Aluminium Functionalized Germanes: Intramolecular Activation of Ge–H Bonds, Formation of a Dihydrogen Bond and Facile Hydrogermylation of Unsaturated Substrates

A series of alkenylhydrogermanes, RR′(H)Ge‐C(AlR′′2)=C(H)‐tBu [4a, R = R′ = Mes, R′′ = tBu; 4b, R = R′ = Mes, R′′ = CH(SiMe3)2; 5a, R = R′′ = tBu, R′ = CH2CH2tBu; 5b, R = tBu, R′ = CH2CH2tBu, R′′ = CH(SiMe3)2], was synthesized by hydroalumination of the respective alkynylgermanes, RR′(H)Ge‐C≡C‐tBu 3, with R′′2Al–H [R′′ = tBu, CH(SiMe3)2]. In the solid state compound 5b showed a contact between Al and the Ge bound H atom (293 pm) which according to quantum chemical calculations results in Ge–H bond activation. In case of less bulky substituents on Al (5a, R′′ = tBu) a mixture of isomers was observed that reacted with Et2NH to yield the simple Lewis acid‐base adduct [(tBu)(tBuCH2CH2)(H)Ge‐C(AltBu2)=C(H)‐tBu](HNEt2) (6). 6 features a dihydrogen bond (Ge–H···H–N) as evident from typical downfield (N–H) and high‐field (Ge–H) shifts in the NMR spectrum, red shifts of νGe‐H and νN‐H in the IR spectrum, a short H···H contact (210 pm) in the solid state and a bond critical point between both H atoms. The presence of a hard Lewis‐acidic Al atom in geminal position to the Ge–H bond in 5a facilitates reactions with Ph2CO, OC4H8N‐C≡N and Ph–NCO under surprisingly mild conditions by hydrogermylation of the heteronuclear π‐bonds and formation of four‐membered Ge–O/N–Al–C (7, 9) or six‐membered Ge–N–C–O–Al–C (8) heterocycles. Reaction with Ph–C≡C–H afforded the cis‐addition product 10, in which the α‐C atom of the new alkenyl group showed a close contact [258.3(2) pm] to the Al atom. Hydroalumination of alkynylgermanes affords Ge–H···Al compounds with weak Al–H interactions and an activation of the Ge–H bond. Treatment with H–NEt2 afforded an adduct with the N atom coordinated to Al and a dihydrogen bond. The presence of Lewis acidic Al atoms in the Ge–H···Al compounds facilitates selective hydrogermylation reactions under mild conditions.