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.