Date of Award
Doctor of Philosophy (PhD)
George G. Stanley
This work describes a study of the mechanism by which a unique homogeneous bimetallic hydroformylation catalyst based on two rhodium metal centers chelated and bridged by a special tetraphosphine ligand works via bimetallic cooperativity. Hydroformylation runs on this bimetallic system which indicate higher rates of production and higher linear to branched product regioselectivity relative to the commercial monometallic Rh/PPh3 catalyst make this system a viable candidate for an almost completely new and effective way of performing hydroformylation catalysis. Numerous complexes have been characterized by in situ FT-IR and NMR spectroscopic studies, as well as X-ray crystallography, and support the presence of dicationic bimetallic complexes as the most active hydroformylation catalyst species. It has been suggested by these studies that the high activity and regioselectivity for this bimetallic hydroformylation catalyst arises from maintenance of the electrophillicity of the rhodium metal centers by the combination of (1) a cationic charge on the metals, (2) strongly donating phosphine ligands that prevent the cationic hydrides from being too acidic, (3) the proper binucleating tetraphosphine ligand structure that enforces an overall optimal coordination geometry about the two rhodium centers, and (4) bimetallic cooperativity that assists in the redistribution of two hydride ligands from a single catalytically inactive Rh(+3) oxidation-state center to two Rh(+2) metal-metal bonded centers and then again for the bimetallic reductive elimination of the final aldehyde product. A new proposed mechanism for bimetallic hydroformylation via this system is presented based on these studies.
Matthews, Rhonda Carter, "In Situ NMR and FT-IR Studies on a Bimetallic Hydroformylation Catalyst." (1999). LSU Historical Dissertations and Theses. 6895.