Structural Diversity and Luminescence Properties of Coordination

Article pubs.acs.org/crystal

Structural Diversity and Luminescence Properties of Coordination Polymers Built With a Rigid Linear Dicarboxylate and Zn(II)/Pb(II) Ion Jhasaketan Sahu, Musheer Ahmad, and Parimal K. Bharadwaj* Department of Chemistry, Indian Institute of Technology Kanpur, 208016, India S Supporting Information *

ABSTRACT: A rigid linear ligand incorporating a carboxylic acid group at either end, p-terphenyl-4,4″-dicarboxylic acid, 2′,5′dimethyl (H2L) has been used to construct four coordination polymers with Zn(II) through solvothermal techniques. The four products formed are formulated as {[Zn3(L)2(OH)2]·8EtOH·4DMF}n (1), {[Zn(L)(bpmp)0.5]·EtOH·1/2DMF}n (2), {[(Zn)0.5(L)0.5(py)0.5]·5/2DMF·EtOH}n (3), and [Zn(L)(HNMe2)]n (4). With Pb(NO3)2 the ligand affords two polymers, {[Pb2(L)1.5(O)0.5]·EtOH·2H2O}n (5) and {[Pb3(L)3(H2O)]·6H2O}n (6). All of these compounds have been characterized by single crystal X-ray diffraction, infrared spectroscopy, thermogravimetry, elemental analysis, and powder X-ray diffraction measurements. Topological analyses reveal that complex 2 is a 6-connected pcu net, while complexes 3 and 4 are simplified as a 4-connected sql net. In the case of complexes 1 and 6, one-dimensional rod-shaped metal carboxylate based secondary building units have been obtained. However, an uncommon structure and topology have been obtained in complex 5. Surprisingly, such type of parallel ladder-like packing is not explored much in coordination polymers. Solid-state photoluminescence studies have been carried out for all of the complexes upon excitation at 347 nm at room temperature.

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INTRODUCTION

to their abundant modes of coordination to the metal ions as well as the ability to show properties such as luminescence. Although unsubstituted carboxylate ligands have been studied comprehensively, less attention has been given to the methylsubstituted polycarboxylate ligands. Introduction of methyl groups in the aromatic polycarboxylates alters the planarity which has a significant effect on the topologies of the final structure of the coordination polymers.13 Here, we have chosen a rigid and symmetric ligand, p-terphenyl-4,4″-dicarboxylic acid, 2′,5′-dimethyl (H2L) (Scheme 1), which can potentially afford various coordination modes as well as diverse architectures. It is also capable of showing luminescence due to the presence of three phenyl rings. Besides, metal ions play an important role in

1

Coordination polymers have attracted considerable attention during the last couple of decades not only due to their fascinating connectivities and topologies but also for their potential applications as functional materials in many fields such as luminescence,2 chemical sensors,3 separation,4 magnetism,5 gas adsorption,6 catalysis,7 and so on. Recently, luminescent coordination polymers have been widely examined for their practical applications in fluorescence sensors,8 nonlinear optics,9 photocatalysis,10 and biomedical imaging.11 Meanwhile, the rational design of coordination polymers is very challenging and depends on the judicious selection of linkers as well as metal ions. In this context, aromatic polycarboxylate linkers12 and their analogues have been extensively utilized to synthesize various coordination polymers having beautiful architectures with excellent physical properties. These aromatic polycarboxylates in particular are very promising candidates due © XXXX American Chemical Society

Received: March 13, 2013 Revised: April 9, 2013

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dx.doi.org/10.1021/cg400376g | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

have illustrated that the addition of different bases can play an important role in controlling the final structures of complexes 1, 3, and 4. Often, successful incorporation of various coligands having different functionalities also allow construction of frameworks with a greater variety of structural motifs, compared with using only one type of ligand.17 Introduction of pyridine-based ditopic coligands18 into the reaction medium can result in a wide variety of interesting architectures. Herein, we report four coordination polymers of Zn(II) having the formulas {[Zn3(L)2(OH)2]·8EtOH·4DMF}n (1), {[Zn(L)(bpmp)0.5]·EtOH·1/2DMF}n (2), {[(Zn)0.5(L)0.5(py)0.5]·5/2DMF·EtOH}n (3), and [Zn(L)(HNMe2)]n (4), and two Pb(II) based coordination polymers, {[Pb2(L)1.5(O)0.5]·EtOH·2H2O}n (5) and {[Pb3(L)3(H2O)]·6H2O}n (6) with the ligand, H2L.

Scheme 1. Representation of the Ligand H2L and Coligand bpmp

the formation of coordination polymers depending upon its coordination tendencies. Often d10 transition metal ions14 such as Zn(II) and Cd(II) are of main interest in luminescence studies due to the lack of a quenching effect such as paramagnetic transition metal ions and high transparency in the UV region. Pb(II) are also used in luminescence coordination polymers. 15 In addition to this, reaction conditions are another important factor affecting the final architecture of the coordination polymers.16 For example, we Table 1. Crystal and Structure Refinement Data for 1−6 complexes

1

2

3

empirical formula formula wt crystal system space group a, Å b, Å c, Å α (°) β (°) γ (°) U, Å3 Z ρcalc g/cm3 μ, mm−1 F(000) refl collected independent refl GOF final R indices [I > 2σ(I)] R indices (all data) complexes

C51H51NO13Zn3 1082.0387 monoclinic C2/m 29.411(4) 24.104(7) 5.966(5) 90.000 100.043(5) 90.000 4165(4) 2 0.731 0.885 932 11264 2398 0.917 R1 = 0.0799, wR2 = 0.2025 R1 = 0.1052, wR2 = 0.2147 4

C33.5H35.5N2.5O5.5Zn 626.5142 triclinic P1̅ 9.728(4) 11.672(5) 14.554(5) 93.466(5) 96.367(5) 93.175(5) 1636.1(12) 2 1.104 0.781 564 8714 4953 1.076 R1 = 0.0523, wR2 = 0.1407 R1 = 0.0617, wR2 = 0.1563 5

C36.5H44.5N3.5O7.5Zn 717.6215 monoclinic C2/m 10.547(5) 19.841(5) 14.599(5) 90.000 102.868(5) 90.000 2978.3(19) 4 1.090 0.851 1008 8026 1752 0.986 R1 = 0.0884, wR2 = 0.2161 R1 = 0.1231, wR2 = 0.2492 6

empirical formula formula wt crystal system space group a, Å b, Å c, Å α (°) β (°) γ (°) U, Å3 Z ρcalc g/cm3 μ, mm−1 F(000) refl collected independent refl GOF final R indices [I > 2σ(I)] R indices (all data)

C24H23NO4Zn 454.8079 orthorhombic Pbca 13.887(5) 9.302(5) 32.649(5) 90.000 90.000 90.000 4217(3) 8 1.429 1.195 1880 21191 3280 1.053 R1 = 0.0472, wR2 = 0.1253 R1 = 0.0561, wR2 = 0.1335

C70H68O19Pb4 2041.9990 monoclinic C2/c 40.205(5) 6.984(7) 26.098(5) 90.000 118.408(5) 90.000 6446(5) 4 2.094 10.491 3832 22245 5451 1.042 R1 = 0.0244, wR2 = 0.0582 R1 = 0.0290, wR2 = 0.0599

C66H62O19Pb3 1780.7152 monoclinic Cc 35.402(5) 15.813(6) 11.424(5) 90.000 90.361(5) 90.000 6395(4) 4 1.735 7.938 3176 19164 7377 1.088 R1 = 0.0300, wR2 = 0.0755 R1 = 0.0306, wR2 = 0.0760

B



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allowed to cool to room temperature at the rate of 1 °C per minute that afforded block-shaped colorless crystals of complex 2 in ∼42% yield. The crystals were washed with DMF, EtOH followed by acetone and air-dried. Anal. Calc. for C33.5H35.5N2.5O5.5Zn: C, 64.22; H, 5.71; N, 5.59%. Found: C, 64.02; H, 5.43; N, 5.47%. IR (KBr, cm−1): 3410(m), 3048(w), 3012(w), 2934(m), 2812(m), 1932(w), 1813(w), 1712(s) 1674(s),1631(s), 1619(s), 1610(s), 1524(m), 1409(s), 1297(m), 1177(m), 1014(m), 981(w), 796(m), 764(m), 714(m). {[(Zn)0.5(L)0.5(py)0.5]·5/2DMF·EtOH}n (3). A mixture containing Zn(NO3)2·6H2O (20 mg, 0.06 mmol), H2L (20 mg, 0.06 mmol), 0.1 mL of pyridine in 2 mL N,N-dimethylformamide (DMF) and 1 mL of EtOH were sealed in a Teflon-lined steel autoclave and heated under autogenous pressure to 100 °C for 48 h. Then it was allowed to cool to room temperature at the rate of 1 °C per minute to produce block-shaped colorless crystals of complex 3 in ∼58% yield . The crystals were repeatedly washed with DMF followed by acetone and air-dried. Anal. Calc. for C36.5H44.5N3.5O7.5Zn: C, 61.09; H, 6.25; N, 6.83%. Found: C, 61.32; H, 6.53; N, 6.47%. IR (KBr, cm−1): 3611(w), 3465(m), 3008(w), 2924(w), 2863(w), 1660(s), 1606(s), 1487(m), 1450(m), 1406(s), 1253(w), 1117(m), 1070(m), 1177(m), 796(m), 767(m), 715(m). [Zn(L)(HNMe2)]n (4). A mixture containing Zn(NO3)2·6H2O (20 mg, 0.06 mmol), H2L (20 mg, 0.06 mmol), 0.2 mL of liquid NH3 in 2 mL of N,N-dimethylformamide (DMF) and 1 mL of EtOH were sealed in a Teflon-lined steel autoclave and heated under autogenous pressure to 130 °C for 72 h. Then it was allowed to cool to room temperature at the rate of 1 °C per minute to produce block-shaped colorless crystals of complex 4 in ∼52% yield. The crystals were washed with DMF, EtOH followed by acetone and air-dried. Anal. Calc. for C24H23NO4Zn: C, 63.38; H, 5.09; N, 3.08%. Found: C, 63.54; H, 5.14; N, 3.02%. IR (KBr, cm−1): 3312(m), 3241(m), 3165(m), 2924(w), 1596(s), 1552(s), 1488(w), 1380(s), 1263(m), 1178(w), 1015(w), 866(m), 798(m), 769(m), 716(m). {[Pb2(L)1.5(O)0.5]·EtOH·2H2O}n (5). A mixture containing Pb(NO3)2 (40 mg, 0.12 mmol) and H2L (20 mg, 0.06 mmol) in 2 mL of EtOH and 1 mL of H2O were sealed in a Teflon-lined steel autoclave and heated under autogenous pressure to 180 °C for 72 h. Then it was allowed to cool to room temperature at the rate of 1 °C per minute to produce block-shaped colorless crystals of complex 5 in ∼64% yield. The crystals were washed with EtOH, water followed by acetone and air-dried. Anal. Calc. for C70H68O19Pb4: C, 41.17; H, 3.35%. Found: C, 41.03; H, 3.39%. IR (KBr, cm−1): 3473(m), 1586(s), 1537(s), 1398(s), 1072(m), 1013(w), 866(s), 829(s), 788(s), 704(w). {[Pb3(L)3(H2O)]·6H2O}n (6). A mixture containing Pb(NO3)2 (40 mg, 0.12 mmol) and H2L (20 mg, 0.06 mmol), 0.1 mL of 1 M NaOH in 3 mL of H2O were sealed in a Teflon-lined steel autoclave and heated under autogenous pressure to 180 °C for 72 h. Then it was allowed to cool to room temperature at the rate of 1 °C per minute to produce block-shaped colorless crystals of complex 6 in ∼56% yield. The crystals were washed repeatedly with water followed by acetone and air-dried. Anal. Calc. for C66H62O19Pb3: C, 44.52; H, 3.51%. Found: C, 44.36; H, 3.41%. IR (KBr, cm−1): 3389(m), 3040(m), 1702(w), 1606(s), 1576(s), 1505(s), 1400(s), 1180(w), 1043(w), 1015(w), 975(w), 851(m), 794(m), 763(m), 711(m).

EXPERIMENTAL SECTION

Materials. Reagent grade 1,4-dibromo-2,5-dimethyl benzene, 4methoxycarbonylphenylboronic acid, tetrakis(triphenylphosphine)palladium, piperazine, 4-(chloromethyl)pyridine hydrochloride, Zn(NO3)2·6H2O, and Pb(NO3)2 were acquired from Aldrich and used as received. All other chemicals and solvents were obtained from S. D. Fine Chemicals, India. Solvents were purified following standard procedures prior to use. Physical Measurements. Infrared spectra were obtained (KBr disk, 400−4000 cm−1) on a Perkin-Elmer Model 1320 spectrometer. Thermogravimetric analysis (TGA) were obtained on a Mettler Toledo Star System (heating rate of 5 °C/min). Microanalyses for the compounds were performed using a CE-440 elemental analyzer (Exeter Analytical Inc.). Solid-state photoexcitation and emission spectra were recorded using double UV−Vis-NIR spectrophotometer (Varian Model Cary 5000) and Jobin Yvon Horiba Fluorolog-3 spectrofluorimeter at room temperature. Variable temperature powder X-ray diffraction patterns for the compounds (CuKα radiation, scan rate 3°/min, 293 K) were collected on a Bruker D8 Advance Series 2 powder X-ray diffractometer. Single-Crystal X-ray Studies. Single crystal X-ray data of complexes 1−6 were collected at 100 K on a Bruker SMART APEX CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The linear absorption coefficients, scattering factors for the atoms, and the anomalous dispersion corrections were referred to from the International Tables for X-ray Crystallography.19 The data integration and reduction were worked out with SAINT20 software. Empirical absorption correction was applied to the collected reflections with SADABS,21 and the space group was determined using XPREP.22 The structure was solved by the direct methods using SHELXTL-9723 and refined on F2 by full-matrix least-squares using the SHELXTL-97 program24 package. All non-H atoms were refined anisotropically except N1 atom of complex 3 and O2, O7, C23, C45, C66 atoms of complex 6. The H-atoms of lattice solvent molecules in complex 5 and the H-atoms of coordinated water molecule in complex 6 could not be located in difference Fourier maps. The H-atoms attached to carbon atoms were positioned geometrically and treated as riding atoms using SHELXL default parameters. Several DFIX commands were used for fixing some bond distances in complexes 1, 3, 5, and 6. Squeeze refinement was performed for 1−3 and 6 using PLATON.25 It shows two EtOH and one N,N-dimethylformamide molecules for 1, one EtOH and half N,N-dimethylformamide molecules for 2, one EtOH and 2.5 N,N-dimethylformamide molecules for 3, while six water molecules are found for 6 per formula weight. The contribution of all the solvent atoms has been incorporated in both the empirical formulas and formula weights of the complexes. The crystal and refinement data are collected in Table 1. Selective bond distances and angles are given in Table S1 (Supporting Information). Synthesis of the Ligands. The ligand H2L and an ancilliary ligand bpmp were synthesized following earlier reported procedures.26,27 {[Zn3(L)2(OH)2]·8(EtOH)·4(DMF)}n (1). A mixture containing Zn(NO3)2·6H2O (40 mg, 0.13 mmol), H2L (20 mg, 0.06 mmol), 0.1 mL of 1 M NaOH in 2 mL N,N-dimethylformamide (DMF) and 1 mL of EtOH were sealed in a Teflon-lined stainless steel autoclave and heated under autogenous pressure to 100 °C for 60 h. It was allowed to cool to room temperature at the rate of 1 °C per minute to produce prism shaped colorless crystals of complex 1 in ∼56% yield. The crystals were washed with hot DMF, water, and finally with acetone and air-dried. Anal. calc. for C51H51NO13Zn3: C, 56.61; H, 4.75; N, 1.29%. Found: C, 56.29; H, 4.39; N, 1.12%. IR (KBr, cm−1): 3611(m), 3382(m), 2926(m), 1647(s), 1595(s), 1548(s), 1406(s), 1181(m), 1099(m), 1016(m), 893(m), 863(m), 846(m), 796(s), 767(s), 714(m). {[Zn(L)(bpmp)0.5]·EtOH·1/2DMF}n (2). A mixture containing Zn(NO3)2·6H2O (40 mg, 0.13 mmol), H2L (20 mg, 0.06 mmol), bpmp (10 mg, 0.04 mmol) in 2 mL of N,N-dimethylformamide (DMF) and 1 mL of EtOH were sealed in a Teflon-lined stainless steel autoclave and heated under autogenous pressure to 100 °C for 72 h. Then it was

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RESULTS AND DISCUSSION

Complexes 1−6, once isolated, are stable in air and insoluble in water and common organic solvents. The IR spectra of all the complexes show strong absorption bands between 1712 cm−1 and 1406 cm−1 attributable28 to coordinated carboxylate groups.29 Broad bands at the region 3610−3312 cm−1 indicate the presence of hydroxyl groups30 as well as lattice and coordinated water molecules.31 Sharp peaks between 1647 cm−1 and 1674 cm−1 in the IR spectra of complexes 1−3 are indicative of the presence of N,N-dimethylformamide molecules. Structural Description. Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the monoclinic C



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Figure 1. (a−e) Coordination modes of L2− in complexes 1−6.

Figure 2. (a−f) Metal coordination modes in complexes 1−6.

space group C2/m, and the asymmetric unit consists of two crystallographically independent Zn(II) sites (one with half occupancy and second one with one-fourth occupancy), half L2− ligand, half OH group in addition to a half DMF molecule and two EtOH molecules in the lattice. Both the carboxylate groups of the L2− ligand adopt μ4-η1:η1:η1:η1 coordination mode in bridging fashion. The coordination modes of L2− are shown in Figure 1a.

The Zn(II) ions form a trimeric unit through bonding from a bridging μ3-OH group and carboxylate group of L2− ligand. In the trimeric unit, Zn(1) sits on the 2-fold axis of symmetry as well as resides on a mirror plane and shows a distorted octahedral coordination environment where as Zn(2) sits on the 2-fold axis of symmetry and shows a distorted tetrahedral geometry. Zn(1) is coordinated to four bridging carboxylates from four different L2− (Zn−O = 1.983(5) Å) occupying the D



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Figure 3. Representation of (a) 3D view of framework with SBUs linked via linear ligand, (b) ball-and-stick model of SBUs, and (c) parallel packing ladder-like rods in complex 1.

equatorial positions and two bridging μ3-OH (Zn−O = 2.039(4) Å) occupying the axial positions. Zn(2) is coordinated to two different carboxylates of L2− (Zn−O = 1.930(3) Å) and two bridging μ3-OH (Zn−O = 2.004(3) Å) as depicted in Figure 2a. All Zn−O bond distances are consistent with the reported Zn based coordination polymers.2 The trimeric units extend to a 3D framework via bridging carboxylate group of L2− ligands and bridging μ3-OH groups. Notably, the open framework in 1 is sustained exclusively by 1-D metal cluster chain running along the c-direction as shown in Figure 3a. This packing gives rise to rhombic-shaped channels along the diagonals with a dimension of ca. 31.65 × 21.21 Å2. Topology analysis of 1 using the Topos software32 indicates that it can be described as parallel packing ladder-like rods as building units. Within these units, both tetrahedral and octahedral units bound to four and two carboxylate groups respectively (Figure 3b). These groups are further coordinated to the metal center via bridging μ3-OH ions. Thus, infinite Zn−O−C rods are stacked parallel as ladders which is further connected by organic linkers to give the sra net (Figure 3c).33 Complex 2 crystallizes in the triclinic space group P1̅ with the asymmetric unit consists of one Zn(II) ion, one ligand L2−, a half of the bpmp coligand besides one EtOH and a half DMF molecules in the lattice. The carboxylate groups of the L2− ligand bind to the metal in a similar fashion as in 1. The metal ion shows a distorted square-pyramidal (τ = 0.02) ZnNO4 coordination from four carboxylate O (Zn−O = 2.024(2)− 2.123(3) Å) from four L2− ligands and one N (Zn−N =

2.039(3) Å) from pyridyl group of the bpmp ligand (Figure 2b). The coordination of carboxlylates to the metal ions leads to a paddle-wheel [Zn2(OCO)4] secondary building unit (SBU) with a Zn···Zn separation of 2.990 Å which further propagates to a 3D framework via N atoms of the flexible bpmp ligand (Figure 4). These ancilliary bpmp ligands act as pillars interconnecting the M4L4 layers in the crystallographic 101 direction. Overall, a 3-fold interpenetrating architecture is

Figure 4. 3D view along the a-axis of complex 2. E



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wheel SBU. Further, these paddle-wheels extend to a 2D gridlike structure by L2− ligands in the crystallographic 201 plane. Because of stacking of the 2D grids involving strong noncovalent CH···π interactions between the pyridine moieties of adjacent layers, an overall 3D supramolecular architecture is obtained (Figures 7 and 8). On considering the rigid linear ligand L2− as line and dimeric Zn2 clusters as one node, complex 3 can be simplified as a 4connected sql net with point symbol {44.62} (Figure 9). Complex 4 crystallizes in the orthorhombic space group Pbca with the asymmetric unit consisting of one Zn(II) ion, one L2− ligand, and a coordinated dimethylamine molecule. Zn(II) shows a distorted tetrahedral coordination from carboxylate O (Zn−O = 1.941(2)−1.989(2) Å) of three different L2− ligands and a N (Zn−N = 2.021(3) Å) atom of dimethylamine produced by in situ hydrolysis of solvent DMF (Figure 2d). Here, the two carboxylate groups of the L2− ligand bind differently to the metal ions viz., μ3-η0:η1:η1:η1 coordination mode (Figure 1b), leading to a 2D sheet-like structure. These 2D sheets are stacked along the b-axis through H-bonding between −NH of the coordinated dimethylamine moiety and the free carboxylate O of L2− ligand from adjacent layers. In addition, 2D sheets are also generated perpendicular to the stacked layers involving CH···π interactions which results in a polycatenated 3D supramolecular architecture (Figure 10). The framework 4 can also be described as 4-connected sql net with point symbol {44.62} (Figure 11). Single-crystal X-ray diffraction analysis reveals that complex 5 crystallizes in the monoclinic space group C2/c. The asymmetric unit consists of two crystallographically independent Pb(II) ions, one and a half L2− ligands, a μ4-O in addition to one EtOH, and two H2O molecules in the lattice. The four Pb(II) ions form a tetranuclear unit [Pb4(μ4-O)]6+ through bonding from one bridging μ4-O. This tetrahedral Pb4O cluster is not very common in the literature.34 Each tetrahedral cluster is linked to six neighboring ones by six L2− ligands. Out of the six L2− ligands four show μ5:η1:η2:η1:η2 and two μ6:η2:η2:η2:η2 L2− binding modes (Figure 1c,d). In the tetramer, Pb1 shows a distorted pentagonal pyramidal geometry with ligation from carboxylate O (Pb−O = 2.520(3)−2.686(3) Å) from four L2− ligands besides the bridging μ4-O (Pb−O = 2.247(3) Å). Pb2 shows a distorted octahedral geometry with bonding from carboxylate O (Pb−O = 2.561(3)−2.736(3) Å) from four L2− ligands and the bridging μ4-O (Pb−O = 2.332(3) Å) (Figure 2e). Bonding of the L2− ligands to the Pb4O clusters lead to a 3D framework (Figure 12). Large channels with the dimension of ca. 37.76 × 13.89 Å2 can be seen in the framework when viewed along the crystallographic b-axis. The channels are not empty but contain water and EtOH molecules in the form of a chain through H-bonding interactions (Figure 13). On considering the rigid linear ligand L2− as lines and Pb4O as a 6-connected node by topological analysis of 6, it can be described as parallel packing ladder-like rods as building units. These rods are further linked by the L2− ligands generating a 3D framework (Figure 14). Complex 6 crystallizes in the monoclinic space group Cc, and the asymmetric unit consists of three crystallographically independent Pb(II) ions, three L2− ligands, one coordinated aqua molecule, and six lattice water molecules. The Pb(II) ions form a trimeric unit through bonding from the carboxylate groups of the L2− ligands. In the trimer, both Pb1 and Pb3 exhibit hexa coordination from carboxylate O (Pb−O = 2.409(6)−2.746(14) Å) of five different L2− ligands. Both

generated because of the interpenetration of three 3D frameworks (Figure 5).

Figure 5. Schematic view showing 3-fold interpenetration in complex 2.

On considering the rigid linear ligand L2− and bpmp coligand as lines and dimeric Zn2 cluster as one node, complex 2 can be simplified as a 6-connected pcu net with point symbol {412.63}(Figure 6).

Figure 6. Topological view of complex 2.

Single-crystal X-ray diffraction analysis reveals that complex 3 crystallizes in the monoclinic space group C2/m. The asymmetric unit consists of a half Zn(II) ion, a half L2− ligand, and a half pyridine molecule besides two and a half DMF and a EtOH molecules in the lattice. Zn(II) exhibits an ideal square pyramidal (τ = 0) ZnNO4 coordination with ligation from four different carboxylate O (Zn−O = 2.023(5)−2.029(4) Å) of L2− ligands occupying the basal positions and one pyridyl N (Zn− N = 1.996(8) Å) (Figure 2c), situated at the apical position. All Zn−O and Zn−N bond distances are within the range reported for Zn(II)-based coordination polymers.2 Here, carboxylate coordination to the metal ions results in a classical paddleF



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Figure 7. Diagram showing (a) 2D grid-like structure along crystallographic 201 plane and (b) stacking fashion of 2D grids in complex 3.

Figure 8. Depiction of CH···π interactions between the pyridine moieties of adjacent layers in complex 3.

ligands (Pb−O = 2.375(7)−2.739(6) Å) and a coordinated aqua (Pb−O = 2.457(8) Å) molecule (Figure 2f). All the bond distance values are within the range reported for Pb(II)-based coordination polymers.34 The carboxylate groups ligate to the metal ions in a different fashion as μ4:η1:η1:η2:η2 and μ6:η2:η2:η2:η2 (Figure 1c,e). This connectivity pattern is repeated infinitely to create Pb−C−O rods along the crystallographic c direction. Topological simplification with Topos software32 shows parallel packing ladder-like rods as building units. These rods are further linked by the L2− ligands, which connect each rod to four neighboring rods generating a 3D framework. The overall structure shows rhombic channels with a dimension of ca. 36.04 × 12.11 Å2 along the crystallographic c-axis (Figure 15). Thermal stabilities of all the six complexes were examined.29 Complex 1 shows weight loss of ∼15.23% (expected = 15.27%) in the temperature range, 60−260 °C corresponding to loss of one DMF and two EtOH molecules. The desolvated framework is found to be stable up to ∼400 °C. Complex 2 exhibits a weight loss of ∼13.02% (expected = 13.18%) in the


Pb1 and Pb3 are in a distorted octahedral coordination environment. However, Pb2 adopts a distorted pentagonal pyramidal geometry through bonding from four different L2− G



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DMF and one EtOH molecules. Decomposition of the complex is accomplished beyond 390 °C. Complex 4 exhibits no weight loss and found to be stable up to ∼360 °C. Complex 5 exhibits a weight loss of ∼8.21% (expected = 8.04%) in the temperature range of 50−320 °C that corresponds to loss of one EtOH and two water molecules. Gradual decomposition of complex 5 takes place only above 410 °C. Complex 6 exhibits a weight loss of ∼6.25% (expected = 6.18%) in the temperature range of 50−250 °C that corresponds to loss of six water molecules. It shows thermal stability up to 400 °C, and gradual decomposition is observed beyond this temperature. PXRD Analysis. Powder X-ray diffraction (PXRD) patterns of complexes 1−6 were in excellent agreement with the simulated patterns. The observed differences in intensity could be due to preferred orientation of the powder samples (Supporting Information, S7−S12). Luminescence Properties. Luminous MOF with carboxylate-based systems and close-shell metal ions have been reported in the literature.35 The solid-state luminescence of complexes 1−6 as well as the free ligand H2L were investigated at room temperature. The results are depicted in Figure 16. It can be seen that the metal-free H2L exhibits an emission band at 403 nm with excitation at 347 nm. Irradiation of crystalline complexes 1−6 with ultraviolet light (λex = 347 nm) at room temperature results in broad unsymmetrical emissions. Since the profile of the emission band is quite similar to that of the metal-free ligand, the emission of 1−6 could be attributed to intraligand (π−π*) emission. The difference in the emission behavior in 1−6 may be due to differences in the nature of encapsulatd guest, crystal packing, and nature of metal clusters. The possibility of cluster-centered transitions can also be ruled out in 1−6 because of the metal···metal separation in the cluster is more than ∼3.6 Å. Compared with the ligand, the complexes 1, 2, 4, 5, and 6 display enhancement in luminescent intensity. Usually, the rigidity imposed by the lattice in metal− organic frameworks constrains the ligands in such a way that is not typically observed for a free complex in solution, which reduces the loss of energy by radiation-less decay and thus photoluminescence intensity and also quantum efficiencies enhanced.36 In addition to this, the quenching of fluorescence

Figure 10. Representation of (a) CH···π interactions between aromatic rings of two different layers, and (b) H-bonding between −NH of the coordinated dimethylamine moiety and the free carboxylate O of L2− ligand in complex 4.


temperature range, 70−260 °C attributable to loss of a half DMF and one EtOH molecules. Decomposition of this complex takes place only above 380 °C. Complex 3 shows a weight loss of ∼31.60% (expected =31.88%) in the temperature range 50−350 °C that corresponds to loss of two and a half

Figure 12. 3D view along the b-axis of complex 5. H



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Figure 13. A representation of water-chain running along the b-axis in complex 5.

Figure 14. (a) 3D view of topological network with SBUs linked via linear ligand, and (b) parallel packing ladder-like rods in complex 5.

Figure 16. Solid-state luminescent spectra at room temperature.

We have chosen a symmetrical rigid carboxylate based organic ligand for construction of coordination polymers with d10 metal ions. Topologically complexes 2, 3, and 4 are normally observed in coordination polymers, while complexes 1 and 6 are unusual rod-type. However, an uncommon structure and topology has been obtained in complex 5. Solid-state emissions of the complexes have been recorded at room temperature. We are in the process of synthesizing similar type ligands for constructing coordination polymers for various applications.

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic data in CIF format, table for selected bond angles and bond distances for complexes 1−6, and complete data for IR, TGA analysis, PXRD and UV/vis. This material is available free of charge via the Internet at http:// pubs.acs.org/.

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Figure 15. Schematic view showing (a) 3D view of framework with SBUs linked via linear ligand and (b) parallel packing ladder-like rods in complex 6.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

intensity is observed in 3 which can be due to less robustness of the 2D framework.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support received from the Department of Science and Technology, New Delhi, India (to P.K.B.) and SRF from the CSIR (to J.S and M.A).

CONCLUSION In conclusion, we have described construction of six new coordination polymers under hydro(solvo)thermal conditions. I



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Article

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