Frustrated Lewis Pairs beyond the Main Group: Cationic Zirconocene–Phosphinoaryloxide Complexes and Their Application in Catalytic Dehydrogenation of Amine Boranes

The cationic zirconocene–phosphinoaryloxide complexes [Cp2ZrOC6H4P(t-Bu)2][B(C6F5)4] (3) and [Cp*2ZrOC6H4P(t-Bu)2][B(C6F5)4] (4) were synthesized by the reaction of Cp2ZrMe2 or Cp*2ZrMe2 with 2-(diphenylphosphino)phenol followed by protonation with [2,6-di-tert-butylpyridinium][B(C6F5)4]. Compound 3 exhibits a Zr–P bond, whereas the bulkier Cp* derivative 4 was isolated as a chlorobenzene adduct without this Zr–P interaction. These compounds can be described as transition-metal-containing versions of linked frustrated Lewis pairs (FLPs), and treatment of 4 with H2 under mild conditions cleaved H2 in a fashion analogous to that for main-group FLPs. Their reactivity in amine borane dehydrogenation also mimics that of main-group FLPs, and they dehydrogenate a range of amine borane adducts. However, in contrast to main-group FLPs, 3 and 4 achieve this transformation in a catalytic rather than stoichiometric sense, with rates superior to those for previous high-valent catalysts.