Recent Advances in Silylene Chemistry: Small Molecule Activation En-Route Towards Metal-Free Catalysis

Understanding the Impact of Group 14 Elements on the Reactivity of [1 + 2] Cycloaddition Reaction between a Cyclic (Alkyl)(amino)carbene Analogue with a Group 14 Element and a Heavy Acetylene Molecule

Computational exploration using the density functional theory framework (M06-2X-D3/def2-TZVP) was undertaken to investigate the [1 + 2] cycloaddition reaction between a five-membered-ring heterocyclic carbene analogue (G14-Rea; G14 = group 14 element) and a heavy acetylene molecule (G14G14-Rea). It was theoretically observed that exclusively Si-Rea, Ge-Rea, and Sn-Rea demonstrate the capacity to participate in the [1 + 2] cycloaddition reaction with the triply bonded SiSi-Rea. In addition, only three heavy acetylenes (SiSi-Rea, GeGe-Rea, and SnSn-Rea) can catalyze the [1 + 2] cycloaddition reaction with Si-Rea. Our theoretical findings elucidated that the reactivity trend observed in these [1 + 2] cycloaddition reactions primarily arise from the deformation energies of the distorted G14G14-Rea. Also, our study reveals that the bonding characteristics of their respective transition states are controlled by the singlet-singlet interaction (donor-acceptor interaction), rather than the triplet-triplet interaction (electron-sharing interaction). Additionally, our work demonstrates that the bonding behavior between G14-Rea and G14G14-Rea is predominantly determined by the filled p-π orbital of G14G14-Rea (HOMO) → the empty perpendicular p-π orbital of G14-Rea (LUMO), rather than the vacant p-π* orbital of G14G14-Rea (LUMO) ← the filled sp2 orbital of G14-Rea (HOMO).

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Tetryliumylidene ions in synthesis and catalysis

Tetryliumylidene ions ([R-E:]+), recognised for their intriguing electronic properties, have attracted considerable interest. These positively charged species, with two vacant p-orbitals and a lone pair at the E(ii) centre (E = Si, Ge, Sn, Pb), can be viewed as the combination of tetrylenes (R2E:) and tetrylium ions ([R3E]+), which makes them potent Lewis ambiphiles. Such electronic features highlight the potential of tetryliumylidenes for single-site small molecule activation and transition metal-free catalysis. The effective utilisation of the electrophilicity and nucleophilicity of tetryliumylidenes is expected to stem from appropriate ligand choice. For most of the isolated tetryliumylidenes, electron donor- and/or kinetic stabilisation is necessary. This minireview highlights the developments in tetryliumylidene syntheses and the progress of research towards their reactivity and applications in catalytic reactions.

N-Heterocyclic silylenes as ambiphilic activators and ligands

Recent developments of the use of N-heterocyclic silylenes (NHSis), higher homologues of Arduengo-carbenes, as ambiphilic activators and ligands are highlighted and a comparison of NHSi ligands with NHC and phosphine ligands is provided.

A silicon-carbonyl complex stable at room temperature

Main-group-element compounds with energetically high-lying donor and low-lying acceptor orbitals are able to mimic chemical bonding motifs and reactivity patterns known in transition metal chemistry, including small-molecule activation and catalytic reactions. Monovalent group 13 compounds and divalent group 14 compounds, particularly silylenes, have been shown to be excellent candidates for this purpose. However, one of the most common reactions of transition metal complexes, the direct reaction with carbon monoxide and formation of room-temperature isolable carbonyl complexes, is virtually unknown in main-group-element chemistry. Here, we show the synthesis, single-crystal X-ray structure, and density functional theory computations of a room-temperature-stable silylene carbonyl complex [L(Br)Ga]₂Si:-CO (L = HC[C(Me)N(2,6-ⁱPr₂-C₆H₃)]₂), which was obtained by direct carbonylation of the electron-rich silylene intermediate [L(Br)Ga]₂Si:. Furthermore, [L(Br)Ga]₂Si:-CO reacts with H₂ and PBr₃ with bond activation, whereas the reaction with cyclohexyl isocyanide proceeds with CO substitution.

DMAP-stabilized bis(silyl)silylenes as versatile synthons for organosilicon compounds

DMAP-stabilized silylenes 1a–c are obtained from the reductive debromination of the corresponding dibromosilanes in the presence of DMAP. Their distinctly different thermal isomerization reactions via C–H bond activation, dearomative ring expansion and silyl migration are discussed. Furthermore, complexes 1 dissociate at elevated temperatures, providing the corresponding free silylenes in situ, which are even capable of single-site activation of H₂. Additionally, a potassium-substituted silicon-centered radical 2 is isolated from overreduction of (^tBu₃Si)₂SiBr₂.

DMAP-stabilized silylenes 1a–c, which are convenient, room temperature stable synthetic equivalents for the corresponding highly reactive free bis(silyl)silylenes are reported.

NHC-Stabilised Silyliumylidene Ions

Donor-stabilised silyliumylidene ions, from the parent [R-Si:]⁺ , are a class of low-valent silicon species which have received increasing research interest in the last several years. This interest began in the fundamental synthesis and characterisation of these compounds, but has since started to include more investigation into their further reactivity after several stable NHC-stabilised silyliumylidene ions were reported. This personal account briefly discusses the history of the still-young field of silyliumylidene ions followed by a more detailed discussion of published work from our group on the further development of silyliumylidene chemistry over the last four years.

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.

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.

An acyclic zincagermylene: rapid activation of dihydrogen at sub-ambient temperature

The first example of a stable zincagermylene, :Ge(TBoN)(ZnL*) (TBoN = N(SiMe₃){B(DipNCH)₂}, Dip = C₆H₃Prⁱ₂-2,6; L* = -N{C₆H₂[C(H)Ph₂]₂Me-2,6,4}(SiPrⁱ₃)) is prepared and shown to have unprecedented reactivity for a germylene, with respect to the activation of dihydrogen. Computational analyses point towards this being partially derived from the electron releasing nature of the amido-zinc fragment, which leads to a narrowing of the HOMO-LUMO gap in the compound.

From Si(II) to Si(IV) and Back: Reversible Intramolecular Carbon-Carbon Bond Activation by an Acyclic Iminosilylene

Reversibility is fundamental for transition metal catalysis, but equally for main group chemistry and especially low-valent silicon compounds, the interplay between oxidative addition and reductive elimination is key for a potential catalytic cycle. Herein, we report a highly reactive acyclic iminosilylsilylene 1, which readily performs an intramolecular insertion into a C═C bond of its aromatic ligand framework to give silacycloheptatriene (silepin) 2. UV-vis studies of this Si(IV) compound indicated a facile transformation back to Si(II) at elevated temperatures, further supported by density functional theory calculations and experimentally demonstrated by isolation of a silylene-borane adduct 3 following addition of B(C₆F₅)₃. This tendency to undergo reductive elimination was exploited in the investigation of silepin 2 as a synthetic equivalent of silylene in the activation of small molecules. In fact, the first monomeric, four-coordinate silicon carbonate complex 4 was isolated and fully characterized in the reaction with carbon dioxide under mild conditions. Additionally, the exposure of 2 to ethylene or molecular hydrogen gave silirane 5 and Si(IV) dihydride 6, respectively.

Iron(II), Cobalt(II), Nickel(II), and Zinc(II) Silylene Complexes: Reaction of the Silylene [iPrNC(NiPr2 )NiPr]2 Si with FeBr2 , CoBr2 , NiBr2 ⋅MeOCH2 CH2 OMe, ZnCl2 , and ZnBr2

Reaction of the donor-stabilized silylene [iPrNC(NiPr₂ )NiPr]₂ Si (1) with FeBr₂ , CoBr₂ , NiBr₂ ⋅MeOCH₂ CH₂ OMe, ZnCl₂ , and ZnBr₂ afforded the respective transition-metal silylene complexes 4-8, the formation of which can be described in terms of a Lewis acid/base reaction (4, 5, 7, 8) or a nucleophilic substitution reaction (6). However, the reactivity profile of silylene 1 is not only based on its strong Lewis base character; the different coordination modes of the two guanidinato ligands (4-6 vs. 7 and 8) add an additional reactivity facet. The paramagnetic compounds 4 and 5 and the diamagnetic compounds 6⋅THF, 7, and 8⋅0.5 Et₂ O were structurally characterized by single-crystal X-ray diffraction. In addition, compound 6⋅THF was studied by ¹⁵ N and ²⁹ Si solid-state NMR spectroscopy, and 7 and 8 were characterized by NMR spectroscopic studies in the solid state (¹⁵ N, ²⁹ Si) and in solution (¹ H, ¹³ C, ²⁹ Si). Compounds 4-8 represent very rare examples of Fe^II , Co^II , Ni^II , and Zn^II silylene complexes. Four-coordinate silicon(II) compounds with an SiN₃ M skeleton (M=Fe, Co, Ni) and M in the formal oxidation state +2 (4-6) have not yet been reported, and five-coordinate silicon(II) compounds with an SiN₄ Zn skeleton (7, 8) are also unprecedented.

Bis(amidinato)- and bis(guanidinato)silylenes and silylenes with one sterically demanding amidinato or guanidinato ligand: synthesis and reactivity

Silylenes 1–4 have great synthetic potential for the synthesis of higher coordinate silicon(ii) and silicon(iv) complexes with unprecedented structural motifs.

A Pt(0) complex with cyclic (alkyl)(amino)silylene and 1,3-divinyl-1,1,3,3-tetramethyldisiloxane ligands: synthesis, molecular structure, and catalytic hydrosilylation activity

A platinum(0) complex bearing a cyclic (alkyl)(amino)silylene and a 1,3-divinyl-1,1,3,3-tetramethyldisiloxane (DVTMS) was synthesized and isolated in the form of colorless crystals.

Basic Reactivity Pattern of a Cyclic Disilylated Germylene

In order to estimate the reactivity of disilylated germylene phosphine adducts, a cyclic version of this compound class was reacted with a number of different reagents. Reactions with the chalcogens sulfur, selenium, and tellurium led to dimers of the heavy ketone analogues. Reactions with water and ethyl bromide proceeded to give the respective oxidized germanol and germyl bromide. Two different reactions with alkynes were observed which led either to a germacyclopropene, by addition of tolane to the germylene, or to a silagermacyclobutene, likely formed by addition of the alkyne across a silagermene. Reaction via the silagermene was also observed in the reaction with benzophenone. Reaction of a germylene phosphine adduct with GeCl2·(dioxane) provided insertion of the silylated germylene into a Ge-Cl bond, leading to a germylated chlorogermylene phosphine adduct.

Cationic Five-Coordinate Bis(guanidinato)silicon(IV) Complexes with SiN4 El Skeletons (El=S, Se): "Heterolytic Activation" of S-S and Se-Se Bonds

The donor-stabilized silylene [iPrNC(NiPr2 )NiPr]2 Si (2) reacts with PhEl-ElPh (El=S, Se) to form the respective cationic five-coordinate bis(guanidinato)silicon(IV) complexes {[iPrNC(NiPr2 )NiPr]2 SiSPh}(+) PhS(-) (4) and {[iPrNC (NiPr2 )NiPr]2 SiSePh}(+) PhSe(-) (5). Compounds 4 and 5 were characterized by crystal structure analyses and NMR spectroscopic studies in the solid state.

Unprecedented silicon(ii)→calcium complexes with N-heterocyclic silylenes

The first N-heterocyclic silylene (NHSi) complexes of any s-block element to date are reported for calcium: [(η⁵-C₅Me₅)₂Ca←:Si(O-C₆H₄-2-^tBu){(N^tBu)₂CPh}] and [(η⁵-C₅Me₅)₂Ca←:Si(N^tBuCH)₂]. The synthesis, structure, bonding analysis by DFT methods and the reactivity towards oxygen containing substrates is discussed.