A Redox-Active Phenothiazine-based Pd₂L₄-Type Coordination Cage and Its Isolable Crystalline Polyradical Cations

Polyradical cages are of great interest because they show very fascinating physical and chemical properties, but many challenges remain, especially for their synthesis and characterization. Herein, we present the synthesis of a polyradical cation cage 1⁴⋅⁺ through post-synthetic oxidation of a redox-active phenothiazine-based Pd₂L₄-type coordination cage 1. It's worth noting that 1 exhibits excellent reversible electrochemical and chemical redox activity due to the introduction of a bulky 3,5-di-tert-butyl-4-methoxyphenyl substituent. The generation of 1⁴⋅⁺ through reversible electrochemical oxidation is investigated by in situ UV/Vis-NIR and EPR spectroelectrochemistry. Meanwhile, chemical oxidation of 1 can also produce 1⁴⋅⁺ which can be reversibly reduced back to the original cage 1, and the process is monitored by EPR and NMR spectroscopies. Eventually, we succeed in the isolation and single crystal X-ray diffraction analysis of 1⁴⋅⁺, whose electronic structure and conformation are distinct to original 1. The magnetic susceptibility measurements indicate the predominantly antiferromagnetic interactions between the four phenothiazine radical cations in 1⁴⋅⁺. We believe that our study including the facile synthesis methodology and in situ spectroelectrochemistry will shed some light on the synthesis and characterization of novel polyradical systems, opening more perspectives for developing functional supramolecular cages.