Supramolecular Isomerism in Cu(II) Pyrazinecarboxylate Coordination Polymers

A new coordination polymer based on Cu(II) and pyrazinecarboxylic acid (pycaH), namely [Cu(pyca)₂]_n (1β), has been obtained by a two-step synthesis and structurally characterized by single-crystal X-ray diffraction analysis. In the crystal, the planar [Cu(pyca)₂] monomeric units are extended into a 2D coordination polymer structure due to the coordination of the oxygen atom of the carboxylic group of two adjacent monomeric units in apical positions resulting in square-bipyramidal coordination surrounding of the copper atom. Compound 1β represents a novel supramolecular isomer compared to another one documented in the Cambridge Structural Database under the refcodes BEYPUQ (1α), where the same monomeric units are linked to a 1D coordination polymer. Hirschfeld surface analysis is performed to gain additional insight into the interactions responsible for crystal packing in both supramolecular isomers. The examination of 2D fingerprint plots has elucidated the most significant intermolecular interactions involved in the stabilization of the crystal network resulting from C─H…O, C─H…N, and C─H…C interactions. DFT calculations of the lattice energies demonstrate a higher degree of stability for supramolecular isomer 1β relative to 1α.