Tris(trimethylsilyl)silanides of the Heavier Alkali Metals—A Structural Study

Anions featuring an aluminium-silicon core with alumanyl silanide and aluminata-silene characteristics

Molecular species containing multiple bonds to aluminium have long been challenging synthetic targets. Despite recent landmark discoveries in this area, heterodinuclear Al-E multiple bonds (where E is a group-14 element) have remained rare and limited to highly polarized π-interactions (Al=E ↔ +Al-E-). Here we report the isolation of three alumanyl silanide anions that feature an Al-Si core stabilized by bulky substituents and a Si-Na interaction. Single-crystal X-ray diffraction studies, spectroscopic analysis and density functional theory calculations show that the Al-Si interaction possesses partial double bond character. Preliminary reactivity studies support this description of the compounds through two resonance structures: one that displays a predominant nucleophilic character of the sodium-coordinated silicon centre in the Al-Si core, as shown by silanide-like reactivity towards halosilane electrophiles and the CH-insertion of phenylacetylene. Moreover, we report an alumanyl silanide with a sequestered sodium cation. Cleavage of the Si-Na bond by [2.2.2]cryptand increases the double bond character of the Al-Si core to produce an anion with high aluminata-silene (-Al=Si) character.

The "Hidden" Reductive [2+2+1]-Cycloaddition Chemistry of 2-Phosphaethynolate Revealed by Reduction of a Th-OCP Linkage

The reduction chemistry of the newly emerging 2‐phosphaethynolate (OCP)⁻ is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(Tren^TIPS)(OCP)] (2, Tren^TIPS=[N(CH₂CH₂NSiPrⁱ₃)]³⁻), with RbC₈ via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(Tren^TIPS)}₆(μ‐OC₂P₃)₂(μ‐OC₂P₃H)₂Rb₄] (5) featuring four five‐membered [C₂P₃] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(Tren^TIPS)(μ‐ORb)}₂] (6) and the known cyclometallated complex [Th{N(CH₂CH₂NSiPrⁱ₃)₂(CH₂CH₂SiPrⁱ₂CHMeCH₂)}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2‐phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP)⁻, rather than cycloaddition products that then decompose with [Th(Tren^TIPS)O]⁻ essentially acting as a protecting then leaving group. In contrast, when KC₈ or CsC₈ were used the phosphinidiide C−H bond activation product [{Th(Tren^TIPS)}Th{N(CH₂CH₂NSiPrⁱ₃)₂[CH₂CH₂SiPrⁱ₂CH(Me)CH₂C(O)μ‐P]}] (3) and the oxo complex [{Th(Tren^TIPS)(μ‐OCs)}₂] (7) were isolated.

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

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

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

Synthesis and Reactivity of a Hypersilylsilylene

Stabilization of an amidinatosilylene with a bulky tris(trimethylsilyl)silyl substituent was realized with the preparation of PhC(NtBu)₂Si{Si(SiMe₃)₃} (1) from PhC(NtBu)₂SiHCl₂ with K{Si(SiMe₃)₃} in more than 90% yield. The highly deshielded ²⁹Si NMR resonance (δ = 76.91 ppm) can be attributed to the absence of a π-donating substituent. The molecular structure of 1 shows a trigonal-planar geometry around the Si^II center with a Si^II-Si^IV bond length of 2.4339(13) Å. A series of reactions of 1 with Me₃NO, S, Se, and Te were performed. While siloxane derivatives (2 and 3) are obtained from reactions with Me₃NO, silachalcogenones (4-6) are formed with other chalcogens. The presence of Si═E (E = S, Se, and Te) bonds in 4-6 have been confirmed by single-crystal X-ray studies. Silaoxirane (7) formation was observed when 1 was treated with acetone, demonstrating the importance of the tris(trimethylsilyl)silyl group to kinetically and thermodynamically protect the silaoxirane derivative with less bulky substituents on the C atom.

Synthesis and structure of the donor-free potassium silanide K[SiPh3]

The donor-free potassium silanide K[SiPh3] was prepared by the reaction of hexaphenyldisilane, Ph3Si–SiPh3, with potassium metal in benzene at room temperature. The solid-state structure, determined by powder X-ray diffraction consists of {K[SiPh3]}2units, which interact with adjacent dimers to form an infinite chain along the crystallographiccaxis (orthorhombic, space groupCmc21,Z=4). The structure features short contacts between the π system of the phenyl rings and the potassium atoms of neighbouring K[SiPh3] units.

Silicon chemistry in zero to three dimensions: from dichlorosilylene to silafullerane

As one of the simplest examples of functionalized Si(ii) species, the SiCl₂/[SiCl₃]⁻ system is not only fundamentally interesting, but also an important starting point for the assembly of oligosilane chains, rings, and clusters.

The Weakly Coordinating Tris(trichlorosilyl)silyl Anion

Closely following the procedure for the preparation of the base-stabilized dichlorosilylene complex NHC^Dipp ⋅SiCl₂ reported by Roesky, Stalke, and co-workers (Angew. Chem. Int. Ed. 2009, 48, 5683-5686), a few crystals of the salt [NHC^Dipp -H⋅⋅⋅Cl⋅⋅⋅H-NHC^Dipp ]Si(SiCl₃ )₃ were isolated, aside from the reported byproduct [NHC^Dipp -H⁺ ⋅⋅⋅Cl^- ], and characterized by X-ray crystallography (NHC^Dipp =N,N-di(2,6-diisopropylphenyl)imidazo-2-ylidene). They contain the weakly coordinating anion Si(SiCl₃ )₃^- , which was also obtained in high yields upon deprotonation of the conjugate Brønsted acid HSi(SiCl₃ )₃ with NHC^Dipp or PMP (PMP=1,2,2,6,6-pentamethylpiperidine). The acidity of HSi(SiCl₃ )₃ was estimated by DFT calculations to be substantially higher than those of other H-silanes. Further DFT studies on the electronic structure of Si(SiCl₃ )₃^- , including the electrostatic potential and the electron localizability, confirmed its low basicity and nucleophilicity compared with other silyl anions.

Alkyne Addition and Insertion Reactions of [(Me3 Si)3 Si]2 Ge⋅PMe3

Silylated germylene-PMe₃ adducts exchange their phosphane moiety smoothly for an N-heterocyclic carbene or isocyanide species to form their respective base adducts. Reaction of the silylated germylene-PMe₃ adducts with monosubstituted alkynes produce germylene adducts with the alkyne inserted into a Ge-Si bond. A computational study of this process provides evidence for the initial formation of a germirene, which rearranges to a vinylgermylene species. The thermodynamic driving force for this reaction is provided by subsequent adduct formation with PMe₃ . Reaction of the PMe₃ adduct of bis[(trimethylsilyl)silyl]germylene with disubstituted alkynes leads to the formation of stable germirenes, which can be isomerized further to silagermetes.

The Tris(pentafluoroethyl)silanide Anion

Tris(pentafluoroethyl)silane, which is conveniently accessible by the treatment of Si(C₂ F₅ )₃ X (X=Cl, Br) with Bu₃ SnH, was successfully employed for hydrosilylation reactions. In the presence of a palladium catalyst, hydrosilylation of phenylacetylene with Si(C₂ F₅ )₃ H affords the product of an α-addition whereas the reaction of trimethylsilylacetylene with the silane leads to the β-trans product. Tris(pentafluoroethyl)silane can be deprotonated by sterically demanding bases such as lithium diisopropylamide at low temperatures to give the corresponding silanide ion. The addition of crown ethers or cryptands to this highly reactive species enabled the isolation and characterization of salt-like tris(pentafluoroethyl)silanide at room temperature.

Introducing a Hydrogen-Bond Donor into a Weakly Nucleophilic Brønsted Base: Alkali Metal Hexamethyldisilazides (MHMDS, M=Li, Na, K, Rb and Cs) with Ammonia

Alkali metal 1,1,1,3,3,3-hexamethyldisilazide (MHMDSs) are one of the most utilised weakly nucleophilic Brønsted bases in synthetic chemistry and especially in natural product synthesis. Like lithium organics, they aggregate depending on the employed donor solvents. Thus, they show different reactivity and selectivity as a function of their aggregation and solvation state. To date, monomeric LiHMDS with monodentate donor bases was only characterised in solution. Since the first preparation of LiHMDS in 1959 by Wannagat and Niederprüm, all efforts to crystallise monomeric LiHMDS in the absence of chelating ligands failed. Herein, we present ammonia adducts of LiHMDS, NaHMDS, KHMDS, RbHMDS and CsHMDS with unprecedented aggregation motifs: 1) The hitherto missing monomeric key compound in the LiHMDS aggregation architectures. Monomeric crystal structures of trisolvated LiHMDS (1) and NaHMDS (2), showing unique intermolecular hydrogen bonds, 2) the unprecedented tetrasolvated KHMDS (3) and RbHMDS (4) dimers and 3) the disolvated CsHMDS (5) dimer with very close intermolecular Si-CH3 ⋅⋅⋅Cs s-block "agostic" interactions have been prepared and characterised by single-crystal X-ray structure analysis.

Crown ether adducts of light alkali metal triphenylsilyls: synthesis, structure and hydrosilylation catalysis

Alkali metal triphenylsilyls [Li(12-crown-4)SiPh₃]·(thf)_0.5, [Na(15-crown-5)SiPh₃]·(thf)_0.5 and [K(18-crown-6)SiPh₃(thf)] were synthesized using 1,1,1-trimethyl-2,2,2-triphenyldisilane and shown to catalyze the regioselective hydrosilylation of 1,1-diphenylethene.

Synthetically important alkali-metal utility amides: lithium, sodium, and potassium hexamethyldisilazides, diisopropylamides, and tetramethylpiperidides

Most synthetic chemists will have at some point utilized a sterically demanding secondary amide (R2 N(-) ). The three most important examples, lithium 1,1,1,3,3,3-hexamethyldisilazide (LiHMDS), lithium diisopropylamide (LiDA), and lithium 2,2,6,6-tetramethylpiperidide (LiTMP)-the "utility amides"-have long been indispensible particularly for lithiation (Li-H exchange) reactions. Like organolithium compounds, they exhibit aggregation phenomena and strong Lewis acidity, and thus appear in distinct forms depending on the solvents employed. The structural chemistry of these compounds as well as their sodium and potassium congeners are described in the absence or in the presence of the most synthetically significant donor solvents tetrahydrofuran (THF) and N,N,N',N'-tetramethylethylenediamine (TMEDA) or closely related solvents. Examples of hetero-alkali-metal amides, an increasingly important composition because of the recent escalation of interest in mixed-metal synergic effects, are also included.

Cyclic Disilylated and Digermylated Germylenes

The preparation of triethylphosphine adducts of cyclic disilylated or digermylated germylenes was achieved by reaction of 1,4-dipotassio-1,1,4,4-tetrakis(trimethylsilyl)tetramethyltetrasilane with GeBr2·(dioxane) and PEt3. Phosphine abstraction with B(C6F5)3 allowed formation of the base-free germylenes, which undergo 1,2-trimethylsilyl shifts to the germylene atom to form the respective silagermene or digermene, which further dimerize in [2 + 2] cycloadditions to tricyclic compounds. The reasons responsible for the germylenes' completely different reactivities in comparison to the previously studied analogous stannylenes and plumbylenes were elucidated in a theoretical study.

Polyamine-stabilized sodium aryloxides: simple initiators for the ring-opening polymerization of rac-lactide

Metalation of 2,4,6-tri(methyl)phenol ((Me)ArOH) and 2,6-di(tert-butyl)-4-methylphenol ((Bu)ArOH) with NaN(SiMe(3))(2) in toluene and in the presence of stoichiometric amounts of the polydentate amines N,N,N',N'-tetramethylethylenediamine (TMEDA) and N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) affords three new sodium aryloxide complexes [Na(μ-OAr(Bu))(TMEDA)](2) (3), [Na(μ-OAr(Me))(PMDETA)](2) (4), and [Na(OAr(Bu))(PMDETA)] (5). Complexes 3 to 5 have been isolated as crystalline materials in reasonable yields and characterized in the solid state by X-ray crystallography and in solution by NMR spectroscopy. Complexes 3 to 5 and the related [tris(2-dimethylaminoethyl)amine] (Me(6)TREN) derivatives [Na(OAr(Me))(HOAr(Me))(Me(6)TREN)] (1) and [Na(OAr(Bu))(Me(6)TREN)] (2), recently prepared in our group, are shown to be active as initiators for the ring-opening polymerization (ROP) of rac-lactide with benzyl alcohol as a co-initiator. However, during the course of the polymerization reactions intrachain and stereorandom transesterification side-reactions were observed under some of the experimental conditions tested.