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-(diphenyl-phosphino)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 H-2 under mild conditions cleaved H-2 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.