Positional Isomerism and Steric Effects in the Self-Assemblies\nof 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 meso-helices when the thioamide NR group is Bu and through dispersive CO···CX (X = O, S, π), S···S, and C−H···S interactions when R is Bu, independently from the substitution pattern in the phenyl ring. The helical structures are exclusively developed through NCSH···O hydrogen bonding. The steric strain imposed by the orthosubstitution pattern has the effect of moving both monothiooxalyl units out of the phenyl plane enabling dimerization through strong NCOH···O intermolecular hydrogen bonds and promotes the formation of meso-helices. The steric demand of the thioamide NR group rules the conformation adopted by metasubstituted derivatives and the self-association arrangement of para-substituted derivatives. Infrared data support the blue-shifted nature of the NCSH···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.