Direct Synthesis of Non-Alkyl Functionalized Bis-Arene Complexes of Rhenium and 99(m)Technetium

Abstract

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.