Importance of the π-Ligand: Remarkable Effect of the Cyclopentadienyl Ring on the Cytotoxicity of Ruthenium PTA Compounds

The water-soluble complexes [(Cp′OR)RuCl(PTA)2] (Cp′OR = η5-1-alkoxy-2,4-di-tert-butyl-3-neopentylcyclopentadienyl; R = Me, Et; PTA = 1,3,5-triaza-7-phosphaadamantane) are considerably more cytotoxic (ca. 2 orders of magnitude) than the cyclopentadienyl analogue [CpRuCl(PTA)2] (i.e., IC50 = 4−10 vs >1000 µM, depending on the cell line). The structure of [(Cp′OMe)RuCl(PTA)2] is reported, together with that of the precursor [(Cp′OEt)Ru(μ-Cl)]2.