Piano-Stool Ruthenium(II) Complexes with Delayed Cytotoxic Activity: Origin of the Lag Time

We have recently reported a series of piano-stool ruthenium(II) complexes of the general formula [RuCl₂(η⁶-arene)(P(1-pyrenyl)R²R³)] showing excellent cytotoxic activities (particularly when R² = R³ = methyl). In the present study, new members of this family of compounds have been prepared with the objective to investigate the effect of the steric hindrance of a bulky phosphane ligand, namely diisopropyl(1-pyrenyl)phosphane (L), on exchange reactions involving the coordinated halides (X = Cl, I). Two η⁶-arene rings were used, i.e. η⁶-methyl benzoate (mba) and η⁶-p-cymene (p-cym), and four complexes were synthesized, namely [RuCl₂(mba)(L)] (1_Cl), [RuI₂(mba)(L)] (1_I), [RuCl₂(p-cym)(L)] (2_Cl), and [RuI₂(p-cym)(L)] (2_I). Unexpectedly, all of the complexes exhibited poor cytotoxic activities after 24 h of incubation with cells, in contrast to the related compounds previously reported. However, it was observed that aged DMSO solutions of 2_I (from 2 to 7 days) exhibited better activities in comparison to freshly prepared solutions and that the activity improved over "aging" time. Thorough studies were therefore performed to uncover the origin of this lag time in the cytotoxicity efficiency. The data achieved clearly demonstrated that compounds 2_I and 2_Cl were undergoing a series of transformation reactions in DMSO (with higher rates for the iodido complex 2_I), ultimately generating cyclometalated species through a mechanism involving DMSO as a coordinated proton abstractor. The cyclometalated complexes detected in solution were subsequently prepared; hence, pure [RuCl(p-cym)(κ²C-diisopropyl(1-pyrenyl)phosphane)] (3_Cl), [RuI(p-cym)(κ²C-diisopropyl(1-pyrenyl)phosphane)] (3_I), and [Ru(p-cym)(κS-dmso)(κ²C-diisopropyl(1-pyrenyl)phosphane)]PF₆ (3_dmso) were synthesized and fully characterized. Remarkably, 3_Cl, 3_I, and 3_dmso are all very efficient cytotoxic agents, exhibiting slightly better activities in comparison to the chlorido noncyclometalated complexes [RuCl₂(η⁶-arene)(P(1-pyrenyl)R²R³)] described in an earlier report. For comparison purposes, the iodido compounds [RuI₂(mba)(dimethyl(1-pyrenyl)phosphane)] (1_I) and [RuI₂(p-cym)(dimethyl(1-pyrenyl)phosphane)] (2_I), bearing the less hindered dimethyl(1-pyrenyl)phosphane ligand, have also been prepared. The cytotoxic and chemical behaviors of 1_I and 1_I were comparable to those of their chlorido counterparts reported previously.