Associate Professor of Chemistry
Vordiplom, Ruhr-Universität Bochum, Germany, 1985
Diplom, Westfälische Wilhelms Universität Münster, Germany, 1988
Dr. rer. nat., Westfälische Wilhelms Universität Münster, Germany, 1991
Postdoctoral Fellow, University of California, Davis, 1992-1998
Assistant Professor of Chemistry, University of Oklahoma, Norman, 1998-2005
Office: 314 Physical Sciences Building
Main Group Chemistry, Main Group Organometallic Chemistry, Inorganic Chemistry, Materials Chemistry, Catalysts
Very Strong Lewis Acids: Low Coordinate Cationic Group 13 Compounds
Lewis acids constitute a very important class of compounds in chemical synthesis. A classic example is AlCl3 as a catalyst in electrophilic substitution of aromatics. The value of the Lewis acidity of a given compound is closely connected to its electron configuration, coordination number and charge. Thus, methods for the increase of the Lewis acidity of a given compound include the reduction of its coordination number or introduction of a positive charge. In many cases higher Lewis acidity is well correlated to higher reactivity. This project aims at the synthesis and investigation of two-coordinate cationic aluminum and gallium compounds. These compounds will be realized by a combination of steric protection, some electronic stabilization and the use of so-called weakly coordinating anions such as fluorinated tetraphenyl borates, fluorinated tetraalkoxyalanates or halogenated carborane anions. The first examples of this type of compounds have been obtained using two large m-terphenyl substituents per metal center.
Due to the large size of the substituents these compounds display low reactivity and Lewis acidity. We have successfully prepared cationic species bearing only one terphenyl substituent such as [TerphGaBu]+. Depending on the size of the terphenyl substituent they either exist as a tight ion pair with close cation•••anion contacts or as solvent separated ions.
The compound [Dipp*GaBu][CHB11Cl11] (above left) slowly alkylizes benzene at room temperature.
Efforts to generate the corresponding aluminum based compounds are under way. Initial results indicate that they catalyze the alkylation of benzene with olefins, the oligomerization of 1-hexene and hydrodefluorination of PhCF3.
9-Heterofluorenes are compounds that are formally derived from the parent organic fluorene by substitution of the CH2 group in the 9-position with another element. Whereas the nitrogen, oxygen and sulfur derivatives are common organic compounds and are well studied, 9-heterofluorenes with other heteroelements such as boron, aluminum, gallium, silicon, germanium, tin, phosphorus or arsenic are relatively rare. Potential applications range from ligands in catalytic systems to building units in OLED's. Our approach utilizes low temperature C-H activation of suitable precursors.
Oxide and Nitride Materials
Oxide and nitride materials possess a very wide area of applications. We are interested in the development of non-hydrolytic low temperature routes to aluminum oxides and aluminum and gallium nitride. The first two materials have been used as support for catalysts, solid phase in chromatography or insulator in the electronic industry. Gallium nitride is a wide bandgap semiconductor, and is thus of interest for the fabrication of blue diodes and lasers. In fact, the new generation of CD and DVD players uses blue lasers based on gallium nitride.
Our current method of choice is the dehalo- or dehydrosilylation and –stannylation. A simplified scheme for the generation of GaN is given below.
Investigations in the gallium nitride project are currently centered on the screening for other leaving groups on Gallium and catalysts to accelerate the R3SnX elimination.
Work on the aluminum oxide system has resulted in the formation of an interesting aluminumoxyhydride, HAlO. This compound combines properties of a ceramic material with those of a chemically reactive reagent. For example, suspensions of HAlO may be used as mild and selective reducing agents for organic substrates.