Compounds with Hexacoordinated Silicon Atoms

Si-N bonds in compounds with hexacoordinate silicon and tricoordinate nitrogen atoms... 

Table 7. Relationship between electronegativites xa A-atoms and pressure required for the synthesis of compounds with hexacoordinated silicon...

The X-ray structure analysis of (salen )SiF2 (8) clearly demonstrates the octahedral coordination of the silicon atom (Fig. 1) [4], There are a number of crystal stractures of bis-chelate compounds with hexacoordinate silicon. Most of these had essentially a tetrahedral arrangement around silicon with the coordinated nitrogen donor atoms capping the tetrahedra at relatively large distances (N-Si between 2.5 and 3.0 A) [5]. 

Scheme 24), the hydride migration from silicon to an adjacent unsaturated imino carbon atom leads to a pentacoordinated silicon complex 89 as final product [170]. For the intermediate 88 a dynamic equilibrium between two conformers 88a and 88b with pentacoordinated Si atom was observed by NMR spectroscopy. For related compounds with hexacoordinated Si atom within a (0,N>2SiMe(H) coordination sphere, the authors observed reversible neutral dissociation of the N-Si dative bond, i.e., an equilibrium between hexa- and pentacoordinated hydrido complexes of silicon [235]. 

Going back to mechanistic studies it is not clear if the reactions of nucleophiles with hexacoordinated silicon compounds are pure nucleophilic substitutions or if they take a more complex route. However there is another challenge to find whether the silicon atom can accept being in heptacoordination. Such a possible situation has been observed with a tricapped tetrahedron structure of a silane which has been proved to be isosteric with the corresponding germane of which the X-ray structure determination has been carried out. 

TABLE 20. Si—O bond lengths in compounds with tetracoordinate silicon and hexacoordinate oxygen atoms... 

FIGURE 23. Histogram of Si—F bond lengths in compounds with a hexacoordinate silicon atom... 

The average Si—Cl bond length in compounds with a hexacoordinate silicon atom was determined from 17 experimental values to be 2.184 A (s.d. 0.02 A, s.m. 0.006 A). The results are shown in Figure 25 and Table 29. 

Summary Our investigations on silicon compounds of etfaylene-MiV"-bi (2 -hydroxyacetophenoneiminate) led to the synthesis and X-ray structure analysis of a new kind of salen complex — hypervalent silicon compounds with a threefold deprotonated salen ligand and an enamine structure. This stmctural unit provides access to new routes for synthesizing hypercoordinate silicon complexes. Addition reactions between various Brpnsted acids and these new pentacoordinate silicon compounds were carried out to precipitate complexes bearing hexacoordinate silicon atoms. 

Transition metal-free hydrosilylation of carbonyl compounds can be realized with the use of Brpnsted or Lewis acids as well as Lewis bases. Alkali or ammonium fiuorides (CsF, KF, TBAF, and TSAF) are highly effective catalysts for the reduction of aldehydes, ketones, esters, and carboxylic acids with H2SiPh2 or PMHS. Lithium methoxide promotes reduction of esters and ketones with trimethoxysilane. A generally accepted mechanism of Lewis base-catalyzed hydrosilylation of carbonyl compovmds involves the coordination of the nucleophile to the silicon atom to give a more reactive pentacoordinate species that is attacked by the carbonyl compound giving hexacoordinate silicon intermediates (or transition states), in which the hydride transfer takes place (Scheme 30) (235). 

Far less compounds have been reported which comprise heavier elements in the coordination sphere of their penta- or hexacoordinated silicon atom. During the past decade significant contributions emerged on the field of silicon coordinatirai chemistry with heavier lone pair donor atoms, and therefore this section will be dedicated to highlight these compounds. 

Hypercoordinate silicon complexes with tetradentate (O, N, N, 0)-chelating ligands of the salen type are expected to exhibit unusual chemical and physical properties because of the higher coordination number of the silicon atom [1,2]. Therefore, several attempts were made to synthesize such compounds [2, 3]. Starting from easily available silicon compounds such as SiCU or other chlorosilanes, conversion with salen type ligands mostly yielded complexes with a hexacoordinate [2, 3] and, in some cases, pentacoordinate silicon atom [4]. Unfortunately, there are only a few examples where the coordination geometry has been confirmed by X-ray structure analysis [2, 4]. 

We prepared the 3,5-dimethylbenzyl-dimethylphosphine complex /ra/75-(3,5-Me2C6H3CH2-PMe2)2SiCl4 (1) by simple mixing of the phosphine [8] with SiCU in the appropriate molar ratio and determined its crystal and molecular structure [9, 10] The resulting structural parameters (Fig 1), especially the Si-P and Si-Cl bond lengths, should be reliable reference values for this class of compounds. In CDCI3 solution, complex 1 maintains its trans hexacoordination of the central silicon atom as follows from its characteristic Si and P NMR resonances (6( Si) = -209.4 ppm (t), Jsi p... 

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. 

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