A novel 2D infinite M₃L₂ cage-based Cd(II) microporous coordination polymer with a tripodal carboxylic acid ligand and solvent-dependent luminescence properties

Abstract

A novel M₃L₂ cage-based microporous coordination polymer assembled from the six-coordinated Cd(II) ion and a flexible tricarboxylic ligand has been designed and synthesized. The solvent-dependent luminescence properties revealed that the coordination polymer has an obvious, enhanced luminescence in the solvents CH₂Cl₂ and CHCl₃.

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Considering that cage-based compounds, especially metal organic complexes, can serve as very effective encapsulation devices for functional guest molecules, their self-assembly and applications have been well studied and summarized.¹ M₃L₂-type cage-like complexes, as the smallest metal organic cages, are commonly synthesized by using a tripodal pyridine-containing or imidazole-containing ligand with low-coordinated metal ions such as Ag⁺, Pt²⁺ and Pd²⁺.² Interestingly, Sun's group has firstly reported the single-crystal structure of a discrete M₃L₂ cage-like complex assembled with a tetrahedral zinc(II) center.³ Later, another discrete M₃L₂ cage was obtained by the assembly of Cu(II) salt and a flexible tri(oxamate)-base ligand.⁴ On the other hand, a M₃L₂ cage containing a silver(I) coordination polymer chain linked by SO₄²⁻ through a non-coordination bond has nearly been assembled.⁵ However, there are no related reports on the use of a carboxyl-containing ligand even with upper 4-coordinated metal ions, such as Zn²⁺ and Cd²⁺, to assemble the 1D, 2D or 3D structures by the stronger coordination bond. This may be due to the various coordinated modes of the carboxyl and the steric hindrance of the highly coordinated metal ions. It is well known that the widely used polycarboxylic ligands play an important role in the construction of various metal organic complexes. So, making full use of polycarboxylic ligands to construct special structural and functional metal organic complexes is very urgent.

The self-assembly of microporous coordination polymers (MCPs) is a highly topical area of current research that has yielded many fascinating architectures and revealed enormous potential applications in catalysis,⁶ the capture and degradation of toxic gases,⁷ NLO optics,⁸ biomedical imaging⁹ and molecular sensors.¹⁰ Among the fruitful products of functional MCPs reported to date, luminescent MCPs can be viewed as the most promising class of materials due to their potential applications. In fact, recently, some luminescent MCPs have been successfully used in the sensing of solvents such as DMF, acetone and water.¹¹ Considering the wide use of MCPs, the study of solvent-dependent luminescence properties in common solvents is necessary.

Herein, we used a bowl-shaped tripodal carboxyl acid ligand, 1,1′,1′′-((2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene))tris (pyridine-1-ium-4-carboxylate) (named L, as shown in Scheme 1),¹² and cadmium ion, which has a distorted octahedral configuration, to construct a M₃L₂ cage expanded by 1,3,5-benzenetricarboxylic acid (H₃BTC) to form a 2D infinite M₃L₂ cage-based framework [(BTC)_2/3(L)_2/3Cd]·(H₂O)_1.5·(DMF)_0.5 (compound 1).§ The 3D structure of compound 1 is formed by the hydrogen bonds and π–π stacking between the 2D planer structures. To the best of our knowledge, this is the first MCP based on a M₃L₂ cage constructed by Cd(II) ions and with flexible tripodal carboxyl-containing ligands. Furthermore, analysis of the solvent-dependent luminescence properties revealed that compound 1 has an obvious, enhanced luminescence in the solvents CH₂Cl₂ and CHCl₃, respectively.

Scheme 1 The structure of L ligand.

Single-crystal X-ray diffraction reveals that compound 1 crystallizes in the centrosymmetric trigonal space group R3̄c, which is associated with the point group D_3d(-3m). The asymmetric unit is comprised by half Cd(II) atom, one-third L ligand, and one-third BTC³⁻ (Fig. S8, ESI†). As shown in Fig. 1a, the Cd atom occupies the centre of a distorted octahedron defined by two carboxylates in bidentate chelating mode from two different BTC³⁻ ligands and two carboxylates in monodentate coordination mode from two independent L ligands. The Cd–O bond lengths range from 2.27(2) to 2.49(7) Å, which are comparable with those reported in other Cd(II)–carboxylic complexes.¹³ The O–Cd–O angles range from 51.5(5) to 139.6(2)°, and the tetrahedron length ranges from 4.03(9) to 5.77(1) Å. In the viewpoint of the L ligand, three methylene-4-carboxylic acid groups of L are oriented towards the same side of the basic phenyl group, which results in a bowl-shaped configuration of L. Two L ligands, just facing opposite, are linked by three Cd(II) ions to form the interesting M₃L₂ cage (Fig. 1a). The diameter of the cage is about 12.6 Å. Then each M₃L₂ cage is linked by six BTC³⁻ to form a 2D coplanar structure (Fig. 1c). In the side of BTC³⁻, each BTC³⁻ is linked by three BTC³⁻, which are coordinated by Cd(II) ions to form a 2D plane (Fig. 1b). So, the resulting M₃L₂-based MCP is obtained due to the contribution of L ligand and BTC³⁻. Because of C–H⋯O hydrogen bonds and π–π stacking (Fig. 2), the 2D coplanar structures link with each other to form the 3D architecture.

Fig. 1 (a) The structure of M₃L₂ cage in compound 1. (b) The 2D planar structure constructed by BTC³⁻ and Cd²⁺. The 2D structures of compound 1 in c direction (c) and b direction (d). The H atoms are omitted for clarity. The solvent molecules were removed from the single-crystal X-ray diffraction data, in order not to be represented in the structure of compound 1.

Fig. 2 The 3D structure of compound 1 results from the interaction of 2D structures in b direction (different colors display different 2D structures). The H atoms and solvent molecules are omitted for clarity; the interactions between the 2D structures are clearly shown in the blown-up illustration. The pink dotted line represents the C–H⋯O hydrogen bond, while the black line is the π–π stacking.

The solid diffuse reflectance spectrum revealed that compound 1 was a wide-gap semiconductor with band gap of 3.45 eV (Fig. S5, ESI†). The photoluminescent properties of L ligand, H₃BTC and compound 1 in solid state were investigated at ambient temperature (Fig. S6, ESI†). Upon excitation at 300 nm, peaks at 372 and 397 nm for H₃BTC, 402 and 460 nm for L ligand, and 399 and 462 nm for compound 1 were found in the photoluminescence spectrum. Compared with the free L ligand, the emission band locations of compound 1 were comparable but with an increasing band height. This may be attributed to the coordination effect of the ligands to Cd ions, which increased the structural rigidity and reduced the nonradiative decay of the intraligand.¹²

The most interesting feature of compound 1 is that its PL spectrum is largely dependent on the solvent molecules, particularly in the case of CH₂Cl₂ and CHCl₃. As shown in Fig. 3, compared with DMF, ethanol, acetone, acetonitrile, methanol and THF, the intensities of compound 1 suspended in CH₂Cl₂ and CHCl₃ were greatly enhanced. The other solvents did not exhibit changes at 395 nm. In order to give a possible fluorescence enhancement mechanism, ¹H-NMR and TGA experiments were conducted on the solvent-exchanged samples. The results of ¹H-NMR revealed that both CH₂Cl₂ and CHCl₃ cannot exchange with the guest molecules in the framework of compound 1 (Fig. S7, ESI†). The result of TGA experiments further proved no exchange between the CH₂Cl₂, CHCl₃ and guest molecules (Fig. S3, ESI†). So, the probable reason may be the solvent-dependent surface-enhanced fluorescence, which has been studied in capped Au or Ag metallic nanostructures, mono-layer organic structures, and quantum dots.¹⁴ Although some fluorescent MOFs relying on guest molecules have been observed previously,¹¹ most examples are in the solvents DMF, acetone and water through solvent exchange. Here, we have presented a rare example of obvious surface-enhanced luminescence in the CH₂Cl₂ and CHCl₃ solvents. To the best of our knowledge, such halomethane solvents for fluorescent MOFs have not previously been reported.

Fig. 3 The PL intensities at 395 nm of compound 1 in suspension using various pure organic solvents under the same testing conditions. The inset picture shows the observed photoluminescence spectra of compound 1 in suspension in organic solvents when excited at 300 nm.

In summary, we have designed and synthesized a 2D infinite M₃L₂ cage-based MCP comprised by six-coordinated Cd(II) ion and a flexible tricarboxylic ligand. The solvent-dependent luminescence properties revealed that compound 1 has an obvious, enhanced luminescence in the CH₂Cl₂ and CHCl₃ solvent. The results could immensely expand the design and synthesis of special cage-based MCPs assembled with well-designed ligand and medium coordinated metal ions, and hence push the application of cage-based MCPs in the functional materials field by encapsulation with functional guest molecules. The guest-functionalized MCPs will have wide applications in nonlinear optics, biological imaging and drug delivery that depend on the function of the guest molecules. Furthermore, some potential halomethane sensors may be found in the field of MCPs. Further work for preparing new, stable cage-based MCPs functionalized with nonlinear optical molecules is in progress.

This work was supported by the National Natural Science Foundation of China (grant no. 21201005, 201201004, 21271004, 21275006, 21273008 and 51372003), the Natural Science Foundation of Anhui Province (grant no. 1308085QB33 and 1208085MB22), and the China Postdoctoral Science Foundation Funded Project (grant no. 2013M541810).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Synthetic conditions, PXRD setup and patterns, CIF file for compound 1, TGA data, solid-state diffuse reflectance spectra, solid-state PL spectra, additional picture of compound 1. CCDC 1402379. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5nj02144h

‡ These authors contributed equally.

§ To the solution of L ligand (0.0340 g, 0.065 mmol) and H₃BTC (0.0210 g, 0.100 mmol) in 4 mL DMF and 1 mL H₂O was added the solution of Cd(NO₃)₂·4H₂O (0.0462 g, 0.150 mmol). Then 1 drop of 6 mol L⁻¹ HCl was added. The mixture was heated to 90 °C in a sealed vial for 72 h, then allowed to cool down to room temperature. The colorless block crystals (0.0174 g) of 1 were obtained by filtration and washed with DMF and CH₂Cl₂ three times, respectively. Yield was 60.42% (based on L ligand). Calcd for C_27.5H_26.5O₁₀CdN_2.5 (M_r = 664.4): C, 49.71; H, 4.02; N, 5.27%. Found: C, 49.90; H, 3.85; N, 5.41%. FT-IR (KBr pellet, cm⁻¹): 3416 s, b, 3114 w, 3053 w, 1619 vs, 1576 vs, 1438 m, 1370 vs, 1250 w, 1206 w, 1161 w, 1129 w, 1104 w, 1043 w, 935 w, 876 w, 799 w, 770 m, 732 m, 696 w.

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