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 beta-hydride elimination and reinsertion, and the beta-hydride elimination step is rate-limiting. The alkyl complexes transfer a beta-hydrogen atom to C=C, C=N, and C=O double bonds and undergo deprotonation by Bronsted acids. The reversible beta-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.