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

Abstract 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@C 81 N, 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@C 81 N 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.