Interplay between the palladium(II) ion, unidentate ligands and hydroxyl groups from a bidentate ligand in a new complex that catalyses hydration and alcoholysis of nitrile and urea‡

Abstract

Palladium(II) complexes containing the bidentate ligand 3,6-dithiaoctane-1,8-diol (dtod) and chloride anions (1) or solvent molecules (2) as two unidentate ligands were synthesized and characterized by ¹H and ¹³C NMR spectroscopy. The complex 1, which is the precursor to the catalyst 2, was also characterized by elemental analysis and X-ray crystallography. The crystal structure of 1 shows square planar co-ordination of two sulfur and two chloride ligands to palladium(II), and also a hydrogen bonding network. The complex 2 catalyses hydration of dichloroacetonitrile to dichloroacetamide and methanolysis of dichloroacetonitrile to (dichloromethyl)methoxymethanimine. The catalysed reactions are as much as 10⁶ times faster than the uncatalysed ones. The hydroxyl groups in the ligand dtod do not directly participate in the hydration and methanolysis reactions of dichloroacetonitrile catalysed by 2. The nitrile is activated for nucleophilic attack by its co-ordination to palladium(II). The nucleophile is the free water molecule or the aqua ligand in the hydration reaction and the free methanol molecule in the methanolysis reaction. Indeed, the latter reaction is first order with respect to methanol concentration. Urea and methylurea co-ordinated to palladium(II) in the complex 2 react with the hydroxyl groups of the dtod ligand and form alkyl carbamates. This unimolecular alcoholysis within the co-ordination sphere of palladium(II) is 240–380 times faster than the bimolecular alcoholysis involving external attack of free ethanol.