Communication: The insertion of silylene in C?H bonds; rate constant limits and the energy barrier

The technique of laser flash photolysis has been used to set limits on the rate constants for the bimolecular reactions of SiH₂ with methane (CH₄) and tetramethylsilane (SiMe₄) at both ambient and elevated temperatures (ca 600 K). These limits show that the energy barriers to insertion reactions of SiH₂ in the C H bonds of CH₄ are at least 45(±6) kJ mol⁻¹ and in the C H and/or Si C bonds of SiMe₄ are at least 23(±6) kJ mol⁻¹. The best thermochemical estimate of the activation energy for SiH₂+CH₄ is 59(±12) kJ mol⁻¹. Reasons for the greatly diminished reactivity of SiH₂ with C H as compared with Si H bonds are discussed. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 393–395, 1999     

Roles of silane coupling agents on properties of silica-filled polychloroprene

Silane coupling agent is used extensively to improve reinforcing efficiency of silica. Recently, many types of silane coupling agents have been developed and their roles on reinforcing improvement have been studied in many types of rubbers. In the present study, roles of the two widely used silane coupling agents, i.e., bis-(3-triethoxysilylpropyl) tetrasulfane (Si-69) and 3-thiocyanatopropyl triethoxy silane (Si-264) were studied in silica-filled polychloroprene (CR). The results reveal that the presence of Si-69 and Si-264 improves significantly the processability of the rubber compounds. The improvement is more pronounced for Si-264 due to its lower molecular weight and, thus, viscosity. Surprisingly, the results show that Si-69 and Si-264 affect cure characteristics differently, i.e., Si-69 somewhat retards cure while Si-264 accelerates cure. However, both Si-69 and Si-264 result in an increased crosslink density of the vulcanizates. The presence of silane coupling agent also enhances the mechanical properties of the vulcanizates due to the combined effects of better filler dispersion, better rubber-filler interaction and increased crosslink density. A thorough look at the results also reveals that the property enhancement is obvious only at low silane loading (approximately 1.5 phr). Further increase of silane loading generally has little influence on properties of the vulcanizates. In the case of Si-264, excessive use of silane could impair certain properties such as modulus and hardness due to the plasticizing effect. Compared with Si-264, Si-69 imparts the rubber vulcanizates with poorer aging resistance. Explanation goes to the sulfur contribution effect of Si-69.     

Covalent grafting of organoalkoxysilanes on silica surfaces in water-rich medium as evidenced by <Superscript>29</Superscript>Si NMR

This paper addresses two questions related to functionalization of silica particles: (1) is the grafting of hydrophobic organoalkoxysilanes on a silica surface possible in water-rich medium and (2) how to prove the formation of covalent bonds with the surface? Trimethylethoxysilane, dimethyldiethoxysilane and methyltriethoxysilane have been reacted with precipitated silica in water-rich medium (water/ethanol 25/75 v/v) and ²⁹Si MAS NMR was used to answer both questions: ²⁹Si chemical shift values of the organosilicon units in the case of trimethylethoxysilane and dimethyldiethoxysilane clearly distinguished between self-condensation reactions and surface reactions through covalent bonds.     

Identification of decomposition reactions for HMDSO organosilicon using quantum chemical calculations

Hexamethyldisiloxane [HMDSO, (CH₃)₃-SiOSi-(CH₃)₃] is an important precursor for SiO₂ formation during flame-based silica material synthesis. As a result, HMDSO reactions in flame have been widely investigated experimentally, and many results have indicated that HMDSO decomposition reactions occur very early in this process. In this paper, quantum chemical calculations are performed to identify the initial decomposition of HMDSO and its subsequent reactions using the density functional theory at the level of B3LYP/6-311+G (d, p). Four reaction pathways—(a) Si O bond dissociation of HMDSO, (b) Si C bond dissociation of HMDSO, (c) dissociation and recombination of Si O and Si C bonds, and (d) elimination of a methane molecule from HMDSO—have been examined and identified. From the results, it is found that the barrier of 84.38 kcal/mol and Si O bond dissociation energy of 21.55 kcal/mol are required for the initial decomposition reaction of HMDSO in the first pathway, but the highest free energy barrier (100.69 kcal/mol) is found in the third reaction pathway. By comparing the free energy barriers and reaction rate constants, it is concluded that the most possible initial decomposition reaction of HMDSO is to eliminate the CH₃ radical by Si C bond dissociation.     

Establishing the difference between colloidal borosilicate and boron-modified colloidal silica via chelation of boron with catechol and dl-tartaric acid

Colloidal borosilicate and boron-modified colloidal silica sols were studied by ¹¹B NMR. Formation of B–O–Si chemical bonds is established in both materials. It is shown that boron present in colloidal borosilicate is stable towards the action of complexing agents catechol and tartaric acid. In contrast, the boron in boron-modified silica is readily complexed by these agents. The results presented herein demonstrate that B–O–Si bonds are homogeneously distributed throughout the colloidal borosilicate disperse phase, while in boron-modified colloidal silica they are concentrated at the surface of colloidal silica particles.     

Spectroscopically tracing the reactions of octahydridosilsesquioxane (H8Si8O12) with organic molecules

This article studies the reactions and mechanisms of H₈Si₈O₁₂ (T₈H₈) molecules with n-propanol, acetone, allyl alcohol, n-butylamine, allylamine, acetic acid, and 1-octene in air, at room temperature, and without catalysts. The reaction between T₈H₈ and n-propanol involves both the highly polarized Si O and Si H bonds and results in cage breakage and forming Q⁴ and Q³ structures with  OC₃H₇ in the reaction product. T₈H₈ also reacts with acetone, and the resultant product possesses Si OCH(CH₃)₂. Allyl alcohol is less reactive to cause T₈H₈ decomposition, and the resultant product contains Si OCH₂CHCH₂ and Si OCH₂(CH₂)₃CHCH₂. However, it is found that basically T₈H₈ does not react with acetic acid and 1-octene. In the reactions of T₈H₈ with n-butylamine and allylamine, the resultant products contain Si NH(CH₂)₃CH₃ and Si NHCH₂CHCH₂, respectively. For the reaction with T₈H₈, allylamine is less active than n-butylamine. Possible mechanisms for the T₈H₈ reactions are discussed.     

A DFT Study on the Mechanism of Rh2II,II‐Catalyzed Intramolecular Amidation of Carbamates

The potential‐energy surfaces of the reactions of dirhodium tetracarboxylate (Rh₂^II,II) catalyzed nitrene (NR) insertion into C H bonds were examined by a DFT computational study. A pure Becke exchange functional (B88) rather than a hybrid exchange functional (B3, BHandH) was found to be appropriate for the calculation of the energy difference between the singlet and triplet Rh₂^II,II–NH nitrene species. Rh₂^II,II–NR¹ (R¹=(S)‐2‐methyl‐1‐butylformyl) is thermodynamically more favorable with a free energy lower than that of Rh₂^II,II–N(PhI)R¹. The singlet and triplet states of Rh₂^II,II–NR¹ have similar stability. Singlet Rh₂^II,II–NR¹ undergoes a concerted NR insertion into the C H bond with simultaneous formation of the N H and N C bonds during C H bond cleavage; triplet Rh₂^II,II–NR¹ undergoes H atom abstraction to produce a diradical, followed by subsequent bond formation by diradical recombination. The singlet pathway is favored over the triplet in the context of the free energy of activation and leads to the retention of the chirality of the C atom in the NR insertion product. The reactivities of the C H bonds toward the nitrene‐insertion reaction follow the order tertiary>secondary>primary. Relative reaction rates were calculated for the six reaction pathways examined in this work.     

Laser-flash photolysis study of dithiobis (tetrazole); reactivities of tetrazole-thio radical

The photo-cleavage of S S bond of 5,5′-dithiobis (1-phenyl-1H-tetrazole) has been studied by the nanosecond-laser flash photolysis method. The transient absorption band at ca. 430 nm was attributed to 1-phenyl-1H-tetrazole-5-thio radical forming by the S S bond fission. For the reaction with conjugated dienes, an addition reaction takes place forming the S C bond, suggesting that unpaired electron of the radical localizes mainly on the S-atom. From the decay rates of the radical, the addition reaction rate constant for 2-methyl-1,3-butadiene is evaluated to be 5.5 × 10⁹ M⁻¹ s⁻¹ in THF at 23°C, which is as fast as diffusion controlled limit. The reactivity of the radical is ca. 100 times higher than that of the PhS˙. The reactivity of the thio radical to O₂ was too low to evaluate, which is one of the characteristics of a S-centered radical. The rate constant for 1,4-cyclohexadienene (1.4 × 10⁸ M⁻¹ s⁻¹) is larger than that of cyclohexene (2.8 × 10⁷ M⁻¹ s⁻¹) suggesting the hydrogen abstraction is a main reaction. The MO calculations have been performed for these radicals to reveal the reason of the high reactivity of the radical. © 1996 John Wiley & Sons, Inc.     

Ab initio study of the topology of the charge distribution of H3SiO(H)AlH3 conformers

The topologic properties of the electronic charge distribution of conformers of H₃SiO(H)AlH₃ molecule hydroxyl groups of zeolites are reported. The studied properties—total density, Laplacian density, and bond ellipticity—were evaluated at the position of the critical points of the O Si, O Al, and O H bonds, by using Hartree–Fock and second‐order Møller–Plesset levels of theory, and the STO/6‐31+G(d,p) standard basis set. For the H₃SiO(H)AlH₃ molecule, four conformers are identified. It is demonstrated that for these conformers, the total density and Laplacian density remain almost constant by effect of the rotations of the T H bonds, T=(Si, Al), around the corresponding O T bonds, respectively. However, these rotations induce sensible variations in the ellipticity at the position of the critical point of the O Al bonds, which are reflected in the OH bond distance, OH vibrational mode, and the stabilization energy of conformers. These results lead to a linear relationship between the magnitude of the bond ellipticity at the critical point of the O Al bonds and the frequency values of the OH bonds, with a correlation coefficient of r²=0.98. In addition, a good linear relationship between the ellipticity of the O Al bond and the pattern of the stabilization energy of conformers was also found. © 1999 John Wiley & Sons, Inc. Int J Quant Chem 76: 1–9, 2000     

Exploration of the Synthetic Potential of Electrophilic Trifluoromethylthiolating and Difluoromethylthiolating Reagents

The electrophilicity parameters (E) of some trifluoromethylthiolating and difluoromethylthiolating reagents were determined by following the kinetics of their reactions with a series of enamines and carbanions with known nucleophilicity parameters (N, s_N), using the linear free‐energy relationship log k₂=s_N(N+E). The electrophilic reactivities of these reagents cover a range of 17 orders of magnitude, with Shen and Lu's reagent 1 a being the most reactive and Billard's reagent 1 h being the least reactive electrophile. While the observed electrophilic reactivities (E) of the amido‐derived trifluoromethylthiolating reagents correlate well with the calculated Gibbs energies for heterolytic cleavage of the X?SCF₃ bonds (Tt⁺DA), the cumol‐derived reagents 1 f and 1 g are more reactive than expected from the thermodynamics of the O?S cleavage. The E parameters of the tri/difluoromethylthiolating reagents derived in this work provide an ordering principle for their use in synthesis.     

Metallacyclopentadienes and related heterocycles via 1,1‐organoboration of alkyn‐1‐ylmetal compounds

Metallacyclopentadienes (metalloles) containing M = Si, Ge, Sn, Pb, Ti, Pt can be prepared by 1,1‐organoboration of alkyn‐1‐ylmetal compounds L_nM CC R¹(R¹ = H, alkyl, aryl, silyl, etc; L depends on M, and can be hydrogen, alkyl, aryl, Cl, Br, amino groups, a chelating diphosphane, and one or more L can be again alkynyl groups). These reactions proceed via activation of the M C bond(s) by an electron‐deficient triorganoborane BR₃ (R = alkyl, aryl; non‐cyclic, monocyclic, bicyclic, and tricyclic boranes), at first intermolecular and then intramolecular. In the course of these reactions, the M C bonds are cleaved, zwitterionic alkynylborate‐like intermediates are formed, in which the metal‐containing fragments are coordinated side‐on to the CC bonds. In most cases, the 1,1‐organoboration reactions tolerate various functional groups at the alkyne as well as at the metal. The characterization of intermediates and final products by X‐ray structural analysis and by multinuclear magnetic resonance spectroscopy (NMR) is documented and described. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:188–208, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20222     

From Silylone to an Isolable Monomeric Silicon Disulfide Complex

The synthesis and characterization of the first bis‐N‐heterocyclic carbene stabilized monomeric silicon disulfide (bis‐NHC)SiS₂ 2 (bis‐NHC=H₂C[{NC(H)C(H)N(Dipp)}C:]₂, Dipp=2,6‐iPr₂C₆H₃) is reported. Compound 2 is prepared in 89 % yield from the reaction of the zero‐valent silicon complex (′silylone′) 1 [(bis‐NHC)Si] with elemental sulfur. Compound 2 can react with GaCl₃ in acetonitrile to give the corresponding (bis‐NHC)Si(S)S→GaCl₃ Lewis acid–base adduct 3 in 91 % yield. Compound 3 is also accessible through the reaction of the unprecedented silylone‐GaCl₃ adduct [(bis‐NHC)Si→GaCl₃] 4 with elemental sulfur. Compounds 2 , 3 , and 4 could be isolated and characterized by elemental analyses, HR‐MS, IR, ¹³C‐ and ²⁹Si‐NMR spectroscopy. The structures of 3 and 4 could be determined by single‐crystal X‐ray diffraction analyses. DFT‐derived bonding analyses of 2 and 3 exhibited highly polar Si S bonds with moderate p_π–p_π bonding character.     

The structure of tetramethyl μ-monothiopyrophosphate

The compound tetramethyl μ-monothiopyrophosphate (C₄H₁₂O₆P₂S) crystallizes in the monoclinic space group C 2/c, with (at -130°C) a = 10.322 Å, b = 8.229 Å, c = 12.062 Å, β = 98.44°, and D_calc = 1.639 g/mL³ and Z = 4. The crystal structure has been determined by single crystal X-ray diffraction to give a final R value of 0.0329 for 614 independent observed reflections [F_˚ > 2.5σ(F_˚)]. The sulfur atom resides on a crystallographic two-fold axis. The P S P bond angle is 105.4° and the P S bond lengths are 2.093 Å. The bond angles around phosphorus range from 99.1° to 118.2°. The terminal PO bond is 1.465 Å, and the methoxyl P O bond is about 1.556 Å. The H₃C O P bond angle is about 119.5°. Many structural features are interpreted in terms of π-bonding to phosphorus. Comparisons with the structures of pyrophosphate and related compounds indicate that the combined effects of increased acuteness of the P S P bond and the increased length of the P—S bonds lead to an increase of about 0.4 Å in the separation of phosphorus atoms in the sulfur-bridging compound. These facts, together with the weakness of the P S bond, must be taken into account in the interpretation of kinetic data for enzymatic reactions of phosphorothiolates as substrates in place of phosphates.     

Modification of Silica by Liquid Polybutadienes Containing Alkoxysilane Groups

Summary : The present work describes a method to modify the surface of silica, reducing its polar character and making it compatible and dispersible into hydrocarbon based elastomers. A liquid low molar mass polybutadiene (PB) was grafted with mercaptopropyltrimethoxysilane (MPTS) via radical addition of the thiol group to the double bonds. The silanized PB was reacted with silica via thermal condensation with its silanol groups. ²⁹Si NMR spectra showed that the condensation reaction of the trifunctional silane involved one or two alkoxy groups, while the third alkoxy group remained unreacted, probably for steric reasons. The characterization of the functionalized silica particles was performed by contact angle measurements and TGA analysis.     

Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving C?H Bond Activation

The insertion of an olefin into a preformed metal–carbon bond is a common mechanism for transition‐metal‐catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal–carbon bond is not present in the precatalyst. The Phillips catalyst, CrO₃ dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first Cr C bond is formed remain unknown. We synthesized well‐defined dinuclear Cr^II and Cr^III sites on silica. Whereas the Cr^II material was a poor polymerization catalyst, the Cr^III material was active. Poisoning studies showed that about 65 % of the Cr^III sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si–(μ‐OH)–Cr^III species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr^III O bonds.     

X-ray photoelectron spectroscopy for detection of the different Si–O bonding states of silicon

X-ray photoelectron spectroscopy (XPS) was used to detect the bonding between a silica particle surface and attached silanes. In addition to the commonly recorded Si 2p spectrum, the Si 1 s level is also accessible when monochromatic Ag Lalpha X-rays are applied. Furthermore, the spectrum of the Si 1 s level shows a fine structure. After spectrum deconvolution, we assigned the fitted spectral peaks to Si-C bonds of the silanes and to the Si-O bonds of the silica network. The recorded Si 1 s spectra were deconvoluted into peaks originating from Si-C bonds and the Si-O-Si silica network. To check the results of spectrum deconvolution, several differently functionalized silanes containing stoichiometric amounts of heteroatoms were applied for silica surface modification. We conclude that spectra deconvolution of the Si 1 s signal is an appropriate means for quantification of surface attached silane molecules.     

Synthesis and structure of bis(dialkylamino)carbeniumdithiocarboxylates

2-Chloro and 2-phenoxy substituted 1,1-bis(diethylamino)ethylenes ( 4a and 4c ) react with elemental sulfur at room temperature to give the inner salt, bis(diethylamino)carbeniumdithiocarboxylate ( 1a ), in excellent yields with extrusion of hydrogen chloride and phenol, respectively, thus providing a new and convenient synthesis of the structurally interesting inner salt. X-ray single crystal structure analysis of 1a reveals that the N C N and S C S planes are nearly vertical to each other with a dihedral angle of 82.0° and that the positive and negative charges are delocalized over the N C N and S C S moieties, respectively. Results of solid-state ¹³C NMR of 1a are also briefly described.     

一种新型硅藻土填充橡胶纳米复合材料研究

橡胶的填料问题一直是人们的研究热点,针对炭黑和白炭黑在橡胶生产中存在的污染问题,本文选用成分结构与白炭黑类似的硅藻土来填充各种橡胶。首先对硅藻土进行了改性,并对不同改性剂改性硅藻土用于填充橡胶进行了研究。结果表明2.5份偶联剂Si69的改性效果最佳。通过机械共混法制备了改性硅藻土/橡胶纳米复合材料,通过力学性能测试确定了比较适合硅藻土填充的橡胶是氟橡胶、三元乙丙橡胶和丙烯酸酯橡胶。绿色环保且价格低廉的硅藻土可以替代白炭黑增强填充氟橡胶、三元乙丙橡胶和丙烯酸酯橡胶。     

Kinetics of hydrolysis and self condensation reactions of silanes by NMR spectroscopy

The hydrolysis of four alkoxy-silane coupling agents, 3-methacryloxypropyl trimethoxy silane (MPMS), 3-mercaptopropyl trimethoxy silane (MRPMS), octyl triethoxy silane (OES) and 3-aminopropyl triethoxy silane (APES) was carried out in an ethanol/water 80/20 (w/w) solution under acidic, alkaline and neutral conditions and followed by ¹H, ¹³C and ²⁹Si NMR spectroscopy. It was found that the kinetic rate of the hydrolysis of the silanes under neutral conditions was very low, except for APES, which displayed the fastest reaction speed. The addition of TEA catalyzed both silane hydrolysis and self condensation reactions. Acidic conditions enhanced the hydrolysis and the ensuing silanol entities were quite stable. In fact, these conditions slowed down the rate of the self condensation reactions, as deduced from in situ ¹H and ¹³C NMR. Thanks to in situ ²⁹Si NMR spectroscopy, the nature of the intermediary species versus reaction time was established.     

Study of the addition of diazoles to vinylsilanes

The reaction of trimethoxy(vinyl)silane with diazoles in the presence of metal lithium was studied. The structure of the reaction products was assigned by NMR spectroscopy (¹H, ¹³C, ²⁹Si, COSY, NOESY, HMBC, and HSQC). An explanation for the different reactivities of reagents (aliphatic amines, diazoles, allylalkylsilanes, vinylalkylsilanes, and vinylalkoxysilanes) was provided.