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).
Four new complexes of formula [Fe(H 2 pcpa) 2 (H 2 O) 2 ] (1), {[Mn(Hpcpa)(H 2 O) 3 ]•1/2H 2 O} n (2), {[Zn(Hpcpa)(H 2 O) 3 ]•1/2H 2 O} n (3) and [Cu 2 (Hpcpa) 2 (bipy) 2 ]•8H 2 O (4) [H 3 pcpa = N-(4-carboxyphenyl) oxamic acid; bipy = 2,2′-bipyridine] have been synthesized and their structures determined by X-ray diffraction. The structure of 1 consists of mononuclear iron(II) units where each iron(II) ion is six-coordinate by two trans-positioned water molecules and two bidentate H 2 pcpa − ligands building a distorted octahedral environment. 2 and 3 consist of neutral zigzag chains of Mn II and Zn II ions respectively, the Hpcpa 2− groups acting as linkers in a bidentate/monodentate coordination mode with three mer-positioned water molecules achieving the six-coordination around the metal centers. Compound 4 is a neutral centrosymmetric dicopper(II) complex where two Hpcpa 2− groups adopting the bidentate/monodentate coordination mode and act as bridges and bidentate bipy molecules act as end-cap ligands, describing a square pyramidal surrounding around each copper atom. Cryomagnetic measurements for 1, 2 and 4 in the temperature range 1.9-300 K reveal the occurrence of a field-induced single-ion magnet (SIM) behavior (1) and weak interchain (J =-0.22 cm-1 , 2) and intradimer (J =-0.39 cm-1 , 4) antiferromagnetic interactions, the Hamiltonian being defined as H =-J S a. S b .
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