Hydrothermal synthesis, crystal structure and magnetic characterization of two 4f–3d heterometallic coordination polymers

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 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).

Two dimensional porous 3d–4f heterometallic coordination polymers constructed by pyridine-2,3-dicarboxylic acid

Four porous Co^IIILn^III 3d–4f heterometallic coordination polymers constructed by the pydcH₂ ligand were hydrothermally synthesized and characterized. The adsorption and catalytic properties of compound Co^IIIDy^III were investigated.

Magnetic properties of the seven-coordinated nanoporous framework material Co(bpy)1.5(NO3)2 (bpy = 4,4'-bipyridine)

The magnetic properties of the porous metal-organic complex Co(bpy)(1.5)(NO(3))(2) (bpy = 4,4'-bipyridine), investigated by SQUID magnetometry, EPR and heat capacity measurements, are reported. The tongue-and-groove structure of this complex is formed by the assembly of T-shaped building blocks, where each Co is bound to three bpy ligands. Co(II) is hepta-coordinated by three N atoms from the bpy units, and four O atoms from two nitrate groups. Experimental results showed a large crystal field effect induced anisotropy with a zero field splitting of Δ = 198 K between the ground and excited Kramers doublets, a factor of two larger than previously reported values in Co(II) hepta-coordinated complexes. EPR revealed orthorhombic crystal field anisotropy, with gyromagnetic principal values of g(1)* = 6.1, g(2)* = 4.2 and g(3)* = 2.2, in an S* = 1/2 effective spin on the ground state Kramers doublet. Ab initio simulations allowed us to assign the anisotropy easy axis of magnetization to the binary symmetry axis of the molecule, aligned with the Co-N apical direction of the T-block.

Poly[hexa-aqua-hexa-kis-(μ-pyridine-2,4-dicarboxyl-ato)tricopper(II)dieuropium(III)]

The asymmetric unit of the title heterometallic coordination polymer, [Cu(3)Eu(2)(C(7)H(3)NO(4))(6)(H(2)O)(6)](n), contains one Eu(III) and two Cu(II) atoms, three pyridine-2,4-dicarboxylate (pdc)(2-) anions and three water molecules. One Cu(II) atom is located on an inversion center and is N,O-chelated by two pdc(2-) anions in the equatorial plane and further coordinated by two carboxyl-ate O atoms from another two pdc anions in the axial positions, with an elongated octa-hedral geometry [Cu-O = 2.409 (3) Å in the axial direction]; the other Cu atom is N,O-chelated by two pdc anions in the coordination basal plane and coordinated by a carboxyl O atom at the apical position with a distorted square-pyramidal geometry [Cu-O = 2.359 (3) Å in the apical direction]. The Eu atom is eight-coordinated with a distorted square-anti-prismatic geometry formed by five carboxyl-ate O atoms from five pdc anions and three water mol-ecules. The carboxyl-ate anions bridge adjacent Eu and Cu atoms, forming the coordination polymer. Inter- and intra-molecular O-H⋯O hydrogen bonding occurs in the structure. π-π stacking further consolidates the crystal structure, the centroid-centroid distance between parallel pyridine rings being 3.367 (2) Å.