Lithium-Templated Formation of Polyhedral Oligomeric Silsesquioxanes (POSS).

A coordination complex, lithium hepta(i-butyl)silsesquioxane trisilanolate (1; Li-T7), a stable intermediate in silsesquioxane (SQ) syntheses, was successfully isolated in 65% yield and found to be highly soluble in nonpolar solvents such as hexane. The structure of Li-T7 was confirmed by NMR, IR spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry, electrospray ionization mass spectrometry, and computational simulation, providing detailed elucidation of the intermolecular self-association of the SQ cage with a box-shaped Li6O6 polyhedron through strong coordination bonds. After acid treatment, Li-T7 undergoes lithium-proton cationic exchange, yielding hepta(i-butyl)silsesquioxane trisilanol (2; H-T7) quantitatively. The high yield of H-T7 seems to be influenced by Li-O bonding in the Li-T7 complex that affects the selective formation of hepta(i-butyl)silsesquioxane trisilanolate and the bulky i-butyl groups which may prevent decomposition or SQ cage-rearrangement even at reflux under alkaline conditions. Single-crystal X-ray crystallography confirms the presence of the dumbbell-shaped SQ partial cages through strong intermolecular hydrogen bonds. Interestingly, lowering the polarity of the reaction solution by adding dichloromethane results in formation of the cubic octa(i-butyl)silsesquioxane (3; T8) cage in a good yield (47%), which is isolated by crystallization from the reaction solution.

Polymerization of ε-Caprolactone Using Bis(phenoxy)-amine Aluminum Complex: Deactivation by Lactide.

Polymerizations of biodegradable lactide and lactones have been the subjects of intense research during the past decade. They can be polymerized/copolymerized effectively by several catalyst systems. With bis(phenolate)-amine aluminum complex, we have shown for the first time that lactide monomer can deactivate the aluminum complex during the ongoing polymerization of ε-caprolactone to a complete stop. After hours of dormant state, the aluminum complex can be reactivated again by heating at 100 °C without the addition of any external chemicals still giving polymer with narrow dispersity. Studies using NMR, in situ FTIR, and single-crystal X-ray crystallography indicated that the coordination of the carbonyl group in lactyl unit was responsible for the unusual behavior of lactide. In addition, the unusual methyl-migration from methyl lactate ligand to the amine side chain of the aluminum complex was observed through intermolecular nucleophilic-attack mechanism.

Synthesis and isolation of methacrylate- and acrylate-functionalized polyhedral oligomeric silsesquioxanes (T8, T10, and T12) and characterization of the relationship between their chemical structures and physical properties.

Novel organic-inorganic hybrid nanobuilding blocks of methacrylate- and acrylate-functionalized polyhedral oligomeric silsesquioxanes were easily prepared via nucleophilic substitution on octakis(3-chloropropyl)octasilsesquioxane, using sodium methacrylate and sodium acrylate, respectively. From a practical standpoint, these cage-rearranged silsesquioxanes (T8, T10, and T12) could be readily isolated in their pure form with conventional silica gel column chromatography. Octakis(3-propyl methacrylate)octasilsesquioxane (T8) is a colorless, crystalline solid with a melting point of 66.7-67.2 °C, while other cage products are colorless viscous liquids at room temperature. Moreover, we report that the chemical structure/physical property relationship of silsesquioxane cages not only is dependent on the symmetry of the inorganic silsesquioxane core at a given temperature but also is dictated by the organic substituent mobility. Structures of the products were confirmed by (1)H, (13)C, and (29)Si NMR spectroscopy and high resolution electrospray ionization mass spectrometry analysis.

Ultrafast synthesis of silicone elastomers using silsesquioxane cages as crosslinkers.

Cross-linked siloxane/silsesquioxane-based elastomers were easily prepared in 15 min via the anionic ring-opening polymerization of cyclotetrasiloxane (D4) and a polyhedral oligomeric silsesquioxane, using K2CO3 as a catalytic base in dimethylformamide at 70 °C. The resulting silicone elastomers have high mechanical strength, good thermal stability, and good superhydrophobic properties.

Synthesis and reactivity of nitrogen nucleophiles-induced cage-rearrangement silsesquioxanes.

Novel phthalimide and o-sulfobenzimide-functionalized silsesquioxanes were successfully synthesized via nucleophilic substitution reactions from octakis(3-chloropropyl)octasilsesquioxane. Surprisingly, the formation of deca- and dodecasilsesquioxanes cages was discovered during substitution with phthalimide, but only octasilsesquioxane maintained a cage in the o-sulfobenzimide substitution reaction. Moreover, we report the electronic effect of nitrogen nucleophiles to promote cage-rearrangement of inorganic silsesquioxane core for the first time. Structures of products were confirmed by (1)H, (13)C, and (29)Si NMR spectroscopy, ESI-MS analysis, and single-crystal X-ray diffraction.

Sonochemical synthesis of microscale Zn(ii)-MOF with dual Lewis basic sites for fluorescent turn-on detection of Al3+ and methanol with low detection limits.

The effects of synthetic methods, modulator types, the content of the modulator, and reaction time on the size and morphology of a microscale Zn-MOF containing dual Lewis basic amino and carbonyl groups, namely [Zn(NH2-bdc)(4,4'-bpy)] {NH2-H2bdc = 2-amino-1,4-benzenedicarboxylic acid and 4,4'-bpy = 4,4'-bipyridine}, were systemically investigated. Uniform octahedral microparticles of Zn-MOF with an average size of 1.7 µm were obtained within an hour at ambient temperature under ultrasound irradiation using sodium acetate as a modulating agent. Interestingly, Zn-MOF has been demonstrated for the dual-functional fluorescent detection of Al3+ and methanol based on a fluorescent turn-on strategy. Very low detection limits (LODs) for Al3+ and methanol were achieved using microcrystalline Zn-MOF and reached 30 nM and 0.7% (v/v), respectively. In comparison to the larger size of the polycrystalline Zn-MOF, the microparticles obtained from the ultrasonic route exhibited an improvement in the detection sensitivity. The dual uncoordinated Lewis basic sites play an important role in the sensitivity and selectivity of the detection. Additionally, the fluorogenic change in the sensing process can be observed by the naked eye under UV-light, allowing preliminary on-field screening.

Anion identification using silsesquioxane cages.

Anthracene-conjugated octameric silsesquioxane (AnSQ) cages, prepared via Heck coupling between octavinylsilsesquioxane (OVS) and 9-bromoanthracene, thermodynamically display intramolecular excimer emissions. More importantly, these hosts are sensitive to each anionic guest, thereby resulting in change of anthracene excimer formation, displaying the solvent-dependent fluorescence and allowing us to distinguish up to four ions such as F-, OH-, CN- and PO4 3- by fluorescence spectroscopy. Depending on the solvent polarity, for example, both F- and CN- quenched the fluorescence emission intensity in THF, but only F- could enhance the fluorescence in all other solvents. The presence of PO4 3- results in fluorescence enhancements in high polarity solvents such as DMSO, DMF, and acetone, while OH- induces enhancements only in low polarity solvents (e.g. DCM and toluene). A picture of the anion recognizing ability of AnSQ was obtained through principal component analysis (PCA) with NMR and FTIR confirming the presence of host-guest interactions. Computational modeling studies demonstrate the conformation of host-guest complexation and also the change of excimer formation. Detection of F-, CN- and OH- by AnSQ hosts in THF is noticeable with the naked eye, as indicated by strong color changes arising from charge transfer complex formation upon anion addition.

Silsesquioxane cages as fluoride sensors.

Pyrene functionalized silsesquioxane cages (PySQ) not only provide significant fluorescence from pyrene-pyrene excimers with a very large Stokes shift (Δλ = 143 nm, 69 930 cm-1) in DMSO but also exhibit fluoride capture results coincidentally with a π-π* fluorescence enhancement. On the other hand, PySQ-F- in THF significantly exhibits π-π* fluorescence quenching and a color change can be observed with the naked eye from light yellow to deep orange by forming a charge-transfer (CT) complex among the pyrenyl rings. Moreover, PySQ selectively captures F- with a response time of <2 min and with a very low detection limit (1.61 ppb), while 19F NMR is used to confirm encapsulation of F- with Δδ = 19 ppm.

Tunable Porosity of Cross-Linked-Polyhedral Oligomeric Silsesquioxane Supports for Palladium-Catalyzed Aerobic Alcohol Oxidation in Water.

Polyhedral oligomeric silsesquioxane (POSS)-based materials, poly-POSS-Tn [n = 8 (1), 10 (2), 12 (3), and mix (4)], were prepared in high yields via free radical polymerization of corresponding pure forms of methacrylate-functionalized POSS monomers, MMA-POSS-Tn (n = 8, 10, 12), and the mixture form, MMA-POSS-Tmix. Powder X-ray diffraction (XRD) spectra and BET analysis indicate that 1-4 are amorphous materials with high surface areas (683-839 m2 g-1). The surface areas and total pore volumes follow the trend: poly-POSS-T12 > poly-POSS-T10 > poly-POSS-Tmix > poly-POSS-T8. In addition, on the basis of Barrett-Joyner-Halenda (BJH) analysis, poly-POSS-T12 contains the highest amount of mesopores. The Pd nanoparticles immobilized on poly-POSS-Tn [n = 8 (5), 10 (6), 12 (7), and mix (8)] are well dispersed with 4-6 wt % Pd content and similar average particle sizes of 6.2-6.5 nm, according to transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDX) and microwave plasma-atomic emission spectroscopy (MP-AES). At 90 °C, the stabilized Pd nanoparticles in 5-8 catalyzed aerobic oxidation of benzyl alcohol to benzaldehyde in 72-100% yields at 6 h using a mixture of a H2O/Pluronic (P123) solution. The PdNp@poly-POSS-T8 catalyst (5) exhibited the lowest catalytic activity, as a result of its lowest surface areas, total pore volumes, and amounts of mesopores. With the catalyst 8, various benzyl alcohol derivatives were converted to the corresponding aldehydes in good to excellent yields. However, with alcoholic substrates featuring electron-withdrawing substituents, high conversions were achieved with 1 equiv of K2CO3 additive and longer reaction times.

Synthesis and isolation of non-chromophore cage-rearranged silsesquioxanes from base-catalyzed reactions.

The nucleophilicity of both ortho- and meta-nitrophenolate anions is strong enough to give substituted products, but their basicity also facilitates cage-rearrangement reactions in polyhedral oligomeric silsesquioxanes (POSS). Anions having a stronger basicity, but weaker nucleophilicity, such as CO32-, gave products only from cage-rearrangement, with the cage expansion products being isolable in multi-gram quantities using conventional column chromatography.

Synthesis of aromatic functionalized cage-rearranged silsesquioxanes (T8, T10, and T12) via nucleophilic substitution reactions.

Organic-inorganic hybrid nano-building blocks of aromatic nitro-, aldehyde-, and bromo-functionalized polyhedral oligomeric silsesquioxanes were easily prepared through nucleophilic substitutions, starting from the reactions between octakis(3-chloropropyl)octasilsesquioxane and phenoxide derivatives. These phenoxide anions not only supply the substitution functions to a silsesquioxane cage, but can also induce a cage-rearrangement leading to the formation of octa-, deca-, and dodecahedral silsesquioxane cages.

One-pot synthesis of halogen exchanged silsesquioxanes: octakis(3-bromopropyl)octasilsesquioxane and octakis(3-iodopropyl)octasilsesquioxane.

Metal halides, solvent effects, phase transfer catalysts, alkylating agent and reaction times were found to have important roles to complete halogen exchange reactions in "one pot" synthesis, starting from octakis(3-chloropropyl)octasilsesquioxane to obtain more reactive halide compounds: octakis(3-bromopropyl)octasilsesquioxane and octakis(3-iodopropyl)octasilsesquioxane. To confirm the complete halogen exchange, the desired products were characterized by (1)H, (13)C and (29)Si NMR spectroscopy, ESI-MS, elemental analysis and single-crystal X-ray diffraction analysis.

Reversible self-assembly of dendrimer based on polyhedral oligomeric silsesquioxanes (POSS).

An acid-base switchable dendritic complex was constructed by self-assembly between dibenzo-24-crown-8 terminated T(10)-POSS dendrimer and dibenzylammonium hexafluorophosphate salt based on T(8)-POSS. The formation and its threading-dethreading property were characterized by (1)H NMR and UV-visible absorption spectroscopy.

Synthesis and characterization of core-shell type Fe3O4 nanoparticles in poly(organosilsesquioxane).

The core-shell type Fe(3)O(4) nanoparticles in poly(organosilsesquioxane) (Fe(3)O(4)@OcTS) were prepared by one-pot synthesis using reverse micelle method. The as-prepared ferrofluid droplets with average size 4-15 nm were in situ encapsulated via polycondensation of molecularly self-assembled octenyltrimethoxysilane (OcTS). The dynamic light scattering and transmission electron microscopy investigations on coated magnetite nanoparticles revealed uniform size of spherical shape and having thin and transparent shells. These nanoparticles showed redispersibility in non-polar solvents without agglomerations due to coated by a layer of hydrophobic shells. The thermogravimetric analysis and infrared spectroscopy suggested the existence of core-shell type. X-ray diffraction confirmed magnetite cores. The superconducting quantum interference device (SQUID) showed that they exhibited superparamagnetic behavior at 300 K and ferromagnetic at 3 K.

Synthesis and characterization of highly pure azido-functionalized polyhedral oligomeric silsesquioxanes (POSS).

Novel azido-functionalized POSS were synthesized, which could serve as nanoprecursors for higher-order molecular construction of organic-inorganic hybrids via"click" chemistry.