Silylene-Functionalized N-Heterocyclic Carbene (Si−NHC)

Synthetic, structural and reactivity studies of a boryl-ethynyl Silylene

The salt metathesis of a boryl-ethynyl lithium salt {[(HCDipN)2]B-CC-Li} with a monochlorosilylene [LSi(:)Cl; L = PhC(NtBu)2] produced an isolable boryl-ethynyl silylene {1; [(HCDipN)2]B-CC-Si(L)}. The Si(II) center in 1 possesses a nonbonding lone pair and forms a covalent bond with the ethynyl group. The characterization of 1 was carried out by multinuclear NMR spectroscopy, single-crystal X-ray structure analysis and DFT calculations. Additionally, a reactivity study of 1 was conducted towards oxygen-containing and aryl C-F substrates.

1,3-Imidazole-Based Mesoionic Carbenes and Anionic Dicarbenes: Pushing the Limit of Classical N-Heterocyclic Carbenes

Classical N-heterocyclic carbenes (NHCs) featuring the carbene center at the C2-position of 1,3-imidazole framework (i.e. C2-carbenes) are well acknowledged as very versatile neutral ligands in molecular as well as in materials sciences. The efficiency and success of NHCs in diverse areas is essentially attributed to their persuasive stereoelectronics, in particular the potent σ-donor property. The NHCs with the carbene center at the unusual C4 (or C5) position, the so-called abnormal NHCs (aNHCs) or mesoionic carbenes (iMICs), are however superior σ-donors than C2-carbenes. Hence, iMICs have substantial potential in sustainable synthesis and catalysis. The main obstacle in this direction is rather demanding synthetic accessibility of iMICs. The aim of this review article is to highlight recent advances, particularly by the author's research group, in accessing stable iMICs, quantifying their properties, and exploring their applications in synthesis and catalysis. In addition, the synthetic viability and use of vicinal C4,C5-anionic dicarbenes (ADCs), also based on an 1,3-imidazole framework, are presented. As will be apparent on following pages, iMICs and ADCs hold potentials in pushing the limit of classical NHCs by enabling access to conceptually new main-group heterocycles, radicals, molecular catalysts, ligands sets, and more.

Activation of CO Using a 1,2-Disilylene: Facile Synthesis of an Abnormal N-Heterocyclic Silylene

Reaction of the 1,2-disilylene, [{ArC(NDip)₂ }Si]₂ 1 (Dip=2,6-diisopropylphenyl, Ar=4-C₆ H₄ Bu^t ), with CO proceeds via insertion of CO into one Si-N bond, and Si-Si bond cleavage, to cleanly give the bis(silylene), {ArC(NDip)₂ }Si(:)O C S i ( : ) ( N D i p )​ 2 C ‾ Ar 2, under ambient conditions. The reaction can be partially reversed when solutions of 2 are subjected to UV irradiation. The five-membered heterocyclic fragment of 2 represents the first silicon analogue of an "abnormal" N-heterocyclic carbene (aNHC), a view which is substantiated by a computational analysis of the compound. Reaction of 2 with [Mo(CO)₆ ] under UV light affords the chelate complex, [Mo(CO)₄ (κ² -Si,Si-2)] 3, while reaction with [Fe(CO)₅ ] gives the unusual silyleneyl bridged complex, [{Fe₂ (CO)₆ }{μ-Si[(NDip)₂ CAr]}₂ ] 4. The same coordination complexes can be accessed by reaction of 1 with [Mo(CO)₆ ] or [Fe(CO)₅ ] under UV light. As is the case for aNHCs, d-block metal complexes of bis(silylene) 2 could prove useful as bespoke catalysts for organic transformations.

Si(II) Cation-Promoted Formation of an Abnormal NHC-Bound Silylene and a cAAC-Silanyl Radical Ion

The reaction of amidinatosilylene LSi(:)Cl [L = PhC(NtBu)₂] with N-heterocyclic carbene I_Ar [:C{N(Ar)CH}₂, where Ar = 2,6-iPr₂C₆H₃] and NaOTf in tetrahydrofuran (THF) facilely afforded a silicon(II) cation [LSi(:)-_aI_Ar]⁺OTf^- (1⁺OTf^-), where I_Ar isomerizes to abnormal N-heterocyclic carbene _aI_Ar, coordinating to the silicon(II) center. Its Ge homologue, [LGe(:)-_aI_Ar]⁺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 I_Ar in THF. In contrast, the replacement of I_Ar with cyclic alkyl(amino) carbene (cAAC) furnished a cAAC-silanyl radical ion [LSi(H)-cAAC]^•+(LiOTf₂)^- [3^•+(LiOTf₂)^-], 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.

Bonding Relationship between Silicon and Germanium with Group 13 and Heavier Elements of Groups 14-16

The topic of heavier main group compounds possessing multiple bonds is the subject of momentous interest in modern organometallic chemistry. Importantly, there is an excitement involving the discovery of unprecedented compounds with unique bonding modes. The research in this area is still expanding, particularly the reactivity aspects of these compounds. This article aims to describe the overall developments reported on the stable derivatives of silicon and germanium involved in multiple bond formation with other group 13, and heavier groups 14, 15, and 16 elements. The synthetic strategies, structural features, and their reactivity towards different nucleophiles, unsaturated organic substrates, and in small molecule activation are discussed. Further, their physical and chemical properties are described based on their spectroscopic and theoretical studies.

Distannabarrelenes with Three Coordinated SnII Atoms

Crystalline 1,4-distannabarrelene compounds [(ADCAr )3 Sn2 ]SnCl3 (3-Ar) (ADCAr ={ArC(NDipp)2 CC}; Dipp=2,6-iPr2 C6 H3 , Ar=Ph or DMP; DMP=4-Me2 NC6 H4 ) derived from anionic dicarbenes Li(ADCAr ) (2-Ar) (Ar=Ph or DMP) have been reported. The cationic moiety of 3-Ar features a barrelene framework with three coordinated SnII atoms at the 1,4-positions, whereas the anionic unit SnCl3 is formally derived from SnCl2 and chloride ion. The all carbon substituted bis-stannylenes 3-Ar have been characterized by NMR spectroscopy and X-ray diffraction. DFT calculations reveal that the HOMO of 3-Ph (ϵ=-6.40 eV) is mainly the lone-pair orbital at the SnII atoms of the barrelene unit. 3-Ar readily react with sulfur and selenium to afford the mixed-valence SnII /SnIV compounds [(ADCAr )3 SnSn(E)](SnCl6 )0.5 (E=S 4-Ar, Ar=Ph or DMP; E=Se 5-Ph).

Recent Advances in Rare Earth Complexes Containing N-Heterocyclic Carbenes: Synthesis, Reactivity, and Applications in Polymerization

N-heterocyclic carbenes (NHCs) are ubiquitous ancillary ligands employed in metal-catalyzed homogeneous reactions and polymerization reactions. Of significance is the use of NHCs as the supporting ligand in second- and third-generation Grubbs catalysts for their application in olefin metathesis and ring-opening metathesis polymerization. While the applications of transition metal catalysts ligated with NHCs in polymerization chemistry are well-documented, the use of analogous rare earth (Ln = Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) catalysts in this area remains under-developed, despite the unique role of rare earth elements in regio- and stereo-specific (co)polymerization reactions. By using hetero-atom-tethered chelating NHCs and, more recently, the employment of other structurally related NHCs, NHC-ligated Ln complexes have proven to be promising and fruitful catalysts for selective polymerization reactions. This review summarizes the recent developments in the coordination chemistry of Ln complexes containing NHCs and their catalytic performance in polymerization.

A crystalline C5-protonated 1,3-imidazol-4-ylidene

The first C5-protonated 1,3-imidazole-based mesoionic carbene (iMIC^Bp) has been isolated and characterized by single-crystal X-ray diffraction.

Chalcogen complexes of anionic N-heterocyclic carbenes

Chalcogen complexes of anionic N-heterocyclic carbenes were isolated as lithium salts and their protonation and oxidation were studied.

Bis(oxazoline)-derived N-heterocyclic carbene ligated rare-earth metal complexes: synthesis, structure, and polymerization performance

Bis(oxazoline)-derived NHC supported rare-earth complexes have been synthesized for the first time and showed good activity for 1-hexene (co)polymerization.