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.