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 = eta(5)-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 mu M, depending on the cell line). The structure of [(Cp'0Me)RuCl(PTA)2] is reported, together with that of the precursor [(Cp'OEt)Ru(mu-Cl)](2).