Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile formation of phosphine-boranes and cationic phosphonium-boranes

Abstract

The concept of “frustrated Lewis pairs” involves donor and acceptor sites in which steric congestion precludes Lewis acid–base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R₃P(C₆F₄)BF(C₆F₅)₂] and [R₂PH(C₆F₄)BF(C₆F₅)₂]. These can be easily transformed into the cationic phosphonium-boranes [R₃P(C₆F₄)B(C₆F₅)₂]⁺ and [R₂PH(C₆F₄)B(C₆F₅)₂]⁺ or into the neutral phosphino-boranes R₂P(C₆F₄)B(C₆F₅)₂. This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann–Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C₆F₅)₃, while the acidity of the phosphine-boranes is diminished.