A neutral germanium-centred hard and soft lewis superacid and its unique reactivity towards hydrosilanes

Abstract

The germanium-centred Lewis superacid Ge(pin^F)₂ (1) was isolated as acetonitrile mono-adduct 1·MeCN and thoroughly characterized by NMR spectroscopy, X-ray crystallography and quantum chemical calculations. Ion abstraction and NMR experiments revealed the hard as well as soft Lewis superacidic nature of 1·MeCN. The title compound readily activates hydrosilanes such as Et₃SiH, which is not feasible for its harder silicon homologue 2·MeCN, and even reacts with Et₃SiF. The strongly coordinating acetonitrile could be abstracted by B(C₆F₅), giving the donor-free Ge(pin^F)₂ (1) and Si(pin^F)₂ (2) which are Lewis superacids. Unlike 1·MeCN, the donor-free 1 efficiently catalyses hydrosilylation of α-methylstyrene by Et₃SiH. For this process, an inverse temperature dependence was observed, i.e. a complete conversion was achieved rapidly when the reaction was cooled to −35 °C, but the reaction stopped at elevated temperatures. Mechanistic investigations, including stoichiometric experiments and quantum chemical calculations, outlined the formation of germylene Ge(pin^F) (3), which acts as the active catalyst. The germylene is formed by reductive elimination of the silylated pinacol from the hydrogermane intermediate, which is obtained by the initial reaction of 1 with Et₃SiH. The inverse temperature dependence of the catalytic reaction could be explained by low entropy associated with the complexation of two cooperating germylenes and the substrates. With this example we introduce an in situ generated Lewis acidic germylene complex for catalytic hydrosilylation of olefins and again exemplify the great potential of main-group-element-based complexes in catalysis.