A Proton-Switchable Bifunctional Ruthenium Complex That Catalyzes Nitrile Hydroboration

A new bifunctional pincer ligand
framework bearing pendent proton-responsive
hydroxyl groups was prepared and metalated with Ru­(II) and subsequently
isolated in four discrete protonation states. Stoichiometric reactions
with H2 and HBPin showed facile E–H (E = H or BPin)
activation across a Ru­(II)–O bond, providing access to unusual
Ru–H species with strong interactions with neighboring proton
and boron atoms. These complexes were found to promote the catalytic
hydroboration of ketones and nitriles under mild conditions, and the
activity was highly dependent on the ligand’s protonation state.
Mechanistic experiments revealed a crucial role of the pendent hydroxyl
groups for catalytic activity.