# The Cytotoxic Activity of Diiron Bis-Cyclopentadienyl Complexes with Bridging C\({}_{3}\)-Ligands
## 2 Results and Discussion  
First, we developed a new and convenient method for the synthesis of the diiron vinyliminium complexes **2a,c,d** from the parent aminocarbyne complexes **1a-b** (Scheme 2, path a). The displacement of the carbonyl ligand by means of trimethylamine-N-oxide was carried out in methanol solution in the presence of the alkyne, and led to isolation of **2a,c,d** in good to high yields, following alumina chromatography. Complexes **2b,e** were instead obtained according to the literature [20; 36]. Using the improved synthetic procedure (Scheme 2), which is performed in one-pot without the needing of anhydrous solvents, the yields of **2a,c,d** are similar to those obtained by means of the two-step synthesis [20; 37]. **Scheme 2. (a) one-pot synthesis of diiron \(\mu\)-vinyliminium complexes from aminocarbyne precursors and terminal alkynes, and (b) DMSO-promoted conversion of **2a** into the monoiron vinyl-aminoalkylidene complex **3a**. Complexes **2a,c,d** were unambiguously identified by means of their IR and 1H NMR spectra (Figures S1-S3), being fully consistent with those reported previously (Figures S1-S3). The IR spectra of **2a-e** (in CH\({}_{2}\)Cl\({}_{2}\)) share a common pattern with three bands, related to the terminal carbonyl ligand, the bridging carbonyl and the iminium function, respectively. For instance, in the case of **2c**, such bands occur at 1990, 1806 and 1685 cm\({}^{-1}\), in the order given. According to 1H NMR spectroscopy, complex **2a** exists as a 2:1 mixture of E and Z isomers related to the nitrogen substituents, due to hampered rotation around the C\({}^{1}\)-N axis, and both isomers share a common cis geometry of the Cp rings. The E/Z isomeric ratio resembles that one recognized following the former procedure. On the other hand, the NMR spectra of **2c,d** display a single set of resonances, ascribable to the cis products. The D\({}_{2}\)O solubility values of **2a-e** were previously reported [20], ranging in between 4.3 \(\times\) 10\({}^{-4}\) (**2e**) and 1.4 \(\times\) 10\({}^{-2}\) (**2c**) molar. The vinyliminium complexes **2a-e** were assessed for their cytotoxic activity against the human lung A549 and human pancreatic BxPC-3 cancer cell lines following 72 h incubation. The results are shown in Table 1, together with previously reported IC\({}_{50}\) values concerning A2780 and A2780 cisR cell lines [20]. Thus, complexes **2b-e** are substantially inactive against the newly investigated cell lines, whereas **2a** displays a significant cytotoxicity, with the IC\({}_{50}\) obtained on the BxPC-3 line not so far from that of the reference drug cisplatin under the same conditions. As explained in the Introduction, the activity of **2a-e** may be imputable, at least in part, to redox processes associated to either a slow degradation of the complexes or their monoelectron reduction, the latter in some cases affording monoiron derivatives as organometallic products. In this regard, during an experiment aimed to test the stability of **2a** in dimethylsulfoxide at 37 \({}^{\circ}\)C, we serendipitously discovered a clean conversion into the monoiron species **3a** and cyclopentadiene (Scheme 2, path b). Complex **3a**, which is almost insoluble in water, was identified based on a comparison of IR and 1H NMR data with those available in the literature [27]. The IR spectrum, in dichloromethane solution, exhibits three characteristic bands due to the carbonyl ligand (1919 cm\({}^{-1}\)), the acyl group (1612 cm\({}^{-1}\)) and the aminoalkylidene moiety (1571 cm\({}^{-1}\)). The 1H NMR spectrum (in the DMSO-d\({}_{6}\) reaction medium; Figure S4) displays a single set of resonances ascribable to the E-configuration of the nitrogen substituents, with the alkenic CH proton resonating at 6.78 ppm.