Low-Valent Mx Al3 Cluster Salts with Tetrahedral [SiAl3 ]+ and Trigonal-Bipyramidal [M2 Al3 ]2+ Cores (M=Si/Ge)

Schnöckel's [(AlCp*)₄ ] and Jutzi's [SiCp*][B(C₆ F₅ )₄ ] (Cp*=C₅ Me₅ ) are landmarks in modern main-group chemistry with diverse applications in synthesis and catalysis. Despite the isoelectronic relationship between the AlCp* and the [SiCp*]⁺ fragments, their mutual reactivity is hitherto unknown. Here, we report on their reaction giving the complex salts [Cp*Si(AlCp*)₃ ][WCA] ([WCA]^- =[Al(OR^F )₄ ]^- and [F{Al(OR^F )₃ }₂ ]^- ; R^F =C(CF₃ )₃ ). The tetrahedral [SiAl₃ ]⁺ core not only represents a rare example of a low-valent silicon-doped aluminium-cluster, but also-due to its facile accessibility and high stability-provides a convenient preparative entry towards low-valent Si-Al clusters in general. For example, an elusive binuclear [Si₂ (AlCp*)₅ ]²⁺ with extremely short Al-Si bonds and a high negative partial charge at the Si atoms was structurally characterised and its bonding situation analysed by DFT. Crystals of the isostructural [Ge₂ (AlCp*)₅ ]²⁺ dication were also obtained and represent the first mixed Al-Ge cluster.

Siliconoid Expansion by a Single Germanium Atom through Isolated Intermediates

The growth of (semi-)metal clusters is pivotal for nucleation processes in gaseous and condensed phases. We now report the isolation of intermediates during the expansion of a stable unsaturated silicon cluster (siliconoid) by a single germanium atom through a sequence of substitution, rearrangement and reduction. The reaction of ligato-lithiated hexasilabenzpolarene LiSi6 Tip5 (1Li⋅(thf)2 , Tip=2,4,6-triisopropylphenyl) with GeCl2 ⋅NHC (NHC=1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene) initially yields the product with exohedral germanium(II) functionality, which then inserts into an Si-Si bond of the Si6 scaffold. The concomitant transfer of the chloro functionality from germanium to an adjacent silicon preserves the electron-precise nature of the formed endohedral germylene. Full incorporation of the germanium heteroatom to the Si6 Ge cluster core is finally achieved either by reduction under loss of the coordinating NHC or directly by reaction of 1Li⋅(thf)2 with GeCl2 ⋅1,4-dioxane.

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.

Revival of Hückel Aromatic (Poly)benzenoid Subunits in Triplet State Polycyclic Aromatic Hydrocarbons by Silicon Substitution

By employing density functional theory (DFT) calculations we show that mono- and disilicon substitution in polycyclic aromatic hydrocarbons, having two to four benzene units, quenches their triplet state antiaromaticity by creating Hückel aromatic (poly)benzenoid subunit(s) and weakly antiaromatic, or almost nonaromatic silacycle. Therefore, such systems are predicted to be globally aromatic in both the ground state and the first excited triplet state. Putting the silicon atom(s) into various positions of a hydrocarbon provides an opportunity to tune the singlet-triplet energy gaps. They depend on the global aromaticity degree which, in turn, depends on the type of aromatic carbocyclic subunit(s) and the extent of their aromaticity. On the basis of the set of studied compounds, some preliminary rules on how to regulate the extent of global, semiglobal and local aromaticity are proposed. The results of this work extend the importance of Hückel aromaticity concept to excited triplet states which are usually characterized by the Baird type of (anti)aromaticity.

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.

Chalcogen-Expanded Unsaturated Silicon Clusters: Thia-, Selena-, and Tellurasiliconoids

Reactions of silylenes with heavier chalcogens (E) typically result in Si=E double bonds or their π-addition products. In contrast, the oxidation of a silylene-functionalized unsaturated silicon cluster (siliconoid) with Group 16 elements selectively yields cluster expanded siliconoids Si7 E (E=S, Se, Te) fully preserving the unsaturated nature of the cluster scaffold as evident from the NMR signatures of the products. Mechanistic considerations by DFT calculations suggest the intermediacy of a Si6 siliconoid with exohedral Si=E functionality. The reaction thus may serve as model system for the oxidation of surface-bonded silylenes at Si(100) by chalcogens and their diffusion into the silicon bulk.

Bonding Relationship between Silicon and Germanium with Group 13 and Heavier Elements of Groups 14-16

The topic of heavier main group compounds possessing multiple bonds is the subject of momentous interest in modern organometallic chemistry. Importantly, there is an excitement involving the discovery of unprecedented compounds with unique bonding modes. The research in this area is still expanding, particularly the reactivity aspects of these compounds. This article aims to describe the overall developments reported on the stable derivatives of silicon and germanium involved in multiple bond formation with other group 13, and heavier groups 14, 15, and 16 elements. The synthetic strategies, structural features, and their reactivity towards different nucleophiles, unsaturated organic substrates, and in small molecule activation are discussed. Further, their physical and chemical properties are described based on their spectroscopic and theoretical studies.

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.

Equilibrium Formation of Stable All-Silicon Versions of 1,3-Cyclobutanediyl

Main group analogues of cyclobutane-1,3-diyls are fascinating due to their unique reactivity and electronic properties. So far only heteronuclear examples have been isolated. Here we report the isolation and characterization of all-silicon 1,3-cyclobutanediyls as stable closed-shell singlet species from the reversible reactions of cyclotrisilene c-Si3 Tip4 (Tip=2,4,6-triisopropylphenyl) with the N-heterocyclic silylenes c-[(CR2 CH2 )(NtBu)2 ]Si: (R=H or methyl) with saturated backbones. At elevated temperatures, tetrasilacyclobutenes are obtained from these equilibrium mixtures. The corresponding reaction with the unsaturated N-heterocyclic silylene c-(CH)2 (NtBu)2 Si: proceeds directly to the corresponding tetrasilacyclobutene without detection of the assumed 1,3-cyclobutanediyl intermediate.

Indirect and Direct Grafting of Transition Metals to Siliconoids

Unsaturated charge‐neutral silicon clusters (siliconoids) are important as gas‐phase intermediates between molecules and the elemental bulk. With stable zirconocene‐ and hafnocene‐substituted derivatives, we here report the first examples containing directly bonded transition‐metal fragments that are readily accessible from the ligato‐lithiated Si₆ siliconoid (1Li) and Cp₂MCl₂ (M=Zr, Hf). Charge‐neutral siliconoid ligands with pending tetrylene functionality were prepared by the reaction of amidinato chloro tetrylenes [PhC(NtBu)₂]ECl (E=Si, Ge, Sn) with 1Li, thus confirming the principal compatibility of such low‐valent functionalities with the unsaturated Si₆ cluster scaffold. The pronounced donor properties of the tetrylene/siliconoid hybrids allow for their coordination to the Fe(CO)₄ fragment.

Atomically Precise Expansion of Unsaturated Silicon Clusters

Small- to medium-sized clusters occur in various areas of chemistry, for example, as active species of heterogeneous catalysis or as transient intermediates during chemical vapor deposition. The manipulation of stable representatives is mostly limited to the stabilizing ligand periphery, virtually excluding the systematic variation of the property-determining cluster scaffold. We now report the deliberate expansion of a stable unsaturated silicon cluster from six to seven and finally eight vertices. The consecutive application of lithium/naphthalene as the reducing agent and decamethylsilicocene as the electrophilic source of silicon results in the expansion of the core by precisely one atom with the potential of infinite repetition.

On the reaction of GeCl2·dioxane with KFeCp(CO)2: isolation and characterization of novel bimetallic clusters

The reaction of GeCl2·dioxane with KFeCp(CO)2 gives the polyhedral cluster compounds Ge6[FeCp(CO)2]61, Ge6[FeCp(CO)2]6Cl23 and Ge6[FeCp(CO)2]4[Fe2Cp2(CO)3]25. Quantum chemical calculations indicate that the substitution of a halide substituent in GeCl2 by FeCp(CO)2 favours the formation of oligomers with Ge-Ge bonds, giving access to polyhedral cluster compounds from easily accessible starting materials.

A Three-Membered Cyclic Phosphasilene

Small unsaturated phosphacycles are versatile reagents owing to their strain and the added functionality of the double bond and the phosphorus lone pair. Herein we report the synthesis and isolation of the smallest possible cyclic phosphasilene as a stable adduct with an N-heterocyclic carbene (NHC). First reactivity studies show a) that the PSi₂ ring is a competent ligand to the Fe(CO)₄ fragment via the phosphorus lone pair and b) that the abstraction of the NHC by BPh₃ results in the rapid head-to-head or head-to-tail dimerization of the PSi₂ unit. The relatively facile NHC cleavage indicates that the P=Si double bond is available for further manipulation.

Anionic Siliconoids from Zintl Phases: R3 Si9 - with Six and R2 Si9 2- with Seven Unsubstituted Exposed Silicon Cluster Atoms (R=Si(tBu)2 H)

Neutral and anionic silicon clusters (siliconoids) are regarded as important model systems for bulk silicon surfaces. For 25 years their formation from binary alkali metal silicide phases has been proposed, but experimentally never realized. Herein the silylation of a silicide, leading to the anionic siliconoids (Si(tBu)₂ H)₃ Si₉ ^- (1 a) and (Si(tBu)₂ H)₂ Si₉ ^2- (2 a) with the highest known number of ligand-free silicon atoms is reported for the first time. The new anions are obtained in a one-step reaction of K₁₂ Si₁₇ /NH₃ (liq.) and Si(tBu)₂ HCl/THF. Electrospray ionization spectrometry and ¹ H, ¹³ C, ²⁹ Si, as well as ²⁹ Si-HMBC (heteronuclear multiple bond correlation) NMR spectroscopy, confirm the attachment of three silyl groups at a [Si₉ ]^4- cluster under formation of 1 a, in accordance with calculated NMR shifts. During crystal growth the siliconoid di-anion 2 a is formed. The single-crystal X-ray structure determination reveals that two silyl groups are connected to the deltahedral Si₉ cluster core, revealing seven unsubstituted exposed silicon cluster atoms with a hemispheroidal coordination. The negative charges -1 and -2 are delocalized over the six and seven siliconoid Si atoms in 1 a and 2 a, respectively.

Designing the Backbone of Hexasilabenzene Derivatives with a High Unimolecular Kinetic Stability

It is an important subject to theoretically predict the kinetic stability of transient species. In this study, we have studied the kinetic stability of hexasilabenzene Si₆ H₆ and its derivatives, that is, decasilanaphthalene Si₁₀ H₈ and Li-substituted hexasilabenzene Si₆ Li₆ , theoretically by the artificial force induced reaction (AFIR) method combined with the rate constant matrix contraction (RCMC) method. Molecular design was further conducted to extend the unimolecular lifetime of hexasilabenzene derivatives. Although both Si₁₀ H₈ and Si₆ Li₆ were shown to possess shorter lifetimes than Si₆ H₆ , we found that the lifetimes of Si₆ Li₆ changed depending on arrangements of Li atoms around the monocyclic Si₆ backbone. Based on this knowledge, we found that a compound of an atomic composition Si₆ H₄ Li₂ with a planar, monocyclic Si₆ backbone has a relatively long unimolecular lifetime. Moreover, substitution of the two Li atoms by Na atoms further increased the lifetime.

Functional Disilenes in Synthesis

Functionalized disilenes are valuable auxiliary tools in preparative chemistry. In the following review the various synthetic potential of disilenes as precursors is presented. Upon consumption of the Si=Si unit in disilenes, cyclic, polycyclic and cluster-like arrangements are obtainable.

Reductive Elimination of Hydrogen from Bis(trimethylsilyl)methyltin Trihydride and Mesityltin Trihydride

Alkyltin trihydride [(Me₃ Si)₂ CHSnH₃ ] was synthesized and the reductive elimination of hydrogen from this species was investigated. A methyl-substituted N-heterocyclic carbene reacts with the organotrihydride in dependence on stoichiometry and solvent to give a series of products of the reductive elimination and dehydrogenative tin-tin bond formation. Besides characterization of the carbene adduct of the alkyltin(II) hydride, a Sn₄ chain was also isolated, encompassing two stannyl-stannylene sites, which are stabilized each as NHC-adducts. Complete dehydrogenation resulted to give either a carbene-stabilized distannyne or a metalloid Sn₉ -cluster salt. Reductive elimination of hydrogen was also achieved with an excess of diethylmethylamine to give the alkyltin(II) hydride as a Lewis base free tetramer [(RSnH)₄ ]. The method of cluster formation at low temperatures by hydrogen elimination was also transferred to the mesityl-substituted tin trihydride MesSnH₃ . In this case [(MesSn)₁₀ ], showing a [5]prismane structure, was isolated in good yield and characterized. NMR spectroscopic features of the propellane-type cluster [Trip₆ Sn₆ ] are reported.

Isolation and Versatile Derivatization of an Unsaturated Anionic Silicon Cluster (Siliconoid)

The characteristic features of bulk silicon surfaces are echoed in the related partially substituted—and thus unsaturated—neutral silicon clusters (siliconoids). The incorporation of siliconoids into more‐extended frameworks is promising owing to their unique electronic features, but further developments in this regard are limited by the notable absence of functionalized siliconoid derivatives until now. Herein we report the isolation and full characterization of the lithium salt of an anionic R₅Si₆‐siliconoid, thus providing the missing link between silicon‐based Zintl anions and siliconoid clusters. Proof‐of‐principle for the high potential of this species for the efficient transfer of the intact unsaturated R₅Si₆ moiety is demonstrated by clean reactions with representative electrophiles of Groups 13, 14, and 15.

Structures and electronic properties of B2Si6−/0/+: anion photoelectron spectroscopy and theoretical calculations

The lowest-energy structures of B₂Si₆^q(q= −1, 0, +1) clusters are a peculiar structure with a silicon atom hanging over a distorted bowl-like B₂Si₅framework. It is characterized with σ or π delocalization in chemical bonding.

Two pentasilahousanes fused together

The organosilicon cluster 1 with two pentasilahousanes fused together was synthesized by the reduction of 1,1,3-trichlorocyclotetrasilane. This reaction involves dimerization and rearrangement of the silicon skeleton. The structure and properties of 1 were studied by X-ray crystallographic and spectroscopic analyses.