Experimental and theoretical study of weak intermolecular interactions in crystalline tertiary squaramides

We report the X-ray solid state structures of two tertiary squaramides, i.e. 3-(diethylamino)-4-ethoxy-cyclobutene-1,2-dione (1) and bis-3,4-(diethylamino)-cyclobutene-1,2-dione (2). Compound 1 forms electrostatically compressed dimers in the solid state. Moreover, compound 2 exhibits a remarkable solid state architecture resembling a lipid bilayer. This supramolecular assembly has been analyzed using high level DFT calculations and Bader's theory of "atoms-in-molecules". The antiparallel CO center dot center dot center dot CO interactions of the cyclobutenedione rings and hydrophobic interactions involving the ethyl chains are crucial for the formation of the bilayer assembly in the solid state.