Three-dimensional hybrid organic-inorganic frameworks based on lanthanide(III) sulfate layers and organic pillars of 1,4-piperazinediacetic acid

Two types of three-dimensional (3D) hybrid organic-inorganic frameworks [Ln2(H2pda)(SO4)3(H2O)4]n [I, Ln = La3+ (1), Pr3+ (2), Nd3+ (3), Sm3+ (4) and Eu3+ (5)] and {[Ln2(H2pda)(SO4)3(H2O)2]⋅2H2O}n [II, Ln = Gd3+ (6) and Dy3+ (7)], where H2pda is 1,4-piperazinediacetic acid, have been obtained by the reactions of H2pda, lanthanide oxides and H2SO4 under similar hydrothermal conditions. All the frameworks are constructed by two-dimensional inorganic lanthanide(III) sulfate layers and organic H2pda pillars. The lanthanide(III) sulfate layers in both types I and II consist of the similar lanthanide(III) sulfate chains, which are built from lanthanide(III) ions and η3,μ3-sulfates, and the different sulfate bridges. The coordination modes of sulfate bridges in I and II are η2,μ2-bidentate and η4,μ2-tetradentate, respectively. Each organic H2pda pillar in two types is connected with four Ln3+ ions in a bis-bidentate syn-anti mode. Compounds 1 and 6 are thermally stable below 180 and 300 °C, respectively. The variable-temperature magnetic studies show that the χM values of 2 and 3 increase on cooling, which may be attributed to the depopulation of Stark levels and/or intramolecular antiferromagnetic coupling between the metal centers.