Dinuclear intermediates in the oxidation of pendant olefinic groups of palladium(II)-co-ordinated Schiff-base ligands

Abstract

The palladium(II) complexes [PdL¹Cl][ClO₄], [PdL²(ClO₄)][ClO₄], and [PdL³(ClO₄)][ClO₄] of the Schiff-base ligands L¹, L², and L³ derived from the condensation of 2,6-diacetylpyridine with 2 mol respectively, of 4-aminobut-1-ene, allylamine, and 1-aminopropane have been prepared. Infrared and ¹H and ¹³C n.m.r. spectra show that in each complex the trimethine ‘N₃’ donor group is co-ordinated to the palladium(II) ion together with one anionic ligand, but that the olefinic groups of the ligand side arms in L¹ and L² are not. However, treatment of the L¹ and L² complexes in solution with 2 equivalents of [Pd(MeCN)₂Cl₂] in the presence of water leads to complexes of the new ligands L⁴ and L⁵ in which the pendant olefinic groups of L¹ and L² have been oxidized to keto-groups. Evidence is presented for a Wacker-type mechanism wherein a key step is the formation of dinuclear intermediates in which the olefinic group of the reactant complexes are co-ordinated to a second palladium(II) ion {from [Pd(MeCN)₂Cl₂]} with concomitant activation to nucleophilic attack by water.