Kinetic Energy Release Distributions as a Probe of Transition-Metal-Mediated H-H, C-H, and C-C Bond Formation Processes: Reactions of Cobalt and Nickel Ions with Alkanes

Article abstract of DOI:10.1021/ja00209a001

Product translational energy release distributions are used to investigate the potential energy surfaces for elimination of H2 and small hydrocarbons from ionic cobalt and nickel complexes with alkanes.The amount of energy appearing as product translation can be used to infer details of the potential energy surfaces in the region of the exit channel and has implications for the ease with which the reverse reactions may occur.The potential energy surfaces for hydrogen and alkane elimination reactions are discussed in view of the very different kinetic energy release distributions observed for these processes.For dehydrogenation reactions, both the shape of the distribution and the maximum kinetic energy release are correlated with the reaction mechanism.For example, the amount of energy apearing in product translation is quite distinctive between reactions known to involve metal-induced 1,2- and 1,4-hydrogen elimination.The selective dehydrogenations of 2-methyl-propane-2-d₁ by Co⁺ and butane-1,1,1,4,4,4-d₆ by Ni⁺ serve, respectively, as models for these processes.A comparison of these translational energy distributions with those observed for loss of H2, HD, and D2 from the dehydrogenation of butan-1,1,1,4,4,4-d₆ by Co⁺ suggest that 1,4-elimination is dominant for the cobalt system and that the observation of different isotopic products results from scrambling processes.All the dehydrogenation processes examined were characterized by kinetic energy release distributions which could not be described by statistical theories.For these reactions, the maximum kinetic energy release approaches the estimated reaction exothermicity.In contrast, the more exothermic alkane eliminations have maximum kinetic energy releases which are less than half the reaction exothermicity, and the distributions can be fit with statistical models.For these processes the excess energy in the activated complex is approximately equal to the reaction exothermicity, suggesting a loose transition state for the disruption of a complex in which the intact alkane to be eliminated is interacting strongly with the metal center.Comparison of experiment with theory yields a Co⁺-propene bond strength of 48 +/- 3 kcal/mol, a Co⁺-CO bond strength of 34 +/- 3 kcal/mol, and a sum of the first and second metal bond strengths in Co(CD3)2⁺ of 110 +/- 3 kcal/mol at 298 K.The latter two values are derived from statistical kinetic energy release distributions observed for the loss of C2D6 and CO, respectively, in the reaction of Co⁺ with aceton-d₆.