Synthesis and magnetic properties of a new family of macrocyclic M II3Ln III complexes: Insights into the effect of subtle chemical modification on single-molecule magnet behavior

Abstract

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 II3Ln III(L Pr)(NO 3) 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 [Cu 3Ce(L Pr)(NO 3) 3(MeOH) 3] and [Cu 3Dy(L Pr)(NO 3) 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 N 2O 2 sites (apical donor either nitrate anion or methanol molecule) of the Schiff base macrocycle. Only the structurally characterized Cu 3Tb 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 Cu II3Ln III macrocyclic complexes prepared, only the Tb III complex is an SMM: the theoretical reasons for this are discussed. © 2012 American Chemical Society.