Conformation and Visual Distinction between Urea and Thiourea Derivatives by an Acetate Ion and a Hexafluorosilicate Cocrystal of the Urea Derivative in the Detection of Water in Dimethylsulfoxide

Structures
of different solvates and solute–solvent interactions
of 4-(3-(4-nitrophenyl)­urido)­benzoate (L1) and methyl-4-(3-(4-nitrophenyl)­thiourido)­benzoate (L2) with different solvents are analyzed.
The solution of L1 with tetrabutylammonium
acetate (TBAA) in dimethylsulfoxide (DMSO) is colorless, but a similar
solution of L2 with TBAA is orange.
On the other hand, respective solutions of these urea and thiourea
derivatives with tetrabutylammonium fluoride (TBAF) in DMSO are orange.
Urea derivative L1 facilitates
the reaction of TBAF with glass to form tetrabutylammonium hexafluorosilicate,
which on further interaction with L1 forms cocrystal 2L1·(TBA)2SiF6. Reorganization of hydrogen-bonded self-assembly
of 2L1·(TBA)2SiF6 in DMSO caused by water is established by a dynamic
light scattering study. With an increase in the amount of water in
the solution, visual color changes from orange to colorless, and the
color changes are reversed upon the addition of a dehydrating agent
such as molecular sieves. Solvates of L1 with DMSO, dimethylformamide (DMF), and dimethylacetamide
are quasi-isostructural. The respective self-assembly of these solvates
differs due to orientations of aromatic rings and the carbomethoxy
group across the thioamide/amide bond. Significant differences in
self-assemblies of the respective DMSO solvate of L1 and L2 are
observed; self-assembly of the former has dimeric subassemblies as
repeat units, whereas the latter has monomeric subassemblies. DMF
solvates of L1 and dimethylacetamide
of L1 are built by dimeric subassemblies
to form self-assembled structures, but these subassemblies differ
in the orientation of the carbomethoxy group across the urea units.