An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules, Including Metal-Free Dehydrogenation of Water

The first single-component N-heterocyclic silylene borane 1 (LSi-R-BMes(2); L = PhC((NBu)-Bu-t)(2); R = 1,12-xanthendiyl spacer; Mes = 2,4,6-Me3C6H2), acting as a frustrated Lewis pair (FLP) in small-molecule activation, can be synthesized in 65% yields. Its HOMO is largely localized at the silicon(II) atom and the LUMO has mainly boron 2p character. In small-molecule activation 1 allows access to the intramolecular silanone-borane 3 featuring a Si=O -> B interaction through reaction with O-2, N2O, or CO2, and formation of silanethione borane 4 from reaction with S-8. The Si-II center in 1 undergoes immediate hydrogenation if exposed to H-2 at 1 atm pressure in benzene, affording the silane borane 5-H-2, L(H-2) Si-R-BMes(2). Remarkably, no H-2 activation occurs if the single silylene LSiPh and Mes(3)B intermolecularly separated are exposed to dihydrogen. Unexpectedly, the pre-organized Si-B separation in 1 enables a metal-free dehydrogenation of H2O to give the silanone-borane 3 as reactive intermediate.