Selective Co-Encapsulation Inside an M6L4Cage

Article abstract of DOI:10.1002/chem.201603017

There is broad interest in molecular encapsulation as such systems can be utilized to stabilize guests, facilitate reactions inside a cavity, or give rise to energy-transfer processes in a confined space. Detailed understanding of encapsulation events is required to facilitate functional molecular encapsulation. In this contribution, it is demonstrated that Ir and Rh-Cp-type metal complexes can be encapsulated inside a self-assembled M₆L₄metallocage only in the presence of an aromatic compound as a second guest. The individual guests are not encapsulated, suggesting that only the pair of guests can fill the void of the cage. Hence, selective co-encapsulation is observed. This principle is demonstrated by co-encapsulation of a variety of combinations of metal complexes and aromatic guests, leading to several ternary complexes. These experiments demonstrate that the efficiency of formation of the ternary complexes depends on the individual components. Moreover, selective exchange of the components is possible, leading to formation of the most favorable complex. Besides the obvious size effect, a charge-transfer interaction may also contribute to this effect. Charge-transfer bands are clearly observed by UV/Vis spectrophotometry. A change in the oxidation potential of the encapsulated electron donor also leads to a shift in the charge-transfer energy bands. As expected, metal complexes with a higher oxidation potential give rise to a higher charge-transfer energy and a larger hypsochromic shift in the UV/Vis spectrum. These subtle energy differences may potentially be used to control the binding and reactivity of the complexes bound in a confined space.

Full text of DOI:10.1002/chem.201603017

DOI: 10.1002/chem.201603017
Full Paper
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Metallocages
Selective Co-Encapsulation Inside an M6L4 Cage
Stefan H. A. M. Leenders,[a] Renꢀ Becker,[a] Tatu Kumpulainen,[a] Bas de Bruin,[a]
Tomohisa Sawada,[b] Taito Kato,[b] Makoto Fujita,[b] and Joost N. H. Reek*[a]
Abstract: There is broad interest in molecular encapsulation
as such systems can be utilized to stabilize guests, facilitate
reactions inside a cavity, or give rise to energy-transfer pro-
cesses in a confined space. Detailed understanding of encap-
sulation events is required to facilitate functional molecular
encapsulation. In this contribution, it is demonstrated that Ir
and Rh-Cp-type metal complexes can be encapsulated
inside a self-assembled M6L4 metallocage only in the pres-
ence of an aromatic compound as a second guest. The indi-
vidual guests are not encapsulated, suggesting that only the
pair of guests can fill the void of the cage. Hence, selective
co-encapsulation is observed. This principle is demonstrated
by co-encapsulation of a variety of combinations of metal
complexes and aromatic guests, leading to several ternary
complexes. These experiments demonstrate that the
efficiency of formation of the ternary complexes depends on
the individual components. Moreover, selective exchange of
the components is possible, leading to formation of the
most favorable complex. Besides the obvious size effect,
a charge-transfer interaction may also contribute to this
effect. Charge-transfer bands are clearly observed by UV/Vis
spectrophotometry. A change in the oxidation potential of
the encapsulated electron donor also leads to a shift in the
charge-transfer energy bands. As expected, metal complexes
with a higher oxidation potential give rise to a higher
charge-transfer energy and a larger hypsochromic shift in
the UV/Vis spectrum. These subtle energy differences may
potentially be used to control the binding and reactivity of
the complexes bound in a confined space.
55% of the size of the interior of the cage.[1] Typically, these
self-assembled hosts are interesting objects for exploring
chemistry with the compounds in their confined spaces.[2–4]
Indeed, examples of capsule-induced selectivity in organic re-
actions have been reported.[5–7] Furthermore, when metal cata-
lysts are captured inside a capsule, the cages resemble to
some extent the second coordination sphere typical of en-
zymes, and as such new reactivity and selectivity may be dis-
played by the encapsulated catalysts.[8–11] Also in the photosyn-
thetic apparatus in nature, the second coordination sphere is
of importance, and in analogy chromophoric guests have been
encapsulated and subjected to detailed studies. As a result, in-
duced charge-transfer (CT) complexes or exciplexes have been
enforced by the close proximity of guests and hosts. While
charge-transfer complexes can be considered as involving only
a weak interaction between the donor and acceptor, they can
often be easily observed and characterized due their intense
colors. It has been demonstrated that selective encapsulation
of guests in preformed hosts, such as cyclodextrins,[12]
cucurbit[8]urils,[13,14] pillar[5]arenes,[15] porous coordination
polymers,[16,17] and metallocages,[18] can be used to obtain new
charge-transfer complexes. Additionally, the formation of
charge-transfer complexes can be used as a driving force to
form new supramolecular assemblies.[19,20] Whereas the
encapsulation of a single guest, such as an organic or metal
complex, has been reported frequently, the selective co-
encapsulation of two different guests is not straightforward.
Nevertheless, this type of assembly is of particular interest as it
Introduction
Supramolecular chemistry has evolved to a stage whereby
a large variety of well-defined structures have been prepared
by assembly of small buildings blocks. Now that control over
the shape and structure of these nano-sized objects has been
well established, the introduction and evaluation of functional
properties is a more pressing concern. Self-assembled cages
typically function as hosts for small molecules that can bind in
their interior. The binding of such guest molecules is generally
based on a combination of noncovalent interactions, including
hydrogen bonds, ionic or p-p interactions, and hydrophobic
effects. Shape complementarity is obviously important, and it
has been established that the ideal size of the guest is about
[a] Dr. S. H. A. M. Leenders, R. Becker, Dr. T. Kumpulainen, Prof. Dr. B. de Bruin,
Prof. Dr. J. N. H. Reek
Van’t Hoff Institute for Molecular Sciences, University of Amsterdam
Science Park 904 1098 XH Amsterdam (The Netherlands)
E-mail: j.n.h.reek@uva.nl
[b] Dr. T. Sawada, T. Kato, Prof. Dr. M. Fujita
Department of Applied Chemistry, School of Engineering
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Supporting information, including experimental detals, and the ORCID iden-
tification number(s) for the author(s) of this article can be found under
http://dx.doi.org/10.1002/chem.201603017.
ꢀ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
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facilitates the occurrence of chemistry between two
components, a prerequisite for metal-catalyzed transforma-
tions. Herein, we report the selective encapsulation of two dif-
ferent guests inside an octahedral metallocage to form ternary
complexes. The ternary complex consists of a Cp-ligated metal
complex that is in close proximity to a flat aromatic guest,
which form a charge-transfer complex as indicated by UV/Vis
spectrophotometry. Different aromatic guests and metal com-
plexes can be used to selectively form the ternary complexes,
and the binding efficiency depends on the properties of the
components. The resulting charge-transfer complex varies ac-
cording to the oxidation potential of the electron-donor metal
complex. Understanding selective co-encapsulation of a metal
complex and substrate in a confined space is important for
future studies on catalysis in such environments.
host–guest complexes are formed by simply mixing the
components.
We investigated whether [(CpMe)Ir(cod)] 2 (Cp=cyclopenta-
diene, cod=1,5-cyclooctadiene) could be encapsulated. Stir-
ring a suspension of 2 and 1a in D2O at 1008C for prolonged
reaction times did not result in significant encapsulation of 2
as no new signals were observed in the NMR spectra. Cage
1 has been reported to bind some guests in the presence of
an appropriate second molecule that is co-encapsulated.[32–34]
In this context, we investigated binding of the metal complex
in the presence of triphenylene (6) as a co-guest. Heating a
suspension of 1a, 2, and 6 in D2O at 1008C for 1 h resulted in
a colored suspension. Interestingly, after cooling the solution
and filtration of the excess guests, a purple solution was
obtained indicating that the ternary complex 1a·2·6 had been
formed (Scheme 1).
Results and Discussion
Selective co-encapsulation and characterization of the
ternary complexes
A self-assembled metallocage, which has been reported previ-
ously (Figure 1), was used as the host for the current studies.
This metallocage has recently been demonstrated to facilitate
exciplex formation with different guests[21–23] and to incorpo-
rate metal complexes, whereupon a charge-transfer band was
observed.[24] The water-soluble octahedral nanosphere
1 (Figure 1) can bind a large variety of guests, mostly on the
basis of hydrophobic effects and interactions with the elec-
tron-deficient sidewalls of its cavity. A broad array of guests
have been accommodated in this molecular container, includ-
ing metal complexes,[24–26] substrates for organic reactions,[27,28]
halogens,[29,30] and a variety of molecules with interesting
UV/Vis spectrophotometric properties.[21–23,31] Generally, the
Scheme 1. Co-encapsulation of guests 2 and 6 in host 1a.
Due to their low solubility in water, neither the iridium com-
1
plex nor the triphenylene could be detected by H NMR spec-
troscopy in D2O solution. In contrast, in the presence of the
cage, their signals were clearly visible. Binding was confirmed
by the clear upfield shifts displayed by the guest molecules in
1
the H NMR spectrum, as depicted in Figure 2. The shielding
caused by the aromatic rings of the cage resulted in typical
shifts of the triphenylene signals of 1.7 and 1.4 ppm, whereas
the signals of the metal complex were shifted by between 2.5
and 3.2 ppm. Another indication of guest encapsulation was
provided by the change of symmetry[34] of the metallocage
from Td to effectively C3 (on the NMR timescale), as was clear
from the 1H NMR spectra. This loss of symmetry occurs be-
cause each guest occupies one half of the capsule. The guests
are closely packed against the triazine panel and on the NMR
timescale the cage loses its Td symmetry[35] (see Supporting In-
formation S4). For the empty cage, all pyridine rings are equiv-
alent so that only two signals are observed for the pyridine
protons. However, for the cage incorporating the two guests,
eight sets of pyridine proton signals are observed. Importantly,
the symmetry of the guest molecules was not affected by the
encapsulation, indicating that these can freely rotate inside the
void of the capsule. Additional support for guest encapsulation
was provided by diffusion-ordered NMR (DOSY, depicted in
Supporting Information S8), which showed that the diffusion
constant of the guests matched that of the capsule.[24] Due to
the enforced close proximity of the respective guests, NOE
Figure 1. Octahedral-shaped metallocage 1 and guests used for co-encapsu-
lation.
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Figure 2. Clear upfield shifts observed in the H NMR spectra of guests 2 and 6: spectrum of a mixture of guests in CDCl3 (a, top) and inside molecular
container 1a in D2O (b, bottom).
signals could be observed between them in the 2D NOESY
spectrum (see Supporting Information S8). In addition, the
guests also displayed NOE contacts with the capsule 1a,
reflecting a tight fit of the compounds in the metallocage.
Attempts to crystallize the ternary product using metallo-
cage 1a did not yield crystalline material suitable for X-ray dif-
fraction analysis. However, when we changed the nanosphere
to its palladium analogue with a 2,2’-bipyridine cis-capping
ligand (metallocage 1b),[36] crystals suitable for X-ray diffraction
analysis were obtained. The data revealed a molecular struc-
ture in which the two guests occupy the void of the sphere
(Figure 3), showing a tight fit of both guests inside the cage.
The guest pair (2·6) was disordered over three positions, hence
only the 33.3% occupancy is displayed to clearly show the
host–guest structure. Although the solid-state structure cannot
be directly compared with the structure in solution, the
disorder over three positions corresponds to the effective
C3 symmetry of the host–guest assembly on the NMR time-
scale, as derived from the NMR spectra in solution at room
temperature.
The molecular structure shows that the triphenylene guest
(6) resides close to the triazine panel. However, it is not
stacked completely parallel to the panel. The distance from 6
Figure 3. Solid-state structure of ternary complex 1b·2·6 (33.3% occupancy).
The metal complex 2 is shown in green and triphenylene 6 is shown in
(central ring) to the central ring of the triazine panel is 3.48 ꢂ,
purple. Solvent molecules and nitrate anions have been omitted for clarity.
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and that from 6 to the iridium atom is 4.84 ꢂ. The average
angle between the pyridine moieties of the triazine panels that
are connected to the palladium center is 86.48. This is similar
to the average of 86.68 previously reported for the crystal
structure of the cage incorporating an adamantoid water
molecule cluster.[36b]
Scope of the co-encapsulation
Following the observation that only the two guests together
could be embedded inside the cavity of the metallocage and
that no individual encapsulation occurs, the scope of the metal
complexes and aromatic moieties that can be co-encapsulated
1
was explored. The H NMR spectrum of 1c·2·6 showed incom-
plete encapsulation as empty cage 1c was still present. Inte-
gration of the pyridine proton signals and comparison of these
with the aromatic signals allowed determination of the effi-
ciency of the formation of the ternary complex, expressed in
terms of percentage of occupied cage; this was 75% for
1c·2·6. Next, the metal complex was changed to the sterically
similar, but electronically distinct, rhodium complex 4, which
resulted in a similar amount of ternary complex (see Table 1).
Scheme 2. Exchange studies of preformed ternary complexes with pyrene
(top) or triphenylene (bottom). These experiments showed the metallocage
to have a preference for binding pyrene.
pyrene at 1008C for 1 h (depicted in Scheme 2, top). After
cooling and filtration of the excess guest, it was clear that ex-
change of the aromatic guest had taken place, and the ratio of
1c·3·6 to 1c·3·7 was determined as 1:6.3. Similarly, a solution
of 1c·3·7 was mixed with triphenylene (6) at 1008C for 1 h. In
this case, the ratio of 1c·3·6 to 1c·3·7 was 1:36.0. Thus, pyrene
easily displaces triphenylene and has a higher affinity for the
cavity of the metallocage.
Table 1. Formation of the ternary complexes in metallocage 1c with
metal donors 2–5 and aromatic guests 6–8. All encapsulation studies
were performed with the same numbers of equivalents of guests for 1 h
at 1008C. Percentages are based on integration of the pyridine 1H NMR
signals.
To further prove that perylene is thermodynamically the
least favored in the void of metallocage 1, a mixture of excess-
es of all three aromatic guests (20 equivalents) was stirred with
3 and 1c. After 1 h at 1008C, this resulted in a ratio of
1c·3·6:1c·3·7 of 1:9.1 and no perylene was encapsulated. Heat-
ing the suspension with the guests for a longer time (6 h)
slightly influenced the ratio (1:10.3) but still did not result in
the encapsulation of 8. This demonstrates that the metallocage
has different affinities for the aromatic guests and that this is
represented in the amount of the ternary complex that is
formed.
Co-encapsulation [%]
Triphenylene (6)
Pyrene (7)
Perylene (8)
[(CpMe)Ir(cod)] (2)
[(Cp)Rh(cod)] (3)
[(CpMe)Rh(cod)] (4)
[(CpMe4)Rh(cod)] (5)
75
84
78
28
98
97
98
81
32
43
37
14
Steric influences on the amount of co-encapsulation were
studied by employing complexes 3 and 5. A small increase in
the formation of ternary complex 1c·3·6 was observed (84%
vs. 78% for 1c·4·6) employing the complex with no methyl
groups on the cyclopentadiene ligand. In contrast, a large de-
crease in co-encapsulation was observed when four methyl
groups were present, resulting in 28% of complex 1c·5·6
(Table 1).
Tuning of the charge-transfer band through co-encapsula-
tion
Intrigued by the distinct colors that were observed with the
different ternary complexes, we proceeded to examine the co-
encapsulated species by UV/Vis spectrophotometry. Upon co-
encapsulation of rhodium complex 4 and triphenylene, the
pale-yellow solution of empty cage 1c turned purple and
a new absorption band was observed at 555 nm (2.23 eV). This
new band is indicative of the formation of a charge-transfer
complex (see Figure 4). To confirm that the cage elicits the
new absorption band, we combined the compounds in chloro-
form. The absence of a charge-transfer band in the UV/Vis
spectrum of this solution demonstrated that pre-organization
in the metallocage is required for exciplex formation.
Interested by this difference in the formation of the co-en-
capsulated species, we changed the aromatic guest to pyrene
(7) or perylene (8). The ternary complexes formed with pyrene
showed a higher amount of co-encapsulation. With guest 7,
high formation of ternary complex was also observed with the
sterically hindered [(CpMe4)Rh(cod)] (81%). Conversely, co-en-
capsulation with perylene resulted in lower amounts of forma-
tion of the ternary complexes, among which that of 1c·5·8
was the lowest, amounting to a mere 14%. The more efficient
formation of ternary complexes with pyrene compared to tri-
phenylene prompted us investigate whether the former guest
could displace the latter. A solution of the triphenylene ternary
complex 1c·3·6 was therefore mixed with 10 equivalents of
The formation of charge-transfer complexes of soluble mole-
cules in supramolecular complexes based on cucurbit[8]uril
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Figure 4. a) UV/Vis absorption spectra indicating the charge-transfer complexes in capsule 1c with triphenylene (250 mm). b) Photograph of the different col-
ored solutions of the ternary complexes, demonstrating that the metal complex changes the
color of the solution (in metallocage 1a).
has been reported previously.[13,15,37] In the current system, for-
mation of the host–guest system is based on extraction of the
guests, which are essentially insoluble in water. The metal
complexes that are co-encapsulated by this metallocage make
it possible to investigate the effect of the redox potential on
the charge-transfer complex. In this regard, various pairs of
guests for the formation of ternary complexes were investigat-
ed. The donor in the charge-transfer complex was expected to
be the metal complex, hence various complexes with different
steric and electronic properties were studied. The same steric
structure was retained and only the electronic properties were
changed by utilizing [(CpMe)Ir(cod)] (2) for the encapsulation
studies. When 2 was co-encapsulated with triphenylene, the
charge-transfer band was shifted to higher energy (513 nm,
2.42 eV; see Table 2) compared to that obtained with the rho-
dium complex (4). This blue-shift of the charge-transfer band
was anticipated as the oxidation potential of 5 is higher than
that of 3, as indicated by the redox potentials of the com-
plexes in dichloromethane determined by cyclic voltammetry
measurements (see Table 2 and Supporting Information S6).
When the electronic properties of the rhodium complex were
further changed by substituting it with four methyl groups
(complex 5) as opposed to no methyl groups (complex 3), it
became apparent that metal complexes with a higher oxida-
tion potential yielded a charge-transfer band with triphenylene
at lower wavelength. To the best of our knowledge, this is the
first time that metal complexes have been used to fine-tune
charge-transfer bands through the formation of ternary com-
plexes, based on their redox potentials. To ascertain whether
similar trends could be observed, we co-encapsulated other ar-
omatic guests. For this purpose, pyrene (7) and perylene (8)
were used to form the ternary complexes. With perylene (8) as
aromatic co-guest, the charge-transfer bands shifted to higher
energies. A recurring trend in the energies was observed, with
the exception of 1c·2·8, for which the energy was difficult to
determine due to overlap with the UV band of perylene. The
use of pyrene resulted in almost quantitative formation of the
ternary complexes (see Table 1), including with the sterically
demanding 5. However, the location of the charge-transfer
band was difficult to determine precisely as it partially over-
lapped with the absorption of pyrene (see Supporting Informa-
tion S5). Although the estimated energies were therefore less
accurate, all CT bands shifted to lower energies as a conse-
quence of the lower-lying LUMO of pyrene. This was consistent
with the metal complex being the electron donor and the aro-
matic compound the acceptor. The resulting charge-transfer
complex may be stabilized by the electron-poor cavity of the
molecular container, but the dominant effect is the charge-
transfer complex formation between the two guests as varia-
tion of either component leads to changes in the bands. Based
Table 2. Charge-transfer energies (DECT
) of the ternary complexes
facilitated by metallocage 1c. Energies are based on fitting of the
charge-transfer peaks in UV/Vis spectrophotometry (lCT). The charge-
transfer energies are in line with the redox properties of the metal
complex (Eox), which were determined by cyclic voltammetry.
Triphenylene (6) Pyrene (7) Perylene (8)
Metal complex
Eox [V][a] DECT [eV]
DECT [eV] DECT [eV]
[(CpMe4)Rh(cod)] (5) À0.28
2.13
2.23
2.34
2.42
1.88
2.22
2.40
2.50
2.38[b]
[(CpMe)Rh(cod)] (4)
[(Cp)Rh(cod)] (3)
0.02
0.07
0.12
2.03[b]
2.01[b]
2.04[b]
[(CpMe)Ir(cod)] (2)
[a] Oxidation potentials were measured using 1 mm solutions of the
metal complex in CH2Cl2 containing 0.1m TBAPF6 at a glassy carbon
working electrode. The potentials are referenced to ferrocene (Fc0/+) and
based on simultaneous fitting of an irreversible wave at multiple scan
rates (0.1/0.3/1.0 VsÀ1); see Experimental Section S6 for more details.
[b] Due to overlap of the CT band with the UV band of the aromatic com-
pound, the exact CT energy is difficult to determine and less accurate.
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We have demonstrated selective co-encapsulation in metallo-
cage 1, the void of which is occupied by one metal complex
and one flat aromatic guest. As the individual guests are not
encapsulated under the applied conditions, this result repre-
sents an example of selective co-encapsulation. The shapes of
the respective guests complement each other and only tog-
ether fill the void of the cage. The amount of co-encapsulation
depends on the guests, and varies with their steric properties.
Exchange of the guests is possible, and offering multiple
suitable guests shows a thermodynamic preference for certain
ternary complexes. In the UV/Vis spectra of these complexes,
clear charge-transfer bands are observed, the energy of which
is controlled by the electronic properties of the donor (metal
complex) and the acceptor (aromatic guest). The void of the
metallocage makes it possible to study the formation of
various ternary complexes, showing the benefits of these
host–guest systems. The possibility of selectively co-encapsu-
lating two guests is important in the future design of novel
host–guest complexes, and could ultimately lead to catalytic
transformations inside these metallocages.
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Keywords: charge-transfer complexes
·
co-encapsulation
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host–guest systems
structures
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supramolecular chemistry
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Received: June 24, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 8
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ꢁ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

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FULL PAPER
Dual encapsulation: M6L4 metallocages
are used to selectively co-encapsulate
metal complexes with an aromatic
guest. The formed ternary assemblies
result in charge-transfer complexes that
can be fine-tuned based on the oxida-
tion potential of the metal complex that
is encapsulated (see figure).
&
Metallocages
S. H. A. M. Leenders, R. Becker,
T. Kumpulainen, B. de Bruin, T. Sawada,
T. Kato, M. Fujita, J. N. H. Reek*
&& – &&
Selective Co-Encapsulation Inside an
M6L4 Cage
&
&
Chem. Eur. J. 2016, 22, 1 – 8
www.chemeurj.org
8
ꢁ 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!