Complexes of dichlorosilylene with allyl chloride and allyl bromide: matrix IR spectroscopy and quantum chemical studies

Formation of donor-acceptor complexes between dichlorosilylene, SiCl2, and allyl halides, AllHal(Hal= Cl, Br) was detected in Ar matrixes using matrix IR spectroscopy. In agreement with the predictions of the performed quantum chemical calculations, only broad unstructured absorption bands contributed by different conformers of the 1 : 1 complexes between SiCl2 and AllHal were observed in IR spectra of matrixes after deposition in the regions of characteristic vibrations of starting reactants. Annealing of matrixes resulted in strong narrowing the bands due to conversions of different conformers into the most stable structures. The predominantly formed conformers in both the reaction systems were those of complexes with SiCl2 coordinated to the Hal atoms of AllHal in the gauche conformations. At the same time, according to the calculations, the complexes with SiCl2 coordination to the double bonds of AllHal can be only slightly less stable than the complexes with coordination to the Hal atoms, and all these basic centers can be considered as comparable in their activity in the complexation. The only products revealed upon photolysis of complexes were the products of silylene insertion into the C–Hal bonds, viz., AllSiCl3 and AllSiCl2Br. Theoretical study of thermal transformations in the SiCl2 + AllHal systems showed that formal insertion of SiCl2 in the C–Hal bonds and its addition to the double bonds of AllHal have low activation barriers of 3–8 kcal mol–1. However, these barriers are too high for these reactions to occur under the matrix isolation conditions.