Pentacoordinate silicon compounds structure

The mechanism of these reactions can be described as a nucleophilic attack on a pentacoordinate silicon compound. Supporting this assumption, neutral or anionic hexacoordinated structures have been isolated and their X-ray structures obtained [5]. 

The literature on these structures is sparse, and generally the articles report preparations of several of them in similar ways. These compounds are constituted by oxygen, sulfur, or nitrogen adducts at the silicon atom, thus providing pentacoordinated silicon compounds or structural analogs (Scheme 33). 

Although anionic, pentacoordinate silicon species have received attention, the isolation of siliconates 194, 103, 195, 196 (Figure 28) by Perozzi, Martin and colleagues (155) has provided an opportunity to study in detail both structure and stereomutation of a pentacoordinate silicon compound (155, 321). This point will be discussed further in Sect. V-C. The crystal structure of 195 shows that the silicon is pentacoordinated with the fluorine and the two carbon atoms forming the equatorial plane of a somewhat distorted trigonal bipyramid (321). It is essentially the same as that proposed for similar siliconates on the basis of NMR data obtained in the solution phase (155). 

Summary The syntheses and properties of derivatives of a new class of zwitterionic monocyclic 5i-silicates with an Si02C2F framework are reported. These molecular pentacoordinate silicon compounds contain bidentate diolato(2-) ligands that derive from benzohydroximic acid, oxalic acid, or glycolic acid. In addition, the crystal structures of these compounds are described. 

Over the past few years, we have synthesized and structurally characterized a series of zwitterionic acyclic and spirocyclic X S/-silicates, such as compounds 1 [1] and 2 [2] (Scheme 1). These zwitterions contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. We have now succeeded in preparing a series of related zwitterionic monocyclic A. S i-silicates. We report here on the syntheses, properties, and crystal structures of compounds 3a/3b-5a/5b (Scheme 1) [3] (for reviews dealing with pentacoordinate silicon compounds, see [4—10]). 

Summary Derivatives of a new class of zwitterionic spirocyclic A, 5/-silicates with an SiO C framework have been prepared. The syntheses and the properties of these molecular pentacoordinate silicon compounds are reported. The crystal structures of two of the zwitterions are described. 

In continuation of our systematic studies on zwitterionic pentacoordinate silicon compounds, we have now succeeded in synthesizing the molecular A, 5/-silicates 5-10 (for reviews dealing with pentacoordinate silicon compounds, see [13-19]). These zwitterions belong to a new class of spirocyclic pentacoordinate silicon compounds with an SiO C skeleton. They contain diolato(2-) ligands derived from acetohydroximic acid and benzohydroximic acid, respectively (tautomers of acetohydroxamic acid and benzohydroxamic acid). Preliminary data concerning 5 have already been published elsewhere [8]. We report here on the syntheses and properties of compounds 5-10. In addition, the crystal structures of 6 and 8 are described. 

In contrast to the well-established chemistry of zwitterionic spirocyclic A, 5/-silicates with an S1O4C framework [1-11] (such as compounds 1 [1], 2 [9], and 3 [7]), only little is known about related zwitterions with an SiOs skeleton (such as compound 4 [11]). We report here on the syntheses, structures, and properties of the zwitterionic X Sr-silicates 5 and 6 (5/O4C skeletons) and their respective isoelectronic analogs 7 and 8 (SiOs skeletons). The studies presented here were carried out with a special emphasis on the comparison of the isoelectronic CH2/O pairs 5/7 and 6/8 (for reviews dealing with pentacoordinate silicon compounds, see [12-18]). 

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. 

In contrast to fluorosilanes with pentacoordinate silicon systems such as 235-237, it is very difficult to find similar compounds of chlorosilanes to establish the Si—Cl bond length in similar cases. X-ray crystal structure is available for only few of these compounds and only typical cases of the shortest and longest Si—Cl bonds are given. The shortest bond lengths are found in compounds where the silicon is bonded to a metal such as... 

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

Compound 86 exists in two diastereomeric forms due to the chiral silicon centers, which are evident in the various NMR spectra (Table XXV). The crystal structure of 86 shows a molecular inversion center, and hence belongs to the meso diastereomer. The geometry around silicon in the crystal is a distorted TBP and the chloride is more than 7 A away from silicon, in accord with a pentacoordinate siliconium chloride structure. The dative Si bonds are exceptionally short in 86 (1.802 and 1.807 A) relative to those in other pentacoordinate complexes.2,3,1115... 

Indeed, the majority of the higher-covalent silicon compounds studied which have an increased coordination number are formed by bi-, tri-, and tetradentate ligands similar effects are apparent with other Group IVA elements. It is likely that the structural factors are responsible for the existence of pentacoordinate pentavalence carbon species expanding the valence shell to 10 (Sect. 3.2). For the same reason that there are about twenty structures of the organotin complexes with seven- and eight-coordination... 

Over the past few years, a series of zwitterionic spirocyclic bis[l,2-benzenediolato(2-)]-organosilicates, such as compounds 1 [Ic] and 2 [la] (Fig. 1), and related zwitterions with 2,3-n q)hthalene-diolato(2-) and symmetrically substituted l,2-benzenediolato(2-) ligands have been structurally charac-terized by X-ray diffraction [1]. The coordination polyhedra around the pentacoordinate silicon atoms of such compounds can be described using the idealized geometries of the trigonal bipyramid (TBP) and the square pyramid (SP). In the solid state, almost all transitions between a... 

Summary The synthesis of the first silicon complexes with the tridentate diketoamine HN[CH2C(rBu)=0]2 (1) is described. The reaction of YSiXs (X = Cl, Br Y = H, Cl, Br, Ph, Vinyl) with 5-aza-2,2,8,8-tetramethylnonane-3,7-dione (1) in the presence of NEtj as an auxiliary base furnished the unusual dinuclear silicon complexes (6-11) having pentacoordinate silicon centers. Theoretical calculations (B3LYP/6-31G ) carried out for the monomeric chloro-substituted silane 7A are consistent with a folded structure and a relatively short and highly polarized Si-N bond (1.742 A Charge Si 0.96 N -0.53). The calculated value for the dimerization energy of 7A is -29 kcal/mol, reflecting the presence of a partial Si-N it-bond. Thus, the novel dimeric silicon compound 7 is the result of a head-to-tail dimerization of formally two Si=N bonds in 7A. 

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]. 

The structure of 3 in solution at room temperature is dynamically pentacoordinated, showing a singlet for both the benzylic protons and the NMe2 group in the H NMR spectrum. A significant upheld shift in the SiNMR spectrum in comparison to mesityltrichlorosilane indicates a highly coordinated silicon compound (Table 1). 

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