Directed Phase Transfer of an FeII4L4 Cage and Encapsulated Cargo

Article abstract of DOI:10.1021/jacs.6b12811

Supramolecular capsules can now be prepared with a wide range of volumes and geometries. Consequently, many of these capsules encapsulate guests selectively by size and shape, an important design feature for separations. To successfully address practical separations problems, however, a guest cannot simply be isolated from its environment; the molecular cargo must be removed to a separate physical space. Here we demonstrate that an Fe^II₄L₄ coordination cage 1 can transport a cargo spontaneously and quantitatively from water across a phase boundary and into an ionic liquid layer. This process is triggered by an anion exchange from 1[SO₄] to 1[BF₄]. Upon undergoing a second anion exchange, from 1[BF₄] to 1[SO₄], the cage, together with its encapsulated guest, can then be manipulated back into a water layer. Furthermore, we demonstrate the selective phase transfer of cationic cages to separate a mixture of two cages and their respective cargoes. We envisage that supramolecular technologies based upon these concepts could ultimately be employed to carry out separations of industrially relevant compounds.

Full text of DOI:10.1021/jacs.6b12811

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Communication
II4
4
Directed Phase Transfer of a FeL Cage and Encapsulated Cargo
Angela Beth Grommet, and Jonathan R. Nitschke
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12811 • Publication Date (Web): 27 Jan 2017
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Directed Phase Transfer of a Fe L Cage and Encapsulated Cargo
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Angela B. Grommet, Jonathan R. Nitschke*
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Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom
Supporting Information Placeholder
4
5,46
tetrafluoroborate ([hmim][BF ])
as both a salt to supply
ABSTRACT: Supramolecular capsules can now be prepared
with a wide range of volumes and geometries. Consequently,
many of these capsules encapsulate guests selectively by size
and shape, an important design feature for separations. To
successfully address practical separations problems, however,
a guest cannot simply be isolated from its environment; the
molecular cargo must be removed to a separate physical
4
–
BF4 anions and as a solvent to act as a receiving phase for
1[BF ]. Furthermore, by exchanging the BF counterion for
SO4 , we can ultimately manipulate cage 1 back into its orig-
–
4
4
2–
inal aqueous environment.
Because our use of non-deuterated [hmim][BF ] in this
study rendered challenging the use of H NMR techniques to
observe cage peaks in ionic solution, we used F NMR to
track an encapsulated, fluorinated guest throughout each
step of the cycle in Figure 1. The guest was chosen to be 1-
fluoroadamantane because it binds strongly and exchanges
slowly within cage 1 in water, [hmim][BF ], and acetonitrile.
In water, no measurable uptake of 1-fluoroadamantane was
4
1
II
19
space. Here we demonstrate that a Fe L coordination cage 1
4
4
can transport a cargo spontaneously and quantitatively from
water across a phase boundary and into an ionic liquid layer.
This process is triggered by an anion exchange from 1[SO ] to
4
1
[BF ]. Upon undergoing a second anion exchange, from
4
4
1
[BF ] to 1[SO ], the cage – together with its encapsulated
4
4
1
9
guest – can then be manipulated back into a water layer.
Furthermore, we demonstrate the selective phase transfer of
cationic cages to separate a mixture of two cages and their
respective cargos. We envisage that supramolecular technol-
ogies based upon these concepts could ultimately be em-
ployed to carry out separations of industrially relevant com-
pounds.
observed by F NMR within the first three hours after adding
the guest (SI Section 5.5). Two weeks of stirring at 273 K were
required for the host-guest complex to approach equilibrium.
While the equilibration of 1-fluoroadamantane ⊂ 1 was more
rapid in acetonitrile (taking three days), no measurable up-
19
take of guest was observed by F NMR within the first 45
min after guest addition (SI Section 5.3). By comparison,
each step in the cycle presented in Figure 1 takes less than 15
min to complete. Because 1-fluoroadamantane ⊂ 1 equili-
brates slowly, and because 1-fluoroadamantane is poorly sol-
uble in water, any free 1-fluoroadamantane peaks in the
NMR may therefore be inferred to result from release of the
encapsulated guest upon decomposition of the cage. We
used the integration of the F peak assigned to encapsulated
1
-14
This study explores the idea of using coordination cages
19
F
to transport guests across a phase boundary, with the ulti-
mate aim of addressing practical separations problems. Be-
1
5-20
cause these cages are not charge neutral,
they derive
19
properties from both cation and anion. Counterions have
previously been shown to play a significant role in the stabil-
1
-fluoroadamantane to gauge the amount of intact cage car-
ried forward in each step of the cycle.
An aqueous solution of 1[SO ] (2.0 mM) was allowed to
21-24
25-35
36
ity,
shape,
solubility, and other physicochemical
37,38
4
properties
of coordination cages. We further demonstrate
equilibrate with 1-fluoroadamantane (15 equiv.) for two
weeks at 293 K. The resulting solution was then filtered to
unencapsulated,
fluoroadamantane (Figure 1a). Upon the addition of hydro-
in this work that exchanging cage counterions can drive
spontaneous and quantitative transport of cages from water
3
9-41
remove
any
non-dissolved
1-
across a phase boundary and into an ionic liquid.
In an
advance over cation transport driven by anionic phase trans-
4
2,43
phobic [hmim][BF ] (1.0 mL) to 1-fluoroadamantane ⊂ 1[SO ]
in water (2.0 mM, 1.0 mL), an ionic liquid layer formed be-
neath the existing water layer (Figure 1b). Because cage
[BF ] is insoluble in water, the cage rapidly underwent anion
exchange from 1[SO ] to 1[BF ], with 1[BF ] partitioning into
the ionic liquid layer upon shaking (Figure 1c; SI Video 1),
leaving the water layer colorless. The presence of a F NMR
peak from free 1-fluoroadamantane (–126.71 ppm) in the ionic
liquid layer indicated that a small portion of cage decom-
posed upon being transferred from the water into the ionic
liquid layer. Nevertheless, 97% of 1-fluoroadamantane ⊂ 1 (-
4
4
fer catalysts,
our strategy allows the simultaneous trans-
fer of selectively-bound guest molecules. To the best of our
knowledge, ours is the first example of the directed transport
of cages and their cargoes between two liquid phases.
1
4
2–
4
4
4
When cage 1 (Figure 1) is prepared as the SO salt, the
4
3
6
complex is soluble in water. Paired with fluorinated anions
(triflate or BF ), however, 1 becomes insoluble in water. This
19
–
4
feature allows the design of a system in which cationic cage 1
can be transported between liquid phases, as shown in Figure
1
. Having previously demonstrated that coordination cages
44
can be soluble and stable in ionic liquids, we now utilize
the hydrophobic ionic liquid 1-hexyl-3-methylimidazolium
1
19
F
120.79 ppm) remained intact (SI Section 6.2). The H and
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NMR spectra of the residual D O layer (SI Section 6.2) re-
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vealed no 1-fluoroadamantane ⊂ 1 remaining in this layer.
The stability of the cage throughout transport from D O to
2
[hmim][BF ] was further confirmed by UV-Vis spectroscopy
4
(SI Section 6.3).
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Figure 1. a) 1-Fluoroadamantane ⊂ 1[SO ] dissolved in water. b) Addition of [hmim][BF ]. c) Upon shaking, 1-fluoroadamantane
4
4
⊂
1 transferred from the water to the ionic liquid layer. d) Upon addition of EtOAc, 1-fluoroadamantane ⊂ 1[BF ] was filtered off
4
n
and redissolved in CD CN. e) Upon addition of [ Bu N][SO ], 1-fluoroadamantane ⊂ 1[SO ] precipitated. f) Solid 1-
3
4
4
4
fluoroadamantane ⊂ 1[SO ] was filtered off and redissolved in water, completing the cycle.
4
1
To manipulate cage 1 back into water, a second anion ex-
change – to convert 1[BF ] back into 1[SO ] – is required.
firmed by H NMR. After 1[BF ] was dissolved in acetonitrile,
acetonitrile-soluble tetrabutylammonium sulfate was added
(50% in water, 4.8 equiv.) to convert 1[BF ] back into 1[SO ].
Anion exchange again caused the cage to precipitate from
solution (Figure 1f). Precipitated 1[SO ] was then isolated via
filtration, washed with diethyl ether (3 × 1 mL) to remove
residual ionic liquid, and redissolved in water (Figure 1a).
4
4
4
Before this conversion may be accomplished, however, 1[BF4]
must be isolated from the ionic liquid layer. This was done
by adding ethyl acetate (10 mL) to the water/ionic liquid bi-
phasic system. The ethyl acetate mixed with [hmim][BF4],
4
4
4
thus lowering the solubility of 1[BF ] in the organic/ionic
4
1
9
liquid layer, and caused it to precipitate from solution (Fig-
The F NMR spectrum of 1-fluoroadamantane ⊂ 1[SO ] in
D O (-120.42 ppm) indicated that 72% of cage from the pre-
vious step remained intact (SI Section 6.5). This final step
thus closes the transport cycle of 1 from water, to an ionic
liquid, and finally back to water.
4
ure 1d). Precipitated cage 1[BF ] was then isolated via filtra-
4
2
tion, washed thoroughly with ethyl acetate (18 × 5 mL) to
remove residual ionic liquid, and redissolved in deuterated
1
9
acetonitrile (1.0 mL), as shown in Figure 1e. The F NMR
spectrum of 1-fluoroadamantane ⊂ 1[BF ] in CD CN (-120.17
4
3
The specific transport cycle outlined in Figure 1 is not the
only possible manifestation of this concept; both the ionic
liquid and the cage can be systematically modified. Since
ppm) indicated that 74% of the cage from the previous step
remained intact (SI Section 6.4), and no peak for free 1-
fluoroadamantane was observed. This observation was con-
[
hmim][BF ] is known to be susceptible to hydrolysis, an
4
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ionic liquid that is more compatible with water can instead
er, a cage has an overall negative charge, transport from wa-
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be chosen. 1-Ethyl-3-methylimidazolium triflimide ([em-
ter to an ionic liquid would necessarily be driven by ex-
change of the cage’s countercations. When hydrophobic im-
idazolium ionic liquids [hmim][BF ] or [emim][NTf ] are
im][NTf ]), for example, is a hydrophobic ionic liquid that is
2
more stable to hydrolysis than [hmim][BF ]. We have previ-
4
4
2
ously shown that [emim][NTf ] is a good solvent for the
added to an aqueous solution of anionic cage [Me N]3 (Fig-
4
2
4
7
fluorinated analog of 1, synthesized with fluorinated sub-
component C instead of B (Figure 2). This fluorinated cage
cannot be used for inter-phase transport, however, as it is
insoluble in water. Neither can 1 be transported into [em-
ure 3), however, no transition from water to the ionic liquid
is observed. While these ionic liquids are only slightly soluble
in water, the combination of anionic cage and imidazolium
cation yields a water-soluble salt. This feature allowed the
design of the system of Figure 3, in which an aqueous mix-
ture of two different cages (2[SO ] and [Me N]3) were sepa-
im][NTf ] – it is insoluble in this ionic liquid and precipitated
2
out of both phases upon anion exchange from 1[SO ] to
4
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[NTf ]. We found, however, that the physicochemical prop-
rated, thereby also separating a mixture of two different en-
capsulated guests.
2
erties of the cage can be tuned by incorporating both B and
C. Combining 4 equivalents of A with 6 equivalents each of
subcomponents B and C, we obtained a mixture of cages
An aliquot of 1-fluorobenzene ⊂ [Me N]3 (0.7 mL, 2.0 mM)
4
in D O was thus added to an aliquot of 1-fluoroadamantane
2
2
[SO ], which is soluble in both water and [emim][NTf ]. The
1
19
4
2
⊂
2[SO ] (0.7 mL, 2.0 mM) in D O (Figure 3a). H and
F
4
2
preparation of 2 thus highlights the utility of subcomponent
self-assembly in preparing materials with tunable properties
NMR confirm that these host-guest complexes were stable in
solution together (SI Section 7.1).
–
solubility, in the present case. The library of cages 2[SO4]
acted in concert to bind 1-fluoroadamantane and were readi-
ly transported together from water into the ionic liquid layer
upon addition of [emim][NTf ]. We anticipate that this strat-
2
egy will be applicable to adapting other cationic, water-
soluble cages for directed phase transfer.
Figure 3. a) 1-Fluoroadamantane
⊂
2[SO4] and 1-
fluorobenzene ⊂ [Me N]3 dissolved in water. b) Upon the
4
addition of [emim][NTf ], 1-fluoroadamantane ⊂ 2 trans-
2
ferred from the water to the ionic liquid layer, whereas 1-
fluorobenzene ⊂ [Me N]3 remained dissolved in water.
4
To this aqueous mixture of host-guest complexes, [em-
im][NTf ] (1.4 mL) was added, initiating the transport of 1-
2
fluoroadamantane ⊂ 1 from the water to the ionic liquid layer
1
9
(Figure 3b). By measuring the F NMR spectra of the result-
ing water and ionic liquid layers, we observed clean separa-
tion of each cage into a different phase: 1-fluorobenzene ⊂
[Me N]3 was found exclusively in the water layer, and 1-
Figure 2. Cage 1 composed of 12 equivalents of B is soluble in
4
fluoroadamantane ⊂ 2[NTf ] was found exclusively in the
water but insoluble in [emim][NTf ]; an analogous cage
2
2
ionic liquid. No peak for encapsulated 1-fluoroadamantane
composed of 12 equivalents of C is soluble in [emim][NTf2]
but insoluble in water; mixed cages 2, prepared from 6
equivalents of B and 6 equivalents of C is soluble in both
1
9
was observed in the F NMR of the water layer, wherein the
1
9
F NMR signal from 1-fluorobenzene ⊂ [Me
N]3 remained
N]3
4
[emim][NTf ] and water.
unchanged. Likewise, no peak for 1-fluorobenzene ⊂ [Me
4
2
was observed in the ionic liquid layer. In the ionic liquid lay-
The cycle described above (Figure 1) is enabled by coun-
teranion exchange of a cationic coordination cage. If, howev-
er, 1-fluoroadamatane was released from 2[NTf ] after the
2
addition of triflic acid (SI Section 8). Phase transfer of cation-
3
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Page 4 of 9
ic cage 2 thus rendered straightforward the clean separation
of two physicochemically similar host-guest complexes.
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2
(
This study demonstrates that anion exchange is capable of
driving the transport of cationic cages between two liquid
phases. Such transport thus provides a new mechanism to
separate physicochemically similar cages and cargoes. Added
layers of complexity may be envisaged, involving the intro-
duction of further solvent layers and cages with varying sol-
vent preferences. This work thus adds to our understanding
of how supramolecular capsules may contribute to solving
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4
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practical separations problems.
Ultimately, supramolecu-
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lar capsules could provide an energy-efficient alternative to
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thermal separations, with different cages selectively encap-
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2-56
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in complimen-
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(
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ASSOCIATED CONTENT
Supporting Information
(
(
18) Wu, N.-W.; Rebek, J. J. Am. Chem. Soc. 2016, 138, 7512.
19) Zhang, G.; Presly, O.; White, F.; Oppel, I. M.; Mastalerz, M.
Synthetic procedures and characterization for cages 1[SO4],
1[BF ], 2[SO ], [Me N]3, and related subcomponents, if appli-
Angew. Chem. Int. Ed. 2014, 53, 5126.
4
4
4
(20) Pandurangan, K.; Kitchen, J. A.; Blasco, S.; Boyle, E. M.;
Fitzpatrick, B.; Feeney, M.; Kruger, P. E.; Gunnlaugsson, T. Angew.
Chem. Int. Ed. 2015, 54, 4566.
cable. Synthetic procedures for ionic liquids [hmim][BF ] and
4
[emim][NTf2]. Guest uptake kinetics measured for 1-
(
21) Bruns, C. J.; Fujita, D.; Hoshino, M.; Sato, S.; Stoddart, J. F.;
Fujita, M. J. Am. Chem. Soc. 2014, 136, 12027.
22) Rizzuto, F. J.; Wu, W. Y.; Ronson, T. K.; Nitschke, J. R. Angew.
fluoroadamantane ⊂ 1[SO ] in water; 1-fluoroadamantane ⊂
4
1[BF ] in acetonitrile. Experimental details of transport cycle
4
(
represented in Figure 1; characterization of Figure 1a, 1c, and
Chem. Int. Ed. 2016, 55, 7958.
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1e. Experimental details of separation represented in Figure 3;
characterization of 3a and 3b. (PDF)
(
24) Zhou, X.-P.; Wu, Y.; Li, D. J. Am. Chem. Soc. 2013, 135, 16062.
Video depicting transport in Figure 1a-c. (AVI)
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(
(
26) Evans, N. H.; Beer, P. D. Angew. Chem. Int. Ed. 2014, 53, 11716.
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This material is available free of charge via the Internet at
http://pubs.acs.org.
Clever, G. H. J. Am. Chem. Soc. 2013, 135, 8476.
(28) Kieffer, M.; Pilgrim, B. S.; Ronson, T. K.; Roberts, D. A.;
Aleksanyan, M.; Nitschke, J. R. J. Am. Chem. Soc. 2016, 138, 6813.
AUTHOR INFORMATION
(
29) Loffler, S.; Lubben, J.; Krause, L.; Stalke, D.; Dittrich, B.; Clever,
G. H. J. Am. Chem. Soc. 2015, 137, 1060.
30) Luo, D.; Zhou, X.-P.; Li, D. Inorg. Chem. 2015, 54, 10822.
Corresponding Author
(
E-mail: jrn34@cam.ac.uk
(31) Luo, D.; Zhou, X.-P.; Li, D. Angew. Chem. Int. Ed. 2015, 54, 6190.
(32) Nakamura, T.; Ube, H.; Miyake, R.; Shionoya, M. J. Am. Chem.
Soc. 2013, 135, 18790.
Notes
(
(
33) Samanta, D.; Mukherjee, P. S. Chem. Eur. J. 2014, 20, 12483.
34) Wu, J.-Y.; Zhong, M.-S.; Chiang, M.-H.; Bhattacharya, D.; Lee,
The authors declare no competing financial interests.
Y.-W.; Lai, L.-L. Chem. Eur. J. 2016, 22, 7238.
(35) Zhu, R.; Lübben, J.; Dittrich, B.; Clever, G. H. Angew. Chem. Int.
Ed. 2015, 54, 2796.
ACKNOWLEDGMENT
This work was supported by the European Research Council
(
36) Bolliger, J. L.; Ronson, T. K.; Ogawa, M.; Nitschke, J. R. J. Am.
(
259352). ABG also acknowledges the Cambridge Trusts for
Chem. Soc. 2014, 136, 14545.
Ph.D. funding.
(
(
37) Chifotides, H. T.; Dunbar, K. R. Acc. Chem. Res. 2013, 46, 894.
38) Yan, X.; Wang, M.; Cook, T. R.; Zhang, M.; Saha, M. L.; Zhou, Z.;
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Reek, J. N. H.; de Bruin, B. Chem. Eur. J. 2013, 19, 10170.
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Phenomenon: Synthesis, Reactivity and Applications of Caged Ions
and Molecules; Springer International Publishing: Switzerland, 2016.
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