Lewis bases, complexes with silylenes

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. 

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,... 

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. 

In a very recent report, Okazaki and coworkers describe the reaction of 34 with hindered isocyanides 35a-c, leading to the first examples of stable silylene-Lewis base complexes 36a-c (equation 78). The latter were isolated as blue-green to blue solids, stable up to 60 °C. Data for these complexes are listed in Table 2. The high-field Si NMR resonances, and the very low values for the coupling constants, are consistent with... 

There is much evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as a Lewis acid are now well established (Fig. 14.3, Table 14.2). These complexes are also described as silaylides, R2Si —X+. Formation of silylene complexes with Lewis bases is conhrmed by a strong blue shift of the n-p transition. Matrix isolated dimesitylsilylene reacts with carbon monoxide to form the complex shown in Eq. The complex absorbs at 354 nm. 

Table IV also shows that the Amax values of the complexes are influenced even more by the substituents at silicon than by the nature of the Lewis base. The smallest blue shift on complexation occurs with silylenes, which are substituted by electron-donating groups such as aryloxy groups. This may be explained by intramolecular stabilization of the silylene by back-bonding of the heteroatom substituent, thus reducing the electrophilicity of the silicon center and weakening the silylene-donor interaction.30...

Scheme 33). In the presence of methanol, 128a-c were formed as the products resulting from protonation of the complex and subsequent addition of the methoxide ion. Weidenbruch et a/.84 also investigated the reactions of a bulky silylene with isocyanides, but no formation of a Lewis-base analogous to 28-30 was observed. 

Since they have both a vacant low-energy orbital and a lone pair, silylenes might behave either as electron pair donors or acceptors. There is scant evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as Lewis acids are now well-established these complexes can also be described as silaylides, R2>Si —B+262. Trinquier has calculated that even SiLL should form a weak complex with 112S i , in which a silane hydrogen binds to the p-orbital of the silylene263. 

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. 

In the reactions with organometallic complexes of f-metals, the reactivity of a pure Lewis base for the 1,3,2(A2)-diazasiloles was observed. Thus, silylene 85 reacts with (Cp)3Ln to give the complexes (Cp)3Ln(85) (Ln = Y, Yb)... 

Quantum-chemical studies showed that the ability to form complexes with Lewis bases decreases on going from silylenes to stannylenes and increases in the following series of n-donor agents amines < phosphines < arsines < stibines189. This series somewhat differs from that proposed based on experimental data188. Calculations were successfully used to predict the absorption maxima shifts on complexation of silylenes with amines190. 

A number of complexes of CAs with Lewis bases (X2E B ) were studied by matrix IR spectroscopy. Their absorptions are collected in Table 9. Data for complexes of silylenes are included in the table for comparison. 

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