Stable N-Heterocyclic Carbene Adducts of Arylchlorosilylenes and Their Germanium Homologues

Adduct Formation, B-H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1) with unsaturated and saturated NHCs as well as CAAC(Me) . Mono-NHC adducts of the type HBcat⋅NHC (NHC=nPr2 Im, iPr2 Im, iPr2 Im(Me) , and Dipp2 Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC(Me) with HBcat yielded the B-H activated product CAAC(Me) (H)Bcat via insertion of the carbine-carbon atom into the B-H bond. The saturated NHC Dipp2 SIm reacted in a 2:2 ratio yielding an NHC ring-expanded product at room temperature forming a six-membered -B-C=N-C=C-N- ring via C-N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene-carbon atom.

Transition metal mediated formation of dicationic diselenides stabilised by N-heterocyclic carbenes: designed synthesis

Designed syntheses of dicationic diselenides have been developed by (i) reduction of dihaloselones (routes A and B) and (ii) by oxidation of selone adducts of metal halides (route C).

25 years of N-heterocyclic carbenes: activation of both main-group element-element bonds and NHCs themselves

N-Heterocyclic carbenes (NHCs) are widely used ligands and reagents in modern inorganic synthesis as well as in homogeneous catalysis and organocatalysis. However, NHCs are not always innocent bystanders. In the last few years, more and more examples were reported of reactions of NHCs with main-group elements which resulted in modification of the NHC. Many of these reactions lead to ring expansion and the formation of six-membered heterocyclic rings involving insertion of the heteroatom into the C-N bond and migration of hydrides, phenyl groups or boron-containing fragments. Furthermore, a few related NHC rearrangements were observed some decades ago. In this Perspective, we summarise the history of NHC ring expansion reactions from the 1960s till the present.

Silicon(i) chemistry: the NHC-stabilised silicon(i) halides Si2X2(Idipp)2 (X = Br, I) and the disilicon(i)-iodido cation [Si2(I)(Idipp)2]. Chem Sci 2015;6:6515-6524. [PMID: 30090270 PMCID: PMC6054044 DOI: 10.1039/c5sc02681d]

An efficient method for the synthesis of the NHC-stabilised Si(i) halides Si₂X₂(Idipp)₂ (2-X, X = Cl, Br, I) was developed, which involves the oxidation of Si₂(Idipp)₂ (1) with 1,2-dihaloethanes. Iodide abstraction from 2-I afforded the unprecedented silicon(i) salt [Si₂(I)(Idipp)₂][B(C₆F₅)₄] (3).

An efficient method for the synthesis of the NHC-stabilised Si(i) halides Si₂X₂(Idipp)₂ (2-X, X = Cl, Br, I; Idipp = C[N(C₆H₃-2,6-iPr₂)CH]₂) was developed, which involves the oxidation of Si₂(Idipp)₂ (1) with 1,2-dihaloethanes. Halogenation of 1 is a diastereoselective reaction leading exclusively to a racemic mixture of the RR and SS enantiomers of 2-X. Compounds 2-Br and 2-I were characterised by single-crystal X-ray crystallography and multinuclear NMR spectroscopy, and their electronic structures were analysed by quantum chemical methods. Dynamic NMR spectroscopy unraveled a fluxional process of 2-Br and 2-I in solution, which involved a hindered rotation of the NHC groups about the Si–C_NHC bonds. Iodide abstraction from 2-I by [Li(Et₂O)_2.5][B(C₆F₅)₄] selectively afforded the disilicon(i) salt [Si₂(I)(Idipp)₂][B(C₆F₅)₄] (3). X-ray crystallography and variable-temperature NMR spectroscopy of 3 in combination with quantum chemical calculations shed light on the ground-state geometric and electronic structure of the [Si₂(I)(Idipp)₂]⁺ ion, which features a Si Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> Si bond between a trigonal planar coordinated Si^II atom with a Si–I bond and a two-coordinate Si⁰ center carrying a lone pair of electrons. The dynamics of the [Si₂(I)(Idipp)₂]⁺ ion were studied in solution by variable-temperature NMR spectroscopy and they involve a topomerisation, which proceeds according to quantum theory via a disilaiodonium intermediate (“π-bonded” isomer) and exchanges the two heterotopic Si sites.

Reactivity of an NHC-stabilized silylene towards ketones. Formation of silicon bis-enolates vs. bis-silylation of the CO bond

The reaction of NHC-stabilized silylaminosilylene 1 with enolizable ketones rapidly led to the regio- and stereoselective formation of silicon bis-enolates in excellent yields under mild conditions.

Base-stabilized silaimine and its donor-free dimer derived from the reaction of NHC-supported silylene with SiCl4

Reaction of the N-heterocyclic carbene (NHC)-stabilized silylene ArN(SiMe₃)Si(IiPr)Cl (1, Ar = 2,6-iPr₂C₆H₃, IiPr = 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene) with SiCl₄ resulted in the formation of three different products ArNSi(IiPr)Cl₂ (2), ArN(SiMe₃)SiCl₃ (3) and (ArNSiCl₂)₂ (4) under different conditions.

Can low-valent silicon compounds be better transition metal ligands than phosphines and NHCs?

We investigated the potential application of low-valent silicon compounds as transition metal ligands by computing the most important ligand properties and compared them to the generally used carbene and phosphine ligands.

Pentacoordinate silicon(IV): cationic, anionic and neutral complexes derived from the reaction of NHC→SiCl4 with highly Lewis acidic (C2F5)2SiH2

Addition of NHC→SiCl4 to the highly Lewis acidic bis(pentafluoroethyl)silane ((C2F5)2SiH2) afforded the salt [(NHC)2SiCl2H][(C2F5)2SiCl3] with pentacoordinate silicon in the cation and the anion. The anion represents the first example of a chlorosilicate structurally characterized in the solid state. In this reaction, the long sought pentacoordinate NHC-adduct of silicochloroform was identified as an intermediate and its crystal structure is presented.

Hydrogen abstraction from organotin di- and trihydrides by N-heterocyclic carbenes: a new method for the preparation of NHC adducts to tin(ii) species and observation of an isomer of a hexastannabenzene derivative [R6Sn6]

N-Heterocyclic carbenes are shown to cleanly abstract dihydrogen from organotin di- and trihydrides to intermediately form the reactive stannylene species [R₂Sn] and [R′SnH], respectively, which undergo further reactions.

NHC-coordinated silagermenylidene functionalized in allylic position and its behaviour as a ligand

Through coordination by an N-heterocyclic carbene (NHC) silagermenylidene (Tip₂Cl)Si-(Tip)SiGe·NHC^iPr2Me2 (4-E/Z; Tip = 2,4,6-ⁱPr₃C₆H₂; NHC^iPr2Me2 = 1,3-ⁱPr₂-4,5-Me₂imidazol-2-ylidene) is available as an E/Z-equilibrium mixture from Tip₂SiSi(Tip)Li and NHC^iPr2Me2·GeCl₂.

A facile access to a novel NHC-stabilized silyliumylidene ion and C–H activation of phenylacetylene

This work presents a facile access to a donor-stabilized monoarylsilyliumylidene ion and its unique reaction with a terminal alkyne.

A Class of Computationally Designed Tri-Coordinate Cyclic Silylenes RSi(µ-R)2AlR2

With the aim of designing novel low-valent silylenes, herein we report the first systematic investigation of the SiAlR5 system. Based on the ‘topology’ isomeric search strategy, four classes of isomers, i.e. RSi(μ-R)2AlR2 1, R3Si-AlR2 2, R2Si(μ-R)AlR2 3, and R2Si-AlR3 4 were identified. For 12 groups with an electron lone-pair, i.e. R = F, NH2,OH, SH, OMe, PMe2, SMe, OEt, PEt2, SEt, OPh, and PPh2, the tri-coordinate silylene 1 is the most stable structure, which contains two sets of intermolecular donor–acceptor interactions. In accordance with the ‘silylene’ term, the Si-centre can undergo both a nucleophilic reaction and insertion reaction. Moreover, coupling AlR3 (R = OR′ and PR′2) (R′ = H, Me, Et, and Ph) with the known cyclic di-coordinate silylene could lead to stable tri-coordinate silylenes. These novel tri-coordinate silylenes strongly welcome laboratory studies in near future.

Five-coordinate silicon(II) compounds with Si-M bonds (M = Cr, Mo, W, Fe): Bis[N,N'-diisopropylbenzamidinato(-)]silicon(II) as a ligand in transition-metal complexes

Reaction of the donor-stabilized silylene 1 with [Cr(CO)6], [Mo(CO)6], [W(CO)6], or [Fe(CO)5] leads to the formation of the transition-metal silylene complexes 2-5, which contain five-coordinate silicon(II) moieties with Si-M bonds (M = Cr, Mo, W, Fe). These compounds were characterized by NMR spectroscopic studies in the solid state and in solution and by crystal structure analyses. These experimental investigations were complemented by computational studies to gain insight into the bonding situation of 2-5. The nature of the Si-M bonds is best described as a single bond.