# The Cytotoxic Activity of Diiron Bis-Cyclopentadienyl Complexes with Bridging C\({}_{3}\)-Ligands
## 1 Introduction  
There is a current great interest in developing new anticancer drugs based on transition metals [1, 2, 3, 4], due to the peculiar properties provided by the presence of the metal center (e.g., redox activity and broad possibility of geometries, stereochemical configurations and kinetic behaviors) [5] and the opportunity to overcome the drawbacks associated to the use of platinum compounds currently employed in clinical treatments against several types of tumors (in particular, severe side effects and acquired resistance towards the drug) [6, 7]. In this regard, iron is an appealing choice as a metal element, being easily available, implicated in diverse biological processes in the human body, essentially nontoxic in many forms and displaying a rich redox chemistry [8, 9]. Monometallic iron complexes with one or two (ferroccenes) cyclopentadienyl ligands (n\({}^{5}\)-C\({}_{5}\)H\({}_{5}\) = Cp) have been widely investigated for their cytotoxicity [10, 11, 12], however these species may suffer from a limitedstructural variability and the absence of water solubility [13; 14], which is a highly desirable characteristic for a drug candidate. Some of us have been involved for longtime in the organometallic chemistry of diiron bis-cyclopentadienyl complexes: with a notable relevance to the natural hydrogenase enzymes, the cooperative diiron core is a key feature, in that it enables reactivity patterns on the bridging site which are otherwise not accessible on homologous monoiron complexes, and thus leads to the stereo- and regio-selective stepwise construction of functionalized organic fragments [15; 16]. The co-presence of Cp (electron donor) and CO (electron acceptor) ligands confers a substantial robustness to the complexes [17], and the fine control of the structural variability makes available a range of compounds with suitable physico-chemical properties for potential biological applications (e.g., appreciable water solubility, amphiphilicity). We recently reported the first studies on the antiproliferative activity of di-organoiron complexes [18; 19], and in particular we found that vinyliminium complexes **2** are substantially water-stable and exhibit a variable in vitro antiproliferative activity depending on the substituents on N, C\({}^{1}\) and C\({}^{2}\)[20; 21; 22]; **2** are usually prepared by means of a straightforward four-step procedure from the commercial [Fe\({}_{2}\)Cp\({}_{2}\)(CO)\({}_{4}\)], via the intermediacy of aminocarbyne complexes **1** (Scheme 1, path a) [20; 23]. Remarkably, IC\({}_{50}\) values in the nanomolar range have been recognized on A2780 and A2780 cisR cells for some complexes of the series **2**. The action of **2** seems multimodal, including the interaction with biomolecules and the interference with cellular redox processes, and in particular the compounds possess the ability to trigger the production of Reactive Oxygen Species (ROS) inside the tumor cells [24; 25; 26]. In principle, two processes might contribute to this phenomenon: (1) a slow, extensive degradation of the diiron scaffold, with the released iron(I) centers rapidly converting into iron(III) oxide [20]; (2) a monoelectron reduction of the cationic complexes, promoting a fragmentation affording iron(0) and the monoiron complexes **3**, which are themselves cytotoxic through the production of ROS and display a significant selectivity with reference to the noncancerous HEK-293 cell line [27]. The optimal laboratory preparation of **3** consists in the reaction of the precursors **2** with an amine (Scheme 1, path b). The neutral diiron complexes **4a-c** are accessible by means of a two-step synthesis consisting in the incorporation of a selenium atom within the vinyliminium moiety, followed by cyclization reaction with internal alkynes bearing electron withdrawing substituents (Scheme 1c) [28]. Complexes **4a-c** are based on a [Fe\({}_{2}\)Cp\({}_{2}\)(CO)\({}_{2}\)] core and comprise an additional bridging ligand which is an alkylidene derivatized with an unusually functionalized selenophene.