Binuclear silicon complexes

Keywords siliconium cations, Hexacoordinate silicon, binuclear silicon complexes... 

Scheme 28 Formation of binuclear silicon complexes as formal products of acetonitrile insertion...

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

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... 

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... 

Especially those metallo-siloxanes bearing an Si-H function at the y silicon have a high synthetic potential, since oxidative addition to electronically unsaturated metal centers gives easy access to Si-0-Si bridged binuclear metal complexes, as illustrated in Eq. 

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. 

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 ... 

The reaction of 1 with 2 (Eq. 2) proceeded like that of XSiQ3 with 2 [2], forming an ethylene-bridged binuclear hexacooidinate complex (5). This result was confirmed by the singlecrystal X-ray analysis shown in Fig. 1 for Sb. The crystal features two equivalent essentially octahedral silicon atoms, with the donor-nitrogens in trans positions relative to each other. 

In these complexes silicon is octahedrally surrounded by six oxygens (three dihydroxybenzenes, didentately bound to Si). According to Weiss et al. (1961), besides these mononuclear silicon complexes, also binuclear complexes (dimers) seem to be formed. 

Thus, in the present article we have shown what the self-assembly-driven growth seems to be due to H-bonding of binuclear heteroligand complex Ni2(0Ac)3(acac)-MP-2H20 with a surface of modified silicone, and further due to directional intermolecular H-bonds, apparently at precipitation of H O molecules, acac, acetate groups, MP [21, 22],... 

A series of binuclear pentacoordinated silicon complexes 85 of diketopiperazine have been synthesized and substituent (or leaving group) effects on the Si-O coordination have been studied for five analogues with X=F, Cl, OTf, Br, and I [226]. Variable-temperature NMR spectroscopy (supported by X-ray crystallography) shows, for the first time in binuclear pentacoordinated silicon complexes, a complex equilibrium with both nonionic (0-Si) and ionic (Si-X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution at the silicon atom in a binuclear pentacoordinated silicon compound (Scheme 21) [226]. 

Iridium siloxide complexes show a similar activity. Catalytic tests performed in the presence of [ Ir( 4-OSiMe3)(cod) 2], with the use of trimethylvinylsilane and dimethylphenylsilane as reactants [59], gave the same type of silicon derivatives as those obtained by Murai and coworkers [58], but the siloxide iridium precursor used appeared to be more efficient under milder conditions. When the [Ir(cod)(PCy3)(OSiMe3)] was used rather than the binuclear iridium siloxide complex, Z-Me3SiCH2CH=CHOSiMe2Ph was obtained exclusively [59],... 

Aldol reactions of silyl enolates are promoted by a catalytic amount of transition metals through transmetallation generating transition metal enolates. In 1995, Shibasaki and Sodeoka reported an enantioselective aldol reaction of enol silyl ethers to aldehydes using a Pd-BINAP complex in wet DMF. Later, this finding was extended to a catalytic enantioselective Mannich-type reaction to a-imino esters by Sodeoka s group [Eq. (13.21)]. Detailed mechanistic studies revealed that the binuclear p-hydroxo complex 34 is the active catalyst, and the reaction proceeds through a palladium enolate. The transmetallation step would be facilitated by the hydroxo ligand transfer onto the silicon atom of enol silyl ethers ... 

Schiff base complex, 32 13 -selenium complex FejScj complexes, 32 348 Fc4Se3 complexes, 32 349-350 Fc4Se4 complexes, 32 348-349 -selenium-nitrosyl complexes, 32 348-350 selenocyanates, 17 295, 296 sequestration in apoferritin, 36 463-464 silicates, Mbssbauer effect of, 6 474-479 -silicon compounds, 3 250 silyl complexes anionic tetracarbonyl, 25 37 binuclear carbonyls, 25 3, 5, 16, 33, 44-45, 116... 

In 1998, a new type of Pd(II) binuclear complex was reported which was effective for Mannich reactions of an imine derived from glyoxylate and anisidine with silicon enolates [38,39]. In these reactions, use of solvents including a small amount of water was essential. It was shown that water played an important role in this system water not only activated the Pd(II) complex to generate a cation complex, but also cleaved the N-Pd bond of the intermediate to regenerate the chiral catalyst. This reaction reportedly proceeded via an optically active palladium enolate on the basis of NMR and ESIMS analyses. A unique binuclear palladium-sandwiched enolate was obtained in the reaction of the p-hydroxo palladium complex with the silyl enol ether [(Eq. (9)]. 

In this paper, we report on the synthesis and structural characterization of a series of related zwitterionic (molecular) spirocyclic X Si-silicates (mononuclear A, Si-silicon(IV) complexes) containing two bidentate l,2-diolato(2-) ligands derived from a-hydroxycarboxylic acids, such as glycolic acid, 2 methyllactic acid, benzilic acid, and citric acid. In addition, some analogous zwitterionic X Ge-germanates (mononuclear X Ge-germanium(rV) complexes) are described. Furthermore, we report on the synthesis and structural characterization of related zwitterionic x Si,X Si -disilicates (binuclear X Si-... 

These compounds are characterized by the presence of two pentacoordinate (formally negatively charged) silicon atoms and two tetracoordinate (formally positively charged) nitrogen atoms. Because of the chiral nature of the trigonal-bipyramidal silicate moieties (see Chapter 2.1), special stereochemical features for these binuclear complexes have to be considered. 

Attempts to synthesize related binuclear X Si-silicon(IV) complexes containing meso-tartraXo-ligands failed [10]. Obviously, for geometrical reasons this particular type of ligand does not allow the formation of such compounds. 

From the electrochemical point of view, an important class of materials is that constituted by aluminosilicates incorporating cobalt, iron, etc., centers. In the case of Fe-based zeolites with Mobil Five structure (FeZSM-5) materials, different forms of iron can coexist. These include isolated ions either in framework positions (isomorphously substituting silicon centers), isolated ions in cationic positions in zeolite channels, binuclear and oligonuclear iron complexes in extra-framework positions, iron oxide nanoparticles (size <2 nm), and large iron oxide particles (FcjOj) in a wide distribution (up to 25 nm in size) located in the surface of the zeolite crystal (Perez-Ramirez et al., 2002). The electrochemistry of such materials will be reviewed in Chapter 8. 

In Kharasch addition to acrylic acid ester that is catalyzed by nickel complexes with silicon-containing dendrimers modified with a chelate aryldiamide ligand, the catalyst activity is lower than that of a low molecular weight analogue, van Koten and coworkers assumed that this is due to the small distance between nickel atoms on the dendrimer surface and to the formation of mixed-valence binuclear complexes [114, 126, 127]. 

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

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