Radical–Radical Recognition: Switchable Magnetic Properties and Re-entrant Behavior

π–π radical interactions have exacting geometry requirements, significantly more stringent than those of a hydrogen bond or a van der Waals interaction. Here, supramolecular synthons based on such radical–radical recognition are employed to generate switchable structural and magnetic properties. Interactions between neighboring paramagnetic ligands of the La­(hfac)3(pyDTDA)2 coordination complex cause a rare re-entrant phase transition (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonato-; pyDTDA = 4-(2′-pyridyl)-1,2,3,5-dithiadiazolyl). Below 100 K, the complex is diamagnetic in the solid state, consistent with an f0 lanthanum­(III) ion and “pancake bonding” of the π-radical ligands. Upon heating, the supramolecular one-dimensional arrangement undergoes two sequential structural phase transitions, observed at ca. 160 and 310 K, successively involving the rupture of half and then the totality of the “pancake bonds” and generating stepwise increases in the paramagnetic susceptibility. We use the structural and magnetic data to develop a theoretical model that clearly predicts the unprecedented re-entrant behavior of this radical-La­(III)-radical complex. Moreover, this microscopic free-energy model demonstrates that any system with essentially noninteracting “dimers” contains within itself the possibility of a distortion associated with re-entrant phase transitions if the lattice is “soft” enough (i.e., low rigidity).