Pillararene incorporated metal-organic frameworks for supramolecular recognition and selective separation

Abstract

Crystalline frameworks containing incorporated flexible macrocycle units can afford new opportunities in molecular recognition and selective separation. However, such functionalized frameworks are difficult to prepare and challenging to characterize due to the flexible nature of macrocycles, which limits the development of macrocycle-based crystalline frameworks. Herein, we report the design and synthesis of a set of metal-organic frameworks (MOFs) containing pillar[5]arene units. The pillar[5]arene units were uniformly embedded in the periodic frameworks. Single crystal X-ray diffraction analysis revealed an interpenetrated network that appears to hinder the rotation of the pillar[5]arene repeating units in the frameworks, and it therefore resulted in the successful determination of the precise pillar[5]arene host structure in a MOF crystal. These MOFs can recognize paraquat and 1,2,4,5-tetracyanobenzene in solution and selectively remove trace pyridine from toluene with relative ease. The work presented here represents a critical step towards the synthesis of macrocycle-incorporated crystalline frameworks with well-defined structures and functional utility.

Fig. 1. Design and synthesis of pillar[5]arene-based MOFs.

a Cartoon representations and chemical structures of ligands and pillar[5]arene-based MOFs: MeP5BPy, MeP5BPPy, H₄TPE, H₄TPPE, zinc node, MeP5-MOF-n (n = 1, 2, 3, 4). b Schematic representations of the transformation from MeP5-MOF-2 to Py@MeP5-MOF-2 upon uptake of Py from a 90:10 v/v (87.3:12.7 mole percentage) Tol/Py mixture. Py = pyridine; Tol = toluene.

Fig. 2. Structures of MeP5-MOF-1, Model-MOF-1, and MeModel-MOF-1.

a Capped-stick representation of structural model MeP5-MOF-1a of MeP5-MOF-1. Here the backbone of MeP5-MOF-1 was determined by SCXRD, while the pillar[5]arene units are the results of structural model. b Cartoon representation of Model-MOF-1 or MeModel-MOF-1. c–f Capped-stick representations of single crystal structures of Model-MOF-1 and MeModel-MOF-1. The TPE ligands are blue, pillared struts are orange, zinc nodes are white, hydrogen atoms and solvent molecules have been omitted for clarity.

Fig. 3. Structures of MeP5-MOF-2, pS-MeP5-MOF-2, and pR-MeP5-MOF-2.

a–i Single crystal structures shown in capped-stick form and cartoon representations of MeP5-MOF-2, pS-MeP5-MOF-2, and pR-MeP5-MOF-2. The TPE ligands are blue, pillared struts are red, zinc nodes are white, and hydrogen atoms have been omitted for clarity.

Fig. 4. Supramolecular recognition studies of MeP5-MOF-1 and MeP5-MOF-2 with PQT and TCN.

Optical microscopy images of MeP5-MOF-1: a before uptake of PQT; b after uptake of PQT; c before uptake of TCN; d after uptake of TCN. Scale bars, 200 μm. The mole ratios of PQT (e) and TCN (f) to struts in MeP5-MOF-1, MeP5-MOF-2, and Model-MOF-1, as inferred from ¹H NMR spectral studies of these MOFs after guest uptake in acetone. g–i PXRD patterns of single crystalline samples of MeP5-MOF-1, MeP5-MOF-2, and Model-MOF-1: I before guest uptake; II after uptake of PQT; III after uptake of TCN. Source data of g–i are provided as a Source data file.

Fig. 5. Structures of (Py)₂@P5, Py@MeP5-MOF-2, and Tol@MeP5-MOF-2.

Illustrated structures of a (Py)₂@P5, b, c Py@MeP5-MOF-2, and d, e Tol@MeP5-MOF-2. The occupancy of Tol in MeP5-MOF-2 is 0.5. Here (a), (d), and (e) are views of single crystal structures, while the Py molecules in (b) and (c) are models based on the single crystal structure of MeP5-MOF-2 and (Py)₂@P5. TPE ligands are blue, pillared struts are red, zinc nodes are white. Hydrogen atoms have been omitted for clarity. Py and Tol are shown in the spacefilling form and color coded by element.