A Theoretical Study of the Reaction of Fe+ with CH3X (X=Cl, Br, I)

The C-X bond functionalization catalyzed by iron and their complexes has attracted much attention. Using density functional theory (DFT), we herein studied the reactivity and mechanism of iron cation (Fe) towards C-X bond cleavage of CH3X (X= Cl, Br, I) at B3LYP/def2-SVP level of theory. The results show that there are two possible pathways available for the title reactions, i.e. the insertion mechanism and the S(N)2 mechanism, respectively. Mechanistically, in the insertion mechanism, the reactions stem from Fe+ attacking on the side of CH3X and results in the generation the products FeX+ and CH3 center dot; whereas in the S(N)2 mechanism, the Fe+ initially attacks the substrate from the back of C-X yielding FeCH3+ and X-center dot. The results show that the sextet and the quartet states of Fe+ demonstrate quite distinct reactivity towards the cleavage of C-X bonds in the most potential pathways, and the quartet pathways are dominant in all the pathways. The relative higher barriers in the S(N)2 pathways results in their lower competitiveness in the title reactions. In addition, our results show that, for all the three reaction systems, the insertion mechanism is exothermic; whereas for the S(N)2 mechanism, only X=I is exothermic, however. Furthermore, the calculations also show that these reactions demonstrate two-state reactivity (TSR) scenario. There are minimum energy crossing points (MECPs) between the sextet and quartet state on potential energy surfaces (PESs) both at the entrance and export sides for the two reaction mechanisms. On the other hand, the electron transfer evolution analysis indicates that the spin polarization plays important role in the stabilization of the potential energy surfaces and as a result, it controls the pathways by which it takes place and the branch ratio of the major products and byproducts. This thorough theoretical study, especially the detailed electronic structure analysis, may provide important clues for understanding the C-X bond activation and theoretical fundamental evidences for iron-based catalysts design.