An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

Abstract

The silicon analogues of an acylium ion, namely, sila-acylium ions 2a and 2b [RSi(O)(NHC)2]Cl stabilized by two N-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO2 using the donor stabilized silyliumylidene cations 1a and 1b [RSi(NHC)2]Cl (1a, 2a; R = m-Ter = 2,6-Mes2C6H3, Mes = 2,4,6-Me3C6H2 and 1b, 2b; R = Tipp = 2,4,6-iPr3C6H2). Structurally, compound 2a features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions 2a and 2b with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of 2a to H2O afforded a stable silicon analogue of carboxylate anion as a dimer form, [m-TerSi(O)O]2(2-)·2[NHC-H](+) (3), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion 2b afforded cyclotetrasiloxanediol dianion [{TippSi(O)}4{(O)OH}2](2-)·2[NHC-H](+) (4). Metric and DFT (Density Functional Theory) evidence support that 2a and 2b possess strong Si═O double bond character, while 3 and 4 contain more ionic terminal Si-O bonds. Mechanistic details of the formation of different (SiO)n (n = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry.

A monotopic aluminum telluride with an Al=Te double bond stabilized by N-heterocyclic carbenes

“…This result is in accordance with a recent NRT study on NHC-stabilized sila-acylium ions that are isoelectronic to the aluminum telluride 3 (ref. 32). Interestingly, though solid batches of 3 can be stored in an inert atmosphere for weeks the compound decomposes in deuterated tetrahydrofurane solution within few days to yield the protonated iminato ligand (L Dip NH, Supplementary Fig.…”

Facile Access to Stable Silylium Ions Stabilized by N-Heterocyclic Imines

“…The difference in the steric bulk of the precursors 1 and 3 leads to different products based on the reaction kinetics. Figure 3 shows the energy profile along the reaction path calculated at the B97-D/6-31G(d) level of theory which has been successfully applied in low-valent silicon compounds [38][39][40][41][42][43][44][45][46][47]. The first step in the reaction is the abstraction of a Me group of a Me 3 Si moiety by the B(C 6 F 5 ) 3 (TS1 Dip ) with the associated energy of +18.9 kcal/mol for 2[MeB(C 6 F 5 ) 3 ] 2 .…”

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

“…We have also reported the application of A/A' for the reduction of CO 2 yielding the first NHC-stabilized silaacylium ions (VII/VII') [59]. In addition, we have demonstrated the importance of kinetic stabilization by the steric bulk of the aryl ligands.…”