A chiral metal–organic framework with polar channels: unique interweaving six-fold helices and high CO₂/CH₄ separation

Abstract

A chiral Cu(II) metal–organic framework, [H₂N(CH₃)₂]₂[Cu(L)]·5.5DMF (1), was constructed using a diisophthalate ligand with an active pyridyl site, 2,6-di(3′,5′-dicarboxylphenyl)pyridine (H₄L). The structure possesses interesting polar channels based on interweaving heterochiral [4 + 2] helices, which contains multiple CO₂ binding sites, leading to highly selective capture of CO₂ over CH₄. Grand canonical Monte Carlo simulations identified the multiple adsorption sites in 1 for CO₂.

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Introduction

Helicity is a highly attractive topological motif because of its fascinating structure and its realistic and potential applications in many fields.¹ Helical chirality is a special chirality exhibited in a one-dimensional form with only one axis of symmetry. Absolute helicity has an inevitable connection with the chirality of molecular building blocks. Inspired by this, many chiral metal–organic frameworks (MOFs) with helical structures have been fabricated artificially by the self-assembly of organic ligands and metal ions since the pioneering work of Lehn,² and various synthetic strategies have been explored. A synthetic strategy that incorporates the ligands with reduced symmetry to construct MOFs has been recently proposed,³ which by increasing the topological complexity offers a largely unexploited approach towards MOF syntheses, particularly in chiral porous MOFs.⁴ Moreover, chiral MOFs built from the achiral ligands not only offer a practical and feasible route to mimic the elegant helical structures in natural superstructures but also provide insight into the mechanisms of chirality transfer and reproduction processes.⁵

MOFs as an emerging class of CO₂ capture and separation materials have been intensively explored.⁶ The recent studies demonstrate that MOFs could be a promising physical adsorbent for CO₂ with a huge potential to replace existing benchmark materials for CO₂ capture at low concentrations and moderate temperatures.⁷ To enhance the CO₂ adsorption capacity and selectivity, one of the effective approaches is to build a framework with suitable pore size, such as by narrowing the pores by interpenetration or altering the lengths of ligands in iso-reticular MOFs.⁸ Another effort underway is to enhance the binding affinity of MOFs for gas, accordingly, some strategies, such as generating open metal sites, forming charged frameworks, and incorporating organic active sites in frameworks have been adopted.⁹ However, it remains an important challenge to realize the balance between large storage capacity and high selectivity of CO₂ for building ideal porous materials.

Based upon our ongoing research on the development of new MOFs containing asymmetrical pyridyl carboxylate ligands for CO₂ capture,¹⁰ we herein constructed a new chiral MOF, [H₂N(CH₃)₂]₂[Cu(L)]·5.5DMF (1), by employing a diisophthalate ligand with a pyridyl site, 2,6-di(3′,5′-dicarboxylphenyl)pyridine (H₄L). 1 possesses unique polar channels assembled from the interweaving heterochiral [4 + 2] helices and displays highly selective capture for CO₂ over CH₄.

Experimental

Materials and general methods

All reagents were purchased commercially and used as-received without further purification. Elemental analyses for C, H and N were performed on a PerkinElmer 2400C elemental analyzer. IR spectra were recorded using KBr pellets on a Nicolet Avatar 360 Fourier transform infrared (FTIR) spectrometer. The solid state circular dichroism (CD) spectra were recorded by using a JASCO Corporation J-715 spectrometer using KBr disks. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8-ADVANCE using Cu Kα radiation (λ = 1.5418 Å). Thermal gravimetric analyses (TGA) were performed under a nitrogen stream using with a heating rate of 5 °C min⁻¹ on a Netzsch TG209F3 equipment. Gas-sorption isotherms were measured by using an ASAP 2020 M adsorption equipment with an automatic volumetric sorption apparatus.

Synthesis of [H₂N(CH₃)₂]₂[Cu(L)]·5.5DMF (1). A mixture of Cu(NO₃)₂·3H₂O (24.2 mg, 0.1 mmol) and H₄L (20.4 mg, 0.05 mmol) and two drops of HNO₃ (0.1 M) in DMF (5 mL) was sealed in a 10 mL screw-capped vial. The vial was placed in an oven at 105 °C for 72 h, and gradually cooled to room temperature at a rate of 5 °C min⁻¹. Blue rod crystals of 1 were collected by filtration and washed with DMF. The yield was ca. 33.3 mg (69.4%, based on H₄L). Anal. Calcd for C_41.5H_63.5CuN_8.5O_13.5: C, 51.87; H, 6.66; N, 12.39. Found: C, 51.81; H, 6.73; N, 12.27. IR (KBr, cm⁻¹): 3421m, 3016w, 2936m, 1948w, 1630s, 1557m, 1509m, 1402s, 1264m, 1189m, 1058w, 1017s, 963w, 868w, 773s, 744m, 697m, 595m, 524w, 478w.

Crystallography

The diffraction data were collected at 296(2) with a Bruker-AXS SMART CCD area detector diffractometer using ω rotation scans with a scan width of 0.3° and Mo Kα radiation (λ = 0.71073 Å). Absorption was corrected using the SADABS program. The structure was solved by direct methods and refined by full-matrix least-squares refinements based on F² with the SHELXTL program.¹¹ Non-H atoms were refined anisotropically with the hydrogen atoms added to their geometrically ideal positions and refined isotropically. The heavily disordered solvent molecules were trapped in the channels of 1 and could not be modeled properly. Thereby the SQUEEZE routine of PLATON¹² was applied to remove the contributions to the scattering from the guests. The final formula was determined by combining single-crystal structure, elemental microanalysis and TGA. Selected crystallographic data and structure refinement results are listed in Table S1.†

Results and discussion

The solvothermal reaction of Cu(NO₃)₂·3H₂O with H₄L in DMF for three days afforded blue prism crystals of 1 (Fig. 1). The single-crystal X-ray diffraction study shows that 1 crystallizes in the chiral P3₁2₁ space group (Flack parameter = 0.12(3)). The solid-state circular dichroism (CD) spectra confirmed the chirality of the bulk material of 1 (Fig. S1†). The asymmetric unit of 1 consists of two independent Cu²⁺ ions, one L⁴⁻ and two [H₂N(CH₃)₂]⁺ cations. Both Cu²⁺ ions with similar square planar geometries are respectively coordinated by four carboxylate O atoms from four different L⁴⁻ ligands. Interestingly, each Cu²⁺ ion as a knot is connected by four L⁴⁻ to form interweaving heterochiral six-fold helices along the c-axis, which is based on the association of parallel four-fold left-handed (Fig. 2a) and two-fold right-handed (Fig. 2b) helical chains, displaying an unusual interweaving [4 + 2] coaxial helical fashion (Fig. 2c). To date, very few MOFs based on the interweaving multi-helices have been reported.^2a,13 To the best of our knowledge, the interweaving [4 + 2] helices in 1 are the first reported so far. Furthermore, the six-fold helices in 1 are enclosed to generate a 1D cylindrical chiral channel with a diameter of 11.055 Å (based on the inner carboxylate O⋯O separations) running along the c-axis (Fig. 2d). The uncoordinated carboxylate O atoms stand in the inner surfaces, suggesting a polar character. Each channel is constructed by the isophthalate subunits of L⁴⁻ with Cu²⁺ ions, and is further connected together with the neighbouring six through the central pyridyl spacers of L⁴⁻, forming a 3D open framework, with 56.6% solvent accessible voids (Fig. 3). Moreover, the strong π⋯π interactions exist in the stacking pyridyl rings and isophthalate subunits (the vertical distance between the face-to-face is 3.367 Å) (Fig. S2†). Topologically, considering the H₄L linker as a branched tetratopic linker, each L⁴⁻ can be treated as a two-linked 3-connected node. Meanwhile regarding the Cu²⁺ ion as a 4-connected node, the framework forms an unprecedented (3,3,4,4)-connected net with the point symbol of (6·8·9)₄(6·8⁴·11)(6·8⁴·12) by Topos¹⁴ (Fig. 4). Notably, H₄L is achiral, the chiral source of 1 should be originate from the significant bending of the L⁴⁻ molecule in 1, which is responsible for the formation of helices. In contrast, L⁴⁻ is almost planar in the reported two MOFs which are achiral frameworks and there is no existence of the helix as well.¹⁵ In addition, the asymmetric characteristic of H₄L may be the other main reason for the chirality occurring in 1.

Fig. 1 Coordination environment of Cu²⁺ in 1, and the microscopy picture of 1.

Fig. 2 Ball and stick models and topology modes of (a) four parallel left-handed helices, (b) two right-handed helices, and (c) and (d) interweaving [4 + 2] helices in 1 along the b- and c-axes.

Fig. 3 3D framework of 1 with 1D hexagonal channels, the green balls in the inner surfaces represent uncoordinated carboxylate O atoms.

Fig. 4 (3,3,4,4)-Connected topological net of 1.

Sorption properties

To examine the porosity, the guest-free phase 1a was obtained by soaking the experimental sample of 1 in CH₃OH for 72 h and subsequent heating at 80 °C under vacuum for 6 h. TGA and IR revealed the complete removal of guest molecules in 1a (Fig. S3 and S4†), and PXRD patterns confirmed the framework integrity of 1a (Fig. S5†). The reversible type-I N₂ sorption isotherm of 1a at 77 K shows the characteristics of microporous material (Fig. 5a). The Brunauer–Emmett–Teller (BET) and Langmuir surface areas are evaluated to be 499 and 569 m² g⁻¹, respectively, and a total microporous volume of 0.216 cm³ g⁻¹ is estimated by the Dubinin–Astakhov (DA) method. The measured pore size distribution (PSD) curve based on the NLDFT model shows the main pore sizes of 4.7–8.1 Å (Fig. S6†).

Fig. 5 (a) Sorption isotherms of 1a for N₂ (77 K), CO₂ (195 K) and CH₄ (195 K); (b) sorption isotherms of 1a: CO₂ at 273 K (a), 285 K (b), and 298 K (c); CH₄ at 273 K (d) and 298 K (e). Solid and open symbols represent adsorption and desorption isotherms, respectively.

The establishment of porosity and polar channels of 1a encourages us to examine its potential application for CO₂ and CH₄ captures at different temperatures. As shown in Fig. 5a, at 195 K, 1a shows similar type-I sorption isotherms for CO₂ and CH₄, but with more adsorption amounts of CO₂ (155.3 cm³ g⁻¹ or 30.5 wt% at 1 bar) relative to that of CH₄ (63.9 cm³ g⁻¹ or 4.6 wt% at 1 bar). Such an adsorption selectivity is more obvious at high temperatures (Fig. 5b). At 1 bar, the CO₂ uptakes are 62.1 cm³ g⁻¹ (12.2 wt%) at 273 K, 54.1 cm³ g⁻¹ (10.6 wt%) at 285 K, and 43.9 cm³ g⁻¹ (8.6 wt%) at 298 K. In contrast, the CH₄ uptakes at 1 bar are 17.2 cm³ g⁻¹ (1.2 wt%) and 9.4 cm³ g⁻¹ (0.7 wt%) at 273 and 298 K, respectively, indicating the significant selective capture of CO₂ over CH₄. To predict CO₂/CH₄ selectivity in 1a for a CO₂/CH₄ binary mixture, the ideal adsorbed solution theory (IAST)¹⁶ was employed on the basis of the adsorption curves of CO₂ and CH₄ at 298 K (Fig. S7†). For CO₂/CH₄ mixtures with the typical feed compositions of landfill gas (CO₂/CH₄ = 50 : 50) and natural gas (CO₂/CH₄ = 10 : 90 and 5 : 95), the CO₂/CH₄ selectivities calculated at 1 bar are 30.8, 23.8 and 23.7, respectively (Fig. 6). Strikingly, a material with a selectivity value of 30.8 at 298 K and 1.0 bar outperforms the most reported MOFs under the same conditions.^6–9 Comparison of the CO₂/CH₄ selectivity in 1a with those of known MOFs which possess good CO₂/CH₄ selectivity under similar conditions (Table 1) indicates that the CO₂/CH₄ selectivity in 1a is significantly high, suggesting potential application in the capture of CO₂ from landfill and natural gas.

Fig. 6 CO₂/CH₄ selectivity at 298 K calculated by the IAST method for different CO₂ concentrations in CO₂/CH₄ binary mixtures.

Table 1 Comparison of CO₂/CH₄ selectivity calculated by the IAST method for the equimolar mixture at 1 bar and 298 K of 1a with the selected MOFs

MOFs	Selectivity	Ref.
a CO2/CH4 = 5 : 95. b CO2/CH4 = 10 : 90.
SIFSIX-3-Zn	108	6a
SIFSIX-2-Cu	42.8	6a
[Cu3(CPT)4(μ3-OH)]·NO3	36	17
UTSA-49	33.7	18
[H 2 N(CH 3 ) 2 ] 2 [Cu(L)]·5.5DMF	30.8, 23.8 , 23.7	This work
MPM-1-TIFSIX	20.3	19
[Cu3(TDPAH)(H2O)3]·13H2O·8DMA	18	20
Cu-TDPDA	13.8	21
[Eu(Hpzbc)2(NO3)]·H2O	12.8, 10.3^a, 10.4^b	22
Mg-MOF-74	11.5	23
[Cu(bpy)2(SiF6)]	10.5	24
JLU-Liu22	9.4, 7.7^a	25
PCN-307	8.8	26
ZIF-93	8.2	27
UiO-66-AD4	8.0	28
PCN-306	7.5	26
SNU-151′	7.2	29
Zr-UiO-67AcOH	6.8	30
ZIF-100	5.9	31
ZJU-60	5.5	32
MOF-177	4.4	33
dia-7i-1-Co	4.1, 4.0^a	34
DMOF	3.2	35

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Such high selectivity for CO₂ may be attributed to the effect of the cationic framework, abundant uncoordinated carboxylate O atoms, free pyridine N atoms, and [H₂N(CH₃)₂]₂⁺ cations, which induce the framework to be highly polar by producing an additional electric field, as a result, improving the interactions between the quadrupolar CO₂ molecule and the framework. The strength of the framework–CO₂ interactions was evaluated by the isosteric heat of adsorption (Q_st) that was calculated with the virial method by fitting the adsorption isotherms at 273, 285 and 298 K (Fig. S8†). The initial Q_st exhibits a maximum of 36.9 kJ mol⁻¹ (Fig. S9†), which is comparable to those of MOFs decorated with typical active sites such as Lewis basic sites (LBSs) and open metal sites (OMSs), and other representative MOFs (Table 2).⁴⁵ Although Q_st displays a gradual decrease with the increasing CO₂ coverage, Q_st still reaches 26.6 kJ mol⁻¹ at the maximum loading of 43.9 cm³ g⁻¹, demonstrating the strong framework–CO₂ interactions, which is responsible for the significant selectivity for CO₂.

Table 2 Comparison of the isosteric heat (Q_st) of 1a with the selected MOFs

MOFs	Q st/kJ mol^−1	Ref.
Bio-MOF-11	45	36
SIFSIX-3-Zn	45	6a
MPM-1-TIFSIX	44.4	19
Mg2(dobpdc)	44	37
Co-MOF-74	37	38
[H 2 N(CH 3 ) 2 ] 2 [Cu(L)]·5.5DMF	36.9	This work
SMT-1	35	39
UTSA-16	34.6	40
[Zn3(pbdc)2]·Hpip	32	41
HKUST-1	30.4	42
Cu(bcppm)	29	43
Cu-btc	28	44
PCN-88	27	39

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GCMC simulations

The interactions of 1a with CO₂ were explored at 298 K by Grand Canonical Monte Carlo (GCMC) simulations (see the ESI†). The density contours of CO₂ obtained at 0.1 kPa reveals that the CO₂ molecules are basically located near the region of Cu–O coordination motifs (Fig. S10a†). At a higher pressure of 100 kPa, CO₂ molecules are mostly adsorbed in proximity to the wall of channels (Fig. S10b†). At 100 kPa, three favourable binding sites, CO₂-I, CO₂-II and CO₂-III, were observed in 1a (Fig. 7). For the first site, CO₂-I is close to the Cu–O coordination units, and is embedded in the middle of two isophthalate units of two L⁴⁻, in which two electropositive carboxylate C atoms come into contact with two electronegative O atoms of CO₂ (Fig. 7a). The C⋯O separations of 3.090 and 3.575 Å approximate with the sum of van der Waals radii of carbon (1.70 Å) and oxygen (1.52 Å), indicating moderate interactions. Meanwhile, the two O atoms of CO₂-I also form C–H⋯O hydrogen bonds (O⋯H = 3.224–3.265 Å) with two methyl H atoms of two [H₂N(CH₃)₂]₂⁺ cations. CO₂-II is located near one isophthalate unit of one L⁴⁻, and through its C and O atoms form the C⋯O (3.441 Å) contact and C–H⋯O (O⋯H = 3.104 Å) hydrogen bond with the uncoordinated carboxylate O atom and phenyl –CH group, respectively (Fig. 7b). The two O atoms of CO₂-II are also involved in the multiple C–H⋯O (O⋯H = 3.162–3.235 Å) and N–H⋯O hydrogen bonds (O⋯H = 3.162 Å) with the methyl and amido H atoms of two [H₂N(CH₃)₂]₂⁺ cations. In contrast, differing from the situations of CO₂-I and CO₂-II, CO₂-III in 1a is not in contact with the host framework, but with the two O atoms is involved in three C–H⋯O (O⋯H = 2.706–3.121 Å) and one N–H⋯O hydrogen bonds (O⋯H = 2.509 Å) with the methyl and amido H atoms of three [H₂N(CH₃)₂]₂⁺ cations (Fig. 7c). In addition, there also exists the intermolecular interactions between CO₂-I and CO₂-II, and between CO₂-II and CO₂-III, in which one O atom of one CO₂ comes into contact with the C atom of the other CO₂ (O⋯C = 3.310 and 3.490 Å) (Fig. 7d).⁴⁶ These simulations clearly indicated that the favourable binding sites are changed from the regions of coordination units to the porous wall, and to the porous center with the increasing of the adsorption amount. Notably, the free pyridine N atom of L⁴⁻, which is blocked by [H₂N(CH₃)₂]₂⁺ cations, is not functionalised as a direct binding site for the CO₂ molecule, however, the [H₂N(CH₃)₂]₂⁺ cations as gas binding sites play an important role in the overall CO₂ coverage.

Fig. 7 (a), (b) and (c) Three simulated favourable CO₂ sorption sites in 1a, forming CO₂ intermolecular interactions (d).

Conclusions

In summary, by utilizing an achiral asymmetrical diisophthalate ligand, an uncommon homochiral anionic framework with 1D cylindrical channels assembled from intriguing interweaving six-fold helices was synthesized. The desolvated framework shows permanent porosity with suitable pore sizes and highly polar porous surfaces, which enable this material to exhibit excellent selectivity for CO₂ over CH₄. GCMC simulations demonstrate the multiple binding sites of CO₂.

Acknowledgements

We are thankful for the support of this work by the NSFC (21471124 and 21531007), the NSF of Shaanxi province (2013KJXX-26 and 2014JQ2049), the PhD Start-up Fund of Northwest A&F University (2452015359), and the Australian Research Council Future Fellowship FT12010072.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures, PXRD, IR, TGA, and sorption isotherm. CCDC 1491604. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6qi00282j

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