FLP-type nitrile activation and cyclic ether ring-opening by halo-borane nonagermanide-cluster Lewis acid–base pairs

Even though homoatomic nine-atom germanium clusters are known for two decades, their chemical properties are still rarely investigated. We now discovered that Zintl ion main group-element clusters possess a reactive lone pair of electrons, and we show a new pathway to bind ligands with functional groups to the [Ge(9)] cluster core through Ge–C bond formation. We report on the reactivity of [Ge(9){Si(TMS)(3)}(2)](2−) (TMS = trimethylsilyl) towards a series of Lewis acidic bromo-boranes. The reaction of [Ge(9){Si(TMS)(3)}(2)](2−) and DAB(o-tol)–Br (DAB = 1,3,2-diazaborolidine; o-tol = 2-methylphenyl) resulted, depending on the reaction protocol, either in the formation of [Ge(9){Si(TMS)(3)}(2)DAB(o-tol)](−) (1a) with direct Ge–B interactions, or in [Ge(9){Si(TMS)(3)}(2)(CH(2))(4)O–DAB(o-tol)](−) (2a) featuring a ring-opened thf moiety. Ring opening reactions occur for all bulkier DAB(R)–Br [R: o-xyl (2,6-dimethylphenyl), Mes (2,4,6-trimethylphenyl), Dipp (2,6-diisopropylphenyl)], DAB(ii)(Dipp)–Br and acyclic ((i)Pr(2)N)(2)BBr without Ge–B bond formation as shown for the structural characterization of the ring-opened products of thf (3, 4) and trimethylene oxide (5). In contrast to thf, the activation of CH(3)CN requires the simultaneous presence of Lewis-acid and Lewis-basic reactants allowing the formation of [Ge(9){Si(TMS)(3)}(2)CH(3)C[double bond, length as m-dash]N–DAB(Mes)](−) (6a). Within the presented compounds, 3 and 4 show an unusual substitution pattern of the three ligands at the [Ge(9)] core in the solid state. The [Ge(9)] cluster/borane systems correspond to intermolecular frustrated Lewis pairs (FLPs), in which the [Ge(9)] cluster with several lone pairs represents the Lewis base, and the borane is the Lewis acid.