LIU Chng-Qing YANG Jin-Xi ZHANG Xin QIN Ye-Yn YAO Yun-Gen

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Organic Carboxylate Ligand Tuned Topological Variations in Three Zn(II) Coordination Polymers: Syntheses, Crystal Structures and Photoluminescent Properties①

LIU Chang-Qinga, bYANG Jin-XiaaZHANG XinaQIN Ye-YanaYAO Yuan-Gena②

a(350002)b(100039)

Three new Zn(II) coordination polymers, namely [Zn2(suc)2(bib)2]n·nH2O (1), [Zn(glu)(bib)]n·4nH2O (2), and [Zn(adp)(bib)]n(3) (bib = 1,4-bis(N-imidazolyl)butane, H2suc = succinic acid, H2glu = glutaric acid, H2adp = adipic acid) have been hydrothermally synthesized and structurally characterized.Compound 1 features a 3D framework with 4-connected hxg-d topological network, compound 2 is comprised of 2D 44-sql-type sheets, and the adjacent 2D sheets are further packed into a 3D supramolecular architecture via intermolecular hydrogen- bonding interactions, and compound 3 is a 3D framework with 4-fold interpenetrating dia topology.The structural comparison of these three compounds demonstrates that the topological variations can be well controlled by employing aliphatic dicarboxylate ligands with different spacer lengths.Moreover, the thermal stabilities and photoluminescent properties of them were also studied in detail.

zinc(II) compound, aliphatic dicarboxylate ligands, hydrothermal reaction, meso-helix, hydrogen bond, photoluminescence;

1 INTRODUCTION

Coordination polymers (CPs), as an emerging class of functional materials, are getting more and more attention because of their appealing frame- works as well as huge potential applications in luminescence sensing, magnetism, gas adsorption and selective separation, heterogeneous catalysis, and so on[1-7].During the self-assembly process, there exist several unpredictable factors, such as coordina- tion geometry of the central metal ion, solvent system, the conformation of organic ligand, pH value, reaction temperature, directing agent and so on, that can significantly affect the final molecular architec- ture, which makes controllable syntheses of crys- talline materials with desired frameworks and properties more difficult[8-11].To realize the con- trollable synthesis at molecular level, the selection of an appropriate synthetic strategy and organic ligand is crucial to construct the extended CPs.

Among the widely used strategies, mixed-ligand self-assembly strategy is one of the most effective methods to construct the CPs with predictable struc- tures[12-16].Generally speaking, mixed carboxylate and dipyridyl or diimidazolyl-based ligands are the preferred organic build blocks in the crystal engineering.Inspired by this strategy, a series of Zn(II) and Cd(II) CPs based on pyridyl or imidazolyl based bridging ligands and aliphatic dicarboxylic acids have been successfully synthesized by our group[17-20].To further expand the study, in this work, we selected three flexible aliphatic dicarboxylic acids with different spacer lengths as the main ligands and 1,4-bis(N-imidazolyl)butane as the auxiliary ligand to assemble the Zn(NO3)2, successfully obtaining three new Zn(II) coordination polymers of 1, 2 and 3.Single-crystal X-ray diffraction analyses revealed that compound 1 features a 3D 4-connected hxg-d topological framework, compound 2 features a 2D 44-sql-type topological framework and compound 3 displays a 3-fold interpenetrating dia topological framework.

2 EXPERIMENTAL

2.1 Materials and equipments

All chemicals were purchased commercially and used without further purification except for the bib ligand.The bib ligand was prepared according to the literature procedures[21].The FT-IR spectra were recorded from KBr pellets in the range of 400～4000 cm-1on a Nicolet Magna 750 FT-IR spectrometer.PXRD patterns were taken on a Rigaku Dmax2500 X-ray diffractometer (Cu-radiation,= 1.54056 ?) with a step size of 0.05°.TGA analysis was measured on a Netzsch STA 449C thermal analyzer at a heating rate of 10 °C·min?1under a nitrogen atmosphere.Elemental analyses (C, H and N) were performed on an EA1110 CHNS-0 CE Elemental Analyzer.Fluorescence spectra of the solid samples were performed on an Edinburgh Analytical instrument FLS920.

2.2 Synthesis of {[Zn2(suc)2(bib)2]·H2O}n (1)

A mixture of Zn(NO3)2·6H2O (60 mg, 0.2 mmol), H2suc (0.024 g, 0.2 mmol), bib (0.039 g, 0.2 mmol), and NaHCO3(0.034, 0.4 mmol) in a molar ratio of 1:1:1:2 and 10 mL H2O was placed in a 23 mL Teflon-lined stainless vessel, then the vessel was sealed and heated to 110 oC.The temperature was held for 60 h, then the vessel was cooled to room temperature over 60 h to lead to the formation of colorless block crystals of 1 (yield: 72% based on Zn).Anal.Calcd.for 1 (C28H38N8O9Zn2): C, 44.13; H, 4.99; N, 14.71%.Found: C, 44.38; H, 5.18, N, 14.48%.IR (solid KBr pellet,/cm-1) for compound 1: 3111(m), 1622(m), 1560(s), 1536(s), 1523(m), 1471(w), 1444(w), 1387(s), 1296(w), 1243(m), 1181(m), 1104(s), 1037(w), 951(m), 883(w), 846(m), 763(m), 753(m), 659(m), 626(m).

2.3 Synthesis of {[Zn(glu)(bib)]·4H2O}n (2)

Compound 2 was synthesized in a similar manner to 1 except that H2glu was used instead of H2suc.Colorless block-like crystals were isolated in 48% yield (based on Zn).Anal.Calcd.for 2 (C15H28N4O8Zn): C, 39.32; H, 6.12; N, 12.23%.Found: C, 39.58; H, 5.98, N, 12.48%.IR (solid KBr pellet,/cm-1) for compound 2: 3128(s), 2944(s), 1605(s), 1535(w), 1460(w), 1446(w), 1407(m), 1374(m), 1340(m), 1265(w), 1149(w), 1108(m), 1093(m), 1062(w), 954(m), 872(w), 788(m), 734(m), 661(m), 628(w).

2.4 Synthesis of {Zn(adp)(bib)}n (3)

Compound 3 was synthesized in a similar manner to 1 except that H2adp was used instead of H2suc.Colorless block-like crystals were isolated in 71% yield (based on Zn).Anal.Calcd.for 3 (C16H22N4O4Zn): C, 48.03; H, 5.50; N, 14.01%.Found: C, 47.89; H, 5.76, N, 14.33%.IR (solid KBr pellet,/cm-1) for compound 3: 3125(s), 2950(m), 1616(s), 1531(s), 1467(w), 1443(m), 1400(s), 1341(w), 1306(m), 1292(s), 1240(m), 1149(w), 1109(s), 1034(w), 949(s), 916(s), 856(m), 791(m), 731(m), 657(s), 636(m).

2.5 Structure determination

The single-crystal structure data for compounds 1～3 were performed on an Oxford Xcalibur E diffractometer (Mo-radiation,= 0.71073 ?, graphite monochromator) at room temperature.Absorption corrections were applied using the SADABS program[22].The structure was solved by direct methods and refined by full-matrix least- squares on2using the SHELXL-97 program[23].All of the non-hydrogen atoms were refined anisotro- pically, and the C-bound H atoms were generated by a riding model on idealized positions.The hydrogen atoms of lattice water molecules of 2 were located from successive Fourier syntheses.However, for 1, no appropriate hydrogen atoms of disordered lattice water molecules were obtained.Crystal data and structure refinement details are summarized in Table 1.Experimental details for the structure determination are presented in Table 1.Selected bond lengths and bond angles are listed in Table S1～S3.

Table 1. Summary of Crystal Data and Structure Refinements for 1～3

=?||F|–|F||/?|F|,wR= [Σ(F2–F2)2/Σ(F2)2]1/2

3 RESULTS AND DISCUSSION

3.1 Description of structure 1

Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the noncentrosym- metric space groupwith a Flack parameter[24, 25]of –0.01(3), indicating a large degree of enantiomeric purity within the crystal.The asymmetric unit of 1 contains two independent Zn(II) cations, two suc2-ligands, two bib ligands and one lattice water molecule (Fig.1a).The two kinds of Zn atoms all take the distorted [ZnN2O2] tetrahedral geometriescoordinating to two oxygen atoms from two individual suc2-ligands and two nitrogen atoms from two different bib ligands.The Zn–O/N bond lengths are in the range of 1.931(12)～2.001(12) ?, and the O/N–Zn–O/N bond angles vary from 94.7(6) to 120.7(7)°, which are similar to those reported in other Zn(II) compounds[19, 20].

Two kinds of bib ligands displayandconformations, respectively.They act as bidentate ligands coordinated to the Zn(II) centers to form an infinite-helix with a pitch of 35.419 ? (Fig.1c).Similarly, bis-monodenate suc2-ligands connect Zn(II) ions into a [Zn(suc)]nsingle-stranded- helix with a pitch of 13.389 ? (Fig.1b).Two types of-helix share the Zn(II) centers to generate a 3D framework (Fig.1d).Topological analysis of com- pound 1 reveals that it is a 4-connected uninodal hxg-d network with point symbol {65·8} (Fig.1e).Because of the flexibility of the suc2-and bib ligand, a non-centrosymmetric {ZnO2N2} center is formed, and combined with the symmetrical bridging ligands which show an unsymmetrical linkage.The arrange- ment of components crystallized into an acentric structure.

Fig.1. (a) Coordination environment of the Zn(II) atom in 1 (Symmetry codes: a = 0.5+, 0.5+,; b = 1.5+, 0.5+,; c =, 1–, 0.5+).(b) and (c) View of the meso-helical chain constructed by Zn(II) atoms and suc2-/bib ligands.(d) Perspective view of the 3D framework along the-axis.(e) Schematic view of the hxg-d topology of structure 1

3.2 Description of structure 2

When the longer ligand glu2-was introduced into the system, compound 2 was obtained.Single-crystal X-ray structural analysis reveals that the asymmetric unit of 2 consists of one independent Zn(II) cation, one glu2?anion, two half bib ligands, and four lattice water molecules.As shown in Fig.2a, each Zn(II) cation is four-coordinated by two carboxylate oxygen atoms from two glu2-anions and two nitrogen atoms from two bib molecules in a distorted tetrahedral geometry.Similar to compound 1, two kinds of bib ligands act as2-bridge coordinated to the Zn(II) centers to yield an infinite 1D-helix with a pitch of 21.494 ?, where the bib ligands exhibit theandconformation and the two imidazole rings of each bib are parallel.These-helixes are further linked by2-glu2-ligands to generate a 2D puckered (4, 4) sheet.

Fig.2. (a) Coordination environment of the Zn(II) cation of 2 (symmetry codes: a = 1+, 1+,; b = 2–, 3–, –; c = 1–, 2–, 1–).(b) (top) View of the meso-helical chain constructed by Zn(II) atoms and bib ligands; (bottom) Perspective view of the (4, 4) network connected by Zn(II) ions, glu2-, and bib ligands.(c) Parallel stacking fashion of the layered frameworks in 2.(d) View of the water hexamer and their coordination environments in 2 (Symmetry codes: e =, 1+,; f = 2?, 2?, ?; g = ?1+, ?1+,; h = 1?, 1?, ?; i= 1?, ?, ?).(e) View of the 3D supramolecular architecture of 2 formed by hydrogen-bonding interactions

A close inspection of the structure discloses a cyclic water tetramer in 2.As depicted in Fig.2d, two crystallographically independent lattice water molecules and their symmetry-related ones (O1w, O1wh, O4wg, O4wf) are interconnected with each other by strong hydrogen bonding to generate a cyclic (H2O)4water cluster.Within the cyclic water tetramer, O1w and O1wh act as both hydrogen bond acceptors and donors, and the O4wg and O4wf serve as hydrogen bond donors.Additional O2we and O2wi showing hydrogen bond acceptors bonded to O1w and O1wh to give the overall water hexamer cluster.Moreover, such water hexamer clusters are connected with carboxyl oxygen atoms (O1, O3) by H-bonding interactions to form an overall 3D supramolecular architecture.In addition, water molecules O3w acted as hydrogen bond donors to link carboxyl oxygen atoms (O2, O4) to sustain the stabilization of the 3D architecture.The H-bonding parameters are summarized in Table 2.

Table 2. Hydrogen-bonding Geometrical Parameters (?, o) of Compound 2

Symmetry codes: e =, 1+,; f = 2?, 2?, ?; g = ?1+, ?1+,; i = 1?, ?, ?; j = ?1+,,

Fig.3. (a) View of the coordinated environment of Zn(II) centers in compound 3.(b) View of the- helical chain constructed by Zn(II) atoms and adp2-ligands.(c) Perspective view of the 3D framework along the-axis.(d) An adamantanoid unit cage unit of 3.(e) Space-filling diagram and Schematic representation of the four interpenetrating adamantanoid cages

3.3 Description of structure 3

When glu2?is replaced with a longer adp2?ligand, a 4-fold interpenetrating dia structure was obtained.In compound 3, the Zn(II) center is situated on a 2-fold axis (site occupancy factor (SOF) = 0.5), and other 2-fold axis of rotation passing through the midponits of C(3)?C(3) and C(8)?C(8) bisected the adp2-anion and bib ligand; thus, the asymmetric unit comprises one-half of the formula.Each Zn(II) cation is four-coordinated, having a distorted tetrahe- dral ZnO2N2environment consisting of two N atoms from two different bib ligands, and two carboxylate oxygen atoms from two different adp2-anions.

The flexible ligand adp2-adoptsconforma- tion to link the Zn(II) ions into a single-stranded-helix with a pitch of 11.783 ? (Fig.3b), which further combines the bib ligands to give a 3D framework with the point symbol 66and the long symbol 62·62·62·62·62·62, typical of a diamondoid topology (Fig.3c).A single adamantanoid frame- work possesses maximum dimensions (the longest intracage distances across the unit along the direc- tions) of 18.78 × 40.27 × 11.78 ?3(2a × 4b × c) (Fig.3d), which allows the other three selfsame nets to penetrate, forming a 4-fold interpenetrating architec- ture without available space (Fig.3e).

3.4 Powder X-ray diffraction (PXRD) analysis and thermal stability analysis

The powder X-ray diffraction (PXRD) experi- ments were carried out to confirm the phase purity.As illustrated in Fig.S1, the measured PXRD pat- terns of 1～3 closely match their simulated spectra from single-crystal data, confirming the phase purity of the compounds.

The thermogravimetric analyses (TGA) were carried out on crystalline samples of 1～3 to study the thermal stability of these compounds.As shown in Fig.S2, compound 1 lost lattice water molecules (obsd.2.49%, calcd.2.37%) in the range of 30～120 °C, and the remaining framework is thermally stable up to 300 °C, at which the framework begins to collapse.The TG curve of compound 2 reveals a steady weight loss between room temperature and 110 °C, corresponding to the departure of lattice watermolecules (obsd.15.34%, calcd.15.73%).Upon further increasing the temperature, the organic ligands start decomposing at 280 °C, forming an unidentified pro- duct.For compound 3, there are not lattice and coor- dination water molecules in the structure, thus the TG curve shows that the structure remains stable up to 290 °C.The major weight loss between 290 and 630 °C is attributed to the decomposition of organic ligands in the product, and the remaining weight corresponds to the formation of ZnO (obsd.20.44%, calcd.19.89%).

3.5 Luminescent properties

Powdered solid samples of 1～3 were examined in order to investigate their photoluminescent properties.As shown in Fig.4, upon excitation of 360 nm, these three compounds are luminescent with the emission bands at 424 nm for 1, 425 nm for 2 and 415 nm for 3, respectively.All showed broad emission profiles with emission maxima occurring in the violet regions, and tails into the blue-violet region.The solid-state emission spectra of the free bib ligand and three kinds of aliphatic carboxylate ligands were also measured to understand the nature of the emission band.The bib ligand shows the maxima emission at 430 nm (ex= 360 nm) which presumably originate from*→or*→transition.Notably, the alipha- tic dicarboxylate ligands are non-fluorescent in the visible light range, as reported previously[26].The resemblance between the emissions of 1～3 and the free bib shows they exhibit similar emission maxima, which indicates that the emissions of 1～3 are probably attributed to the intraligand*→transi- tions.We can see that all Zn(II) atoms in 1～3 are located in a tetrahedral four-coordinated environment, and the imidazole rings of bib ligands show similar-stacking interactions, so the local environments are largely similar, perhaps resulting in similar luminescent.The observed enhancement of the inten- sity for some peaks originated from the coordination action of the dicarboxylate ligands and the N-donor ligands to the Zn(II) ions, which increases the con- formational rigidity of the ligand, thus reducing the loss of energy through a radiationless pathway[27].

Fig.4. Emission and excitation spectra (inset) of 1～3 in the solid-state at room temperature

4 CONCLUSION

In summary, three new CPs based on Zn(II) have been synthesized under hydrothermal conditions.By increasing the spacer length of dicarboxylate, three compounds of 1～3 display various structures ran- ging from 2D 44-sql sheet (2) to 3D hxg-d network (1) and 4-fold interpenetrating dia net (3).It demonstrates that the tecton elongation strategy was efficiently used to fine-tune the resultant structures of the compounds.Thus, the present work can represent a fine example for constructing functional MOFs by the rational choice of organic ligands with specific spacer groups.

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11 January 2018;

17 April 2018 (CCDC 1585770-1585772)

the "Strategic Priority Research Program" of the Chinese Academy of Sciences (XDA09030102), National Key R&D Program of China (2017YFB0307301), and the Science Foundation of Fujian Province

.E-mail: yyg@fjirsm.ac.cn

10.14102/j.cnki.0254-5861.2011-1949