Ionic mononuclear [Fe] and heterodinuclear [Fe,Ru] bis(diphenylphosphino)alkane complexes: Synthesis, spectroscopy, DFT structures, cytotoxicity, and biomolecular interactions.

Iron(II) and Ru(II) half-sandwich compounds encompass some promising pre-clinical anticancer agents whose efficacy may be tuned by structural modification of the coordinated ligands. Here, we combine two such bioactive metal centres in cationic bis(diphenylphosphino)alkane-bridged heterodinuclear [Fe2+, Ru2+] complexes to delineate how ligand structural variations modulate compound cytotoxicity. Specifically, Fe(II) complexes of the type [(η5-C5H5)Fe(CO)2(κ1-PPh2(CH2)nPPh2)]{PF6} (n = 1-5), compounds 1-5, and heterodinuclear [Fe2+, Ru2+] complexes, [(η5-C5H5)Fe(CO)2(μ-PPh2(CH2)nPPh2))(η6-p-cymene)RuCl2]{PF6} (n = 2-5) (compounds 7-10), were synthesized and characterised. The mononuclear complexes were moderately cytotoxic against two ovarian cancer cell lines (A2780 and cisplatin resistant A2780cis) with IC50 values ranging from 2.3 ± 0.5 μM to 9.0 ± 1.4 μM. For 7-10, the cytotoxicity increased with increasing Fe⋅⋅⋅Ru distance, consistent with their DNA affinity. UV-visible spectroscopy suggested the chloride ligands in heterodinuclear 8-10 undergo stepwise substitution by water on the timescale of the DNA interaction experiments, probably affording the species [RuCl(OH2)(η6-p-cymene)(PRPh2)]2+ and [Ru(OH)(OH2)(η6-p-cymene)(PRPh2)]2+ (where PRPh2 has R = [-(CH2)5PPh2-Fe(C5H5)(CO)2]+). One interpretation of the combined DNA-interaction and kinetic data is that the mono(aqua) complex may interact with dsDNA through nucleobase coordination. Heterodinuclear 10 reacts with glutathione (GSH) to form stable mono- and bis(thiolate) adducts, 10-SG and 10-SG2, with no evidence of metal ion reduction (k1 = 1.07 ± 0.17 × 10-1 min-1 and k2 = 6.04 ± 0.59 × 10-3 min-1 at 37 °C). This work highlights the synergistic effect of the Fe2+/Ru2+ centres on both the cytotoxicity and biomolecular interactions of the present heterodinuclear complexes.