Polynuclear dioxolene complexes with redox-active transition metals – novel synthesis routes, characterization and capabilities
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Abstract
Due to their unique redox chemistry, non-innocent dioxolene ligands prove to be valuable and versatile compounds in coordination chemistry. Complexes with this kind of quinoid systems in some cases may exhibit intramolecular electron transfer processes, triggered by external stimuli, the so-called valence tautomerism. Beside their purpose as test-ground for the examination of redox processes, manifold applications for these switchable molecules are found in the fields of sensor devices, data storage or molecular electronics. While previous studies mainly covered mononuclear dioxolene complexes, the present work deals with the synthesis and characterization of polynuclear transition metal complexes with redox-active dioxolene ligands. Due to the presence of multiple redox centers, suchlike complexes offer the possibility for multi-step valence-tautomeric transitions and can therefore be regarded as polymodal molecular switches. In order to examine the ligand bridging mode’s influence on the electron configuration, multiple methods for the targeted synthesis of polynuclear dioxolene complexes were developed: the selforganization of small components under appropriate conditions, the utilization of metallacrown fragments as structuring and preorganizing elements and the use of rigid, bis-chelating ancillary ligands. Based on these methods, multifaceted polynuclear complexes were obtained. Single crystal structure determination, spectroscopic and magnetometric measurements on those compounds allowed the exact determination of the possessed oxidation states for dioxolenes and metal ions. For some synthesized cobalt complexes, thermal valence tautomerism above room temperature was observed. Moreover, spectroscopic studies on suitable complexes showed catalytic activity in the oxidation of catechols to o-quinones with dioxygen, for which the kinetics could be described by the Michaelis–Menten theory. These polynuclear systems therefore serve as model complexes for the metalloenzymes catecholase and tyrosinase, contributing to the mechanistic understanding of the corresponding biological processes.