Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: III. Products of >Si

Permenov  D. G.  Radzig  V. A. 《Kinetics and Catalysis》2004,45(2):273-278

Mechanisms of Reactions of H2O and NH3 Molecules with (≡Si–O)3Si⋅and (≡ Si–O)3Si–⋅ Radicals on the Silica Surface

Spin traps, which are diamagnetic centers (SiO)₂Si, are used to register low-molecular radicals OH, NH₂, and H formed by the reactions of H₂O and NH₃ molecules with the radicals (Si–O)₃Si and (Si–O)₃Si–O stabilized on the silica surface. The experimental data and the results of quantum-chemical calculations for model systems are used to determine the mechanism and thermochemical characteristics of these reactions. A new paramagnetic center (Si–O)₂SiNH₂ was identified on the silica surface, and its radiospectroscopic characteristics are determined.     

Synthesis and structures of silicon-, germanium-, and tin-containing tungsten carbyne complexes (ButO)3W≡C—EPh3 and [(ButO)3W≡C]2EPh2 (E = Si,Ge, Sn)

New tungsten carbyne complexes (Bu^tO)₃WC—SiPh₃, [(Bu^tO)₃WC]₂SiPh₂, [(Bu^tO)₃WC]₂GePh₂, and [(Bu^tO)₃WC]₂SnPh₂ were prepared by the reactions of (Bu^t)₆W₂ with Ph₃SiCC—Pr or Ph₂E(CC—Pr)₂ (E = Si, Ge, Sn) in individual crystalline form in 48—80% yields. The structures of both the (Bu^tO)₃WC—GePh₃ and (Bu^tO)₃WC—SnPh₃ compounds synthesized earlier and the new complexes were established by X-ray diffraction analysis.     

Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: I. Methane Chemisorption on a Reactive Silica Surface

Experimental and quantum chemical study of reactive silica surface methylation is carried out. The main product of the reaction is the (Si–O)₂Si(H)(CH₃) groups, which are formed via a radical-chain process with the participation of methane molecules and paramagnetic and diamagnetic defects on the oxide surface. Spectral (optical and IR) characteristics of the groups participating in the process (Si–O)₂Si^·–CH₃, (Si–O)₂Si(H)(CH₃), and (Si–O)₂Si(CH₃)(CH₃) are determined. Information on the kinetics of separate steps of the process is obtained including rate constants and the activation energies of steps.     

Kinetics of Intramolecular Hydrogen Atom Transfer in the Radicals >Si(O·)(R) (R = H,D, CH3, CD3, and C2H5)

Methods are developed for obtaining oxy radicals by the photodecomposition and thermal decomposition of precursors (Si–O)₂Si(N=N–O^·)(R) and (Si–O)₂Si(O–C^·=O)(R). The mechanism of these processes is established. Kinetic data are obtained for the reaction of hydrogen atom transfer in oxy radicals (Si–O)₂Si(O^·)(R) (R = H, D, CH₃, CD₃, and C₂H₅). The activation energies of hydrogen atom transfer are found for three-, four-, and five-membered transition states: 13.5 ± 1, 18 ± 1, and <10 kcal/mol, respectively. For the reaction of H(D) atom transfer in the (Si–O)₂Si(O^·)(H(D)) radical, the kinetic isotope effect is found. Quantum-chemical calculations were used to determine the structures of transition states in the studied processes. Experimental studies were carried out using ESR spectroscopy.     

Free Radical Reactions of N2O Molecules

Experimental data are presented on the spectral (ESR, IR, and optical) and thermochemical characteristics of a complex between the (Si–O)₃Si^.radical and an N₂O molecule. The rate constants of separate reactions in the systems (Si–O)₃Si^.+ N₂O and (Ge–O)₃Ge^.+ N₂O are found. The results of quantum chemical calculations of potential energy surfaces and spectral characteristics are presented for the following systems: H^.+ N₂O, H₃C^.+ N₂O, H₃Si^.+ N₂O, F₂HSi^.+ N₂O, F₃Si^.+ N₂O, and F₃Ge^.+ N₂O. The latter three systems served as molecular models for experimentally found systems. Based on experimental and theoretical data, the product of N₂O addition to (Si–O)₃Si^.has the structure Si–N=N–O^.. The reactions of free radicals H^., H₃C^., H₃Si^., F₂HSi^., F₃Si^., (Si–O)₃Si^., and (Ge–O)₃Ge^.with N₂O are compared. The spectrum of optical absorbance of the (Si–O)₃Si–O^.radical is recorded and qualitatively characterized.     

Overton-Region IR Registration of (≡Si–O)2Si=O and (≡Si–O)2Si〈O2〉C=O Groups on the Silica Surface

IR spectroscopy in a range of 2050–4000 cm^–1 (the range of overtones and composite frequencies) is used to study the groups (Si–O)₂Si=O and (Si–O)₂SiO₂C=O with different isotopic compositions (¹⁶O, ¹⁸O, ¹²C, and ¹³C). Analysis of the experimental data and quantum-chemical calculations of vibrational spectra for the model compounds are used to identify the IR bands. New data are obtained on the vibrational spectra of these groups. Their identification is shown to be possible in the spectral range that is convenient for the study of silica samples.     

Polysiloxane-Diamine Modified Gel as Precursor to Crystalline/Amorphous Si3N4

New hybrid organic-inorganic materials are prepared by reaction of polymethylhydrosiloxane (PMHS) with aniline (An) or 4,4-diamino,diphenyl methane (APM) through a condensation between NH₂-terminated amines and Si—H groups. The obtained modified-polysiloxane is a liquid polymer (PS-An) in the first case and a cross-linked gel (PS-APM) in the second one. The chemical structures of PS-An and PS-APM were investigated by ²⁹Si nuclear magnetic resonance and were proved to be formed of created (Si—O)₂(Me)Si—N(H)— and unreacted (Si—O)₂(Me)Si—H sites. The pyrolysis of the diamine-based gel has been carried out in N₂ atmosphere up to 1450°C. FTIR, ²⁹Si MAS NMR and X-ray diffraction have shown that Si—O and Si—C bonds are totally broken during pyrolysis and that the final product is formed of crystalline and amorphous silicon nitride in the presence of a free carbon phase.     

Disproportionation of the alkynyl derivatives of alkali metal aluminum hydrides in electron-donor solvents

Conclusions The reaction of disproportionation of mixed hydride alkynyl complexes of aluminum in ether, THF, and hexametapol was studied by²⁷Al NMR, and it was shown that the stability of MAlH(3–n)) (CCR)_n, where M=Li, Na, K, and R=Ph, n-Bu decreases in the order: MAlH(CCR)₃ < MAlH₂(CCR)₂ < MAlH₃(CCR). The tendency of these complexes to disproportionate increases with an increase in the solvation of the cation.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 343–349, February, 1984.We would like to thank A. V. Kisin for recording the²⁷Al NMR spectra.     

Synthesis of novel symmetrical and nonsymmetrical 6-membered heterocycles with pendant electron-rich organoiron substituents

The functionalized complexes [(dppe)Cp^*Fe(CC)]₂-(Py) (Py=2,6-C₅H₃N and 3,5-C₅H₃N (dppe=1,2-bis(diphenylphosphino)ethane) were isolated in good yields from reaction of the chloro complex (dppe)Cp^*FeCl with the protected bis-acetylenic heterocyclic precursor. These electron-rich pyridyl ligands constitute interesting examples of organometallic heterocycles bearing redox-active substituents. Attempts to find an alternative route starting from the alkynyl complex [(dppe)Cp^*Fe(CCH)] and the corresponding dibromopyridines using a Sonogashira cross-coupling reaction are also described. By this route, the monofunctionalized products [(dppe)Cp^*Fe(CC)]-2,6-Py-Br and [(dppe)Cp^*Fe(CC)]-3,5-Py-Br could be cleanly isolated. These compounds open the way to the generation of heteroaromatics featuring nonequivalent alkyne substituents such as [(dppe)Cp^*Fe(CC)]-2,6-Py-[(CC)SiMe₃] or [(dppe)Cp^*Fe(CC)]-3,5-Py-[(CC)SiMe₃] by further coupling.In commemoriation of the centenary of Academician A. N. Nesmeyanov.UMR CNRS 6509 Organométalliques et catalyse: Chimie et Electrochimie Moléculaires, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1207–1218, September, 1999.     

Structure and charge distribution of some alkynoyl cations

We have performedab initio calculations to determine the structure and charge distribution of some alkynoyl cations and their parent alkynoyl fluorides. We have used Mulliken population analysis and a new technique developed by Yáñez, Stewart and Pople. Our results indicate that the mesomeric form O⁺C–CC–R is one of the most important contributors to the structure of these cations, in agreement with experimental conclusions. We have also found that the participation of mesomeric form O=C=C=C⁺ -R is not negligible and increases with -substitution. In the 3-phenylpropynoyl cation substantial delocalization of charge into the phenyl group occurs. Calculations from YSP population analysis are in good agreement with experimental evidence.     

Structural Features of Grafted Phosphates in Mesopores of Si-MCM-41 and Ti,Si-MCM-41 Molecular Sieves

We have used ³¹P NMR (MAS) and IR spectroscopy and also chemical analysis to show that when Si- and Ti,Si-MCM-41 mesoporous molecular sieves are treated with phosphorus oxychloride, surface-anchored silicophosphate and titanophosphate compounds are formed of the type (Si—O) _x —{PO(OH)_3–x } and (Ti—O) _x —{PO(OH)_3–x } (x = 1, 2), and also compounds with the structure of titanium hydrophosphate and pyrophosphate.     

Mechanisms of Heterogeneous Processes in the System SiO2 + CH4: II. Methylation of >Si=O Groups

The methods of optical and IR spectroscopy and quantum chemistry were used to obtain data on the direction and kinetics of the reaction of a silanone (SiO)₂Si=O with a CH₄ molecule and a methyl radical. Two mechanisms of methylation of silanone groups, molecular and free-radical, are studied. Both processes are accompanied by the formation of (SiO)₂Si(OH)(CH₃) groups. The rate constant of the molecular process is determined and its activation energy is estimated (17 kcal/mol). A methyl radical adds to the silicon atom in a silanone group to form the oxy radical (Si–O)₂Si(O)(CH₃). This radical carries a free-radical process of silanone group methylation. The main channel for the pyrolysis of (Si–O)₂Si(OH)(CH₃) groups is their decomposition with the abstraction of a methane molecule. The activation energy of this process is 70 kcal/mol. Quantum chemical methods were employed to obtain data on possible intermediates in the processes studied and these results are used to interpret spectral and kinetic data.     

Ethane hydrogenolysis on silica-supported tantalum hydride. Quantum-chemical study

The structures of the (H₄Si₂O₅)O₂Ta(R, RR"R, RR) clusters (R, R", R = H, Me, Et, Pr; R = CH₂), which model the basic structural units of the catalytic cycle of ethane hydrogenolysis on silica-supported tantalum hydride, were studied by the density functional theory (B3LYP) and the perturbation theory (MP2). The possible structure of active sites was proposed based on comparison of experimental results with calculated data. Ethane hydrogenolysis and metathesis proceed by a mechanism involving the formation of ethylene -complexes and carbenium derivatives of tantalum as intermediates.     

A Spectroscopic Study of an Anhydrous Tetraethyl Orthosilicate-Boric Acid-Ethanol System

Hydrolysis and condensation reactions in an anhydrous sol-gel system comprising tetraethyl orthosilicate (TEOS, Si(OC2H5)4), boric acid (H3BO3) and ethanol have been studied using X-ray diffraction and NMR spectroscopy, and similarities and differences with the more traditional aqueous system are discussed. Boric acid is able to hydrolyse TEOS directly, and subsequent condensation reactions form borosiloxane (=B–O–Si) and siloxane (Si–O–Si) linkages. In an aqueous system, borosiloxanes are unstable to hydrolysis and are formed only upon heat treatment of the gel, a restriction avoided with anhydrous conditions. The anhydrous mixture is stable indefinitely against gelation, but can be readily gelled by addition of NaOH in ethanol. This system may be useful for preparing borosilicate glasses at lower temperatures with good homogeneity.     

Preparation,Structure, and Reactivity of ≡Si– $${\text{N}}^ \cdot$$ –H Radicals

A method is developed for the synthesis of Si– –H radicals on a silica surface and information is obtained by ESR and quantum chemical calculations of model systems on their structure and spectral (radiospectroscopic) characteristics. The reactivity of these radicals toward CO, H₂, and H₂=H₂ molecules is studied. The structure of the Si–HN– =O radical is analyzed, which is the product of CO addition. The kinetic and thermochemical characteristics of processes with the participation of synthesized radicals are determined.     

Effect of alkaline modification of products of thermal vacuum and hydrothermal treatment of NH4Na-Y zeolite according to the IR spectral data

The effect of alkaline modification on the structure of the products of heat treatment of NH₄Na-Y zeolite (53% NH ₄ ⁺ , Si/Al - 2.37) in a vacuum at 573 K and in water vapors at 873 K was investigated with the IR spectrum of vibrations of the zeolite framework in the 400–1200 cm^–1 region. It was shown that the high-frequency shift of the bands in the spectra of the products obtained, the stretching vibration of v_as(TO₄) tetrahedrons in particular (T=Si, Al) at 1023 cm^–1 by 6 and 19 cm^–1, is determined by a decrease in the excess negative charge of the framework due to weakening and hydrolytic splitting of Al-O bonds of the deammoniated units with the formation of bridging Si-O(H)...Al and terminal Si-OHHO-Al hydroxyl groups. Treatment of these samples with an aqueous solution of KOH (pH 13.4) at 293 and 353 K restores the normal framework Si-O-Al bonds at the sites of formation of bridging and terminal hydroxyl groups. In the second case, restoration is hindered by substitution of H⁺ by K⁺ with some silanol groups.Institute of Physical Chemistry, Russian Academy of Sciences, 117915 Moscow. I. V. Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, 199164 St. Petersburg. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 8, pp. 1733–1739, August, 1992.     

Regioselectivity in the Addition of Nucleophilic Reagents to 1-Alkylthio-2-polyfluoroalkylacetylenes

Calculations for HCCH, HCCCF₃, and H₃CSCCCF₃ were carried out using the MP2(f)/6-31G(d) nonempiric quantum-chemical method. The electronic structure and charge density distribution were examined using natural bond orbitals and the results account for the differences in the direction of nucleophilic attack of the triple bond in HCCCF₃ and H₃CSCCCF₃.     

Reaction of the Hexa-1,3,5-triyne Me3SiC≡CC≡CC≡CSiMe3 with Ru4(CO)13(μ 3-PPh): Parallels with the Chemistry of Alkynes and Diynes

Reaction of Ru₄(CO)₁₃(₃-PPh) (1) with the 1,3,5-hexatriyne Me₃SiCCCCC CSiMe₃ under mild thermal conditions affords initially Ru₄(CO)₁₀(-CO)₂{₄-¹,¹,²-P(Ph)C(CCSiMe₃)C(CCSiMe₃) (2), via the facile formation of a P–C bond in a manner similar to that demonstrated previously with alkynes and diynes. The 62-CVE cluster 2 readily decarbonylates to give crystallographically characterised Ru₄(CO)₁₀(-CO)(₄-PPh){₄-¹,¹,²,²-Me₃SiCCC₂CCSiMe₃} (3). Attempts to further incorporate the pendant alkyne moieties in 3 into the Ru₄ coordination environment were partially successful with Ru₄(CO)₁₀(₄-PPh)(₄-¹,¹,³,³-RC₄R') (4, R/R'=SiMe₃/CCSiMe₃) being formed as a minor product together with the unusual toluene coordinated species Ru₄(CO)₇(⁶-C₆H₅Me)(₄-PPh)(₄-¹,¹,³,³-Me₃SiC₄CCSiMe₄) (5). Cluster 3 reacts with an excess of Me₃SiCCCCCCSiMe₃ to give the open chain cluster Ru₄(CO)₉(₃-PPh){₄-²,²,⁴,⁴,-C₄(CCSiMe₃)(SiMe₃)C₄(CCSiMe₃)₃} (6).     

Synthesis and characterization of two oxo-bridged oxo(arylimido) [tris(3,5-dimethylpyrazolyl)borato]molybdenum(V) complexes and crystal structure of [MoTp*(O)Cl](μ-O)[MoTp*(Cl)(≡NC6H4OMe)]

Reaction of the oxo-molybdenum(V) compound, [MoTp*(O)Cl₂], [Tp*=hydrotris(3,5-dimethylpyrazol-1-yl)borate] with p-methoxy and p-nitroaniline in the presence of Et₃N under N₂, afforded the oxo-bridged oxo(arylimido) molybdenum(V) complexes, [MoTp*(O)Cl](-O)[MoTp*(Cl)(NC₆H₄R)] (1, R=OMe; 2, R=NO₂). The new compounds were characterized by elemental analysis, i.r., mass, and ¹H-n.m.r. spectra. The single crystal X-ray crystallographic determination of [MoTp*(O)Cl](-O)[MoTp*(Cl)(NC₆H₄OMe)] was carried out to confirm that there is a Mo—O—Mo bridge and a near linear arylimido group in the structure.