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Reactions of palladium(II) complexes cis-[PdCl2(en)] and cis-[PdCl2(L-HMet-S,N) ], in which en is ethane-1,2-diamine and methionine is an S,N-bidentate ligand, and their aqua analogs with dipeptides glycyl-L-histidine (Gly-His), L-histidylglycine (His-Gly), and the N-acetylated dipeptides MeCO-Gly-His and MeCO-His-Gly have been studied by 1H NMR spectroscopy. In the reactions of cis-[PdCl2(L-HMet-S,N)] and cis-[PdCl2(en)] with Gly-His the formation of [Pd(Gly-His)(L-HMet-S)]+ and [PdCl(Gly-His)] occurs at 1.5 < pH < 3.5. Tridentate co-ordination of Gly-His causes release of the en from cis-[PdCl2(en)] and ring opening of the L-HMet chelate in cis-[PdCl2(L-HMet-S,N) ]. The crystal structure of [PdCl(Gly-His)] shows that the peptide is bound to palladium(II) through imidazole N-3, amide, and amino nitrogen atoms. Tridentate chelation of Gly-His to palladium(II) is unfavorable for the hydrolysis of the peptide. The dipeptide His-Gly co-ordinates to palladium(II) as a bidentate ligand, via the imidazole N-3 and amino nitrogen atoms. This co-ordination mode also is unproductive for the hydrolysis of the peptide. However, at lower pH this complex converts into a hydrolytically active one, in which the dipeptide is bound to palladium(II) via the imidazole N-3 atom only. The dipeptides MeCO-His-Gly and MeCO-Gly-His, in which the terminal amino group is acetylated, exhibit more versatile co-ordination chemistry in the reactions with cis-[PdCl2(en)] and cis-[PdCl2(L-HMet-S,N)] and form complexes in which the imidazole N-1 atom co-ordinates to palladium(II). These studies with model complexes contribute to the understanding of selective cleavage of peptides and proteins by palladium(II) aqua complexes.

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.