Single-molecule magnets (SMMs) have gained attention recently because of parallel/antiparallel bistability of magnetization to a given z direction of a magnetic field when that field is weakened or removed.[1] This bistability is due to an energy barrier to relaxation of the magnetization, which, at low temperatures, becomes significant with respect to ambient thermal energy and prevents loss of magnetic order. Molecules that show relaxation of the magnetization of purely molecular origin are thus described as SMMs.[2] In SMMs based on lanthanide ions the crucial magnetic anisotropy is the result of lifting the degeneracy of the ground states into new sublevels by some ligand-field potentials.Although mononuclear lanthanide SMMs were first generated by Ishikawa and co-workers using two macrocyclic ligands to sandwich the lanthanide ion in a double-decker fashion, [3] they can also be prepared by using a range of acyclic ligands such as polyoxometalates, [4] Schiff bases, [5] radicals, [6] and ketones. [7] There are several mononuclear lanthanide SMMs that also contain one or more 3d metal ions. [8] However, to the best of our knowledge, there are no examples of SMMs containing a single lanthanide ion bound within the cavity of just one organic macrocycle (rather than being sandwiched between two such macrocycles [3,9] or being part of a metallomacrocycle only, that is, no organic macrocycle [10] ). Such a system is highly desirable as it should enhance the nuclearity/geometry control, stability, tunability and solubility of the complex, all of which are important factors for potential future applications. Here we present the first example of such a SMM.The organic macrocycle system we selected for this purpose (Scheme 1) was inspired [11] by those reported by Nabeshima and co-workers [12] and MacLachlan and co-workers.[13]Macrocyclic ligands provide discrete, metal ion binding pockets and thus offer far more predictable cluster nuclearity and structure than acyclic analogues can. Here the [3+3] macrocycle provides three N 2 O 2 pockets for 3d metal ions and one central O 6 pocket for a lanthanide ion, so mixedmetal M 3 Ln tetrametallic macrocyclic complexes are a predictable outcome (in comparison, unpredictable aggregates of 4-12 metal ions were obtained with acyclic ligand analogues [14] ). Macrocycles usually also provide enhanced stability (macrocyclic effect), solubility (vary substituents on periphery, R, to modify solubility) and fine-tunability (vary the periphery, R and n, and choice of M and Ln, whilst retaining the M 3 Ln core) over acyclic analogues. Schiff base links were chosen due to the reversibility of bond formation (facilitates error correction) and our experience with such systems. [15] Initially the dialdehyde 1,4-diformyl-2,3-dihydroxybenzene (1) was combined with a range of aliphatic diamines in 6À , used in this work has R = H and n = 3, and was prepared and complexed in situ to generate the tetrametallic Zn 3 Dy complex 2 (M = Zn, Ln = Dy).
