Rotaxane CoII Complexes as Field‐Induced Single‐Ion Magnets

Mechanically chelating ligands have untapped potential for the engineering of metal ion properties. Here we demonstrate this principle in the context of CoII‐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 CoII complexes to date. The predictable coordination behaviour of the interlocked ligands allowed the magnetic properties of their CoII complexes to be evaluated computationally a priori and our combined experimental and theoretical approach enabled us to rationalize the observed trends. The predictable magnetic behaviour of the rotaxane CoII complexes demonstrates that interlocked ligands offer a new strategy to design metal complexes with interesting functionality. Mechanically interlocked ligands enable predictable metal ion coordination, offering a new strategy to engineer their properties. A series of field‐induced CoII single‐ion magnets illustrates this principle. The predictable coordination behaviour of the rotaxane enables remarkably accurate calculation of their zero‐field splittings, which are in agreement with experimental observations, and can be rationalized in terms of metal–ligand bonding.