The study of intramolecular redox processes in molecules which contain more than one redox-active center provides valuable information about fundamental electron transport phenomena. These processes are often closely correlated with natural biosynthesis phenomena and the development of unique chemical and physical characteristics which allow for the production of novel materials, more efficient catalytic systems, and for molecular electronic devices. Suitable complexes for these investigations may be generally described by the formula (LmM)a(μ- BL)(M’L’n)b (BL = bridging ligand; L, L‘ = terminal ligands). Two main areas of research may be distinguished:
- The variation of the redox-active capping groups MLn allows for a fine-tuning of the redox potential and the electronic configuration of the accessible redox states. Capping groups which feature electrochemically reversible redox reactions are of particular interest, since the study of electron transfer processes by electrochemical methods like cyclic voltammetry requires a reversibility of the investigated redox steps. Metallocene complexes like ferrocene 1 and cobaltocene 2 meet this requirement since both complexes may be converted reversibly into the corresponding monocation by a one-electron oxidation step. The majority of the published studies in this area focusses on ferrocene complexes due to the ease of handling these materials, while examples of bridged dicobaltocenes are still rare despite the fact that the special electron configuration of cobaltocene in which an electron occupies a ligand-centred π* orbital promises electronic interactions over long distances. Hence we are interested in the synthesis of dicobalt complexes in which the metallocene moieties are separated by linkers with extended π systems.
- The variation of the bridging ligand BL in a series of complexes which bear identical redox-active capping groups MLn allows for an assessment of its charge transfer properties. Bridging ligands which derive from rigid rod molecules with extended π systems like oligo-p-phenylenes 3, –thiophenes 4 und –acetylenes 5 are of particular interest, since they represent the main building blocks of many conducting polymers. Since redox-active metallocenyl capping groups are usually attached to the linking group via C-C σ-bonds, the attached cyclopentadienyl rings have to be envisaged as parts of the BL framework. The linkage of cyclopentadienyl metal fragments via annelated arenes as indene 6  and fluorene 7  which may engage the six-membered ring as well as the five-membered ring for coordination, represents an alternative approach to carbon-bridged metallocenes. Polycyclic arenes like decacyclene 8 show a variety of coordination modes and numbers.
Figure 1 presents an example for a dimeric triple-decker complex consisting of two indenyl units linked by a saturated CMe2 bridge, which serve themselves as bridges between two cyclopentadienyl metal units. Heteroatom linkers complement the carbon-based bridging groups and have been the subject of numerous investigations. Furthermore, bridged multinuclear oligometallocenes are accessible if the metallocene moiety bears donor substituents like -OR, -SR, -NR2 oder -PR2, since coordination of these substituents to metal ions affords complexes which link the metallocene capping groups via heteroatom-metal-heteroatom bridges. This synthetic strategy allows for the construction of complex structures that bear a multitude of potentially redox-active centers.
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