Octahedral [Pd6L8]12+ Metallosupramolecular Cages: Synthesis, Structures and Guest-Encapsulation Studies

Article abstract of DOI:10.1002/chem.201702518

Four planar tripyridyl ligands (L_tripy), 1,3,5-tris(pyridin-3-ylethynyl)benzene 1 a, 1,3,5-tris[4-(3-pyridyl)phenyl]benzene 2 a, and the hexyloxy chain functionalized derivatives 1,3,5-tris[(3-hexyloxy-5-pyridyl)ethynyl]benzene 1 b, and 1,3,5-tris[4-(3-hexyloxy-5-pyridyl)phenyl]benzene 2 b, were synthesized and used to generate a family of [Pd₆(L_tripy)₈](BF₄)₁₂ octahedral cages (L_tripy=1 a, b or 2 a, b). The ligands and cages were characterized using a combination of ¹H, ¹³C, and DOSY nuclear magnetic resonance (NMR) spectroscopy, high resolution electrospray mass spectrometry (HR-ESI-MS), infrared (IR) spectroscopy, elemental analysis, and in three cases, X-ray crystallography. The molecular recognition properties of the cages with neutral and anionic guests were examined, in dimethyl sulfoxide (DMSO), using NMR spectroscopy, mass spectrometry and molecular modeling. No binding was observed with simple aliphatic and aromatic guest molecules. However, anionic sulfonates were found to interact with the octahedral cages and the binding interaction was size selective. The smaller [Pd₆(1 a, b)₈]¹²⁺ cages were able to interact with three p-toluenesulfonate guest molecules while the larger [Pd₆(2 a, b)₈]¹²⁺ systems could host four of the anionic guest molecules. To probe the importance of the hydrophobic effect, a mixed water–DMSO (1:1) solvent system was used to reexamine the binding of the neutral organic guests adamantane, anthracene, pyrene and 1,8-naphthalimide within the cages. In this solvent system all the guests except adamantane were observed to bind within the cavities of the cages. NMR spectroscopy and molecular modeling indicated that the cages bind multiple copies of the individual guests (between 3–6 guest molecules per cage).

Full text of DOI:10.1002/chem.201702518

A Journal of
Accepted Article
Title: Octahedral [Pd6L8]12+ Metallosupramolecular Cages: Synthesis,
Structures and Guest Encapsulation Studies
Authors: James David Crowley, Tae Y Kim, Nigel T Lucas, Michael G.
Gardiner, and Lori Digal
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To be cited as: Chem. Eur. J. 10.1002/chem.201702518
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Octahedral [Pd6L8]12+ Metallosupramolecular Cages: Synthesis,
Structures and Guest Encapsulation Studies
Tae Y. Kim,[a] Lori Digal,[a] Michael G. Gardiner,[b] Nigel T. Lucas[a],* and James D. Crowley[a],*
Architectures with hollow cavities, such as capsules and cages,
are of particular interest due to the tunable nature of the internal
cavities. The molecular recognition properties of these systems
have already been exploited to encapsulate drugs,[6]
environmental pollutants[7] and reactive species.[8] Additionally,
the cavities of these cages have been exploited as molecular
reaction flasks[9] and for catalysis.[10]
Abstract: Four planar tripyridyl ligands (Ltripy), 1,3,5-tris(pyridin-3-
ylethynyl)benzene 1a, 1,3,5-tris[4-(3-pyridyl)phenyl]benzene 2a, and
the hexyloxy chain functionalized derivatives 1,3,5-tris[(3-hexyloxy-5-
pyridyl)ethynyl]benzene
1b,
and
1,3,5-tris[4-(3-hexyloxy-5-
pyridyl)phenyl]benzene 2b, were synthesized and used to generate a
family of [Pd6(Ltripy)8](BF4)12 octahedral cages (Ltripy = 1a-b or 2a-b).
The ligands and cages were characterized using a combination of 1H,
13C and DOSY nuclear magnetic resonance (NMR) spectroscopy,
high resolution electrospray mass spectrometry (HR-ESI-MS),
infrared (IR) spectroscopy, elemental analysis, and in three cases, X-
ray crystallography. The molecular recognition properties of the cages
with neutral and anionic guests was examined, in dimethyl sulfoxide
(DMSO), using NMR spectroscopy, mass spectrometry and molecular
modelling. No binding was observed with simple aliphatic and
aromatic guest molecules. However, anionic sulfonates were found to
interact with the octahedral cages and the binding interaction was size
selective. The smaller [Pd6(1a-b)8]12+ cages were able to interact with
three p-toluenesulfonate guest molecules while the larger [Pd6(2a-
b)8]12+ systems could host four of the anionic guest molecules. To
probe the importance of the hydrophobic effect, a mixed water-DMSO
(1:1) solvent system was used to reexamine the binding of the neutral
organic guests adamantane, anthracene, pyrene and 1,8-
naphthalimide within the cages. In this solvent system all the guests
except adamantane were observed to bind within the cavities of the
cages. NMR spectroscopy and molecular modelling indicated that the
cages bind multiple copies of the individual guests (between 3-6 guest
molecules per cage).
While almost any labile metal ion in combination with a
correctly encoded ligand system can be used for the generation
of these metallo-cage systems,[1] square planar palladium(II) ions
have been extensively used to generate metallosupramolecular
architectures.[11] This is due to the favorable combination of kinetic
lability but thermodynamic stability in the PdII−pyridine interaction,
which allows the self-assembly process to proceed in high yields.
In a pioneering discovery, McMorran and Steel,[12] showed that
the combination of “naked” PdII ions and a dipyridyl ligands (Ldipy
)
could be used to assemble a [Pd2(Ldipy)4]4+ cage. Since that work,
the host-guest properties of these small metallosupramolecular
architectures[13] have been extensively examined. The
[Pd2(Ldipy)4]4+ cages have been shown to bind a wide range of
neutral organic[14] and inorganic[15] guests and anions[16] but due
to their small size often they interact with only one or two guest
molecules and this potentially limits the applications of these
systems.
Introduction
Several larger octahedral [Pd6L8]12+ cage systems,[17]
generated from “naked” PdII ions and pseudo-planar tripyridyl
ligands (Figure 1), are known and some have been exploited as
catalysts.[18] However, despite these systems displaying large
accessible cavities, the molecular recognition properties of these
[Pd6L8]12+ cages have hardly been examined.[19] This is surprising
given the rich host-guest chemistry displayed by the related
[M6L4]12+ (where M = PdII or PtII) octahedral cages. These water
soluble octahedral cages are usually generated from
ethylenediamine-capped palladium(II) or platinum(II) ‘corner
pieces’ and planar tripyridyl ligands (Figure 1) forming a semi
open [M6L4]12+ system. Fujita and coworkers have shown that
these cages can bind one to four guest molecules,[20] mainly by
exploiting the hydrophobic effect, and can act as molecular
reaction flasks.[9]
Strategies for the synthesis of two- and three-dimensional
metallosupramolecular architectures[1] are now well understood
and a range of applications for these systems are beginning to
emerge. The biological, [2] photophysical[3] and redox[4] properties
of metallosupramolecular architectures have all been examined.
However, it is the interesting host-guest chemistry[5] of these
metallosupramolecular systems which show the most potential.
[a]
Mr. T. Y. Kim, Ms. L. Digal, Dr. N. T. Lucas and Dr. J. D. Crowley*
Department of Chemistry
University of Otago
PO Box 56, Dunedin, New Zealand
E-mail: jcrowley@chemistry.otago.ac.nz
[b]
Dr. M. G. Gardiner
School of Physical Sciences (Chemistry)
University of Tasmania
Hobart, Australia
Supporting information for this article is given via a link at the end of
the document.
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from the commercially available 1,3,5-tribromobenzene core unit
using either Sonogashira (1a,b) or Suzuki (2a,b) cross coupling
conditions (Scheme 1 and Supporting Information). The ligands
were isolated in moderate to good yields (40-92%) and the
molecular formulations were confirmed via 1H and 13C nuclear
magnetic resonance (NMR) spectroscopy, high resolution
electrospray mass spectrometry (HR-ESI-MS), infrared (IR)
spectroscopy, and elemental analysis (Supporting Information).
Figure 1. Cartoon representations of metallosupramolecular octahedral cage
architectures. Top, semi-open Fujita-type [M6L4]12+ cages and bottom, fully
closed [M6L8]12+ cages.
As part of our interest in the molecular recognition
properties[21] of metallosupramolecular architectures,[22] herein we
report the synthesis and characterization of four rigid, planar C3
symmetric tripyridyl ligands (Ltripy = 1a-b or 2a-b) and the
corresponding [Pd6(Ltripy)8]12+ cages (Figure 1 and Scheme 1).
The tripyridyl systems feature two different linker units, either
alkynyl or phenyl, to connect the coordinating pyridyl groups to
the benzene core of the ligands. Therefore the ligands are
different in length and this generates cages with altered portal and
cavity sizes. This difference in portal and cavity sizes may play a
role in host-guest binding interactions, potentially leading to
differences in guest preference and the number of guest
encapsulated. The molecular recognition properties of the cages
with neutral organic and anionic guest molecules was examined
using NMR spectroscopy, molecular modelling and mass
spectrometry.
Scheme 1. Synthesis of the four trigonal benzene core ligands. (i)
[PdCl2(Ph3P)2], CuI, Et3N, 40 °C. (ii) [PdCl2(dppf)], K2CO3, toluene, EtOH, H2O,
80 °C.
The molecular structures of the ligands 1b and 2b were
unambiguously confirmed using X-ray crystallography (Figure 2
and Supporting Information). Both ligands 1b and 2b crystallized
Results and Discussion
Synthesis of the ligands
̅
in the triclinic P1 space group with the asymmetric unit consisting
The ligands 1,3,5-tris(pyridin-3-ylethynyl)benzene 1a,[23] 1,3,5-
tris[4-(3-pyridyl)phenyl]benzene 2a[24] have been reported
previously. However, we generated the ligands (1a,b and 2a,b)
of one whole ligand. The X-ray diffraction data confirmed the
expected bond connectivity, with three pyridyl units bound in the
1, 3 and 5 positions of the central benzene core unit. Each of the
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ligands adopts an essentially planar conformation in the solid
state and engages in a number of hydrogen bond and π-π
interactions (Supporting Information).
at 50 °C for 2 h. The heating was essential for the formation of the
cage system; simply leaving the components in DMSO solution at
room temperature (RT) for 2 days did not lead to the clean
formation of the octahedral architecture. This behavior is different
to what was observed for related [Pd2L4]4+ cages[21d-g, 21k] which
assemble instantaneously at RT in either acetonitrile (CH3CN) or
DMSO. Presumably, the highly coordinating DMSO solvent and
the longer reaction times at elevated temperatures are required to
facilitate the self-correction process and allow the formation of the
larger octahedral cage systems. The [Pd6(Ltripy)8]12+ cages (Ltripy
= 1a-b or 2a-b) were characterized using a combination of 1H
NMR, DOSY, and HR-ESI-MS data (Figures 3-5 and Supporting
Information).
The 1H NMR spectra (Figure 3 and Supporting Information)
of the [Pd6(Ltripy)8]12+ cages each show a single set of peaks
shifted downfield relative to Ltripy, consistent with complexation to
palladium(II) ions (Figures 3 and Supporting Information). The
sharp, uncomplicated signals observed in the 1H NMR spectra of
[Pd6(Ltripy)8]12+ cages are similar to what was previously observed
for the formation of [Pd2L4]4+[21d-g, 21k] and other palladium(II) based
octahedral cages and are consistent with the formation of a
complex with Oh symmetry in solution.[21e-g, 21k]
The ligand 1b forms hydrogen-bonded dimers in the solid
state involving interactions between a pyridyl nitrogen atom and a
hydrogen atom of the benzene core ring (N3---C4 3.321(1) Å, N3-
--H4 2.40 Å) (Supporting Information). The hexyloxy chains on
adjacent dimers interdigitate forming linear tapes. The linear
tapes are involved in π-π stacking interactions (centroid-centroid
distances 3.69-3.83 Å) (Supporting Information). The ligand 2b
forms slipped stacked supramolecular tapes through π-π
interactions (centroid-centroid distance = 3.78 Å) (Supporting
Information). The supramolecular tapes interact with adjacent
tapes through dispersion interactions between head-to-tail
packed hexyloxy chains (Supporting Information).
a)
b)
Figure 3. Stacked 1H NMR spectra (500 MHz, 298 K) of the free ligand 2a and
[Pd6(2a)8]12+ cage in [D6]DMSO.
Further solution phase evidence for the formation of the desired
discrete cage architectures was obtained from 1H DOSY NMR
spectroscopy (Figure 4 and Supporting Information). Each of the
proton signals in the individual spectra of the [Pd6(Ltripy)8]12+ cages
(D = 0.5-0.6 × 10-10 m2 s-1) and the corresponding ligands (D =
1.7-2.0 × 10-10 m2 s-1) show the same diffusion coefficients (D),
indicating that there is only one species present in solution and
the ratio of the diffusion coefficients of the ligand and palladium
cage in d6-DMSO are approximately 3:1, consistent with the
presence of the larger molecular cage species in solution.
Additionally, a plot of logD against logMW for ligands and
complexes gave a good linear fit (Figure 5).
Figure 2. The molecular structures of ligands 1b and 2b: a) ORTEP[25] diagram
of the X-ray crystal structure of 1b, b) ORTEP[25] diagram of the X-ray crystal
structure of 2b. Ellipsoids are shown at the 50% probability level. Colours: Grey
= carbon, white = hydrogen, red = oxygen, blue = nitrogen.
Synthesis of the [Pd6(Ltripy)8]12+ cages
The [Pd6(Ltripy)8]12+ cages were prepared by mixing
[Pd(CH3CN)4](BF4)2 (6 equiv.) with one the ligands (either 1a or
1b or 2a or 2b, 8 equiv.) in dimethyl sulfoxide (DMSO) and heating
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Figure 4. Overlaid 1H DOSY NMR spectra (500 MHz, [D6]DMSO, 298 K) of
[Pd6(2a)8]12+ cage (blue (top) trace) and free ligand 2a (black (bottom) trace).
Figure 6. HR-ESI-TOF mass spectrum of [Pd6(2a)8](BF4)12 showing signals due
to [Pd6(2a)8(BF4)12-n]
n+, n = 4 – 10. Inset: comparison of experimental (top) and
simulated (bottom) spectra for the signal of [Pd6(2a)8(BF4)3]9+ (m/z = 577.705).
The molecular structure of the [Pd6(2a)8](BF4)12 octahedral
cage was confirmed using X-ray crystallography (Figure 5 and
Supporting Information). A single crystal of [Pd6(2a)8](BF4)12 was
grown by vapour diffusion of ether into a DMSO solution of the
cage. The crystals were small and diffraction was modest thus the
X-ray data was collected at the Australian Synchrotron. The cage
̅
crystallized in the trigonal P3 space group with the asymmetric
unit containing one Pd atom, one complete ligand and a third of a
-
second ligand on a threefold axis. The BF4 anions and co-
crystallized solvent (DMSO and diethyl ether) were too disordered
to be located; their presence was accounted for using the
SQUEEZE program (voids totalled ~7800 Å3/cell and 1866 e-/cell).
Each 2a ligand coordinates to three PdII ions, in a monodentate
fashion, generating the expected [Pd6(2a)8]12+ cage architecture.
The Npy-Pd bond lengths of [Pd6(2a)8](BF4)12 are similar to those
Figure 5. Plot of log(D) against log(MW) for reported ligands and cages
([D6]DMSO, 298 K, 500 MHz) (black dots). The blue dots are previously reported
ligands and [Pd2(L)4]4+, [Pd3(L)4]6+ and [Pd4(L)4]8+ cages.[21a, 21c] Units D: x 10-10
m2 s-1, MW: g mol-1.
Mass spectra (HR-ESI-MS) of the [Pd6(Ltripy)8]12+ cages,
under pseudo cold-spray conditions, in DMSO–CH3CN solution
observed
in
other
[Pd(py)4]2+
moieties
of
similar
displayed
a series of isotopically resolved peaks due to
-
metallosupramolecular architectures (~2.0 Å). The cage displays
a large central cavity with 2.5 nm between the diametrically
opposed PdII ions (Pd1---Pd1′ 25.596(3) Å) and 1.8 nm between
axial and equatorial PdII ions (Pd1---Pd1′′ 18.119(3) Å) metal
centres. The coordinated pyridyl units and the benzene core rings
are almost perpendicular to each other (89°). The phenylene
linkers have greater rotational freedom and are found twisted
~46° relative to the plane of the pyridyl units. The 12 portals into
the internal cavity between the ligands, forming at the edges of
the octahedral cage, are 7.1 Å at the widest point. These openings
should allow relatively easy access to the internal cavity of the
cage for small guest molecules. The portals are large enough for
[Pd6(Ltripy)8](12-n)+ (BF4 )n ions, where n = 3-10, along with small
peaks due to fragmentation of the cage (Figure 6 and Supporting
Information). For example, ions at m/z
=
511.332
660.788
910.213
[Pd6(2a)8(BF4)2]10+
[Pd6(2a)8(BF4)4]8+,
,
577.705
[Pd6(2a)8(BF4)3]9+,
[Pd6(2a)8(BF4)5]7+,
767.756
[Pd6(2a)8(BF4)6]6+, 1109.656 [Pd6(2a)8(BF4)7]5+, 1408.820
[Pd6(2a)8(BF4)8]4+ were observed for the [Pd6(2a)8](BF4)12
octahedral cage (Figure 6).
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the small molecules (DMSO solvent molecules 4.6 Å at the widest
a)
-
point; BF4 counterions 2.0 Å across) and small organic guest
molecules to freely move in and out of the internal cavity. The
[Pd6(2a)8]12+ cages laterally pack with a hexagonal arrangement
displaying edge-to-face interactions between hydrogen atoms of
a pyridyl unit of one ligand interacting with a benzene core ring of
another ligand (hydrogen-centroid distance = 2.847 Å, carbon-
centroid distance 3.579 Å, see Supporting Information), and pack
vertically with a slipped face-to-face π-π stacking interaction
between two benzene core rings (centroid-centroid distance
3.680 Å) and four edge-to-face interactions between two ligands
(hydrogen-centroid distance = 3.121 Å, carbon-centroid distance
4.027 Å, see Supporting Information).
b)
We were unable to generate X-ray quality single crystals of
either of the smaller octahedral cages [Pd6(1a)8](BF4)12 or
[Pd6(1b)8](BF4)12. To provide an estimate of the size of those cage
systems we exploited MMFF molecular modeling (SPARTAN16,
MMFF, Supporting Information). The [Pd6(2a)8]12+ cage was also
modelled and the dimensions compared to the X-ray diffraction
data. The calculated cage dimensions (Pd---Pd1’ 25.048 verses
25.596(3) Å and Pd1---Pd1′′ 17.010-17.877 verses 18.119(3)Å)
were in good agreement with the crystallographic data, a strong
indication the MMFF models could be used to effectively estimate
the size of the smaller octahedral cages. The modelling indicated
that the [Pd6(1a)8](BF4)12 cage was smaller than the
[Pd6(2a)8](BF4)12 system, as expected, the diametrically opposed
PdII ions were separated by 2.1 nm (Pd1---Pd1′ 21.090 Å) and
axial and equatorial PdII ions were approximately 1.45 nm apart
(Pd1---Pd1′′ 14.114-15.063 Å) meaning the alkyne linked cages
have a smaller internal cavity than the aryl linked systems
(Supporting Information).
Figure 7. The molecular structure of the [Pd6(2a)8](BF4)12 shown as a) two
ORTEP[25] diagrams and b) two space filling representations (anions and
solvents molecules were not located). Ellipsoids are shown at the 50%
probability level. Colours: Grey = carbon, white = hydrogen, magenta =
palladium, blue = nitrogen. Bond lengths/interatomic distances (Å): Pd1-N1
2.05(2), Pd1-N3 2.067(9), Pd1-N5 2.07(1), Pd1-N7 2.056(9), Pd1---Pd1'
25.596(3), Pd1---Pd1'' 18.079(3).
Guest encapsulation studies
Having confirmed the formation of the [Pd6(Ltripy)8]12+ cages, we
set out to examine the molecular recognition properties for these
large metallosupramolecular hosts. The cages only showed
appreciable solubility in DMSO and dimethylformamide (DMF);
therefore, the host-guest studies were carried out in these media.
Initially, we choose to examine the neutral organic molecules
adamantane, anthracene, and pyrene, as guests (Figure 6).
These molecules had previously been shown to bind within
related [Pd2(Ldipy)4]4+ cages,[14d, 14e, 14g] [Pd6(L)4]12+ cages[9g-i] and
[Pd6(Ltripy)8]12+ cages,[19b] and it was reasoned that the guests
should interact with our [Pd6(Ltripy)8]12+ cages through either
dispersion (adamantane) forces or π-π interactions (anthracene,
and pyrene). The binding interaction was examined using 1H NMR
spectroscopy ([D6]DMSO, 298 K) with one of the [Pd6(Ltripy)8]12+
cages (1 equiv.) mixed with one of the guests (10 equiv.).
Somewhat surprisingly, none of the aliphatic or aromatic
compounds showed any signs of encapsulation when added to a
DMSO solution of [Pd6(Ltripy)8]12+ cage systems. No complexation
induced shifts (CIS) for the proton resonances for either the
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guests or the host cages were observed indicating that the
organic molecules were not interacting with the octahedral cages
(Supporting Information). All of the guests were small enough that
they should readily fit through the portals of the host cages.
However, to preclude any size-based kinetic exclusion, the
binding experiments were repeated at 80 °C. Again no CIS of
either the host or guest molecules were observed suggesting that
the organic molecules simply do not interact with the
[Pd6(Ltripy)8]12+ cages in neat DMSO. An additional guest, 1,8-
naphthalimide (Figure 6) was chosen to examine this further. 1,8-
Naphthalimide features aromatic units that should be able to
interact with the host cages via π-π and dispersion interactions
and it also contains amide units that could hydrogen bond to the
Figure 6. Neutral and anionic molecules examined as potential guest molecules.
The combined results suggested that even combinations of
hydrogen bonding, π-π and dispersion interactions are not
enough in order to achieve effective binding in the competitive
DMSO solvent environment with these [Pd6(L)4]12+ cages. This
highlights the importance of both the hydrophobic effect[9, 14d, 14e,
14g, 19b, 20]
and anion binding[14a] when designing cationic metallo-
hosts. Anionic sulfonates have been shown to bind in other
16c,
palladium(II) based metallosupramolecular architectures[16b,
1
[Pd(py)4]2+ binding sites of the octahedral hosts. Again, H NMR
16h-k, 21a, 21c, 21f, 27] via a combination of hydrogen bonding and ion-
ion interactions. The presence of ion-ion interactions should
further enhance any potential guest binding within the octahedral
[Pd6(Ltripy)8]12+ cages. Therefore, we explored the used of p-
toluenesulfonate (TsO-) as guest. Addition of NaOTs into a d6-
DMSO solution of one of the cages (either [Pd6(1a-1b)8]12+ or
[Pd6(2a-2b)8]12+) led to CIS of the both the guest and the host
(Supporting Information). When the cages were mixed with 1
equiv. of the TsO- guest, appreciable (0.10-0.15 ppm) shifts of the
endohdral H6 proton of the respective cage systems were
observed. In the presence of 10 equiv. of TsO- the H6 proton
resonance was shifted even further downfield (0.30-0.50 ppm).
This provided strong evidence for endohedral TsO- binding within
the cavities of the cages. However, there was evidence for guest
interaction with the exohedral face of the cages as well with
smaller CIS observed (0.02-0.06 ppm) for the H2 proton
resonance of the respective cage systems. The collected data
suggest that ion assisted hydrogen bonding interactions are
essential for guest interaction with the octahedral cages in the
highly DMSO polar solvent environment.
spectroscopy ([D6]DMSO, 298 K) was used to evaluate guest
binding (Supporting Information). As was observed with the
hydrocarbon guests, no CIS for the proton resonances of the 1,8-
naphthalimide or the host cages were observed indicating that the
neutral guest was not interacting with the octahedral cages
(Supporting Information). While the lack of binding observed for
these guest molecules was initially puzzling, the binding of
- [16d, 16f, 16g, 16k, 21d, 26]
21c]
BF4 ,
DMF[21a,
and DMSO,[21d] through
hydrogen bonding and ion-ion interactions, has been observed in
related metallosupramolecular architectures that feature the
[Pd(py)4]2+ motif. Thus we presume that the DMSO or DMF
-
solvent and the BF4 counter anions, in the absence of the
hydrophobic effect, are out competing the aliphatic or aromatic
compounds and occupy the cage cavity. 19F NMR spectroscopy
of the cages provided some support for this postulate (Supporting
Information). 19F NMR spectroscopy (either CD3CN or d6-DMSO,
298 K) of the each of the [Pd6(Ltripy)8]12+ cages revealed one broad
signal at δ = –150 ppm (Supporting Information) which is shifted
upfield by (~ 2 ppm) compared to the position of the BF4– ions of
NaBF4 in CD3CN (Supporting Information), consistent with the
complexation of the anions within the cage. In [D6]DMSO similar
results are observed, however, the CIS of the BF4– ions is smaller
(0.3 ppm) presumably reflecting competition between the DMSO
There are 12 potential guest binding sites, involving
endohedral or exohedral interaction with the [Pd(py)4]2+ units of
the cages. To gain insight in the stoichiometry of the host-guest
interaction mole-ratio method[28] NMR titrations were carried out.
Following the change in chemical shift of the H6 proton resonance
of the cages, mole-ratio titrations indicated that the smaller
[Pd6(1a-1b)8]12+ cages interact with three TsO- guest molecules,
while the larger [Pd6(2a-2b)8]12+ systems were interacting with four
TsO- anions (Figure 7 and Supporting Information).[29] Thus there
is clearly a size selectivity for the guest binding interaction with
–
solvent molecules and the BF4 counter anions for binding sites
with the cage.
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the different cages. Molecular modelling (SPARTAN16, MMFF,
Supporting Information) of the TsO-/cage host-guest adducts
suggests that the cages can accommodate three ([Pd6(1a-b)8]12+
cages) or four ([Pd6(2a-b)8]12+ cages) guest without any obvious
steric impediment (Supporting Information, Figure S41). The
volumes of the ([Pd6(1a-b)8]12+ and [Pd6(2a-b)8]12+ cages were
estimated to be 1500 and 1900 Å3, respectively (Supporting
Information Figure S40). The volume of the TsO- guest molecule
was calculated to be 154 Å3 (SPARTAN16, Supporting
Information). Therefore the three TsO- guest occupy 30.8% of the
[Pd6(1a-b)8]12+ cage cavities while four TsO- fill 32.4% of the
[Pd6(2a-b)8]12+ cages. While these values are self-consistent they
are less than the 55% expected from Rebek’s rule.[30] This
deviation from the Rebek rule is presumably caused by repulsive
anion-anion interactions between the bound guest molecules.
exterior surface of the octahedral cages can be detected under
the conditions of the mass spectral analysis.
Figure 8. HR-ESI-TOF mass spectrum of [Pd6(2a)8(TsO-)n]12-n showing host-
guest adduct signals with varying numbers of guest molecules associated.
Inset: comparison of experimental (top) and simulated (bottom) spectra for the
signal of [Pd6(2a)8(TsO-)6]6+ (Experimental m/z 949.2144; Simulated m/z
949.1967).
To further probe the importance of the hydrophobic effect
on guest binding we examined the use of mixed water-DMSO
solvent systems for host-guest studies. Preliminary experiments
showed that the [Pd6(1a)8]12+ and [Pd6(2a)8]12+ cages were soluble
in a 1:1 mixture of D2O and [D6]DMSO therefore we used this
solvent system to re-examine binding of the neutral organic
molecules adamantane, anthracene, pyrene and 1,8-
naphthalimide within the cage cavities (Supporting Information).
The binding interaction was examined using 1H NMR
spectroscopy ([D6]DMSO/D2O 1:1, 298 K) with one of the
[Pd6(Ltripy)8]12+ cages (1 equiv.) mixed with one of the guests (10
equiv.). When 10 equiv. of the guests (anthracene, pyrene, and
1,8-naphthalimide) were added to the cages in 1:1 [D6]DMSO
/D2O, suspensions were obtained indicating that the guest
molecules were not completely soluble at higher concentrations
in this solvent mixture. These suspensions were stirred at RT for
16 hours then centrifuged and the solution decanted and analyzed
Figure 7. Mole-ratio titration plots (500 MHz, d6-DMSO, 298 K)[28] showing the
1H chemical shift change of the internally directed proton H6 of the [Pd6(2a)8]12+
(blue data) and [Pd6(1a)8]12+ (black data) cages with increasing equiv. of TsO-
guest (from 0 to 22 equiv.). [Host] = 1.07 mM.
Additionally, we attempted to obtain further information on
the stoichiometry of the host-guest interaction from electrospray
ionization mass spectrometry experiments of the host-guest
mixtures. The [Pd6(Ltripy)8(TsO-)n]12-n adducts were generated by
adding a [D6]DMSO solution of the TsO- guest to a [D6]DMSO
solution of the [Pd6(Ltripy)8]12+ cages at RT. A range of host-guest
adducts
we
observed
in
the
mass
spectra
of
[Pd6(Ltripy)8(TsO-)n]12-n (n = 5-9) with higher stoichiometries than
those observed for the mole ratio titrations. This suggests that the
host-guest interaction is strong and that in addition to the
observing tight endohedral bound guests, the presumably weaker
association of TsO- with the exohedral binding sites on the
1
using H NMR spectroscopy. Unlike what was observed in neat
[D6]DMSO solvent, the anthracene, pyrene, and 1,8-
naphthalimide guest molecules displayed CIS for their proton
resonances indicative of guest binding (Supporting Information).
As observed previously in neat [D6]DMSO, no CIS were observed
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10.1002/chem.201702518
Chemistry - A European Journal
FULL PAPER
with the adamantane guest suggesting a combination of π-π
interactions and the hydrophobic effect were required for guest
uptake.
fill 58%; thus it is likely that the either four or five guests bind in
the [Pd6(2a)8]12+ cage. The volume of anthracene was calculated
to be 200 Å3 (SPARTAN16). Therefore four guests would occupy
53.3% of the smaller cage and five guest molecules would fill
52.6% of the larger cage.
In addition to the CIS of the guest molecules, several proton
resonances of the [Pd6(Ltripy)8]12+ cages shifted in the presence
the guest molecules. The endohdral H6 proton of each cage
undergoes a small (0.05-0.17 ppm) shift, while the proton
resonances of the trisubstituted benzene spacer units (either H10
in 1a or H12 in 2a) shift upfield between 0.10-0.25 ppm consistent
with a π-π interaction (Supporting Information). The proton
resonances of the disubstituted phenyl linker (H8 and H9) of the
[Pd6(2a)8]12+ cage also show upfield shifts of 0.10-0.15 ppm.
While the planar organic guest molecules could potentially bind
on the outside faces of the cages the exohedral H2 protons of
each cage display only very small CIS (0.01-0.05 ppm) indicating
that the guest binding is predominantly within the cage cavity as
would be expected on entropic grounds.
Conclusions
Four planar C3 symmetric tripyridyl ligands (Ltripy = 1a-b and 2a-
b) were synthesized and used to assemble
a family of
[Pd6(Ltripy)8](BF4)12 octahedral cages. The ligands and cages were
characterized using a combination of 1H, 13C and DOSY NMR
spectroscopy, HR-ESI-MS, IR spectroscopy, and elemental
analysis. Additionally, the solid state structures of two of the
ligands (1b and 2b) and the [Pd6(2a)8](BF4)12 cage were
determined using X-ray crystallography. The molecular
recognition properties of the cages with neutral and anionic
guests was examined, in [D6]DMSO and [D7]DMF, using NMR
spectroscopy, mass spectrometry and molecular modelling. No
binding was observed with simple aliphatic and aromatic guest
molecules or aromatic molecules featuring hydrogen bond
acceptors units. However, anionic sulfonates were found to
interact with the octahedral cages and the binding interaction was
size selective. The smaller [Pd6(1a-b)8]12+ cages were able to
interact with three guest molecules while the larger [Pd6(2a-b)8]12+
systems could host four guest molecules.
The proton resonances of the all the guest molecules within
the cages were very broad at 298 K (Supporting Information).
However, heating the host-guest mixtures to 353 K sharpens the
resonances of the 1,8-naphthalimide (in both host cages) and the
pyrene in the smaller cage to allow the host-guest stoichiometry
to be estimated from integration of the guest resonances.
Unfortunately, the proton resonances of anthracene guest
molecules (in both host cages) remained too broad to allow
reliable integration, and as such we were not able to estimate the
host-guest stoichiometry for these systems. The smaller
[Pd6(1a)8]12+ cage was found to bind four 1,8-naphthalimide
guests in [D6]DMSO/D2O 1:1, while the larger [Pd6(2a)8]12+ cage
interacted with six 1,8-naphthalimides. Three pyrene guest were
found to bind with the smaller [Pd6(1a)8]12+ cage, and the larger
[Pd6(2a)8]12+ cage was found to encapsulate five pyrene guest.
Using the cage cavity volume calculated above and volumes for
each guest molecule calculated in SPARTAN16 (Supporting
Information), it was found that the four 1,8-naphthalimide guests
occupy 50.6% of the [Pd6(1a)8]12+ cage cavities while six 1,8-
naphthalimide fill 59.7% of the [Pd6(2a)8]12+ cage. Similarly, three
pyrene fill 44% of the [Pd6(1a)8]12+ cage cavity, while five pyrene
occupy 59% of the [Pd6(2a)8]12+ cage. The values for these neutral
organic guest are consistent with the 55% expected from Rebek’s
rule.[30] Therefore we used the cage and guest volumes to
estimate host-guest stoichiometry for the pyrene (with the larger
cage) and anthracene host-guest adducts. Four pyrene guest
molecules would occupy 46% of the larger cage, while five would
The molecular recognition properties of the cages with
neutral guests were examined, in 1:1 [D6]DMSO/D2O mixtures to
examine the importance of the hydrophobic effect. In this solvent
mixture the anthracene, pyrene, and 1,8-naphthalimide guest
molecules were all observed to bind with the cavity of the cages,
while the adamantane guest still showed no interaction with the
cationic hosts. These results strongly indicate that a combination
of π-π interactions and the hydrophobic effect were required for
the uptake of neutral guests. NMR spectroscopy and molecular
modelling indicated that the cages bound multiple copies of the
individual aromatic guests (between 3-6 guest molecules per
cage).
The reported cage [Pd6(Ltripy)8]12+ systems clearly display
relativity large central cavities that can be used to encapsulate
multiple guest molecules. The ability to bind multiple guest
molecules could potentially be exploited for catalysis and drug
delivery. However, the limited guest binding ability of the reported
systems, in the absence of water, highlights the importance of
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10.1002/chem.201702518
Chemistry - A European Journal
FULL PAPER
both the hydrophobic effect,[9, 14d, 14e, 14g, 19b, 20] and competitive
anion binding[14a] when designing cationic metallo-hosts.
Therefore, to improve the host-guest capabilities of these
[Pd6(Ltripy)8](BF4)12 octahedral cages and enable the binding of a
wider range of guest molecules, we are now targeting systems
decorated with water solubilizing groups and that contain larger,
less competitive anions. Efforts in these directions will be reported
in due course.
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Experimental Section
Experimental Details including synthetic procedures and characterisation,
NMR and MS spectral data, X-ray data (CCDC #: 1531037-1531039) and
MMFF molecular models are available free of charge in the supplementary
information.
Acknowledgements
The authors thanks the Department of Chemistry, University of
Otago for funding. TYK thanks the University of Otago for a PhD
scholarship. Data for the structure of octahedral cage complex
[Pd6(2a)8](BF4)12 was obtained on the MX2 beamline at the
Australian Synchrotron, Victoria, Australia. Dan Preston is
thanked for his assistance generating cartoon diagrams shown in
Figure 1.
Keywords: metallosupramolecular architectures • molecular
recognition • palladium(II) • cages • host-guest chemistry
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This article is protected by copyright. All rights reserved.

10.1002/chem.201702518
Chemistry - A European Journal
FULL PAPER
FULL PAPER
Tae Y. Kim, Lori Digal, Michael G.
Gardiner, Nigel T. Lucas*, James D.
Crowely*
Page No. – Page No.
Metallosupramolecular cages:
Synthesis, structure and guest
encapsulation studies
A combined effort: A series of octahedral cages of varying cavity sizes were
synthesized with PdII and trigonal tripyridyl ligands. In dimethyl sulfoxide (DMSO)
solution the cages only encapsulated anionic guests whereas in a 1:1 D2O:DMSO
solvent mixture neutral aromatic guests could be bound with the cage cavity via a
combination of π-π stacking, dispersion forces and the hydrophobic effect.
This article is protected by copyright. All rights reserved.