Reversible Beta-Hydrogen Elimination of Three-Coordinate Iron(II) Alkyl Complexes: Mechanistic and Thermodynamic Studies

High-spin organometallic complexes have not received extensive mechanistic study, despite
their potential importance as unsaturated intermediates in catalytic transformations. We
have found that, with a suitably bulky bidentate ligand, three-coordinate, high-spin alkyl
complexes of iron(II) are stable. They undergo isomerization and exchange reactions of the
alkyl group through β-hydride elimination and reinsertion, and the β-hydride elimination
step is rate-limiting. The alkyl complexes transfer a β-hydrogen atom to CC, CN, and
CO double bonds and undergo deprotonation by Brønsted acids. The reversible β-hydride
elimination reactions can be used to explore relative M−C bond energies. Competition
experiments and density functional calculations demonstrate an enthalpic preference for
alkyl isomers with iron bound to the terminal carbon of the alkyl fragment. This preference
arises from steric and electronic effects. The steric preference could be overcome with a phenyl
substituent, which steers iron to the benzylic position. A Hammett correlation and density
functional calculations suggest that the substituent effect is attributable to resonance
stabilization of partial negative charge on the alkyl ligand.