Europium(III), Terbium(III), and Gadolinium(III) Oxamato-Based Coordination Polymers: Visible Luminescence and Slow Magnetic Relaxation

Jhonny Willians de Oliveira Maciel; Maykon Alves Lemes; Ana Karoline Valdo; Renato Rabelo; Felipe Terra Martins; Lauro June Queiroz Maia; Ricardo Costa de Santana; Francesc Lloret; Miguel Julve; Danielle Cangussu

doi:10.1021/acs.inorgchem.0c03226

Europium(III), Terbium(III), and Gadolinium(III) Oxamato-Based Coordination Polymers: Visible Luminescence and Slow Magnetic Relaxation

Inorg Chem. 2021 May 3;60(9):6176-6190. doi: 10.1021/acs.inorgchem.0c03226. Epub 2021 Apr 16.

Affiliations

¹ Instituto de Química, Universidade Federal de Goiás, Goiânia, Goiás, Brazil, 74690-900.
² Instituto Federal de Educação, Ciência e Tecnologia Goiano, Iporá, Goiás, Brazil, 76200-000.
³ Departament de Química Inorgànica/Instituto de Ciencia Molecular (ICMol), Facultat de Química de la Universitat de València, València, Paterna, Spain, 46980.
⁴ Instituto de Física, Universidade Federal de Goiás, Goiânia, Goiás, Brazil, 74690-900.

PMID: 33861078
DOI: 10.1021/acs.inorgchem.0c03226

Abstract

The reaction of aqueous solutions of Eu^III, Tb^III, and Gd^III ions with Na₂Hpcpa [H₃pcpa = N-(4-carboxyphenyl)oxamic acid] afforded three new isostructural oxamate-containing lanthanide(III) coordination polymers of general formula {Ln^III₂(Hpcpa)₃(H₂O)₅·H₂O}_n [Ln = Eu (1),Tb (2), and Gd(3)]. Their structure is made up of neutral zigzag chains running parallel to the [101] direction where double syn-syn carboxylate(oxamate)-bridged dilanthanide(III) pairs (Ln1 and Ln2) are linked by three Hpcpa^2- ligands, one of them with the μ-κ²O,O':κO″ coordination mode and the other two with the μ₃-κ²O,O':κO″:κO'''. Additionally, two of those chains are interlinked through hydrogen bonding and π-π type interactions, resulting in a porous structure with channels where water molecules are hosted. The emission properties of 1 and 2 are evaluated as a function of the temperature, exhibiting an emission in red and green, respectively. The external quantum yield for 2 is approximately 7 times that obtained for 1, indicating that the oxamate ligand is a better sensitizer for Tb^III ions. The temperature dependence of the dc magnetic properties of 1-3 reveals a different magnetic behavior depending on the nature of the Ln^III ion. A continuous decrease of χ_MT occurs for 1 upon cooling, and finally χ_MT tends to vanish, as expected for the thermal depopulation of the six magnetic ⁷F_J excited states (J = 1-6) of the Eu^III ion with a nonmagnetic ⁷F₀ ground state. χ_MT for 2 decreases sharply with decreasing the temperature due to the depopulation of the splitted m_J levels of the ⁷F₇ ground state of the magnetically anisotropic Tb^III ion. A very weak antiferromagnetic interaction between the magnetically isotropic Gd^III ions across the double carboxylate(oxamate) bridge is responsible for the small decrease of χ_MT at low temperatures for 3. The dynamic (ac) magnetic properties of 2 and 3 reveal a slow magnetic relaxation with very incipient frequency-dependent χ_M″ signals below 6.0 K (2) and frequency-dependent χ_M″ peaks below 10.0 K (3) under nonzero applied dc magnetic fields, being thus new examples of field-induced single molecule magnets (SMMs).