Síntese, caracterização estrutural e estudo das propriedades magnéticas de compostos de coordenação contendo ligantes do tipo oxamato e metais da primeira série de transição e lantanídeos

Abstract

Molecular magnets are a class of compounds capable of exhibiting spontaneous magnetization, providing them with potential applications as components in data storage and information processing devices. Metal complexes containing oxamate ligands have a long-standing tradition in this field. Two fundamental phenomena associated with this magnetic property are zero-field splitting and the magnetic exchange constant J. The synthesis of novel materials through coordination chemistry, their experimental characterization, and subsequent analysis with computational chemistry tools are critically important. This work was conducted by integrating inorganic synthesis, experimental characterization, and computational analysis using DFT, leading to the investigation of ten new metal complexes containing oxamatebased ligands: cis-[Mn(3-HOpa)2(H2O)2], trans-{[Co(3-HOpa)2(H2O)2]}∙2H2O, Fe(4-MPO)2(H2O)2, Ni(4-MPO)2(H2O)2, Co(4-MPO)2(H2O)2, [Eu(Hphox)3(dmso)2·H2O]n, [Gd(Hphox)3(dmso)2·H2O]n, [Tb(Hphox)3(dmso)2·H2O]n, {[Fe(H2edpba)(dmso)]2(μ-O)}·dmso·H2O, and [Cu(4-HOpa)2(H2O)2]2·2H2O, where 3-Hopa, 4-MPO, Hphox, H4opba, and 4-Hopa represent N-3-hydroxyphenyloxamate, N-4- methoxyphenyloxamate, N-Phenyloxamate, N,N’-1,2-phenylenebis(oxamic acid), and N-4-hydroxyphenyloxamate ligands, respectively. The Mn2+ and Co2+ complexes with the ligand 3-Hopa are hexacoordinated with distorted cis and trans octahedral geometry, respectively. They exhibited slow magnetic relaxation up to 88 K for cis-[Mn(3-HOpa)2(H2O)2] and 7 K for trans-{[Co(3-HOpa)2(H2O)2]}∙2H2O. Complexes with the 4-MPO ligand are hexacoordinated and isostructural, with two ligands and two water molecules occupying axial positions in a trans configuration. Within the crystal lattice, hydrogen bonds form supramolecular coordination polymers in the direction of the shortest distances between metal centers, and these chains are relatively isolated from one another. The magnetic study of these three complexes suggests the presence of significant magnetic interactions along the chain. Fitting the experimental curves to theoretical models for magnetic chains yields J values of -1.44 and -0.78 cm- 1 for the Ni2+ and Co2+ complexes, respectively. DFT calculations corroborate these values. The complexes Eu(Hphox)3(dmso)2·H2O]n, [Gd(Hphox)3(dmso)2·H2O]n, and [Tb(Hphox)3(dmso)2·H2O]n were magnetically studied. The Gd3+ compound exhibited a signal in the out-of-phase susceptibility induced by the application of a dc magnetic field. The dimeric complex {[Fe(H2edpba)(dmso)]2(μ-O)}·dmso·H2O was obtained through the oxidation of a precursor complex. The dimer contains an -oxo bridge linking the Fe3+ centers. Mössbauer spectroscopy confirms the presence of octahedral Fe3+ ions. This complex exhibits a strong antiferromagnetic interaction (J = -226.46 cm-1), in agreement with DFT calculations (J = -207.66 cm-1). In conclusion, the determination of the exchange constant J using the DFT method for the dimer [Cu(4-HOpa)2(H2O)2]2·2H2O was +0.44 cm-1, compared to the experimental value of +0.83 cm-1. This result is promising as it is of a similar order of magnitude as the experimental value, albeit slightly underestimated, thus confirming ferromagnetic ordering. Additionally, it enables the rationalization of the cause of the weak coupling through spin density analysis.