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Synthesis and characterization of organometallic, paramagnetic tervalent titanium compounds and their role in catalysis


Group 4, cationic complexes (Cp2MR)+A-(Cp=cyclopentadienyl, M= Ti, Zr, Hf; A= complex anion) are the last generation of very active olefin polymerisation catalysts. These and related complexes are expected to give full control of catalytic olefin polymerisation process at the molecular level and to lead to completely new materials on the basis of simple olefins. Of the active metals, titanium is for economic reasons most attractive, but the complication seems to be that the metal can have various, energetically not far different, oxidation states. Catalytic activity seems to be limited to cationic, four valent complexes according to knowledge obtained so far. The reason for this, is not clear at all. Neutral, tervalent titanium complexes Cp2TiR meet the basic criteria for catalytic activity. They have a Ti-C bond ready for insertion and a vacant coordination site suited to pre-complex and activate an olefin monomer. So far, no successful application of this type of compounds in catalytic olefin polymerisation has been described. From exploratory work by the Teuben group on Cp*zTiR derivatives it is known that olefins do interact with the metal center but so far insertion into the Ti-C bond and building of a polymer chain has not been observed. In the main part of the project we will concentrate on the question whether it is possible to modify the Cp2TiR system so that it becomes an active polymerisation catalyst. Various approaches will be tried out and some examples will be described in some detail below.
1) modification of the cyclopentadienyl ligands by varying the number and nature of alkyl substituents on the ring is a first target. Some exploratory research indicate that there is a dramatic change in reactivity when e.g. reducing the number of methyl substituents 5 to 4 or 3. 2) Introduction of an ansa-metallocene instead of the regular bis(cyclopentadienyl) ligand system will give the opportunity to close (or open) the Cp-Ti-Cp angle, together with improving the stability of the compounds towards loss of Cp ligands. It is expected that this also will strongly influence the reactivity of the Ri-C bond in Cp2Ti-R compounds. 3) Another approach that will be tried is introduction of bidentate, dianionic ligand systems Cp-X (e.g. Cp-amide, Cpphosphide, Cp-alhoxide, Cp-sulfide, Cp-benzamidinate, others) in which the two anionic functions are linked through a chain of variable length and rigidity. In all sub-projects synthesis of new complexes will be followed by a brief, exploratory investigation of some general aspects of the reactivity of the Ti-C bond with special emphasis on the reactivity toward olefins. As a second sub-project it will be investigated whether it is possible to electrochemically switch on polymerisation activity of compounds Cp2TiR and (Cp-X)TiR by oxidizing these on an electrode in the presence of a-olefins. The cationic species (Cp2TiR)+ and ((Cp-X)TiR)+ thus formed are known to be active catalysts.6
If this is successful it will be an important achievement in itself but it also opens up the perspective on other interesting and possibly commercially attractive perspectives.6 It could be possible to intentionally generate a-olefins of a particular molecular weight by reversing the electrode potential after the polymerisation process has been allowed to proceed for some time. Based on what is known for compounds Cp*2TiR it can be assumed that the neutral compounds Cp2TiR (R= polymer, formed after reversing the potential chain) are inactive for further olefin polymerisation. Instead they will be have a tendency to transfer _-H from the polymer chain to titanium thus generating hydride olefin complexes CpzTi(olefin)H. For Cp*2TiR it has been observed that in this type of complexes the olefin can be replaced easily for another olefin (monomer). The higher a-olefin can diffuse away from the metal center and upon reoxidation a new polymer chain will start to grow. 6


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