Kinetic investigation of reactions of a 3-arylidene-2-thiohydantoin derivative with palladium (II) salts

1H-NMR spectroscopy was used to monitor the reactions of an arylidene 2-thiohydantoin derivative, 3-((phenylmethylene)amino)-2-thioxo-4-imidazolidinone (3), with PdCl2, cis-[PdCl2(dmso-S)2] and K2[PdCl4] in DMSO-d6 in order to elucidate the reaction kinetics and mechanism. The 2-thiohydantoin derivative 3 formed cis-[Pd(3-N,S)(dmso-S)2]+ complex (5) in reactions with PdCl2 and cis-[PdCl2(dmso-S)2], while no reaction with K2[PdCl4] was observed. A two-step mechanism for the reactions of 3 with PdCl2 and cis- -[PdCl2(dmso-S)2] is proposed, in which fast coordination to the side chain nitrogen occurs in the first step, while chelation and coordination to the sulfur atom in the 2-thiohydantoin ring is the second, slower, rate-determining step. The reaction rate constants were calculated and reactivities of the 2-thiohydantoin derivative 3 towards the palladium(II) salts were compared and discussed. Reaction of 3 with cis-[PdCl2(dmso-S)2] was faster than with PdCl2. The investigated palladium(II) salts also react with the solvent, DMSO-d6, and the influence of these side reactions on the outcome and kinetics of the 2-thiohydantoin derivative complexation reaction is discussed in detail. The obtained results of this study can have an impact in explanation of the coordination behavior of antitumor active palladium(II) and platinum(II) complexes.