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

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.