Structures and luminescent properties of a series of Ln–Ag heterometallic coordination polymers

Eight new 4f–4d heterometallic coordination polymers have been synthesized using pyridine-2,6-dicarboxylic acid (H2PDA) as ligand under hydrothermal conditions. {[PrAg(PDA)2(H2O)3]·2.5H2O}n (1) comprises a 2D grid-like net, and the grid structure is assembled through a Pr4Ag2 motif as the building block. The subsequent six complexes [LnAg(PDA)2(H2O)3]·3H2O}n (Ln = Ce3+, 2; Nd3+, 3; Sm3+, 4; Eu3+, 5; Tb3+, 6; Dy3+, 7) are 1D double-chain coordination polymers. In 2–7, the Ln3+ and Ag+ ions are linked by carboxylic O bridges to form the helical chains, while the right-handed and left-handed chains are linked through carboxylic O bridges to form the double-helical chain. Additionally, the lattice H2O molecules linked by hydrogen bonds construct the helical water chain. {[CeAg2(PDA)3(Py)4]·H2O}n (8) (Py = pyridine) contains one Ce4+ ion and displays a 1D wave-like chain, in which partial H2PDA ligands were decomposed to pyridine. Luminescent studies reveal that the emission intensity of 7 significantly increases with the introduction of Mg2+ ions into the system of 7, whereas the addition of other metal ions causes emission intensity to remain the same or even decrease. Therefore, complex 7 may serve as a luminescent probe of Mg2+.