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]22–·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.