Lanthanide metal–organic frameworks constructed by asymmetric 2-nitrobiphenyl-4,4′-dicarboxylate ligand: syntheses, structures, luminescence and magnetic investigations

Abstract

A series of lanthanide metal–organic frameworks, namely [Ln(bpdc-NO₂)_1.5(DMF)(H₂O)]·0.5DMF [Ln = Eu (1), Gd (2), Nd (3), La (4), Ce (5), Pr (6), Sm (7) and Tb (8)] have been prepared solvothermally by reacting 2-nitrobiphenyl-4,4′-dicarboxylic acid (H₂bpdc-NO₂) with related lanthanide(III) nitrates in a mixed solution of DMF, EtOH and H₂O. Single-crystal X-ray analyses reveal that complexes 1–8 are isostructural and crystallize in the P space group. They have the same three-dimensional (3D) architectures, consisting of bpdc-NO₂²⁻ ligands and inorganic chains of {[Ln₂(–COO)₆(DMF)₂(H₂O)₂]}_n. Within the chain, each Ln(III) ion is coordinated with eight oxygen atoms. The two neighbouring Ln(III) ions are bridged by four μ₂-monodentate carboxylate groups to form a [Ln₂(–COO)₄(DMF)₂(H₂O)₂] dinuclear unit. The dimers are further linked together by the other two μ₂-monodentate O–C–O bridges to generate an inorganic chain running along the a axis. Topological analyses suggest that these frameworks possess the 3D tfz-d net with Schläfli symbol of (4³)₂(4⁶6¹⁸8⁴). Solid-state photoluminescent investigations at room temperature reveal that complex 1 exhibits strong red luminescence upon excitation at 466 nm, which arises from the characteristic emissions of the Eu(III) ions, and its lifetime is 622 μs. For complex 3, the strongest emission band of 1055 nm is found in the near-infrared region upon excitation at 582 nm, and its lifetime is 12 μs. Magnetic susceptibility measurements suggest that the magnetic properties of 1 and 3 are mainly originated from the large spin–orbital coupling of Eu(III) and Nd(III) ions, respectively. For Gd(III) complex 2, weak antiferromagnetic interactions mediated between the Gd(III) centers are observed.