Silyl Substituted Carbene Analogues - Preparation and Coordination Chemistry

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 ?-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.

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. The reactivity behavior of cyclic disilylated germylene and stannylene were investigated. While these are very reactive compounds 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. Furthermore the germylenes were investigated towards their behavior in reactions with alkynes and dichlorogermane. Insertion of them into the germanium silicon bond was observed under preservation of the germylene. The behavior of stannylenes and germylenes as ligands to group 11 metals were investigated. These stabilized tetrylenes were reacted with gold-, silver-, and copper cyanide and an astonishing variety of different reaction paths were observed depending on the used metal but also on the silyl skeleton of the ligands. Further investigations were carried out to the examination of compounds of the heavier group 14 elements with lanthanides in the oxidation state +II and +III. Once again the huge difference between cyclic and acyclic silyl substituents could be observed.