Iron-Catalyzed Activation of Carbon-Halogen Bonds

We have studied the cross-coupling reaction of C(spⁿ)-X bonds (n = 1-3; X = F, Cl, Br, I) mediated by the model iron-d⁸ catalyst Fe(CO)₄ with the archetypal model substrates H₃C-CH₂-X, H₂C═CH-X, and HC≡C-X, utilizing relativistic density functional theory at ZORA-OPBE/TZ2P. The barrier of the oxidative-addition step decreases as the C(spⁿ)-X bond varies from X = F to Cl to Br to I and from C(sp³) to C(sp²) to C(sp). Activation strain and energy decomposition analyses uncover that the lowering of the reaction barrier from X = F to I is caused by (i) a weaker C(spⁿ)-X bond that needs to be broken, (ii) enhanced HOMO-LUMO interactions, and (iii) a stronger electrostatic attraction between the catalyst and the substrate due to the more diffuse electron density and higher nuclear charge of the X atom when varying from X = F to I.