Silylene-base complexes

Absorption Maxima Amax (nm) of Selected Silylenes and Silylene-Base Complexes in Solution... 

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

If doubly-bonded compounds >M = X (M = Si, Ge, Sn) are now well known, this is not yet the case for the heavy allenic derivatives >M = C = X. Transient silaallenes >Si = C = C 160-167 or silaazaallene >Si = C = N—168 were postulated some years ago, but the first stable derivative of this type, a formal 3-stanna-l-azaallene >Sn = C = N —, was isolated only in 1992 by Griitzmacher et al169 The compound Tbt(Mes)Si = C = NR170 has also been obtained, but according to the authors it is closer to a silylene Lewis base complex than to a silacumulene. The first stable silaallene was only reported in 1993 by West et al.111 and the first metastable >Si = C = P— was characterized still more recently.172... 

II. Theoretical Studies of Silylene-Lewis Base Complexes. 2... 

Scanning the literature one notices that silylene-Lewis base complexes are drawn either using an arrow, which points from the heteroatom toward the silicon center, thus indicating a dative bond (structure A), or as a 1,2-dipole with a covalent bond (structure B). Because the theoretical results concerning the extent of charge separation in silylene-Lewis base complexes or silaylides are contradictory (Section II,A) we will use both grapical descriptions of these compounds alternatively and interchangeably. 

THEORETICAL STUDIES OF SILYLENE-LEWIS BASE COMPLEXES... 

Only few theoretical studies have been devoted exclusively to the coordination of Lewis bases to silylenes. Most calculational evidence for the formation of silylene-Lewis base complexes was obtained from the computational investigation of the insertion reaction of HiSi into various H-X a bonds, where X is an heteroatom center possessing one or more free... 

The only carbon nucleophile whose coordination to silylenes was investigated theoretically is carbon monoxide. The main interest centers on the question whether this nucleophile forms a Lewis acid-Lewis base complex... 

By far more is known about the UV spectra of silylene-Lewis base complexes in matrices as well as in solution. To observe silylene-Lewis base complexes in matrices, an appropriate silylene precursor, in most cases a cyclic or acyclic oligosilane, is photolyzed at low temperature in an inert hydrocarbon or noble gas matrix, which is doped with small amounts (2-5%) of the respective Lewis base. In a few cases, 2-methyltetrahydrofuran (2-MeTHF), which serves as matrix material as well as a Lewis base, is used. When a rigid matrix such as a 3-methylpentane (3-MP) or 2-MeTHF matrix is used, the formation of the silylene complexes normally requires the matrix to be annealed in order to allow the molecules to diffuse through the softening matrix. In contrast, when the silylene is generated in an a priori soft matrix,... 

As seen from Table IV, the feature most characteristic of the formation of a matrix silylene-Lewis base complex is the hypsochromic shift of the visible absorption band of the silylene on complexation. At this point it is worthwhile to mention that as early as 1982 it was reported28 that the absorption maxima of methyl-substituted silylenes in a nitrogen matrix are shifted to significantly higher energies than those of the same silylenes in an inert argon matrix. However, the cause of the shift difference was not addressed in this publication. 

The only exception to these uniform spectroscopic properties of silylene-Lewis base complexes, i.e., the hypsochromic shift on complexation, is silacarbonyl ylide 27, the absorption maximum of which is shifted to wavelengths longer than that of free dimesitylsilylene (20a) 31 an explanation of this observation is yet to come. 

Whereas UV spectroscopic data are available for base complexes of a variety of matrix-isolated silylenes (Table IV), solution data are reported almost exclusively for complexes of dimethylsilylene (13) (Table V). Again, the complexation of this silylene is accompanied by a significant blue shift of the absorption maximum. Probably due to matrix effects of unknown nature, the Amax values of dimethylsilylene complexes in solution are shifted to wavelengths shorter than those of the corresponding matrix-isolated complexes. 

The laser flash photolysis technique allows the observation of rapid reactions and determines the lifetime of reactive intermediates. Thus, it is not surprising that this method was used extensively to obtain information about the formation, stability, and reactivity of silylene-Lewis base complexes. 

The reactivity of silylene-Lewis base complexes toward a variety of silylene traps such as olefins, alcohols, and silanes is significantly reduced, as can be seen from Table VII. 

This section presents reactions that proceed via the formation of silylene-Lewis base complexes. The material is organized according to the method used for silylene generation. 

Theoretical Studies of Silylene-Lewis Base Complexes. 

The first base-free silylene ruthenium complexes without a 7r-donor stabilization were prepared by Tilley and coworkers by displacement of a triflate ion by a noncoordinating lithium perfluorotetraphenyl borate (equation 34). The base-free silylene... 

From the reactions of carbon monoxide in frozen matrices with Me2Si155 and several arylsilylenes Mes(R)Si (R = Mes, 2,6-diisopropylphenoxy, f-Bu)156, adducts were formed (see Section V.A.2). Theoretical calculations led to consideration of both a linear silaketene and a pyramidal n-donor base complex structure for the Me2Si(CO) adduct249. The observation that warming the carbon monoxide adducts of the arylsilylenes led to the formation of disilenes was interpreted as indicating the formation of a nonplanar complex that could dissociate as do other silylene-n-donor base complexes156. 

Alcohols are also bases toward silylenes. Complexes of alcohols with Mes2Si and other hindered silylenes are observed spectroscopically when a 3-MP matrix containing the silylene and 5% of 2-propanol or 2-butanol is annealed113. Melting of the matrix results in rapid reaction of the complex to give the O—H insertion product (equation 92). These results imply that formation of an acid-base complex is the probable first step in the reaction of silylenes with alcohols and perhaps with many other reagents259. 

As described in Section III.B.2.b, carbon monoxide forms complexes with silylenes which are relatively weak, losing CO easily to give the disilene155 156. Therefore, it is likely that they have the acid-base complex structure 45, in accord with theoretical prediction249, rather than the alternative silaketene structure 46. However, in addition to the absorption bands near 350 nm (Table 4), a weak absorption near 600 nm was observed for matrices containing silylenes and CO. These long-wavelength bands may possibly be due to the silaketene isomers. 

The nature of the bonding in silylene-metal complexes, as compared with the better known metal-carbene complexes, is a question of considerable interest. MO calculations on H2Si=Mo(CO)5 indicate that the Si—Mo bond consists of a cr-donor and --backbond component, like the carbon-metal complexes. The -component is, however, weaker than for metal carbenes251. Infrared C=0 frequencies for the base-free silylene metal complexes support this model. Theoretical considerations of the bonding in silylene-metal complexes are treated more fully in Section IV.E. 

Silylene 59 also behaves somewhat like a phosphine in its interactions with metal carbonyls98,149-376. Typical reactions involve substitution of silylene for CO, to give a silylene-metal complex. Three examples are shown in Scheme 20, and the structure of the nickel complex 75 is displayed in Figure 7149. This complex is both the first silylene-nickel complex, and the first example of a bis-silylene-metal complex free of stabilization by Lewis base donors. 

The first silylene-metal complex with no base stabilization was prepared by extrusion of SiMe2 from a substituted disilane (384), as in Eq. (165) where R is H or Me. 

Silylene 5 is remarkably stable, in sharp contrast to previous silylenes. It was purified by vacuum distillation at 85 °C (1 Torr) and survives heating in toluene solution to 150 °C for many months. The pure compound decomposes only at its melting point, 220 °C. Compound 5 is also less reactive than usual silylenes. It is inert to triethylsilane, diphenylacetylene, or 2,3-dimethylbutadiene, all of which react rapidly with conventional silylenes. Moreover, it does not form acid-base complexes with Lewis bases such as THF and pyridine, although normal silylenes do [19]. However, 5 does react with methanol, water, and dioxygen. 

An X-ray crystal structure has been determined for 7 and is shown in Fig. 3. This compoimd is the first silylene complex of nickel, as well as the first bis-silylene metal complex of any kind without base stabilization [20]. 

Silicon. The first base-stabihzed silylene complex [Cp Ru =Si(Ph)2NCMe (PMe3)2] was reported in 1987, and the first base-free silylene-containing complexes [Cp Ru=Si(SR)2(PMe3)2] (R = Me or Ph) were not synthesized until 1990, much later than for the heavier congeners Ge, Sn, and Pb. Recently, the silylene-containing... 

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