Mechanism and Energetics for Dehydrogenation of Methane by Gaseous Iridium Ions

Ir^+ has been observed as the most efficient transition-metal ion for dehydrogenation of CH_4 in the gas phase. We carried out theoretical studies of the complete reaction profile for this process and find three salient factors responsible for the high reactivity: (i) the ability of Ir^+ to change spin easily, (ii) the strength of the Ir-C and Ir-H bonds, and (iii) the ability of Ir^+ to form up to four covalent bonds. We show that among transition-metal ions Ir^+ is unique in best possessing all three characteristics. The combination of these factors leads to a global minimum for the singlet Ir(H)_2(CH_2)^+ structure, which plays an important role in the activation. On the basis of these results, we suggest solution-phase analogues that may also activate CH_4.