Silane complexes

Both a-silane and a-dihydrogen complexes are useful and in some cases isolable models for the rarely observed a-alkane complexes. Unfortunately, silane complexes of Pt(II) are generally dinuclear (37,122) rendering them less desirable models for Pt(II) a-alkane complexes. In contrast, the closely related dihydrogen complexes of Pt(II) are mostly monomeric (123-126). 

Roewer et al. have used salene-type ligands for the preparation of neutral penta- and hexacoordinated silane complexes.834-836 Interestingly, the reaction of the acid form of the salene ligand reacts with organotrichlorosilanes in the presence of diethylamine to form pentacoordinated enamine silane complexes 849-851, whereas the disodium salt of the salene ligand reacts with phenyltrichlorosilane providing the hexacoordinated chlorosilane complex 852 (Scheme 119).834 The pentacoordinated complex 849 was also obtained when the hexacoordinated complex 852 was reacted with triethylamine (Scheme 119). 

E. Comparison of IHI with Residual a Interactions in Silane Complexes. 290... 

Mn quadrupolar broadening. On heating these compounds decompose via hetero-lytic cleavage of the p -H-Si bond, typical behavior for a cationic silane complex because the silicon center becomes electron deficient upon coordination of the H-Si bond to metal and thus activated toward nucleophilic attack. 

B. Comparison of Multicenter H Si Interactions with I HI and Residual G-Interactions in Silane Complexes... 

In method A, the ligand silane is used to react with surface hydroxyl groups to form a ligand-functional silica, and then a metal complex precursor is allowed to react with the functionalized surface. In method B, a metal-ligand silane complex is first formed in solution and then allowed to react with surface hydroxyls of the support. 

X3SiL ligand silane + M - metal precursor XjSiLM metal-silane complex... 

SiOII surface silane + X3SiLM metal-silane complex - SiOSiLM + IE... 

Cationic silane complexes are normally stable only in solution at low temperature and are rarely isolable (103). Thermally unstable bis(silane) adducts such as [IrHaCr -HSiEts PPhs SbFe (16) were reported by Crabtree to be detectable by NMR at low temperature and show intriguing reactivity, structures, and dynamics (103). 

There are much fewer examples in the literature of the heterolytic cleavage of Si H bonds compared to H-H and even fewer other examples in which a cr-silane complex was observed as an intermediate in the activation. We have synthesized alkyl-substituted diimine ligands with sidearms containing labile functional groups that reserve a vacant site for binding and activation of sigma bonds as shown in Eq. (32) (108,109). 

Brookhart and co-workers found that the cationic a-silane complex [CpFe(CO)(PR3)(HSiEt3)]+ was observable by NMR at RT but only in the presence of excess silane (sacrificial removal of trace H20 as Et3SiOH) and could not be isolated as a solid (122). The [CpFe(CO)(PR3)] fragment catalyzed silane alcoholysis in the presence of the BArF counterion (123). Although rapid deactivation of the catalyst occurred with ethanol as substrate, phenol reacted continuously with turnover numbers up to 80 min-1. It was proposed (Scheme 9)... 

The silane substrate would then displace H2 to give back the starting silane complex for further alcoholysis, and this was determined to be the rate-limiting step. These are all known reactions, and this mechanism and rate-determining step were recently supported by theoretical calculations that showed the heterolysis to be a highly concerted process, i.e., transformation of the a-silane complex to the H2 complex could even take place in a single step, thus circumventing the transient hydride complex (124). It is noteworthy that the mechanism of this reaction involves two different a complexes M(r 2 -Si-H) and M(r 2 -H2). 

Figure 9. Reaction of alcohol on penta-coordinated metal-silane complex.

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