Platinum Diolefin Complexes – Synthesis, Structures, and Cytotoxicity

The synthesis, spectroscopy, structures and chemical reactivity of platinum(II) diolefin complexes cis-[(vertical bar(perpendicular to)vertical bar)PtCl2], cis-[(vertical bar(perpendicular to)vertical bar)PtCl(R)] and cis-[(vertical bar(perpendicular to)vertical bar)Pt(R)(2)] [vertical bar(perpendicular to)vertical bar = chelate diolefin ligand: 1,5-cyclooctadiene (COD), 1,5-dimethylocta-1,5-diene (Me2COD), norbornadiene (NBD), 1,5-hexadiene (HEX), 3-allyloxypropene (All(2)O, diallyl ether), diallylamine (All(2)NH); R = Me, Bn, C6F5, C6F4H-4 (or -5), or CC(4-Me)Ph] have been explored. The relative exchange rates of the cis-[(vertical bar(perpendicular to)vertical bar)PtCl2] complexes towards the diimine ligand diisopropyl-1,4-diazabutadiene (iPr-DAB) increased along the series COD< Me2COD< NBD< HEX< All(2)O by a factor of 4. The presumably dimeric complex [(All(2)NH)PtCl2](2) undergoes a unique rearrangement process in dimethyl sulfoxide (DMSO) solution to yield the dimeric piperazine complex [PtCl(dmso)(C6H10N)](2), which has been characterised by single-crystal XRD. For selected platinum complexes, cytotoxic effects in HT-29 colon carcinoma and MCF-7 breast cancer cell lines were evaluated. For comparison, the dicationic complexes [(COD)Pt(Bn)(L)][PF6](2) with the very labile coligands N-methyl-4,4-bipyridinium (MQ(+)) and N-methyl-1,4-pyrazinium (Mpz(+)) were added to the study. Although the hexadiene complexes [(HEX)Pt(C6F4H-4)(2)] and [(HEX)Pt(C6F4H-5)(2)] show strong cytotoxicity, the introduction of labile diolefin ligands or the labile cationic MQ(+) or Mpz(+) coligands does not generally lead to markedly increased cytotoxicity.