A Helicate-Based Three-State Molecular Switch

Article abstract of DOI:10.1002/anie.201806607

The control of structural transformations triggered by external signals is important for the development of novel functional devices. In the present study, it is demonstrated that helicates can be designed to structurally respond to the presence of differ

Full text of DOI:10.1002/anie.201806607

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: A helicate based three-state molecular switch
Authors: Xiaofei Chen, Thomas Gerger, Christoph Räuber, Gerhard
Raabe, Christian Göb, Iris Oppel, and Markus Albrecht
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201806607
Angew. Chem. 10.1002/ange.201806607
Link to VoR: http://dx.doi.org/10.1002/anie.201806607
http://dx.doi.org/10.1002/ange.201806607

10.1002/anie.201806607
Angewandte Chemie International Edition
COMMUNICATION
A helicate based three-state molecular switch
Xiaofei Chen,[a] Thomas M. Gerger,[a] Christoph Räuber,[a] Gerhard Raabe,[a] Christian Göb,[b] Iris M.
Oppel,[b] Markus Albrecht*[a]
Dedicated to Prof. H. J. Knölker on the occasion of his 60th birthday.
Abstract: The control of structural transformations triggered by
external signals is important for the development of novel functional
devices. In the present study, it is demonstrated that helicates can
be designed to structurally respond to the presence of different
counter ions and to adopt either a compressed or an expanded
structure. Reversible switching is not only possible between those
two states, furthermore, the twist of the aggregate also can be
controlled. Thus, three out of four possible states of a helicate
(expanded/left-handed, expanded/right-handed, compressed/left-
handed) based on an enantiomerically pure ester bridged
dicatecholate ligand are specifically addressed by introduction,
exchange or removal of counter cations. This is used in order to
reversibly switch between the different states or to successively
address them.
Supramolecular chemistry offers approaches towards the
production of dynamic molecular ensembles like catenanes1 or
rotaxanes2 acting as molecular switches,3 rotors4 or muscles.5
All those types of supramolecular ensembles represent devices
showing some dynamic behavior and may be switched to
undergo some structural transformation. Such behavior can be
also observed in oligomers/polymers6 or even in DNA.7
In 2010 Yashima described a helicate8 which sodium dependent
could be switched in size and it was described as a “molecular
spring”.9 In this case the switching process proceeds with
conversion of stereochemistry. Since 2005 we study
hierarchically assembled catecholate based helicates.10,11 (Fig.
1a). In order to obtain a lithium dependent molecular switch, two
monomeric complex units of the hierarchically assembled
helicates are connected by appropriate spacers (Fig. 1b,c). The
stereochemistry of the helicate can be well controlled, if the
enantiomerically pure ligand 4-H4 is used12 and it is possible to
address three different states (compressed left-handed,
expanded right-handed and expanded left-handed). Although
some systems are already described which show inversion of
stereochemistry upon an external input,13 the behavior of the
present switch is unprecedented.
Figure 1. (a) A hierarchically assembled helicate, (b) Ligands 1a/b-H2 and 2-4-
H4 and (c) compressed Li[Li3Ti2(3/4)3] as well as expanded K4[Ti2(3/4)3] or
[Li(2.1.1)]4[Ti2(3/4)3] ((2.1.1) = [2.1.1] cryptand).
[a]
[b]
X. Chen, T. M. Gerger, Dr.C. Räuber, Prof. Dr. G. Raabe, Prof. Dr.
M. Albrecht
Institut für Organische Chemie
RWTH Aachen University
Landoltweg 1, 52074 Aachen, Germany
E-mail: markus.albrecht@oc.rwth-aachen.de
C. Göb, Prof. Dr. I. M. Oppel,
Institut für Anorganische Chemie
RWTH Aachen University
Landoltweg 1, 52074 Aachen, Germany
Figure 2. Structure of the compressed anion [Li3(2)3Ti2]- as found in the crystal
(O: red, Ti: yellow, C and H atoms of the different ligand strands are shown in
magenta, green and blue).
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201806607
Angewandte Chemie International Edition
COMMUNICATION
Synthesis and characterization. Compressed helicates
Li[Li3(2-4)3Ti2] are prepared from titanoyl bisacetylacetonate, 2-
4-H4 and Li2CO3. The crystal structure of Li[Li3(2)3Ti2] (Fig. 2)
reveals that the alkyl spacer connects two catechol units binding
to different titanium(IV) centers but only to one lithium cation.
The corresponding expanded complexes K4[(3,4)3Ti2] are
similarly made using K2CO3 instead of Li2CO3. With ligand 2, no
defined potassium complex is obtained. Due to the close
distance of the charged Ti-complex units of K4[(2)3Ti2], internal
binding of K-cations destabilizes the helicate leading to
undefined material.
1H NMR spectra of compressed Li[Li3(3)3Ti2] ((OCH2) = 3.82
and 2.95 ppm, diastereotopic behaviour) and expanded
K4[(3)3Ti2] ((OCH2) = 4.07 ppm, non-diastereotopic) in DMSO-
d6 are remarkably different. The non-diastereotopic behaviour of
the O-methylene protons in case of the potassium salt is
characteristic for the expanded form in which the titanium
complexes can easily racemize.11
Addition of [2.1.1] cryptand14 to Li[Li3(3)3Ti2] changes the NMR
spectrum significantly ((OCH2) = 4.16 ppm, non-diastereotopic)
revealing that lithium cations have been extracted from the
complex leading to expansion and fast helix inversion.
With the chiral ligand 4-H4 again 1H NMR spectra were obtained
for Li[Li3(4)3Ti2], [Li(2.1.1)]4[(4)3Ti2] and K4[(4)3Ti2] with the
spectra of the compressed complex being very different to the
two expanded ones (Fig. 3a).
For Li[Li3(4)3Ti2] only one peak is observed for [Li3(4)3Ti2]- (m/z =
1900.69) in the negative ESI MS (methanol). The peak vanishes
after addition of an excess [2.1.1] cryptand. In the expanded
potassium salt, some of the cations are easily exchanged by
others (m/z = 1920.62 [H2K(4)3Ti2]-, 1942.60 [HNaK(4)3Ti2]-,
1958.58 [HK2(4)3Ti2]-, 1980.56 [NaK2(4)3Ti2]- or 1996.54
[K3(4)3Ti2]-). However, at least one, probably two of the K+ are
still bound to the complex.
Helicates [(3/4)3Ti2]4- are able to adopt compressed and
expanded forms depending on the counter cations. Based on a
series of X-ray structure analyses11,12 and on the structure of
[Li3(2)3Ti2]- the length in the compressed form is estimated to be
around 10 Å. The expanded form is approximately 24-25 Å long
based on model considerations.
Controlling the twist. S-configurated ligand 4 forms only one
enantiomerically pure diastereoisomer of the compressed
complex Li[Li3(4)3Ti2]. The circular dichroism (CD) spectrum (Fig.
3b) shows a negative Cotton effect at 350 nm and a positive one
at 420 nm. These bands are indicative for -configuration.15 This
is in agreement with observations made for Li[Li3(1b)6Ti2] in
which the methyl substituents are well preorientated to bridge
two catecholates which both coordinate to the same Li+ (see
[Li3(2)3Ti2]- in Fig. 2 and SI).16
Expanded K4[(4)3Ti2] adopts  configuration (right handed twist,
pos. Cotton effect at 340 nm, neg. Cotton effect at 400 nm). The
chiral units are orientated as depicted in Figure 3c, left.13
Potassium cations are coordinating within the expanded helicate
interacting with three catecholate and three carbonyl oxygens
(Fig. 3c, center). The inside orientation of the C=O-units by K+
Figure 3. (a) Parts of the 1H NMR spectra of Li[Li3Ti2(4)3], K4[Ti2(4)3] and
[Li(2.1.1)]4[Ti2(4)3] in DMSO-d6 (a-c mark the resonances of the catechol
protons as depicted in Fig. 1c, * corresponds to the proton at the chiral center);
(b) circular dichroism spectra (CD) of -Li[Li3Ti2(4)3], -K4[Ti2(4)3] and -
[Li(2.1.1)]4[Ti2(4)3] in DMSO-d6; (c) orientations of the ester carbonyl oxygen
atom as influenced by the presence (left) or absence (center) of binding
cations and part of the crystal structure of the potassium titanium model
complex with methylester ligands (right) showing the orientation of the
carbonyl oxygens towards titanium (yellow) enforced by coordination to
potassium cations (purple); (d) schematic representation of possible twists at a
helicate and options to switch between the different states of [(4)3Ti2]4-. Full
conversion of one state to another takes place within 7-8 hours at 30-40°C.
coordination induces
a
right handed twist () and is
demonstrated in the crystal structure of a model compound (Fig.
3c, right).
This article is protected by copyright. All rights reserved.

10.1002/anie.201806607
Angewandte Chemie International Edition
COMMUNICATION
The left handed expanded helicate is obtained by simply
removing potassium or lithium cations which force the carbonyl
oxygen atoms to the interior. Reaction of right handed expanded
K4[(4)3Ti2] with 18crown6 (18C6) or of left handed compressed
Li[Li3(4)3Ti2] with [2.1.1]cryptand (2.1.1) results in M4[(4)3Ti2] (M
= K(18C6) or Li(2.1.1)). No alkali metal cation binds to the
helicate and repulsion occurs between carbonyl and catecholate
oxygen atoms. The carbonyl unit rotates “outwards” (Fig. 3c,
center) and the complexes adopt a left handed twist (Fig. 3b).
Three different states (Fig. 3d) of the helicate with ligand 4 are
available: (i) compressed left-handed (with Li+), (ii) expanded
right-handed (with K+) and expanded left-handed. The potential
of such helicates to be used as mechanical devices highly
depends on the possibility to switch between the three different
states as shown in Figure 3d.
Expansion and compression with retention or inversion.
[(4)3Ti2]4- shows 1H NMR spectra of the compressed left-handed
lithium or of the expanded right-handed potassium salt. Due to
the strong binding of lithium in compressed [Li3(4)3Ti2]- it is easy
to substitute potassium of K4[(4)3Ti2] by adding LiCl. Lithium
cations are removed by addition of [2.1.1]cryptand enabling
interaction of [(4)3Ti2]4- with the potassium cations which are still
present in solution (Fig. 4a). The switching of the structures is
easily followed by 1H NMR (Fig. 4b) as well as by CD
spectroscopy (Fig. 4c) showing that expansion and compression
are accompanied by inversion of the twist.
Switching between all three states of [(4)3Ti2]4- is also possible in
one test tube (Fig. 4d). Addition of [2.1.1] cryptand to left-handed
compressed [Li3(4)3Ti2]- removes lithium ions and left-handed
expanded [(4)3Ti2]4- is obtained. The twist of this complex is
inverted by addition of potassium chloride to obtain right-handed
expanded K4[(4)3Ti2]. Subsequent addition of lithium chloride
leads to the left-handed compressed [Li3(4)3Ti2]- which has been
the starting point of the switching sequence. The switching
process can be easily followed by proton NMR as well as CD
spectroscopy revealing spectra as depicted in Figure 4b,c for the
different species.
In here a helicate is introduced which allows reversible switching
between an expanded or a compressed state. Upon this process
the complex increases its extension from 10 to about 25 Å. This
behavior can be described to be spring type. Compression is
enforced by internal binding of Li-ions but not to an outer
pressure. Removal of the lithium based charge compensation
between the negative complexes results in repulsion and
enforces expansion. This is of course different to the function of
macroscopic springs which are driven by internal tension.
In addition it is possible to invert the helicity of the helicate
during expansion and compression as well as between the
different expanded states. Therefore the present system should
be described as a three state stereochemical switch. The
importance of chirality in chemistry makes switches like this
interesting for the development of e.g. novel catalysts or reaction
platforms.17
Figure 4. (a) Reversible switching between right handed () expanded and
left handed () compressed state of [(4)3Ti2]4- in DMSO-d6 by alternating
addition of LiCl or [2.1.1] cryptand as followed by 1H NMR (b) as well as CD
spectroscopy (c); (d) one cycle of switching between all three possible states
of the helicate in DMSO-d6.
Acknowledgements
This article is protected by copyright. All rights reserved.

10.1002/anie.201806607
Angewandte Chemie International Edition
COMMUNICATION
We thank the Deutsche Forschungsgemeinschaft for funding within
the international graduate school Seleca. X.C. thanks the China
Scholarship Council (CSC) for generous support.
[8]
[9]
(a) J.-M. Lehn, A. Rigault, J. Siegel, J. Harrowfield, B. Chevrier, D.
Moras, Proc. Natl. Acad. Sci. U. S. A. 1987, 84, 2565−2569. (b) C.
Piguet, G. Bernardinelli, G. Hopfgartner, Chem. Rev. 1997, 97,
2005−2062. (c) Albrecht, M. Chem. Rev. 2001, 101, 3457−3497.
(a) K. Miwa, Y. Furusho, E. Yashima, Nat. Chem. 2010, 2, 444-449. (b)
Y. Suzuki, T. Nakamura, H. Iida, N. Ousaka, E. Yashima, J. Am. Chem.
Soc. 2016, 138, 4852-4859. (c) see also B. L. Feringa Nature
Chemistry 2010, 2, 429–430.
Keywords: self-assembly • molecular switch • helicate •
catechol ester • stereochemistry
[10] (a) Review: M. Albrecht, In Synergy in Supramolecular Chemistry; T.
Nabeshima, Ed.; CRC Press: Boca Raton, FL, 2015; Chapter 2, pp
19−32. (b) M. H. W. Lam, S. T. C. Cheung, K.-M. Fung, W.-T. Wong,
Inorg. Chem. 1997, 36, 4618−4619. (c) T. Konno, K. Takuda, T. Suzuki,
K.-I. Okamoto, Bull. Chem. Soc. Jpn. 1998, 71, 1049−1054. (d) X. Sun,
D. W. Johnson, D. Caulder, R. E. Powers, K. N. Raymond, E. H. Wong,
Angew. Chem., Int. Ed. 1999, 38, 1303−1306. (e) Y. Sakata, S. Hiraoka,
M. Shionoya, Chem. - Eur. J. 2010, 16, 3318−3325. (f) S. Hiraoka, Y.
Sakata, M. Shionoya, J. Am. Chem. Soc. 2008, 130, 10058−10059.
[11] (a) M. Albrecht, S. Mirtschin, M. de Groot, I. Janser, J. Runsink, G.
Raabe, M. Kogej, C. A. Schalley, R. Fröhlich, J. Am. Chem. Soc. 2005,
127, 10371-10387. See also (b) M. Albrecht, M. Baumert, J.
Klankermayer, M. Kogej, C. A. Schalley, R. Fröhlich, Dalton Trans.
2006, 4395-4400. (c) M. Albrecht, M. Fiege, M. Baumert, M. de Groot,
R. Fröhlich, L. Russo, K. Rissanen, Eur. J. Inorg. Chem. 2007, 609-616.
(d) M. Baumert, M. Albrecht, H. D. F. Winkler, C. A. Schalley, Synthesis
2010, 6, 953-958. (e) M. Albrecht, E. Isaak, H. Shigemitsu, V. Moha, G.
Raabe, R. Fröhlich, Dalton Trans. 2014, 14636-14643. (f) D. Van
Craen, M. de Groot, M. Albrecht, F. Pan, K. Rissanen, Z. Anorg. Allg.
Chem. 2015, 641, 2222-2227.
[1]
(a) G. Schill, A. Lüttringhaus, Angew. Chem. Int. Ed. 1964, 3, 546-547.
(b) C. O. Dietrich-Buchecker, J. P. Sauvage, J. P. Kintzinger,
Tetrahedron Letters 1983, 24, 5095-5098. (c) P. R. Ashton, T. T.
Goodnow, A. E. Kaifer, M. V. Reddington, A. M. Z. Slawin, N. Spencer,
J. F. Stoddart, C. Vicent, D. J. Williams Angew. Chem. Int. Ed. 1989, 28,
1396-1399. (d) G. Gil-Ramirez, D. A. Leigh, A. J. Stephens, Angew.
Chem. Int. Ed. 2015, 54, 6110-6150. (e) S. P. Black, A. R.
Stefankiewicz, M. M. J. Smulders, D. Sattler, C. A. Schalley, J. R.
Nitschke, J. K. M. Sanders, Angew. Chem. Int. Ed. 2013, 52, 5749–
5752.
[2]
(a) P. L. Anelli, N. Spencer, J. F. Stoddart, J. Am. Chem. Soc. 1991,
113, 5131-5133. (b) J. C. Chambron, C. O. Dietrich-Buchecker, J. F.
Nierengarten, J.-P. Sauvage, J. Chem. Soc., Chem. Commun. 1993,
801–804. (c) C. A. Schalley, K. Beizai, F. Vögtle, Acc. Chem. Res.
2001, 34, 465–476. (d) R. A. Bissell, E. Córdova, A. E. Kaifer, J. F.
Stoddart, Nature 1994, 369, 133–137. (e) R. Jäger, F. Vögtle, Angew.
Chem. Int. Ed. 1997, 36, 1199-1201. (f) M. Asakawa, G. Brancato, M.
Fanti, D. A. Leigh, T. Shimizu, A. M. Z. Slawin, J. K. Y. Wong, F.
Zerbetto, S. Zhang, J. Am. Chem. Soc. 2002, 124, 2939–2950.
J. E. Green, J. W. Choi, A. Boukai, Y. Bunimovich, E. Johnston-
Halperin, E. DeIonno, Y. Luo, B. A. Sheriff, K. Xu, Y. S. Shin, H.-R.
Tseng, J. F. Stoddart, J. R. Heath, Nature 2007, 445, 414-417.
G. S. Kottas, L. I. Clarke, D. Horinek, J. Michl, Chem. Rev. 2005, 105,
1281–1376.
[12] M. Albrecht, E. Isaak, M. Baumert, V. Gossen, G. Raabe, R. Fröhlich,
Angew. Chem. Int. Ed. 2011, 50, 2850-2853.
[3]
[13] (a) S. Akine, S. Sairenji, T. Taniguchi, T. Nabeshima J. Am. Chem. Soc.
2013, 135, 12948-12951. (b) E. Lee, H. Ju, I.-H. Park, J. H. Jung, M.
Ikeda, S. Kuwahara, Y. Habata, S. S. Lee, J. Am. Chem. Soc. 2018,
140, DOI:jacs.8b05751.
[4]
[5]
(a) M. C. Jiménez, C. Dietrich-Buchecker, J.-P. Sauvage, Angew.
Chem. Int. Ed. 2000, 39, 3284-3287. (b) J. Wu, K. C.-F. Leung, D.
Benítez, J.-Y. Han, S. J. Cantrill, L. Fang, J. F. Stoddart, Angew. Chem.
Int. Ed. 2008, 47, 7470-7474. (c) see for comparison E. R. Kay, D. A.
Leigh, F. Zerbetto, Angew. Chem. Int. Ed. 2007, 46, 72-191.
(a) O.-S. Jung, Y. J. Kim, Y.-A. Lee, J. K. Park, H. K. Chae, J. Am.
Chem. Soc. 2000, 122, 9921-9925. (b) E. Berni, B. Kauffmann, C. Bao,
J. Lefeuvre, D. M. Bassani, I. Huc, Chem. Eur. J. 2007, 13, 8463-8469.
E.g. C. G. Baumann, S. B. Smith, V. A. Bloomfield, C. Bustamante,
Proc. Natl. Acad. Sci. USA 1997, 94, 6185-6190.
[14] B. Dietrich, J.-M. Lehn, J.-P. Sauvage, Tetrahedron 1973, 29, 1647-
1658.
[15] M. Albrecht, I. Janser, J. Fleischhauer, Y. Wang, G. Raabe, R. Fröhlich,
Mendeleev Commun. 2004, 14, 250-253.
[16] M. Albrecht, E. Isaak, V. Moha, G. Raabe, R. Fröhlich, Chem. Eur. J.
2014, 20, 6650-6658.
[6]
[7]
[17] D. Van Craen, M. Albrecht, G. Raabe, F. Pan, K. Rissanen, Chem. Eur.
J. 2016, 22, 3255-3258.
This article is protected by copyright. All rights reserved.

10.1002/anie.201806607
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Up and down and left and right. A
triple stranded helicate can be
switched between three different
extended and twisted states. This
behaviour is reversible and several
switching cycles can be performed.
X. Chen, T. M. Gerger, C. Räuber, G.
Raabe, C. Göb, I. M. Oppel, M.
Albrecht*
Page No. – Page No.
A helicate based three-state
molecular switch
Layout 2:
COMMUNICATION
Author(s), Corresponding Author(s)*
((Insert TOC Graphic here))
Page No. – Page No.
Title
Text for Table of Contents
This article is protected by copyright. All rights reserved.