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

Abstract

The redox-active metalloligand (RML) (Et₃NH)₃[Cr^III(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 [Cr^III(F₄Cat)₂(F₄SQ)]²⁻ (F₄SQ = perfluorosemiquinonato), [Cr^III(F₄Cat)(F₄SQ)₂]⁻, and [Cr^III(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 [Cr^III(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 [Cr^III(F₄Cat)₃]³⁻ to act as a RML in solution.