ROLE AND EFFECTS OF HALIDE-IONS ON THE RATES AND MECHANISMS OF OXIDATIVE ADDITION OF IODOBENZENE TO LOW-LIGATED ZEROVALENT PALLADIUM COMPLEXES PD0(PPH3)2

The role and effect of chloride and bromide ions on the intimate mechanisms and rates of oxidative addition of iodobenzene to electrogenerated low-ligated zerovalent palladium complex "Pd0(PPh3)2" are examined and discussed on the basis of P-31 NMR and transient electrochemistry. It is shown that Pd0(PPh3)2 actually consists of a mixture of three anionic species, [Pd0(PPh3)2Cl]2(2-) (A), Pd0(PPh3)2Cl- (B), and Pd0(PPh3)2Cl2(2-) (C), which are in rapid equilibrium within the time scale of their oxidative addition by iodobenzene. This stabilization of low-ligated zerovalent palladium by chloride ions occurs also during the oxidative addition of iodobenzenes to the ubiquitous Pd0(PPh3)4. This shows that the results of this study may be transposed to more classical procedures of generation of the active zerovalent palladium complex Pd0(PPh3)2, and also affords a tentative explanation of the effect of chloride ions in palladium-catalyzed nucleophilic substitutions or in the Heck reaction.