Electrochemical Illumination of Intramolecular Communication in Ferrocene-Containing tris-β-Diketonato Aluminum(III) Complexes; Cytotoxicity of Al(FcCOCHCOCF₃)₃

The series of ferrocene-containing tris-β-diketonato aluminum­(III) complexes [Al­(FcCOCHCOR)₃] (R = CF₃, 1; CH₃, 2; C₆H₅, 3; and Fc = ferrocenyl = Fe­(η⁵-C₅H₅)­(η⁵-C₅H₄), 4) were synthesized and investigated structurally and electrochemically; complex 1 was subjected to cytotoxicity tests. ¹H NMR-spectroscopy distinguished between the mer and fac isomers of 2 and 3. Complex 1 existed only as the mer isomer. A single crystal X-ray crystallographic determination of the structure of a mer-isomer of Al­(FcCOCHCOCF₃)₃, 1, (Z = 4, space group P2₁2₁2₁) demonstrated extensive delocalization of all bonds which explained the pronounced electrochemically observed intramolecular communication between molecular fragments. In contrast to electrochemical studies in CH₂Cl₂/[N­(ⁿBu)₄]­[PF₆], the use of the supporting electrolyte [N­(ⁿBu)₄]­[B­(C₆F₅)₄] allowed identification of all Fc/Fc⁺ electrochemical couples by cyclic and square wave voltammetry for 1–4. For R = Fc, formal reduction potentials of the six ferrocenyl groups were found to be E°′ = 33, 123, 304, 432, 583, and 741 mV versus free ferrocene respectively. Complex 1 (IC₅₀ = 10.6 μmol dm^–3) was less cytotoxic than the free FcCOCH₂COCF₃ ligand having IC₅₀ = 6.8 μmol dm^–3 and approximately 2 orders of magnitude less toxic to human HeLa neoplastic cells than cisplatin (IC₅₀ = 0.19 μmol dm^–3).