Stable Heavier Carbene Analogues

Isolable T-Shaped Planar Silyl Anion

Silyl anions have garnered significant attention due to their synthetic abilities. However, previously reported silyl anions have been limited to either trigonal-pyramidal or trigonal-planar geometries, which confine them primarily as nucleophiles in substitution reactions. Herein, we report the isolation of the unprecedented T-shaped planar silyl anion salt 2 by employment of a geometrically constrained triamido pincer ligand. Theoretical calculations disclosed that the silicon centre in 2 possesses both a lone pair of electrons and an empty 3pz orbital. In addition to nucleophilic substitution reactions with Ph3PAuCl and W(CO)6, 2 readily undergoes oxidative additions with CO2 and 2,6-dimethylphenylisonitrile at room temperature. Furthermore, under mild conditions, compound 2 cleaves Csp2-H, Csp2-H, and H-H bonds in 1,2,4,5-tetrafluorobenzene, an intramolecular iPr group, and dihydrogen, representing the first examples of C-H and H-H activations mediated by a silyl anion, respectively. This work unveils new reactivity of silyl anions owing to the non-classical geometry and electronic structure.

Phospha-bicyclohexene-germylenes exhibiting unexpected reactivity

Introducing phospha-bicyclohexene (BCH)-germylenes (BCHGe's) as a novel, multifunctional compound class: the title compounds 15-18 are obtained from simple salt metathesis reactions of dipotassium germacyclopentadienediides K2[1] with phosphorusdichlorides. The BCHGe's 15-18 are stabilized by homoconjugation of the germanium(ii) centre with the remote C[double bond, length as m-dash]C double bond. Despite substantial thermodynamic stabilization, phospha-BCHGe's are reactive and undergo a reductive elimination of elemental germanium to give the corresponding phospholes. The elimination is a nucleophilic, bimolecular process and is prevented by large substituents. The reaction of phospha-BCHGe's with small electrophiles gives the corresponding phosphonium salts. Oxidation with chalcogens takes place at both the germanium and the phosphorus atom, and after elimination of germanium chalcogenides the corresponding phosphole chalcogenides were isolated. The introduced germylenes exhibit strong nucleophilic but also non-neglectable electrophilic properties.

Antimony Redox Catalysis: Hydroboration of Disulfides through Unique Sb(I)/Sb(III) Redox Cycling

The stibinidene ArSbI (Ar = [2,6-(tBuN═CH)2-C6H3], 1) reacts with S2Tol2 (Tol = p-tolyl) to form ArSbIII(STol)2 (2), which upon treatment with pinacolborane, regenerates 1. These processes unveil an unprecedented antimony redox catalysis involving Sb(I)/Sb(III) cycling for the hydroboration of organic disulfides. Elementary reaction studies and density functional theory calculations support that the catalysis mimics transition metal processes, proceeding through oxidative addition, ligand metathesis, and reductive elimination. The thiophenols and sulfidoborates generated from the hydroboration of disulfides react in situ with α,β-unsaturated carbonyl compounds with the assistance of 1 as a base catalyst. These tandem reactions establish a one-pot synthetic method for β-sulfido carbonyl compounds, in which a stibinidene functions as a redox catalyst and a base catalyst successively, illustrating the versatility and efficiency of antimony catalysis in organic synthesis.

Synthesis of a stable crystalline nitrene

Nitrenes are a highly reactive, yet fundamental, compound class. They possess a monovalent nitrogen atom and usually a short life span, typically in the nanosecond range. Here, we report on the synthesis of a stable nitrene by photolysis of the arylazide MSFluindN3 (1), which gave rise to the quantitative formation of the arylnitrene MSFluindN (2) (MSFluind is dispiro[fluorene-9,3'-(1',1',7',7'-tetramethyl-s-hydrindacen-4'-yl)-5',9''-fluorene]) that remains unchanged for at least 3 days when stored under argon atmosphere at room temperature. The extraordinary life span permitted the full characterization of 2 by single-crystal x-ray crystallography, electron paramagnetic resonance spectroscopy, and superconducting quantum interference device magnetometry, which supported a triplet ground state. Theoretical simulations suggest that in addition to the kinetic stabilization conferred by the bulky MSFluind aryl substituent, electron delocalization across the central aromatic ring contributes to the electron stabilization of 2.

A crystalline stannyne

The synthesis of heteronuclear alkyne analogues incorporating heavier group 14 elements (R1-C≡E-R2, E = Si, Ge, Sn, Pb) has posed a long-standing challenge. Neutral silynes (R1-C≡Si(L)-R2) and germynes (R1-C≡Ge(L)-R2) stabilized by a Lewis base have achieved sufficient stability for structural characterization at low temperatures. Here we show the isolation of a base-free stannyne (R1-C≡Sn-R2) at room temperature, achieved through the strategic use of a bulky cyclic phosphino ligand in combination with a bulky terphenyl substituent. Despite an allenic structure with strong delocalization of π-electrons, this compound exhibits adjacent ambiphilic carbon and tin centres, forming a carbon-tin multiple bond with ionic character. The stannyne demonstrates reactivity similar to carbenes or stannylenes, reacting with 1-adamantyl isocyanide and 2,3-dimethyl-1,3-butadiene. Additionally, its carbon-tin bond can be saturated by Et3N·HCl or cleaved by isopropyl isocyanate.

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).

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Polydentate Amidinato-Silylenes, -Germylenes and -Stannylenes

This review article focuses on amidinatotetrylenes that potentially can (or have already shown to) behave as bi- or tridentate ligands because they contain at least one amidinatotetrylene moiety (silylene, germylene or stannylene) and one (or more) additional coordinable fragment(s). Currently, they are being widely used as ligands in coordination chemistry, small molecule activation and catalysis. This review classifies those that have been isolated as transition metal-free compounds into five families that differ in the position(s) of the donor group(s) (D) on the amidinatotetrylene moiety, namely: ED{R1NC(R2)NR1}, EX{DNC(R2)NR1}, EX{R1NC(D)NR1}, EX{DNC(R2)ND} and E{R1NC(R2)ND}2 (E=Si, Ge or Sn). Those that do not exist as transition metal-free compounds but have been observed as ligands in transition metal complexes are cyclometallated and ring-opened amidinatotetrylene ligands. This article presents schematic descriptions of their structures, the approaches used for their syntheses and a quick overview of their involvement (as ligands) in transition metal-catalysed reactions. The literature is covered up to the end of 2023.

Intramolecularly O,N,O-Coordinated Tin(II) Salts: Syntheses, Structures, Cyclization, and Transition Metal Complexation

We report the syntheses of tin(II) salts of the types [L1SnX]SnX3 [L1=2,6-{(i-PrO)2(O)P}2C5H3N: 1, X=Cl; 2, X=Br], [L2SnCl]SnCl3 [L2=2-{(i-PrO)Ph(O)P}-6-{(i-PrO)2(O)P}C5H3N: 3], [L3SnX]SnX3 [L3=2,6-{MeO(O)C}2C5H3N: 4, X=Cl; 5, X=Br], [L4SnX]SnX3 [L4=2,6-{Et2N(O)C}2C5H3N: 6, X=Cl; 7, X=Br]. These compounds were obtained by addition of SnX2 to the corresponding ligand inducing autoionization of the respective tin(II) halide. The thermal stability of 1, 3, and 4 was elucidated, giving, under ester cleavage and cyclisation, the tin(II) derivatives 8-12. The reaction of [L1SnCl]SnCl3 (1) with W(CO)4(thf)2 afforded the tungsten tetracarbonyl complex [{L1SnCl}{SnCl3}W(CO)4] (13), representing the first example in which a tin(II) stannate anion and a tin(II) stannylium cation simultaneously coordinate to a transition metal centre. The compounds were characterized by single crystal X-ray diffraction analyses and in part by elemental analyses, IR and NMR spectroscopy, electrospray ionization mass spectrometry. DFT calculations accompany the experimental work.

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.

Two-fold addition reaction of silylene to C60: structural and electronic properties of a bis-adduct

The addition reaction of C60 with silylene 1, a silicon analog of carbene, yielded the corresponding bis-adduct 3. The structure of 3 was determined by single-crystal X-ray structure analysis, representing the first example of a crystal structure of a silirane (silacyclopropane) derivative of fullerenes. Electrochemical measurements confirmed that the redox potentials of 3 are shifted cathodically compared to those of the parent mono-adduct 2. Density functional theory (DFT) calculations provided the basis for the electronic properties of compound 3.

Theoretical Investigation on Dialumenes toward Dihydrogen Activation: Mechanism and Ligand Effect

Herein, we report a computation study based on the density functional theory calculations to understand the mechanism and ligand effect of the base-stabilized dialumenes toward dihydrogen activation. Among all of the examined modes of dihydrogen activation using the base-stabilized dialumene, we found that the concerted 1,2-hydrogenation of the Al═Al double bond is kinetically more preferable. The concerted 1,2-hydrogenation of the Al═Al double bond adopts an electron-transfer model with certain asynchrony. That is, the initial electron donation from the H-H σ bonding orbital to the empty 3p orbital of the Al1 center is followed by the backdonation from the lone pair electron of the Al2 center to the H-H σ antibonding orbital. Combined with the energy decomposition analysis on the transition states of the concerted 1,2-hydrogenation of the Al═Al double bond and the topographic steric mapping analysis on the free dialumenes, we ascribe the higher reactivity of the aryl-substituted dialumene over the silyl-substituted analogue in dihydrogen activation to the stronger electron-withdrawing effect of the aryl group, which not only increases the flexibility of the Al═Al double bond but also enhances the Lewis acidity of the Al═Al core. Consequently, the aryl-substituted dialumene fragment suffers less geometric deformation, and the orbital interactions between the dialumene and dihydrogen moieties are more attractive during the 1,2-hydrogenation process. Moreover, our calculations also predict that the Al═Al double bond has a good tolerance with the stronger electron-withdrawing group (-CF3) and the weaker σ-donating N-heterocyclic carbene (NHC) analogue (e.g., triazol carbene and NHSi). The reactivity of the dialumene in dihydrogen activation can be further improved by introducing these groups as the supporting ligand and the stabilizing base on the Al═Al core, respectively.

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.

Access to a peri-Annulated Aluminium Compound via C-H Bond Activation by a Cyclic Bis-Aluminylene

Carbocyclic aluminium halides [(ADC)AlX2]2 (2-X) (X=F, Cl, and I) based on an anionic dicarbene (ADC=PhC{N(Dipp)C}2, Dipp = 2,6-iPr2C6H3) framework are prepared as crystalline solids by dehydrohalogenations of the alane [(ADC)AlH2]2 (1). KC8 reduction of 2-I affords the peri-annulated Al(III) compound [(ADCH)AlH]2 (4) (ADCH=PhC{N(Dipp)C2(DippH)N}, DippH=2-iPr,6-(Me2C)C6H3)) as a colorless crystalline solid in 76 % yield. The formation of 4 suggests intramolecular insertion of the putative bis-aluminylene species [(ADC)Al]2 (3) into the methine C-H bond of HCMe2 group. Calculations predict singlet ground state for 3, while the conversion of 3 into 4 is thermodynamically favored by 61 kcal/mol. Compounds 2-F, 2-Cl, 2-I, and 4 have been characterized by NMR spectroscopy and their solid-state molecular structures have been established by single crystal X-ray diffraction.

An Air-Stable Storage Compound for White Phosphorus: Reversible Addition to a Stannylene and Chemical Release of P4

Controlled insertion into a single P-P bond of white phosphorus (P4) was achieved by employing a diaryl stabilized stannylene, Ar*2Sn (Ar*=2,6-bis(benzhydryl)-4-iPr-phenyl). Conversions of the stannylene with P4 gave a non-pyrophoric, air-stable storage compound, which releases P4 quantitively upon irradiation with light (354 or 455 nm). Alternatively, the phosphorus cage is detached by reacting the storage compound with PhChChPh (Ch=Se, Te). Despite the recent advances in the directed conversion of P4 using main group element compounds, Ar*2Sn constitutes only the second structurally characterized example of a stannylene capable of performing controlled, reversible addition and release of white phosphorus.

Unusual nucleophilic reactivity of a dithiolene-based N-heterocyclic silane

While the dithiolene-based N-heterocyclic silane (4) reacts with two equivalents of BX3 (X = Br, I) to give zwitterionic Lewis adducts 5 and 8, respectively, the parallel reaction of 4 with BCl3 results in 10, a dithiolene-substituted N-heterocyclic silane, via the Si-S bond cleavage. Unlike 5, the labile 8 may be readily converted to 9via BI3-mediated cleavage of the Si-N bond. The formation of 5 and 8 confirms that 4 uniquely possesses dual nucleophilic sites: (a) the terminal sulphur atom of the dithiolene moiety; and (b) the backbone carbon of the N-heterocyclic silane unit.

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).

Advances in disilylation reactions to access cis/trans-1,2-disilylated and gem-disilylated alkenes

Organosilane compounds are widely used in both organic synthesis and materials science. Particularly, 1,2-disilylated and gem-disilylated alkenes, characterized by a carbon-carbon double bond and multiple silyl groups, exhibit significant potential for subsequently diverse transformations. The versatility of these compounds renders them highly promising for applications in materials, enabling them to be valuable and versatile building blocks in organic synthesis. This review provides a comprehensive summary of methods for the preparation of cis/trans-1,2-disilylated and gem-disilylated alkenes. Despite notable advancements in this field, certain limitations persist, including challenges related to regioselectivity in the incorporation and chemoselectivity in the transformation of two nearly identical silyl groups. The primary objective of this review is to outline synthetic methodologies for the generation of these alkenes through disilylation reactions, employing silicon reagents, specifically disilanes, hydrosilanes, and silylborane reagents. The review places particular emphasis on investigating the practical applications of the C-Si bond of disilylalkenes and delves into an in-depth discussion of reaction mechanisms, particularly those reactions involving the activation of Si-Si, Si-H, and Si-B bonds, as well as the C-Si bond formation.

Syntheses of Iminophosphomamido Chlorogermylenes and Their Complexation with a Rhodium(I) Complex

Three-coordinated chlorogermylenes of the type [Ph2 P(RN)2 ]GeCl (3: R=t Bu; 4: R=Dip=2,6-i Pr2 C6 H3 ), which bear an N,N-substituted iminophosphonamide ligand, were synthesized. The coordination behavior of 3 and 4 toward rhodium(I) complex was investigated. When 3 was treated with 1/2 of an equivalent of [RhCl(cod)]2 (cod=1,5-cyclooctadiene), the corresponding chlorogermylene-Rh(I) complex 5 was obtained as orange crystals. In contrast, the reaction of 4 with a half equivalent of [RhCl(cod)]2 under a CO atmosphere resulted in the formation of a five-membered germarhodacycle 7. Compounds 3, 4, 5, and 7 were characterized using NMR spectroscopies and single-crystal X-ray diffraction. Complex 5 can be employed as a catalyst for the hydrosilylation and hydroboration reactions of diphenylacetylene, thus demonstrating the utility of germylene ligands comparable to those of NHCs in the major transition metal catalytic processes.

Amidinatotetrylenes Donor Functionalized on Both N Atoms: Structures and Coordination Chemistry

E(hmds)(bqfam) (E = Ge (1a), Sn (1b); hmds = N(SiMe3)2, bqfam = N,N'-bis(quinol-8-yl)formamidinate), which are amidinatotetrylenes equipped with quinol-8-yl fragments on the amidinate N atoms, have been synthesized from the formamidine Hbqfam and Ge(hmds)2 or SnCl(hmds). Both 1a and 1b are fluxional in solution at room temperature, as the E atom oscillates from being attached to the two amidinate N atoms to being chelated by an amidinate N atom and its closest quinolyl N atom (both situations are similarly stable according to density functional theory calculations). The hmds group of 1a and 1b is still reactive and the deprotonation of another equivalent of Hbqfam can be achieved, allowing the formation of the homoleptic derivatives E(bqfam)2 (E = Ge, Sn). The reactions of 1a and 1b with [AuCl(tht)] (tht = tetrahydrothiophene), [PdCl2(MeCN)2], [PtCl2(cod)] (cod = cycloocta-1,5-diene), [Ru3(CO)12] and [Co2(CO)8] have been investigated. The gold(I) complexes [AuCl{κE-E(hmds)(bqfam)}] (E = Ge, Sn) have a monodentate κE-tetrylene ligand and display fluxional behavior in solution the same as that of 1a and 1b. However, the palladium(II) and platinum(II) complexes [MCl{κ3E,N,N'-ECl(hmds)(bqfam)}] (M = Pd, Pt; E = Ge, Sn) contain a κ3E,N,N'-chloridotetryl ligand that arises from the insertion of the tetrylene E atom into an M-Cl bond and the coordination of an amidinate N atom and its closest quinolyl N atom to the metal center. Finally, the binuclear ruthenium(0) and cobalt(0) complexes [Ru2{μE-κ3E,N,N'-E(hmds)(bqfam)}(CO)6] and [Co2{μE-κ3E,N,N'-E(hmds)(bqfam)}(μ-CO)(CO)4] (E = Ge, Sn) have a related κ3E,N,N'-tetrylene ligand that bridges two metal atoms through the E atom. For the κ3E,N,N'-metal complexes, the quinolyl fragment not attached to the metal is pendant in all the germanium compounds but, for the tin derivatives, is attached to (in the Pd and Pt complexes) or may interact with (in the Ru2 and Co2 complexes) the tin atom.

Facile Access to Cationic Methylstannylenes and Silylenes Stabilized by E-Pt Bonding and their Methyl Group Transfer Reactivity

We describe a family of cationic methylstannylene and chloro- and azidosilylene organoplatinum(II) complexes supported by a neutral, binucleating ligand. Methylstannylenes MeSn:+ are stabilized by coordination to PtII and are formed by facile Me group transfer from dimethyl or monomethyl PtII complexes, in the latter case triggered by concomitant B-H, Si-H, and H2 bond activation that involves hydride transfer from Sn to Pt. A cationic chlorosilylene complex was obtained by formal HCl elimination and Cl- removal from HSiCl3 under ambient conditions. The computational studies show that stabilization of cationic methylstannylenes and cationic silylenes is achieved through weak coordination to a neutral N-donor ligand binding pocket. The analysis of the electronic potentials, as well as the Laplacian of electron density, also reveals the differences in the character of Pt-Si vs. Pt-Sn bonding. We demonstrate the importance of a ligand-supported binuclear Pt/tetrel core and weak coordination to facilitate access to tetrylium-ylidene Pt complexes, and a transmetalation approach to the synthesis of MeSnII :+ derivatives.

Synthesis and Reactivity of Fluorenyl-Based Germanium(II) Compounds

We present the synthesis of alkyl-substituted germylenes GeFlu2, Ge(FluTMS)2, and FluTMSGeCl (Flu = 9H-fluorenyl, FluTMS = 9-trimethylsilyl-9H-fluorenyl) using bulky fluorenyl ring systems and modifications of that. GeFlu2 can only be crystallized as its three-membered ring trimer, whereby the reaction is accompanied by the formation of several byproducts, such as [Li(THF)4][Ge(Ge3Flu7H)]. These results led to the modification of the fluorenyl framework by substitution the one H atom in the 9-position by a TMS group. With the synthesis of the corresponding Li salt LiFluTMS, Ge(FluTMS)2 could be isolated in good yields in a further reaction. The homoleptic Ge(FluTMS)2 is found in its crystalline form as a monomer and thus belongs to the series of monomeric alkyl-substituted germylenes. Also, the corresponding monoalkyl-substituted halogenido germylene was isolated as a four-membered ring tetramer [FluTMSGeCl]4 during an unselective reaction. However, FluTMSGeCl undergoes significant stabilization through the formation of the monomeric phosphane adduct FluTMSGeCl·PEt3, which greatly increases the selectivity of the reaction. During further reactions of Ge(FluTMS)2 with a GeBr solution (toluol/nPr3P), more impressions of the reactivity of Ge(I)X solutions with germylenes were achieved, showing that those germylenes take part in the disproportionation reaction of metastable Ge(I) solutions to give oxidized Ge(IV) compounds like (FluTMS)2GeBr2.

Boryl-substituted low-valent heavy group 14 compounds

Low valent group 14 compounds exhibit diverse structures and reactivities. The employment of diazaborolyl anions (NHB anions), isoelectronic analogues to N-heterocyclic carbenes (NHCs), in group 14 chemistry leads to the exceptional structures and reactivity. The unique combination of σ-electron donation and pronounced steric hindrance impart distinct structural characteristics to the NHB-substituted low valent group 14 compounds. Notably, the modulation of the HOMO-LUMO gap in these compounds with the diazaborolyl substituents results in novel reaction patterns in the activation of small molecules and inert chemical bonds. This review mainly summarizes the recent advances in NHB-substituted low-valent heavy Group 14 compounds, emphasizing their synthesis, structural characteristics and application to small molecule activation.

Germanium(II) Dithiolene Complexes

The 1 : 2 reaction of the imidazole-based dithiolate (2) with GeCl2  • dioxane in THF/TMEDA gives 3, a TMEDA-complexed dithiolene-based germylene. Compound 3 is converted to monothiolate-complexed (5) and N-heterocyclic carbene-complexed (7) germanium(II) dithiolene complexes via Lewis base ligand exchange. A bis-dithiolene-based germylene (8), involving a 3c-4e S-Ge-S bond, has also been synthesized through controlled hydrolysis of 7. The bonding nature of 3, 5, and 8 was investigated by both experimental and theoretical methods.

A planar per-borylated digermene

A tetraboryl digermene synthesized by the reaction between a dianionic digermanide nucleophile and a boron halide electrophile is dimeric both in the solid state and in hydrocarbon solution. It features both a planar 'alkene-like' geometry for the Ge2B4 core, and an exceptionally short GeGe double bond. These structural features are consistent with the known electronic properties of the boryl group, and with lowest energy (in silico) fragmentation into two triplet bis(boryl)germylene fragments.

Fast and scalable solvent-free access to Lappert's heavier tetrylenes E{N(SiMe3)2}2 (E = Ge, Sn, Pb) and ECl{N(SiMe3)2} (E = Ge, Sn)

Iconic Lappert's heavier tetrylenes E{N(SiMe3)2}2 (E = Ge (1), Sn (2), Pb (3)) have been efficiently prepared from GeCl2·(1,4-dioxane), SnCl2 or PbCl2 and Li{N(SiMe3)2} via a completely solvent-free one-pot mechanochemical route followed by sublimation. This fast, high-yielding and scalable approach (2 has been prepared in a 100 mmol scale), which involves a small environmental footprint, represents a remarkable improvement over any synthetic route reported over the last five decades, being a so far rare example of the use of mechanochemistry in the realm of main group chemistry. This solventless route has been successfully extended to the preparation of other heavier tetrylenes, such as ECl{N(SiMe3)2} (E = Ge (4), Sn (5)).

A Guanidine-Supported π-Complex of Germanium Amenable to Intramolecular C-C Cleavage in Arene and Ge Atom Transfer

The germylone dimNHCGe (dimNHC=diimino N-heterocyclic carbene) reacts with azides N3 R (R=SiMe3 or p-tolyl) to furnish the first examples of germanium π-complexes, i. e. guanidine-ligated compounds (dimNHI-SiMe3 )Ge (NHI=N-heterocyclic imine, R=SiMe3 ) and (dimNHI-Tol)Ge (R=p-tolyl). DFT calculations suggest that these species are formed by a Staudinger type replacement of dinitrogen in the azide by a nucleophilic germylone, leading to a transient carbene adduct of iminogermylidene. Heating a solution of compound (dimNHI-SiMe3 )Ge to 70 °C results in extrusion of the iminogermylidene that further aggregates to produce the known [Me3 SiNGe]4 tetramer, whereas the imidazolylidene fragment transforms into an unusual heptatriene species that can be considered as a product of carbene insertion into the C-C bond of a pendant Ar substituent at the imidazolylidene nitrogen of the dimNHC. Reaction of (dimNHI-SiMe3 )Ge with tetrachloro-o-benzoquinone results in the net transfer of a germanium atom and formation of the free diimino-guanidine ligand. This ligand also forms when (dimNHI-SiMe3 )Ge is treated with azide N3 (p-Tol), with the germanium product being [(p-Tol)NGe]n.

Diverse Reactions of Formazanate/Formazan with Tetrylenes: Reduction, C-H Bond Activation, Substitution and Addition

The reactivity of the formazanate potassium salt [LtBu K(thf)] (LtBu= PhNNC(4-t BuPh)NNPh) with the group 14 chlorotetrylenes [{PhC(t BuN)2 }ECl] (E=Si, Ge, Sn) was investigated. Three corresponding compounds with unique configurations were formed, demonstrating the diverse reactivity of the system. In addition to the anticipated salt metathesis reactions of the potassium salt with the chlorine function of tetrylenes, unexpected reduction/insertion steps into the N=N bond of the formazanate (Si, Ge) and subsequent C-H activation (Ge) were also observed. Furthermore, when the neutral formazan ligand [LtBu H] was exposed to silylenes [{PhC(t BuN)2 }SiCl] and [LPh SiNMePy], substitution and addition reactions occurred. These discoveries significantly enrich the diversity of formazanate/formazan redox chemistry, opening up new avenues for exploration in this field.

Sn(II)-carbon bond reactivity: radical generation and consumption via reactions of a stannylene with alkynes

Thermal Sn-C cleavage in the diarylstannylene Sn(AriPr4)2 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) was used to generate ˙Sn(AriPr4) and ˙AriPr4 radicals for alkyne arylstannylation. The radical pair and RCCR' (R = H, R' = Ph; R = Ph, R' = Ph; R = H, R' = C4H9; R = H, R' = SiMe3) in refluxing benzene generate the aryl vinyl stannylene complexes, AriPr4Sn{C(C6H5)-C(H)(AriPr4)} (1), AriPr4Sn{C(C6H5)-C(H)(C6H5)} (2) and AriPr4Sn{C(C4H9)-C(H)(AriPr4)} (3) respectively. For HCCSiMe3, the known distannene {Sn(CCSiMe3)AriPr4}2 (4) was also generated from this new method.

Reactivity of Cyclic (Alkyl)(amino)germylene towards Copper(I) and Gold(I) Complexes

The reactions of cyclic (alkyl)(amino)germylenes (CAAGe) with copper(I) and gold(I) complexes were investigated. CAAGe (1) reacts with CuBr(SMe2 ) leading to a tetrameric germylene complex [CAAGeCuBr]4 (2), whereas CAAGe (3) undergoes Au-Cl bond insertion with LAuCl (L=phosphine or N-heterocyclic carbene) to afford germanium gold(I) complexes (5 and 6). Chlorine abstraction of 6 gives the cationic germylene gold(I) complex 7.

Triplet bismuthinidenes featuring unprecedented giant and positive zero field splittings

Isolation of triplet pnictinidenes, which bear two unpaired electrons at the pnictogen centers, has long been a great challenge due to their intrinsic high reactivity. Herein, we report the syntheses and characterizations of two bismuthinidenes MsFluindtBu-Bi (3) and MsFluind*-Bi (4) stabilized by sterically encumbered hydrindacene ligands. They were facilely prepared through reductions of the corresponding dichloride precursors with 2 molar equivalents of potassium graphite. The structural analyses revealed that 3 and 4 contain a one-coordinate bismuth atom supported by a Bi-C single σ bond. As a consequence, the remaining two Bi 6p orbitals are nearly degenerate, and 3 and 4 possess triplet ground states. Experimental characterizations with multinuclear magnetic resonance, magnetometry and near infrared spectroscopy coupled to wavefunction based ab initio calculations concurred to evidence that there exist giant and positive zero field splittings (>4300 cm-1) in their S = 1 ground states. Hence even at room temperature the systems almost exclusively populate the lowest-energy nonmagnetic Ms = 0 level, which renders them seemingly diamagnetic.

Synthesis and Some Coordination Chemistry of Phosphane-Difunctionalized Bis(amidinato)-Heavier Tetrylenes: A Previously Unknown Class of PEP Tetrylenes (E = Ge and Sn)

The bis(amidinato)-heavier tetrylenes E(bzamP)2 (E = Ge (2a) and Sn (2b); bzamP = N-isopropyl-N'-(diphenylphosphanylethyl)benzamidinate), which are equipped with one heavier tetrylene (germylene or stannylene) and two phosphane fragments (one on each amidinate moiety) as coordinable groups, have been synthesized from the benzamidinum salt [H2bzamP]Cl and GeCl2(dioxane) or SnCl2 in 2:1 mol ratio. A preliminary inspection of their coordination chemistry has shown that their amidinate group can also be involved in the bonding with the metal atoms as tridentate ENP and tetradentate PENP' coordination modes have been observed for the ECl(bzamP)2 ligand of [Ir{κ3E,N,P-ECl(bzamP)2}(cod)] (E = Ge (3a) and Sn (3b); cod = η4-1,5-cyclooctadiene) and the E(bzamP)2 ligand of [Ni{κ4E,N,P,P'-E(bzamP)2}] (E = Ge (4a) and Sn (4b)), which are products of reactions of 2a and 2b with [IrCl(cod)]2 (1:0.5 mol ratio) and [Ni(cod)2] (1:1 mol ratio), respectively. These products contain a 5-membered NCNEM ring that results from the insertion of the metal M atom into an E-N bond of 2a and 2b. Additionally, while iridium(I) complexes 3a and 3b are chloridotetryl derivatives (insertion of the tetrylene E atom into the Ir-Cl bond has also occurred) that have an uncoordinated phosphane group, nickel(0) complexes 4a and 4b contain a tetrylene fragment that, maintaining the lone pair, behaves as a σ-acceptor (Z-type) ligand.

From Sn(II) to Sn(IV): Enhancing Lewis Acidity Via Oxidation

We demonstrate the increased Lewis acidity on going from Sn(II) to Sn(IV) by oxidizing TpMe2SnOTf (OTf = SO3CF3) to TpMe2SnF(OTf)2. Replacement of the fluoride ion in TpMe2SnF(OTf)2 by a triflate, resulting in TpMe2Sn(OTf)3 further enhances the Lewis acidity at tin. 119Sn NMR spectroscopy, modified Gutmann-Beckett test, computational analysis, and catalytic phosphine oxide deoxygenation support the claims.

Aza-Dipyrrinate Stabilized Compounds with a Low-Valent Main Group Element

The possibility of using aza-dipyrromethene (a-DPM) ligands to stabilize compounds containing low-valent main group elements is demonstrated through the isolation of germylenes, a-DPM(p-tol)GeCl (2), a-DPM(Naph)GeCl (6), and a-DPM(Naph)GeN(TMS)2 (7) (tol=tolyl, Naph=naphthyl). Because of the presence of the a-DPM ligand, these germylenes exhibit an absorption maximum at around 640 nm, a highly red-shifted value previously unknown for germylenes.

Reactivity of a Strictly T-Shaped Phosphine Ligated by an Acridane Derived NNN Pincer Ligand

The steric tuning of a tridentate acridane-derived NNN pincer ligand allows for the isolation of a strictly T-shaped phosphine that exhibits ambiphilic reactivity. Well-defined phosphorus-centered reactivity towards nucleophiles and electrophiles is reported, contrasting with prior reports on this class of compounds. Reactions towards oxidants are also described. The latter result in the two-electron oxidation of the phosphorus atom from +III to +V and are accompanied by a strong geometric distortion of the NNN pincer ligand. By contrast, cooperative activation of E-H (HCl, HBcat, HOMe) bonds proceeds with retention of the phosphorus redox state. When using H2 O as a substrate, the reaction results in the full disassembly of H2 O to its constituent atoms, highlighting the potential of this platform for small molecule activation reactions.

Germanium Complexes with ONO Tridentate Ligands: O-H Bond Activation Control According to DFT Calculations

Polydentate ligands are used for thermodynamic stabilization of tetrylenes-low-valent derivatives of Group 14 elements (E = Si, Ge, Sn, Pb). This work shows by DFT calculations how the structure (the presence or absence of substituents) and type (alcoholic, Alk, or phenolic, Ar) of tridentate ligands 2,6-pyridinobis(1,2-ethanols) [^AlkONO^R]H₂ and 2,6-pyridinobis(1,2-phenols) [^ArONO^R]H₂ (R = H, Me) may affect the reactivity or stabilization of tetrylene, indicating the unprecedented behavior of Main Group elements. This enables the unique control of the type of the occurring reaction. We found that unhindered [ONO^H]H₂ ligands predominantly led to hypercoordinated bis-liganded {[ONO^H]}₂Ge complexes, where an E(+2) intermediate was inserted into the ArO-H bond with subsequent H₂ evolution. In contrast, substituted [ONO^Me]H₂ ligands gave [ONO^Me]Ge: germylenes, which may be regarded as kinetic stabilized products; their transformation into E(+4) species is also thermodynamically favorable. The latter reaction is more probable for phenolic [^ArONO]H₂ ligands than for alcoholic [^AlkONO]H₂. The thermodynamics and possible intermediates of the reactions were also investigated.

Open-Shell Germylene Stabilized by a Phenalenyl-Based Ligand

An open-shell germylene 1 stabilized by a phenalenyl-based bidentate ligand was synthesized and characterized. Because of the high thermal stability originating from spin delocalization over the phenalenyl moiety, it was possible to isolate compound 1 in crystalline form by sublimation at ca. 300 °C. Electron spin resonance (ESR) spectra, crystallographic analysis, theoretical calculations, and reactivities with carbon radicals suggest that the spin of 1 is distributed on the phenalenyl moiety, while 1 reacted with C₂Cl₆, PhSSPh, and p-benzoquinone at the germanium center to form Ge-E (E = Cl, S, O) bonds. Furthermore, compound 1 is featured by its reactivity as a "formal germylyne", which allows for the formation of three new σ-bonds or one σ-bond with metal complexation on the germanium center.

Synthesis and structures of homoleptic germylenes and stannylenes with sulfonimidamide ligands

An unusual series of germylenes and stannylenes with homoleptic symmetric and unsymmetric N-substituted sulfonimidamide ligands PhSO(NiPr)(NHiPr) 1 and PhSO(NMes)(NHiPr) 2 have been prepared by protonolysis reaction of Lappert's metallylenes [M(HMDS)₂] (M = Ge or Sn) with two equivalents of the appropriate sulfonimidamide. The homoleptic germylenes [PhSO(NiPr)₂]₂Ge 3 and [PhSO(NMes)(NiPr)]₂Ge 4, and stannylenes [PhSO(NiPr)₂]₂Sn 5 and [PhSO(NMes)(NiPr)]₂Sn 6 were fully characterized by NMR spectroscopy and by X-ray diffraction analysis. DFT calculations have been performed to understand the electronic properties brought by the sulfonimidamide ligand.

Cyclobutenylidene: A Multifaceted Two-Coordinate Carbon Species Obtained via Skeletal Editing of a Cyclopropenylidene

C₄H₄ isomers not only serve as a basis to understand the chemical properties of hydrocarbons but are possible intermediates in combustion and organic reactions in outer space. Cyclobutenylidene (CBY), an elusive C₄H₄ isomer, is often proposed as a key intermediate in transition-metal-catalyzed metathesis and cycloaddition reactions between carbon-carbon multiple bonds. The geometrical structure of cyclobutenylidene predicted by calculations had been debated as whether it should be regarded as a carbocyclic carbene or a strained bridgehead alkene. Here, we report the synthesis of a crystalline cyclobutenylidene derivative, namely, a 3-silacyclobut-2-en-4-ylidene (SiCBY) via "carbene-to-carbene ring-expansion" reaction of an isolable diaminocyclopropenylidene induced by a silicon analogue of a carbene (silylene). The SiCBY exhibits multifaceted electronic properties which are corroborated by its extremely strong electron-donating properties and ambiphilic reactivity toward small gaseous molecules and C-H bonds. This result introduces an exciting strategy as well as a molecular motif to access low-valent carbon species with unusual electronic properties.

Halogen bonds with carbenes acting as Lewis base units: complexes of imidazol-2-ylidene: theoretical analysis and experimental evidence

ωB97XD/aug-cc-pVDZ and ωB97XD/aug-cc-pVTZ calculations were performed on complexes of imidazol-2-ylidene that are linked by halogen bonds. This singlet carbene acts as the Lewis base through a lone electron pair located at the carbon centre. The XCCH, XCN and X₂ units were chosen here as those that interact through the X Lewis acid halogen centre (X = Cl, Br and I); if X = F the complexes are linked by interactions which are not classified as halogen bonds. The properties of interactions that occur in complexes are analyzed using the results of DFT calculations which are supported by parameters derived from the Quantum Theory of Atoms in Molecules, QTAIM, and the Natural Bond Orbital, NBO, approaches. The energy decomposition analysis, EDA, applied here provided additional characteristics of interactions linking complexes analyzed. The majority of complexes are linked by the medium in strength and strong halogen bonds which often possess characteristics typical for covalent bonds. Searches through the Cambridge Structural Database were also performed and structures analogues to complexes analyzed theoretically were found, and these structures are also discussed in this study.

Synthesis and Reactivity of an Anti-van't Hoff/Le Bel Compound with a Planar Tetracoordinate Silicon(II) Atom

For a long time, planar tetracoordinate carbon (ptC) represented an exotic coordination mode in organic and organometallic chemistry, but it is now a useful synthetic building block. In contrast, realization of planar tetracoordinate silicon (ptSi), a heavier analogue of ptC, is still challenging. Herein we report the successful synthesis and unusual reactivity of the first ptSi species of divalent silicon present in 3, supported by the chelating bis(N-heterocyclic silylene)bipyridine ligand, 2,2'-{[(4-^tBuPh)C(N^tBu)]₂SiNMe}₂(C₅N)₂, 1]. The compound resulted from direct reaction of 1 with Idipp-SiI₂ [Idipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Alternatively, it can also be synthesized by a two-electron reduction of the corresponding Si(IV) precursor 2 with 2 molar equiv of KC₁₀H₈. Density functional theory calculations show that the lone pair at the ptSi(II) resides almost completely in its 3p_z orbital, very different from known four-coordinate silylenes. Oxidative addition of MeI to the ptSi(II) atom affords the corresponding pentacoordinate Si(IV) compound 4, with the methyl group located in an apical position. Remarkably, the reaction of 2 with [CuO^tBu] leads to the regeneration of the bis(silylene) arms via Si-Si bond scission and induces the Si(II) → Si(IV) oxidation of the central Si(II) atom and concomitant two-electron reduction of the bipyridine moiety to form the neutral bis(silylene)silyl Cu(I) complex 5.

Gradual Donor Stabilization of a Transient Ferrocene Bridged Bisphosphanyl Phosphenium Cation

A transient phosphenium cation embedded into a [3]ferrocenophane scaffold was formed via chloride abstraction. The cation has been trapped with phosphane, carbene, and silylene donors resulting in stable adducts or bond activation of the ferrocenophane bridge. In the absence of donors, dimerization of the phosphenium cation to the corresponding dication is observed or P-C bond activation with migration of a substituent leading to a putative phosphoniodiphosphene. Using 1,3-di-tert-butylimidazol-2-silylene as the donor, further reaction of the initially formed chlorosilane leads to activation of a P-P bond of the ferrocenophane scaffold with ring expansion of the ansa-bridge. The donor formation and bonding situation are investigated by density functional theory calculations as well as experimental methods (e.g., NMR spectroscopy and X-ray crystallography).

Excited state energy landscape of phosphorescent group 14 complexes

Great progress has been achieved on phosphorescent or photoactive complexes of the Earth-abundant transition metals, while examples for phosphorescent heavy main group element complexes are rare, in particular for group 14 complexes in the oxidation state +II. The known compounds often show only weak phosphorescence with fast non-radiative deactivation. The underlying photophysical processes and the nature of the phosphorescent electronic states have remained essentially unexplored. The present combined photophysical and theoretical study on tin(ii) and lead(ii) complexes E(bpep) with the dianionic tridentate ligand bpep^2- (E = Sn, Pb; H₂bpep = 2-[1,1-bis(1H-pyrrol-2-yl)ethyl]pyridine) provides unprecedented insight in the excited state energy landscape of tetrel(ii) complexes. The tin complex shows green intraligand charge transfer (ILCT) phosphorescence both in solution and in the solid state. In spite of its larger heavy-atom effect, the lead complex only shows very weak red phosphorescence from a strongly distorted ligand-to-metal charge transfer (LMCT) state at low temperatures in the solid state. Detailed (TD-)DFT calculations explain these observations and delineate the major path of non-radiative deactivation via distorted LMCT states. These novel insights provide rational design principles for tetrel(ii) complexes with long-lived phosphorescence.

Kinetically Stabilized Diarylpnictogenium Ions

The newly prepared and fully characterized stibenium and bismuthenium ions [R_ind MesE]⁺ (E=Sb, Bi; R_ind =dispiro[fluorene-9,3'-(1',1',7',7'-tetramethyl-s-hydrindacen-4'-yl)-5',9''-fluorene) were rigorously compared to the previously communicated phosphenium and arsenium ions (E=P, As) as well as the bis(m-terphenyl) pnictogenium ions [(2,6-Mes₂ C₆ H₃ )₂ E]⁺ (E=Sb, Bi). It is demonstrated that the choice of the aryl substituents dramatically effects the molecular structures (e. g. the primary E-C bond lengths) and the electronic structures (e. g. the energy of the LUMOs).

Utilization of a Tris(carbene)borate Ligand for Umpolung Reactivity of a Nucleophilic Tin(II) Cation Salt

We show that a tris(carbene)borate (TCB) ligand, namely [PhB(^tBuIm)₃]^- ([PhB(^tBuIm)₃]^- = phenyltris(3-tert-butylimidazol-2-ylidene)borato), is capable of stabilizing an unprecedented nucleophilic Sn(II) cation salt. Unlike known Sn(II) cations, the strong electron-donating ability of [PhB(^tBuIm)₃]^- makes the cationic tin atom electron-rich, σ-donating yet slightly π-accepting, which allows for the ensuing facile oxidation with o-chloranil and S₈ as well as coordination with coinage metals. The former oxidations give the Sn(IV) cation salts, while the latter reactions produce the metal complexes. The electronic structures of these species are thoroughly probed by quantum chemical computations. These results uncover an added role for TCB ligands in isolating unprecedented p-block species.

Time-resolved, Gas-phase Kinetic and Quantum Chemical Studies of the Reaction of Germylene with Hydrogen Chloride

Time-resolved studies of germylene, GeH₂ , generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene at 193 nm and monitored by laser absorption, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction was studied in the gas phase, mainly at a total pressure of 10 Torr (in SF₆ bath gas) at five temperatures in the range 295-558 K. Experiments at other pressures showed that these rate constants were unaffected by pressure. The second-order rate constants at 10 Torr (SF₆ bath gas) fitted the Arrhenius equation: log(k/cm³  molecule^-1  s^-1 )=(-12.06±0.14)+(2.58±1.03 kJ mol^-1 )/RTln10 where the uncertainties are single standard deviations. Quantum chemical calculations at G4 level support a mechanism in which an initial weakly bound donor-acceptor complex is formed. This can then rearrange and decompose to give H₂ and HGeCl (chlorogermylene). The enthalpy barrier (36 kJ mol^-1 ) is too high to allow rearrangement of the complex to GeH₃ Cl (chlorogermane).

Diverse Reactions of o-Carborane-Fused Silylenes with C≡E (E = C, P) Triple Bonds

The reactivities of o-carborane-fused silylenes toward molecules with C≡E (E = C, P) bonds are reported. The reactions of bis(silylene) [(LSi:)C]₂B₁₀H₁₀ (1a, L = PhC(NtBu)₂) with arylalkynes afforded bis(silylium) carborane adducts 2 and 3, showing a Si(μ-C₂)Si structure with an open-cage nido-carborane backbone. In contrast, the reaction of 1a with a phosphaalkyne AdC≡P (Ad = 1-adamantyl) smoothly furnished compound 4, comprising fused CPSi rings with a C=Si double bond and Si-Si single bond, and the related formation mechanism was investigated by DFT calculations. Furthermore, when monosilylene [(LSi:)C]CHB₁₀H₁₀ (1b) was employed to react with AdC≡P, compound 5 was isolated. The structure of 5 features a 1,2,3-triphosphetene core. All products were characterized by NMR spectroscopy and/or X-ray crystallography.

Recent progress in the chemistry of heavy aromatics

The aromaticity and synthetic application of "heavy benzenes", i.e., benzenes containing a heavier Group 14 element (Si, Ge, Sn, and Pb) in place of skeletal carbon, have been the targets of many theoretical and synthetic studies. Although the introduction of a sterically demanding substituent enabled us to synthesize and isolate heavy aromatic species as a stable compound by suppressing their high reactivity and tendency to polymerize, the existence of a protection group is an obstruction to the development of functional materials based on heavy aromatics. This review will delineate the most recent topics in the chemistry of heavy aromatics, i.e., the chemistry of "metallabenzenyl anions", which are the heavier Group 14 element analogs of phenyl anions stabilized by taking advantage of charge repulsion instead of steric protection.