Carboborosilazane ceramics: initial reactions between TSDE and methylamine – a combined quantum chemical and first principles molecular dynamics study

Cl2BCH(CH3)SiCl3, in words 1-(trichlorosilyl)-1-(dichloroboryl)ethane (TSDE), is a precursor for the synthesis of Si/B/N/C high-demand ceramics. For the preparation of a ceramic of approximate stoichiometry Si2B2N5C4, methylamine is used as a network-forming agent. One way to get information about preferred coordinations in the amorphous network is a quantum chemical investigation of the reactions during the cross-linking of the precursor. For this reason, we have investigated several gas-phase reactions with Møller–Plesset perturbation theory: the aminolyses of TSDE and the decomposition of adducts ((H3C)H2N · SiCl3CH(CH3)BCl2 and (H3C)H2N · BCl2CH(CH3)SiCl3). Our results confirm qualitative chemical intuition: The major primary process in the gas phase is a substitution at the boron center of TSDE. Also, it can be seen that the cleavage of the Si–C or B–C bond in the respective adduct is strongly exothermic but kinetically hindered. The situation in liquid phase has been studied by means of Car–Parrinello molecular dynamics simulations (CPMD). In an excess of CH3NH2, the reaction mechanism of the B–Cl aminolysis changes. Instead of a four-cyclic transition state, a salt-like intermediate occurs. As a result, the reaction includes three minor reaction barriers and becomes highly exothermic. In contrast, the dissociation of the boron adduct is suppressed in an excess of methylamine. A break-up of the Si–C–B unit can therefore not be expected during the first stage of the aminolysis.