688, 696) nm, which are assignable to the 5D0 → 7FJ (J = 0−4) transitions of the Eu(III) ion, respectively. (Figure 11). The spectrum of 3 is dominated by the strongest 5D0 → 7F2 transition arising from an electric dipole transition, resulting in bright red luminescence. In addition, 5D0 → 7F2 transition is extremely sensitive to the chemical environment around the Eu(III) ion, and its intensity increases with the decrease of Eu(III) site symmetry.46,47 As is well-known, the 5D0 → 7F0 transition is strictly prohibited in a field of symmetry, and thus the only one sharp emission band observed corresponding to 5D0 → 7F0 transition at 580 nm signifies that 3 has the non-centrosymmetrical Eu(III) as well as the only one type of Eu(III) species.45 The 5D0 → 7F1 transition is ascribed to a magnetic dipole transition and is affected by the crystal field strength around the Eu(III) ion. The 5D0 → 7F2/5D0 → 7F1 intensity ratio is extensively utilized as a representation of the coordination state and the site symmetry of the lanthanide ion. Therefore, the Eu(III) ion symmetry sites in 3 do not have

Figure 12. Fluorescent excitation (red) and emission (black, λex = 358 nm) spectra of 4 in the solid state.

from a magnetic dipole transition51−53 leads to a strong green luminescence of 4. Meanwhile, a moderate intensity 5D4 → 7F6 transition also is contributive to the strong green luminescence.45 The emission lifetimes (τ) of complexes 3 and 4 were investigated by monitoring the emission decay curves within the 5D0 → 7F2 and 5D4 → 7F5 transition. The observed luminescence decays for complexes 3 and 4 were fitted to a single-exponential function, which suggests the existence of only a single luminescent species at the excited state, giving lifetime values of 1.05 ms for 3 and 1.15 ms for 4 (Figure S5). It is in agreement with the results of the crystal structures analysis and the emission spectra. The quantum yields of 3 and 4 were measured to be 5.34% and 7.86%, respectively, by an absolute integrating-sphere method. These observations indicate that G

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complexes 3 and 4 might be good candidates as photoluminescent materials.

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CONCLUSION A series of novel HMOFs have been hydrothermally synthesized by using a semirigid chiral ligand (H2TMAla) integrating enantiopure alanine and rigid benzoate units in the presence of different auxiliary N-donor ligands. All the complexes include distinctive homochiral left-handed and/or right-handed helical chains, which are assembled by TMAla fragments and metal ions. The phase and enantiomorphism purities of these complexes are proven by PXRD and CD studies, respectively. Complexes 3 and 4 display interesting photoluminescence behaviors. The results demonstrate that the semirigid H2TMAla ligand can act as an effective chiral linker to build novel HMOFs with fascinating helical structures and potential applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b01365. Additional structural figures for the related complexes, the coordination modes of H2TMAla, IR spectra, TGA, powder X-ray diffraction patterns, CD spectra (PDF) Accession Codes

CCDC 1564941−1564945 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +86 021 54340062. ORCID

Qi Yue: 0000-0003-0756-6448 En-Qing Gao: 0000-0002-5631-2391 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC No. 21101064). REFERENCES

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