Silicon—nitrogen bonds transition-metal

In addition to activation of sihcon bonds by fluoride ions as discussed in Section 2.4, silicon-silicon, silicon-carbon, silicon-hydrogen, and silicon-nitrogen bonds are activated by transition metal salts and transition metal complexes. Thus, hydrolysis of silicon-carbon bonds such as in phenyltrimethylsilane 81 can be induced by... 

A less explored area of transition metal catalysis involves bond formation between Group 14 elements and nitrogen. In direct analogy to previously discussed areas of research, silicon-nitrogen bonds can be formed by dehydrocoupling, hydrosilylation, and dehydrogenative silylation. The compounds produced are valuable for use in organic synthesis or as polymer precursors to silicon nitride ceramics. 

The coordination of donors lengthens the Si=N bond and leads to pyramidalization at the silicon centre, as can be seen from the deviation of the sum of angles around the silicon atom from 360° (see examples in Table 17). Coordination of silanimines to transition metal centres also increases the silicon-nitrogen bond length to 1.66-1.69 A, which corresponds to a Si—N single bond. It has already been mentioned above that these metal compounds are best described as metallacycles349,350. 

Such reactions are known to occur at about 2S0°C for monoaminosilanes reacted with a second type of amine [IS]. Amines incorporated in a polysilazane network require higher temperatures for the transamination because of steric hindrance and the necessity to cleave-rebuild simultaneously two Si-N bonds. Transition metal catalysts can reduce the temperature requirements. In early studies [16] of the transition metal-catalyzed polymmzation of silazanes, we observed the catalytic cleavage of Si-N bonds at temperatures below 1S0°C and the ability to metathesize silicon nitrogen bonds. The transamination reactivity occurs at temperatures below that of the carbonization and carbidization of the organic groups. 

It is believed that most of the transition metals are complexed to nitrogen donors, such as are found in amino acids or derivatives of chlorophyll, and that the metals with high ionic potentials, such as beryllium, boron, germanium, titanium, gallium, and major elements such as aluminum and silicon, may be bonded to oxygen donors of degraded lignin. 

The chromium complexes are proved to be silanediyl complexes, as shown by the silicon-transition metal bond lengths (Table 5) and by the extreme low field shift of the 29Si NMR signals (124.9 and 121.2) at 22 °C for R = H and CH2NMe2, respectively (Table 6). The 29Si NMR shifts of these complexes are temperature-dependent due to the hindered rotation of the phenyl ring and dynamic coordination of the nitrogens to the Si atom. 

Compounds containing Silicon-, Germanium-, Tii, and Lead-Nitrogen Bonds.—Molecular nitrogen is reductively silylated by MeaSiCl and lithium metal in the presence of a transition-metal catalyst to give tris(trimethyl-silyl)amine [reaction (101)]. Variable-temperature n.m.r. spectra for a number of compounds containing Si—N bonds in which potentially diastereo-... 

In an attempt to stabilize low-valent silicon compounds, a class of compounds was studied in which N -> Si coordination served to stabilize a double bond between silicon and either sulfur phosphorus , nitrogen", oxygen or a transition metal " (e.g. 107). In these compounds the silicon is formally pentavalent, though it is coordinated to only four atoms. This topic belongs more appropriately to the chapter on silylenes, and is summarized here briefly for the sake of completeness. 

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