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‐BMes₂; L=PhC(N^t Bu)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 O₂, N₂O, or CO₂, and formation of silanethione borane 4 from reaction with S₈. The Si^II center in 1 undergoes immediate hydrogenation if exposed to H₂ at 1 atm pressure in benzene, affording the silane borane 5‐H₂ , L(H₂)Si‐R‐BMes₂. Remarkably, no H₂ activation occurs if the single silylene LSiPh and Mes₃B 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.