A mixed 3d-4f 14-metallacrown-5 complex that displays slow magnetic relaxation through geometric control of magnetoanisotropy.

We describe the synthesis and magnetic properties of a unique mixed 3d-4f 14-metallacrown-5 complex. This is the first metallacrown family to feature μ-O and μ-OH bridges as well as to incorporate a Ln(III) ion into the ring. Alternating-current SQUID magnetometry of the Tb, Dy, and Ho derivatives reveals slow magnetic relaxation, a hallmark property of single-molecule magnets. For the Dy structure (4), an effective energy barrier U(eff) of 16.7 K and a relaxation time of 4.9 × 10(-8) s were calculated. Because of the relatively small total spin, this behavior most likely results from a large magnetoanisotropy, which is controlled through geometric constraints.