Silicon, chelates

An effective route to O Si pentacoordinate silicon chelates consists of the reaction of chloromethyl(dimethyl)chlorosilane (ClCH2SiMe2Cl, 8) with a variety of N- or O-trimethylsilylamides, 5 lactams, ureas, 5... 

Binuclear silicon chelates (55a,c,g,j) were prepared from 1,2-bis(trichloro-silyl)ethane and Af-dimethylamino-O-trimethylsilylimidates (la,c,g,j) in high yields.69 The hexacoordination of these complexes is evident from a crystal structure analysis obtained for 55c (Fig. 23), as well as from their... 

Reaction of functionalized ketones such as a-hydroxy ketones or 1,3-ketones with allyltrifluorosilane in the presence of Et3N has also been investigated [95]. Although catecholate does not play a key role in the pentacoordination of silicon, chelating hexacoordinate intermediates are formed to give allylation products with high yields and selectivity (Sch. 54). 

Our understanding of silicon chelates with organic ligands is still insufficient, and research in this area is sparse. 

Neutral complexes are also known in which silicon is hexacoordinate. Pike and Luongo found that tetracarboxysilanes undergo a general reaction with various 1,3-diketones to yield neutral silicon chelate derivatives 202 (330) (Figure 33). A hexacoordinate structure is assumed on the basis of IR and NMR spectral and chemical evidence (325,330). 

Coupling Constants through a Rapidly Dissociating-Recombining N—>Si Dative Bond in Pentacoordinate Silicon Chelates... 

We have recently reported the first observation and measurement of spin-spin interactions in neutral hexacoordinate silicon chelates [1] which extend across the N— Si coordinative bond and over two, three, and even four bonds. These coupling constants were highly sensitive to small geometrical modifications in the complex. The major geometrical requirement found for spin-spin coupling over two bonds (N—>Si-F or N—>Si-H) in hexacoordinate complexes la-lc was that the corresponding bond angle be 90° or very close to it. 

We now report the first measurements of coupling constants extending over one, two and three bonds across the dative bond in pentacoordinate silicon chelates. Coupling via the N— Si bond was found to persist even at temperatures at which rapid dissociation and recombination of this bond take place [3,4]. 

Binuclear Ethylene-Bridged Silicon Chelates Equilibrium between Neutral Hexacoordinate and Ionic Pentacoordinate Siliconium Complexes... 

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 ... 

A cationic silicon chelate is also formed with 1-hydro.xypyridine yV-oxide. Gardner and Katrizky prepared 24 derivatives of this and the 1-amino analogue (172), and Weiss and Harvey made salts with different anions (173, 174). However, as described in Chapter 1 these chelates, which are apparently stable in water at low pH. decompose rapidly at higher pH. 

The first pure silicon compound identified in a plant was isolated and identified by Weiss and Herzog as a silicon chelate of thujaplicine, an isopropyl tropolone in the conifer Thuja plicata (127) (see also silicon metabolism discussed below). 

FIGURE 10. Synthetic pathways for some neutral pentacoordinate silicon chelates ... 

The tautomeric equilibrium between penta- and hexa-coordinated silicon chelates (Scheme 5.60) was studied by Si NMR spectroscopy [3( Si) = —57.6ppm for penta-coordination and 5( Si) = —134.0ppmfor hexa-coordinationj. The study is very detailed involving the effects of temperature, solvent, substituent, and counterion effects [142]. 

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