Density functional characterization of reactions of bimetallic carbenes PtMCH₂⁺ (M = Pt, Au) with NH₃ in the gas phase

Dehydrogenation and C-N coupling from PtMCH₂⁺ (M = Pt, Au) and ammonia have been investigated by the density functional methodology. The structure and stability of intermediates and ionic products as well as detailed mechanisms for loss of H₂ and degradation of the metal core have been discussed. Calculations reveal that in the case of Pt ₂CH₂⁺ only direct elimination of H₂ from the moiety of CH₂ occurs in the adduct (NH₃)Pt ₂CH₂⁺, and the activation energy for the rate-determining step is about 30 kcal mol^-1. The overall reaction is exothermic by ∼30 kcal mol^-1. The heterometallic carbene PtAuCH₂⁺ exhibits reactivity different from Pt ₂CH₂⁺. The dehydrogenation from C-H and NH ₃ activation can occur in competitive mechanisms, where the channel to loss of H₂ from the stepwise N-H and C-H activation is more favorable than the elimination of H₂ in methylene or in NH ₃ both thermodynamically and dynamically. The ionic product aminocarbene AuPtCHNH₂⁺ reacts with NH₃, giving rise to the loss of H₂ and AuH. Predicted relative energetics and barriers along suggested reaction paths are in reasonable agreement with experimental observations. Different reactivities of PtAuCH₂ ⁺ and Pt₂CH₂⁺ in the gas-phase reactions with NH₃ arise from distinct geometrical and electronic structures of their reactive precursors.