Cationic silylene complex

The displacement of triflate ion from silicon is important as a route to cationic silylene complexes, like the one shown in equation 102. 

Alkoxide anion abstraction by a Lewis acid from phosphorus ligand on a transition metal is one of the most useful methods to prepare cationic phosphenium complexes. It has been reported that OR anion abstraction from an alkoxyalkyl ligand, C(OR)R2, on a transition metal complex leads to the formation of a cationic carbene complex.39-42 It is similarly expected that OR anion abstraction from an alkoxysilyl ligand, Si(OR)R2, would cause the formation of a cationic silylene complex. Therefore, increased attention has been focused on the reaction with a Lewis acid of transition metal complexes bearing both alkoxyalkyl and phosphite ligands and of complexes bearing both alkoxysilyl and phosphite ligands. 

Platinum-silylenes are important to numerous transformations involving organosilicon compounds. Base-stabihzed Fisher-type cationic silylene complex of platinnm(II) may be prepared by anion abstraction from a silyl group, as shown in Scheme 51. The counter anion is cracial... 

Very recently, the coordination chemistry of low valent silicon ligands has been established as an independent, rapidly expanding research area. With the discovery of stable coordination compounds of silylenes [35-38], a major breakthrough was achieved. Within a short time a variety of stable complexes with a surprising diversity of structural elements was realized. Besides neutral coordination compounds (A, B) [35, 36, 38], and cationic compounds (C) [37], also cyclic bissilylene complexes (D) [39,40] exist. A common feature of the above-mentioned compounds is the coordination of an additional stabilizing base (solvent) to the silicon. However, base-free silylene complexes (A) are also accessible as reactive intermediates at low temperatures. 

Triphenylphosphonium ylide reacts with the silylene complex 93 which has a highly electrophilic silicon center, to give the corresponding cationic adduct 94 [115]. The lengthening of the PC bond indicates a loss of the double bond character of the ylide and corresponds to the formation of a tetrahedral silicon center with four covalent bonds (Scheme 28). 

A formally pentavalent cationic silicenium complex, with intramolecular base-stabilization, is the silylene-iron 167, with a Fe=Si double bond. The crystal structure of 167 shows that silicon is essentially tetrahedral in this compound, and the solution 29 Si chemical shift is 8 = 118.3 ppm, compatible with the double bond character rather than with pentacoordination at silicon189. 

In addition to ruthenium, Tilley and coworkers also reported that cationic iridium silylenoid complexes were efficient olefin hydrosilation catalysts [reaction (7.6)].56 This silylene complex catalyzes the hydrosilation of unhindered mono- or disubsti-tuted olefins with primary silanes to produce secondary silanes with anti Markovni-kov selectivity. Iridium catalyst 32 exhibited reactivity patterns similar to those of ruthenium 30 only primary silanes were allowed as substrates. In contrast to 30, cationic iridium 32 catalyzed the redistribution of silanes. Exposing phenylsilane to 5 mol% of 32 in the absence of olefin produced diphenylsilane, phenylsilane, and silane. 

Gas phase ion chemistry has provided the means for obtaining labile intermediates that are thought to play a relevant role in solution chemistry. An example is given by cationic iron-silene and iron-silylene complexes that have been studied as model systems in... 

COMPARISON BETWEEN CATIONIC PHOSPHENIUM COMPLEXES AND SILYLENE COMPLEXES... 

As mentioned above, cationic phosphenium is isoelectronic with carbene, silylene, germylene, stannylene, and plumbylene, because the central element has lone pair electrons and a vacant p orbital as well as two substituents. Therefore, their transition metal complexes have attracted considerable attention. It might be of special interest to compare cationic phosphenium complexes with silylene complexes because phosphorus and silicon are both situated in the third row of the periodic table. 

A cationic phosphenium complex has positive charge. It has been estimated that the positive charge is located on the phosphorus atom to considerable extent and that the central metal is electronically neutral or slightly negative.51,52 With regard to silylene complexes, strong M5+ =Si5 polarization has been reported.53,54 Therefore, these species are expected to form an adduct with a 2e-donor base such as an amine or ether [Eq. (24)]. 

All cationic phosphenium complexes reported to date are base-free species. In contrast, silylene complexes tend to be stabilized by forming base adducts. Most silylene complexes are base-stabilized species54 59 and base-free silylene complexes are still rare.59-63 Typical examples are shown in Scheme 7. A phosphite-phosphenium complex exists in a solid state and even in solution, and does not take an OMe bridging form between the two P atoms,64 whereas a silyl-silylene complex is not detected, but exists as an OMe bridging form between the two Si atoms.65... 

Tilley reported silene-silylene rearrangement in a cationic iridium complex (Eq. 13) [13b]. 

The photolysis of an equimolar amount of SiL 2 and Cp2Mo(PEt3) in hexane results in the formation of the molybdenum-silylene complex Cp2Mo(SiL 2) (3) (Eq. 5). Complex 3 is a deep red, extremly air- and moisture-sensitive solid showing a Si-NMR signal at 139.9 ppm. This is significantly downfield-shifted in comparison both with the free silylene SiL (78.3 ppm [11] and with other silylene complexes ((CO)2Ni(SiL 2)2i 97.5 ppm (CO)5Mo iPhAr 97.5 ppm [12]) indicating a distinct silicenium cation character of 3. 

The first well-characterized silylene complexes stabilized by Lewis bases were reported by Zybill (Equation 13.36) and Tilley (Equation 13.37). In one case, the silylene complex was formed by elimination of a salt and in another case by abstraction of triflate in the presence of Lewis base. The first base-free silylene complexes were prepared by using soft iT-donating thiolate substituents on silicon (Equation 13.38). One thiolate group from a tris(thiolatosilyl) complex was abstracted with TMSOTf, and exchange of a less coordinating tetraphenylborate anion for triflate formed a stable cationic silylene product lacking coordinated base. ... 

The reactivity of metal-silylene complexes is more limited than the reactivity of carbene complexes. The cationic base-stabilized ruthenium-silylene complex in Equation 13.37 does not react with olefins or alkynes to undergo [2-1-2] addition reactions. However, a related complex did undergo [2-1-2] addition reactions with isocyanates, as shown in Equation 13.46. Other reactions of silylene complexes are distinct from those of carbene complexes or those of other conventional organometallic compounds. For example, the reaction of the silylene hydride with an acetylene generates a p-silylvinylarene complex... 

A theoretical basis for the description of the cationic complex [Cp Ru(PR3)2 = = SiR2]+ can also be given. For a d6 CpML2 system, a complete splitting of the three orbitals (octahedral symmetry) is to be expected. Consequently, a coordinated silylene ligand (without any base) should prefer the indicated (Fig. 10) conformation. 

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