Effects of Chain Length on the Mechanism and Rates of Metal-Catalyzed Hydrogenolysis of n-Alkanes

C-C cleavage in C-2-C-10 n-alkanes involves quasi equilibrated C-H activation steps to form dehydrogenated intermediates on surfaces saturated with H atoms. These reactions are inhibited by H-2 to similar extents for C-C bonds of similar substitution in all acyclic and cyclic alkanes and, thus, show similar kinetic dependences on H-2 pressure. Yet, turnover rates depend sensitively on chain length because of differences in activation enthalpies (Delta H double dagger) and entropies (Delta S double dagger) whose mechanistic origins remain unclear. Density functional theory (DFT) estimates of Delta H double dagger and Delta G double dagger for C-C cleavage via >150 plausible elementary steps for propane and n-butane reactants on Ir show that hydrogenolysis occurs via alpha,beta-bound RC*-C*R'double dagger transition states (R = H, CxH2x+1) in which two H atoms are removed from each C*. Calculated Delta H double dagger values decrease with increasing alkane chain length (C-2-C-8), consistent with experiment, because attractive van der Waals interactions with surfaces preferentially stabilize larger transition states. A concomitant increase in Delta S double dagger, evident from experiments, is not captured by periodic DFT methods, which treat low-frequency vibrational modes inaccurately, but statistical mechanics treatments describe such effects well for RC*-C*R double dagger species, as previously reported. These findings, together with parallel studies of the cleavage of more substituted C-C bonds in branched and cyclic alkanes, account for the reasons that chain length and substitution influence Delta H double dagger and Delta S double dagger values and the dependence of rates on H-2 pressure and consequently explain differences in hydrogenolysis reactivities and selectivities across all alkanes.