Interazioni elettroniche metallo-metallo in complessi bimetallici e trimetallici di leganti policiclici aromatici di struttura flessibile e rigida

In the last years many efforts have been directed toward the synthesis and study of ferrocenyl-based conjugated ligands since they can be used as building-blocks for the synthesis of poly-ferrocenyl conjugated systems or as starting material for the synthesis of new homo- and hetero-bimetallic complexes with innovative physical properties. Furthermore, this ferrocenyl-conjugated derivatives represent suitable models for the study of electronic communication between terminal units connected by an unsaturated hydrocarbon bridge in terms of tuning the electron properties and the reactivity of materials. Systems where a metal is connected to a ferrocenyl unit by an unsaturated hydrocarbon bridge, associated with the redox switchable electron donor/acceptor capability of ferrocene/ferrocenium couple, are expected to display attractive properties that depend not only by the nature of the bridge but also by the redox state of the pendant ferrocenyl. Hush analysis of InterValence Charge Transfer band (IVCT) of mixed-valence bimetallic complexes formed by oxidation of one metal unit, has revealed as the magnitude of the electronic coupling is largely dependent on the structure and stereochemistry of the bridging ligand and as it increases if the spacer is forced to adopt a planar geometry. While the charge or energy transfer phenomena in homo- and hetero-bimetallic mixed-valence complexes have been widely investigated, studies of charge and energy transfer processes in systems formed by a ferrocenyl unit linked to an unsatured organic group have received minor attention. In these systems, the ferrocenyl unit, upon of her oxidation, acts as an electron-acceptor group generating new bands in the near-IR region which can be assigned to aryl→ferrocenium transition. The classical electron transfer Hush model, developed for interpretation of IVCT bands of bimetallic and organic molecules, has been recently extended for interpretation of LMCT (Ligand to Metal Carghe Transfer) and MLCT (Metal to Ligand Carghe Transfer) bands to get insight into electron transfer processes between an organic and a metallic group in monometallic complexes. To this purpose, we have prepared the following complexes: (2-ferrocenyl)-indene, (2-ferrocenil)-tetramethylindene, (2-ferrocenyl)-hexamethylindene, (3-ferrocenyl)-indene e (3-ferrocenyl)-hexamethylindene. Furthermore, the indene unit has been replaced by a benzothiophene unit obtaining a couple of isomers: (3-ferrocenyl)- benzothiophene and (2-ferrocenyl)-benzothiophene). Subsequently, complexation with (CH3CN)3Cr(CO)3 of benzene ring of (ferrocenyl)-indenes has lead to a series of hetero-bimetallic Fe/Cr complexes. Availability of couples of monometallic and bimetallic isomers with known geometries with differently methylated indene unit has allowed the detailed investigation of how slight geometric and electronic modifications affect LMCT (in monometallic complexes) and IVCT ( in bimetallic complexes) charge transfer processes. The big interest directed toward the study of bimetallic complexes characterized the last decades has not been accompanied by an equal attention toward the electronic transfer processes in mixed—valence trimetallic systems. This can be due to the difficult synthetic routes to obtain trimetallic species and to the complexity of the interactions that they can present. It is evident that the characterization of the redox properties and the study of the metal-metal interactions in complexes with more than two metallic centres offer the possibility of a better understanding of the electron transfer processes in multinuclear complexes. To this purpose, the research has been extended to the synthesis, characterization and study of a triferrocenylic model system: the anti-[(FeCp)3Td] (Td = trindene) complex.