Binuclear disiliconium salts

The partial and reversible ionization of binuclear hexacoordinate silicon complexes 55a,c,j is described in Section III.A.5.ii. Like the mononuclear siliconium chloride salts, these can also form stable binuclear disiliconium salts (57a,c,j) by replacement of the chloride by other counterions, which are better leaving groups (triflate, bromide, or iodide, Eq. 37).69... 

The salts 82-85 were characterized by their typical pentacoordinate 29Si chemical shifts (Table XXV), by the equivalence of the two chelate rings resulting from the molecular C2 symmetry, and by spectral analogy with a binuclear disiliconium dichloride salt (86) for which a crystal structure (Fig. 46) was obtained. 

In search for new oligomers and polymers containing hypervalent silicon chelates, the reactions of bis(trichlorosilyl)ethane (1) with O- and N-trimethylsilylated hydrazides (2 and 3, respectively) have been studied. The reaction of 1 with 3 was reported to lead to ionic binuclear disiliconium dichloride salts (4, Eq. 1) [1]. It was not obvious how the reaction of 2 with 1 might proceed would it form a neutral binuclear hexacoordinate complex, like the mononuclear analogues [2], or would it form ionic complexes, in analogy to 4 ... 

Summary Binuclear hexacoordinate silicon complexes with two N-+Si dative bonds (per Si atom) have been prepared. The crystal stracture conforms to a distorted octahedron, with the N— Si bonds trans to each other. The binuclear complex undergoes Si-Cl dissociation of one Si Cl bond in CD2CI2 and CDCI3 solution, to form a binuclear, monosiliconium chloride salt, in reversible equilibrium with its precursor. The dissociation and equilibrium reactions are observed by variable-temperature Si NMR spectra. The extent of ionic dissociation increases as the temperature is decreased. The equilibrium population ratio is shifted completely to the di-ionic side at room temperature, by replacement of the chloride by the less nucleophilic triflate anion. The crystal structure of a disiliconium ditriflate shows a) well separated ions, b) that the geometry about silicon is almost an exact square pyramid, and c) that the N- Si bonds are among the shortest coordination bonds of this kind. 

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