| Summary: | A family of compounds with the general formula [Fe(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(CO)(PPh<sub>3</sub>)(NCR)]<sup>+</sup> has been synthesized (NCR = benzonitrile (<b>1</b>); 4-hydroxybenzonitrile (<b>2</b>); 4-hydroxymethylbenzonitrile (<b>3</b>); 4-aminobenzonitrile (<b>4</b>); 4-bromobenzonitrile (<b>5</b>); and, 4-chlorocinnamonitrile (<b>6</b>)). All of the compounds were obtained in good yields and were completely characterized by standard spectroscopic and analytical techniques. Compounds <b>1</b>, <b>4</b>, and <b>5</b> crystallize in the monoclinc P21/c space group and packing is determined by short contacts between the phosphane phenyl rings and cyclopentadienyl (compounds <b>1</b> and <b>4</b>) or π-π lateral interactions between the benzonitrile molecules (complex <b>5</b>). DFT and TD-DFT calculations were performed to help in the interpretation of the experimental UV-Vis. data and assign the electronic transitions. Cytotoxicity studies in MDA-MB-231 breast and SW480 colorectal cancer-derived cell lines showed IC<sub>50</sub> values at a low micromolar range for all of the compounds in both cell lines. The determination of the selectivity index for colorectal cells (SW480 vs. NCM460, a normal colon-derived cell line) indicates that the compounds have some inherent selectivity. Further studies on the SW480 cell line demonstrated that the compounds induce cell death by apoptosis, inhibit proliferation by inhibiting the formation of colonies, and affect the actin-cytoskeleton of the cells. These results are not observed for the hydroxylated compounds <b>2</b> and <b>3</b>, where an alternative mode of action might be present. Overall, the results indicate that the substituent at the nitrile-based ligand is associated to the biological activity of the compounds.
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