Topic: New Catalytic Small Molecule Transformations Based on Metal- and Ligand Redox Activity at Lanthanides: Establishing 4f-Photoredox- and Bioinorganic Chemistries
Speaker: Professor Eric Schelter - University of Pennsylvalnia
Host: Professor Aaron Odom
Date: Monday, March 5, 2018
Time: 11:20 AM
Location: 136 CEM
Lanthanide chemistry is largely defined by the ability of the elements to act as Lewis acids in the +3 oxidation state. Among the lanthanides, cerium is an exception with an established and growing chemistry in the +4 oxidation state. Study of this higher oxidation state of cerium is important in a number of contexts, including separations chemistry and metal-ligand multiple bonds. We have been developing the chemistry of tetravalent cerium for fundamental reasons and for the development of new applications. My group has recently shown that cerium can act as an effective photoredox mediator through absorption of high energy visible or low energy UV light. The fundamental photophysical characteristics, emerging from characteristic 4f-5d electronic transitions, including nanosecond lifetimes, quantum yields >80% in some cases, and strongly reducing excited state reduction potentials, will be discussed. And the application of cerium compounds as photoredox mediators for unique, catalytic reactivities will be described. Cerium is also a remarkable lanthanide element because of its large abundance in the earth’s crust, comparable to copper or nickel. Despite the large abundance, it had been widely believed that cerium and other lanthanides had no role to play in biology. Recently, however, a metalloenzyme was discovered that includes cerium, lanthanum and other early lanthanide elements in its active site. This methanol dehydrogenase, Xoxf, catalyzes the conversion of methanol to formaldehyde and, uniquely, to formate. We have developed a tethered quinolone quinone ligand that replicates the structure and function of the active site of the Xoxf metalloenzyme for the first time. We have used this compound to study the reactivity of alcohol dehydrogenation using a benzylic alcohol test substrate. Our results contribute to understanding about why Nature selects for lanthanides in this case and to understanding the reaction mechanism of the Xoxf methanol dehydrogenase.