Stepwise halide-triggered double and triple catenation of self-assembled coordination cages

Article abstract of DOI:10.1002/anie.201408068

A simple self-assembled [Pd₂L₄] coordination cage consisting of four carbazole-based ligands was found to dimerize into the interpenetrated double cage [3X@Pd₄L₈] upon the addition of 1.5 equivalents of halide anions (X = Cl^-, Br^-). The halide anions serve as templates, as they are sandwiched by four Pd^II cations and occupy the three pockets of the entangled cage structure. The subsequent addition of larger amounts of the same halide triggers another structural conversion, now yielding a triply catenated link structure in which each Pd^II node is trans-coordinated by two pyridine donors and two halide ligands. This simple system demonstrates how molecular complexity can increase upon a gradual change of the relative concentrations of reaction partners that are able to serve different structural roles.

Full text of DOI:10.1002/anie.201408068

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DOI: 10.1002/anie.201408068
Coordination Cages
Stepwise Halide-Triggered Double and Triple Catenation of
Self-Assembled Coordination Cages**
Rongmei Zhu, Jens Lꢀbben, Birger Dittrich, and Guido H. Clever*
Abstract: A simple self-assembled [Pd2L4] coordination cage
consisting of four carbazole-based ligands was found to
dimerize into the interpenetrated double cage [3X@Pd4L8]
upon the addition of 1.5 equivalents of halide anions (X = ClÀ,
BrÀ). The halide anions serve as templates, as they are
sandwiched by four PdII cations and occupy the three pockets
of the entangled cage structure. The subsequent addition of
larger amounts of the same halide triggers another structural
conversion, now yielding a triply catenated link structure in
which each PdII node is trans-coordinated by two pyridine
donors and two halide ligands. This simple system demon-
strates how molecular complexity can increase upon a gradual
change of the relative concentrations of reaction partners that
are able to serve different structural roles.
triggers,[6] a change in pH[7] or electrochemical potential,[8]
and irradiation with light[9] is of high current interest.
With regard to self-assembled coordination cages,[10] we
and others have provided first examples implementing
controllable elements such as light-switchable ligands[11] and
redox-active functionalities[12] within the last couple of years.
In one particular strategy, we realized the self-assembly of
a series of interpenetrated double cages [3BF4@Pd4L’8] based
on banana-shaped bis(pyridyl) ligands L’ and the tetrafluor-
oborate salt of square-planar-coordinated PdII.[12a,13,14] All
structures were found to carry three tetrafluoroborate anions
À
in their one central and two outer pockets. The outer two BF4
ions could be replaced by halide ions following an allosteric
binding mechanism.[13b–e] By using bulky ligands, we further
showed that the choice of an added anionic template residing
in the central pocket controls the selectivity for the subse-
quent binding of further anionic guests in the two outer
pockets of these mechanically coupled double-cage sys-
tems.[13g]
Here, we demonstrate that structurally related but slightly
shorter bis-pyridyl ligands based on a carbazole backbone
react with the palladium salt [Pd(CH3CN)4](BF4)2 to give
stable monomeric cages [Pd2L4] that show no tendency to
dimerize in the presence of tetrafluoroborate anions
(Figure 1). The subsequent addition of stoichiometric
amounts of halide ions was found to lead to the formation
of interpenetrated double cages. Excess amounts of the same
anionic trigger, however, induced a second structural tran-
sition leading to a triply catenated link structure.
Ligand L was synthesized in three steps starting from
carbazole by the Sonogashira cross-coupling reaction of 3,6-
dibromo-9-hexylcarbazole[15] and 3-ethynylpyridine (Fig-
ure 1a). After a 2:1 mixture of ligand L and [Pd(CH3CN)4]-
(BF4)2 in CD3CN had been heated at 708C for 5 h, the
monomeric cage [Pd2L4] was obtained quantitatively as
indicated by 1H NMR spectroscopy (Figure 2a,b) and ESI
mass spectrometry (Figure 3a). The structure of [Pd2L4] was
unambiguously confirmed by the result of a single-crystal X-
ray structure determination (Figure 4a and the Supporting
Information). In contrast to our previous reports on the
formation of cages from slightly longer bis(byridyl) ligands
based on tricyclic dibenzosuberone or phenothiazine back-
bones,[12a,13b,f] no dimerization to give interpenetrated double
cages was observed at this point. We would like to emphasize
that we were able to predict this behavior based on our
collection of X-ray structures[12a,13b,f] and the dimensions
(separation of the pyridine N atoms) of the newly designed
carbazole ligand L that were extracted from a molecular
model (DFT B3LYP/6-31G*). This deviating behavior can be
explained by the fact that the shorter carbazole-based ligand
C
oncentration-dependent changes of the structural or
mechanistic features of complex molecular systems play
a major role in catalysis, pharmacology, and many biological
regulatory processes. In order to gain a basic understanding of
the underlying processes found in such complicated environ-
ments, the field of systems chemistry has evolved where
networks of interacting molecules can be studied in great
detail.[1] Such artificial model systems are often based on
supramolecular self-assembly which utilizes the formation of
complex structures from simple building blocks containing
a
preprogrammed connectivity.[2] Current research foci
include the implementation of functions such as selective
sensing[3] and catalysis.[4] The realization of advanced top-
ologies such as knots and links in the form of self-assembled
architectures is another subtopic in this area.[5] Furthermore,
the temporal and spatial control of structural rearrangements
and encapsulation processes by the addition of external
[*] R. Zhu, J. Lꢀbben, Prof. Dr. G. H. Clever
Institute of Inorganic Chemistry
Georg-August University Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: gclever@gwdg.de
Homepage: http://www.clever-lab.de
Priv.-Doz. Dr. B. Dittrich
Institute of Inorganic and Applied Chemistry
University of Hamburg
Martin-Luther-King-Platz 6, 20146 Hamburg (Germany)
[**] We thank the DFG (CL 489/2-1) and the Fonds der Chemischen
Industrie for financial support. R.Z. thanks the China Scholarship
Council for a PhD fellowship. We thank Dr. M. John for assistance
with NMR spectroscopy and Dr. H. Frauendorf for measuring the
ESI mass spectra.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408068.
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L gives rise to a smaller cavity which does not allow for the
inclusion of two relatively large tetrafluoroborate counter
anions sandwiching the Pd(pyridine)4 plane of a second,
interpenetrating cage unit. A comparison of the length of
ligand L with the lengths of the previously studied systems is
discussed in the Supporting Information. Furthermore, pre-
viously gathered knowledge about the ideal distance between
the Pd(pyridine)4 planes and tetrafluoroborate or halide
anions allowed us to predict the dimerization of the carbazole
monomeric cages in the presence of smaller anions such as
chloride or bromide.[13c,e]
Indeed, after 1.5 equivalents of bromide or chloride
anions had been added to a solution of monomeric cage
[Pd2L4] and the reaction mixture had been heated for 5 h,
NMR spectroscopy (Figure 2c,d) and ESI mass spectrometry
(Figure 3b,c) indicated the formation of an interpenetrated
species [3X@Pd4L8] with three halide anions inside its
pockets. In accordance with our previous reports, double-
cage formation is indicated by a twofold splitting of all NMR
signals and characteristic signal shifts of the pyridine protons
pointing inside the cageꢀs three cavities. In the high-resolution
ESI mass spectra, the double cage can be unambiguously
identified by the signals for the species [3X@Pd4L8]5+,
{[3X@Pd4L8] + BF4}4+, and {[3X@Pd4L8] + 2BF4}3+.
Since an X-ray structure could not be obtained in this case,
a DFT model of the chloride-containing assembly was
prepared based on the spectroscopic results obtained for
[3Cl@Pd4L8] and the crystal structures of the reported double
Figure 1. a) Synthesis of the ligand: i) NBS, DMF, 08C; ii) 1-bromo-
hexane, 50% NaOH, DMSO; iii) 3-ethynylpyridine, CuI, Pd(PPh3)2Cl2,
NEt3, 908C. b) Stepwise assembly of the monomeric cage [Pd2L4], the
halide-templated double cage [3X@Pd4L8] (X=ClÀ, BrÀ), and the triple
catenane {trans-[(PdX2)2L2]}3 (X=BrÀ). c–e) Schematic depiction of the
topologies of a trefoil knot (c), the Borromean rings (d), and the L6n1
link (e) that describes the molecular structure of the triple catenane
{trans-[(PdBr2)2L2]}3.
cages by performing
a geometry optimization on the
wB97XD/def2-SVP level of theory (Figure 4b and the
Supporting Information). In terms of the Pd2+–ClÀ–Pd2+
Figure 2. 1H NMR spectra of a) ligand L, b) the monomeric cage
[Pd2L4], and mixtures resulting from the reaction of 0.7 mm [Pd2L4]
with c) 1.5 equiv of bromide and d) 1.0 equiv of chloride (298 K,
500 MHz, NnBu4 signals at 0.7–3.1 ppm omitted, L=ligand,
M=monomeric cage, D=major double cage, B=minor double cage
[2Cl+Br@Pd4L8]).
Figure 3. ESI-FTICR mass spectra of a) monomeric cage
[nBF4@Pd2L4](4Àn)+ with n=0–2, b) double cage {[3Br@Pd4L8]-
(BF4)n}(5Àn)+ with n=0,1 and c) double cage {[3Cl@Pd4L8](BF4)n}(5Àn)+
with n=0–2 (*=adducts with impurities, B=double cage
[2Cl+Br@Pd4L8]5+).
+
2
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distances, the results of this calculation are in good agreement
with our previously published theoretical and experimental
studies of related double-cage structures.[12a,13]
As can be seen from the NMR spectral analysis, however,
the double cages are in equilibrium with the monomeric cage
and the free ligand (Figure 2c,d). The ratio of the species
[3X@Pd4L8]/[Pd2L4]/L was determined to be 5:3:10 for
bromide and 5:5:2 for chloride[16] by NMR spectroscopy.
The spectrum resulting from the chloride titration reprodu-
cibly showed a minor contamination of a further double cage
which could be identified as the mixed chloride/bromide
species [2Cl + Br@Pd4L8] (= B) by high-resolution mass
spectrometry (see the Supporting Information).
Since no free ligand was present in the solution of the
monomeric cage, the formation of the major component
[3X@Pd4L8] must be accompanied by a partial release of the
ligands by decomplexation, even when stoichiometric
amounts of halide were added. This behavior is in accordance
with our previously reported observations which showed that
also other halide-binding (but tetrafluoroborate-templated)
double cages release the free ligand upon addition of excess
amounts of halide anions.[13b,e] A plausible explanation is the
competition of halide binding inside the cageꢀs pockets with
the direct coordination to the square-planar PdII cations
under displacement of up to two of the pyridine donors. The
resulting PdX2(pyridine)2 motif is well known in supramolec-
ular self-assembly.[17] Probable self-assembled components
apart from the [Pd2L4] and [3X@Pd4L8] cages are the
macrocycle trans-[(PdX2)2L2] and the [2]-catenane {trans-
[(PdX2)2L2]}2 which are formally obtained by replacing two
and four ligands of the monomeric and dimeric cages with
halide substituents, respectively. These neutral species are
expected to be poorly soluble in the polar medium acetoni-
trile and thus account at least in part for the observed
precipitates.
Figure 4. a) Single-crystal X-ray structure of the monomeric cage
[Pd2L4]4+ (counter anions=BF4À), b) DFT-optimized structure of
[3Cl@Pd4L8]5+, and c) X-ray structure of the triple catenane {trans-
[(PdBr2)2L2]}3.
To our surprise, however, a triply catenated compound
{trans-[(PdBr2)2L2]}3 was isolated and a single crystal was
structurally characterized by X-ray diffraction analysis (Fig-
ure 4c). In contrast to the cage structures, the halide-
coordinated PdII nodes in the triply catenated assembly
carry no net charge.[18] Consequently, no further counter
anions are associated with the structure and the palladium
atoms are able to approach each other much closer than in the
charged double cages. The intermetallic distances within the
structures of [3X@Pd4L8] and {trans-[(PdBr2)2L2]}3 are given
in Figure 4 in order to illustrate this effect. The average Pd–Pd
distance in the chloride-templated double cage amounts to
6.73 ꢁ, whereas the average distance in the triple catenane is
4.36 ꢁ. Since the latter distance is significantly greater than
twice the van der Waals radius of Pd (1.63 ꢁ), direct contacts
between the metal centers are not expected to control the
formation of this peculiar product. Nevertheless, the obser-
vation of an almost linear stack of six Pd atoms around which
the bromide substituents are arranged in a helical fashion
indicates that this arrangement is favored over other con-
formations in which the metal centers are not aligned in this
fashion. In our interpretation, the main explanation for the
observed structure is the maximization of the stabilizing p–p
interactions between the ligand backbones in this arrange-
ment. For five of the six ligands, close p-stacking with the
neighboring carbazole moieties is observed in the X-ray
structure. One ligand is twisted away from its nearest
neighbor, which is a packing effect caused by the adjacent
catenane (for a depiction of the crystal packing see the
Supporting Information).
From a topological point of view, this triply catenated
structure is an L6n1 link according to the Thistlethwaite link
table.[19] Figure 1 compares the topologies of the well-known
trefoil knot (Figure 1c) and Borromean rings (Figure 1d),
both of which have been realized in a supramolecular context
before,[5] with the structure of the triple catenane {trans-
[(PdBr2)2L2]}3 obtained here (Figure 1e). Whereas the trefoil
knot consists of only one ring, both other topologies consist of
three individual rings. In the Borromean rings, when one
single ring is removed the other two rings become unlinked.
In the case of the triply catenated structure here, however, all
three rings are interlinked. To the best of our knowledge, this
topology has never been realized by metal-mediated self-
assembly in which the metal centers are components of the
macrocycles.[20]
At different concentrations, the halide trigger has two
distinct roles in this system: First, it mediates the formation of
interpenetrated double cages by templating the dimerization
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of the monomeric precursor cages. In this role, three halide
anions are encapsulated in the three equal-sized pockets of
the double cage, and are thus tightly sandwiched between the
four dicationic palladium centers by Coulomb interactions
yielding an alternating Pd2+-XÀ-Pd2+-XÀ-Pd2+-XÀ-Pd2+ stack.
At a higher concentration of the halide anions, a further
structural conversion kicks in. Under disassembly of the
double cages and partial release of the free ligand, a triply
interdigitated catenane is formed. Here, the halide anions
directly bind to the PdII centers as part of their square-planar
coordination sphere along with displacement of two pyridine
donors.
topology was determined by X-ray analysis. The observation
that three different self-assembled products are formed by
variation of the concentration of a small anionic additive
shows that the use of one multifunctional trigger can be
sufficient to create a notable degree of complexity in
a supramolecular system. The experiments described in this
work required nothing but a simple organic ligand, a palla-
dium source, and a halide salt. In our opinion, this serves as
a prime example of the promising potential of systems
chemistry.[1]
It should be noted that the palladium halides PdCl2 or Experimental Section
The synthesis of ligand L is described in the Supporting Information.
PdBr2 cannot be used to assemble the interpenetrated double
cages [3X@Pd4L8] directly, since these salts would introduce
too much halide into the system right from the start.
Consequently, this situation would be comparable to
a “halide-overtitrated” state of the previous experiment,
which, assuming that the system is in thermodynamic
equilibrium, should not contain double cages. Indeed, the
direct reaction of acetonitrile adducts of the palladium halides
(PdCl2 or PdBr2) with ligand L was found to yield nonpolar
products, which were found to have a much higher solubility
in chloroform than the charged single and double cages.
According to their simple NMR spectra (no signal splitting)
these products are most probably the uncharged macrocycles
trans-[(PdX2)2L2] (X = ClÀ, BrÀ; see the Supporting Informa-
tion).
Cage [Pd2L4] was obtained from ligand L by adding 0.5 equiv of
[Pd(CH3CN)4](BF4)2 in CD3CN and heating the mixture to 708C for
5 h. The halide anions were added as acetonitrile solutions of their
tetrabutylammonium salts and the samples were heated to 708C for
5 h after each titration step before the measurement of the NMR and
ESI mass spectra. Single crystals suitable for X-ray crystal structure
analysis were grown by slow evaporation of solutions of the self-
assemblies in acetonitrile. CCDC-1011498 and -1011499 contain the
supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Received: August 7, 2014
Published online: && &&, &&&&
Keywords: cage compounds · carbazole · host–guest systems ·
.
In conclusion, we have presented an interconverting
system of three self-assembled structures all based on the
same bis(pyridyl) ligand and varying Pd/halide ratios from 1:0
in the case of the monomeric cage [Pd2L4], to 4:3 in the case of
the interpenetrated double cage [3X@Pd4L8], and up to 1:2 in
the case of the triple catenane {trans-[(PdBr2)2L2]}3.
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À
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[18] This complicated the mass spectrometric analysis and obviously
caused low solubility in the polar solvent acetonitrile. Due to the
intricate equilibrium situation, presumably also including the
formation of the macrocycle trans-[(PdX2)2L2] and the [2]cate-
nane {trans-[(PdX2)2L2]}2, no yield for the triple-catenated {trans-
[(PdX2)2L2]}3 reaction product could be determined. An overall
yield for the formation of all {trans-[(PdBr2)2L2]}n (n = 1.3)
species by halide-induced precipitation, however, was estimated
based on elemental analysis (see the Supporting Information).
[19] a) See the “Knot Atlas” on http://katlas.math.toronto.edu/wiki/
L6n1; b) D. W. Summers, Stud. Phys. Theor. Chem. 1988, 54, 67.
[20] For an example of a metal-templated link with the same
topology see: C. Lincheneau, B. Jean-Denis, T. Gunnlaugsson,
Chem. Commun. 2014, 50, 2857.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Coordination Cages
R. Zhu, J. Lꢁbben, B. Dittrich,
G. H. Clever*
&&&& — &&&&
Stepwise Halide-Triggered Double and
Triple Catenation of Self-Assembled
Coordination Cages
A ball-shaped self-assembled coordina-
tion cage [Pd2L4] based on a carbazole
backbone and PdII cations is transformed
into an interpenetrated double cage
[3X@Pd4L8] upon addition of 1.5 equiv of
halide anions (X=ClÀ, BrÀ). Surprisingly,
further addition of the same halide anion
triggered a second structural transfor-
mation to yield the L6n1-linked {trans-
[(PdBr2)2L2]}3.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!