pH variation induced construction of a series of entangled frameworks based on bi- and tri-metallic cores as nodes

Abstract

Reactions of 1,4-bis(pyridin-4-ylmethoxy)benzene (L) and zinc acetate salts with different multi-caboxylate acids: 1,4-benzenedicarboxylic (p-H₂BDC), isophthalic acid (m-H₂BDC) and benzene-1,3,5-tricarboxylic acid (H₃BTC) yield four entangled structures and one 3D supramolecular framework depending on pH variation. The structures of compounds 1–5 were elucidated by single crystal X-ray diffraction. Topology analysis revealed that the entangled structures of compounds 1–4 covered a range of poly-threading, self-penetrating and interpenetrating coordination. Interestingly, the poly-threaded arrays with cyclohexane-like windows in chair conformation in compound 2 was firstly observed. What's more, compound 4 exhibited a rare eight-connected self-penetrating network based on trinuclear zinc clusters as nodes with 4²⁴6⁴ topology. Photoluminescent properties and thermogravimetric analyses for 1–5, were also investigated in detail.

---

Introduction

In recent years, coordination polymers have enjoyed extensive exploration not only stemming from their potential applications ranging from non-linear optics to magnetism, but also from their long-standing fascination of topologies.¹ Entanglements, serving as a crucial subject in the area of supramolecular chemistry, are common in biology as seen in catenanes, rotaxanes, and molecular knots.² A variety of attractive architectures ascribed to this category have been constructed and fully discussed in some excellent papers.³ Interpenetration⁴ recognized as one major theme of an entangled system, has been comprehensively investigated as shown by Batten and Robson.⁵ Differing from interpenetration, self-penetrating⁶ nets are characterized by the presence of the smallest topological rings or knots, which can be catenated by other shortest rings belonging to the same net. However, poly-threading⁷ coordination networks possess periodic analogues of the molecular rotaxanes or pseudo-rotaxanes, meanwhile, showing potential applications in the area of drug delivery vehicles and sensor devices.⁸ Unfortunately, only a limited number of cases with respect to this topological nature have been reported to date.⁹ Therefore, the exploration of new synthetic routes to this class of entanglement architectures is one of the most challenging issues in current synthetic chemistry. Indeed, a systematic investigation to establish useful relationships between structures and properties is unavoidable.

Taking inspiration from previous work, ongoing research in our laboratory has been directed toward the design and synthesis of novel entangled networks.¹⁰ Conformationally, non-rigid ligands are usually the typical building elements for the assembly of interesting entangled structures, thanks to their varied geometries.¹¹ In contrast, the rigid ligands usually result in larger voids that may lead to an intriguing variety of structures and potential applications.¹² Based on an overall consideration, we chose the semi-rigid 1,4-bis(pyridin-4-ylmethoxy)benzene (L) ligand (Scheme 1) for the following reasons: (i) a rigid spacer of a phenyl ring could provide comparative stability as well as enough void within the whole framework; (ii) the freely rotating pyridyl arms may afford variable entanglement skeletons; and (iii) few cases with respect to entangled frameworks based on the L ligand have been reported.¹³ Fortunately, we obtained four entangled frameworks ranging from self-threading to poly-threading arrays by using three types of multi-caboxylate acids as auxiliary ligands. Compound 5 is synthesized under pH variation, which is a 3D supramolecular structure extended by weak interactions between adjacent layers.

Scheme 1 Molecular structure of 1,4-bis(pyridin-4-ylmethoxy)benzene ligand.

Herein, we report the synthesis and characterization of four entangled structures and one 3D supramolecular framework, {[Zn₂(L)₂(HBTC)₂]·H₂O}_n (1), {[Zn₂(L)(HBTC)₂]·H₂O}_n (2), {[Zn₂(L)(m-BDC)₂]H₂O}_n (3), [Zn₃(L)(p-BDC)₃]_n (4), {[Zn₂(L)(HBTC)₂]·H₂O}_n (5), (L = 1,4-bis(pyridin-4-ylmethoxy)benzene; p-H₂BDC = 1,4-benzenedicarboxylic; m-H₂BDC = isophthalic acid; H₃BTC = benzene-1,3,5-tricarboxylic acid). It should be mentioned that both compounds 1 and 2 exhibit the poly-threaded frameworks involving 2D coordination layers with dangling ligands, which are still rare in the system of entangled frameworks. Moreover, the poly-threaded arrays with cyclohexane-like windows in chair conformation in compound 2 are unprecedented. The structure of compound 3 possesses a three-fold interpenetrated 3D network structure with α-Po topology that is built from dinuclear zinc units with a paddle-wheel structure. While, compound 4 is an eight-connected self-penetrating framework based on trinuclear zinc clusters as node with a 4²⁴6⁴ topology. Compound 5 shows a 3D supramolecular structure extended by hydrogen bonds interaction in the adjacent layers.

Experimental section

Materials and physical measurements

All reagents and solvents employed were commercially available and used as received without further purification. The ligand L was synthesized readily by the procedure reported in the literature.¹³ Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 240C elemental analyzer. The Fourier transform infrared (FT-IR) spectra were recorded from KBr pellets in the range 4000–400 cm⁻¹ on a Mattson Alpha-Centauri spectrometer. XRPD patterns were collected on a Rigaku Dmax 2000 X-ray diffractometer with graphite monochromatized Cu Kα radiation (λ = 0.154 nm) and 2θ ranging from 5 to 50°. Solid-state luminescent spectra were measured on a Cary Eclipse spectrofluorometer (Varian) equipped with a xenon lamp and quartz carrier at room temperature. Thermogravimetric analysis (TGA) was performed on a Perkin-Elmer TG-7 analyzer heated from 40 to 800 °C under nitrogen.

Synthesis of {[Zn₂(L)₂(HBTC)₂]·H₂O}_n (1)

A mixture of Zn(Ac)₂·2H₂O (0.25 mmol, 0.055 g), L ligand (0.10 mmol, 0.029 g) and H₃BTC (0.20 mmol, 0.042 g) in H₂O (14 mL) was adjusted to approximately pH ≈ 3–4 with H₂SO₄ (6 M) stirred for 1 h and then transferred and sealed in a 25 mL Teflon-lined stainless steel container, which was heated at 150 °C for 72 h and then cooled down to room temperature at a rate of 5 °C h⁻¹. Colorless crystals of 1 were collected and washed with distilled water and dried in air to give the product; yield, 42.5% (based on Zn^II salts). Elemental analyses calcd (%) for C₅₄H₄₂N₄O₁₇Zn₂ (1149.70): C, 56.36; H, 3.65; N, 4.87. Found C, 56.01; H, 3.10; N, 4.18.

Synthesis of {[Zn₂(L)(HBTC)₂]·H₂O}_n (2)

Compound 2 was prepared in a manner similar to that used to prepare compound 1 at pH ≈ 5.5 instead of pH ≈ 3–4 with H₂SO₄ (6 M). Colorless crystals of 2 were collected and washed with distilled water and dried in air to give the product; yield, 32.5% (based on Zn^II salts). Elemental analyses calcd (%) for C₃₆H₂₆N₂O₁₅Zn₂ (857.37): C, 50.39; H, 3.03; N, 3.27. Found C, 48.56; H, 2.01; N, 2.92.

Synthesis of {[Zn₂(L)(m-BDC)₂]·H₂O}_n (3)

A mixture of Zn(Ac)₂·2H₂O (0.20 mmol, 0.044 g), L ligand (0.10 mmol, 0.029 g) and m-H₂BDC (0.15 mmol, 0.025g) in H₂O (14 mL) was adjusted to approximately pH ≈ 6 with H₂SO₄ (6 M) and NaOH (1 M). Yield, 59.7% (based on Zn^II salts). Elemental analyses calcd (%) for C₃₄H₂₆N₂O₁₁Zn₂ (769.33): C, 53.03; H, 3.38; N, 3.64. Found C, 52.16; H, 3.07; N, 3.23.

Synthesis of [Zn₃(L)(p-BDC)₃]_n (4)

Compound 4 was synthesized in an analogous manner to compound 3 except that m-H₂BDC (0.15 mmol, 0.025g) was replaced by p-H₂BDC (0.30 mmol, 0.050 g). Yield, 50.4% (based on Zn^II salts). Elemental analyses calcd (%) for C₄₂H₂₈N₂O₁₄Zn₃ (980.83): C, 51.39; H, 2.85; N, 2.85. Found C, 52.15; H, 2.24; N, 2.43.

Synthesis of {[Zn₂(L)(HBTC)₂]·H₂O}_n (5)

Compound 5 was prepared in a manner similar to that used to prepare compound 1 at pH ≈ 7 instead of pH ≈ 3–4 with H₂SO₄ (6 M) and NaOH (1 M). Colorless crystals of 5 were collected and washed with distilled water and dried in air to give the product; yield, 49.5% (based on Zn^II salts). Elemental analyses calcd (%) for C₃₆H₂₆N₂O₁₅Zn₂ (857.37): C, 50.39; H, 3.03; N, 3.27. Found C, 49.01; H, 2.01; N, 3.09.

X-Ray crystallography study

Data collection of complexes 1–5 was performed on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at room temperature. All absorption corrections were performed by using the SADABS program. The crystal structure was solved by direct methods and refined with full-matrix least-squares (SHELXTL-97)¹⁴ with atomic coordinates and anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms of aromatic rings were included in the structure factor calculation at idealized positions by using a riding model. Protonated hydrogen atoms attached to nitrogen atoms were bound at idealized positions. In compounds 1, 2 and 3, H atoms of water molecules could not be introduced in the refinement but were included in the structure factor calculation. The detailed crystallographic data and structure refinement parameters are summarized in Table S1 in the ESI.† The IR data of five compounds were provided in the ESI.† Selected bond lengths and angles for complexes 1–5 are given in Table S2 (in the ESI†).

Results and discussion

Structure of {[Zn₂(L)₂(HBTC)₂]·H₂O}_n (1)

Single-crystal X-ray diffraction analysis exhibits that compound 1 crystallizes in the monoclinic space groupP2₁/c and the asymmetric unit of 1 is constructed from two Zn^II ions, two L ligands, two HBTC ligands and one free water molecule. Each Zn^II ion in the dinuclear motif is coordinated by four carboxylic oxygen atoms of HBTC ligands (2.036–2.107 Å) and one nitrogen atom of an L ligand (2.025–2.043 Å)¹⁵ to furnish a square-pyramidal geometry as shown in Fig. 1a. Two crystallographically equivalent Zn^II ions are bridged by four carboxylates bonded in the bridging bis(bidentate) fashion to give a paddle-wheel shaped [Zn₂(CO₂)₄] fragment in which the Zn⋯Zn distance is 2.988 Å. The axis sites of each Zn₂ paddle wheel are occupied by two additional L ligands.

Fig. 1 (a) Coordination environment of Zn^II ions in 1, hydrogen atoms were omitted for clarity (symmetry transformations used to generate equivalent atoms: A, x, 0.5 − y, 0.5 + z). (b) The structure of a single 2D motif showing the dangling arms. (c) Schematic illustration of the mutual poly-threading of the 2D sheets in 1. (d) A schematic illustration of the mutual poly-threading of four layers.

Compound 1 is an extended poly-threaded framework involving 2D coordination layers of square (4,4) topology with dangling ligands. The nodes within the layers are represented by dinuclear units that are bridged by the HBTC ligand and a 2D grid with large rhombic windows (dimensions 9.90 Å × 9.90 Å); dangling L ligands are coordinated in the axial sites of dinuclear nodes and disposed in a mutual anti orientation with respect to the layer plane (Fig. 1b, 1c). The layers are stacked in a parallel manner along the [0 0 1] direction in an ABAB sequence at a distance of approximately 15.18 Å. Each dangling arm has an effective length of about 18.55 Å. As a result, the dangling L ligands of each layer are threaded into the rhombic voids of the two adjacent layers above and below, thus every rhombic window of each layer is threaded by only two dangling ligands that come from opposite directions (this is expected considering that the dangling arms reside on the dinuclear nodes). Finally, this unique simultaneous threading fashion of the adjacent polymeric motifs gives the novel (2D → 3D) poly-threaded array observed for compound 1 (Fig. 1d).

Structure of {[Zn₂(L)(HBTC)₂]·H₂O}_n (2).

X-Ray crystallography shows that compound 2 crystallizes in the monoclinic space groupP2₁/c and the asymmetric unit of 2 contains two Zn^II ions, one L ligand, two HBTC ligands and one isolated water molecules. The Zn^II ions display two coordination modes. One Zn^II (Zn2) ion is coordinated by four oxygen atoms from four HBTC ligands to furnish the distorted tetrahedral coordination geometry. While, the other Zn^II (Zn1) ion is ligated with three oxygen atoms from three HBTC ligands and one nitrogen atoms from one L ligand to furnish the distorted tetrahedral coordination geometry, as shown in Fig. 2a. The Zn–O (1.945 Å–2.076 Å) and Zn–N (2.030 Å) bond distances are all in the normal ranges.¹⁵

Fig. 2 (a) Coordination environment of Zn^II ions in 2, hydrogen atoms were omitted for clarity (symmetry transformations used to generate equivalent atoms: A, −x, 0.5 + y, 1.5 − z; B −x, −y, 1− z). (b, c) The structure of a single 2D motif showing the dangling arms in 2. (d) A schematic illustration of the mutual poly-threading of five layers.

Compound 2 is an extended threaded framework involving 2D coordination layers of hexagonal (6³) topology with dangling ligand. In these layers, two Zn^II ions, related by a two-fold axis, are bridged by two pairs of HBTC carboxylate ends into a dinuclear unit with a Zn(1)⋯Zn(2) distance of 3.435 Å. Three factors can be envisaged to play a role in the generation of mutual poly-threading in 2: (i) the presence within the 2D motif of large hexagonal windows; (ii) the dangling Lgroups, occupying the axial position of each metal atom, are just like open arms nearly perpendicularly protruding from both sides of the sheets; and (iii) all the layers are stacked parallel and arranged in a staggered fashion, and the distance between the layer is shorter than the effective length of lateral arm, thus providing the possibility for the ultimate realization of poly-threading net (as shown in Fig. 2b, 2c). Under this premise, the dangling arms of each layer are threaded into the hexagonal voids of the adjacent layers, in a mutual relationship. Each hexagonal window is therefore penetrated by two L molecules that belong to two different sheets, one entering from one side and the other from the opposite one. This results in a novel 3D poly-threaded array (2D → 3D), originating from the entanglement of three adjacent polymeric units at a time (Fig. 2d).

On the basis of the aforementioned, the most fascinating feature of 2 is that also shows cyclohexane-like windows in chair conformation. Meanwhile, the dangling L ligand can be thought of as a hydrogen atom in the cyclohexane structure (Fig. S1 in the ESI†). Thus, the dangling L ligand exhibits the equatorial bonds with the chair of cyclohexane. Meanwhile, the chair of conformation stabilizes the solid-state structures.

Poly-threaded structures containing finite components are, at present, rare.⁹ The few species known include poly-threaded 0D rings with side arms that give 1D^7b or 2D arrays,^7c as well as molecular ladders with dangling arms that result in (1D → 2D)^17d or (1D → 3D)^7e poly-threaded arrays. Until now, the only known example of (2D → 3D) structures have been reported for poly-threading systems in metal–organic frameworks by us,^9a,b in which the threading involves the two nearest layers only because of the shorter length of the dangling arms. Meanwhile, it should be pointed out that the structures of 1 and 2 are an extended threaded framework involving 2D coordination layers with dangling ligands. However, compound 2 is an unprecedented poly-threaded array with cyclohexane-like windows in chair conformation.

Structure of {[Zn₂(L)(m-BDC)₂]·H₂O}_n (3)

The structure of 3 contains two Zn^II ions, one L ligand, two m-BDC ligands and one isolated water molecule in the asymmetric unit. Compound 3 is a three-fold interpenetrated 3D network structure of α-Po topology that is built from dinuclear Zn₂ units with a paddle-wheel configuration. Fig. 3a illustrates the coordination environment of the Zn^II ions and the six connectivity of the bimetallic unit. Each Zn^II ion in the dinuclear motif is coordinated by three carboxylic oxygen atoms of m-BDC ligands (Zn–O 1.952–1.977 Å) and one nitrogen atom of an L ligand (Zn–N1 2.025 Å)¹⁵ to furnish a distorted tetrahedral geometry. Two crystallographically equivalent Zn^II ions are bridged by two carboxylates with bis(bidentate) fashion to furnish the [Zn₂(CO₂)₄N₂] fragment in which the Zn⋯Zn distance is 3.634 Å. The axis sites of each Zn₂ paddle wheel are occupied by two additional L ligands via a nitrogen atom. Each binuclear unit is connected to six others through four bridging m-BDC and two L ligands to generate an extended neutral 3D network (Fig. 3b), which can also be considered to be constructed from distorted 2D square-grid (4,4) layers of composition [Zn(m-BDC)] pillared by the long L ligand (Fig. S2 in the ESI†). The overall topology of the 3D frame is best described as a compressed α-Po net based on three intersecting (4,4) nets that possesses large distorted cube-like cavities of approximately 10.34 × 10.34 × 21.48 ų (Fig. 3c). The large voids formed by a single 3D network allow incorporation of two other identical networks, thus giving a three-fold interpenetrated α-Po-related network as shown in Fig. 3d.

Fig. 3 (a) Coordination environment of Zn^II ions in 3 with the ellipsoids drawn at the 30% probability level, hydrogen atoms were omitted for clarity (symmetry transformations used to generate equivalent atoms: A, −1 − x, 1 − y, −z; B, x, −y, −0.5 + z). (b) View of one independent primitive cubic net. (c) A single distorted cube-like unit of the α-Po net with the relative dimensions (10.34 × 10.34 × 21.48 Å) of α-Po topology. (d) A schematic illustration of the 3-fold interpenetrated structure.

Structure of [Zn₃(L)(p-BDC)₃]_n (4)

Single-crystal X-ray structure analysis reveals that compound 4 crystallizes in the triclinic space groupP1̄ and the fundamental building unit of 4 consists of one-and-a-half Zn^II ions, one L ligand with half occupation and one-and-a-half p-H₂BDC ligands. Meanwhile, the structure of 4 consists of trinuclear zinc clusters, in which 1.5 crystallographically independent Zn^II ions exhibit different coordination spheres. One Zn^II center (Zn1) lies on a center of symmetry and is coordinated by six carboxylic oxygen atoms from six different p-H₂BDC ligands (Zn–O 2.074–2.150 Å) (Fig. 4a). The other Zn^II center (Zn2) is ligated with three carboxylic oxygen atoms (Zn–O 1.930–2.150 Å) from three p-H₂BDC ligands, one nitrogen atom from an L ligand (Zn–N 2.047 Å).¹⁵ The Zn1 is connected to two adjacent Zn2 centers in a corner-sharing mode to form a [Zn₃O₁₂N₂C₆] core via carboxylate oxygen atoms with a non-bonding Zn⋯Zn distance of 3.633 Å (Fig. S3 in the ESI†). As illustrated in Fig. 4b, 4c, there are eight organic ligands (six p-H₂BDC and two L) surrounding each Zn₃ unit. This, therefore, defines an eight-connected node which is further linked to eight nearest neighbors with distances of 10.660–25.660 Å through eight bridging ligands. This process, when repeated, results in a unique 3D uninodal eight-connected self-penetrating network as shown in Fig. 4d.¹⁶ If the trinuclear zinc clusters take the place of balls, compound 4 shows that it is three types of links of different lengths of type I (20.331 Å), II (11.823 Å) and III (11.476 Å). If we can imagine removing type I (20.331 Å) or II (11.823 Å), then compound 4 shows that it is a three-dimensional six-connected framework with the distorted α-Po topology. Further insight into the nature of this intricate architecture can be obtained by considering the net constructed from two interpenetrating primitive cubic nets (α-Po), which are cross-linked by two extra connections from each node along the cube diagonals (Fig. 4e), and the extension of this structure results in the eight-connected self-penetrating coordination net, in which trinuclear zinc clusters act as eight-connected nodes.

Fig. 4 (a) Coordination environment of Zn^II ions in 4 with the ellipsoids drawn at the 30% probability level, hydrogen atoms were omitted for clarity (symmetry transformations used to generate equivalent atoms: A, −x, 1 − y, −z). (b) Perspective views of the eight-connecting trinuclear zinc clusters as nodes. (c) The symbols of three types of linkers are I (25.66 Å), II (10.66 Å), and III (11.47 Å). (d) Topological representation of 4 showing the 4²⁴6⁴ topology. (e) Two cross-linked interpenetrating α-Po nets.

Structure of {[Zn₂(L)(HBTC)₂]·H₂O}_n (5)

Single-crystal X-ray diffraction analysis reveals that compound 5 crystallizes in the triclinic space groupP1̄. In the asymmetric unit of 5, there are two kinds of crystallographically unique Zn^II ions, one L ligand, two HBTC ligands and one water molecule. The Zn^II ion shows two coordination modes. One Zn^II (Zn1) ion is coordinated by three carboxylic oxygen atoms from three HBTC ligands and one nitrogen atom from an L ligand. Meanwhile, the other Zn^II ion (Zn2) is coordinated by four oxygen atoms from three HBTC ligands and one water molecule to furnish a distorted tetrahedral coordination geometry. The Zn–O (1.913 Å–1.977 Å) and Zn–N (2.046 Å) bond distances are all in the normal ranges,¹⁵ as shown in Fig. 5a. In addition, each HBTC ligand bridges two Zn^II ions to form a cationic [{Zn₄(HBTC)}₄] square with dimensions of 9.66 × 9.40 Å (Fig. S4 in the ESI†). Meanwhile, the coordinated L ligands thread into the void which is constructed by the four HBTC ligands and four Zn^II ions. The extension of the void results in infinite 1D tube structures (Fig. S5 in the ESI†) which are linked in a parallel fashion with the neighboring ones to give rise to a 2D layer (Fig. 5b). In summary, compound 5 can be described as a 3D supramolecular architecture further formed via the hydrogen bonds in the adjacent layers. Moreover, there are the hydrogen-bonding interactions (Fig. 5c) between the coordinated water molecules from one entangled network and carboxylic oxygen atoms from the other in the adjacent 2D layers (dOw⋯O8A, 2.705 Å, A, −x, 2 − y, 1 − z; dOw⋯O18B, 2.675 Å, A, −x, 2 − y, 1 − z; B, −1 + x, y, z), which extend the 2D layers into a 3D supramolecular structure (Fig. 5d). Both the hydrogen bonds and the special pattern make the whole structure of the title compound much tighter. The hydrogen bonding data are summarized in Table S3 (in the ESI†).

Fig. 5 (a) Coordination environment of Zn^II ion in 5 with the ellipsoids drawn at the 30% probability level, hydrogen atoms were omitted for clarity (symmetry transformations used to generate equivalent atoms: A, 2 − x, 1 − y, −z). (b) Polyhedral representation of the 2D metal–organic framework of compound 5. (c) The O–H⋯O hydrogen bonds in the compound 5. (d) Schematic representation of crystal structures in 5 along a axis direction.

Effect of coordination modes of carboxylate ligands

Multidentate carboxylate ligands have been extensively applied in the construction of a rich variety of MOFs because of their diverse coordination modes and structural stability.¹⁷ It is instructive to compare the coordination modes of carboxylate ligands. In 1, the rigid H₃BTC exhibits a bis(bidentate) coordination mode (Fig. S6a in the ESI†). Whereas in 2, the H₃BTC exists bidentate and monodentate modes (Fig. S6b in the ESI†). Notably, both of them have a free carboxylate which does not bridge the metal ions, and this coordination mode of the H₃BTC ligand is very important for the poly-threading structure. In 5, apart from in the bis(monodentate) only binding mode, H₃BTC ligand coordination tends to favor the formation of a dimeric unit, due to coordinating in both bis(monodentate) and bidentate modes, with no free carboxylate (Fig. S6d,e in the ESI†). However, the bidentate and monodentate modes of m-H₂BDC in 3 and the bis(bidentate) modes of p-H₂BDC in 4 are very important in chelating metal ions and can lock their positions into M–O–C clusters and is a key factor for the formation of the high dimension structures¹⁸ (Fig. S6f,g in the ESI†). Thus the coordination modes of carboxylate ligands influence the metal nuclearity and hence the connectivity of the resulting net.

The simulated and experimental PXRD patterns of 1–5 are shown in Fig. S7–S11 (in the ESI†). The simulated and experimental PXRD patterns are in good agreement with each other, indicating the phase purity of the products. The differences in intensity may be due to the preferred orientation of the powder samples.

Luminescent properties

Taking into account the excellent luminescent properties of Zn^II, the solid-state luminescent spectra of 1–5 have been studied at room temperature. Complexes 1, 2 and 3 show the emission maxima at 423 nm (λ_ex = 365 nm), 423 nm (λ_ex = 362 nm), 424 nm (λ_ex = 369 nm). Meanwhile, complex 4 shows the emission maxima at 422 nm (λ_ex = 365 nm), and complex 5 shows the emission maxima at 422 nm (λ_ex = 365 nm). In order to understand the nature of the emission, we analyzed the photoluminescence property of the L ligand and found a strongest emission peak at 394 nm (λ_ex = 325 nm) for L. The emission of the ligands may be assigned to π*→n and π*→π transitions of the intraligands. In comparison with that of the free ligands as shown in Fig. S12 (in the ESI†), the maximum emission wavelengths of complexes 1–5 occur slightly red shifted, which is probably due to intraligand charge transitions.¹⁹ In summary, compounds 1–5 show the potential fluorescence properties in the solid state at room temperature.

Thermal properties

To study the thermal stability of the complexes, thermogravimetric analyses (TGA) were performed on polycrystalline samples under a nitrogen atmosphere with a heating rate of 10 °C min⁻¹. The TG diagram of 1 displays three distinct weight losses, the first one from 70 to 150 °C and corresponds to the loss of the lattice water molecules. The observed weight loss of 1.36% is in agreement with the calculated value of 1.56%. The dehydrated complex showed a weight loss of 7.38%, which is in agreement with the calculated value of 7.65%, corresponding to the loss of two CO₂ molecules in the L ligand. Then, the dehydrated complex is stable up to 329 °C. The frameworks collapsed in the temperature range of 329–626 °C. The remaining weight may correspond to the final products of ZnO in 1 (obsd 14.51%, calcd 13.92%). Compound 2 displays three distinct weight losses. The first one from 40 to 102 °C and corresponds to the loss of the lattice water molecules. The observed weight loss of 1.31% is in agreement with the calculated value of 2.09%. The dehydrated complex weight loss of 3.89% observed is in agreement with the calculated value of 5.13%, corresponding to the loss of one CO₂ molecule the in the L ligand. The frameworks collapsed in the temperature range of 313–519 °C. The remaining weight may correspond to the final products of ZnO in 2 (obsd 18.38%, calcd 18.66%). The TG curve of 4 shows two main weight losses, respectively. The first, of 50.01% from 303 to 331 °C, corresponds to the loss of three p-H₂BDC molecules (obsd 50.77%, calcd 50.16%). The frameworks collapsed in the temperature range of 439–475 °C. The remaining weight may correspond to the final products of ZnO in 4 (obsd 24.51%, calcd 24.46%). The TG curves of 3 and 5 both show one main weight loss. Respectively, the frameworks collapsed in the temperature ranges of 372–623 °C and 374–618 °C. The remaining weight may correspond to the final products of ZnO in 3 (obsd 21.12%, calcd 20.79%) and ZnO in 5 (obsd 19.01%, calcd 18.66%) as shown in Fig. S13 (in the ESI†).

Conclusions

In summary, we have synthesized and characterized five novel compounds utilizing the 1,4-bis(pyridin-4-ylmethoxy)benzene ligand, multi-caboxylate acids and zinc acetate salts. Compounds 1–4 display entangled structures ranging from interpenetration to self-penetrating all the way to poly-threading modes. By adjusting the pH value, the poly-threading architecture in compound 1 was transferred to an unprecedented poly-threading structure of 2, and further pH variation resulted in a 3D supramolecular framework with no entangled motifs as seen in compound 5. The successful isolation of compounds 1–5 not only provides fascinating instances of chemical topology but also enriches the new born poly-threading family.

Acknowledgements

The authors gratefully acknowledge the financial support from the NNSF of China (No. 21001022), The National Grand Fundamental Research 973 Program of China (2010CB635114), Program for New Century Excellent Talents in Chinese University (NCET-10-0282), PhD Station Foundation of Ministry of Education (20100043110003), The Foundation for Author of National Excellent Doctoral Dissertation of P.R. China (FANEDD) (No. 201022), The Science and Technology Development Planning of Jilin Province (201001169, 20100182), The Fundamental Research Funds for the Central Universities (09QNJJ018, 09ZDQD003, 10CXTD001), Open Project Program of Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education.

Notes and references

1. (a) T. F. Liu, D. Fu, S. Gao, Y. Z. Zhang, H. L. Sun, G. Su and Y. J. Liu, J. Am. Chem. Soc., 2003, 125, 13976; (b) R. Kitaura, K. Seki, G. Akiyama and S. Kitagawa, Angew. Chem., Int. Ed., 2003, 42, 428; (c) M. Eddaoudi, H. Li and O. M. Yaghi, J. Am. Chem. Soc., 2000, 122, 1391; (d) A. J. Fletcher, E. J. Cussen, T. J. Prior, M. J. Rosseinsky, C. J. Kepert and K. M. Thoms, J. Am. Chem. Soc., 2001, 123, 10001; (e) S. I. Noro, S. Kitagawa, M. Kondo and K. Seki, Angew. Chem., Int. Ed., 2000, 39, 2082; (f) M. J. Zaworotko, Angew. Chem., Int. Ed., 2000, 39, 3052; (g) X. Y. Wang, L. Wang, Z. M. Wang, G. Su and S. Gao, J. Am. Chem. Soc., 2006, 128, 674.
2. Molecular Catenanes, Rotaxanes and Knots, A Journey Through the World of Molecular Topology, ed. J. P. Sauvage and C. Dietrich-Buchecker, Wiley-VCH, Weinheim, 1999.
3. (a) K. Liang, H. G. Zheng, Y. G. Song, M. F. Lappert, Y. Z. Li, X. Q. Xin, Z. X. Huang, J. T. Chen and S. F. Lu, Angew. Chem., Int. Ed., 2004, 43, 5776; (b) X. H. Bu, M. L. Tong, H. C. Chang, S. Kitagawa and S. R. Batten, Angew. Chem., Int. Ed., 2004, 43, 192; (c) S. A. Bourne, J. J. Lu, B. Moulton and M. J. Zaworotko, Chem. Commun., 2001, 861.
4. (a) Q. M. Wang, G. C. Guo and T. C. W. Mak, Chem. Commun., 1999, 1849; (b) P. Jensen, D. J. Price, S. R. Batten, B. Moubaraki and K. S. Murray, Chem.–Eur. J., 2000, 6, 3186; (c) S. H. Chiu, S. J. Rowan, S. J. Cantrill, J. F. Stoddart, A. J. P. White and D. J. Williams, Chem. Commun., 2002, 2948; (d) M. Eddaoudi, J. Kim, N. L. Rosi, D. T. Vodak, J. Wachter, M. O'Keeffe and O. M. Yaghi, Science, 2002, 295, 469; (e) B. Moulton, H. Abourahma, M. W. Bradner, J. J. Lu, G. J. McManus and M. J. Zaworotko, Chem. Commun., 2003, 1342; (f) A. Galet, V. Niel, M. C. Munoz and J. A. Real, J. Am. Chem. Soc., 2003, 125, 14224; (g) R. Vaidhyanathan, S. Natarajan and C. N. R. Rao, Cryst. Growth Des., 2003, 3, 47; (h) T. J. Prior and M. J. Rosseinsky, Inorg. Chem., 2003, 42, 1564.
5. (a) S. R. Batten and R. Robson, Angew. Chem., Int. Ed., 1998, 37, 1460; (b) S. R. Batten, CrystEngComm, 2001, 3, 67.
6. (a) B. F. Abrahams, M. J. Hardie, B. F. Hoskins, R. Robson and E. E. Sutherland, J. Chem. Soc., Chem. Commun., 1994, 1049; (b) L. Carlucci, G. Ciani, D. M. Proserpio and F. Porta, Angew. Chem., Int. Ed., 2003, 42, 317; (c) B. F. Abrahams, S. R. Batten, M. J. Grannas, H. Hamit, B. F. Hoskins and R. Robson, Angew. Chem., Int. Ed., 1999, 38, 1475; (d) M. A. Withersby, A. J. Blake, N. R. Champness, P. A. Cooke, P. Hubberstey and M. Schröer, J. Am. Chem. Soc., 2000, 122, 4044; (e) M. L. Tong, X. M. Chen and S. R. Batten, J. Am. Chem. Soc., 2003, 125, 16170; (f) L. Carlucci, G. Ciani, D. M. Proserpio and S. Rizzato, J. Chem. Soc., Dalton Trans., 2000, 3821; (g) D. L. Long, R. J. Hill, A. J. Blake, N. R. Champness, P. Hubberstey, C. Wilson and M. Schröer, Chem.–Eur. J., 2005, 11, 1384; (h) X. J. Wang, C. H. Zhan, Y. L. Feng, Y. Z. Lan, J. L. Yin and J. W. Cheng, CrystEngComm, 2011, 13, 684.
7. (a) L. Carlucci, G. Ciani and D. M. Proserpio, Coord. Chem. Rev., 2003, 246, 247; (b) S. Banfi, L. Carlucci, E. Caruso, G. Ciani and D. M. Proserpio, J. Chem. Soc., Dalton Trans., 2002, 2714; (c) G. F. Liu, B. H. Ye, Y. H. Ling and X. M. Chen, Chem. Commun., 2002, 1442; (d) L. Carlucci, G. Ciani and D. M. Proserpio, Chem. Commun., 1999, 449; (e) M. L. Tong, H. J. Chen and X. M. Chen, Inorg. Chem., 2000, 39, 2235.
8. (a) J. P. Sauvage, Acc. Chem. Res., 1998, 31, 611; (b) S. I. Jun, J. W. Lee, S. Sakamoto, K. Yamaguchi and K. Kim, Tetrahedron Lett., 2000, 41, 471.
9. (a) X. L. Wang, C. Qin, E. B. Wang, Y. G. Li, Z. M. Su, L. Xu and L. Carlucci, Angew. Chem., Int. Ed., 2005, 44, 5824; (b) C. Qin, X. L. Wang, L. Carlucci, M. L. Tong, E. B. Wang, C. W. Hu and L. Xu, Chem. Commun., 2004, 1876; (c) L. Carlucci, G. Ciani and D. M. Proserpio, CrystEngComm, 2003, 5, 269.
10. (a) X. L. Wang, C. Qin, E. B. Wang, Z. M. Su, L. Xu and C. W. Hu, Angew. Chem., Int. Ed., 2004, 43, 5036; (b) X. L. Wang, C. Qin, E. B. Wang, Z. M. Su, Y. G. Li and L. Xu, Angew. Chem., Int. Ed., 2006, 45, 7411; (c) X. L. Wang, C. Qin, Y. Q. Lan, K. Z. Shao, Z. M. Su and E. B. Wang, Chem. Commun., 2009, 410.
11. (a) J. Yang, J. F. Ma, Y. Y. Liu and S. R. Batten, CrystEngComm, 2009, 11, 151; (b) G. J. Xu, Y. H. Zhao, K. Z. Shao, Y. Q. Lan, X. L. Wang, Z. M. Su and L. K. Yan, CrystEngComm, 2009, 11, 1842; (c) L. J. Li, G. Yuan, L. Chen, D. Y. Du, X. L. Wang, G. J. Xu, H. N. Wang, K. Z. Shao and Z. M. Su, J. Coord. Chem., 2011, 64, 1578.
12. (a) M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319; (b) G. Ferey, C. M. Draznieks, C. Serre and F. Millange, Acc. Chem. Res., 2005, 38, 217; (c) C. N. R. Rao, S. Natarajan and R. Vaidhyanathan, Angew. Chem., Int. Ed., 2004, 43, 1466.
13. (a) L. P. Zhang, C. K. Lam, H. B. Song and T. C. W. Mak, Polyhedron, 2004, 23, 2413; (b) H. W. Hou, T. Y. Fan, L. P. Zhang, C. X. Du and Y. Zhu, Inorg. Chem. Commun., 2001, 4, 168.
14. G. M. Sheldrick, SHELXL-97, Program for X-ray Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997.
15. (a) E. C. Yang, H. K. Zhao, B. Ding, X. G. Wang and X. J. Zhao, Cryst. Growth Des., 2007, 7, 2009; (b) Y. Y. Liu, J. F. Ma, J. Yang, J. C. Ma and G. J. Ping, CrystEngComm, 2008, 10, 565; (c) K. Z. Shao, Y. H. Zhao, Y. Xing, Y. Q. Lan, X. L. Wang, Z. M. Su and R. S. Wang, Cryst. Growth Des., 2008, 8, 2986; (d) X. M. Zhang, M. L. Tong, M. L. Gong and X. M. Chen, Eur. J. Inorg. Chem., 2003, 138; (e) A. D. Burrows, R. W. Harrington, M. F. Mahon and S. J. Teat, Eur. J. Inorg. Chem., 2003, 766.
16. (a) X. J. Wang, C. H. Zhan, Y. L. Feng, Y. Z. Lan, J. L. Yin and J. W. Cheng, CrystEngComm, 2011, 13, 684; (b) G. S. Yang, Y. Q. Lan, H. Y. Zang, K. Z. Shao, X. L. Wang, Z. M. Su and C. J. Jiang, CrystEngComm, 2009, 11, 274; (c) X. L. Wang, C. Qin, E. B. Wang, Z. M. Su, L. Xu and S. R. Batten, Chem. Commun., 2005, 4789.
17. (a) M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M. O'Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319; (b) G. Ferey, C. Mellot-Draznieks, C. Serre and F. Millange, Acc. Chem. Res., 2005, 38, 217.
18. C. N. R. Rao, S. Natarajan and R. Vaidhyanathan, Angew. Chem., Int. Ed., 2004, 43, 1466.
19. (a) L. Y. Zhang, J. P. Zhang, Y. Y. Lin and X. M. Chen, Cryst. Growth Des., 2006, 6, 1685; (b) Q. Chu, G. X. Liu, Y. Q. Huang, X. F. Wang and W. Y. Sun, Dalton Trans., 2007, 4302; (c) G. H. Wang, Z. G. Li, H. Q. Jia, N. H. Hu and J. W. Xu, Cryst. Growth Des., 2008, 8, 1932; (d) Y. Y. Qin, J. Zhang, Z. J. Li, L. Zhang, X. Y. Cao and Y. G. Yao, Chem. Commun., 2008, 2532; (e) Y. H. He, Y. L. Feng, Y. Z. Lan and Y. H. Wen, Cryst. Growth Des., 2008, 8, 3586.

---

Footnote

† Electronic supplementary information (ESI) available: Structural information, thermogravimetric analysis, spectroscopy details (Fig. S1–S7, Tables S1–S3). CCDC reference numbers 804835–804839. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce05810j

---