This study describes the first use of a silicon­(II) complex, NHC-parent silyliumylidene cation complex [(IMe)2SiH]­I (1, IMe = :C­{N­(Me)­C­(Me)}2) as a versatile catalyst in organic synthesis. Complex 1 (loading: 10 mol %) was shown to act as an efficient catalyst (reaction time: 0.08 h, yield: 94%, TOF = 113.2 h–1; reaction time: 0.17 h, yield: 98%, TOF = 58.7 h–1) for the selective reduction of CO2 with pinacolborane (HBpin) to form the primarily reduced formoxyborane [pinBOC­(O)­H]. The activity is better than the currently available base-metal catalysts used for this reaction. It also catalyzed the chemo- and regioselective hydroboration of carbonyl compounds and pyridine derivatives to form borate esters and N-boryl-1,4-dihydropyridine derivatives with quantitative conversions, respectively. Mechanistic studies show that the silicon­(II) center in complex 1 activated the substrates and then mediated the catalytic hydroboration. In addition, complex 1 was slightly converted into the NHC-borylsilyliumylidene complex [(IMe)2SiBpin]I (3) in the catalysis, which was also able to mediate the catalytic hydroboration.

The reaction of amidinatosilylene LSi(:)Cl [L = PhC­(NtBu)2] with N-heterocyclic carbene IAr [:C­{N­(Ar)­CH}2, where Ar = 2,6-iPr2C6H3] and NaOTf in tetrahydrofuran (THF) facilely afforded a silicon­(II) cation [LSi(:)-aIAr]+OTf– (1 + OTf – ), where IAr isomerizes to abnormal N-heterocyclic carbene aIAr, coordinating to the silicon­(II) center. Its Ge homologue, [LGe(:)-aIAr]+OTf– (2 + OTf – ), was also accessed via the same protocol. For the formation of 1 + , we propose that an in situ-generated Si­(II) cation [LSi(:)]+ under the treatment of LSi(:)Cl with NaOTf may isomerize IAr in THF. In contrast, the replacement of IAr with cyclic alkyl­(amino) carbene (cAAC) furnished a cAAC-silanyl radical ion [LSi­(H)-cAAC]•+(LiOTf2)− [3 •+ (LiOTf 2 ) – ], which may undergo an abstraction of the H radical from THF. All of the products were characterized by nuclear magnetic resonance spectroscopy, electron paramagnetic resonance, and X-ray crystallography, and their bonding scenarios were investigated by density functional theory calculations. These studies provide new perspective on carbene–silicon chemistry.

This study describes the use of an amidinato isopropylmethylamidosilylene [LSiN(Me)iPr] (1, L=PhC(NtBu)2) to catalyze hydroboration of carbonyl compounds. Compound 1 (loading: 5–10 mol%) was shown to be an efficient catalyst for the chemoselective hydroboration of aldehydes (average yield=97 %, average TOF=8.8 h−1) and ketones (average yield=97 %, average TOF=1.7 h−1) with pinacolborane (HBpin) in C6D6 at 90 °C to form borate esters. Mechanistic studies show that the Si lone pair electrons on 1 and the B vacant p orbital of HBpin activates the C=O double bond of aldehydes and ketones, the intermediates of which undergoes hydroboration to yield borate esters and regenerate compound 1.

Li initiated the feasibility of substrate scopes in the catalysis.  I performed the catalytic hydroboration of carbonyl compounds and pyridines.  B.-X. Leong executed the hydroboration of CO2, mechanistic studies, the synthesis of the NHC-parent silyliumylidene cation and shortening of reaction time.

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