The Cytotoxic Activity of Diiron Bis-Cyclopentadienyl Complexes with Bridging C3-Ligands

Diiron bis-cyclopentadienyl bis-carbonyl cationic complexes with a bridging vinyliminium ligand, [Fe2Cp2(CO)(μ-CO){μ-η1:η3-C3(R′)C2HC1NMe(R″)}]CF3SO3 (R = Xyl = 2,6-C6H3Me2, R′ = Ph, R″ = H, 2a; R = Xyl, R′ = R″ = Me, 2b; R = R′ = Me, R″ = H, 2c; R = Me, R′ = 2-naphthyl, R″ = H, 2d; R = Me, R′ = R″ = Ph, 2e), are easily available from commercial chemicals, robust in aqueous media and exert a variable in vitro cytotoxicity against cancer cell lines depending on the nature of the substituents on the vinyliminium ligand. The anticancer activity is, at least in part, associated to fragmentation reactions, leading to iron oxidation and active neutral and well-defined monoiron species. We report an innovative synthetic procedure for the preparation of 2a,c,d, and a facile method to access the monoiron derivative of 2a, i.e., [FeCp(CO){C1(NMeXyl)C2HC3(Ph)C(O)}] (3a). According to IC50 analyses at different times of incubation of the complexes, 3a is significantly faster in inhibiting cell viability compared to its diiron precursor 2a. The neutral complexes [Fe2Cp2(CO)(μ-CO){μ-k1N:k1C:k1C-C3(R′)C2(Se)C1(NMe2)C4(CO2Y)C5(CO2Y)}] (R′ = Y = Me, 4a; R′ = Pr, Y = tBu, 4b; R′ = Y = Et, 4c) are obtained via the two-step modification of the vinyliminium moiety and comprise a bridging selenophene-decorated alkylidene ligand. The antiproliferative activity exhibited by 4a-c is moderate but comparable on the ovarian cancer cell line A2780 and the corresponding cisplatin resistant cell line, A2780cisR. Complexes 4a-c in aqueous solutions undergo progressive release of the alkylidene ligand as a functionalized selenophene, this process being slower in cell culture medium. Since the released selenophenes SeC1{C(O)R′}C2(NMe2)C3(CO2Y)C4(CO2Y) (R′ = Y = Me, 5a; R′ = Pr, Y = tBu, 5b) are substantially not cytotoxic, it is presumable that the activity of 4a-c is largely ascribable to the {Fe2Cp2(CO)2} scaffold.