New Vistas in Metallocenium Chemistry

Metallocenes are an important class of organometallic compounds with many useful applications in chemistry. In this research project, the underdeveloped subclass of cationic metallocenes of cobalt and rhodium will be investigated, aiming at exploiting their intrinsic properties comprising high chemical stability, reversible redox chemistry, cationic charge with concomitant solubility in water, and opposite polarity/electrochemistry in comparison to isoelectronic ferrocene compounds. Thereby new opportunities in redox-catalysis, photo- sensitizing, materials chemistry and anti-cancer agents will arise.

Seventy years ago, the serendipitous discovery of ferrocene marked the foundation of modern organometallic chemistry. Ferrocene is the ancestor of the so-called metallocenes and also the first compound in organometallic chemistry containing the ubiquitous cyclopentadienyl ligand. Based on this seminal discovery, related complexes with all metals across the periodic table of elements were investigated with numerous applications in catalysis, electrochemistry, sensorics, material science and medicinal chemistry were developed. Surprisingly, isoelectronic cobaltocenium salts having similar advantageous properties like ferrocene (air stability, redox activity) have been much less studied. This has to do with the intrinsic cationic charge of the cobaltocenium moiety, inhibiting chemical functionalization of cobaltocenium by standard organic reactions. Within this project, for the first time chemoselective reactions to get access to monofuctionalized cobaltocenium and rhodocenium salts with carboxylate, amino, azido, diazenido, iodo, ethinyl and other functional groups could be realized by new, innovative synthetic protocols. Thereby very valuable key synthons for further research are now available, significantly augmenting the chemical space of metallocenium compounds. A special focus in this area will be exploitation of the advantageous properties of these complexes (high chemical stability, reversible redox chemistry, good solubility in polar solvents, including in water) for various relevant applications, e.g. electrochemically switchable catalysts, organometallic pharmaceuticals, redox-active polymers and other smart materials. First proof-of-principle results could be achieved, e.g. electrochemical reductive water splitting with cobaltocenium-substituted tetrazene complexes, stabilization of mesoionic cobaltocenylidene carbene ligands, catalytic electrophilic hydroamination of alkynes with cobaltocenium-substituted triazolylidene gold complexes and water soluble cobaltocenium-based cytostatic pharmaceuticals. Overall, the projected goals of this research project were achieved fully, thereby laying the foundation for novel metallocenium materials with new and relevant applications.