Boragerma[5]pyramidanes via a Germole-to-Borole Rearrangement

The scope and limitations of a germole-to-borole rearrangement are reported. Double salt metathesis reactions of dipotassium germolediides with amino-, aryl-, and ferrocenyl-substituted boron dihalides allow for the preparation of borole complexes of Ge(II) in synthetically useful yields and quantities. The analogous silole-to-borole transformation is viable but is less selective and of limited synthetic use. The analysis of the molecular and electronic structure of the obtained Ge(II) borole complexes classifies them as boragerma[5]pyramidanes, molecular nido-type clusters. The molecular structure can be fine-tuned as strongly electron-donating substituents at the boron atom induce an opening of the cluster. The boragerma[5]pyramidanes bind via the germanium atom to transition metal complexes and behave as σ-donors with only insignificant π-acceptor abilities. Their reduction with elemental lithium and their reaction with strong nucleophiles such as N-heterocylic carbenes (NHCs) lead to the elimination of germanium and isolation of borole derivatives.