Dimeric Silylene Complexes

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

Also, reactive silylene complexes of iron and chromium can be generated at low temperatures and subsequently derivatized by trapping reagents. In THF as solvent, first labile THF adducts are formed, which are converted to the more stable HMPA adducts. The THF complexes dimerize above —40 °C with loss of THF. The silylene complexes can be utilized for reactions if they are generated in the presence of reagents like dimethylcarbonate. The resulting reaction products... 

Monomeric base adducts of silylene complexes can be transformed into dimeric compounds at elevated temperatures with loss of the donor. This applies also to reactive donor-free compounds. 

Carbene and silylene complexes - Carbene complexes of ruthenium [312] and osmium [313] porphyrins were formed from the neutral dimers [M(P)]2 with diazoalkanes (Eq. 26). 

Removal of the base from a base-stabilized mononuclear silylene complex can induce the dimerization of the generated base-free silylene complex... 

Another reaction in which the dimerization of a mononuclear silylene complex is suggested to play an important role is shown in Scheme ll.58 After the oxidative addition of disilacyclobutene to each iron center, fluorine migration accompanied by the cleavage of the Fe-Fe bond leads to a silyl(silylene) complex as one of the intermediates, which then dimerizes in head-to-tail fashion to form the product. 

Silylene complexes show high reactivities toward nucleophiles such as water, alcohols, ketones, and isocyanates [3, 4]. Some typical reactions of (OC)4Fe=SiR2(HMPA) are summarized in Scheme 1 [2]. The dimerization forming the diferradisilacyclobutane is the only known [2+2]-cycloaddition starting from 1. The photochemical reaction of 1 with 2,3-dimetylbutadiene... 

The free silylene complex 4 is a highly reactive intermediate. Therefore the dimerization reaction occurs without activation energy, leading to the transition state TS4.5. The dimerization is exothermic by 138.9 kJ/mol (Scheme 2). 

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... 

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