Role of Branching on the Rate and Mechanism of C−C Cleavage in Alkanes on Metal Surfaces

The kinetic relevance and rates of elementary steps involved in C−C bond hydrogenolysis for isobutane, neopentane, and 2,3-dimethylbutane reactants were systematically probed using activation enthalpies and free energies derived from density functional theory. Previous studies showed that C−C cleavage in alkanes occurs via unsaturated species formed in fast quasi-equilibrated C−H activation steps, leading to rates that decrease with increasing H2 pressure, because of a concomitant decrease in the concentration of the relevant transition states. This study, together with previous findings for n-alkanes, provides a general mechanistic construct for the analysis and prediction of C−C hydrogenolysis rates on metals. C−C cleavage in alkanes is preceded by the loss of two H atoms and the formation of two C-metal (C−M) bonds for each C and C atom involved in the C−C bond. Metal atoms transfer electrons into the C and C atoms as C−C bonds cleave and additional C−M bonds form. C and C atoms of isobutane, neopentane, and 2,3-dimethylbutane, however, do not lose H atoms before C−C cleavage, and thus, transition states cannot bind the C and C atoms in the C−C bond being cleaved to surface metal atoms. C−H activation occurs instead at C atoms vicinal to the C−C bond, which lose all H atoms and form three C−M bonds. These transition states involve electron transfer into the metal surface, leading to a net positive charge at the C and C atoms; these atoms exhibit sp geometry and resemble carbenium ions at the C−C cleavage transition state, in which they are not bound to the metal surface. These mechanistic features accurately describe measured H2 effects, activation enthalpies, and entropies, and furthermore, they provide the molecular details required to understand and predict the effects of temperature on hydrogenolysis rates and on the location of C−C bond cleavage within a given alkane reagent. The result shown and the conclusions reached are supported by rigorous theoretical assessments for C−C cleavage within about 200 intermediates on Ir surfaces, and the results appear to be applicable to other metals (Rh, Ru, and Pt), which show kinetic behavior similar to Ir.