| Synthetic and Mechanistic Organotransition Metal Chemistry |
Our research program in synthetic and mechanistic organotransition metal chemistry is directed toward finding new ways in which the reactivity of organic molecules is altered in the coordination sphere of metals, and toward better mechanistic understanding of new and known organometallic transformations. Our approach is to prepare new or known molecules to carry out exploratory reactivity studies. Reactions on which we focus are investigated by means of kinetics, isotopic labeling, isotope effects, and structure-reactivity studies. This type of research provides an excellent training for students because they are exposed to synthetic methodology in organic, inorganic and often-highly-air-sensitive organometallic systems, and to many types of spectroscopy and instrumental analysis, especially to multinuclear FT NMR spectroscopy. They are trained to think carefully and in depth about chemical reactivity, to creatively establish hypotheses and to design incisive experiments to test them. In general they are taught to be rigorous yet creative chemical problem solvers.
The current emphasis of our work is on reactions of hydrocarbons in the coordination sphere of complexes of Os, Rh, and Ir with the focus on reactions which disrupt and/or form C-H and C-C bonds. In order to expand the scope of reactivity, we have been exploring this chemistry in the complexes bearing chemically hard ancillary ligands more commonly found in conventional inorganic or Werner complexes. Currently we are largely employing chelating amine ligands such as 1,4,7-triazacyclononane (which we abbreviate "Cn") and the tri-N-methyl analog (which we call "Cn*"). This environment, unconventional for organometallic chemistry, is revealing some interesting effects. Just three examples of the many projects underway or available are given here.
We are examining the "reaction surface" for alkanes in the coordination sphere of Rh and Ir complexes. It is generally believed now that oxidative addition and reductive elimination of alkanes to transition metal centers involve complexes of intact alkanes (sigma complexes") as intermediates, and considerable effect is widely going into trying to discern their properties and to observe them directly. We are investigating their intramolecular reactivity properties with respect to their barriers for oxidative addition, reductive elimination, migration of the metal about the intact alkane, and alkane dissociation, and competition for re-coordination of different hydrocarbons to the rhodium center. All of these processes must be central to any catalytic reaction for the modification or functionalization of alkanes mediated by these metals. Shown below is an example of a rhodium complex under investigation which exhibits migration of rhodium from C1 to C10 of its decyl group. Evidence strongly indicates that this is an intramolecular reaction of an intermediate decane complex of the rhodium cation.
In a different approach, we are seeking to examine electronic effects on this energy surface using neutral complexes of iridium such as those shown below. The neutral alkyl hydrides are much more stable for Iridium than rhodium. Variations of the types shown are planned, and these synthetic projects are underway.
We have previously reported on the unprecedented preparation of polyethylene in water by coordination catalysis using the single component catalyst [Cn*Rh(CH3)(OH)(H2O)]+; however, the rate of polymerization was extremely slow. Although this process may never be of industrial use, we are very interested in learning how to accelerate the polymerization rate in the context of learning how to manipulate catalysis. One of our approaches is to replace the hydroxide ligand with one which is a weaker Bronsted base, but a better p-donor base toward the rhodium. One example is the family of catalysts shown below, where E and Z have the same meaning as above.
Selected publications Zhen,
H.; Wang, C.; Hu, Y.; Flood, T. C. "Reactions
of (Trimethyltriazacyclononane)- Rh(vinyl)3, -Rh(Z-propenyl)3,
and -Rh(vinly)2Me with Protic Acids. The Relative Migratory Aptitude of Methyl and Vinyl Groups to
an (Ethylidene)Rh Alkylidene Carbon."
Zhen, H.; Wang, C.; Hu, Y.; Flood, T. C. "Reactions of (Trimethyltriazacyclononane)- Rh(vinyl)3, -Rh(Z-propenyl)3, and -Rh(vinly)2Me with Protic Acids. The Relative Migratory Aptitude of Methyl and Vinyl Groups to an (Ethylidene)Rh Alkylidene Carbon."Organometallics 1998, 17, 5397-5405.