Chapter 11 - Technetium and Rhenium Complexes with Aromatic Hydrocarbons as Ligands: An Entry into Biomimetic Imaging

Technetium and rhenium as middle transition elements offer a very rich organometallic chemistry, useful for many applications not only in the life sciences. Bioorganometallic compounds are subject of different conceptual approaches, but serve mainly therapeutic purposes for cancer treatment and other diseases. Bioorganometallic complexes exert their activities as, e.g., de novo complexes, i.e., without any obvious structural relationship to biological models. Alternatively, they may comprise ligands or pendent groups for which a function is known. Finally, they can mimic a biological active structure derived from a pharmaceutical lead compound. Group 7 elements include technetium of which metastable radionuclide 99mTc is a daily applied radioisotope in diagnostic nuclear medicine. The combination of rhenium for therapy and technetium for diagnostics with isostructural complexes is, thus, a unique opportunity from within the transition elements. For implementing a biomimetic approach with existing pharmaceuticals or biological compounds, aromatic hydrocarbons such as cyclopentadienyl or phenyl groups are ideal structures as ligands. They are ubiquitous units in bioactive compounds. The two most prominent “copies” of these moieties are cyclopentadienyl and arenes as ligands. Both replace phenyl groups as shown in many examples with, e.g., ferrocene. To transfer the biomimetic approach to group 7 elements, however, requires special conditions. Especially for technetium, any chemistry has to be performed in water, constraints which impose some synthetic challenges but provide new insights and vistas. Chapter 11 focuses on fundamental organometallic chemistry with technetium, related to piano-stool type cyclopentadienyl derivatives for receptor targeting. Subsequent examples demonstrate the practical use of piano-stool complexes of technetium and rhenium for biomimetic purposes. In extension, the chemistry with isoelectronic arene ligands in the form [M(η6-C6R6)2]+)M=99(m)Tc, Re) sandwich complexes will be shown, a new approach in bioorganometallic chemistry with group 7 elements with a perspective towards theranostics. Fundamental chemistry is combined with examples from application. Since the field of aromatic hydrocarbons as ligands in group 7 chemistry is not extensively researched despite its multiple opportunities, the advances in bioorganometallic chemistry presented herein shall inspire an alternative approach considering biomimetic concepts