Supramolecular Chemistry of Cobalt-Corrins

Cobalt corrins are chiral relatives of the cobalt porphyrins, in which the ligand is much less unsaturated and is also ‘contracted’ by one carbon. This type of ligand strongly affects the redox and coordination chemistry of the bound cobalt center. Some lipophilic artificial cobalt corrins have been prepared in tremendous synthetic efforts in the Eschenmoser labs. The natural analogues offer an apparently more straight forward chemical entry to cobalt corrins. However, the natural cobalt corrins are all hydrophilic and highly substituted at their periphery, and they are distantly related to heme and chlorophyll. Although these cobalt corrins are very soluble in water, their natural roles are intimately connected with their essential and rich organometallic chemistry, which is all cobalt-centered.[1] In this lecture, means for specific supramolecular assemblies based on modified natural cobalt corrins are delineated, as well as consequences of the supramolecular restructuring on the organometallic and redox-chemistry. So far, in the excursions to be reported, the cobalt corrin core is not altered and its redox and spectral properties are largely conserved. A unique basis for specific supramolecular assemblies is then provided by the (organometallic) ligand itself, either, when attached to and bridging more than one metal center, or, when providing a structuring interface itself, for the build-up of supra-molecules. In a second general type of supramolecular assemblies the functional groups attached at the peripheral substituents of the natural corrinoids are used in a uniform way. By this means, the direct environment of the cobalt corrin moiety may be affected and structured in a global fashion, generating, e.g., a type of molecular micelles. A third type of supramolecular assembly makes selective use of single sites of the complex, but largely conserved sphere of the peripheral substituents, in order to generate specifically structured cobalt-corrins and their dimers. These latter ones may feature binuclear (organometallic) coordination chemistry, which would allow for synchronized redox processes in a highly structured, ‘second’ sphere around the metal centers. I thank the Austrian Science Foundation (FWF, project P-28892 and earlier projects) for generous financial support. References [1] B. Kräutler, B. Puffer, in Handbook of Porphyrin Science, Vol. 25 (Ed.: K. M. Kadish, Smith, K. M., Guilard, R.), World Scientific, 2012, pp. 133-265.