Rotaxane CoII Complexes as Field-Induced Single-Ion Magnets

Mechanically chelating ligands have untapped potential for the engineering of metal ion properties by providing reliable control of the number, nature and geometry of donor atoms, akin to how a protein cavity controls the properties of bound metal ions. Here we demonstrate this principle in the context of Co^II-based single-ion magnets. Using multi-frequency EPR, susceptibility and magnetization measurements we found that these complexes show some of the highest zero field splittings reported for five-coordinate Co^II complexes to date. The predictable coordination behavior of the interlocked ligands allowed the magnetic properties of their Co^II complexes to be evaluated computationally a priori and our combined experimental and theoretical approach enabled us to rationalize the observed trends. The predictable magnetic behavior of the rotaxane Co^II complexes demonstrates that interlocked ligands offer a new strategy to design metal complexes with interesting functionality.