Understanding the differences between iron and palladium in cross-coupling reactions

Abstract

We aim at developing design principles, based on quantum chemical analyses, for a novel type of iron-based catalysts that mimic the behavior of their well-known palladium analogs in the bond activation step of cross coupling reactions. To this end, we have systematically explored C–X bond activation via oxidative addition of CH₃X substrates (X = H, Cl, CH₃) to model catalysts ^mFe(CO)₄^q (q = 0, −2; m = singlet, triplet) and, for comparison, Pd(PH₃)₂ and Pd(CO)₂, using relativistic density functional theory at the ZORA-OPBE/TZ2P level. We find that the neutral singlet iron catalyst ¹Fe(CO)₄ activates all three C–X bonds via barriers that are lower than those for Pd(PH₃)₂ and Pd(CO)₂. This is a direct consequence of the capability of the iron complex to engage not only in π-backdonation, but also in comparably strong σ-donation. Interestingly, whereas the palladium complexes favor C–Cl activation, ¹Fe(CO)₄ shows a strong preference for activating the C–H bond, with a barrier as low as 10.4 kcal mol⁻¹. Our results suggest a high potential for iron to feature in palladium-type cross-coupling reactions.