Silazanes reactions

Reactions (l)-(5) illustrate known methods for forming silicon-nitrogen bonds of potential use in the formation of oligo- and polysilazanes. The most common method of forming silazanes is via ammo-nolysis or aminolysis as shown in reaction (1) (4,5) ... 

Thus, Andrianov et al. (26) attempted to catalyze polymerization of a number of alkyl and alkyl/aryl cyclosilazanes using catalytic amounts of KOH or other strong bases at temperatures of up to 300°C. In general, the reactions proceed with evolution of NHj, hydrocarbons and the formation of intractable, crosslinked, brittle products even at low temperatures. Contrary to what is observed with cyclotri-siloxanes, no evidence was found for the formation of linear poly-silazanes. Copolymerization of mixtures of cyclosilazanes and cyclosiloxanes gave somewhat more tractable polymers with less evolution of hydrocarbons or ammonia, however very little was done to characterize the resulting materials. 

The coammonolysis of CH3SiHCl2 and CH3(Un)SiCl2 in the indicated ratios was carried out by the procedure as described for the ammonolysis of CH3SiHCl2 [8]. A mixture of cyclic oligomeric silazanes is to be expected in these reactions, [(CH3(H)SiNH)x(CH3(Un)SiNH)y]n, with more than one ring size present. In each preparation, the soluble, liquid products were isolated and used in the base-catalyzed polymerizations. When a ratio CH3SiHCl2/CH3(Un)SiCl2 of x l (x>l) was used in the ammonolysis reaction, the CH3(H)Si/CH3(Un)Si ratio in the soluble ammonolysis product usually was somewhat less than x l. 

In a research program to explore the possibilities of synthesizing useful polymeric materials from silazane (Si-N) containing ring compounds, reactions of the following ring systems were studied ... 

Pioneering work by Verbeek and Winter [12,13] showed that the reaction of alkyl- or aryl-chlorosilanes with ammonia, amines, or amides yields aminosilanes or silazanes which can be converted into polysilazanes (Eqs. 5,6). 

Lekishvili N., Samakashvili Sh. Reactions of polyaddition of dihydride siloxanes to diallyl- silazanes new approaches. Proceedings of Tbilisi State University, 360, 19-23 (2005) (Geo.). 

Submicrometer-size droplets of methyldichlorosilane were produced at cryo-scopic temperatures and reacted with ammonia to give high-molecular-weight solid silazane derivatives in a rapid reaction. On calcination these precursors ended as spherical mixed silicon nitrides of a- and p-Si3N4 (70). 

The Harrod group has investigated dehydrocoupling reactions of silanes with amines, hydrazines, and ammonia. In the presence of catalytic amounts of CuCl, hydrosilanes react with primary amines to give silazanes in high yields.180 For example, the reaction of phenylmethylsilane with benzylamine leads to a mixture of three products, with dependence on initial molar ratio of silane to amine [Eq. (72)]. Equimolar silane to amine ratio leads to formation of 1 as major product, whereas 2 is the main product if a 2 1... 

Reactive centers in a cyclic or chelated alkyl or aryl metal silizane, such as compounds 31-54, are (1) the polar metal-carbon bond, (2) the polar metal-nitrogen bond, and (3) other bonds in the silazane, especially nitrogen-hydrogen bonds. We have been interested in reaction centers (1) and (3) and have... 

To close this section, we describe our observations on the reaction of MeSi-(NrBu[H])3 (26) with the Grignard reagent CH3MgI in benzene/tetrahydro-furan (65). According to Eq. (32), four equivalents of the Grignard reagent always react with the silazane (26), even if the molar ratios are changed. 

The presence of two rather than three Cp rings around the trivalent uranium centers seems to provide an optimum stability to the complex. The synthesis of the Cp UI2(thf)3 and Cp 2UI(thf) was achieved in high yield upon treatment of UI3(thf)4 with the appropriate amount of Cp K (Scheme 17). Further functionalization of Cp UI2(thf)3 was obtained via a metathetic reaction with silazanate. The crystal structure of the resulting Cp U[N(SiMe3)2]2 showed the presence of agostic interactions between the U center and the methyl groups of the silazanate moiety (47). 

Silylation reactions on polysaccharides with chlorosilanes and silazanes were attempted more than 50 years ago resulting in hydrophobic silyl ethers with both increased thermal stability and solubility in organic solvents [376]. The silylation reaction for the protection of hydroxyl groups in mono- and polysaccharides exhibits many advantages, e.g. fast silylation, solubility of silylether in organic solvents suitable for subsequent derivatisation, stability of the resulting silylether under basic conditions but easy deprotection of the silyl moieties by acid hydrolysis or nucleophilic agents like fluoride and cyanide ions [377]. The partial and complete silylation of dextran was studied in detail by Ydens and Nouvel [215-217]. 

When the reaction temperature is raised to temperatures above 873 K, the 3450 cm 1 Si-NH2 band shows a tailing effect. Figure 12.3 illustrates this phenomenon by a FTIR spectrum of Kieselgel 60, thermally pretreated at 973 K and reacted with NH3 at 923 K for 5 hours. A band centred around 3405 cm 1 can be observed, attributable to silazane (Si-NH-Si) species. 

The formation of silazane species is probably due to a secondary reaction of the Si-NH2 species with (strained) siloxane bridges, according to reaction (C) 24... 

When the reaction temperature is raised above 423 K, the reaction mechanism becomes more complex NH4C1 will sublime from the surface and the amine function will gradually convert towards silazane and nitride species. 

In spectrum (b), the symmetrical Si-NH2 stretching vibration (3425 cm1) exhibits a tailing, which becomes a prominent shoulder in spectrum (d). This is due to a band in the region 3400 - 3350 cm1, which can be assigned to the N-H stretching vibration of silazane species. Amine species are progressively converted to silazane species as the reaction temperature rises. 

Fink610 has studied the conversion of Si-NH, functions towards silazane ( = Si-NH-Sis) groups for the reaction of ammonia with untreated silica. 

A significant lowering of the reaction temperatures at which the silazanes and nitrides are created can be observed. This is not the only advantage of chlorosilylation. The reader is referred to figure 12.5, showing an ammonia uptake enhancement factor upon trichlorosilylation > 10. 

In the following cycles, these same reactions will occur repeatedly, finally producing a cross linked network of silazane species, with outstanding amine functions and physisorbed NH4C1. 

The amount of silazane species increases linearly with the number of reaction cycles. The dotted lines in the figure represent the Cl and N concentrations, as calculated by the above estimations. 

It is obvious from this table that approximately 50% of the amine species convert to silazane species, by the reaction mechanisms, discussed in chapter 12. Increasing the reaction temperature from 973 K to 1113 K has no significant impact on the total N concentration, but causes a drastic change in the relative contributions of the different species at the silica surface. At 1123 K, more than 60% of the N containing surface species are present in the form of nitrides. 

The mechanism for the conversion of the silazanes is still very uncertain. Starting from a silazane polymer, Mazdiyasni4 and Glemser5 claimed that the following reactions are responsible for the formation of Si3N4 in bulk ... 

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