New Types of Luminescent Lanthanide Squarato-Aminophosphonates

Hydrothermal reactions of lanthanide nitrates with aminodiphosphonate ligand (H2O3PCH2)(2)-N-CH2-C6H4-COOH, H5L1) and squaric acid afforded a series of isostructural lanthanide(III) squarato-phosphonates, namely, Ln(HL2) (Ln = La, 1; Pr, 2; Nd, 3; Eu, 4; Gd, 5; Tb, 6; Er, 7) (H4L2 = HOOC-C6H4-CH2-N(C4O3H)(CH2PO3H2)), in which a new multifunctional squarato-phosphonate ligand (H4L2) was formed by in situ condensation reaction between the two types of organic ligands. The seven-coordinated lanthanide(III) ions in these compounds are bridged by the squarato-aminophosphonate moieties of the {HL2}(3-) anions into a double layer. Each {HL2}(3-) anion acts as septadentate metal linker, bridging with seven lanthanide ions by using one carboxylate oxygen, three squarato, and three phosphonate oxygens. These double layers are further cross-linked via the coordination of the carboxylate groups into a pillared layered architecture. The Eu, Tb, and Nd compounds exhibit strong luminescence in red light, green light, and near-IR regions, respectively.