Supramolecular isomerism and structural flexibility in coordination networks sustained by cadmium rod building blocks

Bifunctional N-donor carboxylate linkers generally afford dia and sql topology coordination networks of general formula ML₂ that are based upon the MN₂(CO₂)₂ molecular building block (MBB). Herein, we report on a new N-donor carboxylate linker, β-(3,4-pyridinedicarboximido)propionate (PyImPr), which afforded Cd(PyImPr)₂via reaction of PyImPrH with Cd(acetate)₂·2H₂O. We observed that, depending upon whether Cd(PyImPr)₂ was prepared by layering or solvothermal methods, 2D or 3D supramolecular isomers, respectively, of Cd(PyImPr)₂ were isolated. Single crystal X-ray diffraction studies revealed that both supramolecular isomers are comprised of the same carboxylate bridged rod building block, RBB. We were interested to determine if the ethylene moiety of PyImPr could enable structural flexibility. Indeed, open-to-closed structural transformations occurred upon solvent removal for both phases, but they were found to be irreversible. A survey of the Cambridge Structural Database (CSD) was conducted to analyse the relative frequency of RBB topologies in related ML₂ coordination networks in order to provide insight from a crystal engineering perspective.