Preferential activation of primary C-H bonds in the reactions of small alkanes with the diatomic MgO~(+?) cation

摘要

The C-H bond activation of small alkanes by the gaseous MgO ~(+?) cation is probed by mass spectrometric means. In addition to H-atom abstraction from methane, the MgO~(+?) cation reacts with ethane, propane, n- and isobutane through several pathways, which can all be assigned to the occurrence of initial C-H bond activations. Specifically, the formal C-C bond cleavages observed are assigned to C-H bond activation as the first step, followed by cleavage of a β-C-C bond concomitant with release of the corresponding alkyl radical. Kinetic modeling of the observed product distributions reveals a high preference of MgO~(+?) for the attack of primary C-H bonds. This feature represents a notable distinction of the main-group metal oxide MgO~(+?) from various transitionmetal oxide cations, which show a clear preference for the attack of secondary C-H bonds. The results of complementary theoretical calculations indicate that the C-H bond activation of larger alkanes by the MgO~(+?) cation is subject to pronounced kinetic control.