Synthesis and Magnetic Properties of a New Family of Macrocyclic M3IILnIII Complexes: Insights into the Effect of Subtle Chemical Modification on Single-Molecule Magnet Behavior

Thirteen tetranuclear mixed-metal complexes of the hexaimine macrocycle (L-Pr)(6-) have been prepared in a one-pot 3:1:3:3 reaction of copper(II) acetate hydrate, the appropriate lanthanide(III) nitrate hydrate, 1,4-diformyl-2,3-dihydroxybenzene (1), and 1,3-diaminopropane. The resulting family of copper(II)-lanthanide(III) macrocyclic complexes has the general formula Cu(3)(II)Ln(III)(L-Pr)(NO3)(3)center dot solvents (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Tb, Ho, Er, Tm, or Yb). X-ray crystal structure determinations carried out on [Cu3Ce(L-Pr)(NO3)(3)(MeOH)(3)] and [Cu3Dy(L-Pr)(NO3)(3)(MeOH)(3)] confirmed that the large Ln(III) ion is bound in the central O-6 site and the three square pyramidal Cu-II ions in the outer N2O2 sites (apical donor either nitrate anion or methanol molecule) of the Schiff base macrocycle. Only the structurally characterized Cu3Tb complex, reported earlier, is a single-molecule magnet (SMM): the other 12 complexes do not exhibit an out-of-phase ac susceptibility signal or hysteresis of magnetization in a dc field. Ab initio calculations allowed us to rationalize the observed magnetic properties, including the significant impact of subtle chemical modification on SMM behavior. Broken-symmetry density functional theory (BS-DFT) calculations show there is a subtle structural balance as to whether the Cu center dot center dot center dot Cu exchange coupling is ferro- or antiferromagnetic. Of the family of 13 magnetically characterized tetranuclear Cu(3)(II)Ln(III) macrocyclic complexes prepared, only the Tb-III complex is an SMM: the theoretical reasons for this are discussed.