Interactions between the trianionic ligand-centred redox-active metalloligand [CrIII(perfluorocatecholato)3]3− and guest metal ions

The redox-active metalloligand (RML) (Et3NH)3[CrIII(F4Cat)3] (F4Cat = perfluorocatecholato) (1) was synthesized and its interactions with guest metal ions Li+, Mn2+, Fe2+, Co2+, Cu2+, and Zn2+ were examined. Cyclic voltammetry measurements and spectroelectrochemical studies revealed that complex 1 shows three-step ligand-centred one-electron oxidation to consecutively generate [CrIII(F4Cat)2(F4SQ)]2− (F4SQ = perfluorosemiquinonato), [CrIII(F4Cat)(F4SQ)2]−, and [CrIII(F4SQ)3] at −0.12, 0.23, and 0.53 V vs. Ag/Ag+ in dichloromethane, or at −0.21, 0.08, and 0.50 V in acetonitrile (MeCN), respectively. Titration experiments in MeCN revealed that treatment of 1 with Cu2+ leads to the formation of [CrIII(F4Cat)2(F4SQ)]2− and Cu+via a redox reaction. However, when 1 was treated with Li+, Mn2+, Fe2+, Co2+, and Zn2+, further titration experiments revealed that these metal ions coordinated via the lone pairs on the coordinating oxygen atoms of the F4Cat2− moieties in a one-to-one ratio, and binding constants of 3.7 (±0.3) × 104 (Li+), 1.5 (±0.2) × 105 (Mn2+), 2.2 (±0.4) × 105 (Fe2+), 1.9 (±0.2) × 105 (Co2+), and 3.8 (±0.4) × 105 M−1 (Zn2+) were established. Moreover, the oxidation potentials of 1 were positively shifted by 0.08–0.33 V upon addition of guest metal ions. Spectroelectrochemical studies of 1 in the presence of guest metal ions suggested that ligand-centred one- and two-electron oxidation of the RML occurred for Li+, Mn2+, Co2+, and Zn2+, respectively, while guest metal-centred one-electron oxidation was observed for Fe2+. Considering all the aforementioned results, this study demonstrated for the first time the ability of [CrIII(F4Cat)3]3− to act as a RML in solution.