Sergey Sedinkin
M.S. in Chemistry, August, 2009, University of Missouri-St. Louis.
M.S. in Chemistry, June, 2006, Higher Chemical College of the Russian Academy of Sciences

Rational Ligand Design for Potential Applications in Transition Metal Catalysis

Abstract

Strategies to influence the steric and electronic properties of three classes of ligands were investigated. First, a compound class known as amino-dithiaphospholanes was studied. It is structurally related to phosphoramidites which have been successfully applied in catalysis. The synthesis of amino-dithiaphospholanes was envisaged as an approach to electronically tuned phosphoramidites. General access to a variety of structurally modified amino-dithiaphospholanes from commercially available starting materials was developed. The investigation of their chemical reactivities indicated that a combination of electronic, steric and physical properties determined the stability of the target ligands. The electron donating properties of the new ligands were characterized by NMR. They were found to be more basic and, thus, more electon donating than the corresponding phosphoramidites. The coordination chemistry of amino-dithiaphospholanes was investigated by synthesis of a series of iridium and rhodium complexes. Analysis of the physical data obtained for the complexes confirmed the increase in the electron density at the metal centers as a consequence of the increased basicity of the ligands. Second, a series of new, as well as known, phosphinooxazoline (PHOX) ligands was synthesized. A systematic study of structure-property relationships was performed with the PHOX ligands. It was shown that electronic tuning of the ligands is possible by varying the substituents on the phenyl ring. However, the tuning was seen to have unexpected, and often only minor influence on the electronic properties of the metal complexes from those ligands due to the structural divesity and the back-bonding abilities of the PHOX ligands. The efficiency of steric tuning was investigated by evaluation of X-ray structure analysis data of new iron PHOX complexes synthesized for the study. It was shown that the presence of substituents in the position  to the nitrogen atom of the oxazoline ring had the most profound impact on the geometries of the corresponding metal complexes.

Finally, five new multidentate N,O donating ligands (L) were synthesized and characterized. Their coordination chemistry was investigated by preparation of new iron complexes mimicking naturally occuring non-heme enzymes. The general formulation [Fe(L)₂](OTf)₂, [Fe(L)(OTf)₂] or [Fe(L)(OTf)](OTf) was established for the new complexes, and formation of only one isomer was confirmed for one of the complexes. The catalytic activity of the new compounds was demostrated in the oxidation of activated methylene groups and alcohols to the corresponding ketones.

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