Molecular recognition of redox-switchable bis-crown moieties assembled on a dicobalt complex

A new dinuclear complex of cobalt(III) having double bis-crown moieties (1) was synthesized in a one-pot multistep reaction between the dinucleating ligand (H₂L¹), Co(OAc)₂ · 4H₂O, and 4′-aminomethylbenzo-15-crown 5-ether under aerobic conditions. The complex was characterized by UV-vis, IR, NMR, and CSI-mass spectroscopies, and the axial coordination of the 4′-aminomethylbenzo-15-crown 5-ether on both cobalt centers was confirmed. The bis-crown moieties of 1 acted as a potassium ion binding site with a large association constant (K) of 1.97 × 10 ⁵ M^-1 (1:1) caused by the preorganized bis-crown structure. Solvent extraction of K⁺ (potassium picrate) with 1 was investigated by UV-vis and NMR, and the extracted K⁺ was quantified by an atomic absorption/flame emission spectrophotometer. The cyclic voltammogram of 1 showed irreversible redox waves at -0.57 V and +0.16 V versus Ag/AgCl in DMF, which are ascribed to the Co(III) and Co(II) redox couple. The large peak separation (A0.73 V) of this redox couple indicates the association and dissociation of the axial amine on the cobalt centers. The irreversible reduction peak of 1 at -0.57 V was shifted in the positive direction by 80 mV in the presence of the potassium ion, showing the complexation of 1 with the potassium ion. The reversible formation and decomposition of the double bis-crown moieties in 1 caused by the electrical stimulus were also investigated by thin-layer UV-vis spectroelectrochemistry. The switching of the double bis-crown structure of 1 was established over four trials with response to electrical energy.