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– x C 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– x C bonds cleave through α,β,γ- or α,β,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 α,β-bound species with each C atom bound to several surface atoms. These α,β 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– x C 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.