University of Groningen Molecular motors: new designs and applications Roke, Gerrit Dirk IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2018 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Roke, G. D. (2018). Molecular motors: new designs and applications. Rijksuniversiteit Groningen. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 25-11-2020
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University of Groningen
Molecular motors: new designs and applicationsRoke, Gerrit Dirk
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.
Document VersionPublisher's PDF, also known as Version of record
Publication date:2018
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):Roke, G. D. (2018). Molecular motors: new designs and applications. Rijksuniversiteit Groningen.
CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.
Chapter 5 Photoresponsive supramolecular coordination cage based on overcrowded alkenes
Supramolecular coordination cages with a Pd2L4 composition are formed using molecular motors based on overcrowded alkene as a ligand. Characterization of these cages with NMR, HRMS, CD and X-Ray shows that the cages self-sort into homochiral assemblies, which are energetically favored over the diastereomeric complexes as shown by DFT calculations. The photochromic ligands can be switched between three states, each of them having the potential of forming discrete cage complexes, allowing cage-to-cage transformation. Moreover, the complexes were shown to bind a tosylate anion in their cavity.
This chapter will be published as: C. Stuckhardt, D. Roke, W. Danowski, S. J. Wezenberg, B. L. Feringa, manuscript in preparation
The experimental work described in this chapter was performed by C. Stuckhardt as a part
of his Master’s thesis under the guidance of D. Roke.
86
Chapter 5
5.1 Introduction
Supramolecular coordination complexes (SCC’s) represent an exciting class of compounds
which have been used in sophisticated molecular systems.[1–6]
Making use of the vacant
cavity inside these complexes, SCC’s have found applications for drug delivery,[6–8]
supramolecular catalysis,[9–12]
X-ray structure determination[13,14]
and stabilization of
reactive guests.[15–17]
The use of reversible and hence dynamic bonds in supramolecular
chemistry gives rise to systems that allow for error correction, a necessity for self-
sorting,[18–20]
and for adaption to external stimuli such as pH, anions, electric potential,
concentration and light.[4,8,21–24]
Using light to control the shape and function of SCCs is a
very promising strategy as light is a non-invasive stimulus that can be easily controlled in a
spatial and temporal manner as well as in terms of intensity and wavelength, without
producing any waste. The field of photoswitchable SCC’s is, however, underdeveloped.
Systems have been reported where photoisomerization of azobenzene-derived anions
encapsulated in supramolecular palladium complexes caused immediate crystallization.[25]
Moreover, azobenzenes have been used to functionalize both the interior[26]
and
exterior[27]
of SCC’s to photochemically control guest binding and release. Incorporation of
photoswitches into the backbone of the ligands has only been shown with
dithienylethenes, which can be switched between an open and a closed state.[28–30]
These
ligands were used to control host-guest interactions,[31]
structural composition of
coordination cages[32]
and sol-gel transitions.[33]
However, up to now, these are the only
examples of SCCs bearing photoswitchable ligands in the backbone and they are limited to
the use of dithienylethene switches. Introducing photoswitches that have a larger
geometric change upon switching has the potential to induce larger changes in properties.
Employing molecular motors as ligands in SCCs is therefore an interesting strategy, as they
feature a large geometric change upon switching and have the potential to induce chirality
in the complex.[34]
Herein, we report a new photochromic coordination cage with ligands based on molecular
motors (Scheme 5.1a). Cages with a Pd2L4 composition are formed from bent bidentate
bispyridyl ligands and Pd(II) ions with a square planar geometry, which have been widely
studied.[6,35–37]
The photochromic ligands can be switched between three states, each of
them having the potential of forming separate discrete cage complexes, allowing cage-to-
cage transformations (Scheme 5.1b). Moreover, the assemblies were found to be self-
sorting, as only homochiral cages are formed. In addition, two of the cage isomers can
bind a tosylate anion in solution by formation of a host-guest complex.
87
Photoresponsive supramolecular coordination cage based on overcrowded alkenes
Scheme 5.1 a) Schematic representation of a photoresponsive cage with ligands based on
overcrowded alkenes. b) Cage formation of overcrowded alkene switches 1 and 2 and their
isomerization behavior.
5.2 Ligand synthesis and characterization
Ligands Z-1a and E-2a were synthesized by a Suzuki cross-coupling reaction of 3-
pyridinylboronic acid with an E/Z mixture of reported overcrowded alkene precursors
(Scheme 5.2).[38]
The E and Z isomers were readily separated by column chromatography
and identified using 2D NOESY NMR spectroscopy. Enantiopure ligands were synthesized
in the same manner, starting from enantiopure motor 7, which was prepared according to
literature procedures.[38]
88
Chapter 5
Scheme 5.2. Synthesis of overcrowded alkene-based ligands Z-1a and E-2a.
The photochemical and thermal isomerization steps of ligands Z-1a and E-2a were
characterized by detailed 1H-NMR studies (Figure 5.1), revealing the same behavior as
structurally related molecular motors.[39]
When a sample of stable Z-1a was irradiated at
312 nm at -55 °C, a new set of signals appeared, belonging to unstable E-2b (Figure 5.1ii).
This can be seen most clearly for the signals of the protons on the central five membered
ring (Ha-c). The sample was irradiated until no further changes were observed, and at this
photostationary state (PSS) the ratio of E-2b to Z-1a was 91:9. When allowing this sample
to warm to room temperature, this photogenerated isomer undergoes a thermal helix
inversion (THI), quantitatively forming stable E-2a. An Eyring analysis was performed to
obtain the activation parameters for this process. The THI was followed at five different
temperatures ranging from -46 to -26 °C using NMR spectroscopy. A Gibbs free energy
barrier of 72.9 kJ mol-1
was obtained (Table 5.1), slightly lower than the barrier reported
for the unsubstituted parent motor (‡G = 80 kJ mol
-1).
[40]
89
Photoresponsive supramolecular coordination cage based on overcrowded alkenes
Figure 5.1. 1H-NMR spectrum of switching cycle of ligand 1 in CD2Cl2 at -55 °C. i) Stable Z-