Insight into the Mechanism of Dihydrogen-Heterolysis at Cyclopentadienone Iron Complexes and Subsequent C=X Hydrogenation

A bifunctional iron-based catalyst (7), a compound related to the [Fe]- or Hmd-hydrogenase, was studied by using experimental and computational methods. Catalyst 7 bears a protic hydrogen atom at the ligand and a hydridic hydrogen atom at the iron atom, which originate from the heterolytic uptake of H-2, and can be transferred to unsaturated molecules (i.e. carbonyl compounds and imines). Our DFT calculations indicate that a non-classical dihydrogen-iron complex exists, and that the hydrogen uptake is an exergonic process. The hydrogenation of carbonyl compounds proceeds through a synchronous, concerted outer-sphere mechanism, whereas the hydrogenation of the imine 18 is a two-step process. In the first step, the imine is protonated to afford an iminium ion, to which the hydride is then transferred in a second reaction step. Energy hypersurfaces were calculated by using a solvent (toluene) continuum model.