By design: a macrocyclic 3d-4f single-molecule magnet with quantifiable zero-field slow relaxation of magnetization.

Rational modification of the equatorially bound tetranucleating macrocycle in the previously reported SMM complex of the propylene linked macrocycle Cu(II)3Tb(III)(L(Pr))2, to a new butylene linked analogue, is shown to tune the ligand field imposed on the encapsulated Cu(II)3Tb(III) cluster. This results in apical binding of two, rather than one, nitrate ions to the oblate Tb(III) ion, giving enhanced uniaxial anisotropy and SMM properties despite the low symmetry of the Tb(III) site. The resulting complex, Cu(II)3Tb(III)(L(Bu))(NO3)2(MeOH)(H2O)·3H2O, is the first example of a macrocyclic 3d-4f single-molecule magnet that exhibits quantifiable relaxation of magnetization in zero dc field (Δ(eff)/k(B) = 19.5(5) K; τ0 = 3.4 × 10(-7) s). This SMM complex of this new, larger, tetranucleating macrocycle was prepared by the template method from the 3:3:3:1 reaction of 1,4-diformyl-2,3-dihydroxybenzene/diaminobutane/copper(II) acetate/terbium(III) nitrate. Similarly, the analogues, Zn3Tb(L(Bu))(NO3)3·MeOH·H2O·DMF and Cu3La(L(Bu))(NO3)2(MeOH)(H2O)2·H2O·DMF, were prepared in order to facilitate the detailed magnetic analysis. Both copper(II) complexes were also structurally characterized, confirming the expected binding mode: lanthanide(III) ion in the central O6 pocket, and the three copper(II) ions in the outer N2O2 pockets.