Stereoisomers of 1,3,5,7-Tetrahydroxy-1,3,5,7-tetraisopropylcyclotetrasiloxane: Synthesis and Structures in the Crystal

Stereoselective synthesis of different cyclic tetrasiloxane isomers depending on the superacid catalyst employed

In this study, we investigated the hydrolytic condensation of 3-aminopropyldiethoxymethylsilane over different superacid catalysts. We found that cyclic tetrasiloxanes with different stereostructures (Am-CyTS-NNf2 and Am-CyTS-NHf2) could be selectively prepared in high yields (>95%) depending on the superacid catalyst employed (bis(nonafluorobutanesulfonyl)imide or cyclohexafluoropropane-1,3-bis(sulfonyl)imide). The single-crystal X-ray structural analyses of compounds in which amino groups of Am-CyTS-NNf2 and Am-CyTS-NHf2 were protected by the tert-butoxycarbonyl group revealed the formation of all-cis and cis-trans-cis cyclic tetrasiloxanes, respectively.

Thiolated Janus Silsesquioxane Tetrapod: New Precursors for Functional Materials

Herein, we report synthetic strategies for the development of a bifunctional Janus T₄ tetrapod (Janus ring), in which the orthogonal silsesquioxane and organic faces are independently functionalized. An all-cis T₄ tetrasilanolate was functionalized to introduce thiol moieties on the silsesquioxane face and naphthyl groups on the organic face to introduce luminescent and self-organization properties. The stepwise synthesis conditions required to prepare such perfectly defined oligomers via a suite of well-defined intermediates and to avoid polymerization or reactions over all eight positions of the tetrapod are explored via ²⁹Si, ¹³C and ¹H NMR, FTIR and TOF-ESI mass spectroscopy. To the best of our knowledge, this is one of the few reports of Janus T₄ tetrapods, with different functional groups located on both faces of the molecule, thus expanding the potential range of applications for these versatile precursors.

Well-defined cyclic silanol derivatives

Cyclic silanol derivatives (CSDs), possessing siloxane rings consisting of T-unit silicon and oxygen atoms, are considered efficient precursors for the preparation of versatile well-defined building blocks of hybrid materials such as cyclic, cage- or ladder-type SQs. This review provides an outline of the main synthetic routes to numerous stereoregular CSDs with different sizes of siloxane rings since the first example of CSDs reported by Brown et al. in 1965. The typical reaction conditions and chemical shifts of ²⁹Si NMR of all mentioned CSDs in this review are summarized in tables and schemes to recapitulate the state of the art. The synthesis of all-cis-cyclotetrasiloxanes (T₄), the most investigated CSDs, and their functionalization by different organic reactions to access various all-cis-T₄ with functional groups are methodically presented. Moreover, the potential of CSDs in multiple application fields is discussed to show the possible research directions of this family of compounds in the future.

Alkoxy- and Silanol-Functionalized Cage-Type Oligosiloxanes as Molecular Building Blocks to Construct Nanoporous Materials

Siloxane-based materials have a wide range of applications. Cage-type oligosiloxanes have attracted significant attention as molecular building blocks to construct novel siloxane-based nanoporous materials with promising applications such as in catalysis and adsorption. This paper reviews recent progress in the preparation of siloxane-based nanoporous materials using alkoxy- and silanol-functionalized cage siloxanes. The arrangement of cage siloxanes units is controlled by various methods, including amphiphilic self-assembly, hydrogen bonding of silanol groups, and regioselective functionalization, toward the preparation of ordered nanoporous siloxane-based materials.

Stereoisomerization of Cyclic Silanols

Stereoisomerization of readily available all-cis-1,3,5,7-tetrahydroxy-1,3,5,7-tetraisobutylcyclotetrasiloxane (1 a) was carried out under acidic conditions to afford cis-trans-cis (1 b), all-trans (1 c), and cis-cis-trans (1 d) isomers. The compounds in the reaction mixture could be easily separated into 1 a and a mixture of 1 b, 1 c, and 1 d by the treatment with chloroform. Compounds 1 b, 1 c, and 1 d were further separated and isolated, and each structure was identified. The experimental results indicated that the most plausible mechanism is a substitution reaction at the silicon center via a pentacoordinate intermediate without a cyclic siloxane bond cleavage reaction. The obtained isomer 1 a or 1 b further reacted with dichlorodiphenylsilane in the presence of triethylamine to give syn-type laddersiloxane (2 a) or anti-type laddersiloxane (2 b), respectively.

Janus-Cube Octasilsesquioxane: Facile Synthesis and Structure Elucidation

A perfect "Janus-cube" octasilsesquioxane, a nanometer-scale Janus particle with two different types of substituents, was synthesized through the cross-coupling of a "half-cube" cyclic sodium siloxanolate with another half-cube cyclic fluorosiloxane. The structure was confirmed by X-ray crystallography to be a Janus cube. The overall synthesis is simple and does not require drastic separation methods compared with previous methods. The synthesis of the Janus cube demonstrates a novel siloxane bond-forming reaction involving the coupling a silanol salt and fluorosilane. The reaction is mild, does not result in acid generation, and could be applied to the construction of other novel siloxane compounds.

Stereochemically Distinct Cyclotetrasiloxanes Containing 3-Pyridyl Moieties and Their Functional Coordination Polymers

Synthesis of new cyclotetrasiloxane scaffolds containing peripherally functionalized 3-pyridyl moieties, [MeSiO(CH═CH(3)Py)]4 (L(1)) and [MeSiO(CH2CH2(3)Py)]4 (L(2)), and their reactivity studies with certain d(10) metal ions are reported. The ligand L(1) is obtained by the Heck-coupling reaction of tetramethyl tetravinyl tetrasiloxane (D4(vi)) and 3-bromopyridine in the presence of the Pd(0) catalysts. The as-synthesized ligand L(1) shows the presence of three stereoisomers, cis-trans-cis (L(1A)), cis-cis-trans (L(1B)), and all-trans (L(1C)), which are quantitatively separated by column chromatography. Subsequent reduction of L(1A), L(1B), and L(1C) with triethylsilane in the presence of catalytic amounts of Pd/C leads to the formation of the ligands L(2A), L(2B), and L(2C) with retention of stereochemistry due to the precursor moieties. Treatment of ZnI2 with L(1A) gives a one-dimensional coordination framework [(L(1A))4(ZnI2)2]∞, 1. These 1D-chains are further connected by π-π stacking interactions between the pyridyl groups of the adjacent chains leading to the formation of a three-dimensional network with the topology of a PtS net. The reaction of silver nitrate with ligand L(1B) gives a chain like one-dimensional cationic coordination polymer {[(L(1B))4Ag2]·2NO3·H2O·CH3OH }∞, 2, consisting of two different kinds of 32-membered macrocycles. Treatment of the all-trans ligand L(2C) with copper(I) iodide salt results in the formation of a cubane-type Cu4I4 cluster MOF [(L(2C))4Cu4I4]∞, 3, in a two-dimensional 4-connected uninodal sql/Shubnikov tetragonal plane net topology represented by the Schläfli symbol {4(4).6(2)}. This MOF displays a thermochromic luminescence behavior due to Cu4I4 clusters showing an orange emission at 298 K and a blue emission at 77 K.

Synthesis of silanol-rich long-life polysilsesquioxane liquids by cosolvent-free hydrolytic polycondensation of organotrimethoxysilanes followed by aging

Silanol-rich polysilsesquioxane liquids with long-term viscosity stability are formed in a facile manner from organotrimethoxysilane–water binary systems via aging.

A hydrogen bonded molecular capsule versus a 3D network of tripodal organopolysilanols

1,3,5-Triethylbenzene based tripodal trisilanols were synthesized. X-ray, NMR and ESI-mass spectra disclosed the first example of an organotrisilanol molecular capsule based on a 1,3,5-triethylbenzene scaffold which showed encapsulation of H₂O or halogen anions in both the crystalline and solution states.

Synthesis of a multifunctional alkoxysiloxane oligomer

The siloxane bonds of oligomer 1 were not cleaved by hydrolysis, indicating the suitability of 1 for controlled hybrid preparation.

Nonacyclic Ladder Silsesquioxanes and Spectral Features of Ladder Polysilsesquioxanes

Laddersiloxanes, that is, ladder silsesquioxanes with defined structures, could be obtained by stepwise synthesis starting from cyclic silanols. These compounds were shown to have high thermal stability. As an extension of the previous work, the first nonacyclic ladder silsesquioxanes were synthesized by the reaction of bicyclic silanol with tricyclic tetrachloride, which were obtained from cyclic silanols. The structure was confirmed by spectral measurements, and the spectral features of a series of ladder polysilsesquioxanes with determined structures were analyzed.

Solvent-Free Synthesis of Siloxanes and their Usage as Potential Cross-Coupling Reagents

Hexaarylcyclotrisiloxanes were selectively prepared by solvent-free synthesis from diarylsilanediol. These well-known stable compounds were found to undergo palladium-catalyzed cross-coupling reaction with aryl halides in good yields. All of attached aryl groups on silicon atom could be utilized. Furthermore, base-sensitive functionality such as acetyl and nitro groups survived the reaction.

Structural properties and antibacterial effects of hydrophobic and oleophobic sol-gel coatings for cotton fabrics

In a continuation of previous studies, the wetting properties of the hydrophobic diureapropyltriethoxysilane [bis(aminopropyl)-terminated polydimethylsiloxane (1000)] (PDMSU) sol-gel hybrid, which forms washing-resistant water-repellent finishes on cotton fabrics, were further investigated. The addition of 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES) to PDMSU resulted in a highly apolar low-energy surface on aluminum with gammaStotal equal to 14.5 mJ/m2 and a DetlaGiwi value of -82 mJ/m2. Mixed PFOTES-PDMSU finishes applied on cotton fabrics increased the water contact angles (thetaw) from approximately 130 degrees (PDMSU) to 147 degrees, also imparting oleophobicity (thetadiiodomethane=130 degrees, thetan-hexadecane=120 degrees) to the finished cotton fabrics. Washing caused breakage of the coating's integrity as established from SEM, which was attributed to the partial removal of PFOTES from the composite films, also shown by subtractive IR attenuated total reflectance (ATR) and XPS spectral measurements made on washed and unwashed fabrics. The antibacterial properties of the PFOTES-PDMSU-finished fabrics were assessed with the transfer method (EN ISO 20743:2007), revealing that the reduction of Escherichia coli bacteria on unwashed cotton fabrics was nearly 100%. Moreover, for washed (10 times) cotton fabrics a much higher bacterial reduction was noted for the PFOTES-PDMSU finishes (60.6+/-10.8%), surpassing PDMSU (30.4+/-6.1%) and commercial fluoroalkoxysilane (FAS) (21.9+/-5.7%) finishes. The structure of PFOTES-PDMSU gels, xerogels, and the corresponding coatings was investigated by analyzing the 29Si NMR and IR ATR spectra and comparing them with the spectra of PFOTES and octameric (T8) PFOTES based polyhedra. The results revealed the tendency of PFOTES to condense in octameric silsesquioxane polyhedra (T8), coexisting in the PDMSU sol-gel network with cyclic tetramers (T4(OH)4) and open cube-like species (T7(OH)3). The presence of -OH-functionalized PFOTES silsesquioxanes, established even in coatings heat-treated at 140 degrees C (15 min), also explained the excellent washing fastness of PFOTES finishes on cotton fabrics. The regenerative nature of the water- and oil-repellent properties of washed PFOTES-PDMSU-finished cotton fabrics was attributed to the surface mobility of the T8 PFOTES based polyhedra, ousted from the coating interior during consecutive washings.

Siloxanes with Pendent Naphthalene Diimides: Synthesis and Fluorescence Quenching

Cyclic siloxanes with pendent naphthalene diimide groups were synthesized via hydrosilylation to form amorphous electron-accepting compounds. Photophysical measurements and >99.9% fluorescence quenching of well-known p-type polymers by the siloxanes demonstrate that these siloxanes form a new class of highly efficient n-type materials that provide some control over intermolecular interactions.