trans-FeII(H)2(diphosphine)(diamine) complexes as alternative catalysts for the asymmetric hydrogenation of ketones? A DFT study

Abstract

New insights into the structural, electronic and catalytic properties of Fe complexes are provided by a density functional theory study of model as well as real [Fe^II(H)₂(diphosphine)(diamine)] systems. Calculations conducted using several different functionals on the trans- and cis-isomers of [Fe^II(H)₂(S-xylbinap)(S,S-dpen)] complexes show that, as with the [Ru^II(H)₂(diphosphine)(diamine)] complexes, the trans-[Fe^II(H)₂(diphosphine)(diamine)] complex is the more stable isomer. Analysis of the spin states of the trans-[Fe^II(H)₂(diphosphine)(diamine)] complexes also shows that the singlet state is significantly more stable than the triplet and the quintet, as with the [Ru^II(H)₂(diphosphine)(diamine)] complexes. Calculations of the catalytic cycle for the hydrogenation of ketones using two model trans-[M^II(H)₂(PH₃)₂(en)] catalysts, where M = Ru and Fe, show that the mechanism of reaction as well as the activation energies are very similar, in particular: (i) the ketone/alcohol hydrogen transfer reaction occurs through the metal–ligand bifunctional mechanism, with energy barriers of 3.4 and 3.2 kcal mol⁻¹ for the Ru- and Fe-catalysed reactions, respectively; (ii) the heterolytic splitting of H₂ across the MN bond for the regeneration of the Ru and Fe catalysts has an activation barrier of 13.8 and 12.8 kcal mol⁻¹, respectively, and is expected to be the rate determining step for both catalytic systems. The reduction of acetophenone by trans-[M^II(H)₂(S-xylbinap)(S,S-dpen)] complexes along two competitive reaction pathways, shows that the intermediates for the Fe catalytic system are similar to those responsible for the high enantioselectivity of (R)-alcohol in those proposed trans-[Ru^II(H)₂(S-xylbinap)(S,S-dpen)] catalysed acetophenone hydrogenation reaction. Thus the high enantiomeric excess in the hydrogenation of acetophenone could, in principle, be achieved using Fe catalysts.