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Research

Outline

Research in the Clever Lab is driven by fascination for nature’s chemical complexity and the joy of designing and constructing functional devices and materials on a molecular scale.

Our main goals include the development of (multi)functional coordination cages with applications in diagnostics, catalysis, light-harvesting and material science as well as the synthesis of metal-carrying DNA nanostructures with the ability to mimic and interact with biological systems.

 

Overview

Reviews

Cages: "Self-Assembled Coordination Cages based on Banana-shaped Ligands"
M. Han, D. M. Engelhard, G. H. Clever
Chem. Soc. Rev. 2014, 43, 1848.

Metal-DNA: "Metal-base pairing in DNA"
G. H. Clever, M. Shionoya
Coord. Chem. Rev. 2010, 254, 2391.


 

The major tools used in our lab comprise multistep organic synthesis, microwave reactions, photochemistry, automated DNA synthesis and coordination chemistry, backed up by molecular modeling methods. The main characterization techniques we use include NMR spectroscopy, high-res. ESI mass spectrometry, X-ray diffraction, UV-Vis-, CD-and fluorescence spectroscopy, electrochemistry, various chromatographic and electrophoretic methods and surface analytics.

 

We currently focus on the following aspects of supramolecular and bioinorganic chemistry:

 

Coordination Cages

1) Self-assembled Structures with unique Shape, Topology and Dynamics

Conceptual ideas paired with computer-aided design, an eye on synthetic feasibility and state-of-the-art preparative and analytical equipment allow us to synthesize a plethora of coordination cages with different shapes, topologies and dynamic features. Beyond this rational design approach, however, serendipitous results are sometimes giving us delight!

An example from our lab for the first case is the formal reduction of symmetry from a cubic (a=b=c) to a cuboid (a=b≠c) cage by geometry-based design involving the substitution of a 90°-angled bis-pyridyl ligand by a tripodal tris-pyridyl ligand.

 

Box

"Rational Design of a Face-Centred Square-Cuboid Coordination Cage"
M. Han, R. Michel, G. H. Clever
Chem. Eur. J. 2014, 20, 10640.


 

Unexpected was the outcome of the self-assembly of rigid bis-monodentate pyridyl ligands with palladium(II) ions to give an unprecedented intertwined cage [Pd3L6] in which each of the two hemispheres resembles a trefoil knot.

 

Knot

"NMR-Based Structure Determination of an Intertwined Coordination Cage resembling a Double Trefoil Knot"
D. M. Engelhard, S. Freye, K. Grohe, M. John, G. H. Clever
Angew. Chem. Int. Ed. 2012, 51, 4747.


 

Fine-tuning the shape, dynamics and topology of self-assembled cages leads to interesting consequences for guest affinity, selectivity and exchange kinetics. In structurally related interpenetrated double-cages, we were able switch the selectivity for the allosteric binding of guests in the outer two pockets from a) small to b) large by choice of the template (large or small) in the central pocket.

 

Template

"Allosteric Binding of Halide Anions by a New Dimeric Interpenetrated Coordination Cage"
S. Freye, J. Hey, A. Torras-Galán, D. Stalke, R. Herbst-Irmer, M. John, G. H. Clever
Angew. Chem. Int. Ed. 2012, 51, 2191.

"Template Control over Dimerization and Guest Selectivity of Interpenetrated Coordination Cages"
S. Freye, R. Michel, D. Stalke, M. Pawliczek, H. Frauendorf, G. H. Clever
J. Am. Chem. Soc. 2013, 135, 8476.


 

Sophisticated effects such as on/off switching events and non-equilibrium situations can also be implemented. We study such non-trivial guest encapsulation phenomena with our family of interpenetrated double-cages.

 

Acridone

"Triggered Exchange of Anionic for Neutral Guests inside a Cationic Coordination Cage"
S. Löffler, J. Lübben, L. Krause, D. Stalke, B. Dittrich, G. H. Clever
J. Am. Chem. Soc. 2015, 137, 1060.


 

A two-step stimuli-responsive structural reorganization of a monomeric cage into a dimer and further to a triple-catenane was realized using halide anions as triggers.

 

Carbazole

"Stepwise Halide-triggered Double- and Triple-Catenation of Self-Assembled Coordination Cages"
R. Zhu, J. Lübben, B. Dittrich, G. H. Clever
Angew. Chem. Int. Ed. 2015, 54, 2796.


 

2) (Multi)functionalization by Design

Beyond structure design and host-guest chemistry, we study ways of introducing specific functionality into the cages, mainly via chemical modification of the ligands. We construct systems that implement the following features: structural switching, light- and redox-responsiveness, chirality, endohedral modification and catalytic activity.

A coordination cage based on a photochromic dithienylethene (DTE) ligand was prepared in which conformational changes are controlled by irradiation with different wavelengths of light. The light-driven interconversion is clean and reversible, allowing the controlled encapsulation and release of anionic guests such as [B12F12]2−.

 

DTE

"Light-triggered Guest Uptake and Release by a Photochromic Coordination Cage"
M. Han, R. Michel, B. He, Y.-S. Chen, D. Stalke, M. John, G. H. Clever
Angew. Chem. Int. Ed. 2013, 52, 1319.


 

In redox-functional double-cages based on phenothiazine ligands, the eightfold S-monooxygenation of the ligands proceeds with ease, and is accelerated in comparison to the ligand alone. This can be explained by the close packing of the ligands in the self-assembled structure that allows electronic communication and intra-molecular disproportionation. We currently study donor-acceptor systems and light-driven charge separation phenomena based on the concept of densely aggregated redox systems.

 

PTZ

"Assembly and Stepwise Oxidation of Interpenetrated Coordination Cages based on Phenothiazine"
M. Frank, J. Hey, I. Balcioglu, Y.-S. Chen, D. Stalke, T. Suenobu, S. Fukuzumi, H. Frauendorf, G. H. Clever
Angew. Chem. Int. Ed. 2013, 52, 10102.


 

Elaborate ligand design allows for the positioning of endohedral (inside pointing) guest binding sites and other functionalities in self-assembled cages. We study this using oligonorbornane-based architectures that can be synthesized by consecutive electrocyclic reactions in a modular way.

 

Pfeffer

"Modular synthesis of linear bis- and trismonodentate fused [6]polynorbornane-based ligands and their assembly into coordination cages"
M. D. Johnstone, E. K. Schwarze, G. H. Clever and F. M. Pfeffer
Chem. Eur. J. 2015, 21, 3948.


 

Besides the introduction of further types of functionalities we explore methods towards the rational multi-functionalization of self-assembled cavities. Modular combination of recognition and reactive sites promises to lead to new receptors and catalysts. Electronic communication between thoughtfully positioned redox-systems may yield new supramolecular materials for future electronics and photovoltaics.

 

Multi


 

3) Cage-based Materials

We work on strategies for the controlled surface deposition and higher-order aggregation of functionalized cage compounds. Together with implemented switching effects, photo- and redox-addressability and guest encapsulation, novel bulk materials based on (multi)functional cages are developed.

The surface-integrity of our interpenetrated double-cages was recently proven by Surface Enhanced Raman Spectroscopy (SERS) measurements.

 

Raman

"SERS spectroscopic evidence for the integrity of surface-deposited self-assembled coordination cages"
M. Frank, S. Funke, H. Wackerbarth, G. H. Clever
Phys. Chem. Chem. Phys. 2014, 16, 21930.


 

Anion-mediated inter-cage aggregation phenomena have been observed by us in several cases. Currently, we are expanding these effects aiming at stimuli-responsive and self-healing soft materials.

 

Pt5

"Stacked Platinum Complexes of the Magnus’ Salt Type Inside a Coordination Cage"
G. H. Clever, W. Kawamura, S. Tashiro, M. Shiro, M. Shionoya
Angew. Chem. Int. Ed. 2012, 51, 2606.


 

DNA Nanotechnology:

4) Metal-binding G-Quadruplex DNA

Due to their tremendous biological relevance (cell aging, cancer activation…),

G-quadruplexes have moved into the focus of current oligonucleotide chemistry. There is great interest in designing discrete G-quadruplex probes which allow for a control over the de- and re-hybridization thermodynamics, the choice of loop sequence and topology as well as the introduction of non-biogenic functionalities by automated DNA synthesis. Following a bio-artificial hybrid approach, we have developed novel

G-quadruplex structures that can be reversibly stabilized by transition metal ions. In addition, paramagnetic ions such as Cu(II) can serve as EPR-active markers for the examination of yet unknown protein-DNA interactions.

 

GQuad

"Reversible Stabilization of Transition Metal-binding DNA G-Quadruplexes"
D. M. Engelhard, R. Pievo, G. H. Clever
Angew. Chem. Int. Ed. 2013, 52, 12843.


 

3D-Visualization, Animation and Haptic Perception of Supramolecular Structures

We use readily available computer programs for the modeling, 3D visualization and animation of  supramolecular systems to help understanding their three-dimensional structure, topological peculiarities and dynamic features.

 

Pd3L6

Allostery

DNA

 

In addition, we use ball-and-stick models, LEGO construction and 3D printing to visualize our structures and create haptic sensations, both in research and teaching.

 

Modellbau