Siloxides bonding

Although organosilanes appear to react slowly (if at all) with water alone, in the presence of acids or bases (e.g., alkali metal hydroxides), reactions to give a silanol and H2 are rapid, with bases being particularly powerful catalysts. The evolution of H2 in this type of reaction may be used as both a qualitative and a quantitative test for Si-H bonds, and the mechanism of the acid and the base hydrolysis has been discussed in detail (30,31). This hydrolytic method is not very common for the preparation of silanols that are to be isolated, because both acids and bases catalyze the condensation of silanols to siloxanes, and therefore, only compounds containing large substituents are conveniently made in this way. If an anhydrous alkali metal salt is used, a metal siloxide may be isolated and subsequently hydrolyzed to give the silanol [Eq. (10)] (32). 

The polarity of the Al-C bond allows easy derivatization of the five-coordinate aluminum alkyls by alkane elimination (Figure 9). For example, Salen aluminum alkyls LAlMe could be converted to dimeric or polymeric Salen aluminum phosphinates [LAl 02P(H)Ph ] 98 (n = 2 or oo, depending on the Salen ligand backbone)98,99 by reaction with phenyl phosphinic acid, Salen aluminum siloxides LA10SiPh3 by reaction with triphenyl silanol,96 or Salen aluminum alkoxides LAIOR by reaction with an alcohol.100... 

According to the general idea presented by Wolczanski [4], alkoxide and siloxide (alternative to cyclopentadienyl complexes) of transition metals are bonded through a o-type orbital such as the sp hybrid and via Jt-donation of two pjt orbitals perpendicular to the M-O direction (Figure 7.1). 

Characterization of 6 by solid-state NMR spectroscopy as well as elemental analysis (only 28% decrease of rhodium content in samples after 20 cycles in the hydrosilylation) are convincing evidence of the high efficiency of surface rhodium siloxide complexes in the hydrosilylation of carbon-carbon multiple bonds as well as, presumably, in other reactions catalyzed by late transition metal siloxides syn-... 

The heteroleptic B-type ligand sphere offers more steric flexibility. However, chelation by simple donor-functionalities can yield strongly-bonded ligand-metal moieties which are kinetically rather stable, as is shown, for example, in benzamidinate [179] or siloxide complexes [180]. 

X-ray structure analysis revealed a 7-coordinate rare-earth metal center with two asymmetrically / -coordinating tetramethylaluminate ligands, an asymmetrically / -coordinating siloxide ligand and one methyl group of a trimethylaluminum donor molecule (Fig. 28). Such heteroleptic complexes can be regarded as molecular models of covalently bonded alkylated silica surface species. Moreover, isoprene was polymerized in the presence of 1-3 equivalents of diethylaluminum chloride, with highest activities observed for (Cl) (Ln) ratios of 2 1 (Table 12) (Fischbach et al., 2006, personal communication) [150]. 

We simulated thermal decomposition of the supported Cu(acac)2 by removing one of the ligands and the surface silanol proton, then forming a bond between the Cu cation and the surface siloxide. This structure was subjected to the PM3 energy minimization to produce the equilibrium geometry shown in Figure 8. 

An early report of Ti(SiPh3)476, prepared from TiCl4 and KSiPh3, has since been refuted and the compound reformulated as the siloxide Ti(OSiPh3)467. Further studies have shown that early metal-silicon bonds are quite reactive toward oxygen77, underscoring the importance of excluding air in preparations in early metal silyls. 

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