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)(CH2)(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 ((Pr2N)-Pr-i)(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 CH3CN requires the simultaneous presence of Lewis-acid and Lewis-basic reactants allowing the formation of [Ge-9{Si(TMS)(3)}(2)CH3C=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.

Automated summary

Despite being known for two decades, homoatomic nine-atom germanium clusters have been rarely investigated for their chemical properties. This study shows that Zintl ion main group-element clusters have a reactive lone pair of electrons, and introduces a new pathway for binding ligands to the [Ge-9] cluster core through Ge-C bond formation. The reactivity of [Ge-9{Si(TMS)(3)}(2)](2-) towards a series of Lewis acidic bromo-boranes is explored, leading to the formation of different products depending on reaction conditions. Overall, the study highlights the unique substitution patterns and reactivity of these germanium clusters in interaction with borane compounds.