Group 4 Metallocenes in the Oxidation State +3

Over the last years group 4 netallocenes have conquered an important spot as reagents and catalysts. Besides their import position 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 was 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. This is partly caused by the paramagnetic properties, which make detailed NMR studies difficult, but also by synthetic difficulties. Preliminary results to this project have disclosed a novel synthetic access to these compounds which might be very general. As compounds such as Cp2 TiCl have turned out to be extremely versatile reagents in organic synthesis, further development of related reagents should uncover a huge synthetic potential. 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.

Metallocenes of group 4 (titanocens, zirconocenes and hafnocenes) have established themselves as important reagents and catalysts. They were even found to be useful as active agents in the medical treatment of tumors. The main importance of this class of compounds is nevertheless the use as polymerization catalysts (Ziegler-Natta olefin polymerization) and in organic synthesis. Their popularity in this field is associated with mild reaction conditions and high functional group compatibility. Group 4 metallocenes consist of a metal which is tetrahedrally coordinated by two cyclopentadienyl rings and two further substituents. In the current project these substituents are silyl groups. Although silylated metallocenes were already the subject of a number of investigations it is striking that most of these studies concentrated on zirconium, a few on hafnium and almost nothing is known about the respective silylated titanocenes. Also with respect to the oxidation state compounds with oxidation state +4 and +2 are well understood, whereas comparably few information exists about compounds featuring the metal in the oxidation state +3. This is partly caused by the paramagnetic behavior of these compounds which does not allow detailed NMR spectroscopic investigations but also by synthetic difficulties. In the present study, we demonstrated both a novel way for the directed synthesis of metallocenes in the oxidation state +3 but also the direct reduction from +4 to +3. In general the project showed a strong preference of Ti for the oxidation state +3 when sily groups are attached. This preference is even valid for Zr and Hf but not as strong as for Ti. In addition to the interaction between silyl groups and group 4 metallocenes we were also interested in the interaction between group 4 metallocenes and low coordinate group 14 compounds. In the course of the present project was extended to stannylenes and plumbylenes. Especially the group 4-lead compounds are interesting as they constitute the very first examples of compound containing bond between lead and a group 4 element. Overall it can be said that the group 4 group 14 chemistry investigated in this project has greatly expanded our knowledge of this important area and has partly already been published in renowned scientific journals.