Dehydrogenation reactivities of bimetallic species RhMCH₂⁺ (M = Pt, Rh) with different spin multiplicities toward NH₃ in the gas phase: A density functional theory study

The dehydrogenation reactions of bimetallic species RhMCH₂⁺ (M = Pt, Rh) with ammonia have been investigated by carrying out density functional calculations. Equilibrium structures and energies have been determined for intermediates and transition states in these reactions. Present results show that both metal-carbene cations have the reactivity similar to that of Pt₂ CH₂⁺. The variatation of spin multiplicity can further modify significantly their dehydrogenation activities with NH₃. In the reaction of RhMCH₂⁺ (M = Pt, Rh) with NH₃, elimination of H₂ exclusively arises from the moiety of CH₂ and no N-H bond activation of ammonia takes place. The favored dehydrogenation process occurs on the triplet state potential energy surface of RhPtCH₂⁺ with NH₃, followed by intersystem crossing from the triplet to the singlet states, then the Rh-mediated loss of H₂ finally yields the singlet dehydrogenating product with Gibbs free energy ΔG about -37.0 kcal mol^-1 (298.15 K). In the reaction of Rh₂ CH₂⁺ with NH₃, the doublet channel is favorable energetically. The overall low-cost pathway has a Gibbs free energy of ΔG about -26.1 kcal mol^-1 (298.15 K).