Hexacoordinate silicon compounds anionic complexes

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. 

In contrast to the well-established chemistry of anionic and cationic hexacoor-dinate silicon(IV) complexes with SiOe skeletons, neutral hexacoordinate silicon(IV) species with SiOe frameworks are significantly less well explored. Compounds and 60 are examples of this type of compound. 

Hypervalent silicon compounds attract interest from both the structural and reactivity point of view [1]. The azomethine AJV -ethylene-bis(2-hydroxyacetophenoneimine) (salen H2 1), was formed by condensation of ethylenediamine with 2-hydroxyacetophenone. We set out to synthesize hexacoordinate silicon complexes containing the salen ligand. The anion salen is able to chelate the silicon atom through four donor atoms. There are some rare examples of salen-silicon compounds known from the literature [2], but characterization of these compounds seems to be doubtful [3]. Structural aspects are uncertain due to the lack of crystal structure data. 

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. 

Recent studies on hydrazide-based hypercoordinate silicon complexes demonstrated the unusual flexibility of these compounds their tendency to reversibly transform between penta- and hexacoordinate compounds [1, 2] on the one hand and to irreversibly rearrange to more stable complexes [3] on the other. Thus, neutral hexacoordinate bis-chelate complexes (1) undergo reversible ionization in solution (Eq. 1), which is strongly dependent on a variety of factors temperature (ionization is enhanced at low temperatures), solvent (ionization takes place in hydrogen-bond donor solvents such as CHCI3, CH2CI2, and CHFCI2), the nature of the anion, the... 

At the methylidene group carbon atom of 3 a relatively high electron density NMRCCDCls) 5 = 8S.5 ppm) is located which could be caused by the mesomeric electron-releasing effect of the silicon-substituted enamine nitrogen atom. Therefore this methylidene group carbon atom is able to pick up a proton from some acids. Even weak acids react with 3 if a stable Si-0 bond can be formed with the corresponding anion. 1,4-Addition of methanesulfonic acid, picric acid, benzoic acid and hydroquinone to 3 (solution in THE) led to hexacoordinate salen-silicon complexes by precipitation of compounds 4, 5, 6 and 7 respectively (Scheme 1). The Si chemical shifts of the solids obtained (CP/MAS-NMR) are presented in Table 2. 

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