Former FWF-Projects
Hypercoordinated silanes with heavier group 14 element-element bonds (P 26417)
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 σ-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 π-electron systems the π-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 π-system but along a number of σ-bonds. For both types of electron delocalization phenomena it is important that the mol ecule 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 σ-bond electron delocalization. It will be investigated how hyper-coordination can be utilized to control and enhance the conductivity of polysilanes....
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Silyl substituted Carbene analogs - Synthesis and Coordination chemistry (P 25124)
Over the last some 40 years the chemistry of group 14 elements, which due to their four valence electrons exhibit a strong preference for tetravalency has been supplemented by compounds with a divalent bonding behavior, the so called tetrylenes. The main difference between the lightest tetrylenes, the carbenes, and their heavier congeners concerns the electronic ground state. Due to increasing energy differences of s- and p-orbitals of higher elements, hybridization becomes an increasingly less favorable process. Therefore, the two free electrons of a carbene avoid spin pairing and prefer a triplet ground state, while in the heavier tetrylenes these electrons are both found in the s-orbital (lone pair character) which leaves a vacant p-orbital. Theses difference in ground states cause of course hugely different chemical reactivities. A simple attempt to control the reactivity of tetrylenes addresses the orbital energies of the divalent atoms by attaching substituents with different electronegativity. Attaching more electropositive substituents in this context facilitate the hybridization of s and p-orbitals and thus give the tetrylenes even more carbene type character. As carbenes in general tend to dimerize to alkenes for “more carbene like higher tetrylenes” the occurrence of dimers with regular π-bonds is more likely. Recent studies from our group concerned the synthesis and to some extent also the reactivity behaviour of cyclic disilylated germylene, stannylene, and plumbylene derivatives. While these are very reactive compounds of which all of them behaved differently, it was possible to isolate them as base-stabilized adducts. These “instant-tetrylenes” provide us with the opportunity to study not only the free tetrylenes, but also their coordination chemistry. This includes investigating their ambiphilic properties and even more interesting their behaviour as ligands in transition metal chemistry. Preliminary work in this area using early transition metals of group 4 and late metals with d10 electron configuration, gives a strong promise of quite unique chemistry occurring in the coordination sphere of the metals. So far we have were studied only tetrylenes with a very specific tetrasilanylene substituent but for a more thorough understanding of this chemistry the question of variation of the silyl-ligand framework needs to be addressed. ...
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Polygermanium chemistry (P 222678)
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. In connection to these studies we became interested in analogous chemistry of germanium and recognized the unique qualities of 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 Justin Holmes/Cork University College revealed also a quite unusual thermolytic decomposition behavior in supercritical fluids. In one step silylated germanium compounds decompose to nanowires possessing a crystalline germanium core covered by an amorphous siliconoxide surface. 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. ...
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Skeletal Rearrangement Processes in Organometallic Cations of Group 14 Elements (I 669)
Despite the highly interesting electronic properties of polysilanes the chemist’s repertoire for their synthesis of structural manipulation is mostly limited to the Wurtz type coupling reaction. In this connection Lewis acid catalyzed rearrangement reactions represent an interesting method to access molecular structures, which are otherwise difficult to obtain. Closely related to the well know Wagner-Meerwein rearrangement, it is a much more facile process for polysilanes. Although first examples of polysilane rearrangement have been reported almost 40 years ago not much is known about the governing principles. The current project, to be carried out by the groups in Oldenburg and Graz, proposes a joint study comprising new synthetic approaches, investigations of intermediates and reaction mechanisms, both supported by computational methods. Preliminary studies have already shown that the reaction of isomeric polysilanes with aluminum chloride can result in the formation of rather complex structures such as an adamantane type tricyclic polysilane. In addition it was found that germanium containing polysilanes rearrange with the germanium atoms selectively moved to trisilylated positions of the product. Other preliminary results suggest the Lewis acid catalyzed reaction can also be used for the synthesis of polysilanes containing alkylene units and even further for the preparation of complex carbosilanes. The synthetic work in this project, conducted by the Austrian group, will benefit from the theoretical and basic studies (of the German group) which will provide insight into the underlying principles. On the other hand will the synthetic work allow formulating new questions which will be addressed by computational methods. The unique combined synthetic and theoretical expertise of the two groups will allow the best possible treatment of the complex matter of Lewis acid catalyzed rearrangement reactions. ...
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Group 4 Metallocenen in the Oxidation State +3 (P 21346)
Group 4 metallocenes are important as reagents and catalysts. Besides their use as Ziegler-Natta catalysts they are also able to promote the catenation of silicon atoms to polysilanes. For this and other reasons the synthesis and investigation of silylated metallocenes has developed to an important area of research. It is striking that titanium is the least understood metal in this conjunction. Recent studies from our group have shown that titanium silyl compounds show a much stronger preference for the oxidation state +3 than expected. A more detailed investigation has shown that certain silyl groups also promote the stabilization of the oxidation state +3 for zirconium and hafnium. While metallocenes in the oxidation state +4 are extremely well investigated, information about the corresponding compounds in the oxidation state +3 is scarce. Preliminary results to this project have disclosed a novel synthetic access to these compounds which might be very general. A simple access to low-valent zircono- and hafnocenes would be particularly useful to investigate the properties and potential of these compounds in some detail. ...
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Silenes and Disilenes (P 19338)
Over the last years the chemistry of unsaturated silicon compounds (silenes, disilenes, silylenes, disilynes) has experienced a tremendous boost. Stable examples of all classes of compounds could be obtained and the general reactivity pattern have been established. In the course of our studies concerning the chemistry of polysilylanions we could establish additional synthtic routes to disilenes. In addition we could prepare the first examples of group 4 disilene complexes and also the first examples of disilene fluoride adducts. ...
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Molecular substructures of the silicon crystal lattice (P 18538)
The development of microelectronics to ever increasing integration densities will face some physical limits in the near future. One research trend dealing with this scenario involves the development of molecular electronics (unlike the current solid state electronics). One aspect of this research involves the study of small sub-units of the silicon crystal lattice, f.i. the preparation of silicon nano-crystals. ...
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Co-mediated Si-Si bond activation (P 18382)
Cobalt complexed alkynylsilanes and even compounds containing silicon cobalt bonds have been known for some while. In this project a number of new cobalt complexed alkynyloligosilanes are being prepared. These compounds exhibit an unusually strong activation of Si-Si bonds within the oligosilyl fragment. This reactivity is responsible for the formation of a number of unexpected rearrangement products containing bridging silylene ligands. ...
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Chiral Polysilanes (Y 120)
The main achievements of the START-project Y-120 lie in the development of suitable methods for the formation of silicon-silicon bonds. With these reactions at hand we are able to selectively prepare complex compounds. The key topic we have addressed in this connection is the formation of negatively charged polysilane compounds. While the chemistry of silicon compounds, where silicon acts as the positively charged part is very well developed, the area with silicon in the opposite polarisation is only sparsely developed. However, the combination of theses two areas provides a great synthetic tool for the preparation of polysilanes. One apect of polysilanes we were especially interested in is that of cyclic structures. A definite highlight of the project is the elegant synthesis of a first small polysilane molecule, which is a molecular substructure of crystalline silicon. The use of polysilanes as part of electronic devices is still an area, which still has to be developed, however, some important foundations for the targeted synthesis of these compounds have been laid by the work carried out in this project ...
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