Systematic Investigation of the Hydrothermal Syntheses of Pr(III)−PDA (PDA = Pyridine-2,6-dicarboxylate Anion) Metal−Organic Frameworks

A series of novel two-dimensional (2D) and three-dimensional (3D) praseodymium coordination polymers, namely, {[Pr₃(PDA)₄(HPDA)(H₂O)₈]·8H₂O}_n (2), {[Pr₂(PDA)₃(H₂O)₃]·H₂O}_n (3), {[Pr(PDA)(H₂O)₄]·ClO₄}_n (4), and {[Pr₂(PDA)₂(H₂O)₅SO₄]·2H₂O}_n (5) (PDA = pyridine-2,6-dicarboxylic anion), was designed and synthesized under hydrothermal conditions. Complexes 1−3 (chainlike polymer, {[Pr(PDA)(HPDA)(H₂O)₂]·4H₂O}_n (1) was also obtained independently by us, although it has been reported recently by Ghosh et al.) were fabricated successfully by simply tuning the Pr/PDA ratio and exhibited various and intriguing topological structures from a 1D chain to a 3D network. While the synthetic strategy of 5 was triggered and further performed only after 1 was structurally characterized. The complexes were characterized by X-ray single-crystal determination, spectroscopic, and variable-temperature magnetic susceptibility analyses. In complex 2 an unusual nanosized square motif as a building block constructed by eight Pr ions was further assembled into a highly ordered 2D grid compound. In complex 3 the decanuclear Pr metal-based structure as a repeat unit interpenetrated to form a novel 3D polymer. Complex 4 was a 3D network polymer fabricated through a hexanuclear Pr ring as a building block, and ClO₄^- anions as guests were trapped in the cavity. In complex 5 six Pr atoms, two SO₄^2- anions, and carboxylic oxygen bridges constructed an intriguing rectangle structure as a repeat unit in the grid to form a 2D coordination polymer in which the unique bi-bidentate coordination mode of SO₄^2- anion was observed.