Access to metastable [GeH₂] materials via a molecular "bottom-up" approach

We describe the application of a mild, molecular-based, hydride metathesis protocol for the preparation of metastable germanium(II) dihydrides with compositions approaching [GeH₂]. The common starting material for this work [Ge(OBu)₂] was prepared in a high yield and shown to undergo OBu/H exchange at Ge with the hydride sources pinacolborane (HBpin), catecholborane (HBcat), and diisobutylaluminum hydride (DIBAL-H) to give the [GeH₂] materials as yellow to orange solids. Heating one of these [GeH₂] materials to 200 °C affords a narrowing of the optical band gap (from 2.5 eV) and the generation of amorphous Ge. Reaction of [Ge(OBu)₂] with excess H₃B·SMe₂ in toluene at 70 °C provides a convenient route to thin films of amorphous Ge, including its deposition onto soft substrates, such as polyethyleneterephthalate (PET). Accompanying computations give insight into the energetics of OBu/H exchange at Ge, and reveal a general thermodynamic preference for branched structures of [GeH₂] oligomers over linear forms as the Ge chain becomes longer. We also show that [Ge(OBu)₂] is a suitable pre-catalyst for the borylation of aldehydes.