Cooperative GeN Bond Activation in Aluminium‐Functionalised Aminogermanes and Spontaneous Imine Elimination via an Intermediate Germyl Cation

Abstract

Hydrometallation of iPr2 N-Ge(CMe3 )(C≡C-CMe3 )2 with H-M(CMe3 )2 (M=Al, Ga) affords alkenyl-alkynylgermanes in which the Lewis-acidic metal atoms are not coordinated by the amino N atoms but by the α-C atoms of the ethynyl groups. These interactions result in a lengthening of the Ge-C bonds by approximately 10 pm and a comparably strong deviation of the Ge-CC angle from linearity (154.3(1)°). This unusual behaviour may be caused by steric shielding of the N atoms. Coordination of the metal atoms by the amino groups is observed upon hydrometallation of Et2 N-Ge(C6 H5 )(C≡C-CMe3 )2 , bearing a smaller NR2 group. Strong M-N interactions lead to a lengthening of the Ge-N bonds by 10 to 15 pm and a strong deviation of the M atoms from the MC3 plane by 52 and 47 pm, for Al and Ga, respectively. Dual hydrometallation is achieved only with HAl(CMe3 )2 . In the product, there is a strong Al-N bond with converging Al-N and Ge-N distances (208 vs. 200 pm) and an interaction of the second Al atom to the phenyl group. Addition of chloride anions terminates the latter interaction while the activated Ge-N bond undergoes an unprecedented elimination of EtN=C(H)Me at room temperature, leading to a germane with a Ge-H bond. State-of-the-art DFT calculations reveal that the unique mechanism comprises the transfer of the amino group from Ge to Al to yield an intermediate germyl cation as a strong Lewis acid, which induces β-hydride elimination, with chloride binding being crucial for providing the thermodynamic driving force.

σ‐Bond Activation in Aluminium‐Functionalized Alkynylchlorogermanes: Facile Insertion of Isocyanate and Azide into Al–C and Ge–Cl Bonds

“…The Ge–C distances correlate with the hybridization of the C atoms and are shortest for Ge–C≡C, longest for Ge–C vinyl and somewhere in between for Ge–C(Ph) (Table ). The Ge–N bonds are relatively long (193 pm on average) compared to a non‐hydroaluminated (alkynylamino)germane (182 pm), and the Ge atom is almost in plane with the three directly bonded C atoms (sum of the angles ca. 356°).…”

“…The resulting alkenyl derivatives featured a geminal arrangement of Al/Ga and Si/Ge atoms at their C–C double bonds. An interaction of the metal atoms with the functional groups afforded four‐membered MCEX heterocycles with activated Si–X or Ge–X bonds (M = Al, Ga; E = Si, Ge; X = C≡C–R, NR 2 , Cl) . The weakening of the E–X bonds was in selected cases confirmed by quantum‐chemical calculations indicating a decrease of the Wiberg bond indices .…”

“…We intend to activate bonds of Si or Ge atoms to donor groups by utilising intramolecular coordination of the Lewis bases to Lewis‐acidic Al or Ga atoms in order to allow insertion or rearrangement reactions or the generation of silyl or germyl cations. Suitable compounds for such processes were obtained by hydroalumination or ‐gallation of donor‐functionalized alkynylsilanes and ‐germanes . The resulting alkenyl derivatives featured a geminal arrangement of Al/Ga and Si/Ge atoms at their C–C double bonds.…”

“…Interactions of Si‐ or Ge‐bound alkynyl groups with Al or Ga atoms resulted in an activation of Si–C and Ge–C bonds,, which favoured rearrangement to sila‐ and germacyclobutenes (1,1‐carbametallation) , . Intramolecular coordination of NR 2 groups or Cl atoms to the Lewis‐acidic metal atoms facilitated the unique elimination of an imine at room temperature from a (diethylamino)germane or a Cl/ t Bu exchange between Al and Si/Ge atoms (dyotropic rearrangement) , . Quantum‐chemical calculations suggested the formation of silyl or germyl cations as intermediates .…”

“…Intramolecular coordination of NR 2 groups or Cl atoms to the Lewis‐acidic metal atoms facilitated the unique elimination of an imine at room temperature from a (diethylamino)germane or a Cl/ t Bu exchange between Al and Si/Ge atoms (dyotropic rearrangement) , . Quantum‐chemical calculations suggested the formation of silyl or germyl cations as intermediates . The heterolytic cleavage of the relatively stable Si–Cl bond was achieved only with a twice hydroaluminated silane and was supported by the chelating coordination of the leaving Cl atom by two Al atoms …”