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, Catalysis, CO2 Activation and Reduction
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, which is known to every student as a catalyst for 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. Several examples have been 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 carborate 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]+ and [TerphAlEt]+. Depending on the nature of the metal center they either exist as a tight ion pair with close cation•••anion contacts or as solvent separated ions.
While Nature utilizes CO2 as its major carbon source, the industrial use of CO2 as feedstock is still in its infancy. Our strong Lewis acids catalyze the reduction of CO2 with hydrosilanes to mostly methane and toluene depending on the Lewis acid and the solvent employed.
Current efforts focus on the optimization of this system including the synthesis of more stable Lewis acids such as the phenoxide substituted cation shown below. It features strong Al-O bonds and internal π-stabilization through the flanking arene substituents.
Low Oxidation State Group 13 Compounds
Aluminum, Gallium and Indium compounds usually feature the metal in its +3 oxidation state. In recent years the +1 oxidation state has also become accessible, but these compounds suffer from low stability and facile disproportionation reactions. This project aims at the stabilization of the +1 oxidation state by the use of ambiphilic ligands, i.e. ligands with both a donor and an acceptor center. We hypothesize that the donation of electron density from the lone pair at the M+ center into the empty p-orbital at the Lewis acidic boron center of the ligand stabilizes the unusual +1 oxidation state. The long term goal of this project to allow facile switching between the two oxidation states for applications in catalysis similar to the typical oxidative addition/reductive elimination cycles in late transition metal catalysts.
Oxide and Nitride Materials
Oxide and nitride materials possess a very wide range of applications. We are interested in the development of non-hydrolytic low temperature routes to metal oxides, oxynitrides and nitrides.
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.
Attempts to prepare precursors for nitrogen-doped itania (N-doped TiO2), a photocatalyst that responds to visible light, have led to interesting compounds.