Redox-active ligands

Redox reactions are of prime importance in many processes in nature, being it corrosion of metals, photosynthesis, or the respiration chain. In many of these processes, a key element is the combination of two (or more) redox-active components, such as a metal with variable oxidation state and a (organic) substrate which can undergo reduction or oxidation, respectively.

Based on previous research expertise of the group, a new project has been launched in 2007 under the guidance of Dr. Roithová, which deals with the properties of redox-active ligands, particularly phenols and quinones as well as related redox-active arene derivatives. This class of compounds is of extreme relevance in various biochemical and medical aspects, ranging from the vision process over numerous redox enzymes in the human body to rather complex phenomena, such as aging processes.

The research project has two facets which complement each other. On the one hand, the intrinsic physicochemical properties of redox-active ligands shall be probed in the absence of coordinated metal ions in order to quantitatively understand the factors which influence the redox properties in the neutral, ionized as well as protonated or deprotonated forms. On the other hand, complexes of these ligands with metal ions, in particular redox-active metals, will be investigated and probed with respect to their chemical behavior.

The figure below gives an example of a Co^III catecholato complex, which upon heating undergoes a valence isomerism to a Co^II species bound to a semiquinone radical. This change in redox state is associated with remarkable changes of macroscopic properties, e.g. the magnetic behavior, which renders such compounds as attractive candidates not only for chemists but also for material scientists, due to the possible perspectives evolving for the design of molecular switches.

Even more than in the other projects, the input of theory is of extreme importance in this topic, because the experimental detection of intramolecular redox transitions is difficult to probe directly, even though it is reflected in many changes of physical and chemical observables. A drastic change in reactivity upon minor variation of substitution pattern, for example, clearly demonstrates that a major change has taken place, but from reaction kinetics alone, a deduction of the molecular origin of such a phenomenon is rather difficult, if not impossible. Here, contemporary computational chemistry can provide unequivocal answers to the problems posed by experiments.