Tris silicon chelates

Dichloro neutral bis-chelates of hexacoordinate silicon were obtained by two methods directly (35) by transsilylation of Af-dimethylamino-O-(trimethylsilyl)acylimidates (1) with tetrachlorosilane, or by rearrangement as described in the previous section (70). The presence of two chloro ligands was utilized for further substitution, to form hexacoordinate silicon tris-chelates, by reaction with bis(trimethylsilyl)-precursors 71 and 72, as shown in Eqs. (31) and (32). The products (73, 74, and 75), are hexacoordinate neutral silicon tris-chelates, and 75 is the first reported zwitterionic tris-chelate with three different chelate rings.73... 

Liquid phase modifications. Alternatively a porous membrane can be reduced in pore size by a liquid deposition prcx ess where the membrane is dipped into a solution or sol to form deposits inside the membrane pores. For example, a silicon nitride tube with a mean pore diameter of 0.35 pm is first immersed in a solution of aluminum alcoholate (aluminum isopropylate or 2aluminum tris(ethyl acetoacetate) or ethyl acetoacetate aluminum diisopropylate) in an organic solvent (hexane, cyclohexane, benzene, isopropanol, etc.). It is then treated with saturated water vapor to hydrolyze the alcoholate or chelate to form bochmite inside the pores, thus changing the pore diameter to as small as 20 nm [Mitsubishi Heavy Ind., 1984a and 1934b]. Upon calcining at 800X, boehmite transforms into transition-alumina. 

Summary Treatment of 81X4 (X = Cl, Br) with bidentate ligands 2-[(7V-benzylimino)-phenylmethyl]-5-methoxyphenol l H and 8-hydroxyquinoline L H leads to hypercoordinate bis- or tris-chelates of silicon. These complexes were characterized by means of multinuclear NMR spectroscopy. The X-ray crystal structure of (L )2SiCl2 was determined. Furthermore the molecular structure of dichlorobis(8-oxoquinolinato)-silicon (L )2SiCl2 is presented, which is concluded from the comparison of calculated with experimental Si CP/MAS NMR data. 

Compound 3 was obtained in toluene, THF, and CHCI3 from reaction of SiCU and 8-hydroxyquinoline in the stoichiometric ratio of 1 2. In contrast, the analogous reaction with SiBr4 (in toluene) yields the tris-chelate complex 4. Consequently, the spectrum of the products is predominantly influenced by the reactivity of the silicon source, but a significant dependence on the used solvents was not observed. However, this conclusion is contrary to the results for ligand L H (reactions with 2-[(iV-benzylimino)phenylmethyl]-5-methoxyphenol) mentioned above. Compounds 3 and 4 were isolated, but we do not know exactly the configuration of the silicon-ligand sphere. We were not able to isolate the dibromo complex 5 from the mixture with 4. 

At first sight, the dihalido-bis-chelates 3 and 5 exhibit good solubility in DMSO, but the characterization of these compounds indicates their decomposition in this solvent. One product of this decomposition is a silicon complex with an Si chemical shift at -151.8 ppm, which corresponds to the chemical shift of the analogous tris-chelate 4. The solid-state Si NMR spectrum of 4 shows two tightly situated signals probably caused by the meridional and facial product. This supposition is confirmed by the fact that 36 peaks of the aromatic carbon atoms result in the NMR (solution) spectrum. Probably, a transformation between facial and meridional configuration takes place in solution within the NMR time scale of the Si and NMR spectroscopy. 

Compounds 35-37 are silicon(IV) complexes of the tris[salicylato(2-)]-silicate type. " They were prepared by using the same synthetic approach (synthesis of 35 Scheme 24.2) and isolated as the wer-isomers (37 was obtained as wer-37-2THF). In contrast to all the other aforementioned hexa-coordinate silicon(IV) complexes with three five-membered chelate rings, compounds 35-37 contain three six-membered chelate rings. 

Exemplificative of a third type of oxygen-bonded acetylacetonate derivative is the compound formed from the reaction trimethylchloro-silane and acetylacetone (77). The product from this reaction is 2-tri-methylsiloxy-2-pentene-4-one. It is to be noticed that this silicon compound contains a dangling ligand even though several chelated acetyl-acetonates of silicon are known. Dangling complexation of potential... 

Like the alkane solvents discussed previously, CO2 is a poor solvent for nonvolatile hydrophilic molecules. Some enhancement in hydrophile solvation was achieved by addition of cosolvents such as methanol and of complexing agents such as tri- -butyl phosphate [8,54]. Chelating agents have been designed with tails that have highly favorable interactions with CO2, such as fluoroethers and silicones [55]. A much wider range of hydrophilic compounds could be solubilized with surfactants that form reverse micelles and microemulsions in CO2 with polar or aqueous cores. However, extremely few commercial surfactants have tails compatible with the weak van der Waals forces of CO2. This limitation has made the formation of reverse micelles and microemulsions in CO2 more difficult than was the case for the alkane solvents discussed in the previous sections. 

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