Sodium ions directed self-assembly with 3,5-pyridinedicarboxylate (3,5-pdc) and 4-pyridinecarboxylate (4-pc)

A 3D sodium(I) complex driven by the coordination bonds and a 3D hydrogen-bond-sustained network, with empirical formulae Na₂(3,5-pdc)(H₂O)₄]_n (1) and Na₂(4-pc)₂(H₂O)₈]_n (2), respectively, have been synthesized and characterized. X-ray single crystal determination of 1 reveals that two types of hexa-coordinate sodium(I) ions are alternately arranged through three double μ₂-OH₂ bridges and one double μ₂ oxygen bridge coming from one carboxylic oxygen atom of a 3,5-pdc ligand. In comparison to 1, only one kind of six-coordinate sodium ions in octahedral coordination configurations is bridged by double μ₂ aqua bridges in 2 forming a straight line via the similar Na-Na separations. In addition, the fixation of coordinating bonds around the sodium centers in 1 makes pyridine rings parallel to each other and the centroid-centroid separation of 3.539 Å, while in complex 2 pyridine rings are arranged more flexibly merely by the hydrogen bonding interactions associated with its nitrogen atom and a carboxylic group. To the best of our knowledge, 1 is the first 3D framework sustained only by coordination interactions between alkali metals and carboxylates. It is also noted that two types of hexa-coordinate sodium(I) centers are present in complex 1 at the same time and two kinds of topologies (zig-zag and ring) are produced. Complex 2 is an unprecedented 3D sodium(I) network sustained by the hydrogen bonding and the π-π interactions in the absence of coordination forces.