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

Abstract

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 Pt2CH2+. 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 H-2 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 H-2 finally yields the singlet dehydrogenating product with Gibbs free energy Delta G about -37.0 kcal mol(-1) (298.15 K). In the reaction of Rh2CH2+ with NH3, the doublet channel is favorable energetically. The overall low-cost pathway has a Gibbs free energy of Delta G about -26.1 kcal mol(-1) (298.15 K). (c) 2007 Published by Elsevier B.V.