Two isostructural fluorinated metal-organic frameworks with rare rod-packing architecture: syntheses, structures and luminescent properties

2.1 Synthesis and general characterization

Previous work by Bu [30], and Du [31] demonstrated that mixing 1,4-bis(imdazol-1-ylmethyl)benzene (bix) with 1,4-H₂NDC in the presence of Zn(II) and Co(II) ions produced unusual 3D 10-connected MOFs based on pentanuclear SBUs. Encouraged by these results, we were interested in extending these studies employing the use of the perfluorinated analogue Fbix. Compounds 1 and 2 were synthesized by the hydrothermal reactions of Fbix and the corresponding metal sources in the presence of 1,4-H₂NDC. It should be noted that, when we also tried to use NaOH to adjust the pH value of the mixture system, only white precipitates of unknown composition were obtained. Moreover, the synthesis of the compounds was sensitive to the reaction temperature. If the reaction temperature was set, for example, 10°C higher or lower, no suitable crystals were formed. Both complexes are stable at room temperature and insoluble in water and common organic solvents, which is consistent with their polymeric nature. The phase purity of the bulk samples of each complex was confirmed by the powder X-ray diffraction (PXRD) patterns, which matches approximately with the corresponding simulated ones obtained from the single-crystal data (see Fig. 1). For the IR spectra of 1 and 2, bands around 3112 cm⁻¹ are associated with aromatic C–H stretching vibrations. The weak absorption peaks around 2925 cm⁻¹ are the characteristic peaks of the methylene group of the Fbix ligand. The strong absorption bands in the range of 1650–1600 and 1420–1400 cm⁻¹ can be assigned to the asymmetric and symmetric stretching vibrations of carboxylate groups of 1,4-NDC.

Fig. 1:

Experimental and simulated PXRD patterns for complexes 1 (a) and 2 (b).

2.2 Description of the crystal structure

Single-crystal X-ray diffraction analysis has revealed that complexes 1 and 2 are isostructural (see Table 1), and thus herein only the crystal structure of 1 is described in detail as a typical example. Complex 1 crystallizes in the monoclinic space group P2₁/c and shows a 3D network structure. The asymmetric unit of 1 contains two crystallographically independent Cd(II) ions, one Fbix ligand, and two 1,4-NDC dianions. As shown in Fig. 2a, both Cd1 and Cd2 ions are six-coordinated and display a similar CdO₅N octahedral geometry, which is provided by five oxygen atoms from four 1,4-NDC ligands [Cd–O=2.159(3)–2.159(3) Å] and one nitrogen atom from a Fbix ligand [Cd–N=2.230(3) and 2.303(3) Å]. Each 1,4-NDC ligand is μ₄-bridging with one carboxylate group in a μ₂-η²:η¹-chelating/bridging mode and the other adopting a μ₂-η¹:η¹-syn-syn-bridging mode. As shown in Fig. 2b, two Cd(II) ions are linked in sequence by carboxylate groups of 1,4-NDC, forming an infinite 1D rod-like chain along the c axis. Corner-sharing CdO₅N polyhedra are alternately arranged in the rod with separations of adjacent Cd···Cd atoms of 4.087(1) and 4.130(1) Å. Each rod-shaped chain is linked to four neighboring chains through the naphthalene rings of 1,4-NDC ligands to generate a 3D subframework, as shown in Fig. 2c. The Fbix ligand adopts an anti-configuration, which links two rod-like chains along the b axis to construct an ultimate 3D framework (Fig. 2d). Further analysis of the structure reveals a hex-type net topology considering each rod-shaped SBU as a six-connected node (Fig. 2e). It should be noted that, to our knowledge, only limited examples belonging to 6-connected hex net based on infinite rod-shaped SBUs have been reported [8], [32]. Further, examination of the structure with PLATON [33] also shows that there is no solvent-accessible void in the structure of both complexes 1 and 2.

Fig. 2:

Views of (a) the coordination environment of Cd(II) ions in 1 (symmetry codes: #1, −x+1, −y, −z+2; #2, −x+2, y−1/2, −z+5/2; #3, x, −y−1/2, z+1/2; #4, −x+1, y−1/2, −z+3/2; #5, −x+2, −y+1, −z+2), (b) the chain structure of infinite rod-shaped SBUs, (c) the 3D framework by linking rod-shaped SBUs with 1,4-NDC, (d) the 3D structure of 1 (bright green: Fbix; hydrogen atoms have been omitted for clarity), and (e) the six-connected hex net of 1.

2.3 Thermal stability

In order to estimate the thermal stability of complexes 1 and 2, thermogravimetric (TG) analyses were performed under N₂ atmosphere in the temperature range of 30–800°C. The corresponding curves are depicted in Fig. 3. Compounds 1 and 2 exhibit high thermal stability, being stable up to 293 and 296°C, respectively, followed by two consecutive steps of weight loss up to 600°C. This result reveals close thermal decomposition behavior on account of their isomorphic features. The final solids hold weights of ca. 27.0% for 1 and 15.2% for 2 relative to the total sample, which are close to those of CdO (calculated: 26.7%) and MnO (calculated: 14.7%), respectively.

Fig. 3:

TGA curves of complexes 1 and 2.

2.4 Luminescent properties

The emission spectra of complexes 1 and 2 as well as of the free ligand Fbix were examined in the solid state at room temperature (Fig. 4). Excitation of the solid samples at 336 nm leads to the generation of broad fluorescence emission in the range of 465–540 nm. The main emission peak of Fbix is at 493 nm (λ_ex=338 nm), which may be ascribed to the π*→π transition. As for 1, a red-shift (11 nm) on the emission band is observed, which could most probably be attributed to an intraligand and/or ligand-to-ligand charge transfer [34], [35], because the Cd(II) ion is hardly oxidized or reduced, and thus the emission is neither ligand-to-metal charge transfer nor metal-to-ligand charge transfer in nature. The fluorescent emission of 2 with maximum at 495 nm may be attributed to the intraligand transition of coordinated Fbix ligands because similar emission at 493 nm was observed for the free Fbix ligand. Although the free dicarboxylate ligand 1,4-H₂NDC can also exhibit fluorescence (λ_em=508 nm) at room temperature [19], its emission band originating from the π*→n transition is weak, suggesting that the carboxylate ligand has no significant contribution to the fluorescence emission of complexes 1 and 2 in the presence of the N-donor ligand.

Fig. 4:

Solid-state fluorescent emission spectra of complexes 1 and 2 as well as the free ligand Fbix.

In summary, two new perfluorinated MOFs based on mixed Fbix and 1,4-NDC ligands have been synthesized under hydrothermal conditions and structurally characterized. Both complexes are isostructural and feature a rare 3D hex-type framework based on infinite rod-packing SBUs. The Cd(II) MOF displays a strong fluorescence emission in the solid state at room temperature. This work may provide useful information for employing fluorinated N-containing heterocyclic ligands and substituted aromatic carboxylates to assemble new functional MOFs with fascinating topologies and properties.

3.1 Synthesis of [Cd₂(Fbix)(1,4-NDC)₂]_n (1)

A mixture of Cd(NO₃)₂·4H₂O (61.7 mg, 0.2 mmol), Fbix (31.0 mg, 0.1 mmol), 1,4-H₂NDC (43.2 mg, 0.2 mmol), NaNO₃ (18 mg, 0.1 mmol), and water (7 mL) was stirred for 30 min at room temperature and then sealed in a 15 mL Teflon-lined stainless steel container, which was heated to 120°C and held at that temperature for 3 days. After cooling down to room temperature, yellow block-shaped crystals suitable for X-ray diffraction analysis were obtained in ca. 77% yield (37.1 mg, based on 1,4-H₂NDC). – Anal. for C₃₈H₂₂Cd₂F₄N₄O₈ (%): calcd. C 47.37, H 2.30, N 5.82; found C 47.52, H 2.28, N 5.76. – IR (cm⁻¹, KBr pellet): v=3113 (m), 3035 (m), 2974 (w), 2927 (w), 1621 (s), 1549 (m), 1493 (s), 1464 (m), 1418 (s), 1364 (m), 1298 (s), 1258 (m), 1200 (m), 1158 (w), 1089 (w), 931 (w), 774 (m), 753 (w), 616 (w).

3.2 Synthesis of [Mn₂(Fbix)(1,4-NDC)₂]_n (2)

Complex 2 was hydrothermally synthesized by a procedure similar to that of 1 except that Cd(NO₃)₂·4H₂O was replaced by MnCl₂·4H₂O (39.6 mg, 0.2 mmol). Yellow block-shaped crystals of 2 were isolated in ca. 54% yield (22.9 mg, based on 1,4-H₂NDC). – Anal. for C₃₈H₂₂F₄Mn₂N₄O₈ (%): calcd. C 53.79, H 2.61, N 6.60; found C 53.43, H 2.64, N 6.68. – IR (cm⁻¹, KBr pellet): v=3112 (m), 3033 (m), 2972 (w), 2925 (w), 1613 (s), 1550 (m), 1493 (s), 1465 (m), 1412 (s), 1365 (m), 1298 (s), 1261 (m), 1203 (m), 1085 (w), 863 (w), 797 (w), 775 (w), 660 (w).

3.3 X-ray structure determinations

The single-crystal X-ray diffraction measurements were performed on a Bruker Apex II CCD diffractometer at ambient temperature with MoK_α radiation (λ=0.71073 Å). A semiempirical absorption correction was applied using SADABS [36], and the program SAINT was used for integration of the diffraction profiles [37]. The structure was solved by Direct Methods using the SHELXS program of SHELXTL packages and refined anisotropically for all non-H atoms by full-matrix least squares on F² with SHELXL [38], [39], [40], [41]. In general, hydrogen atoms were located geometrically and allowed to ride during the subsequent refinement. Further crystallographic data and structural refinement parameters are summarized in Table 1, and selected bond lengths and angles are listed in Table 2.

CCDC 1496989 and 1496990 contain the supplementary crystallographic data for complexes 1 and 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.