Transition-metal porous coordination polymers with a podand ligand: structure of discrete water clusters and variable-temperature magnetism

Abstract

The podand ligand tris(4-carboxy-2-phenoxyethyl)amine (ptaH₃) readily reacts with Co^II, Mn^II and Ni^II salts at room temperature to afford porous coordination polymers, {Co(ptaH)·5H₂O}_n (1), {[Mn(ptaH)(H₂O)]·4H₂O}_n (2), and {[Ni(ptaH)(4H₂O)]·6H₂O}n, (3). Crystallographic studies performed on these systems reveal that their structures are based on infinite chains of interlinked metallacycles. The bridging carboxylate groups extend the open framework structures, with oval-shaped voids. Supramolecularly assembled water clusters of different sizes, whose structures are proportionate with the size and shape of the MOFs, fill these voids. While discrete (H₂O)₁₀ clusters are present in 1 and 2, both (H₂O)₂ and (H₂O)₁₆ clusters occupy the voids in 3. Magnetic susceptibility measurements, in the temperature range 2-300 K, on 1 and 2 reveal that the metal centers are antiferromagnetically coupled in each case.