Angewandte
Communications
Chemie
How to cite: Angew. Chem. Int. Ed. 2021, 60, 11789–11792
Host-Guest Chemistry
A Cavity-Tailored Metal-Organic Cage Entraps Gases Selectively in
Solution and the Amorphous Solid State
Abstract: Here we report the subcomponent self-assembly of
a truxene-faced Zn4L4 tetrahedron, which is capable of binding
the smallest hydrocarbons in solution. By deliberately incor-
porating inward-facing ethyl groups on the truxene faces, the
resulting partially-filled cage cavity was tailored to encapsulate
methane, ethane, and ethene via van der Waals interactions at
atmospheric pressure in acetonitrile, and also in the amorphous
solid state. Interestingly, gas capture showed divergent selec-
tivities in solution and the amorphous solid state. The selective
binding may prove useful in designing new processes for the
purification of methane and ethane as feedstocks for chemical
synthesis.
gases into large lattice voids or interstitial spaces. Discrete,
soluble molecular containers[6] offer properties that are
complementary to those of crystalline framework materials.
The encapsulation of gases in solution may allow for new
purification strategies to be deployed, for instance by
incorporating containers into separation films, or carrying
out gas separation in solution under flow. The prospect of
these applications has greatly stimulated the development of
the emerging area of porous liquids.[7] Gas encapsulation
within soluble containers may also increase the solubility of
gases in solution,[7a] which may in turn enable these gases to
become the substrates of cage-catalyzed reactions.[8]
Soluble containers bind gases in different ways than do
framework materials.[9] The use of purely organic capsules[10]
for gas binding has been well explored,[11] whereas gas
encapsulation within discrete metal-organic cages[12] has
H
ydrocarbon gases are as ubiquitous as they are industrially
important. Methane is the least environmentally problematic
hydrocarbon fuel, and also an essential raw material for
industry.[1] Ethene is also widely used in industry, primarily in
the production of polyethylene. Ethene is produced primarily
from naphtha or ethane, requiring its separation and purifi-
cation from ethane.[2] The ability to selectively bind these
hydrocarbon gases is crucial for applications such as gas
separation and storage.
been observed in a much more limited set of cases.[13]
A
water-soluble FeII L6 coordination cage was found to encap-
4
sulate SF6 or Xe in water via the hydrophobic effect.[13a,b]
A
FeII L4 tetrahedron that bound cryptophane-111 formed
4
a cage-in-cage host, which further bound Xe.[13c] Recently,
Li et al. presented the entrapment of CO2 by Ni-imidazolate
cages in solution at high CO2 pressure, which also occurred in
the crystalline state of the cages.[13d] The encapsulation of
hydrocarbon gases by metal-organic cages has not been
observed yet, to the best of our knowledge.
Crystalline porous materials, such as metal-organic frame-
works (MOFs)[3] or covalent-organic frameworks (COFs),[4]
have been investigated as adsorbents for gas separation and
storage.[5] The crystal lattices of these materials may take up
Based upon our experience using triazatruxene-contain-
ing cage subcomponents,[13c,14] we sought to explore cages
containing alkylated truxene moieties. The alkyl groups of
these cages were designed to project into and partially fill the
cage cavity. Crucially, the aliphatic character of these groups
provides an internal environment distinct from that of other
metal-organic cages, which are most often lined with aromatic
panels.[12g,15] Such a cavity might thus bind aliphatic hydro-
carbons well, following the principle of “like dissolves like”.
This hypothesis led us to synthesize tetrahedron 1 (Figure 1),
which was shown to be capable of entrapping small hydro-
carbon gases in both solution and the solid state.
Truxene-based subcomponent A was obtained in four
steps from commercially-available starting materials
(Scheme S1). The reaction of subcomponent A (4 equiv)
with zinc(II) bis(trifluoromethane)sulfonimide (triflimide,
Tf2NÀ, 4 equiv) and 2-formylpyridine (12 equiv) in acetoni-
trile afforded tetrahedron 1 (Figures 1 and S4–S14). Its Zn4L4
composition was confirmed by ESI-MS (Figure S11).
[*] J.-L. Zhu,[+] Dr. D. Zhang,[+] Prof. L. Xu, Prof. H.-B. Yang
Shanghai Key Laboratory of Green Chemistry and Chemical Pro-
cesses, School of Chemistry and Molecular Engineering, East China
Normal University
3663 N. Zhongshan Road, Shanghai 200062 (P. R. China)
E-mail: lxu@chem.ecnu.edu.cn
Dr. D. Zhang,[+] Dr. T. K. Ronson, Prof. L. Xu, Prof. J. R. Nitschke
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge CB2 1EW (UK)
E-mail: jrn34@cam.ac.uk
Dr. W. Wang
State Key Laboratory of Structural Chemistry, Fujian Institute of
Research on the Structure of Matter, Chinese Academy of Sciences
Fuzhou 350002 (China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
ꢀ 2021 The Authors. Angewandte Chemie International Edition
published by Wiley-VCH GmbH. This is an open access article under
the terms of the Creative Commons Attribution License, which
permits use, distribution and reproduction in any medium, provided
the original work is properly cited.
Although the clockwise/anticlockwise orientation of the
truxene faces and the handedness of the tris-chelated metal
vertices of the tetrahedron might combine to allow for
different diastereoisomers to form in solution,[13c,14,16] the
Angew. Chem. Int. Ed. 2021, 60, 11789 –11792
ꢀ 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
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