Synthesis Comparative Electrochemistry and Spectroelectrochemistry of Metallocenyl β-Diketonato Dicarbonyl Complexes of Rhodium(I)—Cytotoxicity of [Rh(FcCOCHCOCF3)(CO)2]

Abstract

The metallocenyl-containing β-diketonato rhodium(I) dicarbonyl complexes of [Rh(FcCOCHCOR)(CO)2] where R = CF3, 10; Fc = ferrocenyl = FeII(C5H5)(C5H4), 11; Rc = ruthenocenyl = RuII(C5H5)(C5H4), 12; and Oc = osmocenyl = OsII(C5H5)(C5H4), 13 were synthesized. Complexes 10–13 were then subjected to an electrochemical study utilizing cyclic voltammetry (CV), square wave voltammetry (SWV), and linear sweep voltammetry (LSV) in the non-coordinating solvent/supporting electrolyte medium CH2Cl2/0.1 mol dm−3 [N(nBu)4][B(C6F5)4]. The formal reduction potential for the electrochemical reversible Fc0/+ couples in 10–13 was identified in the range 0.156 ≤ Eo′ ≤ 0.328 V while the electrochemically irreversible osmocenyl and ruthenocenyl oxidations were observed at peak anodic potentials of Epa = 0.640 V and Epa = 0.751 V, respectively. Resolution between the closely overlapping CV-determined Fc0/+ and RhI/II couples was too poor for unambiguous measurement of the RhI/II redox potential, but square wave voltammetry allowed estimates of Eo′ (RhI/II) in the range 0.156 ≤ Eo′ ≤ 0.398 V. FT-IR spectroelectrochemistry confirmed the one-electron oxidation of RhI by the appearance of CO vibrational bands at stretching frequencies, which are associated with rhodium(II) and not rhodium(III). Cytotoxicity tests on 10 (IC50 = 19.2 µM) showed it to be substantially less cytotoxic than the free β-diketone, FcCOCH2COCF3, and [Rh(FcCOCHCOCF3)(cod)].