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

The synthesis of mono- and difunctionalized [Re­(η6-C6H5R)­(η6-C6H6–nRn)]+ (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-C6H6–nRn)]+ (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.