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Temperature-controlled assembly of homochiral metal-organic frameworks from 2D helical layers to 3D frameworks
Journal of Solid State Chemistry 279 (2019) 120967
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Temperature-controlled assembly of homochiral metal-organic frameworks from 2D helical layers to 3D frameworks Zhong-Xuan Xu *, Xu-Ling Bai, Li-Feng Li School of Chemistry and Chemical Engineering, Zunyi Normal College, Zunyi, 563002, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Homochiral metal-organic frameworks Reaction temperature Structural transformation
Two pairs of homochiral metal-organic frameworks (HOMFs), {[Zn((R)-CBA) (1.3- DIB)]⋅H2O}n(1-D), {[Zn((S)CBA) (1.3-DIB)]⋅H2O}n(1-L), [Zn2((R)-CBA)2 (1.3-DIB)2]n (2-D) and [Zn2((S)-CBA)2(1.3-DIB)2]n (2-L), have been synthesized by using lactate derivatives (R)/(S)-H2CBA and N-donor ancillary 1,3-DIB ligands at different reaction temperatures (H2CBA ¼ 4-(1-carboxyethoxy)benzoic acid, 1,3-DIB ¼ 1,3-di(1H-imidazole-1-yl) benzene). Complexes 1-D and 1-L are two-dimensional (2D) layers based on three kinds of chiral helical chains, while complexes 2-D and 2-L are four-connected three-dimensional (3D) frameworks with cds nets. The resulting homochiral complexes have been characterized by PXRD, TGA, IR and CD measurements. Additionally, complexes 1-D and 2D exhibit different photoluminescence behaviors in solid sate compared to ligand (R)-H2CBA. The work has shown that the dimensionality of HMOFs can be regulated by the reaction temperature.
1. Introduction Homochiral metal-organic frameworks (HMOFs) have been the subject of immense interest in the past decades, not only for their fascinating architectures but also their potential applications in chiral separation and asymmetry catalysis [1–6]. Up to now, three general strategies have been employed to construct HMOFs, including use of chiral ligands, spontaneous resolution and asymmetric induction [7–13]. The enantiopure compounds as the primary linkers can impart their chirality to the frameworks to generate homochirality, which has been proved to be the most direct and effective synthetic strategy for HMOFs [14–17]. Therefore, choice of chiral ligands is a key factor to construct HMOFs. However, the reaction temperature, solvent system, pH value and counter ions also can influence the formation of MOFs besides the ligands and metal ions. So it is still difficult to rationalize what reaction conditions obtain target MOFs [18–40]. Among these reaction conditions, the temperature can cause organic ligands to adopt different conformations, coordination modes and so on. Consequently, the reaction temperatures have remarkable influence on the topology and dimensionality of the metal-organic framework structures [41,42]. Furthermore, hydro/solvothermal reactions systems often used to construct metal-organic framework, so the reaction temperature plays an important role in the synthesis of metal-organic frameworks [43,44]. However, to the best of our knowledge, the temperature-controlled structural changes in HMOFs is still quite rare and extremely challenging.
Containing lactic acid and benzoic acid units, the (R)-H2CBA and (S)H2CBA has semirigid skeleton and multiple coordination modes, providing a convenient approach to construct HMOFs [45]. In this work, we continued to employ (R)-H2CBA and (S)-H2CBA as chiral ligands to build HMOFs in the presence of rigid auxiliary nitrogenous ligand 1, 3-DIB. Under hydrothermal reaction condition, two pairs of entirely different HMOFs, namely, {[Zn((R)-CBA) (1.3-DIB)]⋅H2O}n(1-D), {[Zn((S)-CBA) (1.3-DIB)]⋅H2O}n(1-L), [Zn2((R)-CBA)2 (1.3-DIB)2]n (2-D) and [Zn2((S)-CBA)2 (1.3-DIB)2]n (2-L), were synthesized by using the same starting reaction mixture at different reaction temperature (Scheme 1). Their syntheses, crystal structures, chiralities and properties of solid-state photoluminescence are reported below. 2. Experimental section 2.1. General procedures The chiral ligands (R)-H2CBA and (S)-H2CBA were synthesized according to the documented procedures [45]. All other chemical reagents and solvents were purchased commercially and used without further purification. The elemental analysis was performed on a Perhin-Elmer 240C elemental analyzer. The IR spectra were measured on a FTIR-650 FT-IR spectrometer with KBr pellets from 400 to 4000 cm1. Thermal stability studies were performed on a NETSCHZ STA-F3 thermoanalyzer with a heating rate of 20 C/min in N2. The phase purity and crystallinity
* Corresponding author. E-mail address: [email protected] (Z.-X. Xu). https://doi.org/10.1016/j.jssc.2019.120967 Received 21 August 2019; Received in revised form 17 September 2019; Accepted 18 September 2019 Available online 19 September 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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Journal of Solid State Chemistry 279 (2019) 120967
2.3. Synthesis of {[Zn((S)-CBA) (1.3-DIB)]⋅H2O}n(1-L) The same procedure as 1-D, except (S)-H2CBA replaced (R)-H2CBA. Colorless block crystals of 1-L were obtained with a yield of 30% (based on (S)-H2CBA). Anal. Calcd. for C22H20N4O6Zn (%): C, 52.66; H, 4.02; N, 11.17. found (%): C, 49.82; H, 3.84; N, 10.06. IR (KBr, cm1): 3358.6(w), 3137.0(w), 1603.5(s), 1564.4(m), 1512.1(s), 1388.3(s), 1290.5(m), 1231.4(s), 1172.7(w), 944.4(w), 853.3(m), 781.5(m),742.3(w), 657.3(m), 494.4(w). 2.4. Synthesis of [Zn2((R)-CBA)2(1.3-DIB)2]n(2-D) The same procedure as 1-D, except temperature is 120 C. Colorless block crystals of 2-D were obtained with a yield of 50% (based on (R)H2CBA). Anal. Calcd. for C44H36N8O10Zn2 (%): C, 54.62; H, 3.75; N, 11.58. Found (%): C, 51.34; H, 3.34; N, 10.04. IR (KBr, cm1): 3437.2(w), 3110.6(w), 1642.6(s), 1597.1(s), 1512.9(m), 1388.3(m), 1231.4(m), 1172.8(w), 1140.1(w), 1062.2(m), 944.4(w),787.8(m), 676.9(w), 657.3(w).
Scheme 1. The synthetic route of complexes 1-L, 1-D, 2-L and 2-D.
of each complex was checked by powder X-ray diffraction (PXRD) using a Rigaku Dmax2500 diffractometer with Cu Kα radiation (λ ¼ 1.54056 Å) in the range of 5.00–50.00 . Solid-state photoluminescence spectra were performed on a Hitachi FL-4500 fluorescence spectrophotometer. Solid CD spectra were measured on a MOS-450 spectropolarimeter at room temperature.
2.5. Synthesis of [Zn2((S)-CBA)2(1.3-DIB)2]n(2-L) The same procedure as 2-D, except (S)-H2CBA replaced (R)-H2CBA. Colorless block crystals of 2-L were obtained with a yield of 50% (based on (R)-H2CBA). Anal. Calcd. for C44H36N8O10Zn2 (%): C, 54.62; H, 3.75; N, 11.58. Found (%): C, 52.22; H, 3.46; N, 10.26. IR (KBr, cm1): 3420.4(w), 3111.2(w), 1639.3(s), 1597.1(s), 1518.1(m), 1388.4(m), 1231.4(m), 1172.8(w), 1107.6(w), 1062.2(m), 944.4(w), 786.4(m), 676.8(w), 654.9(w).
2.2. Synthesis of {[Zn((R)-CBA) (1.3-DIB)]⋅H2O}n(1-D) Complex 1-D was synthesized by stirring Zn(BF4)2 (35 mg, 0.15 mmol), (R)-H2CBA(21 mg, 0.1 mmol), 1,3-DIB (21 mg, 0.1 mmol) and pyrazine (96 mg, 1.2 mmol) in 6 ml of N,N-dimethylacetamide/ water (1:3) solvent. The resulting mixture was placed in a 23 mL Teflon reactor and kept under autogenous pressure at 90 C for three days. After slowly being cooled to room temperature, colorless crystals were filtered and washed with ethanol and dried in the air. Yield: 40% (based on (R)H2CBA). Anal. Calcd. for C22H20N4O6Zn (%): C, 52.66; H, 4.02; N, 11.17. Found (%): C, 50.24; H, 3.62; N, 9.84. IR (KBr, cm1): 3345.8(w), 3137.0(w),1603.5(s), 1558.0(m), 1512.1(s), 1381.5(s), 1283.7(m), 1231.4(s), 1179.5(m), 1055.4(s), 944.4(w), 853.3(w), 787.8(m), 735.6(w), 657.3(m), 494.4(w).
2.6. X-ray determination All single-crystal X-ray diffraction data were collected on a Rigaku four-circle 003 CCD diffractometer equipped with graphitemonochromated Mo Kα radiation (λ ¼ 0.071073 nm) at room temperature. The crystal structures were solved and refined by full matrixes methods against F2 using SHELXL-2017 program package and Olex2-1.2 software [46,47]. All non-hydrogen atoms located in successive
Table 1 Crystal data and structure refinement details for complexes 1-D, 1-L, 2-D and 2-L. Compound reference
1-D
1-L
2-D
2-L
Chemical formula Formula Mass Crystal system a/Å b/Å c/Å α / β/ γ/ Unit cell volume/Å3 Temperature/K Space group No. of formula units per unit cell, Z Radiation type Absorption coefficient, μ/mm1 No. of reflections measured No. of independent reflections Rint Final R1a values (I > 2σ (I)) Final wRb(F2) values (I > 2σ (I)) Final R1 values (all data) Final wR(F2) values (all data) Goodness of fit on F2 Flack parameter CCDC number
C22H18N4O5Zn⋅H2O 501.79 Monoclinic 7.8700(10) 18.9756(12) 8.2285(7) 90 114.691(14) 90 1116.5(2) 293(2) P21 2 MoKα 1.146 12907 5670 0.0298 0.0303 0.0725 0.0380 0.0756 1.001 0.017(7) 1948488
C22H18N4O5Zn⋅H2O 501.79 Monoclinic 7.8629(10) 18.9525(10) 8.2151(9) 90 114.704(14) 90 1112.2(2) 293(2) P21 2 MoKα 1.151 17810 5855 0.0408 0.0318 0.0739 0.0415 0.0775 1.048 0.007(7) 1948489
0.5(C44H36N8O10Zn2)
0.5(C44H36N8O10Zn2)
483.77 Triclinic 7.9642(3) 10.5601(5) 14.1194(5) 105.445(4) 94.217(3) 109.397(4) 1062.36(8) 293(2) P1 2 MoKα 1.198 24119 10314 0.0288 0.0281 0.0698 0.0327 0.0718 0.977 0.009(5) 1948490
483.77 Triclinic 7.9727(2) 10.5668(3) 14.1226(3) 105.462(2) 94.188(2) 109.367(3) 1064.63(5) 293(2) P1 2 MoKα 1.196 28660 10746 0.0328 0.0276 0.0702 0.0324 0.0726 0.924 0.012(5) 1948491
a b
R1 ¼ Σ||Fo||Fc||Σ|Fo|. wR2 ¼ [Σw(F2o F2c )2/Σ[w(F2o)2]1/2. 2
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Journal of Solid State Chemistry 279 (2019) 120967
Fig. 1. Coordination environment of enantiomers 1-D and 1-L. Symmetric codes: (a) 1-x, 0.5 þ y, -z; (b) 1-x, 0.5 þ y, 1-z; (c) 1-x, 0.5 þ y, 1-z; (d) 1-x, 0.5 þ y, -z.
Fig. 2. Schematic illustration of 1-D and 1-L: (a) the left-handed helical a-chain in 1-D; (b) the right-handed helical a-chain in 1-L; (c) the left-handed helical b-chain in 1-D; (d) the right-handed helical b-chain in 1-L; (e) the 2D layer of 1-D along b-axis; (e) the 2D layer of 1-L along b-axis.
homochiral helical chains. They crystallize in chiral monoclinic space group P21 with Flack parameters of 0.002(5) and 0.018(6), respectively. The near-zero Flack parameters indicate that complexes 1-D and 1-L are homochiral. As illustrated in Fig. 1, the mirror-image structures of 1-D and 1-L showed their enantiomeric nature. So we present mainly the detailed geometric feature of 1-D as a representative. The asymmetric unit of complex 1-D consists of a Zn(II) ion, a deprotonated (R)-CBA2- ligand, a 1,3-DIB ligand and a guest water molecule. The tetrahedral Zn(II) ion center is coordinated by two carboxylic oxygen atoms from two (R)-CBA2- ligands and two nitrogen atoms from two 1,3-DIB ligands.
difference Fourier syntheses and were refined anisotropically. The hydrogen atoms were placed on their calculated positions and treated as riding atoms with default parameters. The crystal and refinement data are summarized in Table 1, while some selective bond distances and angles are listed in Table S1. 3. Results and discussion 3.1. Structures of 1-D and 1-L The single-crystal structure analysis revealed that complexes 1-D and 1-L are 3D supramolecular frameworks containing three kinds of
3
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Journal of Solid State Chemistry 279 (2019) 120967
Fig. 3. Schematic illustration of 1-D and 1-L: (a) the right-handed helical c-chain in 1-D; (b) the left-handed helical c-chain in 1-L; (c) the 2D layer of 1-D along c-axis; (d) the 2D layer of 1-L along c-axis.
and 1,3-DIB ligands are simple linkers. Thus, the framework of 1-D can be topologically represented as a 4-connected sql net with point symbol of (44.62) (Fig. S5) [48].
Three types of helical chains (named as a-chain, b-chain and c-chain) are the striking features of 1-D and 1-L, which were dictated by the chirality of CBA2 ligands (Figs. 2 and 3). As illustrated in Fig. 2a–b, the Zn2þ ions and 1,3-DIB ligands connected together to build left-handed helical a-chain and right-handed helical a-chain along b-axis in compounds 1-D and 1-L, respectively. Moreover, the Zn2þ ions were also bridged by (R)-CBA2- ligands to form left-handed helical b-chain in 1-D, while (S)-CBA2- ligands and Zn2þ ions formed the opposite right-handed helical b-chain in 1-L (Fig. 2b). According to the ratio of 1:1, the a-chains and b-chains construct the 2D layers of 1-D and 1-L along b-axis (Fig. 2c and d). Besides the helical a-chain and b-chain, the (R)-CBA2- ((S)-CBA2-) ligands, Zn2þ ions and 1,3-DIB ligands can also linked together to form right-handed (left-handed) helical c-chain along c-axis in 1-D (1-L) (Fig. 3a–b). The helical c-chain contains four Zn2þ ions, two (R)-CBA2((S)-CBA2-) ligands and two 1,3-DIB ligands per turn, and the pitch lengths is identical to the c-axis length. Similar to helical a-chain and bchain, the helical c-chains can also link together to generate 2D layer framework in 1-D and 1-L along the c-axis, respectively (Fig. 3c–d). The adjacent 2D layers were further packed together to obtain a 3D supramolecular framework (Fig. 4). From the viewpoint of structural topology, the Zn(II) ions can be regarded as 4-connected nodes, while the (R)-CBA2-
Fig. 4. The 3D supramolecular framework of 1-D packed by 2D helical layers. 4
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Journal of Solid State Chemistry 279 (2019) 120967
Fig. 5. Coordination environment of enantiomers 2-D and 2-L. Symmetry codes: (a) 1 þ x, 1 þ y, z; (b) 1 þ x, 2 þ y, 1 þ z; (c) x, 1 þ y, z.
Fig. 6. 3D framework (a) and 4-connected net of complex 2-D (b).
In the framework, the Zn(II) ions act as 4-connected nodes, and the (R)CBA2- ligands and 1,3-DIB ligands are simple linkers. So the whole framework of 2-D should be simplified as a cds net with the point symbol of (65.8) (Fig. 6b) [48].
3.2. Structures of 2-D and 2-L The complexes 2-D and 2-L are also a pair of enantiomers, crystallizing in triclinic chiral space group P1 with Flack parameters of 0.009(5) and 0.012(5), respectively. Herein, we also mainly discuss the structure of 2-D in detail. The asymmetry unit of 2-D contains two zinc ions, two (R)-CBA2- ligands and two 1.3-DIB ligands. Zn1 ion center adopts a slightly distorted tetrahedral geometry connecting two carboxylate oxygen atoms from two (R)-CBA2- ligands and two nitrogen atoms from two 1,3-DIB ligands. Difference from Zn1 ion, Zn2 ion takes on a distorted pentagonal bipyramid geometry definited by three carboxylic oxygen atoms from two (R)-CBA2- ligands and two nitrogen atoms from two 1,3-DIB ligands (Fig. 5). Based on the two coordinated modes, each zinc ion connect two (R)CBA2- ligands and two 1,3-DIB ligands to form a 3D framework (Fig. 6a).
3.3. Reaction temperature effect on the assembly of HMOFs As far as complexes 1-D, 1-L, 2-D and 2-L, the same ligands and metal ions have constructed different frameworks at different reaction temperature. In complexes 1-D and 1-L, the Zn(II) ions were only coordinated by four atoms at 90 C. When the reaction temperature went up to 120 C, Zn(II) ions changed their four coordinations to the coexistence of four and five coordinations in complexes 2-D and 2-L. Furthermore, the guest water molecules are also found to disappear from the frameworks. These results show that high reaction temperature tends to increase the 5
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Journal of Solid State Chemistry 279 (2019) 120967
Fig. 7. The solid-state CD spectra of complexes 1-D, 1-L (a), 2-D and 2-L (b).
been carried out in KCl plates between 200 and 600 nm. The complex 1-D shows a weak positive CD signal at 236 nm and a strong positive CD signal at 278 nm, and a mirror image is observed for 1-L at the same position (Fig. 7a). The CD spectra of 2-D and 2-L are also mirror images of each other, and exhibit single Cotton effects at 252 nm, respectively (Fig. 7b). The above test results show that complexes 1-D and 1-L, and 2D and 2-L are enantiomers, respectively, which are consistent with the structures identified by single crystal structural analysis.
coordination number of the central metal ion and the dimensionality of the HMOFs, and at the same time, to reduce guest molecules. Finally, the dimensionality of HMOFs can be regulated by adjusting the reaction temperature. 3.4. PXRD and TG analysis The powder X-ray diffractions (PXRD) experiments were carried out to examine the purity of 1-D, 1-L, 2-D and 2-L. As shown in Figs. S6–7, the peak positions of the experiments and simulated PXRD patterns are in good agreement with each other, demonstrating that the bulk crystal products are truly representative of the crystal structures. The thermal behaviors of 1-D, 1-L, 2-D and 2-L were also measured between room temperature and 800 C to check their thermal stabilities (Figs. S8–9). The TGA curves of complexes 1-D and 1-L display the weight loss of 3.3% from room temperature to 190 C, which should be attributed to the loss of a guest water molecule (calculated, 3.5%). And then, the whole frameworks begin to decompose over 320 C (Fig. S8). Complexes 2-D and 2-L are very stable and have not any weight losses until 340 C, and then the frameworks begin to decompose (Fig. S9).
3.6. Fluorescence spectra Fluorescence properties of the d10 coordination complexes have received extensive attention for their potential applications in chemical sensors, light-emitting devices, and biomedicine. Hence, the photoluminescences spectra of complexes 1-D and 2-D were measured at room temperature. As shown in Fig. 8, complexes 1-D and 2-D have similar emission peaks with maxima at 463 nm (λex ¼ 396 nm). To better understand the above emission band, the free ligand (R)-H2CBA was also investigated, which displayed an emission band 316 nm (λex ¼ 290 nm). As compared to (S)-H2CBA, emission maxima of 1-L and 2-L have distinct red-shift, which should be ascribed to ligand-to-metal charge transfer (LMCT) or metal-to-ligand transfer (MLCT) [49].
3.5. CD spectra To further investigate the chiroptical activities of complexes 1-D, 1-L, 2-D and 2-L, their solid-state circular dichrosim (CD) experiments have
4. Conclusion In summary, two pairs of HOMFs were prepared by employing lactate derivatives as chiral ligands for assembly with 1,3-DIB and Zn2þ. The 1-D and 1-L are 2D frameworks with three types of helical chains obtained at 90 C, but complexes 2-D and 2-L are 3D frameworks obtained at 120 C。The reaction temperature as structure-directing factor can adjust the multidimensional architectures of HOMFs. This works not only synthesizes two pairs of novel HMOFs but also helps us to further understand the temperature-driven behaviors in the synthesis of HMOFs. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21761036) and the Guizhou Provincial Science and Technology Foundation (20181182). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.120967.
Fig. 8. Photoluminescence of (R)-H2CBA, complexes 1-D and 2-D. 6
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Journal of Solid State Chemistry 279 (2019) 120967
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Journal of Solid State Chemistry journal homepage: www.elsevier.com/locate/jssc
Temperature-controlled assembly of homochiral metal-organic frameworks from 2D helical layers to 3D frameworks Zhong-Xuan Xu *, Xu-Ling Bai, Li-Feng Li School of Chemistry and Chemical Engineering, Zunyi Normal College, Zunyi, 563002, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Homochiral metal-organic frameworks Reaction temperature Structural transformation
Two pairs of homochiral metal-organic frameworks (HOMFs), {[Zn((R)-CBA) (1.3- DIB)]⋅H2O}n(1-D), {[Zn((S)CBA) (1.3-DIB)]⋅H2O}n(1-L), [Zn2((R)-CBA)2 (1.3-DIB)2]n (2-D) and [Zn2((S)-CBA)2(1.3-DIB)2]n (2-L), have been synthesized by using lactate derivatives (R)/(S)-H2CBA and N-donor ancillary 1,3-DIB ligands at different reaction temperatures (H2CBA ¼ 4-(1-carboxyethoxy)benzoic acid, 1,3-DIB ¼ 1,3-di(1H-imidazole-1-yl) benzene). Complexes 1-D and 1-L are two-dimensional (2D) layers based on three kinds of chiral helical chains, while complexes 2-D and 2-L are four-connected three-dimensional (3D) frameworks with cds nets. The resulting homochiral complexes have been characterized by PXRD, TGA, IR and CD measurements. Additionally, complexes 1-D and 2D exhibit different photoluminescence behaviors in solid sate compared to ligand (R)-H2CBA. The work has shown that the dimensionality of HMOFs can be regulated by the reaction temperature.
1. Introduction Homochiral metal-organic frameworks (HMOFs) have been the subject of immense interest in the past decades, not only for their fascinating architectures but also their potential applications in chiral separation and asymmetry catalysis [1–6]. Up to now, three general strategies have been employed to construct HMOFs, including use of chiral ligands, spontaneous resolution and asymmetric induction [7–13]. The enantiopure compounds as the primary linkers can impart their chirality to the frameworks to generate homochirality, which has been proved to be the most direct and effective synthetic strategy for HMOFs [14–17]. Therefore, choice of chiral ligands is a key factor to construct HMOFs. However, the reaction temperature, solvent system, pH value and counter ions also can influence the formation of MOFs besides the ligands and metal ions. So it is still difficult to rationalize what reaction conditions obtain target MOFs [18–40]. Among these reaction conditions, the temperature can cause organic ligands to adopt different conformations, coordination modes and so on. Consequently, the reaction temperatures have remarkable influence on the topology and dimensionality of the metal-organic framework structures [41,42]. Furthermore, hydro/solvothermal reactions systems often used to construct metal-organic framework, so the reaction temperature plays an important role in the synthesis of metal-organic frameworks [43,44]. However, to the best of our knowledge, the temperature-controlled structural changes in HMOFs is still quite rare and extremely challenging.
Containing lactic acid and benzoic acid units, the (R)-H2CBA and (S)H2CBA has semirigid skeleton and multiple coordination modes, providing a convenient approach to construct HMOFs [45]. In this work, we continued to employ (R)-H2CBA and (S)-H2CBA as chiral ligands to build HMOFs in the presence of rigid auxiliary nitrogenous ligand 1, 3-DIB. Under hydrothermal reaction condition, two pairs of entirely different HMOFs, namely, {[Zn((R)-CBA) (1.3-DIB)]⋅H2O}n(1-D), {[Zn((S)-CBA) (1.3-DIB)]⋅H2O}n(1-L), [Zn2((R)-CBA)2 (1.3-DIB)2]n (2-D) and [Zn2((S)-CBA)2 (1.3-DIB)2]n (2-L), were synthesized by using the same starting reaction mixture at different reaction temperature (Scheme 1). Their syntheses, crystal structures, chiralities and properties of solid-state photoluminescence are reported below. 2. Experimental section 2.1. General procedures The chiral ligands (R)-H2CBA and (S)-H2CBA were synthesized according to the documented procedures [45]. All other chemical reagents and solvents were purchased commercially and used without further purification. The elemental analysis was performed on a Perhin-Elmer 240C elemental analyzer. The IR spectra were measured on a FTIR-650 FT-IR spectrometer with KBr pellets from 400 to 4000 cm1. Thermal stability studies were performed on a NETSCHZ STA-F3 thermoanalyzer with a heating rate of 20 C/min in N2. The phase purity and crystallinity
* Corresponding author. E-mail address: [email protected] (Z.-X. Xu). https://doi.org/10.1016/j.jssc.2019.120967 Received 21 August 2019; Received in revised form 17 September 2019; Accepted 18 September 2019 Available online 19 September 2019 0022-4596/© 2019 Elsevier Inc. All rights reserved.
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2.3. Synthesis of {[Zn((S)-CBA) (1.3-DIB)]⋅H2O}n(1-L) The same procedure as 1-D, except (S)-H2CBA replaced (R)-H2CBA. Colorless block crystals of 1-L were obtained with a yield of 30% (based on (S)-H2CBA). Anal. Calcd. for C22H20N4O6Zn (%): C, 52.66; H, 4.02; N, 11.17. found (%): C, 49.82; H, 3.84; N, 10.06. IR (KBr, cm1): 3358.6(w), 3137.0(w), 1603.5(s), 1564.4(m), 1512.1(s), 1388.3(s), 1290.5(m), 1231.4(s), 1172.7(w), 944.4(w), 853.3(m), 781.5(m),742.3(w), 657.3(m), 494.4(w). 2.4. Synthesis of [Zn2((R)-CBA)2(1.3-DIB)2]n(2-D) The same procedure as 1-D, except temperature is 120 C. Colorless block crystals of 2-D were obtained with a yield of 50% (based on (R)H2CBA). Anal. Calcd. for C44H36N8O10Zn2 (%): C, 54.62; H, 3.75; N, 11.58. Found (%): C, 51.34; H, 3.34; N, 10.04. IR (KBr, cm1): 3437.2(w), 3110.6(w), 1642.6(s), 1597.1(s), 1512.9(m), 1388.3(m), 1231.4(m), 1172.8(w), 1140.1(w), 1062.2(m), 944.4(w),787.8(m), 676.9(w), 657.3(w).
Scheme 1. The synthetic route of complexes 1-L, 1-D, 2-L and 2-D.
of each complex was checked by powder X-ray diffraction (PXRD) using a Rigaku Dmax2500 diffractometer with Cu Kα radiation (λ ¼ 1.54056 Å) in the range of 5.00–50.00 . Solid-state photoluminescence spectra were performed on a Hitachi FL-4500 fluorescence spectrophotometer. Solid CD spectra were measured on a MOS-450 spectropolarimeter at room temperature.
2.5. Synthesis of [Zn2((S)-CBA)2(1.3-DIB)2]n(2-L) The same procedure as 2-D, except (S)-H2CBA replaced (R)-H2CBA. Colorless block crystals of 2-L were obtained with a yield of 50% (based on (R)-H2CBA). Anal. Calcd. for C44H36N8O10Zn2 (%): C, 54.62; H, 3.75; N, 11.58. Found (%): C, 52.22; H, 3.46; N, 10.26. IR (KBr, cm1): 3420.4(w), 3111.2(w), 1639.3(s), 1597.1(s), 1518.1(m), 1388.4(m), 1231.4(m), 1172.8(w), 1107.6(w), 1062.2(m), 944.4(w), 786.4(m), 676.8(w), 654.9(w).
2.2. Synthesis of {[Zn((R)-CBA) (1.3-DIB)]⋅H2O}n(1-D) Complex 1-D was synthesized by stirring Zn(BF4)2 (35 mg, 0.15 mmol), (R)-H2CBA(21 mg, 0.1 mmol), 1,3-DIB (21 mg, 0.1 mmol) and pyrazine (96 mg, 1.2 mmol) in 6 ml of N,N-dimethylacetamide/ water (1:3) solvent. The resulting mixture was placed in a 23 mL Teflon reactor and kept under autogenous pressure at 90 C for three days. After slowly being cooled to room temperature, colorless crystals were filtered and washed with ethanol and dried in the air. Yield: 40% (based on (R)H2CBA). Anal. Calcd. for C22H20N4O6Zn (%): C, 52.66; H, 4.02; N, 11.17. Found (%): C, 50.24; H, 3.62; N, 9.84. IR (KBr, cm1): 3345.8(w), 3137.0(w),1603.5(s), 1558.0(m), 1512.1(s), 1381.5(s), 1283.7(m), 1231.4(s), 1179.5(m), 1055.4(s), 944.4(w), 853.3(w), 787.8(m), 735.6(w), 657.3(m), 494.4(w).
2.6. X-ray determination All single-crystal X-ray diffraction data were collected on a Rigaku four-circle 003 CCD diffractometer equipped with graphitemonochromated Mo Kα radiation (λ ¼ 0.071073 nm) at room temperature. The crystal structures were solved and refined by full matrixes methods against F2 using SHELXL-2017 program package and Olex2-1.2 software [46,47]. All non-hydrogen atoms located in successive
Table 1 Crystal data and structure refinement details for complexes 1-D, 1-L, 2-D and 2-L. Compound reference
1-D
1-L
2-D
2-L
Chemical formula Formula Mass Crystal system a/Å b/Å c/Å α / β/ γ/ Unit cell volume/Å3 Temperature/K Space group No. of formula units per unit cell, Z Radiation type Absorption coefficient, μ/mm1 No. of reflections measured No. of independent reflections Rint Final R1a values (I > 2σ (I)) Final wRb(F2) values (I > 2σ (I)) Final R1 values (all data) Final wR(F2) values (all data) Goodness of fit on F2 Flack parameter CCDC number
C22H18N4O5Zn⋅H2O 501.79 Monoclinic 7.8700(10) 18.9756(12) 8.2285(7) 90 114.691(14) 90 1116.5(2) 293(2) P21 2 MoKα 1.146 12907 5670 0.0298 0.0303 0.0725 0.0380 0.0756 1.001 0.017(7) 1948488
C22H18N4O5Zn⋅H2O 501.79 Monoclinic 7.8629(10) 18.9525(10) 8.2151(9) 90 114.704(14) 90 1112.2(2) 293(2) P21 2 MoKα 1.151 17810 5855 0.0408 0.0318 0.0739 0.0415 0.0775 1.048 0.007(7) 1948489
0.5(C44H36N8O10Zn2)
0.5(C44H36N8O10Zn2)
483.77 Triclinic 7.9642(3) 10.5601(5) 14.1194(5) 105.445(4) 94.217(3) 109.397(4) 1062.36(8) 293(2) P1 2 MoKα 1.198 24119 10314 0.0288 0.0281 0.0698 0.0327 0.0718 0.977 0.009(5) 1948490
483.77 Triclinic 7.9727(2) 10.5668(3) 14.1226(3) 105.462(2) 94.188(2) 109.367(3) 1064.63(5) 293(2) P1 2 MoKα 1.196 28660 10746 0.0328 0.0276 0.0702 0.0324 0.0726 0.924 0.012(5) 1948491
a b
R1 ¼ Σ||Fo||Fc||Σ|Fo|. wR2 ¼ [Σw(F2o F2c )2/Σ[w(F2o)2]1/2. 2
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Fig. 1. Coordination environment of enantiomers 1-D and 1-L. Symmetric codes: (a) 1-x, 0.5 þ y, -z; (b) 1-x, 0.5 þ y, 1-z; (c) 1-x, 0.5 þ y, 1-z; (d) 1-x, 0.5 þ y, -z.
Fig. 2. Schematic illustration of 1-D and 1-L: (a) the left-handed helical a-chain in 1-D; (b) the right-handed helical a-chain in 1-L; (c) the left-handed helical b-chain in 1-D; (d) the right-handed helical b-chain in 1-L; (e) the 2D layer of 1-D along b-axis; (e) the 2D layer of 1-L along b-axis.
homochiral helical chains. They crystallize in chiral monoclinic space group P21 with Flack parameters of 0.002(5) and 0.018(6), respectively. The near-zero Flack parameters indicate that complexes 1-D and 1-L are homochiral. As illustrated in Fig. 1, the mirror-image structures of 1-D and 1-L showed their enantiomeric nature. So we present mainly the detailed geometric feature of 1-D as a representative. The asymmetric unit of complex 1-D consists of a Zn(II) ion, a deprotonated (R)-CBA2- ligand, a 1,3-DIB ligand and a guest water molecule. The tetrahedral Zn(II) ion center is coordinated by two carboxylic oxygen atoms from two (R)-CBA2- ligands and two nitrogen atoms from two 1,3-DIB ligands.
difference Fourier syntheses and were refined anisotropically. The hydrogen atoms were placed on their calculated positions and treated as riding atoms with default parameters. The crystal and refinement data are summarized in Table 1, while some selective bond distances and angles are listed in Table S1. 3. Results and discussion 3.1. Structures of 1-D and 1-L The single-crystal structure analysis revealed that complexes 1-D and 1-L are 3D supramolecular frameworks containing three kinds of
3
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Fig. 3. Schematic illustration of 1-D and 1-L: (a) the right-handed helical c-chain in 1-D; (b) the left-handed helical c-chain in 1-L; (c) the 2D layer of 1-D along c-axis; (d) the 2D layer of 1-L along c-axis.
and 1,3-DIB ligands are simple linkers. Thus, the framework of 1-D can be topologically represented as a 4-connected sql net with point symbol of (44.62) (Fig. S5) [48].
Three types of helical chains (named as a-chain, b-chain and c-chain) are the striking features of 1-D and 1-L, which were dictated by the chirality of CBA2 ligands (Figs. 2 and 3). As illustrated in Fig. 2a–b, the Zn2þ ions and 1,3-DIB ligands connected together to build left-handed helical a-chain and right-handed helical a-chain along b-axis in compounds 1-D and 1-L, respectively. Moreover, the Zn2þ ions were also bridged by (R)-CBA2- ligands to form left-handed helical b-chain in 1-D, while (S)-CBA2- ligands and Zn2þ ions formed the opposite right-handed helical b-chain in 1-L (Fig. 2b). According to the ratio of 1:1, the a-chains and b-chains construct the 2D layers of 1-D and 1-L along b-axis (Fig. 2c and d). Besides the helical a-chain and b-chain, the (R)-CBA2- ((S)-CBA2-) ligands, Zn2þ ions and 1,3-DIB ligands can also linked together to form right-handed (left-handed) helical c-chain along c-axis in 1-D (1-L) (Fig. 3a–b). The helical c-chain contains four Zn2þ ions, two (R)-CBA2((S)-CBA2-) ligands and two 1,3-DIB ligands per turn, and the pitch lengths is identical to the c-axis length. Similar to helical a-chain and bchain, the helical c-chains can also link together to generate 2D layer framework in 1-D and 1-L along the c-axis, respectively (Fig. 3c–d). The adjacent 2D layers were further packed together to obtain a 3D supramolecular framework (Fig. 4). From the viewpoint of structural topology, the Zn(II) ions can be regarded as 4-connected nodes, while the (R)-CBA2-
Fig. 4. The 3D supramolecular framework of 1-D packed by 2D helical layers. 4
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Fig. 5. Coordination environment of enantiomers 2-D and 2-L. Symmetry codes: (a) 1 þ x, 1 þ y, z; (b) 1 þ x, 2 þ y, 1 þ z; (c) x, 1 þ y, z.
Fig. 6. 3D framework (a) and 4-connected net of complex 2-D (b).
In the framework, the Zn(II) ions act as 4-connected nodes, and the (R)CBA2- ligands and 1,3-DIB ligands are simple linkers. So the whole framework of 2-D should be simplified as a cds net with the point symbol of (65.8) (Fig. 6b) [48].
3.2. Structures of 2-D and 2-L The complexes 2-D and 2-L are also a pair of enantiomers, crystallizing in triclinic chiral space group P1 with Flack parameters of 0.009(5) and 0.012(5), respectively. Herein, we also mainly discuss the structure of 2-D in detail. The asymmetry unit of 2-D contains two zinc ions, two (R)-CBA2- ligands and two 1.3-DIB ligands. Zn1 ion center adopts a slightly distorted tetrahedral geometry connecting two carboxylate oxygen atoms from two (R)-CBA2- ligands and two nitrogen atoms from two 1,3-DIB ligands. Difference from Zn1 ion, Zn2 ion takes on a distorted pentagonal bipyramid geometry definited by three carboxylic oxygen atoms from two (R)-CBA2- ligands and two nitrogen atoms from two 1,3-DIB ligands (Fig. 5). Based on the two coordinated modes, each zinc ion connect two (R)CBA2- ligands and two 1,3-DIB ligands to form a 3D framework (Fig. 6a).
3.3. Reaction temperature effect on the assembly of HMOFs As far as complexes 1-D, 1-L, 2-D and 2-L, the same ligands and metal ions have constructed different frameworks at different reaction temperature. In complexes 1-D and 1-L, the Zn(II) ions were only coordinated by four atoms at 90 C. When the reaction temperature went up to 120 C, Zn(II) ions changed their four coordinations to the coexistence of four and five coordinations in complexes 2-D and 2-L. Furthermore, the guest water molecules are also found to disappear from the frameworks. These results show that high reaction temperature tends to increase the 5
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Journal of Solid State Chemistry 279 (2019) 120967
Fig. 7. The solid-state CD spectra of complexes 1-D, 1-L (a), 2-D and 2-L (b).
been carried out in KCl plates between 200 and 600 nm. The complex 1-D shows a weak positive CD signal at 236 nm and a strong positive CD signal at 278 nm, and a mirror image is observed for 1-L at the same position (Fig. 7a). The CD spectra of 2-D and 2-L are also mirror images of each other, and exhibit single Cotton effects at 252 nm, respectively (Fig. 7b). The above test results show that complexes 1-D and 1-L, and 2D and 2-L are enantiomers, respectively, which are consistent with the structures identified by single crystal structural analysis.
coordination number of the central metal ion and the dimensionality of the HMOFs, and at the same time, to reduce guest molecules. Finally, the dimensionality of HMOFs can be regulated by adjusting the reaction temperature. 3.4. PXRD and TG analysis The powder X-ray diffractions (PXRD) experiments were carried out to examine the purity of 1-D, 1-L, 2-D and 2-L. As shown in Figs. S6–7, the peak positions of the experiments and simulated PXRD patterns are in good agreement with each other, demonstrating that the bulk crystal products are truly representative of the crystal structures. The thermal behaviors of 1-D, 1-L, 2-D and 2-L were also measured between room temperature and 800 C to check their thermal stabilities (Figs. S8–9). The TGA curves of complexes 1-D and 1-L display the weight loss of 3.3% from room temperature to 190 C, which should be attributed to the loss of a guest water molecule (calculated, 3.5%). And then, the whole frameworks begin to decompose over 320 C (Fig. S8). Complexes 2-D and 2-L are very stable and have not any weight losses until 340 C, and then the frameworks begin to decompose (Fig. S9).
3.6. Fluorescence spectra Fluorescence properties of the d10 coordination complexes have received extensive attention for their potential applications in chemical sensors, light-emitting devices, and biomedicine. Hence, the photoluminescences spectra of complexes 1-D and 2-D were measured at room temperature. As shown in Fig. 8, complexes 1-D and 2-D have similar emission peaks with maxima at 463 nm (λex ¼ 396 nm). To better understand the above emission band, the free ligand (R)-H2CBA was also investigated, which displayed an emission band 316 nm (λex ¼ 290 nm). As compared to (S)-H2CBA, emission maxima of 1-L and 2-L have distinct red-shift, which should be ascribed to ligand-to-metal charge transfer (LMCT) or metal-to-ligand transfer (MLCT) [49].
3.5. CD spectra To further investigate the chiroptical activities of complexes 1-D, 1-L, 2-D and 2-L, their solid-state circular dichrosim (CD) experiments have
4. Conclusion In summary, two pairs of HOMFs were prepared by employing lactate derivatives as chiral ligands for assembly with 1,3-DIB and Zn2þ. The 1-D and 1-L are 2D frameworks with three types of helical chains obtained at 90 C, but complexes 2-D and 2-L are 3D frameworks obtained at 120 C。The reaction temperature as structure-directing factor can adjust the multidimensional architectures of HOMFs. This works not only synthesizes two pairs of novel HMOFs but also helps us to further understand the temperature-driven behaviors in the synthesis of HMOFs. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21761036) and the Guizhou Provincial Science and Technology Foundation (20181182). Appendix A. Supplementary data Supplementary data to this article can be found online at https://do i.org/10.1016/j.jssc.2019.120967.
Fig. 8. Photoluminescence of (R)-H2CBA, complexes 1-D and 2-D. 6
Z.-X. Xu et al.
Journal of Solid State Chemistry 279 (2019) 120967
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