Sistemas moleculares heterometálicos do tipo “metalocoroa” com ácidos derivados hidroxâmicos: síntese, caracterização e determinação estrutural

Abstract

Five 3d-4f heterobimetallic metallacrowns (MCs) with formula of [CeCu5(5mpzHA)5(NO3)2(H2O)8].NO3 (MC1), [Eu2Cu10(5mpzHA)10(NO3)(NO3)1/2(H2O)6]2.3NO3.5H2O.4CH3OH (MC2), [GdCu5(5mpzHA)5(NO3)(H2O)6].2NO3 (MC3), [TbCu5(5mpzHA)5(H3CCOO)(NO3)(H2O)5].NO3 (MC4) and [ErCu5(5mpzHA)5(NO3)2(H2O)5].NO3 (MC5) have been prepared and characterized by IR spectroscopy, powder and single-crystal X-ray diffraction (XRD). In addition, 2 was also characterized by elemental analysis, EPR spectroscopy, magnetic measurements, and the photoluminescent properties were investigated. The MCs were synthesized from a mixture of 5-methyl-2-pyrazinehydroxamic acid (5mpzHA) and Cu(AcO)2.H2O and Ln(NO3)3.xH2O salts (LnIII = Ce, Eu, Gd, Tb, and Er) in methanol under reflux, yielding single crystals after slow solvent evaporation. IR and 1H NMR data indicate the obtention of the 5mpzHA, where a strong νC=O stretching vibrational frequency observed in the infrared spectrum of 5mpza (1710 cm-1) is downshifted in ca. 46 cm-1 for the corresponding hydroxamic acid (1664 cm -1), what can be resulted of the combination of a more pronounced resonance effect caused by the neighboring -NHOH hydroxamic acid moiety (versus -OMe in 5mpza) and the participation of intra- and intermolecular hydrogen bonds. For 5mpzHA, diagnostic νN-H and νO-H stretching vibrational frequencies appear between 3500 cm-1 and 2800 cm-1 , and the absence of the signal in the NMR spectrum in 3.98 ppm indicates the exchange of methyl in 5mpza to hydroxylamine radical in 5mpzHA. IR spectra of the MCs (1-5) show that the C=O stretching bands are shifted ~60 cm-1 indicating the coordination of the 5mpzHA ligand to the metal center. The analysis of single crystal X-ray confirms the formation of the 15-MC-5 heterobimetallic system in MCs 1-5, with five CuII atoms being at the rim of the crown and the EuIII ion occupying the dome of the crown, with water molecules oxygen atoms, and one nitrate anion filling the coordination sphere around the LnIII ion. The 5mpzHA ligands are connected to five CuII ions and one EuIII ion, through μ3-κ 2N,N’:κ 2O,O’:κO’ chelating- bridging mode, forming a crown-like [Cu5LnIII(5mpzHA)5] core. The CuII ions exhibit slightly distorted, square planar, square pyramidal, and octahedral geometries, and the trivalent lanthanides ions are nine-coordinated. Furthermore, the phase purity of the bulk samples MCs (1-3 and 5) was also confirmed by comparison of the experimental and the simulated PXRD patterns, which demonstrated a good coincidence of the Bragg reflection positions. For MC2, the emission and excitation spectra 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 the quenching of the fluorescence.