Hypercoordinated silanes with heavier group 14 element-element bonds

While the chemistry of the lightest group 14 element carbon (organic chemistry) is highly developed, the chemistry of the higher group 14 elements: silicon, germanium, tin, and lead is much simpler and also less developed. However, although the heavier elements lack carbon`s ease to hybridize orbitals and thus its amazing structural and functional variety, they have some unique qualities of themselves to offer. These include stable divalent states, electron delocalization along s-bonds, and a much higher tendency to engage in hyper-coordinate bonding situations. The current proposal aims at investigations combining the last two mentioned features of heavier group 14 elements. In organic chemistry it is long known that in organic molecules with extended p-electron systems the p-electrons are delocalized over more than one (and often many) bonds. This concept of electron delocalization is not only important to understand reactivity of organic molecules, it is also responsible for the property of certain organic molecules to work as conducting material f.i in organic light emitting diodes and other organic circuitry. Not as well known is the fact that catenated heavier analogs of carbon such as polysilanes, -germanes and stannanes also exhibit the feature of electron delocalization. However, electrons in these molecules are not delocalized along an extended p-system but along a number of s-bonds. For both types of electron delocalization phenomena it is important that the molecule is oriented to allow the involved orbitals to overlap effectively. For polysilanes with comparably long Si-Si bonds the rotational barriers to attain certain spatial orientations are very low. Thus, alignment of the main chain needs to be adjusted by some measures such as the introduction of bulky substituent which force the chain into a specific conformation. Other methods entail inclusion into cyclodextrines, incorporation into bicyclic systems. Another property which distinguishes the heavier group 14 elements from carbon is that of hyper-coordination, meaning that these atoms can accommodate interactions with more than four other atoms. The proposed research intends to study the effect of hyper-coordination on s-bond electron delocalization. It will be investigated how hyper-coordination can be utilized to control and enhance the conductivity of polysilanes. The involved Austrian and Russian research groups are specialists in the field of polysilanes and hypercoordinated group 14 elements. They will join forces and utilize their respective expertise to study this new area of material science.

The predictive strength of the periodic table of elements relies on the fact that the reactivity of elements of the same group is very similar. However, the elements of the second period (Li-F) are usually substantially more reactive than their heavier analogues. In group 14 this trend can be seen very clearly: the light element carbon is superior to all other elements in terms of reactivity. Nevertheless, this is not true with regard to the valency of the compounds of the elements. While there is a very strong tendency toward the tetravalent state in carbon compounds, the heavier elements can comparatively easy form both higher and lower valence compounds for different reasons.The present project dealt with penta-coordinate silicon compounds. The main reasons why silicon is capable of forming such compounds are its larger atomic radius and a stronger electropositive character. One of the best known classes of hypercoordinated silanes is that of the siliatranes, where the nitrogen atom of a triethanolamine ligand undergoes an additional interaction with the silicon atom.Although the chemistry of silatranes with comparatively electron-withdrawing substituents is quite well investigated, there were hardly any examples with electron-donating groups previous to our studies. By attaching a tris(trimethylsilyl)silyl group, we were able to produce a first example of an oligosilanylated silatrane. By means of selective trimethylsilyl group substitution, it was possible to prepare a variety of different derivatives. Using these compounds, the influence of the silatranyl group on the property of the ?-bond electron delocalization was investigated. Unfortunately, it was found that the latter is hardly influenced or even slightly diminished.A particularly interesting aspect of the study was the preparation of metallated silatranylsilanes. In these compounds, the silyl group on the silatrane is relatively strongly electron-donating and thus weakens the silicon-nitrogen interaction. These compounds are therefore ideally suited to analyze the Si-N interaction in their entire range theoretically and experimentally.In addition to our studies dealing primarily with the interaction of silicon and nitrogen, we could show that silatranyloligosilanes are interesting ligand systems for silylated lanthanides. Especially divalent lanthanide complexes combine low valence with a large coordination sphere. Since lanthanide ions are strongly Lewis acidic, the formation of very labile and hard to handle solvent complexes is frequently observed. This can be prevented if the ionic ligands provide additional donor sites. Silatranyloligosilanyl substituents offer exactly this desired property and thus facilitate handling of silylated lanthanides substantially.