Two unique (4,5,6)-connected 2D Cd^II coordination polymers based on the 5-nitro-1,2,3-benzenetricarboxylate ligand

Abstract

Two new 2D coordination polymers, [Cd₃(nbta)₂(H₂O)₄] (1) and [Cd₃(nbta)₂(bbi)₂H₂O)₂] (2) (H₃nbta = 5-nitro-1,2,3-benzenetricarboxylate, bbi = 1,1′-(1,4-butanediyl)bis(imidazole)) have been synthesized and structurally characterized by single X-ray structure analysis. The versatile conformations and binding modes of the nbta ligand as well as the coordination environment of the Cd^II atoms result in two unique (4,5,6)-connected 2D Cd^II-coordination polymers, both of which show enthralling helical structures.

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The design and synthesis of coordination polymer frameworks are of great current interest, stemming from their potential applications as functional materials as well as their structural diversity and intriguing topologies.^1–4 Network topology has been proven to be a powerful tool for research of coordination polymers, not only for the importance of reducing complicated frameworks to simple node-and-linker referenced nets but also for guiding the rational design of certain functional materials with desirable properties.^5,6 To date, a variety of network topologies have been realized by real crystal structures since Wells described topologies in his classic monographs on networks decades ago⁷, most of which are based on 3-D networks with three-, four-, or six-connected topologies, such as srs, dia, cds, nbo and pcuetc.⁸ Meanwhile, the simple 2D layers with relevant connectivity are usually constructed from uniform and regular polygons and their corresponding network symbols are designated as 6³, 4⁴, and 3⁶, respectively.⁹ However, mixed connectivity of (4,5,6)-connected 2-D layers have rarely been reported so far.¹⁰

It's well-known that the coordination geometries of the metal centers and the coordination behaviours of the organic ligands are the main keys to achieving the target coordination polymers. Benzene-based multicarboxylic acid ligands, as good candidates for the construction of coordination polymers, have been extensively used.¹¹5-Nitro-1,2,3-benzenetricarboxylate (H₃nbta) ligand, a derivative of 1,2,3-benzenetricarboxylate (H₃bta),¹² remains largely unexplored hitherto in the field of coordination polymers.¹³ However, H₃nbta may provide the potential for constructing unpredictable and interesting network structures due to the existence of a non-coordinating electron-withdrawing nitro-group on the aromatic backbone, which will have a profound impact on the electron density of such a ligand and result in different physical and chemical properties.

Reaction of H₃nbta and Cd^II acetate with or without bbi at 160 °C for 3 days under hydrothermal conditions generates complexes 1 and 2.‡ The compositions were confirmed by elemental analysis, IR spectrum and single crystal X-ray analysis.§¶ The phase purities of the bulk samples were identified by XRPD (Fig. S1 , ESI†)

Single crystal X-ray diffraction analysis (ESI†) reveals that complex 1 crystallizes in a P2₁/c space group, and the asymmetric unit of which contains three crystallographically independent Cd^II centers. As shown in Fig. S2 , ESI†, Cd1 centers a distorted square pyramid, defined by four carboxylic oxygen atoms from four different nbta ligands and one water molecule. The O3, O13, and O18 atoms are weakly coordinated to the Cd1 ion with a Cd–O distances of 2.667(2), 2.679(2), and 2.668(2) Å, respectively. As for Cd2 and Cd3, if we neglect the weak Cd3–O10 bond (2.787(2) Å), both of which have a distorted pentagonal–bipyramidal geometry coordination environment, formed by five carboxylic oxygen atoms from four different nbta ligands and two water molecules with the axial sites O1–Cd2–O11C and O6D–Cd3–O12 angles of 150.61(5) and 152.97(5)°, respectively. (see Table S1, ESI†). The nbta ligands in 1 are completely deprotonated, showing two kinds of heptadentate coordination modes (μ₁–η¹: η¹/μ₂–η¹: η¹/μ₃–η¹: η² and μ₁–η¹: η⁰/μ₁–η¹: η¹/μ₄–η²: η², see Fig. S3, ESI†). In such a ligating manner, the Cd^II centers are connected by the nbta ligands to afford a complicated 2D layered structure containing two types of helical layers (Cd2– and Cd3–nbta helical layer see Fig. 1a). For the first one, two μ₂–O11 atoms bridge Cd2 atoms to form a [Cd₂O₂] dimer, which are then aligned by O₂C7–C1–C2–C8O₂ groups into a 2₁ helical array along the [001] axis. The adjacent right- and left-handed helices are fused to each other by sharing the [Cd₂O₂] dimer to generate a 2D Cd2–nbta helical layer. In the Cd3–nbta helical layer, a pair of O₂C7–C1–C6–C9O₂ groups link the Cd3 atoms to result in a 14-numbered ring, and the adjacent ones are arranged by O₂C17–C11–C12–C18O₂groups to give rise to right- and left-handed helical chains. Interestingly, two same handedness helices from different helical layer interweave to each other by the connectivity of Cd2–O1–C7–O2–Cd3 and Cd1–O to produce double-stranded right- or left-handed helices (see Fig. S4, ESI†). From the viewpoint of topology, the 4-connected Cd2 and 5-connected Cd1, Cd3 inorganic nodes as well as the 6-connected nbta organic nodes interlink into a new (4,5,6)-connected 2D net (see Fig. 1b) with the point symbol of (4⁴.5.6)(3².4⁵.5².6)(3².4⁴.5².6²)(3.4⁸.5².6⁴)(3.4⁸.5³.6³) analyzed by the TOPOS program.

Fig. 1 (a) The 2D layer of 1 containing two types of helical layers (Cd2– and Cd3–nbta helical layer). (b) Schematic representation of the (4,5,6)-connected (4⁴.5.6)(3².4⁵.5².6)(3².4⁴.5².6²)(3.4⁸.5².6⁴)(3.4⁸.5³.6³) topology (Cd1: orange; Cd2: blue; Cd3: purple; nbta ligand: green).

When the flexible bis-(imidazole) ligand bbi was taken into the above system, a distinct 2D helical layer was obtained. Complex 2 crystallizes in a P2₁/c space group, and the asymmetric unit has two different Cd^II atoms. As illustrated in Fig. S5, ESI†, each Cd^II atom is six coordinated but locates in different octahedral coordination geometry. Five O atoms from three nbta ligands and one N atom from the bbi molecule complete the distorted octahedral coordination sphere of Cd1 atom. While Cd2 atom is coordinated by two O atoms from two nbta ligands, two N atoms from two bbi molecules as well as two aqua ligands (see Table S1, ESI† for detailed bond lengths and angles). Different from 1, the completely deprotonated nbta in 2 acts as a hexadentate ligand. Three carboxylic groups adopt μ₁–η¹: η¹, μ₂–η⁰: η² and μ₂–η¹: η¹ bridging modes, respectively (see Fig. S6, ESI†). The bbi ligand acts as a linear bidentate ligand. Based on such bridging modes, the Cd^II atoms are extended by nbta and bbi ligands into a 2D layer (see Fig. 2a). The helical chains, formed by bbi ligands and O₂C8–C2–C3–C9O₂ groups bridging between the Cd^II atoms running along a crystallographic 2₁ axis in the [100] direction with a pitch of 7.61 Å, are observed in the 2D net. The neighbouring right- and left-handed helices are joined together by the connectivity of Cd1–C7O₂. From a topological perspective, all the Cd^II atoms and nbta ligands can be regarded as 4-connected nodes. Thus the 2D net can be described as a trinodal 4-connected net with the Schläfli symbol of (4².5³.6)(5².6².7²)(4².5³.6). However, if the strong hydrogen bonding between carboxylic oxygen atoms and coordinated water molecules [O(9)–H(2W)⋯O(3)^a, symmetry codes: a = −x + 1, −y + 1, −z + 3; H⋯O /O⋯O distances: 2.001/2.784 Å; angle: 157.32°] is considered (see Fig. 2b), the nbta ligands become 5-connecting nodes, [Cd2(H₂O)₂] moieties act as the 6-connected nodes and Cd1 remaining 4-connected nodes. Accordingly, this 2D layer can also be considered as the overall (4,5,6)-connected trinodal net with the Schläfli symbol of (4³.5.6²)(4⁵.5³.6²)(4⁴.5⁶.6⁴.7) (see Fig. 2c).

Fig. 2 (a) 2D helical layer of 2 parallel along the ab plane. (b) The intra-layer hydrogen bonding between carboxylic oxygen atoms and coordinated water molecules (only the pertinent backbones of nbta ligand are shown). (c) Schematic representation of the 4- and (4,5,6)-connected net considering the intra-layer hydrogen bonding (Cd1: orange; Cd2: blue; nbta ligand: green).

Recently, we isolated a pair of 4- and 6-connected 2D supramolecular architectures¹⁴ by the assembly of 1,3-bis(4-pyridyl)propane (bpp) linker with Co^II and different isophthalate tectons. The unique network topologies can be realized by the different linkages of the adjacent 4⁴ layers. Also, for the overall 2D net of 1, if the Cd1 nodes are ignored, it changes into a 4-connected (4³.6³) topology. Of the 4-connected Cd2, Cd3 and nbta nodes, three form the puckering 6³ net (blue and purple in Fig. S7a, ESI†) while the interlayer bridge of the fourth are perpendicularly aligned to the net. In the same way, the (4,5,6) connected net of 2 can be regarded as two corrugated 6³ nets sharing Cd2 nodes with the remaining linker locating at the interlayer (see Fig. S7b, ESI†).

As shown in Table S2, ESI† (two kinds of nbta³⁻ ligands in 1), one outer carboxylic group of each nbta³⁻ ligand is nearly coplanar with the corresponding aromatic rings, while the remaining outer carboxyl group has greatly deviated from the coplanar of the aromatic rings, the dihedral angles between the outer carboxylic groups and the corresponding aromatic rings are 47.2, 31.9, and 131.0°, respectively. This geometry of the nbta³⁻ ligand plays an important role in the formation of the helical structures in 1 and 2. This phenomenon can also be found in complexes {[Cd₃(nbta)₂(bpa)₂]}_n^13c and {[Cd₃(nbta)₂(bpy)₅(H₂O)₂](H₂O)₆}_n^13d reported by our group. Both complexes possess helical and self-penetrating character.

Complexes 1 and 2 are air stable and retain the crystalline integrity at ambient conditions. Thermogravimetric analysis (see Fig. S8, ESI†) of 1 shows the initial weight loss in the temperature range of 140–210 °C, which can be ascribed to the removal of lattice water molecules (observed: 8.2% and calculated: 7.9%). From ca. 340 °C the expulsion of organic components occurs. The TG curve for 2 indicates that the first weight loss of 3.0% (calculated: 2.9%) corresponding to the loss of two coordinated water molecules. Further weight loss indicates the decomposition of coordination framework.

Weak fluorescence emission bands at ca. 471 and 527 nm for both 1 and 2 are observed (see Fig. S9, ESI†). These emissions are neither metal-to-ligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT) in nature, since Cd^II ions are difficult to oxidize or reduce due to their d¹⁰ configuration.¹⁵ In comparison to that of the free H₃nbta, most of the emission maxima of complexes 1 and 2 are significantly shifted (see Fig. S8, ESI†). The shifts of the emission bands are attributed to both the deprotonated effect of H₃nbta and the coordination interactions of the organic ligands to Cd^II ions.¹⁶ More detailed theoretical and spectroscopic studies may be necessary for better understanding of the luminescent mechanism.

In conclusion, two new 2D Cd^II coordination polymers [Cd₃(nbta)₂(H₂O)₄] (1) and [Cd₃(nbta)₂(bbi)₂(H₂O)₂] (2) exhibiting enthralling helical arrays, have been isolated under hydrothermal conditions. Topology analysis shows two unique (4,5,6)-connected networks. A further analysis reveals that both complexes 1 and 2 can be realized as arising from the different linkages of the adjacent 6³ nets. The results demonstrate that the 5-nitro-1,2,3-benzenetricarboxylate tectons are good candidates for constructing interesting coordination frameworks. Subsequent works will be focused on syntheses, structures, and physical properties of more coordination polymers with transition/rare metal ions and H₃nbta or related derivatives.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21073082, 21071074), 2009GGJS-104 and sponsored by Program for Science & Technology Innovation Talents in Universities of Henan Province (2011HASTIT027).

References

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Footnotes

† Electronic supplementary information (ESI) available: Additional structural figures, TG curves and luminescence spectra of 1 and 2. CCDC reference numbers 806327 and 806328. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ra00119a

‡ Synthesis of 1. A mixture of H₃nbta (0.1 mmol, 25.5 mg), Cd(NO₃)₂·4H₂O (0.1 mmol 30.8 mg), KOH (0.1 mmol, 5.6 mg) and H₂O 15 mL were placed in a Teflon-lined stainless steel vessel, heated to 160 °C for 3 days, and then cooled to room temperature over 24 h. Colorless block crystals of 1 were obtained. Yield: 46% (based on Cd). Elemental analysis (%): calcd for C₁₈H₁₂Cd₃N₂O₂₀ C 23.67, H 1.32, N 3.07; found C 23.74, H 1.39, N 3.15. IR (cm⁻¹): 3392m, 1608s, 1560s, 1436s, 1351s, 1112w, 1072m, 927m, 827m, 747m, 713m.Synthesis of 2. 2 was synthesized in the similar way as that described for 1, except that bbi was taken into the reaction. Yield: 34% (based on Cd). Elemental analysis (%): calcd for C₃₈H₃₆Cd₃N₁₀O₁₈ C 36.28, H 2.88, N 11.13; found C 36.35, H 2.941, N 11.18. IR (cm⁻¹): 3355m, 1600s, 1523m, 1434s, 1347s, 1106m, 1082m, 941m, 825m, 740s, 717m.

§ Crystal data for 1: C₁₈H₁₂Cd₃N₂O₂₀, M = 913.50, monoclinic, space groupP2₁/c, T = 296(2)K, a = 14.1956(12) Å, b = 13.5254(11) Å, c = 12.3722(10) Å, β = 98.9920(10), V = 2346.3(3) ų, Z = 4, D_c = 2.586 g cm⁻³, Reflections collected/unique, 17440/4354 [R(int) = 0.0176], R = 0.0175, and wR = 0.0430 (I > 2σ(I)), R = 0.0199, wR = 0.0442 (all data).

¶ Crystal data for 2: C₃₈H₃₆Cd₃N₁₀O₁₈, M = 1257.97, monoclinic, space groupP2₁/c, T = 296(2)K, a = 11.5392(13) Å, b = 25.502(3) Å, c = 7.6065(8) Å, β = 105.9070(10), V = 2152.7(4) ų, Z = 2, D_c = 1.941 g cm⁻³, Reflections collected/unique, 16287/3998 [R(int) = 0.0188], R = 0.0241, and wR = 0.0532 (I > 2σ(I)), R = 0.0279, wR = 0.0548 (all data).

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