Development of redox-switchable catalysts based on chiral 9,9,10,10-tetraaryl-9,10,-dihydrophenanthrenes

• Applicant: Dr. Dieter Schaarschmidt
• Subject Area: Inorganic Molecular Chemistry - Synthesis and Characterisation
• Term: from 2015 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 282983038

The use of tailor-made metal complexes allows adjusting the activity of homogeneously catalysed reactions as well as to achieve a high selectivity. Another possibility to influence these processes is the application of switchable catalysts, which received so far only little attention. Switchable catalysts are characterized by possessing two different states which differ in their catalytic behaviour. The reversible interconversion between these two states can be achieved by applying external stimuli.Transition metal complexes based on chiral 4,5-disubstituted 9,9,10,10-tetraaryl-9,10-dihydrophenanthrenes will be applied in redox-switchable catalytic processes within this research project. The oxidation of the 9,10-dihydrophenanthrenes leads to C-C bond cleavage and yields 2,2'-bis(diarylmethylium)biphenyls. This is accompanied by an increase of the dihedral angle of the biphenyl and will consequently result in a distortion of the coordination geometry around the catalytically active metal atom. It is therefore to be expected that the oxidized and the reduced catalyst show different enantioselectivities in asymmetric processes.Achieving this objective involves the synthesis of different 4,5-disubstituted 9,9,10,10-tetraaryl-9,10-dihydrophenanthrenes and the corresponding metal complexes as well as the determination of their electrochemical characteristics; the oxidation and reduction potentials can be adjusted by the electronic properties of the aromatic substituents in position 9 and 10. The examination of the metal complexes in both oxidation states by X-ray diffraction and nmr spectroscopy allows for the determination of the structural changes caused by oxidation both in the solid state as well as in solution. The synthesized metal complexes will be applied as catalysts in several asymmetric transformations including allylic substitutions. The influence of the redox-switch onto the catalytic activity and the enantioselectivity will be the main focus. Comparing these results with the structural parameters derived before should allow designing a metal complex exhibiting an increased impact onto the enantioselectivity upon redox-switch. In continuation of these investigations titanium complexes will be used for the polymerization of lactide. Stereoblock copolymers of lactide may be accessible by switching between the oxidized and reduced state of the catalyst. The redox-switch shall be achieved by applying an electrical potential using an electrochemical setup.

• DFG Programme: Research Fellowships
• International Connection: United Kingdom
• Host: Professor Dr. Nicholas J. Long