Anomalous Magnetic Hysteresis up to 20 K in a Non-Kramers Light-Lanthanide Single-Molecule Magnet

Light-lanthanides (Ln), despite their widespread uses in commercial permanent magnets, are among the least explored metal elements as building blocks of single-molecule magnets (SMMs) due to their smaller magnetic moments as well as weaker spin–orbit couplings than those of the heavy-Ln counterparts, and so far, only a neodymium (Nd) complex has been reported showing a small magnetic hysteresis at 2 K. Here, we report a low-coordination praseodymium (Pr) complex, namely, Pr@C₈₁N, featuring a non-Kramers trivalent Pr ion entrapped within an azafullerene cage and behaving as the first Pr-based SMM. Although the weak nonaxial ligand field imposed only by the carbon cage on the integer-spin Pr(III) elicits nondegenerate spin states, Pr@C₈₁N shows an anomalous magnetic hysteresis up to 20 K using a field sweep rate of 20 Oe/s, which is the highest temperature among all reported light-Ln SMMs. In addition to the ideally minimized spin–phonon coupling in the low coordination, the origin of such an unexpected magnetic bistability for the non-Kramers complex likely lies in nuclear sublevels arising from a large Pr hyperfine coupling, which feature distinct electronic magnetizations within the highly mixed ground electronic state.