Metal-catalyzed C-C bond cleavage in alkanes: Effects of methyl substitution on transition-state structures and stability

Methyl substituents at C-C bonds influence hydrogenolysis rates and selectivities of acyclic and cyclic C₂-C₈ alkanes on Ir, Rh, Ru, and Pt catalysts. C-C cleavage transition states form via equilibrated dehydrogenation steps that replace several C-H bonds with C-metal bonds, desorb H atoms (H*) from saturated surfaces, and form λ H₂(g) molecules. Activation enthalpies (δH^‡) and entropies (δS^‡) and λ values for ³C- ^xC cleavage are larger than for ²C-²C or ²C-¹C bonds, irrespective of the composition of metal clusters or the cyclic/acyclic structure of the reactants. ³C- ^xC bonds cleave through ∝,ß,γ- or ∝,ß,γ,δ-bound transition states, as indicated by the agreement between measured activation entropies and those estimated for such structures using statistical mechanics. In contrast, less substituted C-C bonds involve ∝,ß-bound species with each C atom bound to several surface atoms. These ∝,ß configurations weaken C-C bonds through back-donation to antibonding orbitals, but such configurations cannot form with ³C atoms, which have one C-H bond and thus can form only one C-M bond. ³C-^xC cleavage involves attachment of other C atoms, which requires endothermic C-H activation and H* desorption steps that lead to larger δH^‡ values but also larger δS ^‡ values (by forming more H₂(g)) than for ²C-²C and ²C-¹C bonds, irrespective of alkane size (C₂-C₈) or cyclic/acyclic structure. These data and their mechanistic interpretation indicate that low temperatures and high H₂ pressures favor cleavage of less substituted C-C bonds and form more highly branched products from cyclic and acyclic alkanes. Such interpretations and catalytic consequences of substitution seem also relevant to C-X cleavage (X = S, N, O) in desulfurization, denitrogenation, and deoxygenation reactions.