Experimental and Theoretical Study of Aromaticity Effects in the Solid State Architecture on Squaric Acid Derivatives

In this manuscript, the aromaticity of the four-membered ring in squaric acid derivatives and, particularly, how it changes when it participates in noncovalent interactions are studied combining experimental observations and theoretical analysis using the nucleus independent chemical shift (NICS). Moreover, the ability of the squaric acid and its derivatives to establish hydrogen bonds, π–π stacking, and other π-interactions (anion−π, lone pair−π, and C–H/π) is related to the increase in the aromaticity of the ring. In addition, several squaric acid derivatives (benzamh)­(sqah)·H2O (1), (pipropamh2)­(sqa)·2H2O (2), (deamh)­(deamsqa) (3), (phpetamh)­(sqah)·(4), (amsqa)­(sqa) (5), (and benzamh)­(benzamsqa)·H2O (6) where benzam = benzylamine; sqah2 = squaric acid; deamsqah = diethylamidosquaric acid; pipropam = 3,3′-(piperazine-1,4-diyl)­bis­(propan-1-amine); deam = diethylamine phpetam = p-hydroxiphenylethylamine and benzamsqah = benzylamidosquaric acid) were synthesized and characterized by single crystal X-ray diffraction analyses.