Frustrated Lewis pairs derived from N-heterocyclic carbenes and Lewis acids

Abstract

The chemistry of frustrated Lewis pairs derived from N-heterocyclic carbenes and a number of Lewis acids has been probed. The combination of 1,3-bis[2,6-(di-iso-propyl)phenyl]-1,3-imidazol-2-ylidene (IDipp) (1) with B(C₆F₅)₃ was shown to give the classical Lewis acid–base adduct (IDipp)B(C₆F₅)₃ (2) which was unreactive. In contrast, the combination 1,3-di-tert-butyl-1,3-imidazol-2-ylidene (3) with B(C₆F₅)₃ proved to form a frustrated Lewis pair, and reacts with H₂ to give the salt [ItBuH][HB(C₆F₅)₃] (4) in high yield. In a similar fashion, addition of (3) to a series of amine-borane adducts including H₃NB(C₆F₅)₃ (5), PhH₂NB(C₆F₅)₃ (6) and PhH₂NB(C₆F₅)₃ (7) led to deprotonation and formation of imidazolium amido-borate salts [ItBuH][H₂NB(C₆F₅)₃] (8), [ItBuH][PhHNB(C₆F₅)₃] (9) and [ItBuH][Ph₂NB(C₆F₅)₃] (10). Similar reactions of the amine-borane adducts EtNH₂B(C₆F₅)₃ (11) tBuNH₂B(C₆F₅)₃ (12) and Et₂NHB(C₆F₅)₃ (13) gave the amidoboranes EtHNB(C₆F₅)₂ (14) tBuHNB(C₆F₅)₂ (15) and Et₂NB(C₆F₅)₂ (16) respectively, with liberation of C₆F₅H and an equivalent of unreacted carbene. Mechanistically these reactions are thought to proceed through transient imidazolium salts similar to (8)–(10). In addition, the combination of carbene (3) and the cation [CPh₃]⁺ in frustrated Lewis pair chemistry has been probed. Reaction of this combination at room temperature results in the immediate formation of [C₃H₂N₂tBu₂(C₆H₅)CPh₂][B(C₆F₅)₄] (17) stemming from carbene attack at a para-carbon of one of the phenyl rings of trityl. Addition of carbene to the benzyl amine adduct of [CPh₃][B(C₆F₅)₄] results in the formation of [ItBuH][B(C₆F₅)₄] (18) and the secondary amine Ph₃CNHCH₂Ph (19), thus providing the first example of an all carbon-based frustrated Lewis pair. Crystallographic data for (2), (4), (9) and (13) are reported.