Synthesis, Crystal Structure, and Photoluminescence of Homodinuclear Lanthanide 4-(Dibenzylamino)benzoate Complexes

Three new binuclear lanthanide complexes of general formula [Ln2(L)6(H2O)4] (Ln = Tb (1), Eu (2), and Gd (3)) supported by the novel aromatic carboxylate ligand 4-(dibenzylamino)benzoic acid (HL) have been synthesized. Complexes 1 and 2 were structurally characterized by single-crystal X-ray diffraction. Both 1 and 2 crystallize in the triclinic space group P1̅, and their molecular structures consist of homodinuclear species that are bridged by two oxygen atoms from two carboxylate ligands via different coordination modes. The discrete bridged dimer of 1 is centrosymmetric and features 8-coordinate terbium atoms, each of which adopts a distorted square-antiprismatic geometry. Both coordination spheres comprise two η2-chelating benzoates, two μ-η1:η1-carboxylate interactions from the bridging benzoates, and two water molecules. By contrast, in complex 2, the Eu3+ ion coordination environment is best described as a distorted tricapped-trigonal prism, each europium ion being coordinated to three η2-chelating benzoate ligands and two water molecules. One of the η2-carboxylate ligands is involved in a further interaction with an adjacent metal, thus rendering the overall binding mode bridging tridentate, μ-η2:η1. Scrutiny of the packing diagrams for 1 and 2 revealed the existence of a one-dimensional molecular array that is held together by intermolecular hydrogen-bonding interactions. The Tb3+ complex 1 exhibits high green luminescence efficiency in the solid state with a quantum yield of 82%. On the other hand, poor luminescence efficiency has been noted for the Eu3+−4-(dibenzylamino)benzoate complex.