NHC-stabilized Al(III) and Ga(III) cationic alkyls: Synthesis, structure and use in hydrosilylation catalysis

Abstract

Cationic Al(III) and Ga(III) species supported by N-heterocyclic carbene (NHC) ligands, (IDipp)AlMe2(PhBr)]+ ([1]+, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) and (IDipp)GaMe2]+ ([2]+), were prepared and structurally characterized as B(C6F5)4− salts via ionization of the corresponding neutral precursors (IDipp)MMe3 (M = Al, Ga) with [Ph3C][B(C6F5)4] in PhBr at room temperature. Both [1][B(C6F5)4] and [2][B(C6F5)4] salt were isolated in high yield and their solid state structures established through X-ray crystallographic studies. Cations [1]+ and [2]+, which are rare examples of structurally characterized tris-organyl Al(III) and Ga(III) cations, stand as potent Lewis acids as experimentally estimated through the Gutmann-Beckett method. These cations were further exploited in hydrosilylation catalysis of alkynes, benzaldehyde and CO2 using HSiEt3 as an hydrosilane source. Hydrosilylation of 1-hexyne, 4-phenylbutyne and phenylacetylene led to the formation of the corresponding Z-selective products 3–5, respectively, while benzaldehyde was converted to PhCH2OSiEt3 (6). Cations [1]+ and [2]+ also slowly catalyze CO2 hydrosilylation with the selective formation of the methanol-equivalent MeOSiEt3.