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.