Spin-Frustrated Trisradical Trication of PrismCage

Organic trisradicals featuring threefold symmetry haveattractedsignificant interest because of their unique magnetic properties associatedwith spin frustration. Herein, we describe the synthesis and characterizationof a triangular prism-shaped organic cage for which we have coinedthe name PrismCage ( 6+ ) and itstrisradical trication TR ( 3(& BULL;+) ). PrismCage ( 6+ ) is composedof three 4,4 & PRIME;-bipyridinium dications and two 1,3,5-phenyleneunits bridged by six methylene groups. In the solid state, PrismCage ( 6+ ) adopts a highly twisted conformation withclose to C (3) symmetry as a result of encapsulatingone PF6 (-) anion as a guest. PrismCage ( 6+ ) undergoes stepwise reduction to its mono-,di-, and trisradical cations in MeCN on account of strong electroniccommunication between its 4,4 & PRIME;-bipyridinium units. TR ( 3(& BULL;+) ), which is obtained by the reductionof PrismCage ( 6+ ) employing CoCp2, adopts a triangular prism-shaped conformation with closeto C (2v ) symmetry in thesolid state. Temperature-dependent continuous-wave and nutation-frequency-selectiveelectron paramagnetic resonance spectra of TR ( 3(& BULL;+) ) in frozen N,N-dimethylformamide indicate its doublet ground state. The doublet-quartetenergy gap of TR ( 3(& BULL;+) ) isestimated to be -0.08 kcal mol(-1), and thecritical temperature of spin-state conversion is found to be ca. 50K, suggesting that it displays pronounced spin frustration at themolecular level. To the best of our knowledge, this example is thefirst organic radical cage to exhibit spin frustration. The trisradicaltrication of PrismCage ( 6+ ) opensup new possibilities for fundamental investigations and potentialapplications in the fields of both organic cages and spin chemistry.