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

Metal-silicon triple bonds: reactivity of the silylidyne complexes [Cp*(CO)2M[triple bond, length as m-dash]Si-Tbb] (M = Cr - W)

The different reactivity pattern of M[triple bond, length as m-dash]Si and M[triple bond, length as m-dash]C bonds (M = transition metal) is illustrated by a series of reactions of the silylidyne complexes [Cp*(CO)2M[triple bond, length as m-dash]Si-Tbb] (1-M) (M = Cr - W; Cp* = η 5-pentamethylcyclopentadienyl; Tbb = 4-tert-butyl-2,6-bis(bis(trimethylsilyl)methyl)phenyl)). Complexes 1-M were obtained selectively from Li[Cp*M(CO)3] and the 1,2-dibromodisilene (E)-Tbb(Br)Si[double bond, length as m-dash]Si(Br)Tbb. The reaction of 1-Mo and 1-W with two equivalents of mesityl isocyanate leads selectively to complex 2-Mo and 2-W, respectively, featuring a novel κ 2 O,O-imidocarbonatosilyl ligand. Ring opening of ethyloxirane occurs rapidly with 1-Mo and leads to the hydrido-enolatosilylidene complex 3-Mo illustrating the Si-centered electrophilicity of the silylidyne complex. Trimethylsilyldiazomethane induces a cleavage of the Mo[triple bond, length as m-dash]Si bond of 1-Mo after a rapid double [2 + 1] cycloaddition of the terminal N-atom, resulting in the first silaamidinato complex 4-Mo. In comparison, the reaction of 1-Mo with mesityl azide gives, after N2 elimination, the Mo-silaiminoacyl complex 5-Mo. All compounds were fully characterized and the isomerism and dynamics of 3-Mo in solution were analysed by a combination of spectroscopic and quantum-chemical studies.

Recent advances in the chemistry of heavier group 14 analogues of carbonyls

Heavier analogues of carbonyls, in the form of "R2E=O" (E = Si, Ge, Sn, Pb), feature a high polar E=O double bond. In contrast to carbonyl compounds, heavier analogues are extremely unstable and prone to proceed head-to-tail oligomerization. Thus, the isolation of such species under ambient conditions is a challenging synthetic target in main group chemistry. In recent years, much progress has been achieved in the synthesis and isolation of a variety of Lewis base/acid, Lewis base-stabilized and even Lewis acid/base free heavier analogues. These compounds exhibit interesting reactivities, such as small molecule activation and metathesis reactions, indicating the potential of heavier analogues in synthetic chemistry. This review summarizes the recent achievements in the chemistry of Lewis base and/or acid stabilized heavier analogues of carbonyls, including synthetic approaches, structural parameters and reactivity of these isolable compounds.

Halogen-Exchange Reactions of Iminophosphonamido-Chlorosilylenes with Alkali Halides: Convenient Synthesis of Heavier Halosilylenes

Halogen-substituted silylenes are an important building block for synthesizing silicon-based low-valent and multiple-bond species. However, the number of reports on heavier halosilylenes that contain bromine and iodine is still limited. Here, we present a convenient synthesis for bromo- and iodosilylenes supported by an iminophosphonamide ligand. The heavier halosilylenes [Ph₂P(^tBuN)₂]SiX (2: X = Br, 3: X = I) were successfully synthesized via the halogen-exchange reaction of chlorosilylene 1 with alkali halides in THF. As a demonstration of the reactivity of 2 and 3, oxidative addition reactions of 2 and 3 with elemental selenium in C₆D₆ afforded the corresponding bromo- (5) or iodosilylene-selone (6) as colorless crystals. The molecular structures of 2, 3, 5, and 6 were fully characterized by spectroscopic means and single-crystal X-ray diffraction analysis. Furthermore, the effects of the halogen atom on the electronic state of halosilylenes 1-3 and halosilylene-selones 4-6 were investigated using density functional theory (DFT) calculations.

Reactivity of organogermanium and organotin trihydrides

The organogermanium and organotin trihydrides (TbbEH₃) [E = Ge (3), Sn (7)] with the Tbb substituent were synthesized by hydride substitution (Tbb = 2,6-[CH(SiMe₃)₂]₂-4-(t-Bu)C₆H₂). Deprotonation of the organoelement trihydrides 3 and 7 was studied in reaction with bases MeLi, BnK and LDA (Bn = benzyl, LDA = lithium diisopropylamide) to yield the deprotonation products (8-11) as lithium or potassium salts. Hydride abstraction from TbbSnH₃ using the trityl salt [Ph₃C][Al(OC{CF₃}₃)₄] gives the salt [TbbSnH₂][Al(OC{CF₃}₃)₄] (12) which was stabilized by thf donor ligands [TbbSnH₂(thf)₂][Al(OC{CF₃}₃)₄] (13). Tintrihydride 7 reacts with trialkylamine Et₂MeN to give as the product of a reductive elimination of hydrogen the distannane (TbbSnH₂)₂ (14). Transfer of hydrogen was observed in reaction of trihydrides TbbEH₃ (E = Ge, Sn) and Ar*GeH₃ with N-heterocyclic carbene (NHC). The NHC adduct TbbSnH(^iPrNHC) (15) was synthesized at rt and the germanium hydrides exhibit hydrogen transfer at higher temperatures to give Ar*GeH(^MeNHC) (16) and TbbGeH(^MeNHC) (17).

An NHC-Mediated Metal-Free Approach towards an NHC-Coordinated Endocyclic Disilene

A convenient metal-free approach towards an N-heterocyclic carbene (NHC)-coordinated disilene 2 is described. Compound 2, featuring the disilene incorporated in cyclopolysilane framework, was obtained in good yield and characterized using NMR spectroscopy and X-ray crystallography. Density functional theory (DFT) calculations of the reaction mechanism provide a rationale for the observed reactivity and give detailed information on the bonding situation of the base-stabilized disilene. Compound 2 undergoes thermal or light- induced (λ=456 nm) NHC loss, and a dimerization process to give a corresponding dimer with a Si10 skeleton. In order to shed light on the dimerization mechanism, DFT calculations were performed. Moreover, the reactivity of 2 was examined with selected examples of transition metal carbonyl compounds.

Reactions of N-heterocyclic carbene-based chalcogenoureas with halogens: a diverse range of outcomes

We have investigated the reactions of chalcogenoureas derived from N-heterocyclic carbenes, referred to here as [E(NHC)], with halogens. Depending on the structure of the chalcogenourea and the identity of the halogen, a diverse range of reactivity was observed and a corresponding range of structures was obtained. Cyclic voltammetry was carried out to characterise the oxidation and reduction potentials of these [E(NHC)] species; selenoureas were found to be easier to oxidise than the corresponding thioureas. In some cases, a correlation was found between the oxidation potential of these compounds and the electronic properties of the corresponding NHC. The reactivity of these chalcogenoureas with different halogenating reagents (Br₂, SO₂Cl₂, I₂) was then investigated, and products were characterised using NMR spectroscopy and single-crystal X-ray diffraction. X-ray analyses elucidated the solid-state coordination types of the obtained products, showing that a variety of possible adducts can be obtained. In some cases, we were able to extrapolate a structure/activity correlation to explain the observed trends in reactivity and oxidation potentials.

Formation of silaimines from a sterically demanding iminophosphonamido chlorosilylene via intramolecular N-P bond cleavage

The sterically demanding iminophosphonamido chlorosilylene [Ph2P(DipN)2]SiCl (Dip = 2,6-diisopropylphenyl) was synthesized and fully characterized using NMR spectroscopy and X-ray crystallography. Substitution reactions of [Ph2P(DipN)2]SiCl with N- and Fe-nucleophiles led to the unexpected formation of the corresponding silaimine derivatives. This process involves the ring-opening rearrangement of three-coordinated silylene intermediates that proceeds via intramolecular N-P bond cleavage.

Phosphine-Stabilized Germasilenylidene: Source for a Silicon-Atom Transfer

A phosphine-stabilized germasilenylidene is synthesized following the pathway of SiCl₄ oxidative addition at a germylene-phosphine Lewis pair. Low-temperature reduction using {(^MesNacnac)Mg}₂ resulted in a chlorosilylene intermediate and finally a molecule exhibiting a Ge═Si: motif. Inside the chelating phosphine-germylene, a low-valent silicon atom is stabilized and was transferred to diazabutadiene to give N-heterocyclic silylenes. Because of the high reactivity of the phosphine-stabilized germasilenylidene, a reaction of two Ge═Si: units was found to yield a Si₂Ge₂-ring molecule exhibiting a germasilene substituted with a silylene.

Probing the tautomerization of disilenes and disilabenzenes with their isomeric silylenes: significant substituent, aromaticity and base effects

Disilene has attracted considerable interest due to the trans-bending geometry which is significantly different from the planar alkene. However, the equilibrium between disilene and isomeric silylsilylene has not been fully understood. Here, we report a density functional theory (DFT) study on this equilibrium. Calculations reveal significant effects of substituent, aromaticity and base. Specifically, the parent disilene is thermodynamically more stable than the isomeric silylene. When the methoxy substituent is introduced, the corresponding silylene becomes thermodynamically more stable, which could be rationalized by the Bent's rule. Interestingly, disilabenzene becomes thermodynamically more stable than the isomeric silylene when the concept of aromaticity is taken into account. Finally, once the base is introduced, the silylene could become thermodynamically more stable than the isomeric disilabenzene. The kinetic effect of the tautomerization with several typical substituents (F, Me and OMe) has also been investigated. Some species with a bridged form have been found to have a higher thermodynamic stability over the nonbridged ones. All these findings could be particularly useful to develop the chemistry of disilenes and silylenes.

Transition metal chemistry of heavier group 14 congener triple-bonded complexes: syntheses and reactivity

The diversification and synthetic utility of carbyne complexes in organometallic chemistry and catalysis are well recognized, but the syntheses of related heavier group 14 alkylidyne complexes are a recent advancement. A wide range of metal-ylidyne M[triple bond, length as m-dash]E (E = Si-Pb) complexes were synthesized and characterized spectroscopically. The synthetic methodology generally involves elimination or substitution chemistry between metallates and suitable group 14 precursors. The reluctance in forming triple bonded complexes makes this field quite fascinating and challenging. This article gives a brief overview of the pioneering reports followed by detailed information on the latest developments of complexes having a triple bond between a metal and heavier group 14 elements (Si, Ge, Sn, and Pb). Their synthesis and chemistry of the earlier reports followed by recent progress in this field will be discussed. Furthermore, their unique structures and bonding properties will be described based on spectroscopic and theoretical studies.

Generation of Bis(ferrocenyl)silylenes from Siliranes

Divalent silicon species, the so-called silylenes, represent attractive organosilicon building blocks. Isolable stable silylenes remain scarce, and in most hitherto reported examples, the silicon center is stabilized by electron-donating substituents (e.g., heteroatoms such as nitrogen), which results in electronic perturbation. In order to avoid such electronic perturbation, we have been interested in the chemistry of reactive silylenes with carbon-based substituents such as ferrocenyl groups. Due to the presence of a divalent silicon center and the redox-active transition metal iron, ferrocenylsilylenes can be expected to exhibit interesting redox behavior. Herein, we report the design and synthesis of a bis(ferrocenyl)silirane as a precursor for a bis(ferrocenyl)silylene, which could potentially be used as a building block for redox-active organosilicon compounds. It was found that the isolated bis(ferrocenyl)siliranes could be a bottleable precursor for the bis(ferrocenyl)silylene under mild conditions.

An Air-Stable Heterobimetallic Si2 M2 Tetrahedral Cluster

Air- and moisture-stable heterobimetallic tetrahedral clusters [Cp(CO)2 MSiR]2 (M=Mo or W; R=SitBu3 ) were isolated from the reaction of N-heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)2 M=Si(SitBu3 )NHC with a mild Lewis acid (BPh3 ). Alternatively, treatment of the NHC-stabilized silylidene complex Cp(CO)2 W=Si(SitBu3 )NHC with stronger Lewis acids such as AlCl3 or B(C6 F5 )3 resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four-membered ring is due to steric bulk.

NHC-Stabilized Silyl-Substituted Chlorosilylene

The first N-heterocyclic carbene (NHC) stabilized silyl-substituted chlorosilylene (1) was isolated via selective dehydrochlorination by NHC from silyl-based Si(IV) precursor tBu₃SiSiHCl₂. Compound 1 can form an iron chlorosilylene complex (2) with an iron carbonyl dimer and undergoes chloride/hydride metathesis to yield a stable NHC-silylene hydride borane adduct (3). Upon treatment with additional NHC, chlorosilylene 1 was converted into silyl-substituted silyliumylidene ions (4).

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Equilibrium Coordination of NHCs to Si(IV) Species and Donor Exchange in Donor-Acceptor Stabilized Si(II) and Ge(II) Compounds

We report the reversible coordination of the N-heterocyclic carbene (NHC), NHC ^iPr₂Me₂ (NHC ^iPr₂Me₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), to silicon(IV)-halides, SiCl₄, MeSiCl₃, Me₂SiCl₂, and Me₃SiCl. Predicted as well as experimentally determined thermodynamic parameters of these equilibria confirm that the complexation constant increases with the Lewis acidity of the silicon halides. In contrast, the more σ-donating N-heterocyclic carbene, NHC^Me₄ (NHC^Me₄ = 1,3,4,5-tetramethylimidazol-2-ylidene), does not show any signs of dissociation from the corresponding SiCl₄ and Me₂SiCl₂ adducts even at higher temperatures. As a consequence, NHC ^iPr₂Me₂ in donor-acceptor stabilized Si(II)- and Ge(II)-dimethyl complexes, NHC ^iPr₂Me₂·GeMe₂·Fe(CO)₄ and NHC ^iPr₂Me₂·SiMe₂·Fe(CO)₄, is readily replaced by NHC^Me₄.

Facile Access to an NHC-Coordinated Silicon Ring Compound with a Si=N Group and a Two-Coordinate Silicon Atom

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si₄ {N(SiMe₃ )Dipp}₄ 1 (Dipp=2,6-diisopropylphenyl) with 5 equiv of the N-heterocyclic carbene NHC^Me4 (1,3,4,5-tetramethylimidazol-2-ylidene) affords a bifunctional carbene-coordinated four-membered-ring compound with a Si=N group and a two-coordinate silicon atom Si₄ {N(SiMe₃ )Dipp}₂ (NHC^Me4 )₂ (NDipp) 2. When 2 reacts with 0.25 equiv sulfur (S₈ ), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si₄ ring, which confirms the silylone character of the two-coordinate silicon atom in 2.

Synthesis of Cyclic Alkyl(amino) Carbene Stabilized Silylenes with Small N-Donating Substituents

Lewis base cAACs stabilized monomeric silylenes with halogen or methyl substituents at the silicon center have not been reported due to the strong σ-donor and π-acceptor character of cAAC. To prepare these monomeric silylenes, we used the silicon(IV) precursors 5 and 6 with a nitrogen donor group L (L=o-C₆ H₄ NMe₂ ). The cAAC-stabilized (cAAC=C(CH₂ )(CMe₂ )₂ N-Ar, Ar=2,6-iPr₂ C₆ H₃ ) silylenes LSiCl(cAAC) (7) and LSiMe(cAAC) (8) were synthesized by reduction of LSiCl₃ and LSiMeCl₂ with two equivalents of KC₈ in the presence of one equivalent of cAAC, respectively. Compounds 7 and 8 were characterized by single-crystal X-ray crystallography, NMR spectroscopy, and elemental analysis. Compounds 7 and 8 are stable in the solid state as well as in solution at room temperature for at least four months under inert conditions.

A β-Diketiminate-Stabilized Sila-Acyl Chloride: Systematic Access to Base-Stabilized Silicon Analogues of Classical Carbonyl Compounds

An oxidation/substitution strategy for the synthesis of silicon analogues of classical organic carbonyl compounds is reported, by making use of a novel β-diketiminate-supported sila-acyl chloride-the first example of such a compound isolated without the use of a stabilizing Lewis acid. Nucleophilic substitution at the Si^IV center allows direct access to the corresponding sila-aldehyde and sila-ester. An alternative approach utilizing the reverse order of synthetic steps is thwarted by the facile rearrangement of the corresponding Si^II systems featuring either H or OR substituents. As such, the isolation of (N-nacnac)Si(O)Cl represents a key step forward in enabling the synthesis of sila-carbonyl compounds by a synthetic approach ubiquitous in organic chemistry.

Synthesis and Characterization of the Germathioacid Chloride Coordinated by an N-Heterocyclic Carbene §

Carboxylic acid chlorides are useful substrates in organic chemistry. Many germanium analogues of carboxylic acid chloride have been synthesized so far. Nevertheless, all of the reported germathioacid chlorides use bidentate nitrogen ligands and contain germanium-nitrogen bonds. Our group synthesized germathioacid chloride, Ge(S)Cl{C6H3-2,6-Tip2}(Im-i-Pr2Me2), using N-heterocyclic carbene (Im-i-Pr2Me2). As a result of density functional theory (DFT) calculation, it was found that electrons are localized on sulfur, and the germanium-sulfur bond is a single bond with a slight double bond property.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Stable unsaturated silicon clusters (siliconoids)

Unsaturated silicon clusters are key intermediates of silicon deposition processes from the gas phase, which is reflected in the recently introduced term "siliconoids" for silicon rich clusters with at least one hemispheroidally coordinated vertex. Unlike the case of metalloid clusters of heavier Group 14 elements, stable homonuclear derivatives containing silicon have become available just recently. This review summarizes the developments since the first report on a stable unsaturated silicon cluster in 2005. The synthesis, structure and reactivity under retention of the unsaturated vertices (i.e. the functionalization) of stable siliconoids is discussed within the broader context of soluble Zintl anions of silicon and metalloid clusters. A structural parameter is introduced as a quantitative measure to characterize the "hemispheroidality" of an unsubstituted vertex of a siliconoid.

NHC-stabilized 1-hydrosilaimine: synthesis, structure and reactivity

A facile, straightforward synthesis of N-heterocyclic carbene (NHC)-stabilized 1-hydrosilaimine starting from a silicon(iv)-precursor is reported. It has been employed for the addition of an O-H bond of water under ambient conditions without any additional catalyst.

Assembly of NHC-stabilized 2-hydrophosphasilenes from Si(iv) precursors: a Lewis acid-base complex

NHC-stabilized 2-hydrophosphasilenes are obtained from 1,2-dihydro-2-chlorophosphasilanes as Si(iv) precursors by a NHC-assisted 1,2-elimination of HCl. The NHC-exchange of these compounds is demonstrated as a proof of donor acceptor bonding between NHC and the silicon centre of the "Si[double bond, length as m-dash]P" moiety. We have also explored the possibility of similar exchanges in NHC-stabilized Si₂ and P₂ compounds. Theoretical DFT calculations were performed to address the nature of Si-P bonding in the NHC-stabilized 2-hydrophosphasilenes.

Cyclic amino(carboranyl) silylene: synthesis, structure and reactivity

A carbene-stabilized cyclic amino(carboranyl) silylene has been prepared and structurally characterized, which can form a Lewis acid–base adduct with borane and undergo cycloaddition reactions with unsaturated molecules such as diphenylacetylene and benzophenone.

Silicon chemistry in zero to three dimensions: from dichlorosilylene to silafullerane

As one of the simplest examples of functionalized Si(ii) species, the SiCl₂/[SiCl₃]⁻ system is not only fundamentally interesting, but also an important starting point for the assembly of oligosilane chains, rings, and clusters.