Mechanism of the Reaction of Gas-Phase Iron Ions with Neutral Olefins

The structures of FeC_nH_2n⁺species formed by reaction of Fe(CO)⁺with olefins (n = 2–14) and cycloalkanes (n = 3–6) in a high-pressure source were examined by using mass spectrometric techniques. The stoichiometry of the complexes was confirmed by high-resolution mass spectrometry, and their structures were probed by collisional activation of the iron-olefin complex in a tandem mass spectrometer (MS/MS). Ethylene, propene, and isobutene retain their structural integrity in complexes with Fe⁺, but all of the other olefins studied rearrange after complexation to a structure with two or more ligands. These rearrangements are generally consistent with a mechanism that begins with addition of an allylic C-C bond to the metal ion followed by abstraction of a β-H atom from the resulting alkyl ligand to form a hydrido π-allyl intermediate. Transfer of the H atom to the allylic fragment produces a bis(olefin) complex. This mechanism dominates unless the required β-H is absent. Possible alternatives to this mechanism are presented for intermediates with no available β-hydrogen. Additional support for the mechanism was obtained by studying the structures of decomposition products of the FeC_nH_2n⁺adducts. These studies involved comparisons of the collision-activated decomposition (CAD) spectra of product ions formed either in the source by bimolecular reactions or by collisional activation of stable FeC_nH_2n⁺(an MS/MS/MS experiment) with corresponding reference spectra. Finally, the generality of the allylic insertion mechanism was tested by examining a series of FeC₈H₁₆⁺ions formed from isomeric octenes and methylheptenes.