Positional Isomerism and Steric Effects in the Self-Assemblies of Phenylene Bis-Monothiooxalamides

The potential interplay of steric and substitution pattern effects of the monothiooxalamide side arms on the structure, conformational features, and self-assembly of a series of phenylene bis-monothiooxalamides was investigated. Herein we have demonstrated that phenylene bis-monothiooxalamides self-associate in the solid state, through intermolecular hydrogen bonding as <i>meso</i>-helices when the thioamide NR group is ^<i>s</i>Bu and through dispersive CO···CX (X = O, S, π), S···S, and C–H···S interactions when R is ^<i>t</i>Bu, independently from the substitution pattern in the phenyl ring. The helical structures are exclusively developed through N_CSH···O hydrogen bonding. The steric strain imposed by the <i>ortho</i>-substitution pattern has the effect of moving both monothiooxalyl units out of the phenyl plane enabling dimerization through strong N_COH···O intermolecular hydrogen bonds and promotes the formation of <i>meso</i>-helices. The steric demand of the thioamide NR group rules the conformation adopted by <i>meta</i>-substituted derivatives and the self-association arrangement of <i>para</i>-substituted derivatives. Infrared data support the blue-shifted nature of the N_CSH···O hydrogen bond. NMR data in solution agree with the extensive intramolecular hydrogen bonding scheme. Results are supported by density functional theory theoretical calculations. Monothiooxalamide unit offers considerable potential as a key moiety for crystal engineering