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

The potential interplay of steric and substitutionpattern effects of the monothiooxalamide side arms on thestructure, conformational features, and self-assembly of a seriesof phenylene bis-monothiooxalamides was investigated. Hereinwe have demonstrated that phenylene bis-monothiooxalamidesself-associate in the solid state, through intermolecularhydrogen bonding as meso-helices when the thioamide NRgroup is sBu and through dispersive COCX (X = O, S, ?),SS, and C?HS interactions when R is tBu, independentlyfrom the substitution pattern in the phenyl ring. The helicalstructures are exclusively developed through NCSHOhydrogen bonding. The steric strain imposed by the orthosubstitutionpattern has the effect of moving both monothiooxalylunits out of the phenyl plane enabling dimerization through strong NCOHO intermolecular hydrogen bonds andpromotes the formation of meso-helices. The steric demand of the thioamide NR group rules the conformation adopted by metasubstitutedderivatives and the self-association arrangement of para-substituted derivatives. Infrared data support the blue-shiftednature of the NCSHO hydrogen bond. NMR data in solution agree with the extensive intramolecular hydrogen bondingscheme. Results are supported by density functional theory theoretical calculations. Monothiooxalamide unit offers considerablepotential as a key moiety for crystal engineering.