Understanding iron-catalyzed alkene hydrogenation: Unraveling the role of spin-state changes

Abstract

Organometallic catalysts based on cheaper first-row transition metals are increasingly popular. Recently one of us described a new set of complexes [PhBP3]Fe-R, R = Me, Bn, CH2CMe3, based on a mono-anionic trisphosphine ligand [PhBP3]. These species catalyze hydrogenation of alkenes and alkynes. Sep., the complexes have also been shown to react slowly with dihydrogen in the presence of a phosphine to provide an Fe(IV) trihydride species [PhBP3]FeH3L. The starting Fe(II) species is paramagnetic, whereas the trihydride is diamagnetic. In this talk, we will present work aimed at elucidating the mechanism of the dihydrogen addn. reaction as well as the hydrogenation, in a bid to develop improved catalysts. It will be shown that all of the key reactions occur over low-spin singlet (or in some cases triplet) transition states, whereas the key intermediates, [PhBP3] Fe-R and [PhBP3]Fe-H, have high-spin quintet ground states. The computational methodol. used to (a) accurately describe the relative energetics of the high- and low-spin states, and (b) characterize the rates of the spin-change events will be described. The key mechanistic conclusion is that the hydrogenation catalytic cycle involves repeated changes from the less reactive but more stable high-spin [Fe]-H and [Fe]-R species to lower spin adducts with H2 or alkene. This has the effect of decreasing reactivity compared to a hypothetical analogous low-spin iron catalyst. However, the latter low-spin catalyst would probably also be inhibited by addn. of extraneous ligands.

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