| Summary: | A series of diiron/tetrairon compounds containing a S- or a Se-function (<b>2a</b>–<b>d</b>, <b>4a</b>–<b>d</b>, <b>5a</b>–<b>b</b>, <b>6</b>), and the monoiron [FeCp(CO){SeC<sup>1</sup>(NMe<sub>2</sub>)C<sup>2</sup>HC<sup>3</sup>(Me)}] (<b>3</b>) were prepared from the diiron μ-vinyliminium precursors [Fe<sub>2</sub>Cp<sub>2</sub>(CO)( μ-CO){μ-η<sup>1</sup>: η<sup>3</sup>-C<sup>3</sup>(R’)C<sup>2</sup>HC<sup>1</sup>N(Me)(R)}]CF<sub>3</sub>SO<sub>3</sub> (R = R’ = Me, <b>1a</b>; R = 2,6-C<sub>6</sub>H<sub>3</sub>Me<sub>2</sub> = Xyl, R’ = Ph, <b>1b</b>; R = Xyl, R’ = CH<sub>2</sub>OH, <b>1c</b>), via treatment with S<sub>8</sub> or gray selenium. The new compounds were characterized by elemental analysis, IR and multinuclear NMR spectroscopy, and structural aspects were further elucidated by DFT calculations. The unprecedented metallacyclic structure of <b>3</b> was ascertained by single crystal X-ray diffraction. The air-stable compounds (<b>3</b>, <b>4a</b>–<b>d</b>, <b>5a</b>–<b>b</b>, <b>6</b>) display fair to good stability in aqueous media, and thus were assessed for their cytotoxic activity towards A2780, A2780cisR, and HEK-293 cell lines. Cyclic voltammetry, ROS production and NADH oxidation studies were carried out on selected compounds to give insights into their mode of action.
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