Porous Organic Cages

Abstract

Porous organic cages (POCs) are a relatively new class of low-density crystalline materials that have emerged as a versatile platform for investigating molecular recognition, gas storage and separation, and proton conduction, with potential applications in the fields of porous liquids, highly permeable membranes, heterogeneous catalysis, and microreactors. In common with highly extended porous structures, such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs), POCs possess all of the advantages of highly specific surface areas, porosities, open pore channels, and tunable structures. In addition, they have discrete molecular structures and exhibit good to excellent solubilities in common solvents, enabling their solution dispersibility and processability─properties that are not readily available in the case of the well-established, insoluble, extended porous frameworks. Here, we present a critical review summarizing in detail recent progress and breakthroughs─especially during the past five years─of all the POCs while taking a close look at their strategic design, precise synthesis, including both irreversible bond-forming chemistry and dynamic covalent chemistry, advanced characterization, and diverse applications. We highlight representative POC examples in an attempt to gain some understanding of their structure-function relationships. We also discuss future challenges and opportunities in the design, synthesis, characterization, and application of POCs. We anticipate that this review will be useful to researchers working in this field when it comes to designing and developing new POCs with desired functions.

Figure 1

Construction of shape-persistent cage 1a as a good siderophore candidate. Reproduced with permission from ref (47). Copyright 1984 Wiley-VCH.

Figure 2

Synthesis of a C₂-symmetry [4 + 8] organic molecular cage 4 by multidentate organic linkers. Reproduced with permission from ref (52). Copyright 2020 Wiley-VCH.

Figure 3

Synthesis of a “tied” porous cage 6. Reproduced with permission from ref (64). Copyright 2014 American Chemical Society.

Figure 4

(a) Synthesis of imine-linked cages 7, 8, 9, and 10 and their structural formulas. Reproduced with permission from ref (73). Copyright 2011 Springer-Nature. (b) Structural formula for the imine-linked cage 11. Reproduced with permission from ref (74). Copyright 2014 The Royal Society of Chemistry. (c) Binary cocrystals of different chiral cages. Reproduced with permission from ref (73). Copyright 2011 Springer-Nature. (d) Synthesis of an imine-linked cage 12. Reproduced with permission from ref (53). Copyright 2011 Wiley-VCH.

Figure 5

(a) Structural formula of bicapped cage 13. Reproduced with permission from ref (85). Copyright 1987 American Chemical Society. (b) Structural formula of trefoil knot 14. Reproduced with permission from ref (86). Copyright 2000 Wiley-VCH.

Figure 6

(a) The synthesis of the tricyclic polyamide cage 17. Reproduced with permission from ref (88). Copyright 1998 Wiley-VCH. (b) The final step in the synthesis of amide cage 19. Reproduced with permission from ref (59). Copyright 2019 Wiley-VCH.

Figure 7

(a) Synthesis of the bicyclophane cages 20 and 21. Reproduced with permission from ref (46). Copyright 1977 Elsevier. (b) Synthesis of D_3h symmetric triangular prism cage 22. Reproduced with permission from ref (99). Copyright 1992 American Chemical Society. (c) Synthesis of calixpyrrole-like cryptand 25. Reproduced with permission from ref (100). Copyright 2001 American Chemical Society. (d) Synthesis of 3D conjugated cage 28. Reproduced with permission from ref (101). Copyright 2017 Wiley-VCH.

Figure 8

Structural formulas of the triptycene-based cages 29 and 30. Reproduced with permission from ref (102). Copyright 2007 American Chemical Society.

Figure 9

(a) Structural formula of the bicyclooxacalixarene 31. Reproduced with permission from ref (104). Copyright 2005 American Chemical Society. (b) Structural formula of the cryptophane cage 32. Reproduced with permission from ref (105). Copyright 2010 American Chemical Society. (c) Structural formula of the cryptophane cage 33. Reproduced with permission from ref (106). Copyright 2011 American Chemical Society. (d) Structural formula of the semirigid cyclophane ExBox⁴⁺. Reproduced with permission from ref (112). Copyright 2013 American Chemical Society. (e) Synthesis of the tetragonal prismatic porphyrin cage TPPCage⁸⁺. Reproduced with permission from ref (114). Copyright 2018 American Chemical Society.

Figure 10

(a) Synthesis of a molecular cage 34. Reproduced with permission from ref (117). Copyright 2013 The Royal Society of Chemistry. (b) Synthesis of molecular cage 35. Reproduced with permission from ref (118). Copyright 2011 The Royal Society of Chemistry. (c) Synthesis of molecular cage 36. Reproduced with permission from ref (119). Copyright 2020 Elsevier. (d) Synthesis of molecular cage 37. Reproduced with permission from ref (120). Copyright 2011 The Royal Society of Chemistry. (e) Synthesis of molecular cage 38. Reproduced with permission from ref (121). Copyright 2012 Wiley-VCH. (f) Synthesis of molecular cage 39. Reproduced with permission from ref (122). Copyright 2018 Wiley-VCH.

Figure 11

(a) Construction of molecular cages 40. Reproduced with permission from ref (123). Copyright 2012 The Royal Society of Chemistry. (b) Construction of molecular cage 41. Reproduced with permission from ref (125). Copyright 2015 The Royal Society of Chemistry. (c) Construction of molecular cage 42. Reproduced with permission from ref (126). Copyright 2015 Wiley-VCH.

Figure 12

(a) Synthesis of molecular cages 43. Reproduced with permission from ref (129). Copyright 2013 American Chemical Society. (b) Synthesis of molecular cage 46₃44₂. Reproduced with permission from ref (130). Copyright 2014 Wiley-VCH. (c) Synthesis of molecular cage 50. Reproduced with permission from ref (132). Copyright 2013 Wiley-VCH. (d) Synthesis of a molecular cage 52. Reproduced with permission from ref (134). Copyright 2017 The Royal Society of Chemistry.

Figure 13

(a) Synthesis of the first boronic cage 53. Reproduced with permission from ref (137). Copyright 2007 American Chemical Society. (b) Synthesis of the insoluble boronic polymer 54a, macrocycle 54b, macrocycle 54c. Reproduced with permission from ref (138). Copyright 2007 American Chemical Society. (c) Formation of cages homo-55 and hetero-55, through a change of solvents. Reproduced with permission from ref (139). Copyright 2009 Wiley-VCH.

Figure 14

(a) Synthesis of cuboctahedral cage 56. Reproduced with permission from ref (140). Copyright 2014 Wiley-VCH. (b) Synthesis of symmetrical cubic cage 57. Reproduced with permission from ref (143). Copyright 2014 The Royal Society of Chemistry. (c) Formation of a giant cage 59. d-TCE: deuteron-tetrachloroethane. Reproduced with permission from ref (146). Copyright 2021 Wiley-VCH.

Figure 15

(a) Synthesis of molecular cage 60. Reproduced with permission from ref (156). Copyright 2008 Wiley-VCH. (b) Synthesis of macrocycle 61. Reproduced with permission from ref (157). Copyright 2008 Wiley-VCH. (c) Synthesis of molecular cage 62 by ball milling. Reproduced with permission from ref (158). Copyright 2009 American Chemical Society. (d) Assembly of molecular cage 63 by the combination of imine condensation and alkene metathesis. TCB: 1,2,4-trichlorobenzene. Reproduced with permission from ref (161). Copyright 2013 American Chemical Society.

Figure 16

Frequently used analytical tools for investigating POCs. (a) PXRD pattern of the trimeric triangular prism cage (TTPCage-1) after desolvation. (b) N₂ gas sorption isotherms at 77 K for a series of [3 + 6] TTP cages. (c) TGA curves of a series of [3 + 6] TTP cages. (d) FTIR spectra of a series of [3 + 6] TTP cages. Reproduced with permission from ref (127). Copyright 2020 American Chemical Society.

Figure 17

Encapsulation of C₆₀ and C₇₀ by the cationic molecular cage TPPCage⁸⁺. Reproduced with permission from ref (114). Copyright 2018 American Chemical Society.

Figure 18

(a) Formation of cocrystal 64 by chiral recognition. (b) H₂ and D₂ adsorption (closed symbols) and desorption (open symbols) isotherms of the cocrystal at different temperatures. (c) D₂/H₂ Isotherm ratio as a function of pressure at different temperatures. (d) Thermal desorption spectroscopy (TDS) recorded on cocrystal 64, obtained after exposure to a 10-mbar 1:1 H₂/D₂ isotope mixture under varying temperatures (T_exp) for a fixed exposure time (t_exp) of 30 min. (e) D₂/H₂ Selectivity as a function of t_exp at 30 K (red), 40 K (blue), and 50 K (green). Reproduced with permission from ref (186). Copyright 2019 American Association for the Advancement of Science.

Figure 19

(a) Spin-coating of a porous cage solution to afford an ultrathin cage-film layer on a porous substrate. (b) A cross-sectional SEM image of amorphous cages coated on Al₂O₃ support. (c) A cross-sectional SEM image of a 50 nm-thick cage 8 thin film coated on Al₂O₃ support. (d) A cross-sectional SEM image of a 300 nm-thick thin film of 8 coated on an alumina support. (e) Photographs of cage thin films spin-coated on glass slides. Reproduced with permission from ref (211). Copyright 2016 Wiley-VCH.

Figure 20

(a) Porous liquids obtained by empty, soluble cage molecules with [15]crown-5 as a solvent. (b) The porous liquids show enhanced solubilities for guest gas molecules. Reproduced with permission from ref (223). Copyright 2015 Springer-Nature.

Figure 21

(a) A reverse double-solvent approach has been developed for encapsulating metal clusters inside the cavities of cage 6 (Figure 3). (b, c) HAADF-STEM Images and (d) particle size of the obtained Pd clusters. The Pd@6 showed excellent catalytic activities for (e) hydrogen generation from ammonia borane, (f) hydrogenation of 4-nitrophenol, and (g) reduction of dyes. Reproduced with permission from ref (244). Copyright 2018 Springer-Nature.