Si complexes

Stable silylene complexes with halogen substituents are a useful starting material for further displacement reactions. Starting from 7 and 11, the Si complexes 22-24 are accessible in high yield. However, the intermediate dichlorosilylene complex... 

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

A Ru—Si complex 169), a cobaloxime derivative 216a), a Ni complex 216), and the reported Pd compound (obtained in an impure state) 158) have also been made [Eqs. (20)-(23)]. 

Fig. 31.2 Typically (DIPAMP, CHIRAPHOS), the intramolecular exchange of Re and Si complexes is several times faster than the dissociation to reform the solvate complex.

The overall observed retention of the enantiomers, and thus the elution order, is based on several kinetically and thermodynamically controlled parameters concerned with stereorecognition nonstereoselective interactions of all partners SO(R), SA(R S), and particularly of the [SO(RI-SA(KI] and [SO(K)-SA(Si] complexes with the achiral stationary phase, also play a role (Figure 21). Therefore the retention order may be reversed for a specific pair of enantiomers depending on whether a covalently bound CSP or a CMPA is applied, but using the same chiral molecule (part) as chiral selector. These general principles, shown schematically for a CLEC system, are further complicated by the complexity of the entire system, hence they are difficult to anticipate and each case must be studied individually. 

TABLE 7. 29Si chemical shifts for dinuclear Si,Si complexes... 

A second method for the preparation of neutral pentacoordinate complexes consists of an exchange reaction between chloromethylchlorosilanes (ClCH2SiMe Cl3 , n = 0-2) and N-trimethylsilylated compounds, as shown for example in equation 2989. The compounds of types 96-99 were prepared by this method89-93. The same reagent was used extensively for the synthesis of O Si complexes, and this is discussed in more detail in the corresponding section (III.B.l). 

Another route to pentacoordinate N Si complexes (100) consists of the substitution of an O-trimethylsilyl group of O-silylated hydrazides by a halogenated silyl group,... 

Method (b), in which ligand exchange between silanes takes place, has been used to obtain both N- -Si (equation 48)210,211 and O—>Si complexes (equation 50)212, depending on the site of trimethylsilylation in the ligand molecule. 

Houdusse, A., Kalabokis, V. N., Himmel, D., Szent-Gyorgyi, A. G., and Cohen, C. (1999). Atomic structure of scallop myosin subfragment SI complexed with MgADP A novel conformation of the myosin head. Cell 97, 459-470. 

Figure 7.12 compares the measured and the theoretical absorption spectra for CH30N0(S i), the latter being obtained in a three-dimensional wavepacket calculation (Untch, Weide, and Schinke 1991a). The energy spacing of the main progression reflects the vibrational frequency of NO in the Si complex. The ratio of the peak intensities is mainly determined by the one-dimensional Franck-Condon factors... 

The right-hand part of Figure 9.12 depicts the final vibrational state distributions of NO obtained in a three-dimensional wavepacket calculation. They reveal a weak propensity for a final vibrational state n = n — 2. Excitation of vibrational state n in the HONO (Si) complex leads preferentially to NO products in vibrational state (n — 2). Because the intermediate potential well is rather deep, 0.362 eV, the breakup of... 

HREELS108 and surface extended X-ray adsorption fine structure (SEXAFS)109 measurements reveal that Si—O—Si complexes exist in the early stage of oxidation. Based on... 

Organosubstituted octasilsesquioxanes (Fig. lb) have also been prepared by cross-metathesis (CM) and silylative coupling of vinylsilsesquioxane with olefins in the presence of the ruthenium carbene complex Cl2(PCy3)2Ru(=CHPh) (Grubbs catalyst) and Ru-H (Ru-Si) complexes, for example, RuHCl(CO)(PCy3)2, respectively [57]. 

Our recent synthetic examinations have confirmed the non-metallacarbene mechanism for Ru-H and Ru-Si complex catalyzed reactions of vinyl alkyl ethers with vinylsilanes,yielding a mixture of ( +Z)-l-silyl-2-(alkoxy)ethenes (Eq. 32). Interestingly, l-silyl-l-(alkoxy)ethene has not been found among the products [51]. 

Formation of an ionic tetracoordinate Si+ complex from an uncharged nucleophile and a functional silane is an exothermic process accompanied by a marked drop in entropy. Many qualitative observations indicated that these complexes are generated more readily at lower temperatures (78,242,252,256). Unfortunately, there are few data on the thermodynamic parameters of complex formation. From the temperature variation of the 29Si resonance position, Bassindale and Stout (252) determined the enthalpy and entropy of the formation of bis(iV,Af-trimethylsilyI)-imidazolium chloride (Table IV, entry 10). A similar procedure permitted Chaudhry and Rummer (242) to determine the enthalpy of formation of complexes of 2-trimethylsilyl-l,l,3,3,-tetramethylguanidine (Table IV, entries 6, 7). 

The tetrahedral Si+ complexes readily exchange the mobile ligand (242,253) of the nucleophile on the silicon center, which was proved for the system shown in Eq. (55) (253). The reaction is rapid as compared with the... 

Tetracoordinate Si+ complexes may undergo transformations in the mobile ligand, since nucleophilic attack of the counterion may be directed toward electrophilic centers located in the ligand group. Some reactions involving these transformations are of importance in synthesis (86,272-274). For example, the silylation of phosphorus alkyl esters has been broadly explored by bioorganic chemists as a convenient method of generation of phosphorus acids [Eq. (57)] (273). The mechanism of the silylation reaction [Eq. (57)] has been well documented for the cases where X is... 

At higher temperatures, and when the alkyne substituent is bulky, the formation of the exo-Re complex (90) is favoured as it removes any steric interaction between the methyl group of the alkene and the R group of the alkyne. In general, the alkene will preferably insert into the Co CH bond over insertion into the Co CR bond for steric reasons, but in the case of the exo-Re complex, insertion into the Co CH bond leads to the methyl group pointing into the complex. In this case, the alkene insertion step is disfavoured. In the exo-Si complex (89), alkene insertion leads to the methyl... 

In summary, trialkylsilylium ions R3Si+ are stabilized by forming tetracoordinated Si complexes of type III. Pentacoordination at Si will only occur if there are just weak steric interactions between substituents R and solvent S. Mostly, this is parallel to a low internal stability of the silylium ion in question. Accordingly, R2HSi(S)2+ complexes can be observed experimentally while this is not possible for trialkyl silyl cations. [43]... 

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