Chiral Self-Discrimination and Guest Recognition in Helicene-Based Coordination Cages

Article abstract of DOI:10.1002/anie.201812926

Chiral nanosized confinements play a major role for enantioselective recognition and reaction control in biological systems. Supramolecular self-assembly gives access to artificial mimics with tunable sizes and properties. Herein, a new family of [Pd₂L₄] coordination cages based on a chiral [6]helicene backbone is introduced. A racemic mixture of the bis-monodentate pyridyl ligand L¹ selectively assembles with Pd^II cations under chiral self-discrimination to an achiral meso cage, cis-[Pd₂L^1P₂L^1M₂]. Enantiopure L¹ forms homochiral cages [Pd₂L^1P/M₄]. A longer derivative L² forms chiral cages [Pd₂L^2P/M₄] with larger cavities, which bind optical isomers of chiral guests with different affinities. Owing to its distinct chiroptical properties, this cage can distinguish non-chiral guests of different lengths, as they were found to squeeze or elongate the cavity under modulation of the helical pitch of the helicenes. The CD spectroscopic results were supported by ion mobility mass spectrometry.

Full text of DOI:10.1002/anie.201812926

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Accepted Article
Title: Chiral Self-Discrimination and Guest Recognition in Helicene-
based Coordination Cages
Authors: Thorben R. Schulte, Julian J Holstein, and Guido H. Clever
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812926
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10.1002/anie.201812926
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Chiral Self-Discrimination and Guest Recognition in Helicene-
based Coordination Cages
Thorben R. Schulte, Julian J. Holstein and Guido H. Clever*[a]
Abstract: Chiral, nano-sized confinements play a major role for
enantioselective recognition and reaction control in biological systems. studied for properties related to their helical chirality.[16] While
Since their discovery in 1912,[15] helicenes have been widely
Supramolecular self-assembly gives access to artificial mimics with
tunable sizes and properties. Here, new family of [Pd2L4]
helicenes have shown appearance in several supramolecular
systems, they have never been used in the construction of
coordination-driven cages.[17] We herein demonstrate, that,
despite their highly twisted appearance, helicene-based bis
monodentate ligands can be used to assemble discrete [Pd2L4]
coordination cages exhibiting chirality-driven effects on their
assembly and guest binding. We further deliver the first example
of a homochiral interpenetrated [Pd4L8] dimer, comprising eight
interlocked helicenes.
a
coordination cages based on a chiral [6]helicene backbone is
introduced. The racemic mixture of bis-monodenate pyridyl ligand L1
assembles with PdII cations under chiral self-discrimination selectively
to an achiral meso cage cis-[Pd2L1P2L1M2]. Enantiopure L1 forms
homochiral cages [Pd2L1P/M4]. Longer derivative L2 forms chiral cages
[Pd2L2P/M4] with larger cavities, able to bind optical isomers of chiral
guests with different affinities. Owing to its distinct chiroptical
properties, this cage can distinguish non-chiral guests of different
lengths, as they were found to squeeze or elongate the cavity under
modulation of the helicenes’ helical pitch. CD spectroscopic results
are supported by ion mobility mass spectrometry. L2 was further found
to yield a unique homochiral, interpenetrated double-cage [Pd4L2P/M8],
as supported by NMR, MS and single crystal X-ray results.
Nanosized cages based on metallosupramolecular self-assembly
have become major players in host-guest chemistry owing to their
structural and functional variability and modular composition.[1]
Recent design-based approaches allow for positioning multiple
building blocks by thermodynamically controlled integrative self-
sorting.[2] In biological host-guest systems, enantioselective
recognition takes a pivotal role due to the inherent homochirality
of most natural compounds. Hence, the formation of synthetic
chiral hosts for enantioselective guest binding is not only of
fundamental interest, but provides the basis for the development
of selective sensors, transporters and catalysts.[3]
Numerous chiral hosts based on covalent macrocyclic
molecules such as cyclodextrins, cyclophanes and calixarenes
have been reported.[4] Chirality has also been reported to facilitate
the assembly of hydrogen-bonded organic cages.[5] More recently,
chiral metallo-supramolecular self-assembled rings and cages
have been introduced as selective receptors and enzyme-like
nanoreactors based on chiral backbones, auxiliaries, the inherent
chirality of stereogenic metal centers or the overall architecture.[6]
Upon metal coordination, racemic mixtures of ligands may
undergo chiral self-sorting,[7] leading to homochiral[8,9,10] or
heterochiral[10,11] assemblies. Beyond their use in enantioselective
recognition, chiral cages based on luminescent metal centers
have been shown to exhibit unique chiroptical properties.[9,12] With
respect to mechanically interlocked coordination cages,[13] reports
covering the implementation of homochirality are still scarce, with
Hardie’s dimer of cyclotriveratrylene-based coordination cages
serving as a notable example.[14]
Figure 1. a), b) Synthesis of ligands L1 and L2 from 2,15-dibromo[6]helicene 3
followed by separation into the P (red color) and M (green color) enantiomers.
Addition of stoichiometric amounts of PdII leads to quantitative formation of
different coordination cages, depending on the ligands’ enantiomeric
composition and length. Racemic L1 exclusively gives C1meso, whereas racemic
L2 leads to
a
statistical mixture of all possible stereoisomers
(PPPM/MMMP/PPMM/PMPM/PPPP/MMMM, shown in grey color). The
enantiopure ligands lead to the chiral coordination cages C1P/M, C2P/M and the
interpenetrated dimer DC2P/M
.
Ligands L1 and L2 were synthesized via Sonogashira cross
coupling reactions from literature-known 2,15-dibromo[6]helicene
(Figure 1) to yield racemic products which were separated into the
enantiomers by chiral HPLC (SI Figure S23).[18] Following our
previously reported routines, the bis-monodentate ligands were
tested for the formation of self-assembled products using
[Pd(CH3CN)4](BF4)2 as the metal source in different polar organic
solvents. Interestingly, in deuterated dmso the racemic mixture of
ligand L1 was found to assemble under chiral self-sorting
[a]
Dr. T. R. Schulte, Dr. J. J. Holstein, Prof. Dr. G. H. Clever
Faculty of Chemistry and Chemical Biology,
TU Dortmund University, Otto-Hahn-Str. 6, 44227 Dortmund
(Germany)
E-mail: guido.clever@tu-dortmund.de
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201812926
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quantitatively to achiral meso cage cis-[Pd2L1P2L1M2] (C1meso),
containing both ligand enantiomers in a 1 : 1 ratio, as confirmed
by 1H (Figure 2a) and NOESY NMR spectroscopy (Figure 4b). For
ωB97XD/def2SVP, PCM solvent: dmso) of C2P/M (20.1 Å) (Figure
4d) compared to C1P/M (10.4 Å) (Figure 4c). In case of racemic L2,
cage formation leads to splitting of all 1H NMR signals into several
sets, indicative for a lack of chiral self-sorting under formation of
a statistical mixture of isomeric species (Figure 2d). This picture
is supported by the clean appearance of the high-resolution ESI
mass spectrum of this mixture showing only peaks assignable to
1
all herein described cages, the H NMR signals of the pyridine
moieties (i.e. protons Ha and Hb) undergo a downfield shift upon
coordination to the palladium(II) cations. The formation of the
meso cage leads to splitting of all 1H NMR signals into two sets of
equal intensity. All signals could be assigned with the help of 2D
NMR techniques (COSY, NOESY, HSQC) indicating that the
upper and the lower half of the ligand L1 have ended up in a
different surrounding upon cage formation (Supporting
Information). The signal splitting can be explained via symmetry
considerations. The halves of the P helicenes and the halves of
the tetracationic [Pd2L2 ]4+ species, superimposable with the
4
spectrum of the homochiral cage C2P/M (SI Figure S14). The
absence of chiral self-discrimination encountered upon cage
formation from racemic L2 can be explained with the increased
distance between the helicene backbones (based on calculated
structures of cages C1 and C2, the closest H−H distance between
two neighboring backbones has increased from 2.39 Å to 6.20 Å).
In contrast to the results obtained in dmso, heating the
enantiopure ligand L2 with palladium(II) cations in acetonitrile was
found to lead to a splitting of all NMR signals into two sets of equal
intensity, thus indicating the formation of a chiral interpenetrated
cage DC2P/M (Figure 2g).[13] In addition, the high-resolution ESI
mass spectrum revealed signals for the dimeric species
M
helicenes facing each other have the same chemical
surrounding resulting in the same chemical shifts for the
corresponding protons. Compared to this, one half of the P
helicene facing the other half of the P helicene (same for M
helicenes) has a different chemical surrounding which explains
1
the twofold splitting in the H NMR spectrum. A tentative trans-
configured cage would not lead to such a splitting of the NMR
signals, since the resulting D2d symmetry would offer two C2 axes
perpendicular to the major C2 axis going through both Pd centers
that would allow converting the upper half of each ligand into its
lower part (SI Figure S7).
[3BF4@Pd4L2 ]5+ (Figure 3c).
8
Figure 2. 1H NMR spectra (400 MHz, dmso-d6, 293 K). a) C1meso, b) L1P/M, c)
homochiral C1P/M, d) statistical mixture of C2 stereoisomers, e) L2P/M, f)
homochiral C2P/M and g) the homochiral interpenetrated cage structure DC2P/M
(here: 600 MHz, acetonitrile-d3, 293 K).
In contrast, the assembly with the enantiopure ligand L1, either in
its P or M form leads to a homochiral cage with no splitting of the
1H NMR signals (Figure 2c). In their high resolution ESI mass
spectra, cage C1meso (SI Figure S9) and enantiopure cages C1P/M
Figure 3. ESI mass spectra of a) cage C1M, b) cage C2M after addition of (1R)-
camphor sulfonate G1R, c) double cage DC2M.
(SI Figure S11) could be identified as tetracationic [Pd2L1 ]4+.
4
Next, chiral guest discrimination of C1P was tested with (1R)
and (1S) camphor sulfonate anions G1R/S, however, no evidence
for uptake of the guests was found (SI Figure S21). Most probable
reason is the limited size of the cavity, known as a critical factor
for guest binding.[19] To permit guest encapsulation, the ligand
structure was extended by including 1,4-phenylene linkers on
both sides to give ligand L2. The elongation of the ligands nearly
doubles the Pd−Pd distance in the modelled structures (DFT
Further structural insight was delivered by X-ray diffraction
methods. Crystals of enantiopure L2 (second HPLC fraction
eluted from a Chiralpak IC column) suitable for X-ray structure
analysis could be obtained by crystallization from dmso (Figure
4e). The asymmetric unit contained twelve individual helicene
ligands, all of which are highly intertwined in a remarkably
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10.1002/anie.201812926
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unordered fashion (SI Figure S25). The absolute configuration
was unambiguously determined as the (P) enantiomer using the
method of Parsons[20] as implemented in SHELXL,[21] yielding an
enantiopurity distinguishing parameter of x = 0.079(8). This
assignment is in agreement with measured circular dichroism
(CD) spectra of this compound as compared to published data on
similarly substituted [6]helicenes and DFT-calculated CD
bands.[22,23]
were observed (SI Figure 22) and the results are summarized as
a comparison of binding isotherms (Δꢀ plot; Figure 5c). Pleasingly,
both guest enantiomers showed different binding behavior when
exposed to the same chiral cage, however, the combination
G1S@C2P showed the same behavior as the enantiomeric system
G1R@C2M, with binding constants of around 560 M−1.[25] The
diastereomeric combinations to this, G1R@C2P and G1S@C2M,
showed a stronger extent of NMR signal shifting and a binding
constant of around 1010 M−1. In the high resolution ESI mass
spectra, the host-guest complexes could be identified as the triple
Single crystals of the dimeric cage species [2PF6@Pd4L2M
]
8
(DC2M, based on the M ligand enantiomer eluting first from the
chiral column), suitable for X-ray structure analysis were obtained
by slow diffusion of diethyl ether into an acetonitrile solution of the
cationic species [G1@Pd2L2 ]3+ (Figure 3b).
4
Furthermore, CD spectra were compared for L1P/M and C1P/M
(SI Figure S24) and L2P/M and C2P/M (Figure 5a), showing a strong
circular dichroism for the ligands and the cages with a positive
Cotton effect for the (P) enantiomers. We next set out to
investigate the potential utilization of the strong CD effect as an
indicator for the discrimination of achiral guests. Since the cages
consist of four helicenes arranged like parallel springs around two
connecting Pd(II) cations, we envisioned that charged guests
encapsulated between these electrostatic anchors should
modulate the helical pitch of the ligand backbones. First, we
compared the effect of binding short 2,7-naphthalene disulfonate
G2 and long 4,4'-biphenyl disulfonate G3 on the CD spectra of
C2P. Difference spectra revealed that encapsulation of the shorter
guest leads to a decrease of CD band intensity around 360 nm
while binding of the longer guest increases intensity of the same
band (Figure 5b). The assumption that such an effect is caused
by tuning the helicenes’ helical pitch was predicted by theoretical
work of Mori and Inoue et al.[22] We were able to confirm this
hypothesis by calculating the relative CD signal intensities of
unsubstituted [6]helicene under variation of its helical pitch within
the limits found in the twelve individual ligands contained in the
solid state structure of L2 (SI Figure S26 and Figure 4e).
–
cage containing PF6 counter anions (Figure 4f). Synchrotron
radiation was required for obtaining diffraction data that could be
solved with direct methods using SHELXT.[24] Again, the absolute
configuration could be unambiguously determined, yielding an
enantiopurity distinguishing parameter of x = −0.02(2). CD data
was found to be in agreement with the literature reported absolute
structure assignment of comparable helicenes.[22,23] The structure
reveals that the double cage features three consecutive pockets,
the two outer ones filled with a PF6− anion. The Pd–Pd distances
are 8.66 Å for the outer pockets and 10.33 Å for the inner cavity.
Figure 4. a) DFT-calculated structure of C1meso, b) NOESY NMR detail of the
C1meso cage supporting the cis ligand arrangement, c) and d) calculated
structures of C1M and C2M. e) One of twelve L2P molecules in the asymmetric
unit of its solid state structure with found min/max. helical pitches. f) X-ray crystal
structure of DC2M, side and top view along the Pd4 axis (Pd: grey, N: blue, C:
green (M enantiomer), red (P enantiomer), P: orange, F: light green).
Figure 5. a) Circular dichroism spectra of ligands L2P/M and cages C2P/M and b)
difference CD spectra (free host CD subtracted from host-guest CD) of G2@C2P
and G3@C2P as well as G4trans@C2P and G4cis@C2P (all in dmso). c)
Comparison of binding isotherms for all four diastereomeric host-guest
combinations G1R/S@C2P/M showing two ‘matched’ and two ‘mismatched’
cases; d) Superposition of mobilograms obtained by trapped ion mobility ESI-
TOF mass spectrometry for host-guest complexes G2@C2P (mobility 1/K0:
1.736 Vs/cm2, CCS: 701 Å2 at m/z 1615.4) and G3@C2P (mobility 1/K0: 1.745
Vs/cm2, CCS: 705 Å2 at m/z 1627.9).
Having the large cavity of monomeric C2P/M in hand, chiral guest
1
discrimination could be shown for the enantiopure cages via H
NMR titration experiments by stepwise addition of camphor
sulfonates G1R and G1S as their tetrabutyl ammonium salts.
Characteristic downfield shifts for the inside-pointing proton Ha
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10.1002/anie.201812926
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Further, direct evidence for a shrinking and expansion of the
cages upon addition of the short and long guest, respectively,
came from trapped ion mobility ESI-TOF mass spectrometry
(timsTOF), which indicates a smaller gas phase collisional cross
section for G2@C2P (701 Å2) than for G3@C2P (705 Å2), even in
a mixture of both host-guest complexes (Figure 5d and SI Figure
S27).[26] We repeated the CD experiment with azobenzene-based
guest G4,[27] either in its cis or trans photoisomeric form (Figure
5b). Remarkably, the effect of band intensity decrease/increase
could be reproduced and allows the differentiation between the
cis and the trans form of achiral azobenzene via CD spectroscopy,
keeping in mind that the free guest itself shows no CD effect. In
addition, observed deviations from the expected band shapes
were attributed to a certain degree of chirality transfer on the
azobenzene chromophore which – in contrast to guests G2 and
G3 – shows significant absorption around 360 nm.
crystallization experiments, Prof. W. Hiller (TU Do), Dr. M. John
(GAU Gö) for help with NMR spectroscopy and L. Schneider (TU
Do), C. Heitbrink, Dr. P. Janning (MPI Do) and Dr. H. Frauendorf
(GAU Gö) for ESI mass spectra. Diffraction data for DC2M was
collected at PETRA III, DESY a member of the Helmholtz
Association
(HGF).
The
authors
thank
Saravanan
Panneerselvam for assistance in using synchrotron beamline P11,
(I-20160736).[29]
Keywords: chirality • anion recognition • host-guest chemistry •
interpenetration • supramolecular chemistry
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In summary, a new family of [Pd2L4] coordination cages based
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Cages C1 and C2 were formed by addition of [Pd(CH3CN)4](BF4)2
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in MeCN at 7 °C. CCDC 1558206 (L2P) and CCDC 1581540
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This work has been supported by the Deutsche Forschungs-
gemeinschaft (CL 489/2-2, RESOLV Cluster of Excellence EXC-
2033 – project number 390677874) and the European Research
Council through ERC Consolidator grant 683083 (RAMSES). We
thank Prof. U. Diederichsen, Prof. L. Ackermann and Prof. L.
Tietze (all Georg-August University Göttingen) for access to CD
and HPLC facilities. We further thank S. Löffler for support with
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10.1002/anie.201812926
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Thorben R. Schulte, Julian J. Holstein
and Guido H. Clever*
Page No. – Page No.
Chiral Self-Discrimination and Guest
Recognition in Helicene-based
Coordination Cages
Chiral [6]helicenes serve as building blocks for the assembly of [Pd2L4] coordination
cages and interpenetrated [Pd4L8] dimers. Depending on the ligand length, chiral
self-discrimination and recognition of enantiomeric guests is observed. Helical pitch
modulation allows the discrimination of non-chiral guests by a combination of CD
spectroscopy and ion mobility mass spectrometry.
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