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

Abstract

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 s Bu and through dispersive COwhen R is t Bu, independently from the substitution pattern in the phenyl ring. The helical structures are exclusively developed through N CS H•••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 N CO H•••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 N CS H•••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.

Mechanochemical Synthesis and Structure of the Tetrahydrate and Mesoporous Anhydrous Metforminium(2+)-N,N′-1,4-Phenylenedioxalamic Acid (1:2) Salt: The Role of Hydrogen Bonding and n→π * Charge Assisted Interactions

“…It is worth mention that this conformation has not been observed among organic cocrystals of H 2 pOXA but is commonly attained by coordination to metals [ 27 ]. The calculated conformational landscape of 1,4-phenylen dioxalyls predicts a very small difference in energy between planar ap - sp and sp - sp conformers of 0.26–0.28 kcal mol −1 , in favor of the former, and an interconversion energy of only 4.80 kcal mol −1 [ 16 ]. Then, the sp - sp conformation exhibited by the HpOXA − moiety is explained because of the stabilization given by hydrogen bonding with metformin that provides the energy to overcome the rotational barrier.…”

“…More recently, both have attracted interest as coordinating ligands for metals [ 14 , 15 ]. Theoretical studies and experimental work [ 16 ] have demonstrated the high flexibility of the oxalic acid derivatives. Both oxalyl carbonyls are usually in anti disposition [ 17 ], but they less frequently adopt the syn disposition, induced by steric constraints [ 18 , 19 ] or by coordination with metals in the form of carboxylates [ 14 ].…”

Microcrystalline solid–solid transformations of conformationally-responsive solvates, desolvates and a salt of N,N′-(1,4-phenylene)dioxalamic acid: the energetics of hydrogen bonding and n/π → π* interactions

“…4 On the other hand, the phenyldioxalamate moiety has become particularly important as a versatile supramolecular building block exploited for the formation of a wide variety of supramolecular architectures. 5 For instance, diethyl N , N ′-1,3-phenyldioxalamates have been shown to form molecular clefts whose conformation and cavity size can be controlled by steric hindrance and hydrogen-bonding effects. 6 Furthermore, they have been used to explore the rotational isomerism of 1,3-benzenediols by cocrystallization.…”