Synthesis and Magnetic Properties of a New Family of Macrocyclic MII3LnIII Complexes: Insights into the Effect of Subtle Chemical Modification on Single-Molecule Magnet Behavior

Thirteen tetranuclear mixed-metal complexes of the hexaimine macrocycle (LPr)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 CuII3LnIII(LPr)­(NO3)3·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(LPr)­(NO3)3(MeOH)3] and [Cu3Dy­(LPr)­(NO3)3(MeOH)3] confirmed that the large LnIII ion is bound in the central O6 site and the three square pyramidal CuII 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···Cu exchange coupling is ferro- or antiferromagnetic. Of the family of 13 magnetically characterized tetranuclear CuII3LnIII macrocyclic complexes prepared, only the TbIII complex is an SMM: the theoretical reasons for this are discussed.