New Computational and Experimental Methods for the Structural Characterization of Homogeneous Catalysts

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With funding from the Inorganic, Bioinorganic and Organometallic Chemistry program, Professor Clark Landis of the University of Wisconsin, Madison will undertake the development of new computational and experimental tools for elucidation of solution structures of organometallic molecules. Landis will develop methods for fitting experimental Nuclear Overhauser Effect timecourses in NMR spectra of organometallic molecules with timecourses that are calculated based on a population of possible conformers. Ensembles of possible conformers will be generated using force field minimization methods combined with systematic or Monte Carlo searching techniques. Solvent mixtures of high viscosity will be used to partially eliminate the loss of NOE due to rapid spin relaxation that normally results from the rapid rotation of small molecules. Secondly, a new empirical force field method (SHAPES) that is specifically designed for describing complex geometries of transition metal containing compounds will be developed. Both techniques will be applied to the determination of structures of homogeneous asymmetric hydrogenation and alpha-olefin catalysts. The ultimate aim of the research is the determination of the reasons behind the high stereoselectivity of these catalysts and the prediction of new and more highly stereoselective catalysts for these and other types of reactions. %%% New methods for the determination of structures of transition metal organometallic molecules in solution will be studied. Complex molecules in solutions are constantly twisting and turning, with consequent changes in overall shape. One of the methods to be developed to study molecular shapes in solution will utilize a variation of nuclear magnetic resonance spectroscopy (Nuclear Overhauser Effect). The existence of various conformational isomers (molecules with different twists) of the dissolved organometallic compounds will be considered in the interpretation of the spectral data so as to provide structural information for all participating isomers. Only an average structure can be deduced using existing methods. The second method to be developed will allow for the determination of structures using molecular mechanics methods that are specifically tailored to include parameters for transition metals. Such techniques are well developed for small organic molecules and proteins but have only begun to be applied to the computation of structures of organometallic molecules. The ultimate objective of the work is to be able to understand the origins of stereoselectivity in reactions catalyzed by organometallic compounds. Such reactions are very important in the chemical industry with applications to the manufacture of commodity, fine and specialty chemicals. An understanding of catalytic stereoselectivity potentially influences issues such as energy and raw materials utilization.