S-H bond activation in hydrogen sulfide by NHC-stabilized silyliumylidene ions

Reactivity studies of silyliumylidenes remain scarce with only a handful of publications to date. Herein we report the activation of S-H bonds in hydrogen sulfide by mTer-silyliumylidene ion A (mTer = 2,6-Mes₂-C₆H₃, Mes = 2,4,6-Me₃-C₆H₂) to yield an NHC-stabilized thiosilaaldehyde B. The results ofNBO and QTAIManalyses suggest a zwitterionic formulation of the product B as themost appropriate. Detailed mechanistic investigations are performed at the M06-L/6-311+G(d, p)(SMD: acetonitrile/benzene)//M06-L/6-311+G(d, p) level of density functional theory. Several pathways for the formation of thiosilaaldehyde B are examined. The energetically preferred route commences with a stepwise addition of H₂S to the nucleophilic silicon center. Subsequent NHC dissociation and proton abstraction yields the thiosilaaldehyde in a strongly exergonic reaction. Intermediacy of a chlorosilylene or a thiosilylene is kinetically precluded. With an overall activation barrier of 15 kcal/mol, the resulting mechanistic picture is fully in line with the experimental observation of an instantaneous reaction at sub-zero temperatures.