Frustrated Lewis Pairs beyond the Main Group: Synthesis, Reactivity, and Small Molecule Activation with Cationic Zirconocene–Phosphinoaryloxide Complexes

The extension of the frustrated Lewis pair (FLP) concept to the transition series with cationic zirconocene–phosphinoaryloxide complexes is demonstrated. Such complexes mimic the reactivity of main group FLPs in reactions such as heterolytic hydrogen cleavage, CO₂ activation, olefin and alkyne addition, and ring-opening of tetrahydrofuran. The interplay between sterics and electronics is shown to have an important role in determining the reactivity of these compounds with hydrogen in particular. The Zr–H species generated from the heterolytic activation of hydrogen is shown to undergo insertion reactions with both CO₂ and CO. Crucially, these transition metal FLPs are markedly more reactive than main group systems in many cases, and in addition to the usual array of reactions they demonstrate unprecedented reactivity in the activation of small molecules. This includes S_N2 and E2 reactions with alkyl chlorides and fluorides, enolate formation from acetone, and the cleavage of C–O bonds to facilitate S_N2 type reactions with noncyclic dialkyl ethers.