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(N t Bu) 2 ; R=1,12‐xanthendiyl spacer; Mes=2,4,6‐Me 3 C 6 H 2 ), 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 , N 2 O, or CO 2 , 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 H 2 O to give the silanone–borane 3 as reactive intermediate.