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. 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.