Photo Credit: Dr. Manish Khandelwal
Activation of Small Molecules and C-H and C-F Bonds using Low-Coordinate Cationic Group 13 Compounds
Lewis acid catalysts are well-established for numerous synthetic procedures at the laboratory or industrial scale. Strategies for the increase of the strength of these Lewis acid catalysts include the decrease of the coordination number, the employment of electronegative substituents, the introduction of a positive charge, the generation of a strained geometry at the metal center or a combination of these. This project focuses on the synthesis of low-coordinate cationic organometallic aluminum compounds [R2Al]+. This can be achieved by the use of halogenated carborane counterions such as [CHB11Cl11]-, which combine a very low basicity with a high chemical stability. The substituents on the aluminum center range from simple alkyls such as ethyl or isobutyl to large and bulky m-terphenyls allowing to fine-tune the catalytic activities and selectivities. Unlike many common Lewis acids these compounds are soluble in aromatic solvents.
A series of these compounds has been synthesized, and their structures were determined by single crystal X-ray diffraction. The interaction of these Lewis acids with olefins was surprisingly weak, but the new compounds were found to be excellent hydrosilylation and moderate olefin hydroamination catalysts. Both reaction types belong to the standard toolbox of a synthetic chemist. Another interesting and promising area concerns the catalytic decomposition of halogenated hydrocarbons including alkylfluorides. Very recently, it has been shown in this laboratory for the first time that Lewis acid catalysts could be employed for the catalytic reduction of carbon dioxide to methane, a potentially important conversion of a cheap waste chemical into a valuable feedstock. The activation of carbon monoxide, another combustion byproduct, will be investigated in the near future.