Rhodium(I) Complexes with Hemilabile N-Heterocyclic Carbenes: Efficient Alkyne Hydrosilylation Catalysts

A series of alkylammonium-imidazolium chloride salts [RImH(CH2)nNMe2]Cl·HCl (R = Me, t-Bu, Mes; n = 2, 3) have been prepared by alkylation of 1-substituted imidazole compounds with the corresponding chloro-alkyl-dimethylamine hydrochloride. These salts are precursors for the synthesis of a library of rhodium(I) complexes containing amino-alkyl functionalized N-heterocyclic carbene (NHC) ligands with hemilabile character by varying the substituent on the heterocyclic ring and the length of the linker with the dimethylamino moiety. The monodeprotonation of alkylammonium-imidazolium salts with NaH in the presence of [{Rh(μ-Cl)(cod)}2] gave the amino-imidazolium salts [RImH(CH2)nNMe2][RhCl2(cod)]. Further deprotonation with NaH under non anhydrous conditions gave the neutral complexes [RhCl(cod)(RIm(CH2)nNMe2)] in good yields. The abstraction of the chloro ligand by silver salts rendered the cationic complexes [Rh(cod)(κ2C,N-RIm(CH2)3NMe2)][BF4] (R = Me, Mes) by coordination of the NMe2 fragment of the sidearm of the functionalized NHC ligands. The catalytic activity of the rhodium complexes in the hydrosilylation of terminal alkynes using HSiMe2Ph has been investigated with Ph-CCH, t-Bu-CCH, n-Bu-CCH, and Et3Si-CCH as substrates. Higher activities were achieved using neutral complexes having small substituents at the heterocyclic ring (R = Me). Excellent selectivities in the β-(Z)-vinylsilane isomer were found in the hydrosilylation of 1-hexyne and predominantly the β-(E) and α-bis(silyl)alkene isomers were obtained in the hydrosilylation of triethylsilylacetylene.