Hexacoordinate silicon complexes intramolecular

VI. NEUTRAL HEXACOORDINATE SILICON COMPLEXES A. Intramolecular Coordination... 

The hexacoordinate silicon complexes discussed here are highly flexible compounds, like some other hypercoordinate silicon compounds,1 6 and undergo a variety of fluxional reorganization reactions, observable by temperature-dependent NMR spectroscopy. The dimethylamino-coordinated bis-chelates 30-38 are particularly suitable for an NMR study of ligand-site exchange processes (inter- or intramolecular), because of the pairs of diastereotopic V-methyl groups, which under certain... 

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

The main methods for the synthesis of hexacoordinate silicon compounds are similar to those for pentacoordinate complexes and were outlined in a recent review6. These methods include (a) addition of nucleophiles (neutral or anionic) to tetracoordinate silanes (b) intermolecular or intramolecular coordination to an organosilane (c) substitution of a bidentate ligand in a tetrafunctional silane. The following discussion focuses mainly on new complexes, reported since the recent reviews6,7 were published. 

An interesting example of an intermolecular complex is the trisilicon complex 194, in which only the central silicon is coordinated to the bidentate donor molecule225. The structure is a regular octahedron, with two tetrahedral termini. The silicon nitrogen bonds are rather short (2.012 and 1.991 A), and are comparable to those of octahedral intramolecular complexes (Table 23). 194 permits a comparison of Si—Cl bonds in a tetrahedral silicon moiety (2.03 to 2.07 A) with Si—Cl bonds trans to the dative bond in a hexacoordinate silicon (2.39 and 2.21 A). As expected, the latter are substatntially longer than the regular covalent bonds. 

A complex system of equilibria of both Si-Cl vs. Si-N dismutation and intramolecular Si-N exchange was observed with NMR spectroscopy in Si(pz )4 and Si(pz )3Cl [106]. The reactirai of Me3Si(pz ) with H2SiCl2 leads to the xmexpected formation of a silicon complex with three silicon atoms [134]. The lateral silicon atoms are pentacoordinated the central silicon atom is hexacoordinated. Furthermore, it was found that the reactimi of HSiCl3 with 3,5-dimethylpyrazole in acetonitrile gives 3,5-dimethylpyrazolyl siUcmi complexes where one (96) or two molecules acetonitrile (97) have been inserted into the braid between silicon and the 3,5-dimethylpyrazolyl ligand (see Scheme 28) [134]. 

Polyhalosilanes form neutral intermolecular hexacoordinate complexes with donor molecules such as pyridine, triethylamine, 2,2 -bipyridine and 1,10-phenanthroline. This topic has recently been reviewed6. It was demonstrated that electronegative substituents on the silicon are essential for the formation of intermolecular complexes. Thus, while SiCLt and Cl2CHSiCl3 react with 1,10-phenanthroline and with 2,2 -bipyridine to form hexacoordinate chelates, MeSiCl3 does not react224,225. For completion we discuss here a few examples, and compare some of the properties of intermolecular complexes with those of the intramolecular complexes. 

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