Silicon complexes hexacoordinate

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

Relatively few ligand types have been used for the formation of neutral hexacoordinate silicon complexes, resulting in several complex types 182-193208 -218. Acetylacetonato (acac) chelates [182, (acac SiXY] were prepared directly from the reaction of... 

Picoline was found to react in chloroform solution with bis(dichlorosilyl)amine to form two hexacoordinate silicon complexes the ionic [H2Si(3-MeC5H4N)4]2+-2Cl and a neutral H2Si(3-MeCsH4N)2Cl2. These complexes are in equilibrium with each other in chloroform solution. The crystal structure of the ionic complex was reported247. 

Important requirements for formation of hexacoordinate silicon complexes is small bulk and a strongly electron-withdrawing character of the substituents at the Si center (171). These conditions are not met in the chlorosilane racemization and solvolysis processes studied (251,285). Also the faster rate of halogen exchange relative to inversion on silicon observed by Cartledge et al. (286) remains in conflict with the hexacoordinate silicon intermediate (A) pathway [Eq. (64)]. 

This paper describes the syntheses, structures, and stereodynamic behavior of the novel hexacoordinate silicon complexes 1—4. 

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... 

A subgroup of the neutral hexacoordinate silicon complexes discussed above is the family of binuclear complexes (55a,c,g,j). 

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

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... 

Stanley Kirschner Resolution and structure proof of a hexacoordinate silicon complex... 

Summary Our investigations on the coordination behavior of ethylene-ACV -bis(2-oxy-4-methoxybenzophenoneiminate) 1, a tetradentate chelating ligand of the salen type, toward diorgano-substituted silicon atoms led to the syntheses and X-ray structure analyses of novel hexacoordinate silicon complexes. The first X-ray structures of a metal-free hexacoordinate dimethyl silane and of a hexacoordinate cw-configurated silicon complex with a salen type ligand are presented. 

Scheme 1. Formation of hexacoordinate silicon complexes with ligand 1. (dimeric complex) (2h)...

Ligand Exchange Mechanism in Novel Hexacoordinate Silicon Complexes 437... 

A convenient method for the preparation of penta- and hexacoordinate silicon complexes was recently developed and reported [1]. The synthesis was based on an exchange reaction between poly-halogenated silanes with the O- or A-trimethylsilylated acylhydrazines and led to the formation of (0-Si)- and (N-Si)- mono and bis- chelate complexes (Scheme 1). 

Scheme 1. Synthesis of penta- and hexacoordinated silicon complexes.

Penta- and hexacoordinate silicon complexes often show very complex ligand exchange reactions, which have been the subject of intense studies [2-5]. In the present paper we analyze the ligand exchange processes and H, N, and Si NMR chemical shifts of a series of pentacoordinate complexes in a search for a better understanding of the exchange mechanisms. Compounds 2a-2h were readily prepared by the reaction shown in Eq. 1. 

Summary The reaction of two equivalents of lithium phosphinomethanides with di- or trifunctional chlorosilanes yields novel five- and six-membered heterocycles by multistep rearrangements or transmetallation reactions. Silaethene intermediates and hypervalent intermediates are likely to be involved. The reaction of one or two equivalents of lithium phosphinomethanide Li[C(PMe2)2(SiMe2Ph)] TMEDA with pTolSiCb and ci QHbcii iCb yields novel penta- and hexacoordinated silicon complexes. Both are the first examples of truly hypervalent organosilicon species with phosphorus donors characterized by X-ray structure determination. 

A coordination bond seems to be very efficient in transmitting spin-spin coupling. This is the case, for instance, in hexacoordinate silicon complexes where two-, three- and four-bond couplings including the dative N —> Si bond in the coupling pathway were reported. ... 

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. 

Summary The first ionic dissociation of the Si-Cl bond in neutral hexacoordinate silicon complexes is reported. An equilibrium reaction between the ionic siliconium chloride and its neutral precursor (dissociation-recombination) is observed. The population ratio can be controlled by temperature or by replacement of the chloro ligand by a triflate group. The reaction enthalpy and entropy of the dissociation are both negative, suggesting that solvent organization facilitates dissociation at low temperature. 

A convenient method for the preparation of neutral bis(N->Si) hexacoordinate silicon complexes has been developed and reported recently, consisting of ligand exchange between a polychlorosilane (1) and 0-trimethylsilyl derivatives of hydrazides (2, Eq. 1) [2]. An attempt to utilize this synthetic route for the preparation of isomeric 0->Si coordinated chelates did not lead to the expected hexacoordinate complexes, but to ionic siliconium chloride salts stabilized by two (O—>Si) dative bonds (5, Eq. 2) [3]. 

The spectral changes are completely reversible with temperature changes, and thus represent an equilibrium dissociation—recombination process, the first reported dissociation of a neutral hexacoordinate silicon complex [5], The Si chemical shifts of the equilibrium mixtures at various temperatures are listed in Table 1. 

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