S–H Bond Activation in Hydrogen Sulfide by NHC-Stabilized Silyliumylidene Ions

Abstract

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-Mes 2 -C 6 H 3 , Mes = 2,4,6-Me 3 -C 6 H 2 ) to yield an NHC-stabilized thiosilaaldehyde B. The results of NBO and QTAIM analyses suggest a zwitterionic formulation of the product B as the most 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 H 2 S 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.

NHC‐Stabilised Silyliumylidene Ions

“…The imidazolium salt could then be separated by repeated washing with hexane, in which the light salt forms a fine suspension while the heavy silyliumylidene salt settles, to give yields of 45–54 %. This yield was later improved to 66 % as dilution enables the separation of the rapidly‐precipitated imidazolium salt prior to crystallisation of the silyliumylidenes salt …”

“…This yield was later improved to 66 % as dilution enables the separation of the rapidly-precipitated imidazolium salt prior to crystallisation of the silyliumylidenes salt. [34] It is worth noting that Sasamori, Tokitoh and Matsuo have prepared arylsilyliumylidene ions by addition of NHC to an appropriate disilene precursor Ar(Br)Si=Si(Br)Ar, [20] and recently reported that our route is also effective for this, although the separation of by-product is more difficult in their case. [23] The X-ray diffraction (XRD)-determined structure of 1 a gave a sum of bond angles from the silicon centre and the three carbon atoms bound to it of 310.28, which shows substantial pyramidalisation.…”

“…In 2018 we reported the synthesis of a thiosilaaldehyde, 9, by reaction of 1a with H 2 S. [34] Treatment of an acetonitrile solution of 1a with a 1 M H 2 S solution at À20 8C almost instantly generates compound 9 (Scheme 9), which can be separated from the imidazolium salt by-product by fractional crystallisation to give an isolated yield of 54 %. The less bulky 1b under the same conditions appeared to react analogously but could not be isolated, presumably due to its lower kinetic stability.…”

The Quest for Stable Silaaldehydes: Synthesis and Reactivity of a Masked Silacarbonyl

“…[13] Exploration of the reactivity of silyliumylidene (1)t owards chalcogens and H 2 Se nabled an access to NHC-stabilized heavierc halcogen silaacylium derivatives [14] and thiosilaaldehyde. [15] Herein, we report our study devotedt ot he synthesis of as ilaaldehyde, isolated as a Si,O-donor-acceptor complex, and its unique reactivity showing strong relationship to the chemistry of carbonyl congeners ( Figure 1b).…”

Coordination Chemistry of Silicon

“…They also discuss the isolation of arylsilyliumylidene ions through the dehydrobromination with four NHC equivalents. In the course of the reactivity study on the NHC-coordinated silyliumylidene ion, Porzelt et al describe the activation of the S-H bond in hydrogen sulfide by the arylsilyliumylidene ion, resulting in the formation of an NHC-coordinated thiosilaaldehyde [8]. DFT (density functional theory) calculations have been employed to examine the zwitterionic character of NHC-coordinated thiosilaaldehyde, and the reaction mechanism for the formation has also been computationally investigated.…”