Acyclic Germylones: Congeners of Allenes with a Central Germanium Atom

The
cyclic alkyl­(amino) carbene (cAAC:)-stabilized acyclic germylones
(Me2-cAAC:)2Ge (1) and (Cy2-cAAC:)2Ge (2) were prepared utilizing
a one-pot synthesis of GeCl2(dioxane), cAAC:, and KC8 in a 1:2:2.1 molar ratio. Dark green crystals of compounds 1 and 2 were produced in 75 and 70% yields, respectively.
The reported methods for the preparation of the corresponding silicon
compounds turned out to be not applicable in the case of germanium.
The single-crystal X-ray structures of 1 and 2 feature the C–Ge–C bent backbone, which possesses
a three-center two-electron π-bond system. Compounds 1 and 2 are the first acyclic germylones containing each
one germanium atom and two cAAC: molecules. EPR measurements on compounds 1 and 2 confirmed the singlet spin ground state.
DFT calculations on 1/2 revealed that the
singlet ground state is more stable by ∼16 to 18 kcal mol–1 than that of the triplet state. First and second
proton affinity values were theoretically calculated to be of 265.8
(1)/267.1 (2) and 180.4 (1)/183.8
(2) kcal mol–1, respectively. Further
calculations, which were performed at different levels suggest a singlet
diradicaloid character of 1 and 2. The TD-DFT
calculations exhibit an absorption band at ∼655 nm in n-hexane solution that originates from the diradicaloid
character of germylones 1 and 2.