Tuning Reduced States of Group 4 and Rare Earth Metals

Some 65 years ago a new chapter of chemistry was opened by the incidental discovery of ferrocene. While in the initial publications the compounds structure was not assigned correctly, later scientists at Harvard University and at the TU Munich recognized that ferrocene constitutes the first member of a completely new class of compounds, the metallocenes. At that time the bonding interaction in ferrocene between a metal atom (iron) and an organic -bond was completely new and it revolutionized the way of our perception of the bonding between metal atoms and organic groups. The organic group in ferrocene is the cyclopentadienyl ligand which nowadays can be found in numerous interesting compounds, many of them being catalysts for industrially important reactions. Interestingly enough the popularity of the cyclopentadienyl ligand is not caused by its spectacular reactivity. The group is valued much more as a spectator ligand, which constitutes part of a metal complex by providing electron density but does not interfere directly with the actual reaction. Cyclopentadienyl complexes of all transition metals (metallocenes) and lanthanides (lanthanocenes) are known. However, the chemistry of the metallocenes was studied in much more detail due to the fact that transition metals were considered of superior value for chemical transformations compared to lanthanides. The latter were thought to be chemically incapable because of their strong preference for the oxidation state +3. However, over the last years several studies emerged which showed that both lanthanide complexes and group 4 metallocenes in reduced oxidation states are able to form complexes with dinitrogen and other notoriously non-reactive small molecules. Careful analysis of these complexes shows that their reactivity is intimately connected to the energy levels of the highly reactive reduced oxidation states of these metals which can be manipulated by proper choice of substituted cyclopentadienyl ligands. Recent work by Evans and co-workers has shown that it is possible to prepare stable cyclopentadienyl complexes of all lanthanide complexes in the oxidation state +2. Given the redox properties of naked lanthanide ions this should be impossible. However, Evans work suggests that the additional electron of several of the Ln(II) compounds is not located in an f- orbital but rather in a d-orbital. The energetic accessibility of this d-orbital is caused by the ligand field of a silylated cyclopentadienyl ligand. The main intention of the current proposal is therefore to study the effect of silylated cyclopentadienyl ligands and of the related silole, germole and stannole ligands on the energy levels of d-orbitals in the pursuit of understanding whether this is a general effect and how it can be exploited for new reactions.

Recent years have seen a surprising reassessment of the chemistry of the so-called rare earth (RE) elements. Previously, their chemical properties were assumed to be very similar with only the ionic radii being different. With respect to catalysis these elements were considered largely inferior to transition metals. Today numerous catalytic processes have been developed based on RE elements. Our contribution to this field is the study of silyl lanthanoids, a class of highly reactive compounds. While there are 17 RE elements, most of the known silyl complexes were synthesized with samarium and ytterbium. Our recent investigation involved a system suitable for all elements, highlighting some compounds with hitherto unknown bonds between silicon and RE elements lanthanum and praseodym. Another part of the project was concerned with the study of siloles, a class of cyclic silicon compounds with interesting photophysical properties, which is used for instance for the production of organic light emitting diodes (OLEDs) in electronic displays. Our contribution to this field involves the attachment of polysilanes to siloles. Polysilanes are an inorganic class of compounds with delocalized electrons. Our new class of compounds a deliberate fine-tuning of the photophysical properties of siloles A long-term goal of our research group is to establish a toolbox of chemical compounds and transformations, for the construction of complex organosilicon compounds. The main focus in these efforts was on the use of silanides, negative charged silicon compounds. When combined with positively polarized silicon compounds (electrophiles), they form silicon-silicon bond, which is the single most important transformation in polysilane chemistry. One particularly desirable property of silanides is functional groups, which allow further transformations after Si-Si bond formation. Our latest achievement in this field is the facile formation of silanides with a fluoride or chloride attached close to the negatively charge silicon atom. These compounds are ambiphilic, which means that they are able to react with positively and negatively charged reagents, rendering them as versatile building blocks Hypercoordinated silicon compounds is another area of silicon chemistry, we are studying. Silicon being larger than carbon can accommodate additional ligands to extend the traditional tetravalent bonding situation. The presence of an additional Lewis basic group exerts some (weakening) influence on the other bonds. This can be exploited either for bond activation or as switch of physical properties. After working mainly with silatranes, which are compounds where one nitrogen atom is confined to interact with a silicon atom by three handles, we have recently started to study compounds with a higher molecular flexibility. Such compounds might be used as switches in the context of the development of molecular circuitry for future electronic devices.