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.