Organogermanium chemistry

Organogermanium chemistry is the science of chemical species containing one or more C–Ge bonds. Germanium shares group 14 in the periodic table with carbon, silicon, tin and lead. Historically, organogermanes are considered as nucleophiles and the reactivity of them is between that of organosilicon and organotin compounds. Some organogermanes have enhanced reactivity compared with their organosilicon and organoboron analogues in some cross-coupling reactions. The great majority of organogermanium compounds are tetrahedral with the formula GeR4-nXn where X = H, Cl, etc. Ge-C bonds are air-stable, although Ge-H bonds can undergo air-oxidation. The first organogermanium compound, tetraethylgermane, synthesized by Winkler in 1887, by the reaction of germanium tetrachloride with diethylzinc. More commonly, these Ge(IV) compounds are prepared by alkylation of germanium halides by organolithium and Grignard reagents. The method has been applied to surfaces terminated with Ge-Cl bonds. Some organogermanes are prepared by nucleophilic substitution or Pd-catalyzed cross-coupling reactions. Hydrogermylation provides another route to organogermanium compounds. Akin to hydrocarbons and polysilanes, many organogermanium compounds are known with Ge-Ge bonds. An early example is hexaphenyldigermane, . It is prepared by Wurtz coupling of the bromide: Many cyclic polygermanes are known, e.g. . Triphenylgermanol ((C6H5)3GeOH) is a colorless solid. Like the isostructural silanol, it engages in hydrogen bonding in the solid-state. Compounds with multiple bonds to Ge are usually highly reactive or require bulky organic substituents for their isolation. This situation follows from the double bond rule. Digermynes only exist for extremely bulky substituents. According to X-ray crystallography, the C–Ge≡Ge–C core of digermynes is bent. Such compounds are prepared by the reduction of bulky arylgermanium(II) halides. Compounds containing Ge=C (germenes) double bonds require bulky organic substituents for their isolation.