Three 3D lanthanide–organic frameworks with sra topology: syntheses, structures, luminescence and magnetic properties

Abstract

Three 3D lanthanide–organic frameworks (LOFs), [Ln(HL)(DMF)]·2DMF (Ln = Eu (1), Gd (2), Dy (3); H₄L = 3,3′-dihydroxy-4,4′-biphenyldicarboxylic acid), have been prepared by solvothermal reaction of Ln(NO₃)₃·6H₂O and H₄L in DMF–H₂O mixed solvent. Crystallographic data show that 1–3 are isomorphous and crystallize in the monoclinic space group P2₁/c. Each Ln(III) is eight-coordinated to seven O atoms from five (HL)³⁻ ligands and one O atom from a DMF molecule in a dodecahedron geometry. The adjacent Ln(III) ions are linked by the carboxylate groups of the (HL)³⁻ ligands to form a 1D inorganic rod-shaped [Ln(CO₂)₃]_n chain as a secondary building unit (SBU). The infinite 1D chains are interconnected by the biphenyl groups, giving rise to a 3D sra framework containing 1D open rhombic channels with dimensions of 18.4 × 11.6 Å along the a axis. The present LOF is the first sra-net built from Ln–O–C rods with only one ligand. The ligand (HL)³⁻ shows an unusual septadentate coordination mode with one bound hydroxy group. Furthermore, 1 exhibits characteristic luminescence of Eu³⁺ upon 394 nm excitation. The investigation of magnetic properties reveals the weak antiferromagnetic interactions (J₁ = −0.018(14) and J₂ = −0.021(13) cm⁻¹) between Gd(III) ions in 2 and θ = −1.62(2) K in 3 due to thermal depopulation of the Stark levels of Dy(III) ions and/or the possible antiferromagnetic interactions between Dy(III) ions.