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.