Ein cyclisches Silylon (“Siladicarben”) mit einem elektronenreichen Silicium(0)-Atom

An Isolable Bis(Germylene)-Stabilized Plumbylone

We report herein the synthesis of a stable plumbylone (3) by reduction of a bromodigermylplumbylene (2) with 2.2 equiv of potassium graphite (KC₈ ). The molecular structure of 3 was established by a single-crystal X-ray diffraction study and features a two-coordinated Pb center with an acute Ge-Pb-Ge bond angle. Computational studies showed that this complex (3) possesses a singlet electronic ground state with a Pb⁰ center. Its high thermal stability can be most likely ascribed to the delocalization of π electrons over the Ge-Pb-Ge moiety. A preliminary reactivity study demonstrates that complex 3 can deliver Pb⁰ atoms to an organic azide producing a tetrameric imido complex [(PbNDipp)₄ ] (Dipp=2,6-ⁱ Pr-C₆ H₃ , 4) and perform a metathesis reaction with GeCl₂ ⋅dioxane to produce a bis(germylene)-stabilized germylone (5), highlighting the synthetic utility of 3.

The Heaviest Bottleable Metallylone: Synthesis of a Monatomic, Zero-Valent Lead Complex ("Plumbylone")

The elusive plumbylone {[SiII (Xant)SiII ]Pb0 } 3 stabilized by the bis(silylene)xanthene chelating ligand 1, [SiII (Xant)SiII =PhC(NtBu)2 Si(Xant)Si(NtBu)2 CPh], and its isolable carbonyl iron complex {[SiII (Xant)SiII ]Pb0 Fe(CO)4 } 4 are reported. The compounds 3 and 4 were obtained stepwise via reduction of the lead(II) dibromide complex {[SiII (Xant)SiII ]PbBr2 } 2, prepared from the bis(silylene)xanthene 1 and PbBr2 , employing potassium naphthalenide and K2 Fe(CO)4 , respectively. While the genuine plumbylone 3 is rather labile even at -60 °C, its Pb0 →Fe(CO)4 complex 4 turned out to be relatively stable and bottleable. However, solutions of 4 decompose readily to elemental Pb and {[SiII (Xant)SiII ]Fe(CO)3 } 5 at 80 °C. Reaction of 4 with [Rh(CO)2 Cl]2 leads to the formation of the unusual dimeric [(OC)2 RhPb(Cl)Fe(CO)4 ] complex 6 with trimetallic Rh-Pb-Fe bonds. The molecular and electronic structures of 3 and 4 were established by Density Functional Theory (DFT) calculations.

Synthesis and Reactivity of N-Heterocyclic Silylene Stabilized Disilicon(0) Complexes

Synthesis and reactivity of disilicon(0) complexes are of fundamental and application importance. Herein, we report the development of an N-heterocyclic imino-substituted silylene (1), which has strong σ-donating ability and is significantly sterically hindered. The one-pot reaction of this silylene with [IPr→SiCl₂ ] (IPr=1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) and KC₈ (2 equiv) in THF at -30 °C leads to a silylene-ligated disilicon(0) complex (2), isolated as red crystals in 60 % yield. Characterization data and DFT calculations show that the trans-bent Si₄ skeleton in 2 features a Si⁰ =Si⁰ double bond with significant π-π bonding and one lone pair of electrons on each of these two Si⁰ atoms. Complex 2 reacts readily with phenylacetylene, producing a structurally intriguing silatricyclic complex 6,8-diaza-1,2,5-trisilatricyclo-[3.2.1.0^2,7 ]-oct-3-ene (3), and revealing new aspects of low-valent silicon chemistry.

A Genuine Stannylone with a Monoatomic Two-Coordinate Tin(0) Atom Supported by a Bis(silylene) Ligand

The monoatomic zero-valent tin complex (stannylone) {[SiII (Xant)SiII ]Sn0 } 5 stabilized by a bis(silylene)xanthene ligand, [SiII (Xant)SiII =PhC(NtBu)2 Si(Xant)Si(NtBu)2 CPh], and its bis-tetracarbonyliron complex {[SiII (Xant)SiII ]Sn0 [Fe(CO)4 ]2 } 4 are reported. The stannylone 5 bearing a two-coordinate zero-valent tin atom is synthesized by reduction of the precursor 4 with potassium graphite. Compound 4 results from the SnII halide precursor {[SiII (Xant)SiII ]SnII Cl}Cl 2 or {[SiII (Xant)SiII ]SnBr2 } 3 through reductive salt-metathesis reaction with K2 Fe(CO)4 . According to density functional theory (DFT) calculations, the highest occupied molecular orbital (HOMO) and HOMO-1 of 5 correspond to a π-type lone pair with delocalization into both adjacent vacant orbitals of the SiII atoms and a σ-type lone pair at the Sn0 center, respectively, indicating genuine stannylone character.

Recent Developments in the Chemistry of Pn(I) (Pn=N, P, As, Sb, Bi) Cations

The Group 15 Pn(I) cations (Pn=N, P, As, Sb and Bi), which are isoelectronic with the donor-stabilized carbones, have emerged recently. Despite the presence of two lone pair of electrons, the Pn(I) cations are weakly nucleophilic due to their inherent positive charge. Strongly electron-donating supporting ligands including zwitterionic forms have been used to enhance their Lewis basicity. Furthermore, the chelating effect of cyclic ligand systems proved effective in increasing their nucleophilicity. The strategies involved in successfully isolating the fleeting Sb(I) and Bi(I) cations as the recent most achievements in this field have been discussed. The syntheses, structure, bonding situations and reactivity of the Pn(I) cations are discussed. An outlook on the periodic trends and future applications of these electronically unique electron-rich cationic moieties have been provided.

Low-Valent Germanylidene Anions: Efficient Single-Site Nucleophiles for Activation of Small Molecules

Rare mononuclear and helical chain low-valent germanylidene anions supported by cyclic (alkyl)(amino)carbene and hypermetallyl ligands were synthesised by stepwise reduction from corresponding germylene precursors via stable and isolable germanium radicals. The electronic structures of the anions can be described with ylidene and ylidone resonance forms with the Ge-C π-electrons capable of binding even weak electrophiles. The germanylidene anions reacted with CO2 to give μ-CO2 -κC:κO complexes, a rare coordination mode for low-valent germanium and inaccessible for the related neutral germylones. These results implicate low-valent germanylidene anions as efficient single-site nucleophiles for activation of small molecules.

A Zwitterionic Heterobimetallic Gold-Iron Complex Supported by Bis(N-Heterocyclic Imine)Silyliumylidene

The facile synthesis of the first bis-N-heterocyclic imine-stabilized chlorosilyliumylidene 1 is reported. Remarkably, consecutive reaction of 1 with PPh3 AuCl and K2 Fe(CO)4 gives rise to the unique heterobimetallic complex 1,2-(Mes NHI)2 -C2 H4 -ClSiAuFe(CO)4 (4). The overall neutral complex 4 bears an unusual linear Si-Au-Fe structure and a rare anagostic interaction between the d10 -configured gold atom and a CH bond of the mesityl ligand. According to the computational analysis and 57 Fe Mössbauer spectroscopy, the formal Fe-oxidation state remains at -II. Thus, the electronic structure of 4 is best described as an overall neutral-yet zwitterionic-heterobimetallic "Si(II)+ -Au(I)+ -Fe(-II)2- "-silyliumylidene complex, derived from double anion exchange. The computational analysis indicates strong hyperconjugative back donation from the gold(I) atom to the silyliumylidene ligand.

(NHC)Si═C═N-R: A Two-Coordinated Si0-Isocyanide Compound as Si(NHC) Transfer Reagent

Experimental and theoretical studies are reported of the first two-coordinated Si⁰-isocyanide compound (SIDipp)Si═C═N-Ar^Mes (1: SIDipp (NHC) = C[N(Dipp)CH₂]₂, Ar^Mes = 2,6-dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr(CNAr^Mes)(SIDipp)][B(Ar^F)₄] (2[B(Ar^F)₄], Ar^F = B(C₆H₃-3,5-(CF₃)₂)₄) with KC₈. 2[B(Ar^F)₄] was obtained from SiBr₂(SIDipp) after bromide abstraction with an equimolar mixture of Na[B(Ar^F)₄] and Ar^MesNC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B(Ar^F)₄] reacts with SiBr₂(SIDipp) via isocyanide exchange to afford the disilicon(II) salt [Si₂Br₃(SIDipp)₂)][B(Ar^F)₄] (3[B(Ar^F)₄]), the reaction leading to an equilibrium that favors 3[B(Ar^F)₄] (K_eq(298 K) = 10.6, ΔH° = -10.6 kJ mol^-1; ΔS^° = -16.0 J mol^-1 K^-1). 3[B(Ar^F)₄] was obtained selectively from the 2:1 reaction of SiBr₂(SIDipp) with Na[B(Ar^F)₄] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar C_NHC-Si-C-N skeleton with a V-shaped geometry at the dicoordinated Si⁰ center, a slightly bent Si═C═N core, a C_NHC-Si-C_CNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and Ar^Mes substituents leading to axial chirality and the presence of two enantiomers, (R_a)-1 and (S_a)-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)(Cl)Si═GeAr^Mes (4) from 1 and Ge(Ar^Mes)Cl.

Stabilization of the Elusive Antimony(I) Cation and Its Coordination Complexes with Transition Metals

Upon stabilization by 5,6-bis(diisopropylphosphino)acenaphthene to form compound 1, the fugitive antimony (I) cation exhibited nucleophilic behavior towards coinage metals. Compound 1 was strategically synthesized at room temperature from SbCl₃ , the bis(phosphine), and trimethylsilyl trifluoromethanesulfonate taken in a 1:2:3 ratio, whereby the bis(phosphine) plays the dual role of a reductant and a supporting ligand. The generation of 1 involves two-electron oxidation of the ligand to form a P-P bonded diphosphonium dication. Compound 1 was separated from this dication to give both products in pure form in moderate yields. Despite the overall positive charge, the Sb^I site in 1 was found to bind to metal centers, forming complexes with Au^I , Ag^I and Cu^I . Compound 1 reduced Cu^II to Cu^I and formed a coordination complex with the resulting Cu^I species. The effects of the electron-rich bis(phosphine) and the constrained peri geometry in stabilizing and enhancing the nucleophilicity of 1 have been rationalized through computational studies.

Changing the Reactivity of Zero- and Mono-Valent Germanium with a Redox Non-Innocent Bis(silylenyl)carborane Ligand

Using the chelating C,C'-bis(silylenyl)-ortho-dicarborane ligand, 1,2-(RSi)2 -1,2-C2 B10 H10 [R=PhC(NtBu)2 ], leads to the monoatomic zero-valent Ge complex ("germylone") 3. The redox non-innocent character of the carborane scaffold has a drastic influence on the reactivity of 3 towards reductants and oxidants. Reduction of 3 with one molar equivalent of potassium naphthalenide (KC10 H8 ) causes facile oxidation of Ge0 to GeI along with a two-electron reduction of the C2 B10 cluster core and subsequent GeI -GeI coupling to form the dianionic bis(silylene)-supported Ge2 complex 4. In contrast, oxidation of 3 with one molar equivalent of [Cp2 Fe][B{C6 H3 (CF3 )2 }4 ] as a one-electron oxidant furnishes the dicationic bis(silylene)-supported Ge2 complex 5. The Ge0 atom in 3 acts as donor towards GeCl2 to form the trinuclear mixed-valent Ge0 →GeII ←Ge0 complex 6, from which dechlorination with KC10 H8 affords the neutral Ge2 complex 7 as a diradical species.

Bis(silylene)-Stabilized Monovalent Nitrogen Complexes

The first series of bis(silylene)-stabilized nitrogen(I) compounds is described. Starting from the 1,2-bis(N-heterocyclic silylenyl) 1,2-dicarba-closo-dedocaborane(12) scaffold 1, [1,2-(LSi)2 C2 B10 H10 ; L=PhC(Nt Bu)2 ], reaction with adamantyl azide (AdN3 ) affords the terminal N-μ2 -bridged zwitterionic carborane-1,2-bis(silylium) AdN3 adduct 2 with an open-cage dianionic nido-C2 B10 cluster core. Remarkably, upon one-electron reduction of 2 with C8 K and liberation of N2 and adamantane, the two silylene subunits are regenerated to furnish the isolable bis(silylene)-stabilized NI complex as an anion of 3 with the nido-C2 B10 cluster cage. On the other hand, one-electron oxidation of 2 with silver(I) yields the monocationic bis(silylene) NI complex 4 with the closo-C2 B10 cluster core. Moreover, the corresponding neutral NI radical complex 5 results from single-electron transfer from 3 to 4.

Stabilization of Linear C3 by Two Donor Ligands: A Theoretical Study of L-C3 -L (L=PPh3 , NHCMe , cAACMe )*

Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L-C3 -L with L=PPh3 (1), NHCMe (2, NHC=N-heterocyclic carbene), and cAACMe (3, cAAC=cyclic (alkyl)(amino) carbene). The calculations predict that 1 and 2 have equilibrium geometries where the ligands are bonded with rather acute bonding angles at the linear C3 moiety. The phosphine adduct 1 has a synclinal (gauche) conformation whereas 2 exhibits a trans conformation of the ligands. In contrast, the compound 3 possesses a nearly linear arrangement of the carbene ligands at the C3 fragment. The bond dissociation energies of the ligands have the order 1<2<3. The bonding analysis using charge and energy decomposition methods suggests that 3 is best described as a cumulene with electron-sharing double bonds between neutral fragments (cAACMe )2 and C3 in the respective electronic quintet state yielding (cAACMe )=C3 =(cAACMe ). In contrast, 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh3 )2 ]+ or [(NHCMe )2 ]+ and negatively charged [C3 ]- fragments in the respective doublet state.

Facile Access to Dative, Single, and Double Silicon-Metal Bonds Through M-Cl Insertion Reactions of Base-Stabilized SiII Cations

Silicon(II) cations can offer fascinating reactivity patterns due to their unique electronic structure: a lone pair of electrons, two empty p orbitals and a positive charge combined on a single silicon center. We now report the facile insertion of N-heterocyclic carbene (NHC)-stabilized silyliumylidene ions into M-Cl bonds (M=Ru, Rh), forming a series of novel chlorosilylene transition-metal complexes. Theoretical investigations revealed a reaction mechanism in which the insertion into the M-Cl bond with concomitant 1,2-migration of a silicon-bound NHC to the transition metal takes place after formation of an initial silyliumylidene transition-metal complex. The mechanism could be verified experimentally through characterization of the intermediate complexes. Furthermore, the obtained chlorosilylene complexes can be conveniently utilized as synthons to access Si-M and Si=M bonding motifs bonds through reductive dehalogenation.

NHC-Stabilised Silyliumylidene Ions

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

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

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

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

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

Azines: synthesis, structure, electronic structure and their applications

Azines (2,3-diaza-1,3-butadienes): structure, electronic structure, tautomerism, and their applications in organic synthesis, medicinal chemistry and materials chemistry.

Tetrylones: An Intriguing Class of Monoatomic Zero-valent Group 14 Compounds

Tetrylones (ylidones) represent a class of zero-valent group 14 compounds with the general formula EL₂ (E=C, Si, Ge, Sn, or Pb; L=neutral σ-donating ligand), wherein the tetrel atom, E(0), possess its four valence electrons in the form of two electron lone pairs, and is moreover coordinated by two ligands (L) via donor-acceptor interactions (L→E←L). This review focuses on the synthesis, structure, reactivity, and computational examination of the isolable heavier tetrylones (Si, Ge, Sn) that have been discovered recently. A comprehensive review on carbone chemistry is beyond the scope of this review. It should also be noted that tetrylones contain two different types of lone pairs, that is, one that exhibits p-type and one that exhibits s-type characteristics. Different behavior should thus be expected when these lone pairs react with Lewis acids.

Can Remote N-Heterocyclic Carbenes Coordinate with Main Group Elements? Synthesis, Structure, and Quantum Chemical Analysis of N+ -Centered Complexes

Remote N-heterocyclic carbenes (rNHCs), such as N-methyl-4-pyridylidene, are known to form coordination complexes with TMs. Herein, it is established that rNHCs can also coordinate to the N⁺ centre. Synthesis of some novel divalent N^I complexes with the general formula (rNHC)→N⁺ ←(NHC) and (rNHC)→N⁺ ←(rNHC) was achieved, and X-ray diffraction studies supported the coordination bond character between the rNHCs and the N⁺ centre. Quantum chemical analysis established the presence of divalent N^I character at the central nitrogen in these systems.

Titanium Imido Complexes via Displacement of -SiMe3 and C-H Bond Activation in a Ti(III) Amido Complex, Promoted by a Cyclic (Alkyl)(Amino) Carbene (cAAC)

A strong σ-donating cyclic (alkyl)(amino) carbene (cAAC) triggers rearrangement of the silyl(aryl) amido ligand -N(SiMe3)Dipp (Dipp = 2,6-diisopropylphenyl) in the coordination sphere of titanium(III) to afford a novel zwitterionic titanium imido complex with a TiCH2SiMe2[cAAC] linkage. Reduction of this species produces a new DippN=Ti imido complex containing a cAAC-centered radical species, characterized by single crystal diffraction analysis and electron paramagnetic resonance spectroscopy.