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

Abstract

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 [Ge9] cluster core through Ge-C bond formation. We report on the reactivity of [Ge9{Si(TMS)3}2]2- (TMS = trimethylsilyl) towards a series of Lewis acidic bromo-boranes. The reaction of [Ge9{Si(TMS)3}2]2- and DABo-tol-Br (DAB = 1,3,2-diazaborolidine; o-tol = 2-methylphenyl) resulted, depending on the reaction protocol, either in the formation of [Ge9{Si(TMS)3}2DABo-tol]- (1a) with direct Ge-B interactions, or in [Ge9{Si(TMS)3}2(CH2)4O-DABo-tol]- (2a) featuring a ring-opened thf moiety. Ring opening reactions occur for all bulkier DABR-Br [R: o-xyl (2,6-dimethylphenyl), Mes (2,4,6-trimethylphenyl), Dipp (2,6-diisopropylphenyl)], DAB(ii)Dipp-Br and acyclic (iPr2N)2BBr 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 CH3CN requires the simultaneous presence of Lewis-acid and Lewis-basic reactants allowing the formation of [Ge9{Si(TMS)3}2CH3CN-DABMes]- (6a). Within the presented compounds, 3 and 4 show an unusual substitution pattern of the three ligands at the [Ge9] core in the solid state. The [Ge9] cluster/borane systems correspond to intermolecular frustrated Lewis pairs (FLPs), in which the [Ge9] cluster with several lone pairs represents the Lewis base, and the borane is the Lewis acid. This journal is