Our Research interests [pdf]
Our research interests are in the area of Organic and Organometallic Chemistry. Organometallic chemistry is the study of compounds having metal-carbon bonds. Carbon-containing (organic) molecular fragments that are bonded to metal atoms often have unique geometries and reactivities on account of the metal's electronic properties. Organometallic compounds are important in catalysis, medicine, and the construction of molecular scale devices (nanoscience).
Research goal: Catalysts!
Catalysts have a huge economical impact on all different kind of syntheses. Catalysts increase the efficiency and selectivity of chemical reactions, and they help saving ressources and energy. Organometallic compounds often have unique reactivities on account of the metal's electronic properties. and they can serve as catalysts in organic transformations. It is our goal to find novel organometallic archtiectures, which are catalytically active and easy to handle.
Olefin- and Alkyne-Metathesis in the Coordination Spheres of Transition Metal Complexes
Ring closing metathesis is a powerful tool for the synthesis of small or large ring systems. The application of the ring closing metathesis reaction (applying e.g. Grubb's olefin metathesis catalyst or Schrock's alkyne metathesis catalyst) in the coordination sphere of transition metal complexes provides a novel route to macrocyclic metal complexes. These metal complexes are interesting classes of compounds that might serve as starting point for molecular scale devices (Scheme 1).
I designed and synthesized several novel molecular architectures such as metal-containing big molecular ring systems or protected molecular wires. Those compounds might be a starting point for the construction of "molecular machines", such as motors, rotors or gyroscopes. Published here, here and here. Alkyne metathesis was applied as well
Diastereoselective Assembly of Phosphametallocenes
Phosphametallocenes are an interesting class of compounds, which have similar properties than metallocenes. In phosphametallocenes, one carbon of the coordinated cyclopentadienyl ligand is replaced by phosphorus, forming a phospholylligand. Due to the phosphorus in the ring system, unsymetrically substituted phospholylligands form two diastereomers (rac and meso).
The lone pair of the phosphorus in the coordinated phospholylligand is capable of coordination to a transition metal such as Rhodium (Scheme 2). It turned out that combination of a rac/meso mixture of a phosphazirconocene I with a diasteropure Rhodium complex leads to a single diastereomer of a bimetallic Rh-Zr complex II. A rac/meso conversion occurs on Zr, either before or after coordination, giving diasteroselectivity. The rac meso isomerization of chiral phosphametallocenes most likely follows a ring-slip/inversion/ring-slip mechanism (Hollis, T. K.; Wang, L.-S.; Tham, F. J. Am. Chem. Soc. 2000, 122, 11737).
The coordinated phosphametallocene undergoes a ring slip from hapto5 coordination to hapto1 coordination followed by an inversion of the ligand. Ring slip back to a hapto5-coordinated phosphyl-ligand affords an isomer different from the starting material.
This slip/inverion/slip mechanism takes place either before or after the bimetallic assembly of I to II.
Novel Carbene Complexes and their Application as Catalysts in Organic Synthesis
Carbenes are a relatively new class of compounds which contain divalent carbon atoms and a lone pair. Carbenes can serve as ligands in transition metal complexes and influence their electronic properties. Some of those complexes are excellent catalysts in reactions such as hydrosilylation (Scheme 3) and hydroamination.
Hydrosilylation (addition of silanes to multiple C-C bonds) constitutes an interesting way to silanes. Silanes are important building blocks in organic synthesis. However, typically the addition of silanes to triple bonds provides a mixture of E, Z and alpha isomers. It turned out that the novel carbene complexes 1 and 2 are capable of catalyzing hydrosilylation (Scheme 3). After 1 h at 80 deg C, typically the (Z)-isomer was obtained for terminal alkynes, and the (E) isomer for internal ones. The catalysts are compatible with alcohols. Further applications are under investigation. Two publications, one here and one here.