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.

[Re(η6-arene)2]+ as a highly stable ferrocene-like scaffold for ligands and complexes

“…The irreversible nature of the two oxidation processes in 3a + suggests a ligand-and a metal-based oxidation, as observed for other sandwich complexes with functionalities bound to the arene rings. 5,7 In 3b + with two electron-withdrawing groups on the arenes, the metal-based oxidation is probably beyond the solvent window and only the oxidation of the aldehydes occurs. Meola et al reported the mixed-ring complex [Re(η 6 -C 6 H 6 )(η 5 -C 5 H 4 COH)] which also comprises an aldehyde.…”

“…The symmetry of all NMR spectra and therefore the free rotation around the rhenium axis is found for all [Re(arene) 2 ] + synthesized by our group so far. [5][6][7]16,17,27 Pyridines are prototypical ligands in many catalytic processes, e.g. for proton reduction in photocatalysis.…”

“…[4][5][6] Direct syntheses with functionalized arenes were reported as well, however, the high reactivity of AlCl 3 limits the compatibility to some aniline-type arenes. 7 Therefore, post-synthetic modifications are often required to introduce functionalities into the basic [Re(η 6 -C 6 H 6 ) 2 ] + framework, a comparable procedure as done with ferrocene. Because of its stability and great versatility in terms of derivatization, ferrocene has become landmark in organometallic chemistry and has played a fundamental role in many areas of chemistry.…”

“…The symmetry of all signals observed in the NMR spectra (see ESI †) supports the notion of free rotation at room temperature. [5][6][7]16,17 Electrochemistry showed that Re(I) is reversibly oxidized at E°1 /2 > +1.3 V (Re I /Re II ). Irreversible reductions take place at low potentials <−2.0 V, both vs. Ag/AgCl.…”

“…Irreversible reductions take place at low potentials <−2.0 V, both vs. Ag/AgCl. 5,7 Compared to ferrocene, Re(I) bis-arene derivatives are much more difficult to oxidize and are generally stable against atmospheric oxygen ( Fig. S1 and S2 †).…”