Sandwich complexes are important building blocks in medicinal inorganic chemistry for group 6 and 8 elements but are almost unknown for the manganese triad. We present the syntheses and full characterization of the mixed-arene Tc sandwich complexes [Tc(η-hmbz)(η-CH-NH)](PF) and [Tc(η-hmbz)(η-CH-Br)](PF). Both comprise functionalities for conjugation to targeting molecules or for being included as substructures in pharmaceutically active lead compounds. Since η-benzene ligands are too stably bound to be replaced with incoming ligands, we prepared naphthalene complexes [Re(η-CH)(η-napht)] and [Re(η-napht)]. Their reactivities towards substitution are increased and one or both naphthalene ligands can be replaced with mono- or multi-dentate ligands. Combining the features of Tc and Re may lead to a molecule-based theranostic approach.

The synthesis of mono- and difunctionalized [Re(η-CHR)(η-CHR)] (n = 0, 1; R = COOH, Br) complexes starting from [Re(η-benzene)] is described. The lithiation of [Re(η-benzene)] with n-BuLi leads preferentially to the neutral, alkylated product [Re(η-CH)(η-CH-Bu)] but not to the expected deprotonation of the arene ring. Deprotonation/lithiation with LDA gave the mono- and the dilithiated products in situ. Their reactions with 1,1,2,2-tetra-bromoethane (TBE) or with CO, respectively, gave [Re(η-CHBr)(η-CH)], [Re(η-CHBr)], or [Re(η-CHCOOH)(η-CH)], [Re(η-CHCOOH)]. These functionalized derivatives of [Re(η-benzene)] represent novel precursors for the synthesis of bioconjugates to bioactive structures, comparable to [Co(Cp)] or [Fe(Cp)]. Different model compounds [Re(η-CHR)(η-CHR)] (n = 0, 1; R = -SCHPh, -NHPh, -CONHCHPh, -CH-COdpa) were synthesized via amide bond formation and nucleophilic aromatic substitution. These conjugates were fully characterized including X-ray structure analyses of most products. For all complexes, the H NMR arene proton signals are strongly upfield-shifted as compared to those of the noncoordinated arenes. The electrochemical analyses show an irreversible, probably substituent-centered oxidation, which contrasts the cyclic voltammetry of the underivatized complexes where oxidation is fully reversible. The stability of the core and the reactivity of the substituents make these bis-arene complexes useful precursors in medicinal inorganic chemistry, comparable to cobaltocenium or ferrocene.

The contraction of coordinated aromatic hydrocarbons is a rare reactivity pattern in organometallic chemistry. We describe the conversion of a bromobenzene coordinated to a Re center into a cyclopentadienyl aldehyde. Under mildly alkaline conditions, the expected phenol complex is formed with Re and Tc but under strong basic conditions; ring contraction occurs in close to quantitative yields for Re only. A mechanism for this unprecedented reaction is proposed based onH and H NMR spectra and density functional theory calculations.

The synthesis of technetium and rhenium bis-arene sandwich complexes, [M­(η6-C6H6)2]+, requires AlCl3 according to Fischer–Hafner conditions. Its high reactivity limits syntheses to alkylated aromatic hydrocarbons but is generally incompatible with aromatic hydrocarbons, bearing heteroatoms in their side chains “R”. In a nuclear medical context, a synthesis to complexes of the [M­(η6-C6H5-R)2]+ type directly from [ReO4]− or [99mTcO4]− is desirable because postsynthetic derivatizations are not possible for 99mTc. Here, we present direct syntheses to [M­(η6-C6H5-R)2]+, starting from [ReO4]− or [99(m)TcO4]− with “C6H5-R” representing differently substituted anilines such as N-methylaniline, N,N-dimethylaniline, diphenylaniline (triphenylamine), and phenylpiperidine. Depending on the reaction conditions, mixed-ring sandwich complexes of the [Re­(η6-arene)­(η6-C6H6)]+ type are obtained with diphenylaniline and triphenylmethane. The X-ray crystal structures of the complexes are presented together with their photophysical and electrochemical properties. Aniline type complexes show ligand-centered, irreversible ReI/II oxidations, whereas sandwich complexes without heteroatoms exhibit reversible, metal-centered oxidation. Weak fluorescence at 365 nm was found for [M­(η6-C6H5-NPh2)2]+. The synthesis of the [99(m)Tc­(N,N-dimethylaniline)2]+ analogue was performed as a proof for applicability in technetium chemistry.

The labeling of (bio)molecules with metallic radionuclides such as 99mTc demands conjugated, multidentate chelators. However, this is not always necessary since phenyl rings can directly serve as integrated, organometallic ligands. Bis‐arene sandwich complexes are generally prepared by the Fischer–Hafner reaction. In extension of this, we show that [99mTc(η6‐C6R6)2]+‐type complexes are directly accessible from water and [99mTcO4]−, even using arenes incompatible with Fischer–Hafner conditions. To unambiguously confirm the nature of these unprecedented 99mTc complexes, their rhenium homologous have been prepared by substituting naphthalene ligands in [Re(η6‐C10H8)2]+ with the corresponding phenyl groups. The ease with which highly stable [99mTc(η6‐C6R6)2]+ complexes are formed under standard labeling conditions enables a multitude of new potential imaging agents based on commercial pharmaceuticals or lead structures.