Iron-Catalyzed\nActivation of Carbon–Halogen\nBonds

We have studied the cross-coupling reaction of C(<i>sp</i><sup>n</sup>)–X bonds (n = 1–3; X = F, Cl,\nBr, I) mediated\nby the model iron-d<sup>8</sup> catalyst Fe(CO)<sub>4</sub> with the\narchetypal model substrates H<sub>3</sub>C–CH<sub>2</sub>–X,\nH<sub>2</sub>CCH–X, and HCC–X, utilizing\nrelativistic density functional theory at ZORA-OPBE/TZ2P. The barrier\nof the oxidative-addition step decreases as the C(<i>sp</i><sup>n</sup>)–X bond varies from X = F to Cl to Br to I and\nfrom C(<i>sp</i><sup>3</sup>) to C(<i>sp</i><sup>2</sup>) to C(<i>sp</i>). Activation strain and energy\ndecomposition analyses uncover that the lowering of the reaction barrier\nfrom X = F to I is caused by (i) a weaker C(<i>sp</i><sup>n</sup>)–X bond that needs to be broken, (ii) enhanced HOMO–LUMO\ninteractions, and (iii) a stronger electrostatic attraction between\nthe catalyst and the substrate due to the more diffuse electron density\nand higher nuclear charge of the X atom when varying from X = F to\nI.