Exploring the Mechanism for the Synthesis of Silsesquioxanes. 3. The Effect of Substituents and Water

Ultraviolet Light Catalyzed Gelation of 3-Methacryloxypropyltrimethoxysilane via Altered Silicate Spatial Structure

The gelation of 3-methacryloxypropyltrimethoxysilane (MAPTMS) is much more difficult to achieve in conventional conditions. This article describes a novel and concise approach to acquire transparent and firm hybrid gel material by one step promptly without photoinitiator or other tetraalkoxysilane. MAPTMS was hydrolyzed in acidified aqueous solution, which became homogeneous sol in 3 min, and then the sol was irradiated with UV light for a few minutes to form gel. The experimental results indicated that MAPTMS sol gelled in the presence of UV-irradiation was mainly attributed to altering Si-O-Si skeleton structure through hydroxyl radicals, and the gelation originated from the hydrolytic polycondensation of MAPTMS rather than the polymerization of methacryloxy substituent groups. The hydroxyl radicals could break the Si-O-Si ring structure to form cross-linker like species, and these cross-linkers chemically joined linear chains together to form the gel network. This investigation offers not only the photoinduced gelation strategy for MAPTMS sol but also the new insight into the effect of UV-irradiation on the sol-gel process of organotrialkoxysilanes.

D 5h [PhSiO1.5]10 synthesis via F− catalyzed rearrangement of [PhSiO1.5]n. An experimental/computational analysis of likely reaction pathways

Fluoride catalyzed rearrangement of PhSiO_1.5 favoring [PhSiO_1.5]₁₀.

A new, higher yielding synthetic route towards dodecaphenyl cage silsesquioxanes: synthesis and mechanistic insights

Cage dodecaphenylsilsesquioxane (T12-Phenyl) was synthesized in a one batch, mildly basic aqueous solution under room temperature conditions using a trialkoxysilane precursor. Significant improvements in synthetic yield (>95%) were observed compared with previous reports. Kinetic studies of the hydrolysis of phenyltrimethoxysilane were conducted and the condensation was monitored by (29)Si NMR which revealed the presence of a transient, intermediary T1 species as the pathway to dodecaphenylsilsesquioxane spherulites, and the tendency for T12 structures over T8, T10, and other substructures was explained through MM2 simulations.

Characterization and Some Insights into the Reaction Chemistry of Polymethylsilsesquioxane or Methyl Silicone Resins

Structural characterization of a polymethylsilsesquioxane (PMSQ) and a DT-type methyl silicone resin (MeDT) has been carried out by various instrumental analyses including GPC, NMR, gas chromatography, and gas chromatography-mass spectrometry. Although the PMSQ had aMwaround 5000, the resin contained a significant amount of low molecular weight species consisting of T2[MeSi(OH)O2/2] and T3[MeSiO3/2] units, ranging from to including many isomers. One isomer of was isolated of which structure was determined as a cage structure. The species are supposed to consist mainly of cyclotetra- and cyclopentasiloxanes, but presence of strained rings such as cyclotrisiloxane rings also was suggested. In MeDT, species in which the T2units in the molecules from PMSQ is replaced with D2[Me2SiO2/2] were found, for example, , suggesting that general silicone resins consist of similar structures as silsesquioxanes. The Mark-Houwink exponent for these methyl resins was~0.3, indicating the molecular shape to be compact. Investigation on the formation chemistry of the cubic octamers indicates that siloxane bond rearrangement is an important mechanism in the molecule build-up process.

Theoretical study of the mechanisms of the hydrolysis and condensation reactions of silicon and titanium alkoxides: similarities and differences

Stationary states for hydrolysis reactions in M(OCH(3))(4) + nH(2)O (M = Si, Ti; n = 1-3) systems are optimized at the B3LYP and MP2 levels with the Wachters basis set for titanium and the cc-pVDZ set for other atoms. Geometries of these states for M = Ti are characterized by trigonal bipyramidal (water molecules in front-side position) and octahedral coordination (for back-side position). Barrier heights for hydrolysis and condensation are substantially lower than those for silicon in keeping with experimental results. The lowering of the barrier heights on the addition of water molecules in the front-side position (reduction of hydrogen bond strain) exceeds that of the back-side addition (catalytic effect) for both M = Si and Ti, but the difference diminishes with n. The influence of oligomerization of titanium alkoxides on the rate of hydrolysis is studied on the model of the interaction of a Ti(2)(OCH(3))(8) dimer with one and two water molecules. It was shown that only terminal methoxy groups are exposed to hydrolysis and therefore the dimeric structure is retained in the process of the substitution of terminal methoxy groups. Barrier heights for terminal hydrolysis do not differ significantly from those of monomers. Barrier heights for condensation reactions obtained for the 2M(OMe)(n)(OH)(4-n) + H(2)O model system, are substantially (by ca. 10 kcal mol(-1)) lower for M = Ti and in both silicon and titanium species demonstrate a steady growth with n.

Role of structures with penta- and hexacoordinate silicon in the nucleophile-catalyzed hydrolysis of tetramethoxysilane

The mechanism of the base catalyzed hydrolysis of tetramethoxysilane (TMOS), proposed earlier on the basis of experimental data, is assessed by theoretical methods, i.e. MP2 and B3LYP with 6-31G(d) and Dunning correlation-consistent basis sets. Models considered involve one and two hydrolyzing water molecules attacking (MeO)(4)Si in the frontside position with a nucleophile (NH(3) and OH(-)) in the backside position. This approach allowed us to simulate uniformly the catalytic action of weak and strong bases. It was shown that the presence of a base in the backside position considerably lowers the activation barrier for hydrolysis. The inclusion of the additional water molecule which results in a substantial lowering of the barrier for uncatalyzed hydrolysis does not change noticeably the catalytic effect. In both one- and two-water molecules models the structure of the transition state in the presence of a nucleophile becomes nearly octahedral in which hexacoordinate silicon and four oxygens of the TMOS moiety have a planar arrangement.

Steric Stabilization of a Monomeric Proalumatrane: Experimental and Theoretical Studies

Treatment of tripodal tris(3-tert-butyl-2-hydroxy-5-methylbenzyl)amine (L) with 1 equiv of trimethylaluminum in toluene gave the stable proalumatrane (AlL) (1) [wherein L = tris(3-tert-butyl-5-methyl-2-oxidobenzyl)amine] featuring a distorted trigonal monopyramidal four-coordinate aluminum geometry. An analogous reaction uses the less sterically congested isomer of L, namely, tris(5-tert-butyl-2-hydroxy-3-methylbenzyl)amine provided dimeric (AlL')2 (2) [wherein L' = tris(5-tert-butyl-3-methyl-2-oxidobenzyl)amine], which contains two bridging alumatrane moieties possessing five-coordinate TBP aluminum geometries. Reaction of AlL with water provided the adduct H2O. AlL (3), a species that is representative of a coordinatively stabilized intermediate in the hydrolysis of an aluminum alkoxide. Theoretical calculations revealed that considerable stabilization energy is obtained by the coordination of a water molecule to the tetracoordinate aluminum in AlL and that this result is consistent with the postulate that the Lewis acidity of AlL exceeds that of boron trifluoride, despite the presence of the transannular N-->Al bond in AlL.

Supramolecular Silanol Chemistry in the Gas Phase. Topological (AIM) and Population (NBO) Analyses of Hydrogen-Bonded Complexes between H3SiOH and Selected O- and N-Acceptor Molecules

Hydrogen bonding of the type SiO-H...A (A = O, N) has been studied in the gas phase for simple H3SiOH.acceptor complexes with the acceptor molecules being O(H)SiH3, OH2, O(H)CH3, O(CH3)2, O(CH3)SiH3, O(SiH3)2, NH3, N(CH3)H2, N(CH3)2H, N(CH3)3, N(CH3)2C6H5, and NC5H5, respectively, at the B3LYP/6-311+(2d,p) level of theory, using Bader's atoms in molecules (AIM) and Weinhold's natural bond orbital (NBO) methodology. For all complexes (except H3SiOH.N(CH3)2C6H5) the complex energy Eadd. is a good estimate for the hydrogen bond energy EHB, which is generally higher in N-acceptor complexes (-5.52 to -7.17 kcal mol-1) than in O-acceptor complexes (-2.09 to -5.06 kcal mol-1). In case of H3SiOH.N(CH3)2C6H5, EHB and Eadd. differ by the energy associated with the loss of n(N)-->pi conjugation in N(CH3)2C6H5 upon complex formation. EHB shows no correlation with O...A distances and the red shifts Deltanu(OH) of the OH-stretching vibrations when different acceptors are compared, although both parameters are commonly used to estimate the strength of the hydrogen bond from spectroscopic and diffraction data. A good linear correlation of the hydrogen bond energy EHB has been established with parameters derived from the AIM and NBO analyses, namely, the electron densities rho(HA) and rho(OH) at the H...A and O-H bond critical points (BCPs) and the NLMO bond orders BONLMO(HA) of the H...A bonds of the H3SiOH.acceptor complexes as well as the change of natural charges DeltaqNPA(O) at the O-donor atom upon H3SiOH.acceptor complex formation. Hydrogen bonding of the type SiO-H...A (A = O, N) has been also studied in the related cyclic multiple H3SiOH.acceptor complexes (H3SiOH)3, (H3SiOH)2.NC5H5, and (H3SiOH)4, respectively, at the same level of theory. Cooperative hydrogen bonding is evident for all cyclic multiple H3SiOH.acceptor complexes, whereby the strongest concomitant strengthening of the hydrogen bonds is observed for (H3SiOH)4 and (H3SiOH)2.NC5H5.

Hydrolysis of Fluorosilanes: A Theoretical Study

Hydrolysis and condensation of simple trifluorosilanes, HSiF3 and MeSiF3, was studied by quantum mechanical methods. Hydrolysis of fluorosilanes is highly endothermic. The Gibbs free energy of the first reaction step in the gas phase is 31.4 kJ/mol, which corresponds to an equilibrium constant of 10(-6). Hydrolysis of the subsequent fluorine atoms in trifluorosilanes is thermodynamically more unfavorable than the first step of substitution. No significant difference in thermodynamics of hydrolysis was found between HSiF3 and MeSiF3. The activation energy for hydrolysis by a water dimer is significantly lower than that for hydrolysis by a water monomer. The former reaction is also less unfavorable thermodynamically, due to a high binding energy of the HF-H2O complex formed as a product of hydrolysis. Self-consistent reaction field (SCRF) calculations show that hydrolysis of trifluorosilanes in aqueous medium has lower activation energy than in the gas phase. It is also thermodynamically less unfavorable, due to better solvation of the products. Homofunctional condensation of HSiF2OH is thermodynamically favored. The equilibrium mixture for hydrolysis/condensation of RSiF3 in water is predicted to contain ca. 2.3% disiloxane (HF2Si)2O, if 100-fold excess of water relative to silane is assumed. Further hydrolysis of (HF2Si)2O is negligible. The thermodynamics of fluorosilane hydrolysis contrasts with that of chlorosilanes, where both hydrolysis and condensation are strongly favorable. Moreover, in the case of trichlorosilanes each subsequent hydrolysis step is more facile, leading to the product of full hydrolysis, RSi(OH)3.

Cages, Baskets, Ladders, and Tubes: Conformational Studies of Polyhedral Oligomeric Silsesquioxanes

The conformational flexibility of a series of cage, basket, ladder, and tube polyhedral oligomeric silsesquioxanes (POSS) has been examined using the Low Mode:Monte Carlo conformational search method in conjunction with the MM3/GBSA(CHCl3) surface. An ensemble of low energy structures was generated and used to explore the molecular shape and flexibility of each system. The results indicate that, except for the ladder molecule, the incompletely condensed systems that are studied are relatively rigid. Even in cases where the molecule is able to adopt numerous low energy conformations, the overall shape remains cage-like and the conformations differ only by small angles or substituent orientations. The ladder molecule is the most flexible and this ensemble clusters into two families: one that is cage-like and the other that is more open and ladder-like. The conformational flexibilities in the gas and solvent phases, as approximated using the GBSA continuum solvent model, are very similar.

Exploring the Mechanism for the Synthesis of Silsesquioxanes. 4. The Synthesis of T8

In an attempt to explore the mechanism for the synthesis of silsesquioxanes (POSS), the entire reaction processes from HSi(OH)3 to T8, one of the most common and stable POSS was investigated with electronic structure theory calculations, including electron correlation effects. In addition, the effect of some controlling factors, which play important roles in the reaction mechanism, is discussed.

Experimental and Computational Studies of Trialkylaluminum and Alkylaluminum Chloride Reactions with Silica

Reactions of trimethylaluminum, triethylaluminum, and diethylaluminum chloride and ethylaluminum dichloride with silica gel have been studied experimentally by infrared spectroscopy and elemental analysis. The silica gel was subjected to different pretreatments to alter surface functionalities prior to reaction. In all cases the extent of surface modification reaction follows the trend unmodified > 600 degrees C pretreated > hexamethyldisilazane (HMDZ) pretreated > 600 degrees C/HMDZ pretreated. All of the aluminum compounds studied completely react non-hydrogen-bonded silanols, while also reacting with hydrogen-bonded silanols and siloxanes. Primarily monomeric surface species result from the surface modification reaction. Ethylaluminum chlorides preferentially react with silanols through cleavage of the Al-C bond rather than the Al-Cl bond. Singly bonded Si(s)-O-AlCl(2) surface species are readily synthesized by reaction of ethylaluminum dichloride with HMDZ-pretreated silica gel. Bridged bonded (Si(s)-O)(2)-AlCl surface species are readily synthesized by reaction of diethylaluminum chloride with HMDZ-pretreated silica gel. Computational ab initio studies of the cluster Si(4)O(6)(OH)(4) as a model to study the reaction of monomeric and dimeric methylaluminum dichloride with a silica silanol are also described. Comparison of the potential energy surface (PES) of monomer and dimer indicates that the energetics favor monomer reaction, consistent with experimental results. The energy cost in the dimer reaction is primarily from cleavage of a bridged Al-Cl bond upon adsorption. This does not occur when the monomer adsorbs. A comparison of the PES for the two reaction pathways resulting from cleavage of either an Al-Cl or Al-C bond indicates that while the former reaction is slightly kinetically favored (E(a) = 23.1 kJ/mol for Al-Cl bond cleavage versus E(a) = 31.1 kJ/mol for Al-C bond cleavage), the latter is strongly thermodynamically favored with an overall free energy difference between the two reaction pathways of 135 kJ/mol favorable to Al-C bond cleavage. These reactions are thermodynamically controlled.

Water-Coordinated Neutral Silane Complex: A Frozen Intermediate of Hydrolysis of Alkoxysilanes

1-Aquo- and 1-ethyloxonio-5-carbasilatranes 2 and 3 were synthesized. X-ray crystallographic analysis revealed that 2 has a nearly ideal trigonal bipyramidal structure, in which the water molecule is located at the apical position. This water molecule is exchangeable with the other Lewis bases, such as H218O and HMPA. Theoretical calculations suggested that 2 would acquire the stabilization energy (20 kcal mol-1) by complexation. Deprotonation of 2 by pyridine and not by m-nitroaniline showed that the pKa value is smaller than that of free water, and that the water molecule is activated by complexation.

Experimental and Theoretical Evidence for Homogeneous Catalysis in the Gas-Phase Reaction of SiH2 with H2O (and D2O): A Combined Kinetic and Quantum Chemical Study

Time-resolved kinetic studies of the reaction of silylene, SiH(2), with H(2)O and with D(2)O have been carried out in the gas phase at 297 K and at 345 K, using laser flash photolysis to generate and monitor SiH(2). The reaction was studied independently as a function of H(2)O (or D(2)O) and SF(6) (bath gas) pressures. At a fixed pressure of SF(6) (5 Torr), [SiH(2)] decay constants, k(obs), showed a quadratic dependence on [H(2)O] or [D(2)O]. At a fixed pressure of H(2)O or D(2)O, k(obs) values were strongly dependent on [SF(6)]. The combined rate expression is consistent with a mechanism involving the reversible formation of a vibrationally excited zwitterionic donor-acceptor complex, H(2)Si...OH(2) (or H(2)Si...OD(2)). This complex can then either be stabilized by SF(6) or it reacts with a further molecule of H(2)O (or D(2)O) in the rate-determining step. Isotope effects are in the range 1.0-1.5 and are broadly consistent with this mechanism. The mechanism is further supported by RRKM theory, which shows the association reaction to be close to its third-order region of pressure (SF(6)) dependence. Ab initio quantum calculations, carried out at the G3 level, support the existence of a hydrated zwitterion H(2)Si...(OH(2))(2), which can rearrange to hydrated silanol, with an energy barrier below the reaction energy threshold. This is the first example of a gas-phase-catalyzed silylene reaction.