Reversible beta-hydrogen transfer between Fe(C2H5)(+) and HFe(C2H4)(+): A case of two-state reactivity?

The iron ethyl cation, Fe(C2H5)(+), and its tautomer, the ethene complex of the iron hydride cation HFe(C2H4)(+), have been examined computationally using a hybrid of density functional theory and the Hartree-Fock approach (BECKE3LYP). The quintet Fe(C2H5)(+) ((5)A') corresponds to the global minimum of the [Fe,C2H5](+) potential energy hypersurface. Fe(C2H5)(+) can interconvert via beta-hydrogen transfer into HFe(C2H4)(+) (5A'), which is ca. 13 kcal mol(-1) less stable. The transition structure (TS) associated with their mutual interconversion on the quintet surface requires 36 kcal mol(-1) relative to Fe(C2H5)(+). However, this barrier may be circumvented by a reaction path on the energetically low-lying triplet surface in which the corresponding transition structure for beta-H transfer is 8 kcal mol(-1) lower in energy than the quintet TS. Thus, the path of minimal energy requirement connects the quintet species Fe(C2H5)(+) and HFe(C2H4)(+) via the triplet surface such that spin inversion is part of the reaction coordinate. Agostic interaction, which is only possible in the low-spin system, constitutes an essential factor for this unprecedented reaction mechanism. Further support to this interpretation is provided by mass spectrometric experiments which demonstrate that the interconversion Fe(C2H5)(+)reversible arrow HFe(C2H4)(+) is facile and occurs well below the respective dissociation asymptotes.