The interaction of antitumor active vanadocene dichloride with sulfur-containing amino acids

Vanadocene dichloride (1) reacts with sulfur-containing amino acids, cysteine and methionine, giving new complexes with five- or six-membered chelate ring, but the structure of isolated compounds is affected by the pH value of the reaction mixture. Methionine reacts with aqueous 1 in the pH range of 3–8 affording chelate structure [Cp2V(N,O-met)]Cl (4). Similar reaction with cysteine gives two different products depending on pH. In the acidic solution, the complex [Cp2V(O,S-cys)]Cl (2) is present, whereas in neutral media the compound [Cp2V(N,S-cys)] (3) could be identified. On inspection of spectroscopic measurements, particularly EPR and vibrational spectroscopy, it is evident that sulfur atom of amino acid is bonded directly to the vanadium atom of [Cp2V]2+ moiety. For the purpose of comparison the complexes [Cp2V(O,S-mpa)] (5) and [Cp2V(N,S-csam)]+ (6a) with related chelating ligands, 3-mercaptopropionic acid (mpa) and cysteamine (csam), respectively, were prepared and spectroscopically characterized. The structure of the complex [Cp2V(N,S-csam)]BPh4 (6b) was also determined by X-ray diffraction analysis.