A combined crystallographic and theoretical study of weak intermolecular interactions in crystalline squaric acid esters and amides

We report the synthesis and X-ray solid state structures of five squaric acid derivatives, i.e. a zwitterionic compound, namely 3-hydroxy-4-(2-pyridin-2-yl-ethylamino) cyclobut-3-ene-1,2-dione (1), a squaramide monoester, 3-ethoxy-4-(2-pyridin-2-yl-ethylamino) cyclobut-3-ene-1,2-dione (2), two differently solvated (EtOH and DMSO/water) disquaramides3,4-bis((4-hydroxyphenethyl)amino)cyclobut-3-ene-1,2-dione (3 and 4, respectively), and a mixed hydrogen squarate and disquarate 2-(2- aminoethyl) pyridinium salt (5). All compounds form interesting supramolecular assemblies in the solid state that have been analyzed using high level DFT calculations and Bader's theory of "atoms-in-molecules". An intricate combination of ion-pair and H-bonding interactions along with pi-pi stacking and anion-pi contacts of the cyclobutenedione rings is crucial for the formation of the supramolecular assemblies in the solid state.