The chemistry of organo silanetriols

Silsesquioxane Cages under Solvent Regimen: The Influence of the Solvent on the Hydrolysis and Condensation of Alkoxysilane

This study investigates the formation mechanisms of oligomeric phenyl silanols, focusing on polyhedral oligomeric silsesquioxane (POSS) and double-decker silsesquioxane (DDSQ) derivatives. Combining literature reports and crystal structures of solvated derivatives obtained in our laboratory, we show that the solvent choice significantly influences their structures. POSS-based silanols prefer aprotic solvents like THF, preserving dimerization, while double-deckers form stable architectures in protic solvents like isopropanol. This discrepancy arises from different stabilization mechanisms. Our findings enhance our understanding of hydrolytic condensation involving trimethoxyphenylsilane and suggest aprotic solvents for efficient reactions with POSS-based silanols.

Oligo-Condensation Reactions of Silanediols with Conservation of Solid-State-Structural Features

Oligo- and polysiloxanes are usually prepared by condensation reactions in solvents without control of stereochemistry. Here we present a solventless thermal condensation of stable organosilanols. We investigated the condensation reactions of organosilanediols with different organic substituents, having in common at least one aromatic group. The condensation kinetics of the precursors observed by NMR spectroscopy revealed a strong dependence on temperature, time, and substitution pattern at the silicon atom. SEC measurements showed that chain length increases with increasing condensation temperature and time and lower steric demand of the substituents, which also influences the glass transition temperatures (Tg) of the resulting oligo- or polymers. X-ray diffraction studies of the crystalline silanediols and their condensation products revealed a structural correlation between the substituent location in the crystalline precursors and the formed macromolecules induced by the hydrogen bonding pattern. In certain cases, it is possible to carry out topotactic polymerization in the solid-state, which has its origin in the crystal structure.

Synthesis of SiN/SiS-heterocycles via the reactions of a bis-silylene with isocyanate/isothiocyanate molecules

Reactions of o-carborane-fused bis-silylene 1 with isocyanate/isothiocyanate molecules furnished a series of SiN/SiS-heterocycles, which show distinct styles of cyclization and were theoretically studied.

Facile preparation of organosilanes from benzylboronates and gem-diborylalkanes mediated by KOtBu

Methods to efficiently synthesize organosilanes are valuable in the fields of synthetic chemistry and materials science. During the past decades, boron conversion has become a generic and powerful approach for constructing carbon-carbon and other carbon-heteroatom bonds, but its potential application in forming carbon-silicon remains unexplored. Herein, we describe an alkoxide base-promoted deborylative silylation of benzylic organoboronates, geminal bis(boronates) or alkyltriboronates, allowing for straightforward access to synthetically valuable organosilanes. This selective deborylative methodology exhibits operational simplicity, broad substrate scope, excellent functional group compatibility and convenient scalability, providing an effective and complementary platform for the generation of diversified benzyl silanes and silylboronates. Detailed experimental results and calculated studies revealed an unusual mechanistic feature of this C-Si bond formation.

Photocatalytic C-Si Bond Formations Using Pentacoordinate Silylsilicates as Silyl Radical Precursors: Synthetic Tricks Using Old Reagents

A visible-light-induced photocatalytic C-Si formation strategy has been disclosed by uncovering the reactivity of Martin's spirosilane-derived pentacoordinate silylsilicates as silyl radical precursors. The hydrosilylation of a broad spectrum of alkenes and alkynes, as well as the C-H silylation of heteroarenes, has been demonstrated. Remarkably, Martin's spirosilane was stable and could be recovered via a simple workup process. Furthermore, the reaction proceeded well using water as the solvent or low-energy green LEDs as an alternative energy source.

Photocatalytic Generations of Secondary and Tertiary Silyl Radicals from Silylboranes Using an Alkoxide Cocatalyst

Silyl radicals are valuable species to prepare diverse organosilicon compounds. However, unlike stable tertiary silyl radicals, the use of secondary silyl radicals has been problematic in silylation reactions due to their instability. Here, we present photocatalytic in situ generations of both secondary and tertiary silyl radicals by one-electron oxidation of ate complexes, formed from silylboranes and an alkoxide cocatalyst, achieving highly efficient hydrosilylation and deuterosilylation of electron-rich alkenes and dienes as well as electron-deficient alkenes. The theoretical studies show that anionic borate complexes activated with an alkoxide have lower oxidation potentials than neutral borate complexes, allowing the formation of secondary silyl radicals. The calculated reaction pathways reveal that anionic conditions using the conjugate acid-base pair of NaOEt (cocatalyst) and EtOH (solvent) are the key to expanding the scope of silyl radicals and alkenes.

Thermally stable polyfluorinated monoalkoxysilanetriols and dialkoxydisiloxanetetrols

The reaction of the polyfluorinated lithium triarylmethanolates Ar₃COLi (Ar = C₆F₅, 3,5-(CF₃)₂C₆H₃) with SiCl₄ provided the monosubstituted products Ar₃COSiCl₃ (1a, Ar = C₆F₅; 1b, Ar = 3,5-(CF₃)₂C₆H₃). The hydrolysis of 1a and 1b produced the silanetriols Ar₃COSi(OH)₃ (2a, Ar = C₆F₅; 2b, Ar = 3,5-(CF₃)₂C₆H₃) without the aid of an HCl scavenger. The reaction of two equivalents of Ar₃COLi (Ar = C₆F₅, 3,5-(CF₃)₂C₆H₃) with (Cl₃Si)₂O afforded the disubstituted products [(C₆F₅)₃COSiCl₂]₂O (3a) and {[(3,5-(CF₃)₂C₆H₃)₃CO]SiCl₂}₂O (3b), the hydrolysis of which gave the corresponding disiloxanetetraols [(C₆F₅)₃COSi(OH)₂]₂O (4a) and [(3,5-(CF₃)₂C₆H₃)₃COSi(OH)₂]₂O (4b). At high concentrations in the presence of HCl, 2b undergoes controlled condensation to yield 4b. In the solid-state, 2a, 4a and 4b are mainly associated by hydrogen bonds of the type SiO-H⋯O(H)Si whereas the competing SiO-H⋯O(C)Si type was not observed.

Synthesis and Hydrogen-Bond Patterns of Aryl-Group Substituted Silanediols and -triols from Alkoxy- and Chlorosilanes

Organosilanols typically show a high condensation tendency and only exist as stable isolable molecules under very specific steric and electronic conditions at the silicon atom. In the present work, various novel representatives of this class of compounds were synthesized by hydrolysis of alkoxy‐ or chlorosilanes. Phenyl, 1‐naphthyl, and 9‐phenanthrenyl substituents at the silicon atom were applied to systematically study the influence of the aromatic substituents on the structure and reactivity of the compounds. Chemical shifts in ²⁹Si NMR spectroscopy in solution, correlated well with the expected electronic situation induced by the substitution pattern on the Si atom. ¹H NMR studies allowed the detection of strong intermolecular hydrogen bonds. Single‐crystal X‐ray structures of the alkoxides and the chlorosilanes are dominated by π‐π interactions of the aromatic systems, which are substituted by strong hydrogen bonding interactions representing various structural motifs in the respective silanol structures.

Trifunctional Silyl Groups as Anchoring Units in the Preparation of Luminescent Phosphole-Silica Hybrids

A synthetic strategy to β-silylphospholes with three methoxy, ethoxy, chloro, hydrido, or phenyl substituents at silicon has been developed, starting from trimethoxy, triethoxy, or triphenyl silyl substituted phenyl phosphanides and 1,4-diphenyl-1,3-butadiyne. These trifunctional silylphospholes were attached to the surface of uniform spheric silica particles (15 μm) and, for comparison, to a polyhedral silsesquioxane (POSS)-trisilanol as a molecular model to explore their luminescent properties in comparison with the free phospholes. Density functional theory calculations were performed to investigate any electronic perturbation of the phosphole system by the trifunctional silyl anchoring unit. For the immobilized phospholes, cross-polarization magic-angle-spinning NMR measurements (¹³C, ²⁹Si, and ³¹P) were carried out to explore the bonding situation to the silica surface. Thermogravimetric analysis and X-ray photoelectron spectroscopy measurements were performed to approximate the amount of phospholes covering the silica surface. Identity and purity of all novel phospholes have been established with standard techniques (multinuclear NMR, mass spectrometry, and elemental analysis) and X-ray diffraction for the POSS derivative.

On the Potential of Silicon as a Building Block for Life

Despite more than one hundred years of work on organosilicon chemistry, the basis for the plausibility of silicon-based life has never been systematically addressed nor objectively reviewed. We provide a comprehensive assessment of the possibility of silicon-based biochemistry, based on a review of what is known and what has been modeled, even including speculative work. We assess whether or not silicon chemistry meets the requirements for chemical diversity and reactivity as compared to carbon. To expand the possibility of plausible silicon biochemistry, we explore silicon's chemical complexity in diverse solvents found in planetary environments, including water, cryosolvents, and sulfuric acid. In no environment is a life based primarily around silicon chemistry a plausible option. We find that in a water-rich environment silicon's chemical capacity is highly limited due to ubiquitous silica formation; silicon can likely only be used as a rare and specialized heteroatom. Cryosolvents (e.g., liquid N2) provide extremely low solubility of all molecules, including organosilicons. Sulfuric acid, surprisingly, appears to be able to support a much larger diversity of organosilicon chemistry than water.

Sila-Peterson Reaction of Cyclic Silanides

Sila-Peterson type reactions of the 1,4,4-tris(trimethylsilyl)-1-metallooctamethylcyclohexasilanes (Me₃Si)₂Si₆Me₈(SiMe₃)M (2a, M = Li; 2b, M = K) with various ketones were investigated. The obtained products strongly depend on the nature of the ketone component. With 2-adamantanone 2a,b afforded the moderately stable silene 3. 3 is the first example of an Apeloig–Ishikawa–Oehme-type silene with the tricoordinate silicon atom incorporated into a cyclopolysilane framework and could be characterized by NMR and UV spectroscopy as well as by trapping reactions with water, methanol, and MeLi. The reaction of 2b with aromatic ketones also follows a sila-Peterson type mechanism with formation of carbanionic species. With 1,2-diphenylcyclopropenone 2b reacted by conjugate 1,4-addition to give a spirocyclic carbanion. In most cases the underlying reaction mechanism could be elucidated by the isolation and characterization of unstable intermediates and final products after proper derivatization.

Unusual Racemization of Tertiary P-Chiral Ferrocenyl Phosphines

Tertiary phosphines are generally known to withstand inversion under moderate conditions. In this work, a remarkable racemization process of three P-chiral ferrocenyl phosphines is reported. Subjected to conventional column chromatography as highly enantioenriched compounds, they greatly experienced racemization when collected at the column outlet within minutes. Initially, attention was drawn to this unusual inversion behavior after observing that the superb enantiomeric excess of these ligands (>95 % ee in all cases) was almost lost in their corresponding ruthenium(II) complexes. Successively excluding possible racemization causes, these P-chiral ferrocenyl phosphines were found to undergo a significant, acid-catalyzed racemization process at room temperature within a few minutes. This process is mainly observed during standard column chromatography by using conventional silica or alumina, but can also be triggered deliberately by addition of certain acids. Therefore, the stereochemical preservation of P-chiral phosphines during their purification may per se not always be guaranteed, since column chromatography is the most frequently used technique for purifying such types of compounds.

Comparative study of the influence of linear and branched alkyltrichlorosilanes on the removal efficiency of polyethylene and polypropylene-based microplastic particles from water

Microplastics are a global environmental pollution. Due to this fact, new solutions are needed to reduce the amount in various aquatic environments. A new concept introduced by Herbort and Schuhen from the year 2016 describes the agglomeration of microplastics in water using silicon-based precursors. In the study presented here, alkyltrichlorosilanes with different linear and branched alkyl groups and a chain length between 1 and 18 carbon (C-) atoms are investigated for their suitability to fix microplastics (mixtures of polyethylene (PE) and polypropylene (PP)) and to form larger agglomerates. As the alkyl group has a major influence on the reaction rate and agglomeration behavior, we present here the extensive data collection of the evaluation of the best case. The removal efficiency is determined gravimetrically. The reaction behavior and the fixation process are characterized by hydrolysis kinetics. ²⁹Si-MAS-NMR spectroscopy, IR spectroscopy, and thermogravimetry (TGA) are used to characterize the chemical composition of the agglomerates. Finally, the use of optical coherence tomography (OCT) allows the visualization of the formed agglomerates. The results show that the different alkyl groups have a strong impact on the suitability of the alkyltrichlorosilanes for the agglomeration, as they influence the hydrolysis and condensation kinetics in water and the affinity to the microplastics. Best suited for microplastic removal were intermediate chain length between 3 and 5 C-atoms.

Deposition of Cellulose-Based Thin Films on Flexible Substrates

This study investigates flexible (polyamide 6.6 PA-6.6, polyethylene terephthalate PET, Cu, Al, and Ni foils) and, for comparison, stiff substrates (silicon wafers and glass) differing in, for example, in surface free energy and surface roughness and their ability to host cellulose-based thin films. Trimethylsilyl cellulose (TMSC), a hydrophobic acid-labile cellulose derivative, was deposited on these substrates and subjected to spin coating. For all the synthetic polymer and metal substrates, rather homogenous films were obtained, where the thickness and the roughness of the films correlated with the substrate roughness and its surface free energy. A particular case was the TMSC layer on the copper foil, which exhibited superhydrophobicity caused by the microstructuring of the copper substrate. After the investigation of TMSC film formation, the conversion to cellulose using acidic vapors of HCl was attempted. While for the polymer foils, as well as for glass and silicon, rather homogenous and smooth cellulose films were obtained, for the metal foils, there is a competing reaction between the formation of metal chlorides and the generation of cellulose. We observed particles corresponding to the metal chlorides, while we could not detect any cellulose thin films after HCl treatment of the metal foils as proven by cross-section imaging using scanning electron microscopy (SEM).

Coordination Polymers based on the Neutral Ditopic Ligand (C6H4PO(OCH3)2)2 Involving some f-Block Elements

An improved synthesis using microwave heating affords (C6H4PO(OCH3)2)2 in excellent isolated yield (95 %). The ligand properties of this bisphosphonateester were explored towards hard metal centers M2+ (M = Ca, UO2) and M3+ (M = La, Ce, Sm, Eu) resulting in coordination polymers, for which the reduction of ionic size of the central metal atom resulted in lower-dimensional structural motifs as opposed to higher dimensional networks obtained for the larger ions. All coordination polymers were characterized by single-crystal X-ray diffraction, IR spectroscopy, and combustion analysis. The ligand was furthermore characterized with multinuclear NMR spectroscopy.

Si10Cu6N4 Cage Hexacoppersilsesquioxanes Containing N Ligands: Synthesis, Structure, and High Catalytic Activity in Peroxide Oxidations

The synthesis, composition, and catalytic properties of a new family of hexanuclear Cu(II)-based phenylsilsesquioxanes are described here. Structural studies of 17 synthesized compounds revealed the general principle underlying their molecular topology: viz., a central metal oxide layer consisting of two Cu₃ trimers is coordinated by two cyclic [PhSiO_1.5]₅ siloxanolate ligands to form a skewed sandwich architecture with the composition [(PhSiO_1.5)₁₀(CuO)₆]²⁺. In addition to this O ligation by the siloxanolate rings, two opposite copper ions are additionally coordinated by the nitrogen atoms of corresponding N ligand(s), such as 2,2'-bipyridine (compounds 1-9), 1,10-phenanthroline (compounds 10-13), mixed 1,10-phenanthroline/2,2'-bipyridine (compound 14), or bathophenanthroline (compounds 15-17). Finally, the charge balance is maintained by two HO^- (compounds 1-7, 10-13, and 15-17), two H₃CO^- (compound 8), or two CH₃COO^- (compounds 9 and 14) anions. Complexes 1 and 10 exhibited a high activity in the oxidative amidation oxidation of alcohols. Compounds 1, 10, and 15 are very efficient homogeneous catalysts in the oxidation of alkanes and alcohols with peroxides.

Building on Sub-Arctic Soil: Geopolymerization of Muskeg to a Densified Load-Bearing Composite

The marshy water-saturated soil typical of the sub-Arctic represents a considerable impediment to the construction of roads, thereby greatly hindering human habitation and geological excavation. Muskeg, the native water-laden topsoil characteristic of the North American sub-Arctic, represents a particularly vexing challenge for road construction. Muskeg must either be entirely excavated, or for direct construction on muskeg, a mix of partial excavation and gradual compaction with the strategic placement of filling materials must be performed. Here, we demonstrate a novel and entirely reversible geopolymerization method for reinforcing muskeg with wood fibers derived from native vegetation with the addition of inorganic silicate precursors and without the addition of extraneous metal precursors. A continuous siloxane network is formed that links together the muskeg, wood fibers, and added silicates yielding a load-bearing and low-subsidence composite. The geopolymerization approach developed here, based on catalyzed formation of a siloxane network with further incorporation of cellulose, allows for an increase of density as well as compressive strength while reducing the compressibility of the composite.

Reactivity of the Sterically Demanding Siloxanediol Mes2 Si(OH)(μ-O)Si(OH)Mes2 Towards Water and Ether Molecules

A series of isotopologues of the siloxanediol Mes₂ Si(OH)(μ-O)Si(OH)Mes₂ (3 a) (Mes=2,4,6-trimethylphenyl) were synthesized by reactions of the corresponding disiloxane precursors Mes₂ Si(μ-O)₂ SiMes₂ (2 a), Mes₂ Si(μ-¹⁷ O)₂ SiMes₂ (2 b) or Mes₂ Si(μ-¹⁸ O)₂ SiMes₂ (2 c) with an excess of H₂ O, H₂¹⁷ O or H₂¹⁸ O. NMR and IR signal assignments for the siloxanediols in benzene are supported by quantum-chemical calculations, which indicate small energy differences between trans and cis conformers, the latter of which exhibits an intramolecular hydrogen bond. ¹ H NMR as well as IR data suggest the presence of a mixture of both conformers in C₆ D₆ . Hydrogen-bonded adducts of Mes₂ Si(OH)(μ-O)Si(OH)Mes₂ with ethers such as diethylether, dimethoxyethane or dioxane were observed in the solid state, where they form polymeric chain-like structures. The latter appear to be stable only in the crystal. ¹⁷ O{¹ H} NMR and IR data in THF solution suggest an interaction of 3 a with at least one THF molecule, whereas diethylether appears not to interact. Water adducts form neither in solution nor in the solid state as indicated by NMR and ATR IR data. ¹⁷ O{¹ H} NMR and ESI-MS experiments illustrate the remarkably high stability of the siloxanediols towards water and show no evidence for intra- or intermolecular oxygen-exchange reactions. In marked contrast, a stepwise exchange of all three oxygen atoms-including the one in the Si-O-Si bridge-occurred in the gas phase, when [Mes₂ Si(¹⁸ OH)(μ-¹⁸ O)Si(¹⁸ O)Mes₂ ]^- was treated with H₂ O in the hexapole of an ESI FT-ICR mass spectrometer. The scrambling between the bridging and the other oxygen atoms likely proceeds through cyclic Si₂ O₂ intermediates.