Deceptive similarities in the reactions of Fe⁺ and Co⁺ with linear nitriles in the gas phase

The gas-phase reactions of the transition-metal ions Fe⁺ and Co⁺ with linear C(4)-C(12) nitriles are reported. In spite of an overall similar reactivity pattern, a more detailed analysis, based on the study of labeled nitriles, reveals distinct differences with regard to the mechanisms of elimination of alkenes and alkanes. For both metal ions, hydrogen and alkenes are generated from linear C(4)-C(12) nitriles, and the intermediates are formed via oxidative addition to terminal and internal C-H bonds. For the RCN/Fe⁺ system insertion in an internal C-H bond commences at position C(8) of the nitrile; for the analogous RCN/Co⁺ system, the oxidative addition to an internal C-H bond starts at position C(7) of the nitrile. Similarly, alkane formation is different for the two transition-metal ions. For RCN/Fe⁺ the generation of alkanes is observed for nitriles having at least eight carbon atoms; in contrast, the elimination of alkanes from RCN/Co⁺ is already observed for C(6) nitriles. Alkane elimination seems to follow the conventional mechanism (i.e., oxidative addition to a C-C bond, β-hydrogen transfer, and reductive elimination) for the RCN/Co⁺ system, whereas for the RCN/Fe⁺ complex there exists an additional mechanism. This mechanism corresponds to the loss of H₂ from an internal position of the alkyl chain followed by the elimination of an alkene. Some possible origins of the different behavior of Fe⁺ vs. Co⁺ are discussed.