A Hydride-Substituted Homoleptic Silylborate: How Similar is it to its Diborane(6)-Dianion Isostere?

The B-nucleophilic 9H-9-borafluorene dianion reacts with 9-chloro-9-silafluorene to afford air- and moisture-stable silylborate salts M[Ar 2 (H)B-Si(H)Ar 2 ] (M[HBSiH], M=Li, Na). Li[HBSiH] and Me 3 SiCl give the B-pyridine adduct Ar 2 (py)B-Si(H)Ar 2 ((py)BSiH) or the chlorosilane Li[Ar 2 (H)B-Si(Cl)Ar 2 ] (Li[HBSiCl]) in C 6 H 6 -pyridine or THF. In both cases, the first step is H - abstraction at the B center. The resulting free borane subsequently binds a py or thf ligand. While the py adduct is stable at room temperature, the thf adduct undergoes a 1,2-H shift via the cyclic B(μ-H)Si intermediate BHSi, which is afterwards attacked at the Si atom by a Cl - ion to give Li[HBSiCl]. DFT calculations were employed to support the proposed reaction mechanism and to characterize the electronic structure of BHSi. Treatment of Li[HBSiCl] with the N-heterocyclic carbene IMe introduces the neutral donor at the Si atom and leads to Ar 2 (H)B-Si(IMe)Ar 2 (HBSi(IMe)), a donor-acceptor-stabilized silylene.