Density functional characterization of reactions of bimetallic carbenes PtMCH2+ (M = Pt, Au) with NH3 in the gas phase
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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 H-2 and degradation of the metal core have been discussed. Calculations reveal that in the case of Pt2CH2+ only direct elimination of H-2 from the moiety of CH2 occurs in the adduct (NH3)Pt2CH2+, and the activation energy for the rate-determining step is about 30 kcal mol(-1). The overall reaction is exothermic by similar to 30 kcal mol(-1). The heterometallic carbene PtAuCH2+ exhibits reactivity different from Pt2CH2+. The dehydrogenation from C-H and NH3 activation can occur in competitive mechanisms, where the channel to loss of H-2 from the stepwise N-H and C-H activation is more favorable than the elimination of H-2 in methylene or in NH3 both thermodynamically and dynamically. The ionic product aminocarbene AuPtCHNH2+ reacts with NH3, giving rise to the loss of H-2 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.