Clustering and activation in reactions of CoCp with hydrogen and methane

Gas-phase clustering reactions of CoCp⁺ with H₂ and with CH₄ were investigated using temperature-dependent equilibrium experiments. In both systems, the CoCp⁺ ion was found to form strong interactions with two ligands. The first and second H₂ groups cluster to CoCp⁺ with bond energies of 16.2 and 16.8 kcal/mol, respectively, while the first and second CH₄ groups cluster to CoCp⁺ with bond energies of 24.1 and 12.1 kcal/mol, respectively. These bond energies are in good agreement with those determined by density functional theory (DFT). Molecular geometries for the four clusters determined with DFT are also presented. Weak experimental bond energies of 0.9 kcal/mol for the third H₂ and 2.2 kcal/mol for the third CH₄ clustering to CoCp⁺ suggest these ligands occupy the second solvation shell of the ion. In addition to clustering in the methane system, H₂-elimination from CoCp(CH₄)₂⁺ was observed. The mechanism for this reaction was investigated by collision-induced dissociation experiments and DFT, which suggest the predominate H₂-elimination product is (c-C₅H₆)Co⁺---C₂H₅. Theory indicates that dehydrogenation requires the active participation of the Cp ring in the mechanism. Transfer of H and CH₃ groups to the C₅-ring ligand allows the metal center to avoid the high-energy Co(IV) oxidation state required when it forms two covalent bonds in addition to its interaction with a C₅-ring ligand.