A series of interdigitated Cd(II) coordination polymers based on 4,6-dibenzoylisophthalic acid and flexible triazole ligands

Abstract

A series of Cd(II) coordination polymers has been obtained through hydrothermal self-assembly. During self-assembly, the flexibility and the length of the ligands play an important role in the construction of interdigitated molecular structures, which supports an alternative approach for designing such networks with the use of spacers.

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The design and synthesis of novel coordination polymers (CPs) have attracted more and more attention for their potential applications in fluorescence imaging, nonlinear optics, magnetism and catalysis.¹ As an important class of materials in this field, entangled CPs in particular have attracted great interest due to their intriguing structures and special properties.² Up to now, some reviews about entangled systems have been written by Robson, Batten, Ma, Ciani and Leigh.³ Robson and his co-workers were the first to describe interdigitation, where side arms of the ligand are necessary for the construction of the entangled system.^3a Moreover, some other terminologies, such as polycatenation, polythreading, polyknotting and polymeric chains, have also been defined.^3c In principle, the entanglement can be driven by carefully selecting the coordination geometry of the metal centers and the organic ligands with flexible coordination modes.⁴ In the past few decades, much effort has been devoted to the design and synthesis of novel entangled networks.⁵ However, it still remains a challenge to clarify which factor controls the construction of the entangled system.

According to the literature,⁶ multi-carboxylate aromatic ligands have been widely used due to their flexible coordination modes. In this regard, 4,6-dibenzoylisophthalic acid (H₂L)⁷ should be a good candidate for the construction of entangled CPs. To begin with, the two carboxylate groups of H₂L can bridge metal ions to form various frameworks through its versatile coordination modes. Then, a bulky benzoyl group on the benzene ring can rotate around the C–C bond to generate various geometric conformations. Last but not least, flexible non-covalent interactions can be expected, i.e. hydrogen bonds, π⋯π stacking and X–H⋯π (X = C, N, O) interactions, due to the carbonyl groups and the middle benzene ring, which is expected to play an important role in the guidance of the entangled self-assembly. On the other hand, flexible N-donor ligands with excellent coordination abilities have also been proven to be suitable for the construction of entangled networks.⁸

Herein, we try to utilize H₂L and flexible N-donor ligands, 1,6-di(1H-1,2,4-triazol-1-yl)hexane (L₁), 1,4-bis(pyridin-4-ylmethyl)piperazine (L₂) and 1,3-bis((1H-1,2,4-triazol-1-yl)methyl)benzene (L₃) (Scheme S1†), to assemble entangled coordination polymers with Cd(II) ions under hydrothermal conditions. Three new coordination polymers, namely, [Cd(L)(L₁)] (1), [Cd(L)(L₂)] (2) and [Cd(L)(L₃)] (3),¹ have been successfully synthesized and characterized.⁹ Both 1 and 2 feature a 2D → 3D interdigitated network with a 4⁴-sql topology proven by single-crystal X-ray diffraction analyses (Table S2†), whilst 3 exhibits a 3⁶-hxl topological network without any interdigitation. Additionally, the fluorescence properties of 1–3 were also studied in detail.

Complex 1 crystallizes in the triclinic space group P1̄. The asymmetric unit of 1 comprises one crystallographically independent Cd(II) ion, one L²⁻ anion and one L₁ ligand. As shown in Fig. 1a, the Cd1 ion is coordinated by four O atoms (O1, O2#1, O3, and O4) from the three L ligands (Cd–O 2.220(2)–2.493(2) Å) and two N atoms (N1 and N4) from the two L₁ ligands (Cd–N 2.285(4)–2.295(5) Å), forming a distorted CdN₂O₄ octahedral geometry. Viewing it along the a-axis, the Cd ions of 1 are connected by μ₃-η¹:η¹:η¹:η¹-bridging L²⁻ linkers to generate a 1D {CdL} chain with an adjacent Cd⋯Cd distance of 4.439 and 7.329 Å (Fig. 1b and S3†). L₁ ligands link the neighboring chains into a 2D layered structure with a 4⁴-sql topology (Fig. 1c).¹⁰ Interestingly, the bulky benzoyl groups of the L²⁻ anions act as lateral arms projecting beyond both sides of the layers. And, thanks to these bulky benzoyl groups, the adjacent 2D sheets of 1 interdigitate to generate a 2D → 3D interdigitation network (Fig. 1d). Moreover, the weak C20–H20⋯O6 hydrogen bonding interactions should be helpful to guide the self-assembly, and stabilize the 3D supramolecular architecture of 1.

Fig. 1 (a) Coordination environment of the Cd ion of 1. Symmetry codes: #1, 1 − x, 1 − y, 1 − z. (b) View of the 1D {Cd(L)}_n chain of 1. (c) Schematic depiction of the 4⁴-sql layer of 1. (d) The 3D supramolecular structure of 1 contains a feature of 2D → 3D interdigitation. The C20–H20⋯O6 hydrogen bonding interactions are shown by dashed red lines.

With regard to compound 2, due to the more rigid L₂, a different framework was obtained. 2 crystallizes in the monoclinic space group P2₁/c. The asymmetric unit of 2 contains one crystallographically independent Cd(II) ion, one L²⁻ anion and two half-L₂ molecules. Similar to compound 1, the Cd1 ion is six-coordinated by four O atoms (O1, O2, O3#1, and O4#2) from the three L ligands (Cd–O 2.227(3)–2.408(3) Å) and two N atoms (N1 and N4) from the two L₂ ligands (Cd–N 2.311(4)–2.330(4) Å), forming a distorted CdN₂O₄ octahedral geometry (Fig. S1a†). The Cd ions of 2 are also connected by L²⁻ linkers to form 1D {CdL}_n chains, which are further linked by L₂ ligands to generate a 2D layered structure with a 4⁴-sql topology (Fig. S1b and S1c†).¹⁰ However, different to 1, the layers of 2 are alternately arranged in an ABA fashion (Fig. S1d†). As the lateral arms of the bulky benzoyl groups project beyond both sides of the layers, the 2D layers of 2 interdigitate with each other to generate a 2D → 3D interdigitation (Fig. S1d†). Meanwhile, the weak C31–H31a⋯O6 hydrogen bonds should also be helpful to guide and stabilize the 3D supramolecular architecture of 2.

L₃ is shorter and more rigid than both L₁ and L₂, which may be not helpful to guide the interdigitation of compound 3. In the crystal structure of 3, the asymmetric unit contains one crystallographically independent Cd(II) ion, two half-L²⁻ anions and two half-L₃ molecules. The Cd ions are six-coordinated by four O atoms (O1, O2, O3, and O4#1) and two N atoms (N1 and N4) to generate a distorted CdN₂O₄ octahedral geometry and are then connected by the L²⁻ linkers to form 1D {CdL}_n chains (Fig. S2a and S2b†). It is worth noting that although similar coordination environments of Cd ions and 1D {CdL}_n chains to those of compounds 1 and 2 can be found in 3 (Fig. S3†), the significantly different framework was formed after self-assembly. In 3, 1D {CdL}_n chains are connected by L₃ molecules to form a 2D layered structure with a 3⁶-hxl topology (Fig. 2a).¹⁰ As the flexibility was reduced gradually from L₁ to L₃, the tilt angles of the L²⁻ anions (opposed to the plane of the 2D layer) decreased steadily from 1 to 3 (Fig. 3). Thus, no interdigitation of the 2D sheets can be found in compound 3 (Fig. 2b). This result indicates that the entanglement of the framework can be efficiently adjusted by intentionally designing the flexibility of the bridging ligands.

Fig. 2 (a) Schematic depiction of the 3⁶-hxl layer of 3. (b) View of the 3D supramolecular structure of 1 based on C25–H25a⋯O6 hydrogen bonding interactions (dashed black lines).

Fig. 3 View of the tilt angles of the H₂L molecules in (a) 1, (b) 2 and (c) 3.

The purity of the title compounds was characterized by powder X-ray diffraction (PXRD) (Fig. S4–S6†). The experimental PXRD patterns corresponded well with the results simulated from the single crystal data, indicating high purity of the synthesized samples. Thermogravimetric analysis was carried out for compounds 1–3, in order to investigate their thermal stability (Fig. S7†). The experiments were performed under a N₂ atmosphere with a heating rate of 10 °C min⁻¹. All of the three samples exhibit a similar thermal decomposition process with only one decomposition step, and the organic groups start to decompose gradually from 287 °C for 1, 284 °C for 2 and 265 °C for 3.

The solid-state fluorescence spectra of 1–3 (Fig. 4) and the free ligands H₂L, L₁, L₂ and L₃ (Fig. S8†) were recorded at room temperature. The main emission bands of the free ligands H₂L, L₁, L₂ and L₃ are at 454 (λ_ex = 373 nm), 445 (λ_ex = 327 nm), 461 (λ_ex = 327 nm), and 458 nm (λ_ex = 372 nm), respectively. These emission bands can be assigned to the π* → n or π* → π transitions as previously reported. The emission spectra of compounds 1–3 exhibit emission maxima at 500 nm (λ_ex = 350 nm), 522 nm (λ_ex = 350 nm) and 508 nm (λ_ex = 380 nm), respectively. The emission bands of compounds 1–3 are similar to those of the free ligands. Since the Zn(II) and Cd(II) ions are difficult to oxidize or to reduce due to their d¹⁰ configuration, the emissions of compounds 1–3 are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal charge transfer (LMCT) in nature.¹¹ Thus, the emission may be assigned to the transitions of both H₂L and N-donor ligands. Compared with the emission spectra of H₂L and N-donor ligands, red shifts of the emission bands for 1–3 have been observed, probably due to the deprotonated effect and the coordination interactions of H₂L and N-donor ligands to Cd(II) ions.^11,12

Fig. 4 Emission spectra of 1–3 in the solid state at room temperature.

Conclusions

In summary, three Cd(II) coordination polymers have been synthesized under hydrothermal conditions in the presence of H₂L and flexible N-donor ligands. Both 1 and 2 feature a 2D → 3D interdigitated network with a 4⁴-sql topology under the guidance of flexible L₁ and L₂. In comparison, compound 3 shows a 3⁶-hxl topological framework without any interdigitation due to the lower flexibility of L₃. It can be deduced that the length and flexibility of the ligands can significantly influence the interdigitation level of the crystal structure. In other words, the more flexible and longer the ligands, the more able they are to obtain interdigitated structures. This observation provides an alternative approach to the systematic assembly of interdigitated structures for coordination polymers.

Acknowledgements

This work was supported by the NNSFC (no. 20871023), the Jilin Provincial Science Research Foundation of China (no. 20101549), and the XiaoNei Foundation of Changchun University of Technology. The 12th Five-Year Plan for Science & Technology Research was sponsored by the Department of Education of Jilin Province (no. 130 and 146, 2013).

Notes and references

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9. A typical synthesis method: a mixture of CdCl₂·2.5H₂O (22.8 mg, 0.1 mmol), H₂L (37.4 mg, 0.1 mmol), N-donor ligands (22.3 mg for L₁, 26.8mg for L₂ and 24.3 mg for L₃, 0.1 mmol) and H₂O (10 mL) was stirred for 30 min, and the pH of the mixture was adjusted to 5.5 with 0.1 mol L⁻¹ NaOH solution. Then it was sealed in a 23 mL Teflon reactor and heated at 160 °C for 72 h. After cooling to room temperature at a rate of 3 °C h⁻¹, colorless crystals were isolated by filtration, washed with water, and air-dried. For 1: about 60% yield based on Cd. Anal. Calcd for C₃₂H₂₈CdN₆O₆ (705.01): C, 54.52; H, 4.00; N, 11.92%. Found: C, 54.61; H, 3.93; N, 11.85%. IR (KBr, cm⁻¹): 3061 (w), 2940 (w), 2865(w), 1671 (s), 1600 (s), 1476 (w), 1428 (m), 1382 (s), 1346 (s), 1250 (m), 1136 (w), 1011 (w), 918 (w), 885 (m), 816 (m), 761 (m), 710 (m), 671(w), 621 (w), 524 (w), 482 (w). For 2: about 42% yield based on Cd. Anal. Calcd for C₃₈H₃₂CdN₄O₆ (753.08): C, 60.60; H, 2.96; N, 7.44%. Found: C, 60.68; H, 3.03; N, 7.35%. IR (KBr, cm⁻¹): 3040 (w), 2941 (w), 2813 (w), 1668 (s), 1617 (s), 1578 (w), 1480 (m), 1402 (s), 1346 (m), 1261 (w), 1135 (w), 1061 (w), 1009 (w), 947 (m), 914 (m), 850 (m), 801 (m), 693 (m), 540(w), 488 (w). For 3: about 55% yield based on Cd. Anal. Calcd for C₃₄H₂₄CdN₆O₆ (725.00): C, 56.33; H, 3.34; N, 11.59%. Found: C, 56.41; H, 3.43; N, 11.65%. IR (KBr, cm⁻¹): 3437 (W), 3110 (w), 1669 (s), 1598 (s), 1521 (s), 1433 (m), 1381 (s), 1261 (m), 1132 (w), 1014 (w), 976 (w), 886 (m), 815 (m), 734 (m), 671 (m), 526 (w).
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Footnote

† Electronic supplementary information (ESI) available: Materials and general methods, single-crystal X-ray crystallography, Fig. S1–S8 and Tables S1–S6. CCDC 975867 (1), 890982 (2) and 975868 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra00246f

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