The role of Pd colloids as catalysts in the phosphane-free methoxycarbonylation of iodobenzene

Abstract

The catalytic activity of PdCl₂(cod) (1) in the methoxycarbonylation of iodobenzene (at 40–70 °C, 1–5 atm of CO, in methanol solution) increased remarkably when tetra-n-butylammonium salts [ⁿBu₄N]X (X = Br, Cl, I, BF₄, PF₆) were added to the system. XRD analyses confirmed that under methoxycarbonylation reaction conditions palladium was reduced to a Pd colloid of nanometer size, 2.0–5.6 nm, which was responsible for the catalytic activity. In the absence of ammonium salts fast deactivation of the colloid was observed, even when PVP (polyvinylpyrrolidone) was added as a stabilizer. Catalytic tests performed with isolated Pd colloid demonstrated its high catalytic activity in methoxycarbonylation when used together with [ⁿBu₄N]X salts. A mechanism of palladium reduction to Pd nanoparticles is proposed.