Evidence for an equilibrium between neutral and cationic arylpalladium(II) complexes in DMF. Mechanism of the reduction of cationic arylpalladium(II) complexes

In DMF, neutral arylpalladium(II) complexes ArPdX(PPh3)(2) (Ar = Ph; X = Cl, Br, I, OAc) are involved in an equilibrium with the cationic complex ArPd(PPh3)(2)(DMF)(+) and the anion X-. The values of the equilibrium constants K-X have been determined and the affinity of X- for ArPd(PPh3)(2)(DMF)(+) is found to be in the order: Cl- > Br- > I- > AcO-. Therefore, albeit the usual reactions with nucleophiles are performed from the neutral complex, the real reactive intermediate might well be the cationic complex. The electrochemical reduction of cationic complexes ArPd(PPh3)(2)(+) affords biaryl, and the mechanism of this reaction has been investigated, It proceeds via transient arylpalladium (I) complexes.