Polygermane chemistry

Due to the unusual property of sigma bond electron delocalization, long chains consisting of silicon atoms, so called polysilanes, are the subject of intense research since the 1960s. Also polystannanes, which are composed of tin atoms, possess this property and are thus studied for their potential as molecular conductors. For some reason chains made of germanium, the element between silicon and tin, were not recognized as a prime target of research. This strange fact reflects a general disinterest in germanium chemistry. For a long time germanium was considered to be a dull hybrid of silicon and tin, without having much chemical potential. Also our own work over the last years was focused mainly on the element silicon. Studying the chemistry of polysilyl anions and cations we were able to develop synthetic methods for the construction of structurally varied polysilane molecules. In connection to these studies we became interested in analogous chemistry of germanium. Quickly we recognized the unique qualities of the germanium compounds, which were not only chemically different but were found to differ also with respect to structural properties from the previously studied polysilanes. Investigations in collaboration with colleagues from Cork University College; Ireland revealed also a quite unusual thermolytic decomposition behavior in supercritical fluids. In one step our silylated germanium compounds decompose to nanowires possessing a crystalline germanium core covered by an amorphous siliconoxide surface. The planned studies of the project will, based on our experience with similar silicon compounds, explore the reactivity of analogous germanium compounds. Germylanions with and without additional silyl groups will be used as building blocks for larger molecular entities. Conversion of these compounds to cations leads to structural rearrangement. The thus obtained molecules will be used to build structurally defined germanium clusters. Similarly to the mentioned nanowires, these clusters, which can be further functionalized, are of high interest as materials for various applications of nano-technology.

The research project polygermanium chemistry dealt with a number of different aspects of germanium (Ge), which in the periodic table of the elements (PSE) is located between silicon and tin. It is, therefore, a good semiconductor, because of its lower abundance, however, it has been studied to a much smaller extent.The first project part was concerned with molecular chains consisting of germanium atoms. Such compounds can be regarded as molecular wires. However, their synthesis is challenging. Our synthetic studies were concerned with the preparation of building blocks that can be linked to form larger structures. An important aspect of this synthesis is that conductivity in these molecules along the main chain is only given for certain spatial orientations. This property might be used to construct a molecular switching device. Another part of the project also dealt with the semiconductor aspects of germanium. By decomposition of small molecules with Ge-Ge bonds in the presence of indium nano-particles germanium nanowires were obtained with a terminal indium tip. Such materials are of technological interest, among others, as electrode materials in lithium ion batteries.The arguably largest part of the project was devoted to compounds of divalent germanium. Although germanium usually occurs in compounds possessing four bonds (tetravalent), it is possible to obtain also divalent compounds. These are very reactive but possess a number of interesting properties. In particular, they show a pattern of reactivity which otherwise only the elements of the transition metals exhibit. The latter typically act as catalysts in countless synthetically interesting reactions. In order to study the properties of the divalent germanium compounds, it is necessary to stabilize them so they will not react with themselves. This could be achieved by addition of suitable bases, which engage in a weak interaction with them. The addition of certain reagents displaces the base and the germanium compounds exhibit reactivity pattern similar to transition metals. This property holds in the promise that by custom synthesis metal-free catalytically active compounds can be created, which might be regarded as alternatives or supplements to conventional metal-based catalysts.As an extension of the area just described, experiments were performed in which divalent germanium compounds react with metals. The main reason for the unusual properties of the divalent germanium compounds is their ability to act both as an acid and as a base. This way they can form bonds with electron rich compounds such as alkynes as well as with electron poor compounds such as metal ions. How this can be used to manipulate the reactivity of metal complexes is one of the exciting questions that need to be addressed.