Chloride triggered reversible switching from a metallosupramolecular [Pd2L4]4+ cage to a [Pd2L2Cl4] metallo-macrocycle with release of endo- and exo-hedrally bound guests

Article abstract of DOI:10.1039/c5cc02226f

A metallosupramolecular [Pd₂L₄]⁴⁺ cage can be cleanly converted into a [Pd₂L₂Cl₄] metallo-macrocycle upon addition of chloride ions. The process is reversible, treatment of the [Pd₂L₂Cl₄] macrocycle with silver(i) ions regenerates the [Pd₂L₄]⁴⁺ cage. Additionally, it is shown that guest molecules could be released on chloride triggered cage dis-assembly and taken up anew on re-assembly. This journal is

Full text of DOI:10.1039/c5cc02226f

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Chloride triggered reversible switching from a
metallosupramolecular [Pd2L4]4+ cage to a [Pd2L2Cl4] metallo-
macrocycle with release of endo- and exo-hedrally bound guests
Received 00th January 20xx,
Accepted 00th January 20xx
Dan Preston,a Alyssa Fox-Charles,a Warrick K. C. Loa and James D. Crowleya*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A metallosupramolecular [Pd2L4]4+ cage can be cleanly converted unchanged (Fig. 1a). An alternative method is to modify the host
into a [Pd2L2Cl4] metallo-macrocycle upon addition of chloride and therefore its binding ability while leaving the guest molecule
ions. The process is reversible, treatment of the [Pd2L2Cl4] unchanged (Fig. 1b-d). There are number of ways in which this can
macrocycle with silver(I) ions regenerates the [Pd2L4]4+ cage. be achieved. Yam and co-workers9 have shown that Pt-containing
Additionally, it is shown that guest molecules could be released on guest molecules can be released from [Pt2L2]2+ metallo-rectangles
chloride triggered cage dis-assembly and taken up anew on re- by protonation of the endotopic pyridine units that line walls of the
assembly.
cavity of the assembly. Clever and co-workers have developed a
2-
[Pd2L4]4+ cage that binds a [B12F12
] anion. Irradiation of the host-
The ready synthesis1 and molecular recognition properties of
metallosupramolecular assemblies have resulted in these systems
being examined for a variety of potential applications.2 The host-
guest properties of these metallosupramolecular assemblies have
been exploited for a variety of proposes, including molecular
storage of reactive species,3 the binding of environmental
pollutants,4 catalysis,5 and the delivery of biologically relevant
molecules including drugs.6 The ability of metallosupramolecular
assemblies to selectively bind guest molecules within their internal
cavities is clearly extremely important for these applications.
However, just as useful as guest encapsulation, but harder to attain,
is the ability to controllably release the guest from the cavity when
required. There are two different approaches to induce guest
release from metallosupramolecular assemblies: (1) alteration of
the guest and its binding affinity for the host (Fig. 1a), or (2)
modification of the host and its binding affinity for the guest (Fig.
1b-d). Examples of both approaches have been demonstrated
previously. A number of groups have shown that neutral guest
molecules will be expelled from cationic host cages upon
guest adduct with UV (365 nm) induces an isomerization of the
cage’s ligands and expels the guest from the host cavity.10
Transmetallation has also been exploited to trigger guest release.
Addition of Cu(I) ions to a Cd(II) M4L2 assembly leads to the release
of encapsulated anionic croconate guest molecules.11 In these
examples the metallosupramolecular assemblies’ overall three-
dimensional connectivity is retained but the stimulus alters the
host’s binding ability and leads to release of the guest molecule
(Fig. 1b).
introduction of positive charge onto the guest, either by
7
protonation or oxidation.6a,
Others have shown that light
switchable guest molecules can be released from a cationic host
cage upon UV irradiation.8 In these systems the properties of the
guest are altered by some stimulus while the host remains
Figure 1. Generic cartoon representations of stimuli-responsive guest release from
metallosupramolecular assemblies; a) stimuli induced alteration of the guest, b) stimuli
induced alteration of the host binding affinity for the guest with retention of the host’s
three-dimensional structure, c) complete dis-assembly of the host architecture d)
partial dis-assembly of the host structure.
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Guest release can also be achieved by exploiting the dynamic palladium complex in d7-dimethyformamide (DMF) was
nature of the metal-ligand interaction to either completely (Fig. 1c) approximately 2:1, consistent with the formation of a larger
or partially (Fig. 1d) dis-assemble the metallosupramolecular host. molecular cage species. Mass spectra (HR-ESMS) of the cage C in
Nitschke and co-workers have developed a family of tetrahedral DMF–CH3CN solution displayed a series of isotopically resolved
[Fe4L6]8+ cages which can bind a variety of neutral guest molecules. peaks due to [Pd2L4](4-n)+ (BF4)n (where n = 1-3) ions along with
Treatment of these cages with either acid or tris(2- peaks due to fragmentation of the cage structure (ESI†).
ethylamino)amine leads to the complete dis-assembly of the cages, Additionally, the molecular structures of the ligand (L) and the cage
4
releasing encapsulated cyclohexane12 or SF6 guest molecules. (C) were determined unequivocally by X-ray diffraction (Fig 2b and
Others have developed systems that operate in a similarly fashion. ESI†).
Yoshizawa and co-workers synthesised
macrocycle from an anthracene-containing dipyridyl ligand and
a
[Ag2L2]2+ metallo-
13
showed that it can strongly bind the fullerene C60
. UV irradiation
of the [C60⊂Ag2L2]2+ host-guest adduct causes the reduction of the
Ag(I) ions to Ag(0) leading to the disintegration of the metallo-
macrocycle and release of the encapsulated C60. Using the same
anthracene-containing dipyridyl ligand Yoshizawa and co-workers
generated
a
[Hg2L4]4+ cage system.14 Like the Ag(I) metallo-
macrocycle the [Hg2L4]4+ cage is capable of encapsulating C60 or C70
fullerenes. Addition of two equivalents of Hg2+ causes the
[C60⊂Hg2L4]4+ cage to rearrange to form two [Hg2L2]4+ metallo-
macrocyles and release of the fullerene guest. Reformation of the
[Hg2L4]4+ cage and re-binding of the fullerene was achieved through
addition of two equivalents of free ligand to the metallo-macrocycle
fullerene mixture.
As part of our interest in functional metallosupramolecular
assemblies15 we have developed a [Pd2L4]4+ cage system, utilising
‘banana-shaped’ 2,6-bis(pyridin-3-ylethynyl)pyridine ligands, which
can bind the anti-cancer drug cisplatin.16 We have shown
previously, that the cage can be completely dis-assembled using an
excess of 4-(N,N-dimethylamino)pyridine (DMAP) or chloride (Cl-) to
release the cisplatin guest. We herein report the formation of a
new, more soluble, [Pd2L4]4+ cage using a diglyme substituted
tripyridyl ligand, L. This new system can be reversibly toggled
between the cage and its corresponding [Pd2L2Cl4] metallo-
macrocycle by the sequential addition of tetrabutylammonium
chloride ([NBu4]Cl) and AgBF4 as stimuli. Furthermore, it is
demonstrated that the cage can interact both endo- and exo-
hedrally with guest molecules which can be released from the cage
(Fig. 2a) upon treatment with chloride.
The ligand (L, Fig. 2b), substituted with diglyme chains on the
terminal pyridine rings for improved solubility, was synthesised in
good yield (71%) using standard procedures (ESI†).17 Addition of
[Pd(CH3CN)4](BF4)2 (1 eq.) to an acetonitrile (CH3CN) solution of L (2
eq.) gave the cage, [Pd2L4](BF4)4 (C) in good yield (Fig. 2b and ESI†).
1
The identities of the ligand and the cage were confirmed by H, 13C Figure 2. a) A cartoon showing the chloride induced conversion of a [Pd2L4] (BF4)4 (C)
into a metallo-macrocycle [Pd2L2Cl4] (M) with release of both endo and exo-hedrally
and DOSY NMR and IR spectroscopies and high resolution
electrospray mass spectrometry (HR-ESMS) (Fig. 3 and ESI†). The 1H
bound guest molecules; b) the molecular structures of the ligand (L), cage (C) and
metallo-macrocycle (M). The scheme (b) shows the formation of
C from L and
NMR spectrum of C (Fig. 3b and ESI†) showed a single set of peaks,
[Pd(CH3CN)4](BF4)2 and the reversible formation of M using [NBu4]Cl and AgBF4 as
stimuli. Colours: carbon, grey; chloride, green; nitrogen, blue; oxygen, red; palladium,
with the proton resonances of the terminal pyridine (Hc-e) shifted
downfield relative to L (Fig. 3a and b and ESI†), consistent with magenta. Hydrogen atoms, counterions and solvent molecules omitted for clarity.
complexation of L to the palladium(II) ions. Further solution phase
Addition of four equivalents of tetrabutylammonium chloride to a
evidence for the formation of the desired discrete cage architecture
was obtained from the 1H DOSY NMR of L and C. Each of the proton
d7-DMF solution (0.75 mM) of the cage C resulted in the formation
of new two species (Fig. 3c). The chemical shifts and diffusion
signals in the individual spectra show the same diffusion
coefficients (DL = 4.59 x 10-10 m2 s-1, DC = 2.16 x 10-10 m2 s-1),
coefficient for one of the two species in solution were identical to
those of the ligand L, suggesting that chloride ions were displacing
indicating that there is only one species present in each solution
some of the ligands L from the palladium(II) cage complex forming a
(ESI†). The raꢀo of the diffusion coefficients of the ligand and
2 | J. Name., 2012, 00, 1-3
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mixture of free L and a new neutral metallo-macrocycle [Pd2L2Cl4] consistent with
COMMUNICATION
C
interacting all four guest molecules
(M). The proton chemical shifts and diffusion coefficient (DM = 3.10 simultaneously; two cisplatin’s bound in the cage’s central cavity
x 10-10 m2 s-1) for the new species were intermediate between those and two mesylate anions binding on the exo-hedral faces of the
of C and L, consistent with the formation of the neutral, chloride- cage
containing complex M. Vapour diffusion of diethyl ether into a DMF
solution containing a mixture of M and L provided X-ray quality
crystals of the new palladium complex unambiguously confirming
the formation of the neutral metallo-macrocycle architecture (Fig.
2b and ESI†).18
Interestingly, efforts to prepare M independently by reacting
[Pd(CH3CN)2Cl2] and L resulted in only insoluble polymeric products
indicating that the preorganisation provided by the cage C was
essential for the clean generation of the metallo-macrocycle.
Treatment of the d7-DMF solution (0.75 mM) of M and L with four
equivalents of AgBF4 abstracted the chloride ions from M and led to
the reformation of the [Pd2L4](BF4)4 cage (C). Thus we have
generated a stimuli-responsive system that can be toggled cleanly
between two different metallosupramolecular architectures, a cage
and a metallo-macrocycle, by the addition or removal of chloride
ions.
We have previously shown that related dipalladium cages are
able to bind both neutral cisplatin16, 19 and anionic20 mesylate (MsO-
) guest molecules.21 1H NMR spectra of mixtures of the new diglyme
substituted cage and these guest molecules indicated that C retains
the host-guest capability of the previously reported palladium(II)
cage systems and bind both cisplatin and mesylate (Fig. 3).
Mixing cisplatin (excess) and [Pd2L4](BF4)4 in d7-DMF solution
results in a downfield shift and broadening of the proton resonance
Hc of the cage NMR (Fig. 3h and 3i, Δδ = 0.05 ppm), indicative of
Figure 3. Stacked, partial 1H NMR spectra (500 MHz, d7-DMF, 298 K) showing a) free
cisplatin encapsulation as we have observed previously.14, 16 In d7-
DMF the shift is smaller than those observed in CD3CN solution14, 16
ligand L, b) cage C, c) addition of [NBu4]Cl (4 eq.) giving macrocycle M and ligand L, d)
addition of AgBF4 (4 eq.) giving C, e) addition of [NBu4]OMs (2 eq.), f) addition of
indicating that the interaction is weaker. 1H NMR titrations showed
[NBu4]Cl (4 eq.) giving M and L, and liberated MsO- anions, g) addition of AgBF4 (4 eq.)
leading to reformation of host-guest adduct [(DMF)2(MsO-)2⊂C]2+, (h) C, (i) addition of
cisplatin (excess), giving [(cisplatin)2⊂C]4+ host-guest adduct, j) addition of [NBu4]Cl (4
eq.) giving M, L, and liberated cisplatin molecules, and k) addition of AgBF4 (4 eq.)
that the interaction between
C and cisplatin, in the highly
competitive DMF solvent, is extremely weak (K1 = 2±1 M-1, K2 = 5±2
M-1, ESI†).
resulting in the reformation of the host-guest adduct [(cisplatin)2⊂C]4+
.
Similar NMR experiments confirmed the interaction of mesylate
anions with C. A 1H NMR mole-ratio22 titration of C with the
mesylate guest indicated that two MsO- guests bind to the cage
(ESI†). Addiꢀon of tetrabutylammonium mesylate (2 eq.) to a d7-
DMF solution of the cage C caused downfield shifts of the proton
resonances Hc and He. The shift of proton He (Δδ = 0.45 ppm) was
the largest (Fig. 3e), suggesting that the MsO- anions are binding on
Having confirmed the guest binding abilities of C we set out to see
if the chloride induced structural rearrangement to the metallo-
macrocycle
M would lead to guest release. Addition of
tetrabutylammonium chloride (4 eq.) to d7-DMF solutions
containing one of the host-guest adducts (either [(cisplatin)2⊂C]4+, [
(DMF)2(MsO-)2⊂C]2+ or [(cisplatin)2(MsO-)2⊂C]2+) resulted in
essentially identical 1H NMR spectra (Fig. 3f, 3j and ESI†). The
proton chemical shifts and diffusion coefficient of M and L in these
spectra were identical to those observed when C was reacted with
tetrabutylammonium chloride (Fig. 3c) indicating that the metallo-
macrocycle M and the ligands L do not interact with either neutral
cisplatin or anionic mesylate guests. Thus chloride triggered
formation of M is accompanied by guest release (either two
cisplatin, two mesylates or all four guests).23 Presumably the
mesylate anions do not interact with the metallo-macrocycle M
because this neutral complex lacks the strong cation-anion
interactions that are present in the [(DMF)2(MsO-)2⊂C]2+ host-guest
adduct. The lack of interaction of cisplatin with M can potentially be
ascribed to the trans orientation of the central pyridyl hydrogen
bond donors of the metallo-macrocycle. This new orientation
the exo-hedral face(s) of C. The association constants (K1
=
1000±100 M-1, K2 = 180±20 M-1), for the interaction of the mesylate
anions with C were determined using 1H NMR ꢀtraꢀon (ESI†), the
values are similar to those observed in related host-guest systems.21
Slow vapour diffusion of diethyl ether into
a DMF solution
containing C and ten equivalents of tetrabutylammonium mesylate
resulted in X-ray quality crystals of the host-guest adduct. The
molecular structure (ESI†) confirmed that two mesylate anions bind
on the two exo-hedral faces of the dipalladium complex and two
DMF molecules fill the central cavity of the cage giving
[(DMF)2(MsO-)2⊂C]2+ host-guest adduct.
This interesting behaviour allows the possibility of C binding two
cisplatin and two mesylate anions at the same time. Pleasingly the
1H NMR spectrum (ESI†) of a d7-DMF solution containing cisplatin
(excess), tetrabutylammonium mesylate (2 eq.) and [Pd2L4]4+ was
a
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ammine ligands of cisplatin in a chelating fashion. Addition
of
d) M. M. J. Smulders, S. Zarra and J. R. Nitschke, J. Am. Chem. Soc.
,
AgBF4 (4 eq.) to these mixtures of M, L and guest molecules
resulted in the reformation of the cage C and restored the host-
guest interactions (Fig. 3g and 3k).
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In conclusion we have developed
a new diglyme
substituted [Pd2L4]4+ cage which can be cleanly converted into
[Pd2L2Cl4] metallo-macrocycle upon the addition of four
equivalents of chloride. The initial [Pd2L4]4+ cage can be
quantitatively reformed by treating the mixture of [Pd2L2Cl4] 12. P. Mal, D. Schultz, K. Beyeh, K. Rissanen and J. R. Nitschke,
Angew. Chem., Int. Ed., 2008, 47, 8297-8301.
and 2L with Ag(I) ions. The cage showed the capacity to bind
13. N. Kishi, M. Akita, M. Kamiya, S. Hayashi, H.-F. Hsu and M.
two cisplatin molecules within the internal cavity of the
Yoshizawa, J. Am. Chem. Soc., 2013, 135, 12976-12979.
architecture and two mesylate anions on the exterior face, or
all four guests simultaneously. Additionally, it was
demonstrated that the guest molecules, bound either endo-
(within the cage cavity) or exo-hedrally on the exterior face of
the Pd(II) cage, were released into solution on chloride
triggered cage dis-assembly and taken up anew on re-
assembly.
As Pd(II)-based systems24 are one of the most common
classes of metallosupramolecular architectures we are now
examining if this chloride triggered cage dis-assembly can be
applied to other larger Pd(II)-containing assemblies. The ability
14. N. Kishi, M. Akita and M. Yoshizawa, Angew. Chem., Int. Ed., 2014,
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to
controllably
release
or
recall
guests
from
17. a) V. Gudipati, D. P. Curran and C. S. Wilcox, J. Org. Chem., 2006,
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18. a) Recently, Clever and co-workers have shown that halide ions can
trigger the conversion of [Pd2L4]4+ cage into a triply catenated link
structure see; b) R. Zhu, J. Luebben, B. Dittrich and G. H. Clever,
Angew. Chem., Int. Ed., 2015, 54, 2796-2800.
metallosupramolecular architectures has potential in wide-
ranging applications such as drug delivery, catalysis and
environmental remediation.
This work was supported by an Otago Medical Research
Foundation Laurenson Award (LA307). The authors thank
Department of Chemistry, University of Otago for additional
funding. DP and WKCL thank the University of Otago for PhD
scholarships. DP thanks Otago Medical Research Foundation
for a McQueen Summer Studentship.
19. a) J. E. M. Lewis, A. B. S. Elliott, C. J. McAdam, K. C. Gordon and
J. D. Crowley, Chem. Sci., 2014, 5, 1833-1843; b) J. E. M. Lewis, C.
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4 | J. Name., 2012, 00, 1-3
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