Exhaustively Trichlorosilylated C1 and C2 Building Blocks: Beyond the Müller–Rochow Direct Process

Challenges in the Synthesis and Processing of Hydrosilanes as Precursors for Silicon Deposition

Hydrosilanes are highly attractive compounds, which can be processed as liquids with printing technology to amorphous silicon films on nearly any solid substrate. The silicon layers can be processed for electronic devices like transistors or thin-film solar cells. The endothermic character of hydrosilanes with their positive enthalpies of formation results in favorable properties for processing. The larger the molecules, the lower their decomposition temperature and the higher their photoactivity. Cyclic hydrosilanes such as cyclopentasilane and cyclohexasilane can be easily deposited. The branched neopentasilane is more difficult to deposit but yields better-quality films after processing. The key challenge is the complex synthesis of the precursors and the hydrosilanes. The available preparative methods are presented in this review and their advantages and disadvantages are evaluated. The following synthesis methods are presented and discussed in this article: Wurtz coupling and other reductive coupling processes, dehydrogenative coupling of silanes, plasma synthesis of chlorinated polysilanes, amine- or chloride-induced disproportionations, and transformation of monosilane to higher silanes. Plasma synthesis is already carried out today as a continuous industrial process. The most effective synthesis methods in the laboratory are currently amine- and chloride-induced disproportionations. There is a great need to further optimize the syntheses of hydrosilanes and to develop new simple synthesis variants.

Regioselective Derivatization of Silylated [20]Silafulleranes

Silafulleranes with endohedral Cl^- ions are a unique, scarcely explored class of structurally well-defined silicon clusters and host-guest complexes. Herein, we report regioselective derivatization reactions on the siladodecahedrane [nBu₄N][Cl@Si₂₀(SiCl₃)₁₂Cl₈] ([nBu₄N][1]), which has its cluster surface decorated with 12 SiCl₃ and 8 Cl substituents in perfect T_h symmetry. The room-temperature reaction of [nBu₄N][1] with excess iBu₂AlH in ortho-difluorobenzene (oDFB) furnishes perhydrogenated [nBu₄N][Cl@Si₂₀(SiH₃)₁₂H₈] ([nBu₄N][2]) in 50% yield; the non-pyrophoric [2]^- is the largest structurally authenticated (by X-ray diffraction) hydridosilane known to date. A simple switch from pure oDFB to an oDFB/Et₂O solvent mixture suppresses core hydrogenation and results in the formation of [nBu₄N][Cl@Si₂₀(SiH₃)₁₂Cl₈] ([nBu₄N][3]). In addition to the exhaustive Cl/H exchange at all 44 Si-Cl bonds of [1]^- and the regioselective 36-fold silyl group hydrogenation, we achieved the simultaneous introduction of Me substituents at all 8 SiCl vertices along with the conversion of all 12 SiCl₃ to SiH₃ groups by treating [nBu₄N][1] with Me₂AlH/Me₃Al in oDFB ([nBu₄N][Cl@Si₂₀(SiH₃)₁₂Me₈], [nBu₄N][4]; 73%). Quantum-chemical free-energy calculations find an S_N2-Si-type hydrogenation of the exohedral SiCl₃ moieties in [1]^- (trigonal-bipyramidal intermediate) slightly preferred over metathesis-like S_Ni-Si substitutions (four-membered transition state). Cage hydrogenation likely occurs via S_Ni-Si processes. The experimentally demonstrated influence of an Et₂O co-solvent, which drastically increases the respective reaction barriers, is attributed to the increased stability of the resulting iBu₂AlH-OEt₂ adduct and its higher steric bulk compared to free iBu₂AlH.

A Hydride-Substituted Homoleptic Silylborate: How Similar is it to its Diborane(6)-Dianion Isostere?

The B-nucleophilic 9H-9-borafluorene dianion reacts with 9-chloro-9-silafluorene to afford air- and moisture-stable silylborate salts M[Ar₂ (H)B-Si(H)Ar₂ ] (M[HBSiH], M=Li, Na). Li[HBSiH] and Me₃ SiCl give the B-pyridine adduct Ar₂ (py)B-Si(H)Ar₂ ((py)BSiH) or the chlorosilane Li[Ar₂ (H)B-Si(Cl)Ar₂ ] (Li[HBSiCl]) in C₆ H₆ -pyridine or THF. In both cases, the first step is H^- abstraction at the B center. The resulting free borane subsequently binds a py or thf ligand. While the py adduct is stable at room temperature, the thf adduct undergoes a 1,2-H shift via the cyclic B(μ-H)Si intermediate BHSi, which is afterwards attacked at the Si atom by a Cl^- ion to give Li[HBSiCl]. DFT calculations were employed to support the proposed reaction mechanism and to characterize the electronic structure of BHSi. Treatment of Li[HBSiCl] with the N-heterocyclic carbene IMe introduces the neutral donor at the Si atom and leads to Ar₂ (H)B-Si(IMe)Ar₂ (HBSi(IMe)), a donor-acceptor-stabilized silylene.

Dope it with germanium: selective access to functionalized Si5Ge heterocycles

The Cl^- diadduct [nBu₄N]₂[A·2Cl] of the mixed cyclohexatetrelane (SiCl₂)₅(GeMe₂), A, is accessible from Me₂GeCl₂, 6 eq. Si₂Cl₆, and 2 eq. [nBu₄N]Cl in one step (96%). Free, tenfold functionalized A can be released from the primary product by decomplexation with AlCl₃ (78%). Insight into the assembly mechanism of [nBu₄N]₂[A·2Cl] and the reactivity of A is provided.

Photochemical Alkene Hydrophosphination with Bis(trichlorosilyl)phosphine

Bis(trichlorosilyl)phosphine (HP(SiCl₃)₂, 1) was prepared from [TBA][P(SiCl₃)₂] ([TBA]2, TBA = tetra-n-butylammonium) and triflic acid in 36% yield. Phosphine 1 is an efficient reagent for hydrophosphination of unactivated terminal olefins under UV irradiation (15-60 min) and gives rise to bis(trichlorosilyl)alkylphosphines (RP(SiCl₃)₂, R = (CH₂)₅CH₃, 88%; (CH₂)₇CH₃, 98%; (CH₂)₂C(CH₃)₃, 76%; CH₂Cy, 93%; (CH₂)₂Cy, 95%; CH₂CH(CH₃)(CH₂)₂CH₃, 82%; (CH₂)₃O(CH₂)₃CH₃, 95%; (CH₂)₃Cl, 83%; (CH₂)₂SiMe₃, 92%; (CH₂)₅C(H)CH₂, 44%) in excellent yields. The products require no further purification beyond filtration and removal of volatile material under reduced pressure. The P-Si bonds of prototypical products RP(SiCl₃)₂ (R = -(CH₂)₅CH₃, -(CH₂)₇CH₃) are readily functionalized to give further phosphorus-containing products: H₃C(CH₂)₇PCl₂ (56%), [H₃C(CH₂)₅P(CH₂Ph)₃]Br (84%), H₃C(CH₂)₇PH₂ (61%), H₃C(CH₂)₅P(O)(H)(OH) (81%), and H₃C(CH₂)₅P(O)(OH)₂ (55%). Experimental mechanistic investigations, accompanied by quantum chemical calculations, point toward a radical-chain mechanism. Phosphine 1 enables the fast, high-yielding, and atom-efficient preparation of compounds that contain phosphorus-carbon bonds in procedures that bypass white phosphorus (P₄), a toxic and high-energy intermediate of the phosphorus industry.

Diels-Alder Cycloaddition with CO, CO2, SO2, or N2 Extrusion: A Powerful Tool for Material Chemistry

Phenyl, naphthyl, polyarylphenyl, coronene, and other aromatic and polyaromatic moieties primarily influence the final materials' properties. One of the synthetic tools used to implement (hetero)aromatic moieties into final structures is Diels-Alder cycloaddition (DAC), typically combined with Scholl dehydrocondensation. Substituted 2-pyranones, 1,1-dioxothiophenes, and, especially, 1,3-cyclopentadienones are valuable substrates for [4 + 2] cycloaddition, leading to multisubstituted derivatives of benzene, naphthalene, and other aromatics. Cycloadditions of dienes can be carried out with extrusion of carbon dioxide, carbon oxide, or sulphur dioxide. When pyranones, dioxothiophenes, or cyclopentadienones and DA cycloaddition are aided with acetylenes including masked ones, conjugated or isolated diynes, or polyynes and arynes, aromatic systems are obtained. This review covers the development and the current state of knowledge regarding thermal DA cycloaddition of dienes mentioned above and dienophiles leading to (hetero)aromatics via CO, CO2, or SO2 extrusion. Particular attention was paid to the role that introduced aromatic moieties play in designing molecular structures with expected properties. Undoubtedly, the DAC variants described in this review, combined with other modern synthetic tools, constitute a convenient and efficient way of obtaining functionalized nanomaterials, continually showing the potential to impact materials sciences and new technologies in the nearest future.

The power of trichlorosilylation: isolable trisilylated allyl anions, allyl radicals, and allenyl anions

Treatment of hexachloropropene (Cl₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C(Cl)–CCl₃) with Si₂Cl₆ and [nBu₄N]Cl (1 : 4 : 1) in CH₂Cl₂ results in a quantitative conversion to the trisilylated, dichlorinated allyl anion salt [nBu₄N][Cl₂CC(SiCl₃)–C(SiCl₃)₂] ([nBu₄N][1]). Tetrachloroallene Cl₂CCCCl₂ was identified as the first intermediate of the reaction cascade. In the solid state, [1]⁻ adopts approximate C_s symmetry with a dihedral angle between the planes running through the olefinic and carbanionic fragments of [1]⁻ of CC–Si//Si–C–Si = 78.3(1)°. One-electron oxidation of [nBu₄N][1] with SbCl₅ furnishes the distillable blue radical 1˙. The neutral propene Cl₂CC(SiCl₃)–C(SiCl₃)₂H (2) was obtained by (i) protonation of [1]⁻ with HOSO₂CF₃ (HOTf) or (ii) H-atom transfer to 1˙ from 1,4-cyclohexadiene. Quantitative transformation of all three SiCl₃ substituents in 2 to Si(OMe)₃ (2^OMe) or SiMe₃ (2^Me) substituents was achieved by using MeOH/NMe₂Et or MeMgBr in CH₂Cl₂ or THF, respectively. Upon addition of 2 equiv. of tBuLi, 2^Me underwent deprotonation with subsequent LiCl elimination, 1,2-SiMe₃ migration and Cl/Li exchange to afford the allenyl lithium compound Me₃Si(Li)CCC(SiMe₃)₂ (Li[4]), which is an efficient building block for the introduction of Me, SiMe₃, or SnMe₃ (5) groups. The trisilylated, monochlorinated allene Cl₃Si(Cl)CCC(SiCl₃)₂ (6), was obtained from [nBu₄N][1] through Cl⁻-ion abstraction with AlCl₃ and rearrangement in CH₂Cl₂ (1˙ forms as a minor side product, likely because the system AlCl₃/CH₂Cl₂ can also act as a one-electron oxidant).

Treatment of hexachloropropene (Cl₂CC(Cl)–CCl₃) with Si₂Cl₆ and [nBu₄N]Cl (1 : 4 : 1) in CH₂Cl₂ results in a quantitative conversion to the trisilylated, dichlorinated allyl anion salt [nBu₄N][Cl₂CC(SiCl₃)–C(SiCl₃)₂] ([nBu₄N][1]).

Selective One-Pot Syntheses of Mixed Silicon-Germanium Heteroadamantane Clusters

Si x Ge y alloys are emerging materials for modern semiconductor technology. Well-defined model systems of the bulk structures aid in understanding their intrinsic characteristics. Three such model clusters have now been realized in the form of the Si x Ge y heteroadamantanes [0] , [1] , and [2] via selective one-pot syntheses starting from Me 2 GeCl 2 , Si 2 Cl 6 , and [nBu 4 N]Cl. Compound [0] contains 6 GeMe 2 and 4 SiSiCl 3 vertices, while one and two of the GeMe 2 groups are replaced by SiCl 2 moieties in compounds [1] and [2] , respectively. Chloride ion-mediated rearrangement quantitatively converts [2] into [1] at room temperature and finally into [0] at 60 °C, which is not only remarkable in view of the rigidity of these cage structures but also sheds light on the assembly mechanism.

Synthesis and structure of the λ 6Si-silicate [Cs(18-crown-6)]2[Si(OSO2CH3)6]

The λ 6Si-silicate [Cs(18-crown-6)]2[Si(OSO2CH3)6] (1) was synthesized by treatment of Si2Cl6 with Cs[OSO2CH3] in the presence of 18-crown-6. Compound 1 is the first example of a λ 6Si-silicate with a methanesulfonate ligand. It was characterized by NMR spectroscopy and by single-crystal X-ray diffraction. The solid-state structure of 1 consists of discrete [Si(OSO2CH3)6]2– anions and two [Cs(18-crown-6)]+ cations (triclinic space group, P 1 ¯ $P\overline{1}$ , Z = 1).

A Mild One-Pot Reduction of Phosphine(V) Oxides Affording Phosphines(III) and Their Metal Catalysts

The metal-free reduction of a range of phosphine(V) oxides employing oxalyl chloride as an activating agent and hexachlorodisilane as reducing reagent has been achieved under mild reaction conditions. The method was successfully applied to the reduction of industrial waste byproduct triphenylphosphine(V) oxide, closing the phosphorus cycle to cleanly regenerate triphenylphosphine(III). Mechanistic studies and quantum chemical calculations support the attack of the dissociated chloride anion of intermediated phosphonium salt at the silicon of the disilane as the rate-limiting step for deprotection. The exquisite purity of the resultant phosphine(III) ligands after the simple removal of volatiles under reduced pressure circumvents laborious purification prior to metalation and has permitted the facile formation of important transition metal catalysts.

Building up Strain in One Step: Synthesis of an Edge-Fused Double Silacyclobutene from an Extensively Trichlorosilylated Butadiene Dianion

The exhaustive trichlorosilylation of hexachloro‐1,3‐butadiene was achieved in one step by using a mixture of Si₂Cl₆ and [nBu₄N]Cl (7:2 equiv) as the silylation reagent. The corresponding butadiene dianion salt [nBu₄N]₂[1] was isolated in 36 % yield after recrystallization. The negative charges of [1]²⁻ are mainly delocalized across its two carbanionic (Cl₃Si)₂C termini (α‐effect of silicon) such that the central bond possesses largely C=C double‐bond character. Upon treatment with 4 equiv of HCl, [1]²⁻ is converted into neutral 1,2,3,4‐tetrakis(trichlorosilyl)but‐2‐ene, 3. The Cl⁻ acceptor AlCl₃, induces a twofold ring‐closure reaction of [1]²⁻ to form a six‐membered bicycle 4 in which two silacyclobutene rings are fused along a shared C=C double bond (84 %). Compound 4, which was structurally characterized by X‐ray crystallography, undergoes partial ring opening to a monocyclic silacyclobutene 2 in the presence of HCl, but is thermally stable up to at least 180 °C.

Syntheses and Molecular Structures of Liquid Pyrophoric Hydridosilanes

Trisilane, isotetrasilane, neopentasilane, and cyclohexasilane have been prepared in gram scale. In-situ cryo crystallization of these pyrophoric liquids in sealed capillaries on the diffractometer allows access to the single crystal structures of these compounds. Structural parameters are discussed and compared to gas-phase electron diffraction structures from literature and with the results from quantum chemical calculations. Significantly higher packing indices are found for the silanes compared to the corresponding alkanes. Radiation with ultraviolet light (365 nm) and parallel ESR (EPR) measurement shows that cyclohexasilane is easily split into radicals, which subsequently leads to the formation of branched and chain-like oligomers. The other compounds form no radicals under these conditions. NMR spectra of all four compounds have been recorded.

Disilane Cleavage with Selected Alkali and Alkaline Earth Metal Salts

The industry-scale production of methylchloromonosilanes in the Müller-Rochow Direct Process is accompanied by the formation of a residue, the direct process residue (DPR), comprised of disilanes Me_n Si₂ Cl_6-n (n=1-6). Great research efforts have been devoted to the recycling of these disilanes into monosilanes to allow reintroduction into the siloxane production chain. In this work, disilane cleavage by using alkali and alkaline earth metal salts is reported. The reaction with metal hydrides, in particular lithium hydride (LiH), leads to efficient reduction of chlorine containing disilanes but also induces disproportionation into mono- and oligosilanes. Alkali and alkaline earth chlorides, formed in the course of the reduction, specifically induce disproportionation of highly chlorinated disilanes, whereas highly methylated disilanes (n>3) remain unreacted. Nearly quantitative DPR conversion into monosilanes was achieved by using concentrated HCl/ether solutions in the presence of lithium chloride.

Orthophosphate and Sulfate Utilization for C-E (E = P, S) Bond Formation via Trichlorosilyl Phosphide and Sulfide Anions

Reduction of phosphoric acid (H₃PO₄) or tetra- n-butylammonium bisulfate ([TBA][HSO₄]) with trichlorosilane leads to the formation of the bis(trichlorosilyl)phosphide ([P(SiCl₃)₂]^-, 1) and trichlorosilylsulfide ([Cl₃SiS]^-, 2) anions, respectively. Balanced equations for the formation of the TBA salts of anions 1 and 2 were formulated based on the identification of hexachlorodisiloxane and hydrogen gas as byproducts arising from these reductive processes: i) [H₂PO₄]^- + 10HSiCl₃ → 1 + 4O(SiCl₃)₂ + 6H₂ for P and ii) [HSO₄]^- + 9HSiCl₃ → 2 + 4O(SiCl₃)₂ + 5H₂ for S. Hydrogen gas was identified by its subsequent use to hydrogenate an alkene ((-)-terpinen-4-ol) using Crabtree's catalyst ([(COD)Ir(py)(PCy₃)][PF₆], COD = 1,5-cyclooctadiene, py = pyridine, Cy = cyclohexyl). Phosphide 1 was generated in situ by the reaction of phosphoric acid and trichlorosilane and used to convert an alkyl chloride (1-chlorooctane) to the corresponding primary phosphine, which was isolated in 41% yield. Anion 1 was also prepared from [TBA][H₂PO₄] and isolated in 62% yield on a gram scale. Treatment of [TBA]1 with an excess of benzyl chloride leads to the formation of tetrabenzylphosphonium chloride, which was isolated in 61% yield. Sulfide 2 was used as a thionation reagent, converting benzophenone to thiobenzophenone in 62% yield. It also converted benzyl bromide to benzyl mercaptan in 55% yield. The TBA salt of trimetaphosphate ([TBA]₃[P₃O₉]·2H₂O), also a precursor to anion 1, was found to react with either trichlorosilane or silicon(IV) chloride to provide bis(trimetaphosphate)silicate, [TBA]₂[Si(P₃O₉)₂], characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. Trichlorosilane reduction of [TBA]₂[Si(P₃O₉)₂] also provided anion 1. The electronic structures of 1 and 2 were investigated using a suite of theoretical methods; the computational studies suggest that the trichlorosilyl ligand is a good π-acceptor and forms σ-bonds with a high degree of s character.