Reversible β-hydrogen transfer between Fe(C 2H 5) + and HFe(C 2H 4) +: case of two-state reactivity?

Abstract

The iron ethyl cation, Fe(C 5H 5) +, and its tautomer, the ethene complex of the iron hydride cation HFe(C 2H 4) +, have been examined computationally using a hybrid of density functional theory and the Hartree-Fock approach ( Becke3LYP). The quintet Fe(C 2H 5) +( 5A′) corresponds to the global minimum of the [Fe,C 2,H 5] + potential energy hypersurface. Fe(C 2H 5) + can interconvert via β-hydrogen transfer into HFe(C 2H 4) + ( 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 Fc(C 2H 5) +. However, this barrier may be circumvented by a reaction path on the energetically low-lying triplet surface in which the corresponding transition structure for β-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(C 2H 5) + and HFe(C 2H 4) + 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 providedby mass spectrontetric experiments which demonstrate that the interconversion Fe(C 2H 5) + ⇄ HFe(C 2H 4) + is facile and occurs well below the respective dissociation asymptotes.