Spin-Frustrated Trisradical Trication of PrismCage

Organic trisradicals featuring threefold symmetry have attracted significant interest because of their unique magnetic properties associated with spin frustration. Herein, we describe the synthesis and characterization of a triangular prism-shaped organic cage for which we have coined the name PrismCage and its trisradical trication─TR. PrismCage is composed of three 4,4'-bipyridinium dications and two 1,3,5-phenylene units bridged by six methylene groups. In the solid state, PrismCage adopts a highly twisted conformation with close to C₃ symmetry as a result of encapsulating one PF₆^- anion as a guest. PrismCage undergoes stepwise reduction to its mono-, di-, and trisradical cations in MeCN on account of strong electronic communication between its 4,4'-bipyridinium units. TR, which is obtained by the reduction of PrismCage employing CoCp₂, adopts a triangular prism-shaped conformation with close to C₂ symmetry in the solid state. Temperature-dependent continuous-wave and nutation-frequency-selective electron paramagnetic resonance spectra of TR in frozen N,N-dimethylformamide indicate its doublet ground state. The doublet-quartet energy gap of TR is estimated to be -0.08 kcal mol^-1, and the critical temperature of spin-state conversion is found to be ca. 50 K, suggesting that it displays pronounced spin frustration at the molecular level. To the best of our knowledge, this example is the first organic radical cage to exhibit spin frustration. The trisradical trication of PrismCage opens up new possibilities for fundamental investigations and potential applications in the fields of both organic cages and spin chemistry.