A novel 1D independent metal–organic nanotube based on cyclotriveratrylene ligand

Abstract

A novel nanotubular metal–organic framework [Cu₃L₂(EtOH)(Py)₂(H₂O)₂]·9DEF·8H₂O (CTV-Cu) was prepared by assembly of the rigid, trigonal pyramidal ligand tri-(4-carboxy-phenyl)-trimethoxycyclobenzylene (H₃3L) and Cu(NO₃)₂·3H₂O viasolvothermal reaction. The Cu(II) ions in the crystal serve as square and linear molecular building blocks, thus forming a framework with ternary topology. One single open-ended nanotube contains two identical orthogonally interpenetrated two-nodal nets. This framework exhibits permanent microporosity and relatively high surface area.

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Introduction

Making materials with appealing structures or extraordinary functions by “self-assembly” has become a major endeavour for chemists. Metal–organic frameworks (MOFs) built by connecting metal ions or clusters with organic linkers, are a rapidly growing class of hybrid porous networks.^1,2 The metal clusters used as building blocks will provide specific rigid molecular geometries that aid in directing the construction of functional materials. Through elaborate design, materials with various structures and functions can be obtained, and their applications cover the area of gas storage,³ separation,⁴ catalysis,⁵ sensing⁶ and drug delivery.⁷ One of the most common secondary building units (SBUs) formed by metals and carboxylates is the square paddle-wheel binuclear Cu cluster. Through elaborate control of vertex geometry and binding motif of the organic linkers, a variety of infinite MOFs and discrete MOPs (metal–organic polyhedra) with different dimensionalities, functionalities and pore metrics based on Cu₂(CO₂)₄ “paddle-wheel” are obtained.⁸

Since the discovery of carbon nanotubes (CNTs) in 1991,⁹ many nanotubular structures constructed by elements other than carbon have been synthesized and investigated.¹⁰ A particularly interesting and challenging subject in metal–organic polymers is the design and construction of metal–organic nanotubular structures. So far, the coordination approach has been proved to be very effective. For example, the metal–organic nanotubes (MONTs) based on coordinative bonds and metal ions have a great development during the past decades, many nanotubular MOFs with open channels have been reported.¹¹ While in most cases, all the nanotubular units in the MOFs only act as subunits, which are further bridged by other ligands to build 2D or 3D structures.¹² A common strategy to construct 1D independent MONTs is that the organic ligands first link metal ions to generate a polygon as the subunit, and then, the subunits are connected by the second organic ligands to form 1D nanotubular framework (three-component strategy).¹³ The 1D independent coordination-driven nanotubes can also be obtained by using bent geometric organic ligands and the square planar binding metal ions (two-component strategy).¹⁴

Cyclotriveratrylene (CTV) is a macrocyclic host with a shallow bowl shaped electron-rich cavity, which could be derived into unique ligands with pyramidal shape and convergent binding modes.¹⁵CTV based ligands can be coordinated to transition metals to form coordination polymers or discrete metallo-supramolecular assemblies.^16–18 By introducing carboxyl groups to the CTV derivatives, we could found a possibility of utilizing proper SBUs to construct interesting metal–organic assmblies.¹⁸ But there is not a single example of CTV based MOF with accessible permanent porosity. According to the reticular chemistry,² the geometry analysis of the CTV skeleton suggested that C₃-tri-(4-carboxy-phenyl)-trimethoxycyclo-benzylene might be a suitable tritopic 90° corner ligand, that can be used as a cubic corner building unit.¹⁹ (Scheme 1) Here we utilize the square paddle-wheel Cu cluster and C₃-symmetrical CTV-based tritopic ligand H₃3L as organic linker to prepare novel frameworks.

Scheme 1 Ligand H₃3L. (a) Chemical structure; (b) topological view as a 90° corner unit.

Experimental

Materials and measurements

¹H NMR and ¹³C NMR spectra were recorded at ambient temperature on a Bruker (400 MHz) NMR spectrometer. Elemental analyses for C, H and N were carried out with a Flash EA 1112 elemental analyzer. Thermogravimetric analyses were performed on a Perkin-Elmer TGA-7 apparatus with a temperature ramping rate of 10 °C min⁻¹ from room temperature to 550 °C. Powder X-ray data were collected using a Rigaku D/max2500 diffractometer with Cu-Kα radiation (λ = 1.54056 Å) over the 2θ range of 2–60° at room temperature. Gas sorption experiment was carried out with a Micrometrics ASAP 2020 instrument. Ligand H₃3L was synthesized according to the literature methods.^17d All chemicals were used as received without further purification unless stated otherwise.

Synthesis of CTV-Cu

A mixture of 9.8 mg H₃3L and 16 mg of Cu(NO₃)₂·3H₂O was dissolved in a 20 mL Teflon vial containing 1 mL DEF (N,N′-diethylformamide), 1 mL ethanol, and 0.5 mL H₂O. The mixture was stirred at room temperature for 10 min, and then 30 μL of pyridine was added. The capped vial was sealed and heated at a rate of 0.1 °C min⁻¹ to 105 °C, held at 105 °C for 48 h, then cooled at a rate of 0.05 °C min⁻¹ to room temperature. The resulting green block-shaped crystals were collected, washed with DEF and a small amount of ethanol, allowed to dry in air overnight. Yield: 16 mg (81% yield based on ligand H₃3L). They were stable in air, insoluble in DEF and ethanol. The product was formulated as [Cu₃L₂(EtOH)(Py)₂ (H₂O)₂]·9DEF·8H₂O on the basis of elemental analysis. Anal. Calcd. for: C 60.45, H 6.94, N 5.27; found: C 60.43, H 6.34, N 5.28.

X-ray crystallographic analysis

Suitable single crystal of CTV-Cu (green block-shaped crystal, size: 0.34 mm × 0.26 mm × 0.18 mm) was carefully chosen under an optical microscope and glued onto a thin glass fiber. X-ray diffraction data on single crystals were collected on a Rigaku RAXISRAPID diffractometer with graphite monochromated Mo-Kα radiation (λ_Mo/Kα = 0.71073 Å) at 173 K. Absorption corrections were carried out using “multi-scan” of the Crystalclear program.²¹ The structures were solved by direct method using SHELXS-97²¹ and refined by full-matrix least-square methods on F² using SHELXL-97.²² All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were attached in a riding model. SQUEEZE technique in PLATON was used to remove the solvent contribution in the final refinement.

Gas adsorption analysis

Before measurement, the sample of CTV-Cu was soaked in ethanol for 3 days to remove guest molecules, then filtrated and dried at room temperature. The sample was pretreated under high vacuum at 60 °C overnight to remove all the residue solvents in the channels. About 80 mg of the desolvated sample was used for the adsorption measurement.

Results and discussion

Structure description

A single-crystal X-ray diffraction analysis of CTV-Cu reveals an infinite 1D nanotubular channel that crystallizes in a space group of P4₂/nmc. The asymmetric unit determined by the X-ray analysis contains two CTV ligands, three Cu(II), one ethanol, two pyridines and two water molecules. The nanotubular structure was constructed by cuboid units with a space range of 24.1 Å × 12.8 Å.‡Copper ions have two different binding modes: the square binuclear “paddle-wheel” clusters and the five-coordinated square-pyramid. Cu ions in the binuclear “paddle-wheel” clusters (Cu1 and Cu2) have a distance of 2.666 Å. One ethanol and one water molecule are coordinated on the axial positions of the cluster, at outer and inner of the cavity, respectively. Cu3 is five-coordinated by two pyridine rings (half-occupancy), one water molecule and two monodentate carboxylates from two L enantionmers in distorted square-pyramidal geometry (Fig. 1a). Three carboxyl groups of H₃3L are all deprotonated and every L serves as a 3-c 90° corner node to coordinate with two bicopper clusters and one copper(II) ions through its two carboxylates in bidentate mode and one in monodentate mode, thus generating a 1D ternary nanotube²³ (Fig. 1b).

Fig. 1 (a) Stick view of a cuboid unit formed by four CTV ligands, four paddle-wheel Cu₂(CO₂)₄ clusters, and two linear Cu linkers. (b) 1D channel of CTV-Cu. C gray, N deep blue, O red, Cu cyan. All hydrogen atoms are omitted for clarity.

Compared with the 3D interwoven MOF formed by trigonal-planar 3-c organic linker 4,4′,4′′-benzene-1,3,5-triyl-tribenzoate (BTB) and Cu₂(CO₂)₄,^8f the reduction of dimensionality of our structure was mainly due to the convergent binding mode of L ligand as well as the square-pyramidal binding mode of Cu3.

The linear binding mode of Cu3 ions with L enlarged the distance between racemic ligands. As is usually found in metal–organic frameworks containing large cavities that allow interpenetration,²⁴ this 1D network is orthogonal catenated (Fig. 2a,b). The hydrogen-bond interactions between the axial water and the carbonyl O of the monodentate carboxylate (2.456 Å) could be the driving force of catenated packing. Topological analysis was carried out using TOPO4.0,²⁵ which reveals that one single nanotube contains two identical interpenetrated two-nodal nets (Fig. 2a,b). These two nets are symmetrically related by the 4₂ symmetry operation and this interpenetration belongs to the subclass of IIa with Z_n = 2. The Schläfli symbol for the single net is {4;6²}2{4²;6⁴} and can be considered as a fragment of FeSi₂ net by viewing along the a-axis. Two adjacent independent nanotubes are closely stacked mainly through π⋯π interaction in a parallel-displaced model with a shortest distance of 3.41 Å, and some other weak interactions are also involved, such as C–H⋯O hydrogen bond (2.50 Å) between methoxyl H and dicopper cluster O, and C–H⋯π interactions (2.04 Å) between methylene H and phenyl of CTV. This arrangement yields an overall 1D-periodic network with two kinds of pores. The first is the tube channel that along the c-axis, surrounded by CTVs and Cu3 (pore A), and partially occupied by the disordered pyridines. The second one is along the a(b)-axis surrounded by CTV, Cu3 and paddle-wheel Cu clusters (pore B). (Fig. 2c).

Fig. 2 (a) Stick view of the 1D interpenetrated networks; (b) topology of interpenetration; (c) stick view of the framework with Connolly surface.

Adsorption property

From the SQUEEZE calculation in PLATON, the total solvent accessible void volume of CTV-Cu is 12265 ų. N₂ sorption isothermal measurement was then executed on the desolvated sample at 77 K to study the accessibility of the voids. The isotherm in Fig. 3 shows a reversible type I behaviour without hysteresis upon desorption, indicating an open framework with accessible permanent microporosity. The Brunauer–Emmett–Teller (BET) and the Langmuir surface area of CTV-Cu is 594 m² g⁻¹ and 785 m² g⁻¹, respectively, with the relative pore volume of 0.227 cm³ g⁻¹. The Horvath–Kawazoe (HK) model indicates a media pore diameter of 6.04 Å in CTV-Cu, which is consistent with the crystal structure.

Fig. 3 Nitrogen gas sorption isotherm at 77 K for CTV-Cu (filled circles, adsorption; open circles, desorption).

Thermal stability analysis

To investigate the thermal stability of the framework and structure integrity after guest removal, powder X-ray diffraction studies (PXRD) and thermogravimetric analysis (TGA) were carried out. PXRD reveals that, upon guests exchange and evacuation, the material maintains the framework periodicity (Fig. 4, red), which confirms the architectural stability of the framework in the absence of guest molecules. If the desolvated sample was further heated and evacuated at the temperature of 120 °C in high vacuum, coordinated ethanol, water and pyridine are steadily lost, and cause the framework to collapse along with the loss of long-range order (Fig. 4, blue). The thermal gravimetric analysis (TGA) in air, on the as-synthesized sample shows that there is a weight loss of 36% below 250 °C corresponding to the liberation of free guest molecules (9 DEF molecules and 8 water molecules) (see ESI†).

Fig. 4 Powder X-ray data of CTV-Cu. The data collected after guest-exchange (red) can be well indexed with almost the same unit cell by Le Bail fitting (black). The simulated pattern is shown in green (bottom). The framework is collapsed if heated at 120 °C in vacuum (blue). A Le Bail fitting gives the unit cell as a = 40.50(4) and c = 18.02(2) with R_p = 3.32.

Conclusions

In conclusion, with a tritopic 90° corner C₃-symmetric macrocyclic binding unit, we have prepared a novel interpenetrated 1D nanotube based on the square Cu₂(CO₂)₄ cluster and five-coordinated square-pyramidal copper(II) ions. From the synthetic point of view, this research creates a new way to design 1D independent ternary nanotubes. This material is the first CTV based metal–organic framework with permanent porosity. Further analysis on this material is underway in our laboratory.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (20773114, 20932004), the Major State Basic Research Development Program of China (2007CB8008005, 2011CB932501), the Ministry of Health of China (2009ZX09501-006), and the Chinese Academy of Sciences (KJCX2-YW-H13).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Additional experimental procedure and further structure diagrams. CCDC reference number 817121. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ce05316g

‡ Crystal data for CTV-Cu: C₁₀₂H₈₆Cu₃N₂O₂₁, M = 1864.33, tetragonal, a = 40.284(6) Å, c = 17.660(3) Å, V = 28659(8) ų, T = 173(2)K, space groupP42/nmc, Z = 8, 77142 reflections measured, 12828 independent reflections (R_int = 0.1047). The final R₁ values were 0.1006 (I > 2σ(I)). The final wR(F²) values were 0.2562 (I > 2σ(I)). The final R₁ values were 0.1259 (all data). The final wR(F²) values were 0.2742 (all data). The goodness of fit on F² was 1.137.

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