Promotion effect of the protonated amine arm of a ruthenium complex on hydride migration to CO2:: A density functional study

The hydride migration to the carbon of CO2 on the experimentally synthesized novel N-H . . .H-Ru H-bonded complex (eta (5)-C5H4(CH2)(3)NMe2H+)RuH(dppm), 2, was examined by means of the hybrid density functional method (B3LYP), assuming the following two generally considered reaction paths: (a) the abstraction of the hydride ligand by an incoming CO2 without direct coordination of CO2 to the Ru atom, and (b) the insertion of CO2 to the Ru-H bond with the eta (2)-CO precoordination of CO2 to the Ru atom. Without the contribution of the protonated amine arm of the Ru complex, the reaction is uphill in path a, while 8.1 kcal/mol exothermic in path b, requiring the energy barrier of 6.8 kcal/mol. However, when the proton bonded to N of the amine arm has a H-bonding with an oxygen of CO2, the energetics in path a completely changes, and the potential energy surface becomes quite smooth, where only the small energy barrier of 2.6 kcal/mol is needed. On the other hand, in path b although the potential energy surface of the reaction is stabilized by about 20-21 kcal/mol, its feature does not change even with the participation of the amine arm with the H-bonding. Consequently, it was found that path a was more facile than path b because the function of the ptotonated amine arm is only effective for path a. This promotion effect of the protonated amine arm in path a originates from the enhancement of the electrophilicity at the carbon of CO2 by the H-bonding of the proton attached to N of the amine arm with one of the CO2 oxygens.