Heterobimetallic metallacrown of EuIIICuII5 with 5-methyl-2-pyrazinehydroxamic acid: Synthesis, crystal structure, magnetism, and the influence of CuII ions on the photoluminescent properties

Abstract

A 3d-4f heterobimetallic metallacrown with the formula of [EuCu5(5mpzHA)5(NO3)3/2(H2O)6]2·3NO3·4H2O·5CH3OH (MC1) has been prepared using the stirring method from a mixture of 5-methyl-2-pyrazinehydroxamic acid (5mpzHA), Cu(AcO)2·H2O, and Eu(NO3)3·5H2O salts in methanol under reflux, which yielded single crystals after slow solvent evaporation. Single crystal X-ray diffraction analysis shows a crown-like [EuCu5(5mpzHA)5] core with the 15-MC-5 system, with five CuII and one EuIII ions occupying the rim and the dome of the crown, respectively. Water molecules oxygen atoms and one nitrate anion fill the coordination sphere around the EuIII ion, which exhibits a MFF-9 shape. The MC1 is made up of two [EuCu5(5mpzHA)5(NO3)3/2(H2O)6] metallacrown units connected by one coordinated nitrate ion, where intramolecular hydrogen bonding interactions generate aR86(18) supramolecular macrocycle leading a two-dimensional supramolecular network. The emission and excitation spectra of MC1 show the absence of EuIII emission and f-f transitions, probably due to the paramagnetic nature of the CuII ions which play a key role in quenching the fluorescence in MC1. The observed magnetic properties are attributed to antiferromagnetic interactions among the CuII ions within the metallacrown rim plus the depopulation of EuIII excited Stark sublevels. This magnetic behavior was also supported by the EPR spectroscopy which presents a superposition of signals of the termally populated state 7F1 of EuIII and CuII ions at room temperature and a sole signal of CuII ion at 4 K.