Coal-Tar Dye-based Coordination Cages and Helicates

Article abstract of DOI:10.1002/anie.202015246

A strategy to implement four members of the classic coal-tar dye family, Michler's ketone, methylene blue, rhodamine B, and crystal violet, into [Pd₂L₄] self-assemblies is introduced. Chromophores were incorporated into bis-monodentate ligands using piperazine linkers that allow to retain the auxochromic dialkyl amine functionalities required for intense colors deep in the visible spectrum. Upon palladium coordination, ligands with pyridine donors form lantern-shaped dinuclear cages while quinoline donors lead to strongly twisted [Pd₂L₄] helicates in solution. In one case, single crystal X-ray diffraction revealed rearrangement to a [Pd₃L₆] ring structure in the solid state. For nine examined derivatives, showing colors from yellow to deep violet, CD spectroscopy discloses different degrees of chiral induction by an enantiomerically pure guest. Ion mobility mass spectrometry allows to distinguish two binding modes. Self-assemblies based on this new ligand class promise application in chiroptical recognition, photo-redox catalysis and optical materials.

Full text of DOI:10.1002/anie.202015246

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Accepted Article
Title: Coal-Tar Dye-based Coordination Cages and Helicates
Authors: Irene Regeni, Bin Chen, Marina Frank, Ananya Baksi, Julian
J. Holstein, and Guido H. Clever
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202015246
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Coal-Tar Dye-based Coordination Cages and Helicates
Irene Regeni,[a] Bin Chen,[a,b] Marina Frank,[a] Ananya Baksi,[a] Julian J. Holstein,[a] and Guido H.
Clever*[a]
Dedicated to the memory of Prof. Dr. François Diederich
[a]
[b]
I. Regeni, Dr. B. Chen, Dr. M. Versäumer (née Frank), Dr. A. Baksi, 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
Current Affiliation: Dr. B. Chen, State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-
X), Soochow University, Suzhou 215123, China.
Abstract: A strategy to implement four members of the classic coal-
tar dye family, Michler´s ketone, methylene blue, rhodamine B and
crystal violet, into [Pd2L4] self-assemblies is introduced.
Chromophores were incorporated into bis-monodentate ligands using
piperazine linkers that allow to retain the auxochromic dialkyl amine
functionalities required for intense colors deep in the visible spectrum.
Upon palladium coordination, ligands with pyridine donors form
lantern-shaped dinuclear cages while quinoline donors lead to
strongly twisted [Pd2L4] helicates in solution. In one case, single
crystal X-ray diffraction revealed rearrangement to a [Pd3L6] ring
structure in the solid state. For nine examined derivatives, showing
colors from yellow to deep violet, CD spectroscopy discloses different
degrees of chiral induction by an enantiomerically pure guest. Ion
mobility mass spectrometry allows to distinguish two binding modes.
Self-assemblies based on this new ligand class promise application
in chiroptical recognition, photo-redox catalysis and optical materials.
The so-called “coal-tar dyes” (such as di- or triarylmethanes,
xanthenes and phenothiazine derivatives) belong to the first
block-buster products of the emerging chemical industry in the
19th century. Until today, they are produced on large scales and
find widespread application as redox and pH indicators,[19]
photosensitizers,[20] dye-based laser materials,[21] for tissue
staining[22] and imaging purposes,[23] aside from their industrial
application for coloring e.g. fabrics, polymers, paper, hair and food.
In the context of self-assembled systems, however, they turn out
to be underrepresented as integral structural elements and have
1
been merely used as additives, e.g. as photosensitizers for O2
generation.[24] Non-covalently encapsulated in metal-organic
hosts, such dyes were for example employed for proton reduction
in combination with cobalt nodes in the architecture.[25]
Full integration of these chromophores into bridging positions
of metallo-supramolecular structures, however, requires
functionalization with at least two distinct coordinating moieties.
When examining the general structure of the herein studied dyes
(box in Figure 1), it becomes apparent that the attachment of
typical linkers such as alkynes or arenes is not feasible. In
particular, removing the auxochromic dialkyl amines would not be
tolerated, as they are fundamental parts of the delocalized π-
system, giving rise to intense colors in the visible region.
Organic chromophores have been integrated into supramolecular
architectures to equip them with various functions. For example,
artificial mimics for solar energy conversion often use porphyrin
arrays as light harvesting units, capable of energy transfer and
charge separation.[1] Organic materials based on the non-
covalent arrangement of compounds with specific photophysical
properties have been proposed as active components for displays,
light sources and optoelectronic circuits.[2] Numerous examples
for receptors with optical read-out, photoswitchable self-
assemblies and light-controlled molecular machines have been
reported.[3] Recently, photo-redox chemistry started to take
advantage of the supramolecular organization of organic or metal-
based chromophores into functional systems, e.g. for catalytic
oxidation of alcohols[4] or cycloadditions to anthracenes.[5] Dye-
based assemblies have further been studied for bio-imaging[6] and
medicinal applications such as photodynamic therapy.[7]
With respect to discrete coordination cages with an accessible
cavity (such as the popular [Pd2L4] systems),[8] radical reactions
have been realized in hosts where side-walls can be brought to
excited states by irradiation with light.[9] Cages have also been
used as photosensitizers for singlet O2 generation,[10] and as
reagents for photochemical hydrogen formation.[11] Most reported
systems, however, are based on chromophores that absorb in the
UV to short-wavelength visible region, giving rise to colorless or
yellowish compounds. Exceptions include assemblies where
highly colored charge-transfer complexes are part of metal
nodes,[12] or metallo-ligands[13] and architectures based on
photochromic diarylethenes (convertible between faint yellow and
deep blue photoisomers)[14] perylenes,[15] porphyrins,[16]
triarylamines[17] and BODIPY.[18]
Figure 1: a) Design of of dye-based, bis-monodentate ligands (DYE-P) as
opposed to ligand with alkyne bridges; b) formation of [Pd2L4] cages; c) organic
chromophores chosen for this study: Michler’s ketone MK, crystal violet CV,
methylene blue MB (in blue/oxidized/cationic and colorless/reduced/uncharged
states) and rhodamine B RB (in pink/zwitterionic and colorless/lactone forms).
1
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Here we show, how the implementation of piperazine linkers
between coal-tar dye-based backbones (Michler’s ketone MK,
crystal violet CV, methylene blue MB, rhodamine B RB) and metal
coordinating donors allows to retain the typical chromophore
characteristics (Figure 1a).[26] Furthermore, as aliphatic linkers,
piperazines should electronically decouple backbones from
donors and at the same time not interfere with coordination to the
metal centers, despite their ethylene diamine-like substructure.
Two choices of donor groups (3-pyridyl P and 8-isoquinolinyl Q)
were examined, giving rise to eight (plus one derivative) ligands
that were all found to form intensely colored Pd(II)-mediated
cages or helicates (ligand names are composed of backbone and
donor label; see Figure S1 in the SI for all ligand structures). We
Ligand RB-P was synthesized in only two steps by a Buchwald-
Hartwig amination of literature-described tosylated fluorescein
with commercially available 3-piperazyl-pyridine. Following the
same synthetic pattern, ligand MK-P was obtained from 4,4’-
dihydroxybenzophenone. Ligand CV-P was synthesized through
a Grignard reaction of 4-MgBr-N,N-dimethylaniline on ligand
MK-P. The synthesis of MB-P followed a procedure already
reported[27] for substituted MB derivatives. Both CV-P and MB-P
carry one positive charge and, as a final step of the synthesis,
their halogen counter anions were exchanged for non-
–
–
coordinating anions (BF4 or NO3 ) by metathesis reactions with
the corresponding silver salts.
Quantitative formation of cages [Pd2(RB-P)4], [Pd2(MK-P)4],
[Pd2(MB-P)4] and [Pd2(CV-P)4] was achieved by mixing
respective ligands RB-P, MK-P, MB-P and CV-P in a 2 : 1 ratio
with palladium(II) salts [Pd(CH3CN)4](BF4)2 or Pd(NO3)2 in
DMSO-d6 at 70 °C for 15 min. In full accordance with our
previously reported Pd(II)-based cages,[28] coordination of ligands
to the metal centers was indicated by downfield shifting of the
pyridine 1H NMR signals (in particular Ha and Hb, SI and Figure
2a for [Pd2(RB-P)4]). 1H DOSY NMR and high-resolution ESI
mass spectrometry further supports the formation of the desired
products (SI and Figure 2b). Slow diffusion of toluene in the
DMSO solution of [Pd2(MK-P)4] (Figure 2c), EtOAc in the DMSO
solution of [Pd2(RB-P)4] (Figure 2d), and of MTBE in the DMF
solution of [Pd2(CV-P)4] (Figure 2e) afforded single crystals
suitable for X-ray diffraction. In the structure of [Pd2(MK-P)4], all
piperazines show a disorder concerning their chair-type ring flip
(occupancies ∼50:50), revealing a certain flexibility of the linkers.
The crystal structure of [Pd2(RB-P)4] reveals two possible
orientations for the spirolactone group in each ligand, giving rise
to four possible isomers (Figure S46) which is consistent with the
broadened 1H NMR spectrum observed for this cage.
further show, how
a chiral anionic guest translates its
configurational information onto the assemblies and is able to
discriminate between lantern-shaped pyridyl cages and twisted
isoquinolinyl helicates.
MB is known to be easily reduced to its colorless leuco form,
prompting us to investigate whether this behavior is retained in
cage [Pd2(MB-P)4]. Indeed, treating cage [Pd2(MB-P)4] with Zn
powder induced partial decoloration and revealed a statistical
distribution of oxidized (MB-P) and reduced (MBR-P) ligands by
ESI MS analysis, with observed signals described by the formula
{[Pd2(MB-P)x(MBR-P)4-x](BF4)4+x-z}z+(x=1..3; z=2..4; Figure S63).
Next, we introduced isoquinoline donors in order to bestow
the [Pd2L4] assemblies with a helical twist (Figure 3). According to
the above mentioned procedures, ligands MK-Q, RB-Q, MB-Q,
CV-Q were synthesized using 8-(piperazin-1-yl)isoquinoline.
Corresponding [Pd2(DYE-Q)4], helicates were assembled as
described for the pyridine derivatives, but differ from the latter by
their characteristic dynamic behaviour: As previously reported,[29]
[Pd2L4] coordination cages featuring inward-pointing isoquinolin-
8-yl donors adopt a helical structure with ligands twisting around
the Pd2-axis, resulting in M- and P-configured chiral conformers.
Figure 3b) depicts the 1H NMR spectrum of [Pd2(MB-Q)4] at
different temperatures in DMSO-d6. At r.t., the -CH2- protons of
the piperazine moieties split into eight broad signals, resulting
from slow ring flip dynamics and their diastereotopic nature within
the chiral environment. At 75 °C all eight signals coalesce into two
broad peaks, as expected for a situation where both the
piperazine ring flip as well as the overall cage twist are fast on the
NMR time scale. We assumed these dynamics to be dependent
on the solvent. Indeed, the 1H NMR spectrum of [Pd2(MB-Q)4] in
D2O at r.t. shows only two signals (Figure S99), indicating faster
exchange dynamics in the aqueous medium.
Figure 2: a) 1H NMR spectra in DMSO-d6 of ligand RB-P and cage
[Pd2(RB-P)4]; b) ESI-MS spectrum of cage [Pd2(RB-P)4]. Side view of X-ray
crystal structures of c) [Pd2(MK-P)4], d) [Pd2(RB-P)4] (only one occupancy of
disordered backbone shown) and e) [Pd2(CV-P)4]; f) DFT (B3LYP/def2-SV(P))
model of [Pd2(MB-P)4]. Counter anions and solvent molecules omitted.
2
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For [Pd2(MK-Q)4], a similar NMR behaviour was observed (Figure
S94). By slow vapor diffusion of toluene into the DMSO solution
of [Pd2(MK-Q)4], small yellow crystals suitable for synchrotron X-
ray diffraction were obtained. While ESI-MS and DOSY NMR
clearly support the dinuclear [Pd2(MK-Q)4] stoichiometry in
solution (Figures S90-93; model Figure 3c), the obtained solid-
state structure surprisingly revealed formation of a three-
membered ring [Pd3(MK-Q)6] (Figure 3e). We assume this
structural rearrangement to occur in the course of the
crystallization process, facilitated by the conformational flexibility
of the MK backbone.
Next, we analyzed cage [Pd2(RB-Q)4]. Its 1H NMR spectrum at r.t.
is broadened due to signal coalescence. At higher temperatures,
some signals sharpen, while at lower temperatures signals
sharpen but show more complicated splitting patterns. Since its
ESI-MS analysis indicated the exclusive formation of [Pd2(RB-
Q)4], we assign the convoluted NMR behavior to the isomerism of
the spirolactones (as discussed for [Pd2(RB-P)4]), in combination
with dynamic interconversion between two chiral conformers
(Figure S97). A most interesting feature of RB is its solvent
polarity-dependent transformation between
a
colorless
spirolactone form and an intensely pink zwitterionic open form
(Figure 1). Indeed, exposing a colorless solution of [Pd2(RB-P)4]
in DMSO (Reichardt solvent polarity ET = 0.44)[30] to increasing
N
N
N
amounts of acetonitrile (ET = 0.46), methanol (ET = 0.76) or
N
water (ET = 1.00) leads to stepwise increase of the typical pink
color (Figure 4a). Since corresponding NMR spectra showed
strong signal broadening, hampering quantification of the fraction
of opened chromophores, we converted ligand RB-P into ligand
RE-P in which the colored open form is secured by esterification
of the carboxylic moiety. The corresponding cage [Pd2(RE-P)4]
could then be formed and unambiguously characterized as
described for the other systems (see section 3.2.5 in the SI).
Next, we recorded 1H DOSY NMR spectra of the colored
DYE-P and DYE-Q ligands and corresponding assemblies to
compare their solution state dimensions. Interestingly, while
ligands DYE-Q are larger than DYE-P in terms of their larger
donor group, [Pd2(DYE-Q)4] helicates are characterized by
slightly smaller hydrodynamic radii as compared to their DYE-P
siblings (Table S2). This observation is in line with the X-ray/DFT
structure results, showing a more globular shape with larger cavity
for the [Pd2(DYE-P)4] cages and twisted structures with smaller
and less accessible cavities for the [Pd2(DYE-Q)4] helicates
(Figures 2, 3 and S115).
Figure 4: Photos of 0.7 mM solutions of a) [Pd2(RB-P)4] from pure DMSO to
DMSO:D2O 9:1 (left to right). b) UV-Vis absorption spectra of all nine colored
assemblies in DMSO; c) 0.7 mM DMSO solutions (order as labels in b).
Figure 3: a) Generic bis-isoquinolinyl ligand (DYE-Q) forming a helical cage in
two enantiomeric forms; b) 1H NMR spectra in DMSO-d6 of MB-Q and [Pd2(MB-
Q)4] at indicated temperatures. DFT models (B3LYP/def2-SV(P) of c) [Pd2(MK-
Q)4] and d) [Pd2(MB-Q)4]; e) top and side views of X-ray crystal structure of
[Pd3(MK-Q)6]. Counter anions and solvent molecules omitted.
Owing to the systems’ 3D chromophore arrangement and strong
absorption in the visible spectrum (Figures 4b and c), we
examined their application in the chiral recognition of small
3
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molecules. First, encapsulation of (R)-camphor sulfonate (CSA)
in cages [Pd2(RB-P)4] and [Pd2(RE-P)4] in DMSO could be verified
by H NMR spectroscopy (shift of inward-pointing host protons)
[Pd2(MB-Q)4], showing that the guest has a stronger preference
to interact with the pink globular cage rather than the blue helical
one. The clearly distinguishable CD signatures of both cages
allowed to follow this discrimination process in a titration
experiment: while the induced CD band at 574 nm increases until
one equivalent of guest is added, the one at 693 nm shows its
steepest ascend after the 1:1 stoichiometry is reached (Figure 5b).
We ascribe this to the propensity of (R)-BINSO3 to bind inside the
cage cavity of the more globular [Pd2(DYE-P)4] systems (as
confirmed by 1H NMR titrations), while for the quinolinyl-based
helicates, we propose rather weak outside binding. In fact, titration
of (R)-BINSO3 into the [Pd2(MB-Q)4] solution leads to a mere
decrease of all NMR signals, indicative for an anion-induced
aggregation of the cationic assemblies. Moreover, while ESI-MS
spectra of (R)-BINSO3 with cages [Pd2(DYE-P)4] show only peaks
for the host-guest complexes, for helicates [Pd2(DYE-Q)4], the
respective peaks are either not found at all (for [Pd2(MK-Q)4] and
[Pd2(RB-Q)4]) or are rather small and accompanied by multiple
peaks of the free host. Here, trapped ion mobility mass
spectrometry (TIMS) proved helpful to differentiate the proposed
binding modes: While inside encapsulation led to contraction of
the cage-guest complex, outside association caused a collisional
cross section (CCS) increase, instead (Figure 5c and SI).
1
and ESI-MS spectrometry. However, neither of these host-guest
complexes showed a CD signature in the region 250-800 nm,
which would be indicative of a significant guest-to-host chirality
transfer. As similar observations were made for all other seven
cages (see SI for ESI-MS and CD spectra), we assume that the
rather small, singly charged (R)-CSA molecule is not able to
induce preferred population of one diastereomeric host-guest
complex over the other.[31] We then tested the larger, double
negatively charged guest (R)-1,1'-binaphthyl-2,2'-disulfonate
(BINSO3). Equimolar solutions of each of all nine colored cages
together with (R)-BINSO3 were prepared in DMSO and a
selection of CD and UV-Vis absorption spectra is shown in Figure
5a. In spectral regions were the cages (but not the guest) absorb,
induced CD signals originate from guest-to-host chirality transfer
(for further spectra see the SI).
In conclusion, we report nine [Pd2L4] assemblies based on a new
family of functional bis-monodentate ligands, designed from coal-
tar dyes, bridged via auxochromic piperazines to two types of N-
donors. Pyridine-based ligands afford lantern-shaped cages with
accessible cavities while isoquinoline-based ligands lead to
racemic mixtures of strongly twisted helicates. All assemblies
were shown to maintain the electronic structure of the parental
dyes which was exploited in the chiral recognition of small anions.
NMR and CD titration as well as ion mobility mass spectrometry
indicate that the chiral anions interact via an encapsulation mode
for the cages and outside-association mode for the helicates. This
new class of dye-functionalized assemblies, spanning a color
spectrum from yellow to blue/violet, based on chromophores with
a well-established application range, sets the stage for the
development of photo-redox catalysts with confined reaction
chambers, light harvesting devices as well as diagnostic tools for
the chiroptical detection of biomolecules and -polymers.
Acknowledgements
I.R. thanks the Fonds der Chemischen Industrie (FCI) for a Kekulé
PhD fellowship. We thank the European Research Council for
support through ERC Consolidator grant 683083 (RAMSES). This
work was funded by the Deutsche Forschungsgemeinschaft
(DFG) under Germany’s Excellence Strategy – EXC 2033 –
390677874 – RESOLV. We thank Jerome Günes, Chuan Dong,
Lorenz Hündgen, Laura Schneider, André Platzek for help in
syntheses and analyses and Prof. Dr. Lars Schäfer, Ruhr-Uni
Bochum, for assisting with computations. Diffraction data of
[Pd2(RB-P)4], [Pd2(CV-P)4] and [Pd3(MK-Q)6] was collected at
PETRA III, DESY (Hamburg, Germany) a member of the
Helmholtz Association. We thank Eva Crosas, Sofiane Saouane
and Anja Burkhardt for assistance at beamline P11 (I-20180412,
I-20191452, I-20180990).[32]
Figure 5: a) CD and UV-Vis absorption spectra of 18.75 μM cage (indicated in
the legend) solution with 1 equiv. of guest (R)-BINSO3 in DMSO at 25 °C. b) CD
intensities at 574 nm and 693 nm of a 18.75 μM 1:1 mixed solution of cages
[Pd2(RE-P)4] and [Pd2(MB-Q)4] at increasing amounts of (R)-BINSO3 in DMSO
at 25 °C. c) Trapped ion mobility experiments showing guest encapsulation for
cage [Pd2(RE-P)4] vs. outside association for helicate [Pd2(MB-Q)4].
Next, we examined differential guest binding preference by
adding (R)-BINSO3 to a kinetically-trapped, narcissistic 1:1
mixture of lantern-shaped [Pd2(RE-P)4] and helical-shaped
4
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10.1002/anie.202015246
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
Somewhere over the rainbow: take a dye, give it arms and a metal, cages will please your eye! A family of classic organic dyes,
known since the advent of modern chemistry, was integrated into 3-dimensional, self-assembled nano objects via a novel piperazine-
based linker strategy that conserves the auxochromic substituents. The dynamic nature of the lantern-shaped cages and helicates
allows to recognize enantiopure guests via chirality transfer to the host, detected by circular dichroism response far in the visible region.
Researcher Twitter username: @guido_clever
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This article is protected by copyright. All rights reserved.