TEMPO Radical-Functionalized Supramolecular Coordination Complexes with Controllable Spin–Spin Interactions

The topic of noncovalent spin–spin interactions is of increasing general interest in supramolecular radical chemistry. In this report, a series of exo- and endo-TEMPO radical-functionalized metallacycles 1–4 and metallacages 5 and 6 were constructed via coordination-driven self-assembly, wherein the number, location, and distance of the spins were precisely controlled. Their intriguing spin–spin interactions were systematically investigated by electron paramagnetic resonance (EPR) and were well interpreted at the molecular level assisted by X-ray crystallography analysis. The results revealed their distinct spin–spin interactions in the solution state, wherein the spin–spin interaction of metallacycle 3 was much stronger than that of the other five assemblies mainly due to its shorter intramolecular spin–spin distance. In the solid state, 1–6 exhibited obvious spin–spin (dipole–dipole) interactions because of the close arrangement of TEMPO units as indicated in their single crystals. Specifically, a large zero-field splitting (ZFS; D = 17.5 mT) was observed in the crystalline form of metallacycle 4, which arose from the strong intermolecular spin–spin coupling. Interestingly, when the counterion of PF6– in 4 was changed to BF4–, the BF4– counterion analog 4a also exhibited a large ZFS, but the ZFS originated from the intramolecular spin–spin interaction due to a small variation in its crystal conformation. Moreover, the reversible on–off switching of the ZFS in 4 and 4a via the crystal-to-amorphous transformation induced by mechanical grinding and solvent vapor stimuli was also successfully realized. The unique and controllable inter- and intramolecular spin–spin interactions in this work reveal new insights for the understanding and manipulation of spin–spin interactions and may open up a new way to develop organic spin materials in the future.