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<sup>II</sup><sub>3</sub>­Tb<sup>III</sup>­(<b>L</b><sup><b>Pr</b></sup>)]­(NO<sub>3</sub>)<sub>2</sub>, to a new butylene linked analogue, is shown to tune the ligand field imposed on the encapsulated Cu<sup>II</sup><sub>3</sub>Tb<sup>III</sup> cluster. This results in apical binding of two, rather than one, nitrate ions to the oblate Tb<sup>III</sup> ion, giving enhanced uniaxial anisotropy and SMM properties despite the low symmetry of the Tb­(III) site. The resulting complex, [Cu<sup>II</sup><sub>3</sub>­Tb<sup>III</sup>­(<b>L</b><sup><b>Bu</b></sup>)­(NO<sub>3</sub>)<sub>2</sub>­(MeOH)­(H<sub>2</sub>O)]­(NO<sub>3</sub>)·3H<sub>2</sub>O, is the first example of a macrocyclic 3d–4f single-molecule magnet that exhibits quantifiable relaxation of magnetization in zero dc field (Δ<sub>eff</sub>/<i>k</i><sub>B</sub> = 19.5(5) K; τ<sub>0</sub> = 3.4 × 10<sup>–7</sup> 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, Zn<sub>3</sub>Tb­(<b>L</b><sup><b>Bu</b></sup>)­(NO<sub>3</sub>)<sub>3</sub>·MeOH·H<sub>2</sub>O·DMF and [Cu<sub>3</sub>­La­(<b>L</b><sup><b>Bu</b></sup>)­(NO<sub>3</sub>)<sub>2</sub>­(MeOH)­(H<sub>2</sub>O)<sub>2</sub>]­(NO<sub>3</sub>)·H<sub>2</sub>O·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 O<sub>6</sub> pocket, and the three copper­(II) ions in the outer N<sub>2</sub>O<sub>2</sub> pockets.