Towards tuning the packing and entanglement of zigzag coordination chains by terminal ligands

Solvothermal reactions of trans-stilbene-4,4′-dicarboxylic acid (H₂STDC) and zinc(ii) acetate in the presence of systematically varied terminal ligands afforded a series of supramolecular architectures with formula [Zn(STDC)(py)₂]•py (1), [Zn(STDC)(bipy)(H ₂O)]•0.5py•H₂O (2), [Zn(STDC)(biql)] (3), [Zn(STDC)(phen)]•solv (solv = DMSO, 4a; DMF, 4b), where py = pyridine, bipy = 2,2′-bipyridine, biql = 2,2′-biquinoline, phen = 1,10-phenathroline. X-Ray analyses revealed that all the compounds consist of infinite 1D zigzag polymer chains. Investigations based on intermolecular interactions illustrate that the chelate terminal ligands play a critical role in determining the packing/entangling modes of the chains and the porosity of the final three-dimensional architectures. In compounds 1 and 2, the weak hydrogen bonding and/or π-π stacking interactions assemble the parallel chains into diamond nets with four- and two-fold interpenetration, respectively. In compound 3, the hydrogen bonding and π-π stacking interactions collaborate to arrange the chains in two different directions, generating a 3D supramolecular architecture with high catenation. The most interesting packing occurs in 4. Extensive π-π stacking interactions involving the terminal and bridging ligands arrange the chains in four different directions, and the chains are hierarchically entangled to produce an unprecedented 3D microporous framework with high stability. Based on comparative investigations, the effects of the terminal and bridging ligands on the packing of zigzag chains have been discussed. The reversible guest inclusion properties of 2 and 4 have also been demonstrated.