Crystal structures, gas storage and magnetic properties of lanthanide-organic frameworks built up from dicarboxylates, [Ln2(2,5-pydc)2(2,5-pipdc)(H2O)2]n (Ln = Ce, Pr, Eu) and (H2pip)n[Ln2(2,6-pydc)4(H2O)2]n (Ln = Ce, Pr, Eu, Sm)

Abstract

We have synthesized various kinds of lanthanide complexes, [Ln2(2,5-pydc)2(2,5-pipdc)(H2O)2]n (Ln = Ce (MOF 1), Pr (MOF 2), Eu (MOF 3); 2,5-pydc = 2,5-pyridinedicarboxylate, 2,5-pipdc = 2,5-piperazinedicarboxylate), and (H2pip)n[Ln2(2,6-pydc)4(H2O)2]n (Ln = Ce (MOF 4), Pr (MOF 5), Eu (MOF 6), Sm (MOF 7); 2,6-pydc = 2,6-pyridinedicarboxylate, H2pip = piperazinediium) under hydrothermal conditions. The X-ray crystallographic analysis shows that [Ln2(2,5-pydc)2(2,5-pipdc)(H2O)2]n forms three-dimensional coordination network, while (H2pip)n[Ln2(2,6-pydc)4(H2O)2]n forms one-dimensional coordination polymer, whose different coordination network structure is controlled by the positional isomer of pyridinedicarboxylate (2,5-pydc, 2,6-pydc). The magnetic properties and gas storage (H2 and CO2) capacities were also investigated, and among the synthesized compounds MOF 7 had the highest gas storage capacities for H2 and CO2. From the analysis of magnetic susceptibility measurements, the Weiss constants for Ce, Pr, and Sm complexes were estimated. In the case of Pr complexes, MOF 2 and MOF 5, the magnetic susceptibility deviates from the Curie-Weiss law below 20 K and 10 K, respectively, which is the precursor phenomenon of antiferromagnetic ordering. In the case of Sm(III) complex, MOF 7, the magnetic susceptibility multiplied by temperature (χT) as a function of temperature between 300 K and 2 K was theoretically analyzed and the spin-orbit coupling constant (λ) and the energy levels of ground multiplet, 6HJ, were determined.