Molecular nanostructures based on polyfunctional clathrochelate complexes

This thesis describes the design and synthesis of clathrochelate metalloligands for the construction of molecular nanostructures. The low synthetic effort, versatility and stability of the clathrochelate metalloligands makes them particularly suited to the construction of coordination structures.

Chapter 2 shows it is possible to control the geometry and the composition of metallasupramolecular assemblies via the aspect ratio of their ligands. Functionalized clathrochelate complexes with variable aspect ratios were used as rod-like metalloligands. Cubic FeII8L12 cages were obtained from a metalloligand with an intermediate aspect ratio. By increasing the length or by decreasing the width of the ligand, the self-assembly process resulted in the clean formation of tetrahedral FeII4L6 cages instead of cubic cages.

Chapter 3 describes a simple one-step protocol, which allows large bipyridyl functionalized double clathrochelate metalloligands with an overall bent shape to be synthesized from easily accessible and/or commercially available starting materials. The ligands were used to construct PdII2L4-type coordination cages of unprecedented size. Furthermore, evidence is provided that these cages may be stabilized by close intramolecular packing of lipophilic ligand side chains.

In Chapter 4, even larger triple clathrochelate metalloligands were used to form two PdII6L8-type coordination cages. With molecular weights of more than 15 kDa and Pd···Pd distances of up to 4.2 nm, these complexes are among the largest palladium cages described to date.

In the 5th chapter, the stability of five different [Pdn(N donor)m]2n+ assemblies was examined by performing disassembly experiments with pyridine and with trifluoroacetic acid. Pyridine-induced disassembly was found to be most pronounced for Pd complexes containing N-donor ligands of low basicity. At the same time, these assemblies displayed high acid resistance. The contrasting stability in the presence of acid or pyridine can be used for the pH-controlled switching between different metallosupramolecular structures.

The final research section (Chapter 6) shows that the addition of a metastable-state photoacid to solutions containing metal-ligand assemblies renders the systems light responsive. Upon irradiation, proton transfer from the photoacid to the ligand is observed, resulting in disassembly of the metallasupramolecular structure. In the dark, the process is fully reversed. Light-induced switching was demonstrated for six different metal ligand assemblies containing PdII, PtII or RuII complexes and bridging polypyridyl ligands. The methodology allows the liberation of guest molecules using light as a stimulus.

Syntheses and Applications of Boronate Ester-Capped Clathrochelate Complexes

Ophélie Marie Planes

This thesis describes the syntheses of novel boronate ester-capped clathrochelate complexes and their application in supramolecular chemistry, materials science, and chemical biology. Chapter 2 shows the synthesis of long Fe(II) dinuclear clathrochelates, up to 3 nm, with low synthetic effort. Their ability to be used as building blocks in supramolecular chemistry is evidenced by the formation of a Cu4L4-type coordination cage, and by the formation of a 2-dimensional metal-organic framework (MOF). The incorporation of clathrochelate complexes with terminal pyridyl groups into pillared MOFs by using solvent-assisted ligand exchange is described in Chapter 3. The strong basicity of clathrochelates with pyridyl groups makes them particularly suited for this approach. In Chapter 4, the structural characterization of novel cobalt clathrochelates is described. A first example of a Co(II) clathrochelate with potentially redox non-innocent phenanthrenequinone dioximato groups is reported. Chemical reductions of cobalt clathrochelates were carried out, and ligand effects are investigated by crystallography. Moreover, in the Chapter 5, clathrochelates with elongated Ïâdioximate groups are described, along with preliminary attempts of using them as three-way junction (TWJ) stabilizers. The synthesis of clathrochelate complexes with a chiral dioximato ligand and terminal pyridyl, bromo or carboxylic acid groups are described in Chapter 6. Potential applications of this new metalloligand in supramolecular chemistry are investigated.

EPFL2021

Self-assembly of multiligand supramolecular architectures at metal surfaces

Alexander Langner

Self-assembly of organic molecules represents an efficient and convenient bottom-up approach for the structural functionalization of surfaces at the nanometer scale. The great potential of assembling supramolecular architectures from organic molecules lies in the vast choice of building blocks that are accessible. This allows to predefine and to direct the molecular organization through the steric and electronic information stored in the molecules. A large variety of homotopic supramolecular structures can be achieved, however limited in complexity. The full realization of highly developed surface architectures for designable chemical functions and physical properties may depend critically on a higher level of complexity, which could be satisfied by utilizing a mixture of different molecular building blocks. Within this thesis, the self-organization of multi-component systems at well-defined metal surfaces is investigated by scanning tunneling microscopy. The mixtures consist of different representatives of linear polyaromatic ligands with carboxylate or pyridyl derived functional groups. The assembly is directed by hydrogen bonding, metal-organic complex formation or a combination of both, i.e. a hierarchical interaction scheme. The aim of this thesis is threefold: First, the homotopic self-assembly of different pyridyl ligands is investigated as a basis for the multi-component systems. One-dimensional chains as well as two dimensional (2D) networks can be formed by copper-pyridyl coordination motifs. The influence of the substrate on these structures will be discussed in detail. Moreover, the self-organization process of multi-components is investigated at the fundamental level. Since functional groups primarily discriminate the different molecular species during the assembly and with that assure a distinguished arrangement, selective bonding schemes are essential for the design of highly ordered supramolecular architectures. Two suitable selective coordination motifs are identified, an iron-carboxylate and a copper-pyridyl complex node. In ligand mixtures, strong preference of one metal species is observed at surfaces, which would not be expected in solution. In addition, redundant mixtures (i.e. ligands of different size but same functional group) serve as model systems to investigate the dynamic self-organization process of modular multicomponent systems, e.g. self-selection or error tolerance, directly with nanometer accuracy. Self-selection is observed if the molecular recognition is steered by a highly reversible coordination bond, while a more robust bonding fosters structural adaption and tolerance to the introduced error (i.e. redundant mixture). These experiments underline the importance of reversibility of the interactions steering the self-assembly process into highly ordered structures. Finally, the extended possibilities due to the multi-component approach for the construction of advanced architectures and structural control are explored: Complex and sophisticated supramolecular networks can be directly synthesized out of simple molecules at the surface. A hierarchical assembly is presented, where a coordination motif defines a primary unit, while hydrogen bonding steers the 2D organization. The secondary level of interaction (hydrogen bonding) can be exclusively addressed, leading to distinct 2D arrangement of the undisturbed primary coordination units. A concept of structural control by rational design is presented, where geometric network parameters can be modified by the replacement of the appropriate molecular species. Due the multi-component nature of the systems, a finer tuning of the structure is enabled (independent substitution of each component) compared to homotopic assemblies. Moreover, a continuation of the multi-component concept is demonstrated, where organizational control is imposed to a homotopic system by adding an additional molecular species. It is demonstrated that a long range order and thermal stability is imposed onto meta-stable Cu-pyridyl coordination chains by cooperative assembly with a carboxylate species.

EPFL2009

Low-dimensional supramolecular architectures at metal surfaces

Sebastian Stepanow

This thesis is concerned with supramolecular architectures assembled at metal surfaces. The investigations pursued two objectives. On the one hand, the focus was placed on the fabrication of low-dimensional surface-supported network structures by applying the concepts of conventional three-dimensional supramolecular chemistry at surfaces. Three main driving forces were explored for the construction of low-dimensional assemblies: hydrogen bonding, metal coordination and ionic interactions. The realized structures were characterized by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. In particular, the interplay between the adsorbate-substrate coupling and lateral adsorbate interactions was addressed in the experiments. Further, the electronic and magnetic properties of the fabricated coordination structures were investigated. This characterization was done by x-ray absorption experiments at the synchrotron facility in Grenoble. Moreover, the response of the metal units and open cavity structures to the adsorption of large organic and small gas molecules was studied by temperature controlled STM experiments. The first part of the thesis deals with hydrogen bonded supramolecular assemblies. Simple aromatic carboxylic acids were employed to study the assembly principles and in particular effects arising from the substrate and molecular structure. The investigation of TPA (1,4-benzenedicarboxylic acid) adlayers on Cu(100) at different substrate temperatures revealed how the protonation status of the carboxyl moieties affects the topology of the assemblies. Further, the influence of the molecular backbone functionality and symmetry is addressed in comparative investigations on TPA and PDA (2,5-pyrazinedicarboxylic acid) as well as BDA (4,4'-biphenyldicarboxylic acid) and SDA (4',4" trans-ethene-l,2-diyl-bisbenzoic acid) adlayers on Cu(100). In the second part of the thesis, metal-ligand interactions were employed for the construction of supramolecular architectures at a Cu(100) surface. The concepts of conventional coordination chemistry were successfully applied at the surface. Iron atoms in conjunction with rod-like aromatic molecular linkers comprising carboxyl, pyridyl and hydroxyl functional groups assemble into a rich variety of open network structures with distinctly arranged mono- and diiron coordination centers and well-defined cavities. The results show that the adsorbate-substrate coupling considerably affects the local coordination environment of the iron units when the molecular backbone length is varied. The occurrence of structural isomerism in open network structures is addressed in a comparative study of heterofunctional and homofunctional linker molecules. It is shown that the former ligand allows only one topologically pure structure while the other linker forms two phases with different topology. The hydroxyl ligand has been used to construct distorted hexagonal coordination networks containing threefold coordinated single iron atoms. The comparison between the networks on Cu(100) and Ag(111) surfaces shows marked differences, which are attributed to adsorbate-substrate interactions. Finally, a novel design strategy for the fabrication of two-dimensional coordination structures at surfaces is introduced by using alkali metal ions in conjunction with carboxylate ligands. The interaction between the components is dominated by electrostatic forces and is of intermediate bond strength as in the case of hydrogen bonding and transition metal coordination. It is expected, that substrate symmetry and registry plays a larger role in the network formation, because the ionic interaction is not directional. The chemical as well as magnetic properties of the fabricated coordination networks are examined in the last part of the thesis. The adsorption of C60 molecules on open network structures reveals how cavity size and chemical functionality affect the bond strength to the guest molecules as well as the number of accommodated C60 molecules. Since transition metal clusters, and in particular diiron units, are the catalytic active centers in various proteins, the reactivity of various mono- and diiron coordination structures to molecular oxygen was investigated in temperature controlled STM experiments. In particular, the focus was placed on the structural relaxation upon gas adsorption and chemical reaction. At last, the magnetic properties of the metal coordination centers are addressed in XMCD experiments. The iron units are found to be paramagnetic at temperatures down to 5 K and show distinct magnetic anisotropy and magnetization loops. These experiments demonstrate that surface-supported metal coordination structures are a promising class of materials for applications in the field of heterogenous catalysis and surface magnetism.