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Catalysis of Early Transition Metals

Dr. Perdita Arndt

The research involves the coordination chemistry as well as the catalytic features of metallocene and non-metallocene complexes (particularly Ti and Zr). We use the obtained results to evaluate new catalytic reactions and processes and also to optimize existing procedures.

Titanocene and zirconocene complexes of bis(trimethylsilyl) acetylene (Figure 1) are used as reagents, because they release the coordinative and electronically unsaturated metallocene fragment "Cp’₂Ti" or "Cp’₂Zr" after dissociation of the alkyne under mild conditions ¹.

The results obtained so far should be exemplified under three different topics:

• Exotic five-ring metallacycles of titanium and zirconium (Figure 2)
• Unusual cleavage of C-F bonds
• Polymerisation and oligomerisation catalysis of olefins

By formal abstraction of methyl anions from dimethyl complexes of early transition metals Cp’₂MMe₂ (M = Ti, Zr; Cp’ = substituted or unsubstituted η⁵-cyclopentadienyl) using strong Lewis acids like B(C₆F₅)₃, certain ion pairs [(Cp’₂ZrR)]⁺[RB(C₆F₅)₃]^- are formed. These polar complexes can act as highly reactive catalysts for olefin polymerisations. Therefore we also investigated the reaction of a number of different metallocenes with B(C₆F₅)₃ ^{2, 3}. Most frequently deactivation reactions leading to the poisoning of these catalysts can be attributed to the C₆F₅ group transfer to the metal center, followed by transfer of a fluoride to the metal. We have been able to show for the first time, that the transition metal fluoride can be reactivated by using Al-H sources, giving Zr-H and Al-F as products ⁴.

Different mono-Cp complexes of the type Cp’ML_n (with M = Ti, Zr, Cp’ = cyclopentadienyl dimethylsilylamide and L = different ligands) are currently under investigation as “constrained catalysts” (CGC) for their potential and activity in polymerisation reactions.

Another emphasis is put on the investigation of non-metallocene complexes with stabilized and nonstabilized phosphorus ylides as monodentate ligands ^{5, 6}. In addition chelate ligands are investigated as interesting alternatives compared to the Cp ligand systems. A project supported by the DFG within the Schwerpunktprogramm 1166 is engaged with the use of phosphorus ylide complexes of lanthanides in catalytic reactions. Within this project chelate ligands containing an ylide and a hemilabile donor group are evaluated for the complexation of rare earth elements and also the use in catalytic reactions. Different lanthanide complexes with metallated ylides of the type „PCPC“  ([Ph₂P-CH=PPh₂-CHR]^-) are utilized in certain catalytic processes like the ROP of ε-caprolactone.  

1. U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 2003, 22, 884.
2. U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Eur. J. Inorg. Chem. 2004, 22, 4739.
3. U. Rosenthal, V. V. Burlakov, P. Arndt, W. Baumann, A. Spannenberg, Organometallics 2005, 24, 456.
4. P. Arndt, U. Jäger-Fiedler, M. Klahn, W. Baumann, A. Spannenberg, V. V. Burlakov, U. Rosenthal, Angew. Chem. 2006, 118, 4301; Angew. Chem. Int. Ed. 2006, 45, 4195.
5. A. Spannenberg, W. Baumann, U. Rosenthal, Organometallics 2000, 19, 3991.
6. A. Spannenberg, B. H. Müller, U. Rosenthal, Z. Kristallogr. NCS 2005, 220, 581.