Self-Assembled Zn(II) Coordination Complexes Based on Mixed V

Article pubs.acs.org/crystal

Self-Assembled Zn(II) Coordination Complexes Based on Mixed V‑Shaped Asymmetric Multicarboxylate and N‑Donor Ligands Wei Yang, Chiming Wang, Qi Ma, Xuenan Feng, Hailong Wang, and Jianzhuang Jiang* Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: Hydrothermal reaction between Zn(OAc)2·2H2O and three asymmetric semirigid V-shaped multicarboxylate ligands H3L1−3 with the help of a 4,4'-bipyridine (4,4'-bpy) or 1,4-bis(imidazol-1-ylmethyl)benzene (bix) linker led to the isolation of six new coordination polymers, including [Zn3(L1)2(4,4′-bpy)2]n·(H2O)2n (1), [Zn3(L2)2(4,4′-bpy)(H2O)2]n·(H2O)2n (2), [Zn3(L3)2(4,4′-bpy)2(H2O)4]n·(H2O)6n (3), [Zn3(L1)2(bix)3]n·(H2O)7n (4), [Zn3(L2)2(bix)3]n·(H2O)4n (5), and [Zn3(HL3)2(bix)2]n (6), where H 3L1, H 3L2, H3L3 ligands represent 3-(2-carboxyphenoxy)phthalic acid, 4-(2carboxyphenoxy)phthalic acid, 3-(4-carboxyphenoxy)phthalic acid, respectively. Single crystal X-ray diffraction analysis reveals a three-dimensional (3D) network for 1 and 3−5 but a two-dimensional (2D) structure for 2 and 6. Despite the construction from the polymetallic chains connected by the 4,4′-bpy ligands for both compounds 1 and 2, a 3D architecture was revealed for the former species while a 2D configuration for the latter one. Complex 3 contains open nanotube building units composed of sole 44-numbered metallomacrocycles. For 4, the 20-numbered metallomacrocycle subunits linked by Zn ions give a 1D chain, which further form a 3D polymeric structure with the help of the other cyclic-shaped subunits made from the bix ligands and Zn ions. A 3D framework of 5 is generated from the 2D sheets simplified as a (6,3) net bound by the bix ligands. Compound 6 shows a 2D corrugated framework simplified as a (4,4) net assembled by the bix ligand and dinuclear zinc unit as node. These results seem to suggest that the diversity in the building subunits formed in 1−6 actually originates from the intrinsic nature of the three asymmetric Vshaped tricarboxylate ligands together with the tunable coordination geometry and molecular configurations of ligands by the Ndonor ligand employed. In addition, the thermal stability and luminescence properties for the series of six complexes have also been investigated.

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INTRODUCTION Rational design and synthesis of coordination polymers have stimulated a wide range of research interests over the past few decades owing to their rich structural aesthetics and functionalities.1,2 Recently, considerable progress in coordination chemistry and crystal engineering has been achieved in functional metal−organic frameworks (MOFs) with new structural topology.3,4 However, it still remains a great and long-term challenge to predict exactly the molecular structure and property of coordination polymers because of many subtle factors involved in the formation and crystallization process such as the geometries of the metal ions (node) and ligands (linker), supramolecular interaction, and reaction conditions.5 As a consequence, understanding the assembly principle of node and linker by elaborately using a series of isomeric organic ligands with a versatile molecular framework and binding modes but slightly structural difference, where the structures for obtained coordination polymers are able to be effectively controlled and tuned, has been one focus of crystal engineering.6 Fortunately, the availability of the family of V-shaped multidentate O-donor ligands with a semirigid molecular framework containing two benzene rings connected by a nonmetallic atom (C, O, S, and N atoms) seems to provide a © XXXX American Chemical Society

good opportunity to work in this regard due to the free rotation of the benzene rings. Many interesting structures including helices and interpenetrating networks with potential applications in the field of separation, absorption, catalysts, and sensors have been constructed based on such kinds of ligands with the same number of carboxylic substituents at symmetrical positions of semirigid V-shape molecular framework.7−9 In contrast to the extensive studies over the coordination compounds formed from symmetrical V-shaped organic ligands, asymmetrical semirigid V-shaped multidentate Odonor ligands with different numbers of carboxylic substituents at each benzene ring of the central molecular framework have been relatively less investigated.10 Despite the isolation of interesting coordination polymers comprised of polymetallic chain and nanotube subunits on the basis of preliminary study employing mixed V-shaped asymmetric multicarboxylate and N-donor ligands reacting with cobalt or nickel ion, it seems still significant to provide more novel organic−inorganic hybrid complexes based on the other metal ions with a different Received: May 13, 2013 Revised: July 28, 2013

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dx.doi.org/10.1021/cg4007372 | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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configurations of ligands by the N-donor ligand employed. The thermal stability and luminescent properties of these six zinc coordination polymers have also been comparatively investigated.

assembly principle toward further clarifying the relationship between the symmetry of V-shaped multidentate O-donor ligands and the structures of coordination polymer. In the present work, semirigid V-shaped asymmetrical tricarboxylate ligand 3-(2-carboxyphenoxy)phthalic acid (H3L1) and its two derivatives 4-(2-carboxyphenoxy)phthalic acid (H3L2) and 3-(4-carboxyphenoxy)phthalic acid (H3L3) with different positional carboxyl group(s) were elaborately selected and employed to assemble zinc coordination complexes Scheme 1. In the presence of 4,4′-bipyridine (4,4′-

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EXPERIMENTAL SECTION

All the reagents and solvents employed in the present work were obtained from commercial sources and used directly without further purification. The three ligands H3L1−3 and bix linker were synthesized according to the reported procedure.10a,11,12 [Zn3(L1)2(4,4′-bpy)2]n·(H2O)2n (1). The mixture of Zn(OAc)2· 2H2O (0.132 g, 0.6 mmol), 4,4′-bpy (0.0156 g, 0.1 mmol), H3L1 (0.0302 g, 0.1 mmol), and H2O (15 mL) were sealed in 25 mL Teflonlined stainless steel reactor, heated to 150 °C, and held for 72 h. After the mixture was cooled to 50 °C at a descent rate of 10 °C/h, the oven was shut off and kept for another 10 h, leading to the separation of perfect colorless block-shaped crystals suitable for X-ray diffraction by filtration with the yield of 0.0452 g, 79% (based on L1 ligand). Anal. Calcd. for C50H34N4O16Zn3: C 52.54, H 3.00, N 4.90. Found: C 52.90, H 3.12, N 5.21. IR/cm−1 (KBr): 3070(m), 1593(s), 1535(s), 1411(m), 1387(s), 1247(s), 1073(m), 807(m). [Zn3(L2)2(4,4′-bpy)(H2O)2]n·(H2O)2n (2). Zn(OAc)2·2H2O (0.132 g, 0.6 mmol), 4,4′-bpy (0.0156 g, 0.1 mmol), H3L2 (0.0302 g, 0.1 mmol), and H2O (15 mL) as starting materials were sealed in 25 mL Teflon-lined stainless steel reactor and heated to 150 °C for 72 h, then slowly cooled to room temperature with the yield of 0.0291 g, 57% (based on L2 ligand). Anal. Calcd. for C40H30N2O18Zn3: C 46.97, H 2.96, N 2.74. Found: C 47.26, H 2.82, N 2.99. IR/cm−1 (KBr): 3380(m), 1599(s), 1575(s), 1362(s), 1269(s), 1071(m), 813(m). [Zn3(L3)2(4,4′-bpy)2(H2O)4]n·(H2O)6n (3). A mixture of Zn(OAc)2· 2H2O (0.132 g, 0.6 mmol), 4,4′-bpy (0.0156 g, 0.1 mmol), H3L3 (0.0302 g, 0.1 mmol), and H2O (15 mL) as starting materials were sealed in 25 mL Teflon-lined stainless steel reactor and heated to 140 °C for three days and then slowly cooled to room temperature. Colorless needle crystals suitable for X-ray diffraction analysis were obtained after the reactor was cooled to room temperature from reaction temperature with the yield of 0.0296 g, 46% (based on L3 ligand). Anal. Calcd. for C50H50N4O24Zn3: C 46.66, H 3.92, N 4.35. Found: C 46.61, H 3.87, N 4.33. IR/cm−1 (KBr): 3408(m), 1609(s), 1564(s), 1471(m), 1385(s), 1242(s), 1069(m), 816(m).

Scheme 1. Schematic Molecular Structures of H3L1−3 and bix Ligands

bpy) or 1,4-bis(imidazol-1-ylmethyl)benzene (bix) ligand as the secondary building block, all these three tricarboxylates (L1−3) are assembled into zinc complexes with two- and threedimensional (2D and 3D) networks including [Zn3(L1)2(4,4′bpy)2]n·(H2O)2n (1), [Zn3(L2)2(4,4′-bpy)(H2O)2]n·(H2O)2n (2), [Zn3(L3)2(4,4′-bpy)2(H2O)4]n·(H2O)6n (3), [Zn3(L1)2(bix)3]n·(H2O)7n (4), [Zn3(L2)2(bix)3]n·(H2O)4n (5), and [Zn3(HL3)2(bix)2]n (6). Systematic and comparative studies on the structures reveal that the diversity in the building subunits formed in 1−6 actually originates from the intrinsic nature of the three asymmetric V-shaped tricarboxylate ligands together with the tunable coordination geometry and molecular

Table 1. Crystal Data and Structure Refinements of Complexes 1−6

a

complex

1

2

3

4

5

6

formula FW crystal system space group a b c α β γ volume Z Dcald /g cm−3 μ/mm−1 F000 Rint I > 2θ Rw2 I > 2θ R1a Rw2b S

C50H34N4O16Zn3 1142.92 monoclinic C2/c 19.9604(9) 17.7159(6) 14.7330(7) 90.00 120.641(6) 90.00 4482.4(3) 4 1.695 1.677 2320 0.0346 0.0711 0.0620 0.0665 0.984

C40H30N2O18Zn3 1022.77 triclinic P1 9.2930(6) 10.1006(6) 10.9952(6) 99.790(4) 105.709(5) 977.52(10) 2126.11(12) 1 1.737 1.911 3245 0.0504 0.0816 0.1242 0.1303 1.062

C50H50N4O24Zn3 1287.02 monoclinic C2/c 35.892(2) 8.2785(3) 19.2066(13) 90.00 110.656(7) 90.00 5340.1(5) 4 1.598 1.426 2624 0.0564 0.1364 0.0864 0.1491 1.028

C72H70N12O21Zn3 1635.51 triclinic P1̅ 10.5680(4) 13.7987(5) 25.5505(8) 94.475(3) 97.521(3) 98.646(3) 3633.7(2) 2 1.495 1.066 1688 0.0541 0.1207 0.0932 0.1329 0.974



R1 = Σ||Fo| − |Fc||/Σ|Fo|. bRw2 = [Σw(Fo2 − Fc2)2/Σw(Fo2)2]1/2. B



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Figure 1. (A) The coordination geometry for Zn(II) atoms in 1 with the 30% probability level; all hydrogen atoms and water molecules have been omitted for clarity. (B) Nanotube unit assembled from cages. (C) The 3D framework of 1. [Symmetry codes: (i) 2 − x, y, 2.5 − z for O1A; (ii) x, 1 − y, −0.5 + z for O4B; (iii) 1 − x, y, 0.5 − z for N2C; (iv) −0.5 + x, −0.5 + y, −1 + z for O2D and O3D; (iv) 1.5 − x, −0.5 + y, −1.5 − z for O2E and O3E]. [Zn3(L1)2(bix)3]n·(H2O)7n (4). The mixture of Zn(OAc)2·2H2O (0.0439 g, 0.2 mmol), bix (0.0476 g, 0.2 mmol), H3L1 (0.0302 g, 0.1 mmol), and H2O (15 mL) was sealed in 25 mL Teflon-lined stainless steel reactor, which was heated to 120 °C and held for 72 h and then cooled to 50 °C at a descent rate of 10 °C/h. Finally, the oven was cut off and kept for another 10 h, perfect colorless block-shaped crystals suitable for X-ray diffraction analysis were separated by filtration with the yield of 0.0375 g, 47% (based on L1 ligand). Anal. Calcd. for C72H66N12O19Zn3 (4-2H2O): C 54.06, H 4.16, N 10.51. Found: C 54.45, H 3.99, N 10.21. IR/cm−1 (KBr): 3440(m), 1601(s), 1527(s), 1448(m), 1374(s), 1239(s), 1093(m), 824(s). [Zn3(L2)2(bix)3]n·(H2O)4n (5). By utilizing a similar procedure used to prepare 4 only with H3L2 (0.0302 g, 0.1 mmol) instead of H3L1 (0.0302 g, 0.1 mmol) as the starting material, colorless blocked crystals suitable for X-ray diffraction analysis were obtained after the reactor was cooled to room temperature from 120 °C with the yield of 0.0555 g, 71% (based on L2 ligand). Anal. Calcd. for C72H62N12O17Zn3: C 55.31, H 4.00, N 10.75. Found: C 55.26, H 4.01, N 10.71. IR/cm−1 (KBr): 3040(m), 1602(s), 1523(s), 1422(m), 1363(s), 1227(s), 1093(m), 840(m). [Zn3(HL3)2(bix)2]n (6). A mixture of Zn(OAc)2·2H2O (0.0439 g, 0.2 mmol), bix (0.0476 g, 0.2 mmol), H3L3 (0.0302 g, 0.1 mmol), and H2O (15 mL) as starting materials were sealed in 25 mL Teflon-lined stainless steel reactor and heated to 120 °C for three days, then slowly cooled to room temperature. Colorless blocked crystals suitable for Xray diffraction analysis were obtained after the reactor was cooled to room temperature from 120 °C with the yield of 0.0368 g, 61% (based on L3 ligand). Anal. Calcd. for C58H44N8O14Zn2: C 57.68, H 3.67, N 9.28. Found: C 57.90, H 3.73, N 9.25. IR/cm−1 (KBr): 3061(m), 1702(s), 1562(s), 1503(m), 1389(s), 1240(s), 1097(m), 854(m). Physical Measurements. Elemental analyses were carried out with an Elementary Vario El. The infrared spectroscopy on KBr pellets was performed on a Bruker Tensor-37 spectrophotometer in the region of 4000−400 cm−1. TGA was measured on a Perkin-Elmer TG-

7 analyzer heated from 30 to 800 °C under air. Steady-state fluorescence spectroscopic studies were performed on an F4500 (Hitachi). The slit width was 5.0 nm for excitation and 2.5 nm for emission. The photon multiplier voltage was 700 V. Single Crystal X-ray Diffraction Determination. Data were collected on a Oxford Diffraction Gemini E system with Mo Kα radiation (λ = 0.71073 Å). Final unit cell parameters were derived by global refinements of reflections obtained from integration of all the frame data. The collected frames were integrated by using the preliminary cell-orientation matrix. CrysAlisPro Agilent Technologies software was used for collecting frames of data, indexing reflections, and determining the lattice constants; CrysAlisPro Agilent Technologies for integration of intensity of reflections and SCALE3 ABSPACK for absorption correction, The structures were solved by the direct method (SHELXS-97) and refined by full-matrix leastsquares (SHELXL-97) on F2.13 Anisotropic thermal parameters were used for the non-hydrogen atoms and isotropic parameters for the hydrogen atoms. Hydrogen atoms were added geometrically and refined using a riding model. ‘DFIX’ command was used to restrain the hydrogen atoms of water molecules in complexes 1−5. Crystallographic data and other pertinent information for all the complexes are summarized in Table 1. Selected bond distances and bond angles with their estimated standard deviation are listed in Table S1 (Supporting Information). CCDC 784586 for 1, 938863 for 2, 709627 for 3, and 938864−938866 for complexes 4−6, respectively, contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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RESULTS AND DISCUSSION Synthesis of the Complexes 1−6. As shown in Scheme 1, three tricarboxylate ligands were chosen and used to assemble complexes with help of the N-donor ligand including 4,4′-bpy C



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coordination mode to link the crystallographically independent Zn1 and Zn2 ions, leading to a cage, Figure S1D (Supporting Information). The cage expands infinitely to generate a 1D nanotube via the linkage of carboxylic group in a syn-anti coordination mode, Figure 1C. The perspective diagram of molecular structure in this compound is a 3D metal−organic framework, Figure 1C. Crystal Structures of Complex 2. Single crystal X-ray diffraction analysis reveals that compound 2 features a 2D architecture completed by the polymetallic chains linked by 4,4′-bpy ligands. There are three Zn(II) ions, one 4,4′-bpy molecule, and two L2 ligands, and two coordination and solvent water molecules in the asymmetric unit of 2. As can be found in Figure 2A, the Zn1 and Zn3 ions locate in a distorted

and bix. In the present study, complexes 1−6 were prepared from the hydrothermal reaction between corresponding ligand (L) and Zn(OAc)2·2H2O together with N-donor secondary ligand. The reactions between corresponding tricarboxylate ligands and Zn(OAc)2·2H2O salt without any N-donor secondary ligand were also performed at a different temperature, giving only some precipitates. However, when the auxiliary N-donor ligand 4,4′-bpy or bix was introduced, perfect single crystals of six complexes were obtained. Nevertheless, introduction of the alkali (NaOH, KOH, and NH3·H2O) with a molar ratio of 3:1 to the ligand introduced for the purpose of deprotonating protons in the presence of 4,4′-bpy and bix ligands led to the isolation of all these complexes but with a reduced yield and poor crystal quality (except 6), indicating the important role of pH value on the formation and the crystallization of coordination polymers. This is also true for the reaction temperature. Actually, compounds 4−6 can be obtained only at a relative lower temperature below 120 °C. IR Spectra. In the IR spectra of complexes 1−6, the asymmetric and symmetric stretching vibrations of semirigid Vshaped ligands span in the range of 1562−1602 cm−1 and 1362−1389 cm−1, respectively, due to the formation of the Zn(II)−O coordination bond of the carboxylic oxygen atom in the ligands. This result is in line with that reported for Cd analogues based on the V-shaped ligands.14 The band observed at 1702 cm−1 for 6 indicates the incomplete deprotonation of carboxyl group(s). Crystal Structure of Complex 1. There are two kinds of crystallographically independent Zn(II) ions in 1. Zn1 ion is in a pyramidal coordination environment built from one nitrogen atom of 4,4′-bpy ligand and four oxygen atoms from carboxylic groups in the monodentate or syn-anti coordination mode. Zn2 ion locates in a distorted octahedral coordination sphere with two nitrogen atoms of 4,4′-bpy ligands occupying the two neighboring positions and four oxygen atoms at remaining positions from carboxylic groups in a syn-anti mode. Through the Zn2 ion [site occupancy factor (SOF) = 0.5] as an inverse center, Figure 1A, a linear trinuclear Zn subunit is formed by two crystallographically equivalent Zn1 and Zn1A ions in terminal positions and one Zn2 ion in the middle position, Figures S1A and S2B (Supporting Information). The neighboring trinuclear subunits are further connected by 2carboxyl groups coordinating to Zn1 and Zn2 ions, forming a 1D infinite chain surrounded by the two lateral organic ligands in a trans-coordination conformation. The linkage mode of the adjacent trinuclear Zn(II) clusters to form 1D chain of 1 is different from the previously reported Co(II) compound composed of trinuclear clusters probably due to the smaller ionic radius of Zn(II) than Co(II).10a It is worth noting that when the benzene ring with two neighboring carboxyl groups of the L1 ligand toward the right direction is located on the plane, the remaining monocarboxyl group in the remaining benzene ring points out of the plane; such a molecular configuration of the L1 ligand is defined as the cis-type, while the reverse is defined as the trans-type mode. This is also true for the other two ligands. In compound 1, the 4,4′-bpy ligands form tubelike secondary building units with the help of carboxyl groups and Zn ions. First, four 4,4′-bpy ligands, four 3-carboxylic groups in a synanti coordination mode, and eight Zn(II) ions construct a 52numbered metallomacrocycle basic building unit, Figure S1C (Supporting Information). The neighboring metallomacrocycles are connected by the carboxylic group in a syn-anti

Figure 2. (A) The coordination environment for Zn(II) atoms in 2 with 30% probability level; all hydrogen atoms and water molecules have been omitted for clarity. (B) A 2D packing structure of this compound in plane ac. [Symmetry code: (i) 1 + x, y, z for O7A, O6A, O3A, and O11A; (ii) 1 + x, −1 + y, 1 + z for N2B].

tetrahedral coordination geometry completed by one nitrogen atom from 4,4′-bpy ligand and three oxygen atoms from three carboxylic groups of L2 ligands. The distortion of tetrahedral sphere is revealed on the basis of the calculated τ4 parameter introduced by Houser (τ4 = 0.85 and 0.89 for Zn1 and Zn3 ions, respectively).15 The coordination geometry of Zn2 ion is completed by four oxygen atoms and two oxygen atoms from water molecules. The Zn−O and Zn−N bond lengths are in line with those of reported compounds consisting of O−Zn−N segenments.16 The L2 ligands in 2 adopt a trans-type and a cistype molecular conformation with a dihedral angle of 88.88° and 83.69°, respectively, between the two benzene rings in the central V-shaped molecular framework. The 2′-, 3-, and 4carboxyl groups of one L2 ligand exhibit monodentate, syn-anti, and μ2-η2-η0 coordination modes, respectively, and these carboxyl substituents in the other type of ligand employ the monodetate and syn-syn mode to assemble Zn(II) ions, D



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1D infinitely screwed ribbon chain along the b axis, Figure 3B. The neighboring metallomacrocycles are arranged approximately perpendicularly, Figure S2A (Supporting Information). Actually, there is also the other type of chain constructed from the neighboring macrocycles along the c axis, Figure S2B (Supporting Information). These 44-membered metallomacrocycles are connected by the 2-carboxyl groups of the L3 ligand to form a chain with the Zn1 ion as node. As shown in Figure 3C, the 2-carboxyl group of the above-mentioned metallamacrocycle (red color in the figure) is just like an arm coordinating to Zn2 ion of macrocycle (green color in the figure) in a monodentate mode, and the Zn2 ion of the ring (green color in the figure) is also linked by 2-carboxyl group of the bottom red ring, Figure 3D. A similar situation exists in the two rings in red and another ring in blue in the figure. Such kind of linkage in cooperation with the connection mode in the ribbon chain between the basic 44-membered ring units induces the formation of a relatively intricate 3D supramolecular architecture. All rings are stacked along the b axis, giving an open nanotube (11.48 × 16.57 Å). To the best of our knowledge, the 3D structure of complexes constructed by the sole single rings is still very scarcely reported in the coordination polymers. In this complex, the formation of a 44-membered ring is considered to associate with the L3 ligand with carboxylic groups arranged in a suitable angle. This is also different from the reported nickel complex with the nanotube unit due to the different assembly principle of the metal ions. Crystal Structure of Complex 4. The asymmetric unit of 4 comprises three kinds of Zn(II) ions, three types of chelating bix ligand, and two types of L1 ligands. For these crystallographically independent four-coordinated Zn(II) ions, their tetrahedral coordination geometries are constructed from two nitrogen atoms of bix ligands and two oxygen atoms from two monodentate carboxylic groups, Figure 4A. By introducing the bix secondary ligand, the coordination interaction between Zn(II) ions and oxygen atoms of carboxylic groups of L1 ligand expresses more information stored in this semirigid V-shaped ligand, leading to the formation of metallomacrocycle and chain subunits, Figure 4B,C. Two L1 ligands of 4 bind two symmetryrelated Zn1 or Zn3 ions in a trans-type molecular conformation, forming a 20-numbered metallomacrocycles; the 3-carboxyl groups link the Zn2 ions to give a 1D chain subunit, Figure 4C. It is worth noting that there is the other macrocylcle unit with 78-numbered atoms composed of the six bix ligands and Zn ions in this compound, Figure 4B. The two basic building units are compiled by these three Zn ions, leading to a 3D network. The difference in the structure between 1 and 4 indicates the effect of the flexibility of N-donor ligand on tuning the coordination geometry and molecular configuration of L1 ligand and therefore the structure of coordination polymer, Figure 4C.17 Crystal Structure of Complex 5. Compound 5 has been synthesized by introducing the semirigid N-donor bix instead of rigid species of 4,4′-bpy as a secondary ligand to assemble Zn complexes with L2 ligand, which exhibits a complicated 3D molecular structure with 2D coordination sheets pillared by bix ligands building units. All the three Zn ions are fourcoordinated by two nitrogen atoms of bix ligands and two oxygen atoms of different L2 ligands, Figure 5A. The six ligands and six zinc ions form an irregular hexagon, and these units further repeat into a 2D sheet. Each L2 ligand can be regarded as a three-connected node; the secondary structure can be described as a (6,3) net,18 Figure S3 (Supporting Information).

inducing the formation of an infinite 1D polymetallic chain, Figure 2B. The adjacent chains are associated by the 4,4′-bpy ligands connecting the Zn1 and Zn3 ions, giving a 2D sheet in the ac plane, Figure 2B. Crystal Structure of Complex 3. The structure of 3 features a 3D framework with tubelike subunits containing open channel assembled by stacked metallomacrocycles. There exist two types of crystallographically independent Zn(II) ions, one kind of 4,4′-bpy ligand, and one type of L3 ligand in this compound, Figure 3A. Zn1 ion is in a distorted pyramid

Figure 3. (A) The coordination sphere for Zn(II) atoms in 3 with the 30% probability level; all hydrogen atoms and water molecules have been omitted for clarity. (B) 1D ribbon chain subunit of 3. (C) The connection mode of metallomacrocycles observed in the formation nanotublar unit of 3. (D) 3D molecular framework in 3 with open nanotube units. [Symmetry codes: (i) 1 − x, −y, 1 − z for O14A; (ii) x, −1 + y, z for N4B; (iii) x, −1 − y, 0.5 + z for N8C; (iv) 1 − x, −1 − y, 1 − z for O6D and O7D].

coordination environment completed by two oxygen atoms of different carboxylic groups, two oxygen atoms from water molecules, and one nitrogen atom of 4,4′-bpy ligand. Zn2 ion is six-coordinated with a distorted octahedral geometry formed by four oxygen atoms of two bidentate chelating carboxylic groups and two nitrogen atoms from 4,4′-bpy ligands. The sole basic building unit of 3 is a 44-membered metallomacrocycle, which is constructed from four Zn(II) ions bridged by two L3 ligands and 4,4′-bpy linkers arranged in an alternate manner. The diagonal Zn1−Zn1A and Zn2−Zn2A separations are 17.204 and 18.272 Å, respectively, Figure 3B. The dihedral angle of the two benzene rings for L3 ligand is 83.29°. The adjacent metallomacrocycles are linked by Zn2 ions as node, forming a E



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Figure 4. (A) The asymmetric unit of 4 with the 30% probability level; all hydrogen atoms and water molecules have been omitted for clarity. (B) A 78-numbered metallomacrocycles in 4. (C) 3D packing structure of 4. [Symmetry codes: (i) 1 − x, −y, 1 − z for O14A; (ii) x, −1 + y, z for N2B; (iii) 1 − x, 1 − y, −z for N8C; (iv) −x, 1 − y, −z for O6D].

Each hexagon grid is filled with two bix ligands coordinating four zinc ions to stabilize the crystal structure. These sheets are further linked by two kinds of bix ligands, giving a complicated 3D structure, Figure 5B. Crystal Structure of Complex 6. As can be found in Figure 6A, this compound is composed of one kind of Zn ion, two types of bix ligands, and one partly deprotonated L3 ligand. Zn ion possesses a tetrahedral coordination geometry constructed from two nitrogen atoms of bix ligands and two oxygen atoms from 2- and 3-carboxylic groups of L3 ligands, which is grown into a dinuclear unit via an inverse center. The adjacent dinuclear blocks in a bc plane are connected by the bix ligands, forming a 2D sheet. The dinuclear Zn unit is treated as a node; the sheet can be described as a typical (4,4) net,19 Figure 6B. The neighboring sheets are further packed into a 3D supramolecular structure via the hydrogen bonding interaction between the partly deprotonated 4′-carboxyl group and deprotonated species (2-carboxyl substituent), Figure S4 (Supporting Information). Thermal Analysis and XRD Pattern. The thermal behavior for compounds 1−6 was investigated to reveal their thermal stability. TGA experiments were performed on pure

single crystal sample of 1−6 under air atmosphere with a heating rate of 10 °C/min in the range of 25−600 °C. The thermal curves are shown in Figure S5 (Supporting Information). As can be seen, complex 1 displays a slight weight loss owing to the release of one crystalline water molecule in the range from room temperature to 210 °C (obsd. 3.0%, calcd. 3.1%). The organic linkers then decompose from 330 to 500 °C (obsd. 75.4%, calcd. 75.5%). The coordination and solvent water molecules in compound 2 are found to be lost continuously in the range of 25−220 °C (obsd. 7.0%, calcd. 7.1%). For compound 3, the TGA curve indicates that the solvent water molecules are lost by the stage from room temperature to 202 °C (obsd. 8.3%, calcd. 8.4%), and the coordination water molecules are then removed in the range of 201−305 °C (obsd. 5.9%, calcd. 5.6%). In addition, the organic ligands are decomposed in the temperature range of 354−510 °C with the ZnO as remaining residue (obsd. 19.3%, calcd. 19.0%). According to the TGA curve of 4, the crystalline water molecules are lost in the range of 25−245 °C (obsd. 5.6%, calcd. 7.7% on the basis of the completely seven water molecules in 4), indicating the removal of the partly solvent water molecules at room temperature. The bix and L ligands organic components break down in the range of 290−550 °C. F



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observed in the range of 290−580 °C (obsd. 86.1%, calcd. 86.5%). In addition, the purity of the complexes 1−6 was also supported by the powder X-ray diffraction analysis result, Figure S6 (Supporting Information). Luminescent Property of Complexes 1−6. The luminescence spectra of 1−6 have been recorded in the solid state at room temperature with the excitation wavelength λex = 350 nm for the former three complexes and λex = 370 for the latter three species. Despite the use of different tricarboxylic ligands and secondary ligands for these six complexes, their solid emission spectra are very similar with the wide emission band observed at 456−461 nm, Figure 7. On the basis of

Figure 5. (A) The asymmetric unit of 5 with the 30% probability level; all hydrogen atoms and water molecules were omitted for clarity. (B) 3D supramolecular architecture of 5. [Symmetry codes: (i) x, 1 + y, z for O4A; (ii) x, −2 + y, z for N8B; (iii) −1 + x, y, −z for O7C; (iv) x, −1 + y, z for O10D]. Figure 7. Luminescence emission band of compounds 1−6.

previous result reported on the emission of metal-free H3L1−3, 4,4′-bpy, and bix ligands, the largely red-shifted emission of these complexes around 460 nm relative to metal free ligand H3L1−3 with emission at ca. 400 nm might be due to LMCT.20 The Effect of Coordination Geometry of Tricarboxylate Ligands H3L1−3 and Secondary N-donor Ligands and on the Subunit Structure of 1−6. Despite the elucidation of the slight difference in the coordination mode and conformation of these three ligands (L) with carboxyl group(s) at different positions on the semirigid V-shaped framework in different complex as detailed above, the intrinsic reason for the formation of diverse building units in 1−6 has not yet been clarified. It therefore appears necessary to further exploit the effect of the coordination geometry of the semirigid V-shaped tricarboxylate ligands as well as the secondary Ndonor ligand on the subunit structure. This will also be helpful for the rational design and synthesis of coordination polymers based on the molecular function requirement. All the compounds 1−3 (based on L and 4,4′-bpy ligands) exhibit the complicated 3D architecture. The semirigid ligands play the key role in constructing the building blocks. As can be seen in Scheme 2, the basic trinuclear metal cluster units and further 1D polymetallic chain in 1 and 2 are composed of L1 and L2 ligands, respectively, and metallomacrocycle composed of 44membered atoms in 3 is assembled from L3 ligand with the help of 4,4′-bpy. By introducing the bix ligand instead of the 4,4′bpy species, the versatile characteristics of the semirigid Vshaped ligands are further expressed in constructing the coordination polymers. The dinuclear unit, 1D chain, and metallomacrocycle are observed in complexes 4−6 (based on different L ligand and bix linker), revealing that the coordination mode and molecular configuration of mulitdentate O-donor components could be tuned by the secondary Ndonor ligand.

Figure 6. (A) The ORTEP diagram of coordination sphere for Zn(II) atom in 6 with the 30% probability level; all hydrogen atoms have been omitted for clarity. (B) A 2D coordination sheet in 6. [Symmetry code: (i) 1 − x, 2 − y, −z for O3A].

The release of water molecules in 5 was observed in the range from room temperature to 112 °C (obsd. 2.1%, calcd. 2.3%). The residual composition of this compound is then decomposed in the range of 300−590 °C (obsd. 82.4%, calcd. 82.1%). The deposition of the organic composition of 6 without any coordinated and solvent water molecule was G



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linker with cis-coordination mode. Another derivative of the L1 ligand with the 2′-carboxyl group shifted into the 4′-position of the benzene ring (namely, L3 ligand) was applied to synthesize the mixed ligand coordination compounds, and the basic 44membered metallomacrocycle units formed by L3 and 4,4′-bpy species are observed in 3, which then are connected to give the ribbon chain and nanotube secondary units depending on diverse connection modes of the asymmetrically attached carboxylic substituents and the appropriate bend angle for the semirigid V-shaped framework of L3 ligand. This also indicates that the carboxyl groups separated by a long distance seem to limit the formation of the diverse subunits for the sole semirigid V-shaped ligands. This point is further confirmed by the simple dinuclear building blocks revealed for 6, Scheme 2E. Both 4,4′-bpy and bix derivatives ligands have been well employed as an important N-donor secondary linker in cooperation with other aromatic polycarboxylic acids in preparing coordination polymers with mixed ligands, usually exhibiting a versatile coordination role and forming various subunits to tune the structure of coordination polymer.20 This is also true in the present case for the construction of zinc complexes 1−6 with asymmetric semirigid V-shaped tricarboxylate ligands as the starting material. In the former three compounds 1−3, the 4,4′-bpy ligand plays as an excellent linker to expand the low-dimensional building blocks into a 3D structure. In compounds 1, the 4,4′-bpy ligands are assembled by condensed metal ions in the polymetallic chain into another metallomacrocycle unit with the help of the carboxylic groups, first forming a distorted cuboid cage and then expanding into a 1D nanotube secondary unit. Similar to the effect of the 4,4′bpy ligand in 1, the rodlike 4,4′-bpy species of 2 connect the adjacent 1D polymetallic chains into a 2D structure. The rigid rodlike 4,4′-bpy ligand cooperates with the semrigid V-shaped ligand L3 to assemble Zn complexes, leading to the formation of the 44-numbered macrocycle and 3D structure of 3, stabilizing the crystal structure of this compound. In order to compare the effect of the secondary ligand on the macroscopic supramolecular structure of complexes, a semirigid secondary species of bix was introduced to prepare the complexes. An obvious structural difference was found between 4 in good contrast with 1. This is also true for 5 relative to 2 and 6 in comparison with 3. Despite the linker role played by the bix ligand, this

Scheme 2. The Building Units Constructed from the Three Ligands L1, L2, and L3 in 1−6a

a

A and B for 1 and 2, C, D, and E for 3, F and G for 4, H for 5, and F for 6.

Actually, the free rotation of the two benzene framework around the bridged etheric oxygen atom in these three ligands with the asymmetrically attached carboxylic groups successfully is considered to result in the diverse subunits. Despite the transtype mode employed for L1 and L2 ligands, the polymetallic chains in 1 and 2 are still fabricated due to the three asymmetric carboxyl substituents with a close distance, Figure 8A−C. The obvious difference in the assembly information stored in the L2 ligand relative to the L1 linker is absolutely expressed by using the bix secondary ligand. The compound 4 based trans-type L1 and bix ligands contains 20-numbered macrocyle subunits and a 1D chains due to the 2-, 2′-, and 3carboxyl groups of the L1 ligand with a monodentate coordination mode, unlike 2- and 3-carboxyl groups of this O-donor ligand in 1 exhibiting μ2-η1-η1 connected mode. When the L2 ligand was used with the 2- and 3-carboxyl groups of L1 linker moved into 3- and 4-positions of semirigid V-shaped skeleton to fabricate Zn coordination polymers with help of the bix ligand, the irregular macrocycles of 5 are molded by the L2

Figure 8. The coordination modes of the three ligands L1−3 in 1−6 (A for 1, B and C for 2, D for 3, E and F for 4, G and H for 5, and I for 6). H



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secondary composition exhibits two kinds of coordination modes in these three compounds. It is worth noting that the macrocycle blocks are found in 4 instead of the chain-like subunits composed of Zn ions and bix ligands with two kinds of molecular configuration in 5, Figure S7 (Supporting Information), indicating the position effect of carboxylic groups on tuning the structure of coordination polymer. The semirigid bix ligand in 6 is not coordinated with the L3 ligand to fabricate interesting subunits because the intrinsic flexibility is a disadvantage to stabilize the crystal structure of coordination polymers with a relatively flexible O-donor aromatic acid.

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CONCLUSION In summary, three asymmetric semirigid V-shaped tricarboxylate ligands have been exploited to construct various Zn(II) coordination polymers with a polynuclear metal unit, polymetallic chains, metallomacrocycle, and nanotube as secondary building units in cooperation with 4,4′-bpy or bix ligands. Correlation between the diverse subunits formed and the building unit of the V-shaped tricarboxylate ligands has been systematically and comparatively studied, revealing the important role of the coordination geometry and molecular framework of V-shaped asymmetric multicarboxylate in the formation of the functional coordination polymers.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files (CIF), diagrams of the structures of 1 and 3, bix coordination mode, refinement information on the crystal structure, selected bond distances and angles, and TGA curves of compounds 1−6. This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Key Basic Research Program of China (Grant Nos. 2013CB933402 and 2012CB224801), Natural Science Foundation of China, Beijing Municipal Commission of Education, and University of Science and Technology Beijing is gratefully acknowledged.

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REFERENCES

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Self-Assembled Zn(II) Coordination Complexes Based on Mixed V

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