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

Abstract

The reaction of aqueous solutions of EuIII, TbIII, and GdIII ions with Na2Hpcpa [H3pcpa = N-(4-carboxyphenyl)­oxamic acid] afforded three new isostructural oxamate-containing lanthanide­(III) coordination polymers of general formula {LnIII 2(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 Hpcpa2– ligands, one of them with the μ–κ2 O,O′:κO″ coordination mode and the other two with the μ3-κ2 O,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 TbIII ions. The temperature dependence of the dc magnetic properties of 1–3 reveals a different magnetic behavior depending on the nature of the LnIII ion. A continuous decrease of χM T occurs for 1 upon cooling, and finally χM T tends to vanish, as expected for the thermal depopulation of the six magnetic 7F J excited states (J = 1–6) of the EuIII ion with a nonmagnetic 7F0 ground state. χM T 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 TbIII ion. A very weak antiferromagnetic interaction between the magnetically isotropic GdIII ions across the double carboxylate­(oxamate) bridge is responsible for the small decrease of χM T at low temperatures for 3. The dynamic (ac) magnetic properties of 2 and 3 reveal a slow magnetic relaxation with very incipient frequency-dependent χM″ signals below 6.0 K (2) and frequency-dependent χM″ peaks below 10.0 K (3) under nonzero applied dc magnetic fields, being thus new examples of field-induced single molecule magnets (SMMs).

Lanthanide(iii)-oxamato complexes containing Nd3+ and Ho3+: crystal structures, magnetic properties, and ab initio calculations

“…13 More recently, zigzag one-dimensional coordination polymers of general formula {Ln III 2 (Hpcpa) 3 (H 2 O) 5 ·H 2 O} n [H 3 pcpa = N -(4-carboxyphenyl)oxamic acid] with an octacoordinated lanthanide sphere were reported. 14 The Tb 3+ and Gd 3+ polymers behaved as SMMs at lower temperatures, and the Eu 3+ and Tb 3+ complexes exhibited photoluminescent properties. 14…”

“…14 The Tb 3+ and Gd 3+ polymers behaved as SMMs at lower temperatures, and the Eu 3+ and Tb 3+ complexes exhibited photoluminescent properties. 14…”

“…Focusing on lanthanide oxamate complexes, a few examples of crystal structures and magnetic properties of these compounds have been described in the literature. 10–14 The first one consists of a one-dimensional chain Me 4 N[Dy(HL) 4 ]·2CH 3 CN, in which HL = N -(2,6-dimethylphenyl)-oxamic acid and Me 4 N + = tetramethylammonium cation. 10 This compound presents four ligands coordinated to the dysprosium ions resulting in a DyO 8 coordination sphere and 2D hydrogen-bonded polymer with a herringbone net topology.…”

“…12 The cytotoxic activity of lanthanide complexes containing the ligand ( N -(2,6-dimethylphenyl)oxamate (Hpma − )) resulting in compounds of formula {[Ln(Hpma) 3 (MeOH)(H 2 O)]·2MeOH} n (Ln = Ce 3+ and Nd 3+ ), {[Gd 2 (Hpma) 6 (MeOH) 4 ]·6MeOH} n , {[Er 2 (Hpma) 6 (MeOH)(H 2 O) 3 ]·2MeOH} n and [Ln 2 Na 2 (Hpma) 8 (EtOH)(H 2 O) 6 ] n (Ln = Nd 3+ and Gd 3+ ) against breast cancer (MCF-7), endometrial carcinoma (HEC-1A), and human monocytic (THP-1) cell lines were evaluated. 13 Among them, the complexes containing Nd 3+ and Gd 3+ ions of formula [Ln 2 Na 2 (Hpma) 8 (EtOH)(H 2 O) 6 ] n present one-dimensional zig-zag chains of the –Ln–O–C–O–Ln-type, 14 whereas the other complexes present mononuclear or dinuclear structures with eight or nine-coordinated environments. 13 More recently, zigzag one-dimensional coordination polymers of general formula {Ln III 2 (Hpcpa) 3 (H 2 O) 5 ·H 2 O} n [H 3 pcpa = N -(4-carboxyphenyl)oxamic acid] with an octacoordinated lanthanide sphere were reported.…”

Solvothermal Synthesis, Temperature-Dependent Structural Study, and Magnetic Characterization of a Multipolydentate Oxamate-Based 2D Coordination Network

“…Investigating the reactivity of these multipolydentate ligands will further expand the morphology of the coordination architectures it is possible to generate and access with oxamate ligands. For example, the use of amino acid based oxamate ligands gives access to new porous frameworks with remarkable properties. − On the other hand, phenolic and benzoic derivatives of the oxamate ligands seem to pose a greater synthetic challenge for the observation of fully coordinating ligands and/or the preparation of heterometallic systems. − Following our work on the 2-hydroxyphenyloxamate ligand under bench conditions, we had investigated solvothermal conditions to succeed in crystallizing copper­(II)-based heterobimetallic compounds. This proved efficient with both the 2-hydroxyphenyloxamate and the 2-carboxyphenyloxamate, yet only 1D coordination polymers were obtained. , To increase the dimensionality of our edifices, we have naturally moved to a para -substitued phenyloxamate, and started to investigate the 4-(ethyloxamate)­benzoic acid, H 2 Et-paba.…”