Alkyne-substituted mixed-metal clusters

One of the main reasons of interest in organometallic clusters is their ability in stabilizing small fragments by multimetallic coordination. In the context of the well-known cluster-surface analogy, organic ligands bound on metal clusters are valuable models for fragments chemisorbed on metal surface, and helped to clarify their reactivity and spectroscopic data. Alkyne ligands have been in the past and still are widely exploited in metal cluster chemistry both for applicative purposes and basic research. Indeed these ligands are particularly suited for interacting with several metal sites (from 1 up to 4), can donate a variable number of electrons and then can give rise to many different bonding modes. Moreover the applicative potentiality of derivatized clusters can be enhanced by using functionalized alkynes. Possible structural variations are further amplified by the use of mixed – metal clusters and, indeed, if metals of different groups are used, unknown coordination modes and/or peculiar reactivity of the ligand can be expected. For these reasons, we are currently studying the synthesis of alkyne-substituted clusters of Fe-Rh and Fe-Ir. The reaction of tri- and tetra-nuclear Fe-Rh clusters invariably produced the triangular cluster [Fe2Rh(CO)8PhCCPh]-, which is best prepared from [Fe2Rh(CO)10]- and diphenylacetylene in refluxing THF. Despite its formal insaturation, the cluster is very stable, and can be easily isolated and characterized by X-ray diffraction. The C-C bond is laying perpendicular to a Fe-Fe edge of the cluster (see figure): a coordination mode unusual in these compounds. The Fe-Ir cluster [Fe2Ir2(CO)12]2- also reacts with diphenylacetylene, in refluxing MeCN, yielding the dianion [Fe2Ir2(CO)10PhCCPh]2-. The solid state structure of this complex was determined, and shown to consist of a butterfly arrangement of atoms, with the Ir-Ir bond at the hinge; the two C atoms bind to Ir with  bonds and to Fe with  bonds. We are currently exploring, by cyclic voltammetry, the redox aptitude of these clusters, which are expected to enhance their stability after coordination of the alkyne. Moreover, we are trying to isolate the products observed after reactions with acids, CO and phosphine. 13C NMR characterizations are also in course.