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

The reaction of aqueous solutions of Eu-III, Tb-III, and Gd-III ions with Na(2)Hpcpa [H(3)pcpa = N-(4-carboxyphenyl)oxamic acid] afforded three new isostructural oxamate-containing lanthanide(III) coordination polymers of general formula {Ln(2)(III)(Hpcpa)(3)(H2O)(5)center dot 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 mu-kappa O-2,O':kappa O" coordinationmode and the other two with the mu(3)-kappa O-2,O':.O.:.O'''. Additionally, two of those chains are interlinked through hydrogen bonding and pi-pi 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 Ln(III) ion. A continuous decrease of chi(T)(M) occurs for 1 upon cooling, and finally chi(T)(M) tends to vanish, as expected for the thermal depopulation of the six magnetic F-7(J) excited states (J = 1-6) of the EuIII ion with a nonmagnetic F-7(0) ground state. chi(T)(M) for 2 decreases sharply with decreasing the temperature due to the depopulation of the splitted m(J) levels of the F-7(7) 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 chi(T)(M) at low temperatures for 3. The dynamic (ac) magnetic properties of 2 and 3 reveal a slow magnetic relaxation with very incipient frequency-dependent chi(M)'' signals below 6.0 K (2) and frequency-dependent chi(M)'' peaks below 10.0 K (3) under nonzero applied dc magnetic fields, being thus new examples of field-induced single molecule magnets (SMMs).