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] cluster core through Ge-C bond formation. We report on the reactivity of [Ge{Si(TMS)}] (TMS = trimethylsilyl) towards a series of Lewis acidic bromo-boranes. The reaction of [Ge{Si(TMS)}] and DAB -Br (DAB = 1,3,2-diazaborolidine; -tol = 2-methylphenyl) resulted, depending on the reaction protocol, either in the formation of [Ge{Si(TMS)}DAB ] () with direct Ge-B interactions, or in [Ge{Si(TMS)}(CH)O-DAB ] () featuring a ring-opened thf moiety. Ring opening reactions occur for all bulkier DAB-Br [R: -xyl (2,6-dimethylphenyl), Mes (2,4,6-trimethylphenyl), Dipp (2,6-diisopropylphenyl)], DAB(ii)-Br and acyclic ( PrN)BBr without Ge-B bond formation as shown for the structural characterization of the ring-opened products of thf (, ) and trimethylene oxide (). In contrast to thf, the activation of CHCN requires the simultaneous presence of Lewis-acid and Lewis-basic reactants allowing the formation of [Ge{Si(TMS)}CHC[double bond, length as m-dash]N-DAB] (). Within the presented compounds, and show an unusual substitution pattern of the three ligands at the [Ge] core in the solid state. The [Ge] cluster/borane systems correspond to intermolecular frustrated Lewis pairs (FLPs), in which the [Ge] cluster with several lone pairs represents the Lewis base, and the borane is the Lewis acid.