A Generic One-Pot Route to Acyclic Two-Coordinate Silylenes from Silicon(IV) Precursors: Synthesis and Structural Characterization of a Silylsilylene

Preparation of a high-coordinated-silicon-centered spiro-cyclic compound

Silicon compounds containing silicon-silicon bond with a variety of unusual oxidation states are quite important, because their high reactivity leads to the formation of a variety of silicon compounds. The isolation of such compounds with unusual oxidation states requires a resilient synthetic strategy. Herein, we report the synthesis of a silicon based spirocyclic compound containing a hyper-valent silicon atom and a silicon-silicon bond. The computational calculations employing natural bond orbital (NBO) analysis and energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) reveal that the nature of bonding between the silicon atoms is of an electron sharing nature.

Silylenes with a Small Chalcogenide Substituent: Tuning Frontier Orbital Energies from O to Te

The general synthesis of heteroleptic acyclic silylenes with a bulky carbazolyl substituent (dtbpCbz) is detailed and a series of compounds with a chalcogenide substituent of the type [(dtbpCbz)SiE16R] (E16R=OtBu, SEt, SePh, TePh) is reported. With the bulky carbazolyl substituent present, the chalcogenide moiety can be very small, as is shown by incorporating groups as small as ethyl, phenyl or tert-butyl. For the first time, the electronic properties of the silylene can be tuned along a complete series of chalcogenide substituents. The effects are clearly visible in the NMR and UV/Vis spectra, and were rationalised by DFT computations. The reactivity of the heaviest chalcogenide-substituted silylenes was probed by reactions with trimethylphosphine selenide and the terphenyl azide TerN3 (Ter=2,6-dimesitylphenyl).

Ferrocene-Based N-Heterocyclic Silylenes: Monomeric Silanechalcogenones, Silanimines, Silirenes, and Insertion Products with P4

The stable ferrocene-based N-heterocyclic silylenes fc[(N{B})2Si] (A; fc=1,1'-ferrocenylene, {B}=(HCNDipp)2B, Dipp=2,6-diisopropylphenyl) and fc[(NDipp)2Si] (B) are compared in a study focussing on their reactivity towards a range of small to moderately sized molecular substrates, viz. P4, S8, Se8, MesN3 (Mes=mesityl), RC≡CH, and RC≡CR (R=Ph, SiMe3). The Dipp-substituted congener B exhibits a more pronounced ambiphilicity and is sterically less congested than its 1,3,2-diazaborolyl-substituted relative A, in line with the higher reactivity of the former. The difference in reactivity is obviously due more to electronic than to steric reasons, as is illustrated by the fact that both A and B react with the comparatively bulky substrate MesN3 under mild conditions to afford the corresponding silanimine fc[(N{B})2Si=NMes] and fc[(NDipp)2Si=NMes], respectively. The heavier ketone analogues fc[(N{B})2Si=E] (E=S, Se, Te) are readily available from A and the corresponding chalcogen. In contrast, the reaction of the more reactive silylene B with elemental sulfur or selenium is unspecific, affording product mixtures. However, fc[(NDipp)2Si=Se] is selectively prepared from B and (Et2N)3PSe; the Te analogue is also accessible, but crystallises as head-to-tail dimer.

Diverse Reactivity of Amidinate-Supported Boron Centers with the Hypersilyl Anion and Access to a Monomeric Secondary Boron Hydride

Diverse reactivity of the bulky tris(trimethylsilyl)silyl substituent [Si(SiMe3)3], also known as the hypersilyl group, was observed for amidinate-supported dichloro- and phenylchloroborane complexes. Treatment of the dichloroborane with potassium tris(trimethylsilyl)silyl led to the activation of the backbone β-carbon center and formation of saturated four-membered heterocyclic chloroboranes R'{Si(SiMe3)3}C(NR)2BCl [R' = Ph, R = Cy (3); R' = Ph, R = iPr (6); R' = tBu, R = Cy (8)], whereas the four-membered amidinate hypersilyl-substituted phenyl borane 4 {PhC(NCy)2B(Ph)[Si(SiMe3)3]} was observed for the case of an amidinate-supported phenylchloroborane. The highly deshielded 11B NMR spectroscopic resonance and the distinct difference in the 29Si NMR spectrum confirmed the presence of a σ-donating hypersilyl effect on compounds 3, 6, and 8. Reaction of 3 with the Lewis acid AlCl3 led to the formation of complex 11 in which an unusual cleavage of one of the C-N bonds of the amidinate backbone is observed. Nucleophilic substitution at the boron center of saturated chloroborane 3 with phenyllithium generated the phenylborane derivative 12, whereas the secondary monomeric boron hydride 13 was observed after treatment with alane (AlH3). All compounds (2-13) have been fully characterized by NMR spectroscopy and single-crystal X-ray structure determination studies.

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.

Approach to the "Missing" Diarylsilylene: Formation, Characterization, and Intramolecular C-H Bond Activation of Blue Diarylsilylenes with Bulky Rind Groups

The treatment of the bulky Rind-based dibromosilanes, (Rind)2SiBr2 (2) [Rind = 1,1,7,7-tetra-R1-3,3,5,5-tetra-R2-s-hydrindacen-4-yl: EMind (a: R1 = Et, R2 = Me) and Eind (b: R1 = R2 = Et)], with two equivalents of tBuLi in Et2O at low temperatures resulted in the formation of blue solutions derived from the diarylsilylenes, (Rind)2Si: (3). Upon warming the solutions above -20 °C, the blue color gradually faded, accompanying the decomposition of 3 and yielding cyclic hydrosilanes (4) via intramolecular C-H bond insertion at the Si(II) center. The molecular structures of the bulky Eind-based 3b and 4b were confirmed by X-ray crystallography. Thus, at -20 °C, blue crystals were formed (Crystal-A), which were identified as mixed crystals of 3b and 4b. Additionally, colorless crystals of 4b as a singular component were isolated (Crystal-B), whose structure was also determined by an X-ray diffraction analysis. Although the isolation of 3 was difficult due to their thermally labile nature, their structural characteristics and electronic properties were discussed based on the experimental findings complemented by computational results. We also examined the hydrolysis of 3b to afford the silanol, (Eind)2SiH(OH) (5b).

Preparation of a Compound with a SiII -SiIV -SiII Bonding Arrangement

Herein, we report the synthesis of a rare bis-silylene, 1, in which two SiII atoms are bridged by a SiIV atom. Compound 1 contains an unusual SiII -SiIV -SiII bonding arrangement with SiII -SiIV bond distances of 2.4212(8) and 2.4157(7) Å. Treatment of 1 with Fe(CO)5 afforded a dinuclear Fe0 complex 2 with two unusually long Si-Si bonds (2.4515(8) and 2.4488(10) Å). We have also carried out a detailed computational study to understand the nature of the Si-Si bonds in these compounds. Natural bond orbital (NBO) and energy decomposition analysis-natural orbital for chemical valence (EDA-NOCV) analyses reveal that the Si-Si bonds in 1 and 2 are of an electron-sharing nature.

Stabilization of NH- Group Adjacent to Naked Silicon(II) Atom in Base Stabilized Aminosilylenes

Aminosilylene, comprising reactive NH- and Si(II) sites next to each other, is an intriguing class of compounds due to its ability to show diverse reactivity. However, stabilizing the reactive NH- group next to the free Si(II) atom is challenging and has not yet been achieved. Herein, we report the first examples of base stabilized free aminosilylenes Ar*NHSi(PhC(Nt Bu)2 ) (1 a) and Mes*NHSi(PhC(Nt Bu)2 ) (1 b) (Ar*=2,6-dibenzhydryl-4-methylphenyl and Mes*=2,4,6-tri-tert-butylphenyl), tolerating a NH- group next to the naked Si(II) atom. Remarkably, 1 a and 1 b exhibited interesting differences in their reactivity upon heating. With 1 a, an intramolecular C(sp3 )-H activation of one of the benzhydryl methine hydrogen atoms to the Si(II) atom produced the five-membered cyclic silazane 2. However, with 1 b, a rare 1,2-hydrogen shift to the Si(II) atom afforded a silanimine 3, with a hydride ligand attached to an unsaturated silicon atom. Further, the coordination capabilities of 1 a were also tested with Ru(II) and Fe(0) precursors. Treatments of 1 a with [Ru(η6 -p-cymene)Cl2 ]2 led to the isolation of a η6 -arene tethered complex [RuCl2 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si-η6 -arene}] (4), whereas with the Fe(CO)5 precursor a Fe(0) complex [Fe(CO)4 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si}] (5) was obtained. Density functional theory (DFT) calculations were conducted to shed light on the structural, bonding, and energetic aspects in 1-5.

Geometrically Compelled Silicon(II)/Silicon(IV) Donor-Acceptor Interaction Enables the Enamination of Nitriles

Discovering new bonding scenarios and subsequently exploring the reactivity contribute substantially to advance the main group element chemistry. Herein, we report on the isolation and characterization of an intriguing class of the hydrido-benzosiloles 2-4. These compounds exhibit a side arm of the amidinatosilylenyl group, featuring unidirectional silicon(II)/silicon(IV) donor-acceptor interaction on account of the geometric constraint. Furthermore, the reactions involving 2-4 with nitriles yield the tricyclic compounds that edge-fused of the Si-heteroimidazolidine-CN2 Si2 , silole-C4 Si, and phenyl-C6 -rings (5-13). These compounds are manifesting a unique reaction that the silicon(II)/silicon(IV) interaction enables the enamination of the α-H-bearing nitriles. The reaction mechanism involved in H-shift under oxidative addition at silylene followed by hydrosilylation of a ketenimine intermediate was revealed by density function theory (DFT) calculations.

Tin(II) cations stabilized by non-symmetric N,N',O-chelating ligands: synthesis and stability

A series of novel non-symmetric neutral N,N',O-chelating ligands derived from the α-iminopyridine 2-(C(R¹)N(C₆H₃-2,6-iPr₂))-6-(R²R³PO)C₅H₃N (L1: R¹ = H, R² = R³ = Ph; L2: R¹ = Me, R² = R³ = Ph; L3: R¹ = H; R² = Ph, R³ = EtO; L4: R¹ = Me, R² = Ph, R³ = EtO; L5: R¹ = H, R² = R³ = iPrO; L6: R1 = Me, R² = R³ = iPrO) were synthesized. Ligands L1-6 were reacted with SnCl₂ and Sn(OTf)₂ with the aim of studying the influence of different R²R³PO functional groups on the Lewis base mediated ionization of SnCl₂ and Sn(OTf)₂. While all ligands L1-6 provided the corresponding ionic tin(II) complexes [L1-6 → SnCl]⁺[SnCl₃]^- (1-6), only ligands L1, L4 and L6 were able to stabilize tin(II) dications [L1,4,6 → Sn(H₂O)][OTf]₂ (7-9). The auto-ionized compounds [L3-6 → SnCl]⁺[SnCl₃]^- possessing ethylphenyl phosphinate and diisopropylphosphite substituents undergo elimination of EtCl and iPrCl, respectively, yielding compounds 10-13. These can either be interpreted as neutral tin(II)phosphinate chloride (10, 11) and tin(II)phosphonate chloride (12, 13), respectively, containing Sn-O and Sn-Cl bonds, and a PO → SnCl₂ interaction, or as zwitterionic compounds, where the positive charge of the central tin atom is compensated by an [OSnCl₂]^- anion. Finally, DFT studies were performed to better understand the steric and electronic properties of the ligands L1-6 as well as the nature of the bonds in the resulting products, with a particular focus on complexes 10-13.

Room Temperature Intermolecular Dearomatization of Arenes by an Acyclic Iminosilylene

A novel nontransient acyclic iminosilylene (1), bearing a bulky super silyl group (-Si^tBu₃) and N-heterocyclic imine ligand with a methylated backbone, was prepared and isolated. The methylated backbone is the feature of 1 that distinguishes it from the previously reported nonisolable iminosilylenes, as it prevents the intramolecular silylene center insertion into an aromatic C-C bond of an aryl substituent. Instead, 1 exhibits an intermolecular Büchner-ring-expansion-type reactivity; the silylene is capable of dearomatization of benzene and its derivatives, giving the corresponding silicon analogs of cycloheptatrienes, i.e. silepins, featuring seven-membered SiC₆ rings with nearly planar geometry. The ring expansion reactions of 1 with benzene and 1,4-bis(trifluoromethyl)benzene are reversible. Similar reactions of 1 with N-heteroarenes (pyridine and DMAP) proceed more rapidly and irreversibly forming the corresponding azasilepins, also with nearly planar seven-membered SiNC₅ rings. DFT calculations reveal an ambiphilic nature of 1 that allows the intermolecular aromatic C-C bond insertion to occur. Additional computational studies, which elucidate the inherent reactivity of 1, the role of the substituent effect, and reaction mechanisms behind the ring expansion transformations, are presented.

Frustrated Lewis pair-ligated tetrelenes

The reactivity of [PB{SiX₂}] (X = Cl, Br; PB = 1,2-ⁱPr₂(C₆H₄)BCy₂; E = Si, Ge) adducts is described, with an initial focus on reduction attempts to access [PB{E}]_x species; however, in all cases only free PB ligand was formed as the soluble product. Moreover, computations were performed to evaluate the energy penalty associated with EX₂ dissociation from the PB chelates. Moving up the periodic table, the formal methylene adduct [PB{CH₂}] was isolated and its reactivity was compared with its heavier element congeners of [PB{EH₂}]. We also introduce new phosphine-borane frustrated Lewis pair (FLP) chelates and explore preliminary coordination chemistry with these ligands.

Room-Temperature-Observable Interconversion Between Si(IV) and Si(II) via Reversible Intramolecular Insertion Into an Aromatic C-C Bond

An easily isolable silacycloheptatriene (silepin) 1 b was synthesized from the reaction of a N-heterocyclic imino (IPrN) substituted tribromosilane IPrNSiBr₃ with the sterically congested bis(trimethylsilyl)triisopropylsilyl silanide KSi(TMS)₂ Si(ⁱ Pr)₃ (BTTPS). In solution, the Si(IV) silepin 1 b is in a thermodynamic equilibrium with the acyclic Si(II) silylene 1 a. The relative concentration of the Si(II) or Si(IV) isomers can be controlled by temperature variation and observed by variable temperature NMR and UV/Vis spectroscopy. DFT calculations show a small reaction barrier for the Si(II)⇌Si(IV) interconversion and a small energy gap between the Si(II) and Si(IV) species. The reactivity of 1 a/b is demonstrated on a variety of small molecules.

Synthesis, Characterization and Reactivity of a σ‐Donating Ni 0 ‐Stabilized Silyliumylidene Ion

The synthesis of a silyliumylidene cation complex 2 stabilized by a Ni⁰‐based donating ligand is reported. Experimental and theoretical studies demonstrate that the highly electrophilic Si^II center is stabilized by a dative Ni→Si σ‐interaction and π‐donations from the amino‐ and Ni‐moieties. Due to the energetically close frontier orbitals localized on the Si and Ni atoms, complex 2 presents a competitive reactivity at Si and Ni sites.

Tetracyclic silaheterocycle formed through a pericyclic reaction cascade including a two-fold intramolecular C–C bond activation

Reductive debromination of the tribromoamidosilane 2 gave the tetracyclic silaheterocycle 3 through a unique reaction cascade involving unprecedented two-fold intramolecular cycloaddition by transient silylenes. Experimental and computational analyses of the reaction mechanism allowed the identification of the key intermediates that lead to the silaheterocycle 3 or, alternatively, to the cyclotrisilene 19.

Reductive debromination of the tribromoamidosilane 2 gave the tetracyclic silaheterocycle 3 through a unique reaction cascade involving unprecedented two-fold intramolecular cycloaddition by transient silylenes.

Pyridylpyrrolido ligand in Ge(II) and Sn(II) chemistry: synthesis, reactivity and catalytic application

In our previous communication, we have reported the synthesis of a new chlorogermylene (B) featuring a pyridylpyrrolido ligand. This study details the preparation of a series of new germylenes and stannylenes starting from B. A transmetallation reaction between B and SnCl₂ led to the analogous chlorostannylene (1) with the simultaneous elimination of GeCl₂. This is a very unusual example of transmetallation between two elements of the same group. The preparation of 1via lithiation led to the formation of 2 as a side product, where the ortho C-H bond of the pyridine ring was activated and functionalized with a ⁿBu moiety. Subsequently, B and 1 were used as precursors to generate germylene (4) and stannylene (5) featuring tris(trimethylsilyl)silyl (hypersilyl) moieties. We also prepared tetrafluoropyridyl germylene (6) by reacting 4 with C₅F₅N with the simultaneous elimination of (Me₃Si)₃SiF by utilizing the fluoride affinity of the silicon atom. As there is scarcity of Sn(II) compounds as single-site catalysts, we investigated 5 as a catalyst towards the hydroboration of aldehydes, ketones, alkenes and alkynes. All the compounds have been characterized by single-crystal X-ray diffraction and by state of the art spectroscopic studies.

Synthesis and Reactivity of a Neutral Homocyclic Silylene

Isolation of the neutral homocyclic silylene 2 is possible via amine ligand abstraction with potassium graphite (KC₈) and subsequent reaction with SiMe₃Cl from a bicyclic silicon(I) amide J. This reaction proceeds via an anionic homoaromatic silicon ring compound 1 as an intermediate. The twofold‐coordinated silicon atom in the homocyclic silylene 2 is stabilized by an allyl‐type π‐electron delocalization. 2 reacts in an oxidative addition with two equivalents of MeOH and in cycloadditions with ethene, phenylacetylene, diphenylacetylene and with 2,3‐dimethyl‐1,3‐butadiene to afford novel functionalized ring compounds.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Single‐Site and Cooperative Bond Activation Reactions with Ylide‐Functionalized Tetrylenes: A Computational Study

Due to their transition metal‐like behavior divalent group 14 compounds bear huge potential for their application in bond activation reactions and catalysis. Here we report on detailed computational studies on the use of ylide‐substituted tetrylenes in the activation of dihydrogen and phenol. A series of acyclic and cyclic ylidyltetrylenes featuring various α‐substituents with different σ‐ and π‐donating capabilities have been investigated which demonstrate that particularly π‐accepting boryl groups lead to beneficial properties and low barriers for single‐site activation reactions, above all in the case of silylenes. In contrast, for the thermodynamically more stable germylenes and stannylenes an alternative mechanism involving the active participation of the ylide ligand in the E−H bond (E=H or PhO) activation process by addition across the element carbon linkage was found to be energetically favored. Furthermore, the boryl substituted tetrylenes allowed for a further activation pathway involving the active participation of the boron element bond. These cooperative mechanisms are especially attractive for the heavier cyclic ylidyltetrylenes in which the loss of the protonated ylide group is prevented due to the cyclic framework. Overall, the present studies suggest that cyclic ylide‐substituted germylenes and stannylenes bear huge potential for cooperative bond activations at mild conditions which should be experimentally addressed in the future.

A Striking Mode of Activation of Carbon Disulfide with a Cooperative Bis(silylene)

The reactivity of the 1,4-substituted bis(silylenyl)terphenylene 1, 1,4-[ortho-(LSi)C₆ H₄ ]₂ C₆ H₄ , (L=RC(NtBu)₂ , R=Ph, Mes) towards CS₂ is reported. It results in a dearomatization of the phenylene ring, affording the 1,3-substituted cyclohexadiene derivative 2. According to DFT calculations, a transient silene containing a Si=C bond capable of π(C=C) addition at the aromatic phenylene ring is a key intermediate. In contrast, addition of CS₂ to the biphenyl-substituted mono-silylene ortho-(LSi)C₆ H₄ -C₆ H₅ 3 leaves the aromatic π-system intact and forms, in a [1+2] cycloaddition reaction, the corresponding thiasilirane 4 with a three-membered SiSC ring. Further experimental studies led to the isolation of the novel mesoionic five-membered Si₂ S₂ C heterocycle 6, which reacts with CS₂ under C-C bond formation. All isolated new compounds were fully characterized and their molecular structures determined by single-crystal X-ray diffraction analyses.

Entering new chemical space with isolable complexes of single, zero-valent silicon and germanium atoms

Monatomic zero-valent silicon and germanium complexes (silylones and germylones), stabilised by neutral donating ligands, emerged only recently as a new class of low-valent group 14 element compounds. Featuring four valence electrons in the form of two lone pairs at a single site, silylones and germylones represent a molecular resting state of single Si and Ge atoms, which are typically only observed at high temperature in the gas phase or in interstellar matter. These species are capable of transferring single Si and Ge atoms to unsaturated substrates and acting as building blocks for novel group 14 species. After introducing this type of compound and the examples known to date, this feature article highlights some chelating bis N-heterocyclic carbene (bis(NHC)) and bis N-heterocyclic silylene (bis(NHSi)) supported Si⁰ and Ge⁰ complexes, for which a range of unprecedented reactivity has been discovered. The characteristic behaviour of these silylones and germylones discussed here consists of (i) coordination to Lewis acids, (ii) oxidation with elemental chalcogens, (iii) bond activation of common organic substrates and inert small molecules; and (iv) homocoupling of the Si⁰ and Ge⁰ centres. This wealth of reactivity has opened the door to a series of Si and Ge compounds, which would be otherwise difficult to realise.

Reactivity of a Phosphinoamidinate-Stabilized Disilylene toward H-X Bonds

An efficient method for the preparation of a phosphinoamidinate-supported disilylene was developed, and its reactivity toward H-E bonds (E = elements from Groups 13-15) was studied. With HBpin, transfer of the ligand from silicon to boron was observed to afford (NP)Bpin. Reaction with a silane (H₃SiPh) took place only at elevated temperatures, at which point oxidative addition of the N-P bond of the NP-ligand to one of the silicon atoms of the disilylene occurred prior to Si-H addition involving the remaining silylene center. In contrast, reaction of the disilylene with phosphine, HPPh₂ furnished the phosphidosilylene (NP)SiPPh₂ (16) together with a highly transient species that, on the basis of a trapping experiment with H₃SiPh, is proposed to be the hydridosilylene (NP)SiH, 17. Interestingly, 16 reacts with HPPh₂ to give the diphosphine (PPh₂)₂, most likely via a direct σ-bond metathesis process. The aforementioned products have been characterized by multinuclear NMR and single-crystal X-ray diffraction studies.

Unsymmetrical sp2 -sp3 Disilenes

Disilenes with differently coordinated silicon atoms are not known. Here, we have shown the high yield synthesis of a range of disilenes (2-4 and 6) upon reaction of a hypersilyl silylene PhC(NtBu)₂ SiSi(SiMe₃ )₃ (1) with aliphatic chlorophosphines. The most striking characteristic of these disilenes is the presence of two differently coordinated Si atoms (one is three-coordinated, the other four-coordinated). The analogous reaction with Ph₂ PCl did not afford the desired disilene, but, surprisingly, led to the first tetraphosphinosilane (8). DFT calculations were performed to understand the bonding in disilenes and differences in reactivity of the complexes.

N-Heterocyclic silylenes in coinage metal chemistry: an account of recent advances

This article intends to highlight and comprehensively summarize the recent developments in the field of silylene-coinage metal chemistry. Recent years have witnessed exponential growth in the utilization of N-heterocyclic silylenes as ligands in transition metal chemistry. Still, silylene-coinage metal complexes have only started to appear very recently. Particular attention is focused on the synthetic approaches to silylene-coinage metal complexes and their unusual properties derived from the spectroscopic and crystallographic data. Recent studies have demonstrated that silylene-coinage metal complexes exhibit catalytic efficiency towards hydrosilylation, copper-catalyzed alkyne azide cycloaddition (CuAAC), and glycosidation reactions. Although the chemistry of silylene-coinage metal complexes has only begun to blossom, these findings justify the need for a review at this stage of development. This article will summarize the previous work on silylene-coinage metal complexes followed by recent advances and conclude with future possibilities.

Distinctly different reactivity of bis(silylenyl)-versus phosphanyl-silylenyl-substituted o-dicarborane towards O2, N2O and CO2

In stark contrast to the reactivity of the bis-silylenyl dicarborane CB-Si2 (1) [CB = ortho-C,C'-C2B10H10, Si = PhC(tBuN)2Si] towards O2, N2O, and CO2, yielding the same dioxygenation product CB-Si2O2 (2) with a four-membered 1,3,2,4-disiladioxetane ring, the activation of the latter small molecules with the phosphanyl-silylenyl-functionalised CB-SiP (3) {P[double bond, length as m-dash]P[N(tBu)CH2]2} affords with O2 the CB-Si([double bond, length as m-dash]O)P([double bond, length as m-dash]O) silanone-phosphine oxide (4), with N2O the CB-Si([double bond, length as m-dash]O)P silanone-phosphine (5), and with CO2 the CB-Si(O2C[double bond, length as m-dash]O)P silicon carbonate-phosphine (6) and CB-C([double bond, length as m-dash]O)OSiOP ester (7), respectively.

A Stable Homoleptic Divinyl Tetrelene Series

The synthesis of the new bulky vinyllithium reagent (Me IPr=CH)Li, (Me IPr=[(MeCNDipp)2 C]; Dipp=2,6-iPr2 C6 H3 ) is reported. This vinyllithium precursor was found to act as a general source of the anionic 2σ, 2π-electron donor ligand [Me IPr=CH]- . Furthermore, a high-yielding route to the degradation-resistant SiII precursor Me IPr⋅SiBr2 is presented. The efficacy of (Me IPr=CH)Li in synthesis was demonstrated by the generation of a complete inorganic divinyltetrelene series (Me IPrCH)2 E: (E=Si to Pb). (Me IPrCH)2 Si: represents the first two-coordinate acyclic silylene not bound by heteroatom donors, with dual electrophilic and nucleophilic character at the SiII center noted. Cyclic voltammetry shows this electron-rich silylene to be a potent reducing agent, rivalling the reducing power of the 19-electron complex cobaltocene (Cp2 Co).

Evidence of the Lewis-Amphoteric Character of Tris(pentafluoroethyl)silanide, [Si(C2 F5 )3 ]. Angew Chem Int Ed Engl 2021;60:12124-12131. [PMID: 33617080 PMCID: PMC8252080 DOI: 10.1002/anie.202016455]

According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non-fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C2 F5 )3 ]- ion. With these findings and a HOMO-LUMO gap of 4.80 eV similar to N-heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis-amphoteric character similar to silylenes. Deprotonation of Si(C2 F5 )3 H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C2 F5 )3 ]- anion, which add to benzaldehyde, benzophenone, CS2 , and CO2 in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.

Synthesis and Reactivity of Acyclic Germanimines: Silyl Rearrangement and Cycloadditions

We report the synthesis of aromatic germanimines [(HMDS)₂Ge═NAr] (Ar = Ph, Mes, Dipp; Mes = 2,4,6-Me₃C₆H₂, Dipp = 2,6-iPr₂C₆H₃) and an investigation into their associated reactivity. [(HMDS)₂Ge═NPh] decomposes above -30 °C, while [(HMDS)₂Ge═NDipp] engages in an intramolecular reaction at 60 °C. [(HMDS)₂Ge═NMes] was shown to rearrange via a 1,3-silyl migration to give [(HMDS){(SiMe₃)(Mes)N}Ge(NSiMe₃)] in a 1:7 equilibrium mixture at room temperature. These latter germanimines react with unsaturated polar substrates such as CO₂, ketones, and arylisocyanate via a [2 + 2] cycloaddition pathway.

How metallylenes activate small molecules

We have studied the activation of dihydrogen by metallylenes using relativistic density functional theory (DFT). Our detailed activation strain and Kohn-Sham molecular orbital analyses have quantified the physical factors behind the decreased reactivity of the metallylene on going down Group 14, from carbenes to stannylenes. Along this series, the reactivity decreases due to a worsening of the back-donation interaction between the filled lone-pair orbital of the metallylene and the σ*-orbital of H2, which, therefore, reduces the metallylene-substrate interaction and increases the reaction barrier. As the metallylene ligand is varied from nitrogen to phosphorus to arsenic a significant rate enhancement is observed for the activation of H2 due to (i) a reduced steric (Pauli) repulsion between the metallylene and the substrate; and (ii) less activation strain, as the metallylene becomes increasingly more predistorted. Using a rationally designed metallylene with an optimal Group 14 atom and ligand combination, we show that a number of small molecules (i.e. HCN, CO2, H2, NH3) may also be readily activated. For the first time, we show the ability of our H2 activated designer metallylenes to hydrogenate unsaturated hydrocarbons. The results presented herein will serve as a guide for the rational design of metallylenes toward the activation of small molecules and subsequent reactions.

Synthesis and Coordination Behavior of a New Hybrid Bidentate Ligand with Phosphine and Silylene Donors

This work describes the synthesis and coordination behavior of a new mixed-donor ligand PhC(NtBu)2 SiC6 H4 PPh2 (1) containing both silylene and phosphine donor sites. Ligand 1 was synthesized from a reaction of ortho-lithiated diphenylphosphinobenzene (LiC6 H4 PPh2 ) with chlorosilylene (PhC(NtBu)2 SiCl). Treatment of 1 with Se and GeCl2 resulted in SiIV compounds 2 and 3 by selective oxidation of the silylene donor. This strong σ-donor ligand induces dissociation of CuCl and PhBCl2 leading to formation of ionic complexes 4 and 5 respectively. The reaction of 1 with ZnCl2 and AlCl3 resulted in the formation of chelate complexes 5 and 7, respectively, while treatment with EtAlCl2 and GaCl3 forms monodentate complexes 8 and 9. X-ray analysis of 4 showed that the copper is in the spiro center of the two five-membered rings. Moreover, the copper(I)chloride has not been oxidized but dissociates to Cu+ and [CuCl2 ]- . All the compounds are well characterized by mass spectrometry, elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction studies.

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.

Activation of small molecules by cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes): a theoretical study

Quantum chemical calculations have been carried out on a series of skeletally modified cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes) to understand their ligand properties and reactivity towards the activation of a variety of small molecules. The installation of boron or silicon atoms into the ring framework of these silylenes/germylenes led to a dramatic increase in their σ-basicity while the incorporation of ylidic moieties resulted in a sharp reduction of their π-acidity although it did help in increasing the electron donation ability. The calculated values of energy barriers for the activation of H-H, N-H, C-H and Si-H bonds by many of the cyclic silylenes considered here are found to be comparable to those for experimentally evaluated systems, indicating the potential of these computationally designed molecules in small molecule activation and calling for synthetic efforts towards their isolation. Furthermore, activations employing CAAGes are found to be more demanding than those with CAASis which may be attributed to the significantly lower Lewis basicity of the former than the latter.

Pentamethylcyclopentadienyl-substituted hypersilylsilylene: reversible and irreversible activation of C[double bond, length as m-dash]C double bonds and dihydrogen

Recent studies of low-valent main group species underscore their resemblance to transition metal complexes with regards to the ability to activate small molecules. Here, we report synthesis and full characterisation of the persistent (hypersilyl)(pentamethylcyclopentadienyl)silylene Cp*[(Me3Si)3Si]Si: as well as its unique reactivity. Silylene Cp*[(Me3Si)3Si]Si: activates dihydrogen to give the corresponding dihydrosilane Cp*[(Me3Si)3Si]SiH2 at particularly mild conditions as well as ethylene to afford the three-membered cyclic silirane c-Cp*[(Me3Si)3Si]Si(H2CCH2). The addition of N-heterocyclic carbene NHC (NHC = 1,3,4,5-tetramethyl-imidazol-2-ylidene) to dihydrosilane Cp*[(Me3Si)3Si]SiH2 induces the reductive elimination of Cp*H, which according to DFT calculations is thermodynamically preferred over H2 elimination. With NHC, Cp*[(Me3Si)3Si]Si: forms a typical donor-acceptor complex with concomitant change in hapticity of the Cp* ligand from η2 to η1 (σ). In contrast, the reaction with the N-heterocyclic silylene c-[(CH[double bond, length as m-dash]CH(tBuN)2]Si: leads to an unusual "masked" disilene with the former Cp* ligand bridging the two silicon centres. The heterodimer is stable in the solid state, but dissociates reversibly to the constituting silylene fragments in solution.

Small Molecule Activation by Two-Coordinate Acyclic Silylenes

In recent decades, the chemistry of stable silylenes (R2Si:) has evolved significantly. The first major development in this chemistry was the isolation of a silicocene which is stabilized by the Cp* (Cp* = η5-C5Me5) ligand in 1986 and subsequently the isolation of a first N-heterocyclic silylene (NHSi:) in 1994. Since the groundbreaking discoveries, a large number of isolable cyclic silylenes and higher coordinated silylenes, i.e. Si(II) compounds with coordination number greater than two, have been prepared and the properties investigated. However, the first isolable two-coordinate acyclic silylene was finally reported in 2012. The achievements in the synthesis of acyclic silylenes have allowed for the utilization of silylenes in small molecule activation including inert H2 activation, a process previously exclusive to transition metals. This minireview highlights the developments in silylene chemistry, specifically two-coordinate acyclic silylenes, including experimental and computational studies which investigate the extremely high reactivity of the acyclic silylenes.

Cooperative N-H bond activation by amido-Ge(ii) cations

N-heterocyclic carbene (NHC) and tertiary phosphine-stabilized germylium-ylidene cations, [R(L)Ge:]+, featuring tethered amido substituents at R have been synthesized via halide abstraction. Characterization in the solid state by X-ray crystallography shows these systems to be monomeric, featuring a two-coordinate C,N- or P,N-ligated germanium atom. The presence of the strongly Lewis acidic cationic germanium centre and proximal amide function allows for facile cleavage of N-H bonds in 1,2-fashion: the products resulting from reactions with carbazole feature a tethered secondary amine donor bound to a three-coordinate carbazolyl-GeII centre. In each case, addition of the components of the N-H bond occurs to the same face of the germanium amide function, consistent with a coordination/proton migration mechanism. Such as sequence is compatible with the idea that substrate coordination via the pπ orbital at germanium reduces the extent of N-to-Ge π donation from the amide, thereby enhancing the basicity of the proximal N-group.

Phosphorus-ylides: powerful substituents for the stabilization of reactive main group compounds

Phosphorus ylides are 1,2-dipolar compounds with a negative charge on the carbon atom. This charge is stabilized by the neighbouring onium moiety, but can also be shifted towards other substituents thus making ylides strong π donor ligands and hence ideal substituents to stabilize reactive compounds such as cations and low-valent main group species. Furthermore, the donor strength and the steric properties can easily be tuned to meet different requirements for stabilizing reactive compounds and for tailoring the properties and reactivities of the main group element. Although the use of ylide substituents in main group chemistry is still in its infancy, the first examples of isolated compounds impressively demonstrate the potential of these ligands. This review summarizes the most important discoveries also in comparison to other substituents, thus outlining avenues for future research directions.

Oxidation reactions of a versatile, two-coordinate, acyclic iminosiloxysilylene

Due to their outstanding reactivity, acyclic silylenes have emerged as attractive organosilicon alternatives for transition metal complexes on the way to metal-free catalysis. However, exploration of their reactivity is still in its infancy, as only a few derivatives of this unique compound class have been isolated so far. Here, we present the results of an extensive reactivity investigation of the previously reported acyclic iminosiloxysilylene 1. Divalent silylene 1 proved to be a versatile building block for a plethora of novel organosilicon compounds. Thus, not only the activation of the rather challenging targets NH₃ and P₄ could be achieved, but also the conversion into a reactive donor-free silaimine, which itself turned out to be a useful reagent for small molecule activation. In addition, 1 served as an excellent precursor for gaining access to donor-stabilized heavier carbonyl compounds. Our results thus provide further insights into the chemistry of low-valent silicon at the interface between carbon and transition metals.