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

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 CH ₃ X substrates (X = H, Cl, CH ₃ ) to model catalysts ^m Fe(CO) ₄ ^q (q = 0, -2; m = singlet, triplet) and, for comparison, Pd(PH ₃ ) ₂ and Pd(CO) ₂ , using relativistic density functional theory at the ZORA-OPBE/TZ2P level. We find that the neutral singlet iron catalyst ¹ Fe(CO) ₄ activates all three C-X bonds via barriers that are lower than those for Pd(PH ₃ ) 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, ¹ Fe(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.