A series of transition metal–organic frameworks: crystal structures, luminescence properties, and sensitizing for luminescent Ln(III) ions in aqueous solution

Abstract

A series of coordination polymers, namely, [Cd(DPDC)(BPP)·H₂O]·H₂O (1), Cd₄(O-OBA)₄(BPP)₂ (2), [Zn₂(DPDC)₂(BTB)(H₂O)₂]·H₂O (3), [Zn(O-OBA)(BPP)]·0.5H₂O (4), [Zn(O-OBA)(BPP)]·H₂O (5), Cu(DPDC)(BTB) (6) and [Co(O-OBA)(BTB)H₂O]·0.5H₂O (7) (DPDC = 2,2′-diphenyldicarboxylate, O-OBA = 2,2′-oxybis(benzoate), BPP = 1,3-di(4-pyridyl)propane, and BTB = 1,4-bis(1,2,4-triazol-1-yl)butane) were hydrothermally synthesized and structurally characterized. 1 forms a quadrilateral grid like network with {CdO₄N₃} polyhedra. 2 exhibits a 2D architecture containing four types of {Cd1O₇}, {Cd3O₇}, {Cd2O₄N₂} and {Cd4O₄N₂} polyhedra. 3 features a ladder like double chain that consists of Zn1–DPDC- and Zn2–DPDC-chains with {Zn1O₄N} and {Zn2O₄N} polyhedra. 4 possesses a helical 2D framework consisting of Zn2–O-OBA- and Zn1–BPP–Zn2–BPP-helical chains involving {Zn1O₂N₂} and {Zn2O₂N₂} tetrahedra. 5 consists of Zn–BPP–Zn–O-OBA-double helical chains with {ZnO₂N₂} tetrahedra. 6 has a (4, 4) 2D framework formed by Cu–DPDC- and Cu–BTB-chains involving {CuO₄N₂} octahedra. 7 consists of a 1D zigzag chain where the {CoO₄N₂} octahedra are bridged by BTB ligands, while the O-OBA behaves as a terminal ligand. Complex 4 shows solvent-dependent luminescence and can be used to detect nitrobenzene via the quenching effect. Moreover, complex 4 can notably sensitize luminescent lanthanide ions in aqueous solution to emit their characteristic emissions.

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1. Introduction

Metal–organic frameworks (MOFs) have diverse intriguing architectures and potential applications in gas storage and catalysis, as well as in magnetic materials, sensors, and optics.¹ Thus, the design and synthesis of MOFs attract much interest in chemistry and materials science. MOFs are obtained by combining well-designed metal nodes and organic linkers.² The combined intrinsic properties of metal cations and organic ligands can result in diverse structures and properties for a broad range of applications. Specific properties, such as luminescence, are influenced by metal cations and organic ligands.^1e,f Transition metal cations exhibit a variety of coordination geometries, and can be used in a variety of crystal architectures. Furthermore, a variety of transition metals have been used in cases where linker-based luminescence is observed.³ For example, transition metal ions without unpaired electrons, such as Zn(II) and Cd(II), can yield ligand-based fluorescence.^3c–k In addition, some fluorescent probes of transition-based MOFs have been developed accordingly. Such probes offer important applications in biological and environmental systems.^1g Multifunctional ligands containing O- and N-donors, such as carboxylic acids and N-ligands, have been exploited in constructing MOFs because these ligands may induce diversity in coordination modes. Moreover, polycarboxylates can bridge metal centers to form multitudinous frameworks through different bridging modes.^2a,b Multitopic N-ligands, usually introduced into MOFs, are another kind of useful building block. In addition, flexible N-ligands can construct interesting coordination networks owing to the conformational freedom of these ligands.⁴ 1,4-Bis(1,2,4-triazol-1-yl)butane (BTB)⁵ and 1,3-bis(4-pyridyl)propane (BPP)⁶ are examples of flexible ligands. Carboxylate ligands and N-ligands can cooperate by adjusting their coordination and conformations to generate the final framework, which may exhibit diverse topological structures and properties. This study chose 2,2′-diphenyldicarboxylic acid (H₂DPDC)/2,2′-oxybis(benzoic acid) (O-H₂OBA) and BTB/BPP as bridging ligands to construct seven complexes, namely, [Cd(DPDC)(BPP)H₂O]·H₂O (1), Cd₄(O-OBA)₄(BPP)₂ (2), [Zn₂(DPDC)₂(BTB)(H₂O)₂]·H₂O (3), [Zn(O-OBA)(BPP)]·0.5H₂O (4), [Zn(O-OBA)(BPP)]·H₂O (5), Cu(DPDC)(BTB) (6) and {Co(O-OBA)(BTB)H₂O]·0.5H₂O (7). Zn(II), Cd(II), Cu(II) and Co(II) ions are favored to feature a wide range of coordination numbers from 4 to 7, thereby forming different topologies. 2,2′-H₂DPDC^7a–c and O-H₂OBA^7d,e are semi-rigid ligands composed of two benzene rings and often demonstrate strong fluorescence properties for π⋯π conjugation effect, and the two benzene rings can twist to meet the requirements of the coordination geometries of metal atoms during assembly. The highly flexible BTB and BPP ligands present different conformations (TTT, GTG, and GTT conformations for BTB^5a,b whereas TT, TG, GG and GG′ for BPP^6b,c) with respect to the relative orientations of the CH₂ groups. Complexes 1–7 show diverse structures based on the different coordination geometries of metal centers and on the different coordination modes and ligand conformations. Complex 4 was selected as an example to further investigate the luminescence property and the possible potential sensing application for organic small molecule and metal cations. Most interestingly, complex 4 enhanced the fluorescence of luminescent Ln(III) ion.

2. Experimental

2.1 Materials and physical measurements

Commercially available reagents are used as received without further purification. Elemental analyses (C, H, N) were performed on an Elementar Vario EL analyzer. Infrared (IR) spectra were measured on a Bruker Tensor37 spectrophotometer using the KBr pellets technique. Experimental powder X-ray diffraction (PXRD) was carried out on a PANaytical X'Pert PRO MPD diffractometer for CuK_α radiation (λ = 1.5406 Å), with a scan speed of 2° min⁻¹ and a step size of 0.02° in 2θ. PXRD patterns (Fig. S1†) of compounds 1–7 confirm their purity. Thermogravimetric analyses (TGA) were carried out on a HCT-2 thermal analyzer at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C in air. Solid state and liquid state fluorescence spectra were recorded on an FL7000 fluorescence spectrophotometer (Japan Hitachi company) at room temperature. The lifetimes were measured at room temperature on FLS920 Steady State & Time-resolved Fluorescence Spectrometer (Edinburgh Instrument).

2.2 Synthesis of complexes 1–7

Synthesis of [Cd(DPDC)(BPP)·H₂O]·H₂O (1). A mixture of 3CdSO₄·8H₂O (0.1 mmol, 25.8 mg), H₂DPDC (0.1 mmol, 24.2 mg), BPP (0.1 mmol, 19.8 mg) and NaOH (0.15 mL, 1 mol L⁻¹) in H₂O (8 mL) was sealed into a 25 mL Teflon cup and heated at 120 °C for 3 days. The mixture was then cooled to room temperature, and the colorless crystals were obtained. Yield: 55%, based on 3CdSO₄·8H₂O. Elemental analysis (%) calcd for C₂₇H₂₅CdN₂O₆: C 55.29, H 4.27, N 4.78. Found (%): C 55.35, H 4.31, N 4.82. Selected IR (KBr pellet, cm⁻¹): 3440.14 (vs), 1613.59 (m), 1561.75 (s), 1502.80 (m), 1404.09 (vs), 1226.61 (w), 1107.83 (w), 1069.23 (w), 1016.22 (w), 854.38 (w), 793.57 (w), 752.41 (w), 682.90 (w), 617.87 (w).

Synthesis of Cd₄(O-OBA)₄(BPP)₂ (2). Complex 2 was synthesized by applying the same synthetic procedure as that for 1 except that the H₂DPDC was replaced by O-H₂OBA and the temperature was 150 °C. Yield: 55%, based on 3CdSO₄·8H₂O. Elemental analysis (%) calcd for C₈₂H₆₀Cd₄N₄O₂₀: C 52.59, H 3.20, N 2.99. Found (%): C 52.34, H 3.18, N 3.13. Selected IR (KBr pellet, cm⁻¹): 3435.57 (vs), 1606.09 (s), 1584.10 (vs), 1545.54 (s), 1476.30 (m), 1447.91 (m), 1385.66 (vs), 1231.06 (m), 1158.26 (w), 1099.02 (w), 891.30 (w), 859.68 (w), 802.85 (w), 757.72 (m), 661.22 (m), 511.48 (w).

Synthesis of [Zn₂(DPDC)₂(BTB)(H₂O)₂]·H₂O (3). Complex 3 was synthesized by applying the same synthetic procedure as that for 1 with 3CdSO₄·8H₂O and BPP replaced by ZnSO₄·7H₂O and BTB. Yield: 52%, based on ZnSO₄·7H₂O. Elemental analysis (%) calcd for C₃₆H₃₄Zn₂N₆O₁₁: C 50.38, H 3.96, N 9.79. Found (%): C 50.52, H 4.01, N 9.67. Selected IR (KBr pellet, cm⁻¹): 3435.38 (s), 3123.50 (m), 1606.89 (vs), 1531.17 (m), 1438.21 (m), 1358.24 (s), 1280.41 (m), 1137.03 (m), 1048.22 (w), 998.22 (m), 850.03 (w), 750.02 (m), 718.065 (w), 685.06 (w), 646.51 (m), 537.19 (w), 454.30 (w).

Synthesis of [Zn(O-OBA)(BPP)]·0.5H₂O (4). A mixture of ZnSO₄·7H₂O (0.1 mmol, 28.7 mg), O-H₂OBA (0.1 mmol, 25.8 mg), BPP (0.1 mmol, 19.8 mg) and NaOH (0.13 mL, 1 mol L⁻¹) in H₂O (10 mL) was sealed into a 25 mL Teflon cup and heated at 150 °C for 3 days. The mixture was then cooled to room temperature, and the colorless crystals were obtained. Yield: 55%, based on ZnSO₄·7H₂O. Elemental analysis (%) calcd for C₅₄H₄₆Zn₂N₄O₁₁: C 61.26, H 4.35, N 5.29. Found (%): C 61.03, H 4.32, N 5.05. Selected IR (KBr pellet, cm⁻¹): 3434.92 (vs), 1619.75 (s), 1398.47 (vs), 1224.78 (w), 1098.54 (w), 748.85 (w), 668.96 (w).

Synthesis of [Zn(O-OBA)(BPP)]·H₂O (5). Complex 5 was synthesized by applying the same synthetic procedure as that for 4 without addition of NaOH. Yield: 53%, based on ZnSO₄·7H₂O. Elemental analysis (%) calcd for C₂₇H₂₄ZnN₂O₆: C 60.23, H 4.46, N 5.20. Found (%): C 60.11, H 4.33, N 5.53. Selected IR (KBr pellet, cm⁻¹): 3426.01 (m), 1618.73 (vs), 1508.79 (w), 1476.14 (w), 1447.02 (m), 1434.56 (m), 1392.37 (m), 1372.21 (vs), 1242.48 (s), 1156.57 (w), 1098.03 (w), 1070.37 (w), 1031.21 (m), 891.12 (w), 846.98 (w), 829.23 (w), 799.35 (w), 749.55 (m), 721.75 (m), 668.08 (w), 627.28 (w), 525.99 (w), 493.40 (w).

Synthesis of Cu(DPDC)(BTB) (6). A mixture of CuSO₄·5H₂O (0.1 mmol, 24.9 mg), O-H₂OBA (0.1 mmol, 25.8 mg), BTB (0.1 mmol, 19.2 mg) and NaOH (0.2 mL, 1 mol L⁻¹) in H₂O (10 mL) was sealed into a 25 mL Teflon cup and heated at 100 °C for 3 days. The mixture was then cooled to room temperature, and the colorless crystals were obtained. Yield: 58%, based on CuSO₄·5H₂O. Elemental analysis (%) calcd for C₂₂H₂₀CuN₆O₄: C 53.23, H 4.03, N 16.94. Found (%): C 53.15, H 4.12, N 16.72. Selected IR (KBr pellet, cm⁻¹): 3435.38 (m), 3129.68 (vs), 1618.82 (m), 1400.55 (vs), 1283.75 (w), 1128.74 (m), 996.87 (m), 854.46 (w), 678.07 (w), 533.03 (w).

Synthesis of [Co(O-OBA)(BTB)H₂O]·0.5H₂O (7). A mixture of CoCl₂·H₂O (0.1 mmol, 23.8 mg), O-H₂OBA (0.1 mmol, 25.8 mg), BTB (0.1 mmol, 19.2 mg) and NaOH (0.2 ml, 1 mol L⁻¹) in H₂O (8 mL) were sealed into a 18 mL Teflon cup and heated at 80 °C for 3 days. The mixture was then cooled to room temperature, and the colourless block crystals were collected. Yield: 44%, based on CoCl₂·6H₂O. Elemental analysis (%) calcd for C₄₄H₄₆Co₂N₁₂O₁₃ (%): C, 49.40; H, 4.30; N, 15.72. Found (%): C, 49.16; H, 4.12; N, 15.26. IR (cm⁻¹): 3413.31 (s), 3126.11 (vs), 1618.16 (s), 1525.45 (w), 1470.94 (m), 1400.49 (vs), 1281.83 (w), 1208.87 (m), 1130.10 (s), 994.20 (m), 857.42 (w), 759.76 (w), 681.97 (w), 664.65 (w), 621.49 (w), 532.91 (m).

2.3 X-ray crystallographic study

The X-ray single crystal data collections for the seven complexes were performed on a Bruker Smart Apex II CCD diffractometer equipped with a graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 293(2) K. Semiempirical absorption correction was applied using the SADABS program.^8a The structures were solved by direct methods and refined by full matrix least squares method on F² using SHELXS 97 and SHELXL 97 programs.^8b,c

2.4 Sensing and sensitizing experiments

The solvent sensing experiment: well-ground complex 4 powder (3 mg) was immersed in different organic solvents (3 mL), treated by ultrasonication for approximately 30 min, and then aged for three days. The Ln(III) ion sensitizing experiment: well-ground complex 4 powder (3 mg) was immersed in Ln(III), (Sm(III), Eu(III), Tb(III), Dy(III) 10⁻² mol L⁻¹) aqueous solutions (3 mL), treated by ultrasonication for approximately 30 min, and then aged for three days.

3. Results and discussion

3.1 Crystal structure

A summary of the crystallographic data and details of the structure refinements of complexes 1–7 are listed in Table S1.† Selected bond distances and angles are listed in Table S2.† A scheme containing simple chemical representations of the ligands and their configurations in complexes 1–7 is shown in Scheme S1.†

The structures of [Cd(DPDC)(BPP)H₂O]·H₂O (1), and Cd₄(O-OBA)₄(BPP)₂ (2).
Structure of [Cd(DPDC)(BPP)H₂O]·H₂O (1). Complex 1 crystallizes in the monoclinic, space group P2₁/c and features a 2D structure (Fig. 1). The asymmetric unit of 1 comprises one Cd(II) ion, one DPDC, one BPP ligand, one coordination water molecule and free water molecule (Fig. 1a). The Cd(II) is in a {CdO₄N₃} distorted pentagonal bipyramidal fashion by four O atoms of two DPDC ligands, two nitrogen atoms of BPP ligand and one water molecule. The Cd–O (carboxylate) bond distances are in the ranges of 2.311(5)–2.630(5) Å. Cd–N bond distances are 2.331(5) and 2.379(6) Å. Cd–O (water) bond distance is 2.287(5) Å. The DPDC ligand as linear linker adopts μ₁:η¹η¹/μ₁:η¹η¹ mode to link Cd(II) centers to form Cd–DPDC-linear chain with the distance Cd⋯Cd of 9.528(6) Å along b axis. The BPP ligand displays TT conformation with the N⋯N distance of 10.244(2) Å and link Cd(II) centers to form Cd–BPP-chain with the distance Cd⋯Cd of 14.660(4) Å and Cd⋯Cd⋯Cd angle of 176.040(8)°, which is almost in the straight line along a axis. The two types of chains are conjoined into a quadrilateral grid like 2D layer running parallel to the bc crystal plane (Fig. 1b). Considering each Cd(II) center as a node, the 2D framework can be seen as a four-connected (4-c) plane net with a (4, 4) topology. The adjacent planes in a ABCD fashion are connected by O–H⋯O/C–H⋯O hydrogen bonds between lattice/coordinated water molecules and DPDC ligands to form the 3D supramolecular network, and the hydrogen bond distances of O⋯O and C⋯O are 2.7067(6) Å and 3.6410(1) Å respectively (Fig. S2†). There are small pores containing uncoordinated water molecules.

Fig. 1 The structure of 1. (a) The coordination environment of the Cd(II) ion, (b) a quadrilateral grid like 2D layer, showing Cd–DPDC- and Cd–BPP-chain. All hydrogen atoms are omitted for clarity. Symmetry code: (A) x, −1 + y, z. (B) −1 + x, 0.5 − y, −0.5 + z.

Structure of Cd₄(O-OBA)₄(BPP)₂ (2). Compared with complex 1, a O-OBA was used as the substitute of DPDC, and interesting complex 2 was obtained. Complex 2 crystallizes in monoclinic crystal system with space group Cc and is constructed from a 2D structure and is quite different from 1 (Fig. 2). The asymmetric unit of 2 comprises four Cd(II) ions, four O-OBA and two BPP ligands (Fig. 2a). The most interesting structural feature of 2 is the presence of four different types of coordination environments of Cd(II) centers. This is a rare example of having the four different types of Cd(II) centers in the Cd(II) compounds.⁹ Cd1 and Cd3 ions are both pentagonal bipyramidally coordinated in a {CdO₇} arrangement, with the O atoms provided by four O-OBA ligands. Cd2 and Cd4 manifest a {CdO₄N₂} octahedral environment, with O atoms from two O-OBA ligands and N from two BPP ligands. The Cd–O bonds are in the 2.221(9)–2.435(9) Å range. Cd–N bond distances are in the ranges of 2.256(11)–2.272(11) Å. The O-OBA ligands with two different coordination modes are found: μ₁:η¹η¹/μ₂:η¹η² and μ₂:η¹η²/μ₂:η¹η². The BPP ligands are in TT conformations with the N⋯N distances of 9.107(4) Å. The BPP ligands link Cd2 and Cd4 ions to form a Cd2–BPP–Cd4 chain. The COO groups establish connections between four neighboring Cd(II) centers, leading to the formation of a Cd1⋯Cd2⋯Cd3⋯Cd4⋯ chain. Interestingly, the Cd1⋯Cd2⋯Cd3⋯Cd4⋯ chains are further joined together by the Cd2–BPP–Cd4 chains, leading to the formation of a 2D self-penetrating motif (Fig. 2b). In other words, the two types of chains are conjoined into (4, 4) quadrilateral grid like layer running parallel to the ab crystal plane, whose quadrilateral windows are constructed by two Cd2, two Cd4, one Cd1, one Cd3, four COO groups and BPP ligands (Fig. 2c). The Cd⋯Cd distances are 3.890(0) Å (Cd1⋯Cd2), 4.040(3) Å (Cd2⋯Cd3) and 3.876(6) Å (Cd3⋯Cd4) and 3.995(6) Å (Cd4⋯Cd1) by COO groups. The Cd⋯Cd (Cd2⋯Cd4) distances through BPP ligands are 12.343(6) and 12.040(1) Å, respectively. Interestingly, left/right-handed – helical chains are found in the 2D network, which composed of [Cd2–BPP–Cd4–COO–Cd1–COO] and [Cd4–BPP–Cd2–COO–Cd3–COO] units, respectively, with the pitch of 15.692 Å along a axis (Fig. 2d). The adjacent 2D networks in AB fashion are connected by the C–H⋯O hydrogen bonds with C⋯O distance of 3.296(0) Å between O-OBA ligands. Thus, a 3D supramolecular microporous architecture is formed (Fig. S3†).

Fig. 2 The structure of 2. (a) The coordination environment of the four types of Cd(II) centers, (b) 2D framework, (c) 2D self-penetrating motif, (d) [Cd2–BPP–Cd4–COO–Cd1–COO] (L) and [Cd4–BPP–Cd2–COO–Cd3–COO] (R)-handed helix. All hydrogen atoms are omitted for clarity. Symmetry code: (A) 0.5 + x, −0.5 + y, z. (B) −0.5 + x, −0.5 + y, z. (C) x, −1 + y, z.

Structures of [Zn₂(DPDC)₂(BTB)(H₂O)₂]·H₂O (3), [Zn(O-OBA)(BPP)]·0.5H₂O (4) and [Zn₂(O-OBA)₂(BPP)₂]·H₂O (5).
Structure of [Zn₂(DPDC)₂(BTB)(H₂O)₂]·H₂O (3). Complex 3 crystallizes in monoclinic crystal system with a space group of P2₁/c and is constructed from a 1D chain structure (Fig. 3). The asymmetric unit of 3 comprises two Zn(II) ions, two DPDC ligands, one BTB ligand, two coordination water molecules and one free water molecule (Fig. 3a). The two crystallographically independent Zn(II) centers are in {ZnO₄N} square-pyramidal coordination geometry, defined by three oxygen atoms from two DPDC ligands, one nitrogen atom from BTB ligand and one water molecule. The Zn–O (carboxylate) bond distances are in the ranges of 1.954(4)–2.490(4) Å. However, the Zn1–O2 (carboxylate) and Zn2–O6 (carboxylate) bond distances of 2.490(4) and 2.471(4) Å, respectively, are the largest, because four-membered ring Zn1O₁C₁O₂ and Zn2O₆C₂₃O₇ have stronger strain, and O2 and O6 as bridged simultaneously link two different Zn(III) ions. Zn–N bond distances are 2.029(4) and 2.023(4) Å, respectively. Zn–O (water) bond distances are 2.015(4) and 2.009(3) Å. Unlike 1, the DPDC ligands as linear linkers adopt μ₁:η¹η⁰/μ₁:η¹η¹ mode and link Zn1 and Zn2 centers to form Zn1–DPDC- and Zn2–DPDC-linear chains, respectively. The two chains are connected into a double chain structure via the BTB ligands with GTG conformation (Fig. 3b). The double chain looks like a ladder containing Zn1–DPDC- and Zn2–DPDC-chains and BTB spacers. The distances of Zn1⋯Zn1, Zn2⋯Zn2 and Zn1⋯Zn2 are 7.950, 7.950 and 12.485 Å. The coordination environments of the Zn1 and Zn2 ions in 3 are devoid of an inversion center. The chain motifs are stacked in an AA fashion via interchain hydrogen bonds and are further connected into 3D microporous supramolecular framework occupied by crystallization water molecules (Fig. S4†). The O–H⋯O noncovalent interactions exist between lattice/coordinated water molecules and carboxylate O atoms of DPDC ligands with the bond distances of O⋯O are in the ranges of 2.665(0)–2.973(9) Å.

Fig. 3 The structure of 3. (a) The coordination environment of the two crystallographically independent Zn(II) centers, (b) a ladder like double chain. All hydrogen atoms are omitted for clarity. Symmetry code: (A) x, −1 + y, z. (B) x, 1 + y, z.

Crystal structures of [Zn(O-OBA)(BPP)]·0.5H₂O (4) and [Zn(O-OBA)(BPP)]·H₂O (5). The reaction system with the same ZnSO₄·7H₂O : O-OBA : BPP molar ratio of 1 : 1 : 1 at the same reaction temperature (150 °C), complexes 4 and 5 were obtained in the presence/absence of 0.1 mL 1 mol L⁻¹ NaOH. The two complexes crystallize in monoclinic crystal system with space group of P2₁/c. Although the Zn(II) species with the same organic ligands in the two complexes. However, show distinct structures.
Structure of [Zn(O-OBA)(BPP)]·0.5H₂O (4). Complex 4 forms a 2D structure (Fig. 4). The asymmetric unit of 4 comprises two Zn(II) ions, two O-OBA ligands, two BPP ligands and free water molecule (Fig. 4a). The two crystallographically independent Zn(II) centers are four-coordinated to two oxygen atoms from two O-OBA ligands and two nitrogen atoms from BPP ligands in a [ZnO₂N₂] distorted tetrahedral arrangement. The Zn–O bond distances are in the ranges of 1.935(2)–1.964(2) Å, Zn–N bond distances are in the ranges of 2.015(3)–2.050(3) Å. The O-OBA ligands adopt μ₁:η¹η⁰/μ₁:η¹η⁰ coordination mode. The BPP ligand displays TT conformation with the N⋯N distance of 9.946(9) Å. Zn1 and Zn2 ions are linked by the O-OBA and BPP ligands to form a 2D structure (Fig. 4b). Interestingly, neighboring Zn1 ions are joined into a centrosymmetric binuclear unit (Zn1–O-OBA)₂ ring (Zn1⋯Zn1, 5.168(4) Å) (Fig. 4c) while neighboring Zn2 ions are linked by the O-OBA ligands to form a Zn2–O-OBA-left-handed double-helical chain, which has a repeat unit consisting of two [Zn2–O-OBA-] units with a pitch of 13.102 Å along b axis (Zn2⋯Zn2, 6.580(4) Å) (Fig. 4d). However, the Zn1 and Zn2 ions are linked by the BPP ligands to form a Zn1–BPP–Zn2–BPP-left/right-handed – helical chains (Zn1⋯Zn2, 13.429(4) Å). This chain motif involves an alternation of the Zn1, BPP and Zn2 with a repeat distance of 39.307 Å along b axis (Fig. 4e). Notably, two adjacent Zn1–BPP–Zn2–BPP-left/right-handed helical chains intertwine into a double-flexural helix chain. So, this 2D structure of 4 can be described as a helical framework. The crystallization water molecules are tightly H-bonded to the layer, resulting in the 3D supramolecular crystal packing. The hydrogen bond distances of O⋯O are 2.832(0) and 2.798(8) Å (Fig. S5†).

Fig. 4 The structure of 4. (a) The coordination environment of the two crystallographically independent Zn(II) centers, (b) 2D framework, (c) centrosymmetric binuclear unit (Zn1–O-OBA)₂ ring, (d) Zn2–O-OBA-helix, (e) Zn1–BPP–Zn2–BPP-double-flexural helix chains. All hydrogen atoms are omitted for clarity. Symmetry code: (A) 1 − x, 3 − y, −z. (B) 2 − x, 0.5 + y, 0.5 − z. (C) 2 − x, 1.5 + y, 0.5 − z.

Structure of [Zn(O-OBA)(BPP)]·H₂O (5). Unlike 4, 5 is a 1D chain structure where the asymmetric unit comprises one Zn(II), one O-OBA, and one BPP ligand along with one free water molecule (Fig. 5a). The Zn(II) represents the [ZnO₂N₂] distorted tetrahedral coordination with two oxygen atoms from two O-OBA ligands and two nitrogen atoms from BPP ligands. The Zn–O bond distances are in the ranges of 1.923(3)–1.929(3) Å, Zn–N bond distances are in the ranges of 2.035(3)–2.047(3) Å. The coordination mode of O-OBA ligand in 5 is similar to that in 4, the two carboxylate groups are in μ₁:η¹η⁰/μ₁:η¹η⁰ mode to link two Zn(II) ions. However, the BPP ligand displays TG conformation with the N⋯N distance of 9.928(9) Å. The neighboring two Zn(II) ions are linked by the two O-OBA ligands to give an cyclic ring of a centrosymmetric binuclear (Zn–O-OBA)₂ unit (Zn⋯Zn, 4.953(4) Å) (Fig. 5b). The neighboring (Zn–O-OBA)₂ units as SUBs are linked by the double BPP ligands to produce an double chain (Fig. 5c). The Zn⋯Zn distance by BPP is 12.259(3) Å. The Zn–BPP–Zn–O-OBA-double helices are observed along the b-axis in the crystal structure. The helices have a repeat unit consisting of two Zn(II) centers, two BPP ligands and two O-OBA ligands with a pitch of 12.259 Å (Fig. 5d). A 3D supramolecular network is formed based on O–H⋯O and C–H⋯O hydrogen bonds. The former [d(O⋯O) = 2.840(2) Å] involves the crystallization water molecules and COO groups of O-OBA anions. The latter [d(C⋯O) = 3.413(3) Å] involves the crystallization water molecules and C–H of O-OBA and BPP ligands (Fig. S6†).

Fig. 5 The structure of 5. (a) The coordination environment of the Zn(II) ion, (b) centrosymmetric binuclear unit (Zn1–O-OBA)₂ ring, (c) 1D framework, (d) the Zn–BPP–Zn–O-OBA- (L) and (R)-handed helix. All hydrogen atoms are omitted for clarity. Symmetry code: (B) 1 − x, 2 − y, 1 − z. (C) x, 1 + y, z.

Structure of Cu(DPDC)(BTB) (6). Complex 6 crystallizes in the monoclinic, space group P2₁/c and features a 2D structure with an asymmetric unit consisting of one Cu(II) ion, one DPDC and one BTB ligand (Fig. 6a). The Cu(II) ion manifests a slightly distorted octahedral [CuO₄N₂] coordination environment, with four O atoms of two different DPDC ligands and two nitrogen atoms of BTB ligand. The Cu–O bond distances are in the ranges of 1.944(2)–2.719(2) Å, Cu–N bond distances are 1.982(2) and 1.988(2) Å, respectively. The six Cu–O/N bonds show “4 + 2” type interactions, the Cu⋯O2 and Cu⋯O4 distances of 2.514(2) and 2.719(2) Å are significantly longer than those in basal plane, since there are Jahn–Teller distortions for d⁹ metal ion. Like 1, the DPDC ligands as linear linkers adopt μ₁:η¹η¹/μ₁:η¹η¹ mode while the BTB ligands display GTT conformation to link Cu(II) centers to form a quadrilateral grid like layer with the Cu⋯Cu of 6.089 and 13.534 Å running parallel to the ac crystal plane (Fig. 6b). Interestingly, Cu–DPDC–BTB-double-handed helical chains are found, which has a repeat unit consisting of two [Cu–DPDC–BTB-] units with a pitch of 27.042 Å (Fig. 6c). The 2D motifs are further connected into 3D supramolecular architecture by C–H⋯π noncovalent interactions between C17–H(17B) group of BTB and C8–C13 ring of DPDC with the distance of 3.220 Å and C6–H6 group of DPDC and N1–N3, C15–C16 ring of BTB with the distance of 2.693 Å (Fig. S7†).

Fig. 6 The structure of 6. (a) The coordination environment of the Cu(II) ion, (b) a quadrilateral grid like layer, showing Cu–DPDC- and Cu–BTB-chain, (c) Cu–DPDC–Cu–BTB- (L) and (R)-handed helix. All hydrogen atoms are omitted for clarity. Symmetry code: (A) x, 0.5 − y, 0.5 + z. (B) −1 + x, 0.5 − y, 0.5 + z.

Structure of {Co(O-OBA)(BTB)H₂O]·0.5H₂O (7). Complex 7 crystallizes in the monoclinic, space group C2/c. The asymmetric unit of 7 comprises Co(II) ion, one O-OBA ligand, one BTB ligand, one coordination water molecules and free water molecules (Fig. 7a). Co(II) ion is six-coordinated in a slightly distorted octahedral [CoO₄N₂] coordination environment with two carboxylate O atoms (d(Co–O) = 2.052(2) Å) and one ether O atom (d(Co–O) = 2.224(2) Å) of one O-OBA ligand, one water O atom (d(Co–O) = 2.047(3) Å), and two N atoms (d(Co–N) = 2.104(5) Å) of BTB ligand. The BTB ligand displays GTG conformation to link Co(II) centers to form Co–BTB-zigzag chain structure with the distance Co⋯Co of 12.256 Å and Co⋯Co⋯Co angle of 142.52° along b axis (Fig. 7b). O-OBA acts as a terminal ligand using one of the two O atoms of carboxylate group and ether oxygen atom to chelate the Co(II) ion while the other O atoms of carboxylate groups is idle, which is different from 2, 4 and 5. The adjacent 1D chains in AB fashion are further connected by O–H⋯O hydrogen bonds between water molecules and carboxylate O atoms from O-OBA ligands to form a 3D supramolecular framework and the bond distances of O⋯O are 2.803(2) and 3.306(2) Å (Fig. S8†).

Fig. 7 The structure of 7. (a) The coordination environment of the Co(II) ion, (b) a 1D zigzag chain. Symmetry code: (A) −x, y, 1.5 + z.

3.2 Discussion

The above studies indicate that the structural features of MOFs are the direct results of the coordination geometry of metal centers and of the ligand conformations. The seven MOFs show diverse architectures containing various structural motifs, quadrilateral grid like 2D network (1, 2), a ladder double like chain (3), a helical 2D framework (4), double chain (5), and a 1D zigzag chain (7). Cu(II), Zn(II), Cd(II) and Co(II) ions possess the variable coordination numbers and adopt different coordination geometries. Zn(II) center in tetrahedral and/or square-pyramidal coordination geometry, Cd(II) in pentagonal bipyramidal and/or octahedral fashion, Cu(II) and Co(II) in octahedron. Cu(II) is Jahn–Teller active d⁹ metal ion, its complex displays Jahn–Teller distortion. The Zn(II)/Cd(II) ions exhibit a d¹⁰ configuration. In addition, Zn(II) shows low coordination numbers of 4 and 5, whereas Cd(II) possesses high coordination numbers of 6 and 7 owing to its large metal radius. Among the two Cd(II) complexes, Cd(II) ions display different coordination geometries. In 1, one crystallographically independent Cd(II) center exists, namely, {CdO₄N₃}-pentagonal bipyramidal fashion. In 2, four crystallographically independent Cd(II) centers exist, namely, {Cd1O₇} and {Cd3O₇} pentagonal bipyramidal fashion, along with {Cd2O₄N₂} and {Cd4O₄N₂} octahedral fashion. Interestingly, 4 and 5 have identical chemical composition (the difference consists only in the number of free water molecules in the two complexes) but different crystal structures. The templating effect of NaOH on the formation of frameworks 4 and 5 is observed. DPDC and O-OBA are semi-rigid ligands and their carboxylate groups adopt different bridging modes to link metal centers. O-OBA demonstrates higher flexibility than DPDC, and two aromatic carboxylic acid moieties can freely twist its benzene rings to meet the requirements of the coordination geometries of metal atoms during assembly. DPDC/O-OBA ligands provide a very wide range of binding modes, including bis(bidentate-chelating), monodentate/bidentate-chelating, bis(bridging-chelating), bis(monodentate), and bidentate-chelating/bridging-chelating coordination modes (Scheme S1a†). These modes are realized depending on the properties of the metal ions, and on the properties of any auxiliary ligands that complete the metal coordination sphere. BPP is a long and flexible building ligand and adopts two different configurations (TT and TG), whereas BTB demonstrates higher flexibility and displays two configurations (GTG and GTT) (Scheme S1b†). Interestingly, left/right-handed – helical chains composed of [Cd2–BPP–Cd4–COO–Cd1–COO] and [Cd4–BPP–Cd2–COO–Cd3–COO] in 2, Zn2–O-OBA-left-handed double-helical chain and Zn1–BPP–Zn2–BPP-left/right-handed – helical chains in 4, Zn–BPP–Zn–O-OBA-double helices in 5, and Cu–DPDC–BTB-double-handed helical chains in 6 are formed. It seems that these helices are a direct consequence of the high flexibility of BPP and BTB. Thus, utilizing Zn(II), Cd(II), Cu(II) and Co(II) ions as centers and DPDC/O-OBA, together with flexible BPP/BTB as bridging ligands, we constructed a series of the first MOFs to be reported.

3.3 Thermogravimetric analysis

Thermogravimetric analysis (TGA) of complexes 1–7 were studied from room temperature to 800 °C (Fig. S9†). For 1, a 2.87% weight loss occurs from 58 to 139 °C, which can be attributed to loss of the water molecule (calcd 3.07%). A second weight loss from 239 to 509 °C is found, which leads to the decomposition of the framework and the total mass loss is 79.0%. For 3, the first weight loss of 8.41% occurs in the range 105 to 157 °C, corresponding to the loss of all water molecules (calcd 8.38%). A further mass loss of 82.70% from 276 °C to 501 °C is caused by the decomposition of the framework. For 4 and 5, the trivial weight loss of about 1.72% for 4 and 3.27% for 5 takes place from 65 to 107 °C for 4 and 69 to 114 °C for 5, corresponding to loss of water molecule (calcd 1.70% for 4, 3.35% for 5). The complex framework decomposes from 250 to 501 °C (4) and 260 to 506 °C (5), the total mass loss is 84.7% (4) and 85.0% (5). For 2 and 6, no weight loss is observed until 271 °C for 2 and 240 °C for 6. The framework collapses in the temperature range of 271–498 °C for 2 and 240–509 °C for 6, with a total mass loss of 73.00% (2) and 83.80% (6). For 7, the gradually weight loss of about 5.15% takes place from 44 to 132 °C corresponding to the loss of all water molecules (calcd 5.05%). Further heating led to the decomposition of whole framework within 190–466 °C and the total weight loss is 86%.

3.4 Luminescent property

The fluorescent spectra of the seven complexes and the free ligands in the solid state were recorded at room temperature (Fig. S10†). Complexes 6 and 7 containing Cu(II)/Co(II) are almost no emission. This is probably due to Cu(II)/Co(II) with unpaired electrons, which can be efficient quenchers. However, complexes 1–5 containing Zn(II)/Cd(II) having d¹⁰ configurations show similar emissions, which come from the intraligand π*–π transition of the ligands.^1e In this study, complex 4 was selected as an example to further investigate the luminescence properties and the potential application of sensing. The emission spectrum of 4 in the solid state displays a broad band at 432 nm (Fig. 8). Compared with that of the ligands, the luminescence of 4 is stronger and, is mainly attributed to the coordination effect.¹⁰ Interestingly, the emission spectra of 4 dispersed in different solvents show a solvent-dependent luminescence. Fig. 9 illustrates the emission spectra of 4 dispersed in different solvents, such as N,N-dimethylformamide (DMF), formaldehyde, benzene, toluene, p-xylene, ethanol, o-xylene, chloroform, acetonitrile, m-xylene, 1,4-dioxane, water, isopropanol, triethylamine, pyridine, acetone and nitrobenzene (NB). The emission spectra of 4 dispersed in different solvents are similar to that of 4 in the solid state, the luminescence intensities are different (Fig. 10). Complex 4 in common solvents, such as DMF, formaldehyde, benzene, toluene, p-xylene, shows strong emission, indicating that complex 4 has good luminescence ability in these solutions. However, NB was proved to the distinctive emission quenching effect. The result suggests that complex 4 shows selectivity toward NB and is a selective luminescent probe for the pollutant molecule NB. To explore the detection limit of 4 as an NB probe, we prepared a series of suspensions of 4 immersed in DMF containing different NB concentrations (50, 100, 200, 300 and 400 ppm), and we recorded their luminescence spectra. When the NB content is gradually increased, the luminescence intensity of 4 gradually decreases (Fig. 11). Upon adding 50 ppm NB, the luminescence significantly decreases. The quenching efficiency is estimated to be 54% (the quenching efficiency is defined by (I₀ − I)/I₀ × 100%, where I₀ and I are the luminescence intensities of 4 before and after adding NB, respectively^3c). With the presence of 100–300 ppm of NB, the quenching efficiency is approximately 68–92%. These data indicates that Zn-MOF (4) is a potential highly sensor for NB. The PXRD pattern (Fig. S11†) of 4 immersed in DMF with NB is basically consistent with that of 4, indicating the structural stability of 4. NB is possibly adsorbed on the surface of the Zn-MOF, which does not change the framework structure of Zn-MOF. The possibility of a quenching mechanism for NB is assumed to originate from the electron transfer from Zn-MOF to NB. The ligands of Zn-MOF have rich electron. The nitro group has electron-withdrawing property, and the NB is the electron acceptor. It has been confirmed by molecular orbital theory, the LUMO energy level of NB represents can accept electron.^3k The result indicates the NB with an electron deficient functional group can capture electron from Zn-MOF framework. Thus, upon excitation, the excited state electron transfers from the Zn-MOF framework to NB molecule, resulting in the luminescence quenching of Zn-MOF. Notably, Zn-MOF can be reused and can be used as a recyclable sensing for NB (Fig. S12a†). Furthermore, the recycled compound 4 is still stable, as confirmed by powder X-ray diffraction (Fig.S13a†). Thus, the Zn-MOF has potential application in sensor.

Fig. 8 The solid state emission spectra of complex 4 and the free ligands.

Fig. 9 Emission spectra of complex 4 dispersed in different solutions (λ_ex = 310 nm).

Fig. 10 The max emission intensities of complex 4 dispersed in different solvents (λ_ex = 310 nm).

Fig. 11 Emission spectra of complex 4 dispersed in DMF with different concentrations of nitrobenzene (λ_ex = 310 nm).

The Zn-MOF framework (4) containing electron-rich π-systems shows a good ligand-based luminescence. Thus, we explored the possibility of sensitizing the luminescence of Ln(III) ions using Zn-MOF. Complex 4 was immersed in Ln(NO₃)₃ (Ln(III) = Eu(III), Tb(III), Sm(III), Dy(III)) aqueous solution containing an Ln(III) concentration of 10⁻² mol L⁻¹, yielding 4@Ln(III) suspensions, and the emission properties of 4@Ln(III) suspensions were measured at room temperature. These emission spectra exhibit the characteristic transitions of the corresponding Ln(III) ions. The 4@Tb(III) displays strong emissions of Tb(III) ion, whereas 4@Ln(III) (Ln(III) = Eu(III), Sm(III), and Dy(III)) demonstrates weak emissions of Ln(III) ions. The emission bands at 490, 544, 586, and 622 nm for 4@Tb(III) are attributed to the ⁵D₄ → ⁷F_J (J = 6–3) transitions of Tb(III) (Fig. 12a). The most intense transition is exhibited by ⁵D₄ → ⁷F₅ transition (at 544 nm), which produces green emission. A green color luminescence can be clearly seen by the naked eye under 365 nm UV-light. The ⁵D₄ Tb(III) lifetime value is 0.37 ms from its luminescent decay profile by fitting the data with a monoexponential curve (Fig. S14†). The emissions at 480 and 575 nm for 4@Dy(III) originate from the ⁴F_9/2 → ⁶H_J (J = 15/2, 13/2) transitions of the Dy(III) ion (Fig. 12b). 4@Eu(III) produces emissions at 592 and 615 nm, corresponding to the ⁵D₀ → ⁷F_J (J = 1, 2) transitions of Eu(III) ion (Fig. 12c). 4@Sm(III) displays emissions at 561, 597 and 650 nm, which are assigned to the ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2) transitions of Sm(III) ion (Fig. 12d). Compared with that of Ln(III), the characteristic emission intensity of Ln(III) from the 4@Ln(III) is enhanced. However, compared with that of 4, the emission intensity at 418 nm of the ligands is reduced. Ln(III) electronic transitions are forbidden by parity (Laporte) selection rules, leading to weak absorbance and very weak luminescence. However, an organic fluorophore can produce an antenna effect, and the antenna efficiently transfers energy to Ln(III) accepting levels to trigger their emission that make Ln(III) luminescence. Ln(III) ions dispersed in the Zn-MOF aqueous solutions can emit the characteristic fluorescence emissions of Ln(III), implying the existence of an efficient ligand-to-Ln(III) energy transfer process. The PXRD patterns (Fig. S11†) of the Zn-MOF dispersed in Ln(III) aqueous solutions are consistent well with the pattern from the Zn-MOF crystal data showing the crystal of Zn-MOF is still maintained, which indicates that the Ln(III) ions do not change the framework structure. Ln(III) may be adsorbed on the surface of the Zn-MOF particles. Similar phenomenon is found in the literature.^11c The Ln(III) luminescence from 4@Ln(III) can be attributed to the energy transfer mechanism, to the absorbed energy, which is transferred from the Zn-MOF framework into the excited states of the Ln(III) ion, resulting in Ln(III) fluorescence.^11c Comparison of the luminescence behavior of the Eu(III), Tb(III), Sm(III) and Dy(III) ions in Zn-MOF water clearly show that the Zn-MOF sensitizes Ln(III) luminescence much more for the Tb(III) than for the other Ln(III) ions because of the lower susceptibility of the Tb(III) ion to solvent-induced nonradiative decay processes. Interestingly, the included Ln(III) can be released after washing with water, and the spectrum exactly matched that of Zn-MOF. This finding shows that Zn-MOF can be repeatedly use to sensitize the Ln(III) luminescence (Fig. S12b†). The recycled compound 4 is still stable, which is confirmed by powder X-ray diffraction (Fig. S13b†). These results confirmed that Zn-MOF can act as an antenna and can sensitize the luminescence of Ln(III), especially for Tb(III) ion. We provide herein a promising distinct method, that is, the use of transition MOF-based pathway to achieve luminescence of Ln(III) in aqueous environment, which is a rare example of luminescent lanthanide materials.¹¹

Fig. 12 Emission spectra of 4@Ln(III)/water (Ln(III) = Tb(III) (a), Dy(III) (b), Eu(III) (c), and Sm(III) (d). 1 × 10⁻² mol L⁻¹) (λ_ex = 310 nm). Inset for (a), image under 365 nm UV-radiation of 4 water dispersed before and after in Tb(III) water solution. Inset for (c and d), local magnification (×30).

4. Conclusion

The combination of semi-rigid dicarboxylate ligands DPDC/O-OBA incorporating N-donor flexible ligands BPP/BTB with different metal centers afforded seven compounds, [Cd(DPDC)(BPP)H₂O]·H₂O (1), Cd₄(O-OBA)₄(BPP)₂ (2), [Zn₂(DPDC)₂(BTB)(H₂O)₂]·H₂O (3), [Zn(O-OBA)(BPP)]·0.5H₂O (4), [Zn(O-OBA)(BPP)]·H₂O (5), Cu(DPDC)(BTB) (6) and {Co(O-OBA)(BTB)H₂O]·0.5H₂O (7). These complexes exhibit diverse structural features based on the different coordination geometries of metal centers and on the different coordination modes and conformations of ligands. The combination of H-bonds and/or π-stacks led to diverse supramolecular architectures. Complex 4 can be used to detect nitrobenzene via quenching effect, especially can sensitize the Ln(III) luminescence.

Acknowledgements

The authors are grateful to the National Natural Science Foundation of China (21471104), the Science and Technology Program, Beijing Municipal Education Commission (KM201510028006) and Scientific Research Base Development Program of the Beijing Municipal Commission of Education.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1465150–1465156. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra08419b

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