Thermal activation of N-H bonds by transition-metal oxide cations: does a hierarchy exist in the first row?

The thermal reactions of first-row transition-metal oxide cations [MO](+) (M=Sc-Ni, Zn) with ammonia have been studied by gas-phase experiments and computational methods. The activation of N-H bonds is brought about by the monoxides of the middle and late 3d metals Mn-Ni and Zn. The two primary reaction channels correspond to dehydration, which leads to [M(NH)](+), and hydrogen-atom abstraction to form [M(OH)](+). Oxygen-atom transfer from [MO](+) to NH(3) to produce neutral or ionized hydroxylamine was observed as a minor channel for some of the late transition-metal oxides. The computational analysis of these reactions, which was aimed at elucidating the reaction mechanisms and to uncover possible periodic trends across the first row, have been performed for the couples [MO](+) /NH(3) (M=Sc-Zn). Dehydration is found to be endothermic for the oxides of scandium to vanadium and exothermic for the other systems. Hydrogen-atom abstraction becomes exothermic starting with [MnO](+) and, finally, oxygen-atom transfer is feasible for the cationic oxides of nickel to zinc.