Salt-Enhanced Oxidative Addition of Iodobenzene to Pd: An Interplay Between Cation, Anion, and Pd–Pd Cooperative Effects

Halide
salts facilitate the oxidative addition of organic halides
to Pd(0). This phenomenon originates from a combination of anionic,
cationic, and Pd–Pd cooperative effects. Exhaustive computational
exploration at the density functional theory level of the complexes
obtained from [Pd0(PPh3)2] and a
salt (NMe4Cl or LiCl) showed that chlorides promote phosphine
release, leading to a mixture of mononuclear and dinuclear Pd(0) complexes.
Anionic Pd(0) dinuclear complexes exhibit a cooperativity between
Pd(0) centers, which favors the oxidative addition of iodobenzene.
The higher activity of Pd(0) dimers toward oxidative addition rationalizes
the previously reported kinetic laws. In the presence of Li+, the oxidative addition to mononuclear [Pd0L­(Li2Cl2)] is estimated barrierless. LiCl coordination polarizes
Pd(0), enlarging both the electrophilicity and the nucleophilicity
of the complex, which promotes both coordination of the substrate
and the subsequent insertion into the C–I bond. These conclusions
are paving the way to the rational use of the salt effects in catalysis
for the activation of more challenging bonds.