Silylium Ion/Phosphane Lewis Pairs

The reactivity of a series of silylium ion/phosphane Lewis pairs was studied. Triarylsilylium borates 4[B(C6F5)(4)] form frustrated Lewis pairs (FLPs) of moderate stability with sterically hindered phosphanes 2. Some of these FLPs are able to cleave dihydrogen under ambient conditions. The combination of bulky trialkylphosphanes with triarylsilylium ions can be used to sequester CO2 in the form of silylacylphosphonium ions 12. The ability to activate molecular hydrogen by reaction of silylium ion/phosphane Lewis pairs is dominated by thermodynamic and steric factors. For a given silylium ion increasing proton affinity and increasing steric hindrance of the phosphane proved to be beneficial. Nevertheless, excessive steric hindrance leads to a breakdown of the dihydrogen-splitting activity of a silylium/phosphane Lewis pair.