Influence of ligand and nuclearity on the cytotoxicity of cyclometallated C^N^C platinum(II) complexes

A series of cyclometallated mono- and di-nuclear platinum(II) complexes and the parent organic ligand, 2,6-diphenylpyridine 1 (HC^N^CH), have been synthesized and characterized. This library of compounds includes [(C^N^C)Pt(II)(L)] (L = dimethylsulfoxide (DMSO) 2 and triphenylphosphine (PPh3) 3) and [((C^N^C)Pt(II))2(L`)] (where L` = N-heterocycles (pyrazine (pyr) 4, 4,4,`-bipyridine (4,4`-bipy) 5 or diphosphine (1,4-bis(diphenylphosphino)butane (dppb) 6). Their cytotoxicity was assessed against four cancerous cell lines and one normal cell line, with results highlighting significantly increased antiproliferative activity for the dinuclear complexes (4-6), when compared to the mononucleated species (2 and 3). Complex 6 is the most promising candidate, displaying very high selectivity towards cancerous cells, with selectivity index (SI) values > 29.5 (A2780) and > 11.2 (A2780cisR), and outperforming cisplatin by > 4-fold and > 18-fold respectively.