The reaction of p-isopropylbenzaldehyde thiosemicarbazone [p-is.TSCN], 1, with palladium(II) acetate and potassium tetrachloroplatinate yielded two tetrameric orthopalladated isomers, [Pd(p-is.TSCN)]4 (complexes 2 and 3), and the platinum analogue [Pt(p-is.TSCN)]4 (complex 4), respectively. All of these complexes contain the thiosemicarbazone bonded as a terdentate ligand to the metallic atom, through the thiol sulfur, the azomethinic nitrogen and the ortho carbon of the p-isopropylphenyl ring to which the imine group is attached to as deduced from the study of the IR, NMR, and XRD spectra of complexes 2 and 4. Complexes 2 and 4 crystallize in the centrosymmetric monoclinic space group C2/c, with Z = 8. Unit cell parameters for complex 2 are as follows: a = 25.742(5) A, b = 19.560(4) A, c = 24.199(5) A, beta = 101.70(3)o. Unit cell parameters for complex 4 are as follows: a = 25.8728(19) A, b = 19. 5053(14) A, c = 24.0899(16) A, beta = 101.305(2)o. As can be deduced from the NMR study, the palladated isomers 2 and 3 interconvert in DMSO which may be a consequence of the existence in both complexes of a flexible eight-membered ring with alternating Pd-S atoms. The testing of the cytotoxic activity of these compounds against several human and murine cell lines sensitive and resistant to cisplatin (cis-DDP) suggests that compounds 2, 3, and 4 may be endowed with important anticancer properties since they elicit IC50 values in the microM range as does the clinically used drug cis-DDP, and, moreover, they display cytotoxic activity in tumor lines resistant to cis-DDP. The analysis of the interaction of these novel tetrameric cyclometalated compounds with DNA suggests that they form DNA interhelical cross-links.

In the present paper we report the synthesis and structural IR and 1H and 13C NMR characterization of four platinum(II) and palladium(II) cyclometalated complexes of the formula [Pd(4-ClC6H4NC(COC6H5)C6H4)X]2, where X = OAc (2) or Cl (3), and [Pt(4-ClC6H4NC(COC6H5)C6H4)X]2, where X = Cl (4) or OAc (5). The acetate-bridged compounds 2 and 5 have an open-book structure. The chloro-bridged compounds 3 and 4 have an unfolded structure. Complex 2 crystallizes in the centrosymmetric triclinic space group P1̄, with Z = 2. Unit cell parameters are as follows:  a = 11.765(3) Å, b = 12.862(4) Å, c = 15.022(5) Å, α = 84.08(2)°, β = 78.56(2)°, γ = 83.13(2)°. All these compounds were screened against MDA-MB 468 (breast carcinoma) and HL-60 (leukemic) tumor cells. The low IC50 values of compound 5 (1.05 μM) and of compound 4 (1.58 μM) against MDA-MB 468 and of 0.82 μM (5) and 1.37 μM (4) against HL-60 relative to cis-DDP (2.67 and 2.07 μM, respectively) and other cyclometalated compounds suggest that the synthesized compounds may be regarded as having potential antitumor properties. The data suggest that the antiproliferative activity of compound 5 may correlate with its covalent binding to DNA and the induction of important modifications on the helix.

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