Bis-Arene Complexes [Re(η⁶-arene)₂]⁺ as Highly Stable Bioorganometallic Scaffolds

The synthesis of mono- and difunctionalized [Re(η⁶-C₆H₅R)(η⁶-C₆H_6-nR_n)]⁺ (n = 0, 1; R = COOH, Br) complexes starting from [Re(η⁶-benzene)₂]⁺ is described. The lithiation of [Re(η⁶-benzene)₂]⁺ with n-BuLi leads preferentially to the neutral, alkylated product [Re(η⁶-C₆H₆)(η⁵-C₆H₆-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 CO₂, respectively, gave [Re(η⁶-C₆H₅Br)(η⁶-C₆H₆)]⁺, [Re(η⁶-C₆H₅Br)₂]⁺, or [Re(η⁶-C₆H₅COOH)(η⁶-C₆H₆)]⁺, [Re(η⁶-C₆H₅COOH)₂]⁺. These functionalized derivatives of [Re(η⁶-benzene)₂]⁺ represent novel precursors for the synthesis of bioconjugates to bioactive structures, comparable to [Co(Cp)₂]⁺ or [Fe(Cp)₂]. Different model compounds [Re(η⁶-C₆H₅R)(η⁶-C₆H_6-nR_n)]⁺ (n = 0, 1; R = -SCH₂Ph, -NHPh, -CONHCH₂Ph, -C₆H₅-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 ¹H 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.