Four new water soluble molybdenocene complexes were synthesized in aqueous solution at pH 7.0. The new species, [(η 5 -C 5 H 5 ) 2 Mo(L)]Cl (L= 6-mercaptopurine, 2-amino-6-mercaptopurine, (-)-2-amino-6-mercaptopurine ribose and 6-mercaptopurine ribose), were characterized by spectroscopic methods. NMR spectroscopic data showed the presence of two coordination isomers, S(6), N(7) and S(6), N(1), in aqueous solution, being S(6), N(7) the most stable. The antiproliferative activities of the new species were investigated in HT-29 colon and MCF-7 breast cancer cell lines. The incorporation of molybdenocene (Cp 2 Mo 2+ ) into the thionucleobases/ thionucleosides decreases their cytotoxic activities in HT-29 colon cancer cell line. In contrast, in the MCF-7 cell line, [Cp 2 Mo(2-amino-6-mercaptopurine)]Cl showed a high cytotoxic activity. This is most likely a consequence of the enhanced lipophilic character on the thionucleobase combined with synergism between Cp 2 Mo 2+ and the thionucleobase ligand.

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