Although the compounds were designed to target delivery of the cytotoxic ferrocene group to cells overexpressing estrogen receptors, they showed significant in vitro activity against both hormone–dependent MC–7 cells and triple–negative MDA–MB–23 breast cancer cells (IC50 = 0.7 μM and IC50 = 0.6 for 1, respectively). This evidence concerns the gene ESR1 and breast carcinoma.