Synthesis, structure and properties of an octahedral dinuclear-based Cu₁₂ nanocage of trimesoyltri(L-alanine)

Abstract

The reaction of a three-armed ligand of trimesoyltri(L-alanine) (TMTAH₃) with CuCl₂ gave [Cu₁₂(TMTA)₈(H₂O)₁₂]·8CH₃CH₂OH·40H₂O (1) as expected. It features paddlewheel dinuclear secondary building blocks, six of which are connected by eight TMTA³⁻ linkers, forming a Mo₆C1₈⁴⁺-like dinuclear-based octahedral nanocage. The magnetic studies revealed the presence of antiferromagnetic interactions between the Cu(II) ions in 1. The desolvated sample of 1 presents nice selectivity on the adsorption of H₂ and CO₂ over CH₄.

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Introduction

Metal–organic nanocages, which are also called metal–organic polyhedrons and coordination cages,¹ have attracted great attention of chemists because of their aesthetic topologies² and potential applications in fields such as gas storage, separation, catalysis, sensing, and drug delivery.³ In comparison to metal–organic frameworks, they show much better solubility in many solvents.^3d Thus, a lot of effort has been devoted to designing and constructing metal–organic nanocages under control.⁴ However, their rational design and syntheses with desired properties are still a great challenge for chemists. Thus we take the challenge to develop the synthetic strategy of nanocages.

As well known, Mo₆Cl₈⁴⁺ has an octahedral Mo₆¹²⁺ skeleton (Scheme 1a),⁵ in which the three Mo atoms in each triangular face are further consolidated by one μ₃-Cl⁻ ion. Based on this structure, we have ever contemplated that it might also be extended into metal–organic nanocage compounds with the Mo vertexes replaced by four-connecting polynuclear secondary building blocks⁶ and the μ₃-Cl⁻ ions replaced by three-armed organic linkers as shown in Scheme 1b. Thus the design of appropriate organic linkers is of great importance in building such cages.⁷ Three factors are to be taken into consideration for this linker. The first one is that the flexibility of the linker should be controlled in a certain range to sustain a stable cage. Usually, the rigid and semi-rigid linkers show much advantage over flexible ones in the construction of such nanocages out of the stability reasons. Another factor is to fuse some linking groups onto the organic linkers to help to form the polynuclear secondary building blocks. As well known, carboxylate groups are often to be a nice selection for its strong linking ability and versatile linking modes.⁸ The third one is to keep it in mind that the designed organic linkers must consolidate three polynuclear secondary building blocks.

Scheme 1 (a) A schematic show of Mo₆Cl₈⁴⁺; (b) a schematic show of the designed cage based on the structure of Mo₆Cl₈⁴⁺ with L and M_n representing three-armed linker and polynuclear secondary building block, respectively.

Based on these considerations, we designed a semi-rigid three-armed organic linker of trimesoyltri(L-alanine) (TMTAH₃) as shown in Scheme 2, which possesses the above-mentioned three characters and can link three polynuclear building blocks just like the chloride ion in Mo₆C1₈⁴⁺. Herein, we report a new discrete metal–organic nanocage of [Cu₁₂(TMTA)₈(H₂O)₁₂]·8CH₃CH₂OH·40H₂O (1). Its structure, magnetic properties and gas adsorption properties were investigated.

Scheme 2 A schematic show of the three-armed linker of trimesoyltri(L-alanine) (TMTAH₃) in this study.

Experimental section

Materials and physical measurements

Trimesoyltri(L-alanine) (TMTAH₃) was prepared according to a literature method reported by our group.⁹ The other materials and reagents in this study were used as obtained commercially without further purification. Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400II CHN elemental analyzer. The infrared spectra were recorded from KBr pellets in the range of 4000–400 cm⁻¹ with a Perkin-Elmer spectrum one FTIR spectrometer. The powder X-ray diffraction (PXRD) data were collected on a Rigaku D/max 2500v/pc diffractometer with Cu-Kα radiation (λ = 1.5418 Å). All magnetic measurements were carried out on a Quantum Design MPMS-XL7 SQUID magnetometer. The experimental susceptibility data were corrected for the diamagnetic contribution calculated from Pascal constants, and the diamagnetism of the sample and sample holder were taken into account. The adsorption isotherms of H₂ at 77 K and CO₂ and CH₄ at 298 K were measured with BELSORP-HP volumetric adsorption equipment from Bel Japan, Inc.

Synthesis of 1

A solid mixture of TMTAH₃ (0.1 mmol, 0.0423 g) with CuCl₂·2H₂O (0.3 mmol, 0.0519 g) was dissolved in a mixed solvent of DMF (2 mL) and C₂H₅OH (8 mL). After being stirred for 20 minutes, 1 mol L⁻¹ KOH (0.2 mL) were added. The resulted mixture was further stirred for 3 hours and then filtrated. The filtrate was allowed to evaporate the solvent at room temperature for seven weeks, giving green crystals which were collected, washed with ethanol, and then air-dried for six hours. Yield: 0.0176 g, 26% (calculated based on TMTAH₃). Anal. calcd (%) for C₁₆₀H₂₉₆Cu₁₂N₂₄O₁₃₂ (1): C, 35.39; H, 5.49; N, 6.19. Found: C, 35.70; H, 5.29; N, 6.42. IR (KBr pellet, cm⁻¹): 3402 (m), 1646 (s), 1535 (m), 1453 (m), 1414 (m), 1393 (m), 1378 (m), 1344 (w), 1327 (w), 1289 (m), 1207 (w), 1151 (w), 1124 (w), 1106 (w), 1074 (w), 1053 (w), 695 (w).

X-ray crystallography

Crystallographic data for all complexes were collected on a Bruker SMART CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å). Absorption correction was applied by using the multi-scan program SADABS.¹⁰ The structure was solved by direct methods of SHELXS-97 and refined by full-matrix least-squares against F² using the SHELXTL package.¹¹ The non-H atoms were refined again initially using isotropic and later anisotropic thermal displacement parameters. Complex 1 is very easy to volatilize the lattice guest molecules while exposed in air, which easily leads to the disorder of the guest molecules. Thus these crystalline solvent molecules could not be modeled properly and were removed by the SQUEEZE routine in PLATON.¹² It shows a solvent-accessible volume of 11 761.7 ų (43.4%) per unit cell (27 100.0 ų). The final formula was determined from the SQUEEZE results combined with the TG analysis and elemental analysis. The hydrogen atoms of coordinated water molecules were located in a difference Fourier map and refined isotropically in the final refinement cycles. The other hydrogen atoms were placed in calculated positions and refined using a riding model. Crystallographic data for compound 1 are given in Table S1.† Selected bond lengths and bond angles are given in Table S2.†

Results and discussion

Syntheses and general characterizations

The titled compounds were separated from the reaction of CuCl₂ with TMTAH₃ in the presence of KOH at room temperature by routine solution method. The optimized solvent is the mixed solvent of DMF with ethanol. The use of other solvents in this reaction could not give the titled compounds as expected. The change of CuCl₂ into the other Cu(II) salts such as Cu(NO₃)₂ and Cu(ClO₄)₂ could also produce the titled compound, but with much poorer quality of the crystals. The experimental PXRD pattern of the microcrystals of 1 agrees well with the simulated one based on its crystal structure (Fig. S1†), confirming the phase purity of 1. The TG analysis (Fig. S2†) of 1 reveals a fast weight loss upon heating to 90 °C, followed by a plateau in the temperature range of 90–180 °C. The weight loss of 19.3% below 138 °C corresponds to the loss of all solvated molecules (calcd 20.06%). After the plateau, the weight loss is first fast and then slow, which might be due to the collapse of the cage framework. The weight loss is not complete even when the sample was heated to 936 °C.

Crystal structure

The single crystal X-ray analysis indicates that complex 1 crystallizes in the tetragonal space group P4₂2₁2. The coordination asymmetrical unit of 1 contains six crystallographically independent Cu(II) ions, four TMTA³⁻ ligands and six coordinated water molecules as shown in Fig. S3.† All Cu²⁺ ions in 1 are five-coordinated in square pyramidal geometries by four carboxylato O atoms from four different TMTA³⁻ ligands in the square plane and one water molecule in the apical position. The equatorial Cu–O bond distances (1.944(5)–1.999(5) Å) are shorter than the axial Cu–O bond distances (2.131(5)–2.410(5) Å), which is attributed to the Jahn–Teller (JT) effect of Cu²⁺ ions. Two of such Cu(II) ions are bridged by four carboxylate groups from four different TMTA³⁻ ligands, forming a paddlewheel binuclear [Cu₂(COO)₄(H₂O)₂] unit with a Cu⋯Cu distance of 2.6292(11)–2.6362(11) Å. All TMTA³⁻ ligands in 1 behave as μ₆:η⁶-bridges, linking six Cu(II) ions from three [Cu₂(COO)₄(H₂O)₂] units. Six paddlewheel [Cu₂(COO)₄(H₂O)₂] units are consolidated by eight TMTA³⁻ ligands to form a [Cu₁₂(TMTA)₈(H₂O)₁₂] cage with a diameter of about 23.0(1) Å as shown in Fig. 1, S4 and S5.† It has a pore with an effective diameter of 5.7(1) Å (Fig. S5†). Each Cu(II) ion in this cage is coordinated by four TMTA³⁻ linkers, which in return act as six-connecting linkers. This kind of connection for the whole cage can be depicted in Fig. 2a–c. Regarding the [Cu₂(COO)₄(H₂O)₂] unit as one node, the TMTA³⁻ ligand can be viewed as a three-connecting linker and each [Cu₂(COO)₄(H₂O)₂] node interacts with four three-connecting linkers. This kind of linkage results in the construction of a Mo₆C1₈⁴⁺-like neutral octahedral cage (Fig. 2d), which is similar to the documented coordination cages.^1b,13 These nanocages pack in the lattice in a face-centered cubic style as depicted in Fig. 3 and S6.†

Fig. 1 A perspective view of the cage viewed down the C₃ symmetry axis (a) and the C₄ axis (b).

Fig. 2 Topological views of 1 from different directions. Red spheres represent TMTA³⁻ ligands, green spheres represent the Cu atoms in (a–c) and Cu₂ binuclear units in (d).

Fig. 3 A schematic show of the packing diagram of the octahedral cages (cyan spheres) of 1.

Magnetic studies

The magnetic properties of 1 were investigated by the solid state magnetic susceptibility measurements in the temperature range 2–300 K in a DC field of 1000 Oe, which are shown in Fig. 4 in the form of χ_mT and χ_m versus T plots. It presents a χ_mT value of 4.51 cm³ K mol⁻¹ at 300 K, which is nearly equal to that (4.50 cm³ K mol⁻¹) calculated for twelve spin-only non-interacting Cu(II) ions. Upon cooling, the χ_mT value decreases gradually to 1.07 cm³ K mol⁻¹ at 2 K. The profile of the χ_mT versus T plot suggests the presence of strong antiferromagnetic interactions between the metal ions in the title complex,¹⁴ which is mainly due to the intradinuclear magnetic interactions in the paddlewheel dinuclear [Cu₂(COO)₄(H₂O)₂] building units. It agrees well with the complexes containing paddlewheel dinuclear [Cu₂(COO)₄(H₂O)₂] secondary building units which show strong antiferromagnetic interactions between the intradinuclear Cu(II) ions through the face-on d_x²–y² orbitals by the four equatorial-to-equatorial carboxylate bridges.¹⁵

Fig. 4 Plots of χ_m and χ_mT vs. T for 1.

Gas sorption

The structural analysis of 1 reveals a volume of 11 761.7 ų (43.4%) per unit cell (27 100.0 ų) for free solvent molecules and a void with effective diameter of 5.7(1) Å in the octahedral cage. There are also some functionalities of –C(=O)–NH– on the inner wall of the cage which favors to interact with other guest molecules.¹⁶ All of these features suggest that 1 might present gas adsorption properties. Thus the adsorption of H₂, CO₂ and CH₄ gases for compound 1 was studied by measuring the corresponding adsorption isotherms using a volumetric gas-sorption apparatus. The polycrystalline samples of 1 were activated at 100 °C in a dynamic vacuum for 6 h before the adsorption measurements. The activated sample of 1 shows selectivity for gas adsorption. No apparent adsorption of CH₄ was found for the pre-activated samples of 1. However, it can adsorb a certain amount of H₂ and CO₂ as revealed in their adsorption isotherms (Fig. 5). The high pressure hydrogen adsorption isotherm of 1 was measured at 77 K, which gives an uptake of 29.47 cm³ g⁻¹ (0.27 wt%) at 20.4 atm. The high pressure CO₂ adsorption isotherm of 1 was collected at 298 K, which presents an uptake of 15.68 cm³ g⁻¹ (3.09 wt%) at 21.9 atm. It shows nearly the same increasing rate upon increasing the gas pressure below 8.87 atm for the adsorption of both H₂ and CO₂. The further increasing gas pressure leads to the same increasing rate for the adsorption of CO₂, however a much higher increasing rate for the uptake of H₂. The marked hysteresis loops are observed for both H₂ and CO₂ adsorption isotherms of 1, which reveals that the desolvated sample of 1 allows both H₂ and CO₂ to be stored at a pressure lower than that they are charged.^9,17 Although the adsorption of H₂ and CO₂ for the desolvated sample of 1 is not high as expected, it really shows nice sorption selectivity for the gas molecules.

Fig. 5 H₂ and CO₂ adsorption isotherms of 1. Filled and open circles represent adsorption and desorption branches, respectively. Connecting traces are guides for eyes.

Conclusions

In conclusion, a novel Cu₁₂ nanocage was built from the linkage of six paddlewheel dinuclear [Cu₂(COO)₄(H₂O)₂] secondary building units by the three-armed linker of trimesoyltri(L-alanine). It features a Mo₆C1₈⁴⁺-like structure. The magnetic investigation revealed the presence of dominant antiferromagnetic interaction between the adjacent metal ions of the title compound. The desolvated title compound shows nice selectivity for gas molecules as revealed by the adsorption measurements of H₂, CO₂ and CH₄ gases. More examples for developing the synthetic strategies of aimed meta-organic nanocages are under way in our group.

Acknowledgements

We thank the financial supports by National Natural Foundation of China (Grant No. 21261004 and 21271050), Guangxi Natural Science Foundation of China (Grant No. 2013GXNSFGA019008 and 2013GXNSFAA019039).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: PXRD patterns, TG curve, additional structural figures, and tables for crystallographic data and selected bond lengths and angles. CCDC 1439771. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra26357c

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