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 ¹C and ²C atom involved in the C-C bond. Metal atoms transfer electrons into the ¹C and ²C atoms as C-C bonds cleave and additional C-M bonds form. ³C and ⁴C 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 ³C and ⁴C atoms in the C-C bond being cleaved to surface metal atoms. C-H activation occurs instead at ¹C 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 ³C and ⁴C atoms; these atoms exhibit sp² 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.