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^II₃­Tb^III­(L^Pr)]­(NO₃)₂, to a new butylene linked analogue, is shown to tune the ligand field imposed on the encapsulated Cu^II₃Tb^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^II₃­Tb^III­(L^Bu)­(NO₃)₂­(MeOH)­(H₂O)]­(NO₃)·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]­(NO₃)·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.