Synthesis and structural characterization of 1-arylimidazole-2-thiones and N,N′-aryldiethoxyethylthioureas with electronically diverse substituents: a manifold of hydrogen bonding networks

Abstract

The 1-arylimidazole-2-thiones, (Hmim^Ar) [Ar = 3,4,5-C₆H₂(OMe)₃, 2,4-C₆H₃(NO₂)(OMe), 2,4,6-C₆H₂Cl₃ and 3,5-C₆H₃(CF₃)₂], which feature electronically diverse substituents, may be obtained via acid-catalyzed ring closure of the corresponding N,N′-aryldiethoxyethylthiourea derivatives, ArN(H)C(S)N(H)CH₂CH(OEt)₂, (H₂detu^Ar), which in turn are obtained via treatment of aminoacetaldehyde diethyl acetal, H₂NCH₂CH(OEt)₂, with the respective arylisothiocyanates (ArNCS). The molecular structures of all of the above N,N′-aryldiethoxyethylthioureas and 1-arylimidazole-2-thiones have been determined by X-ray diffraction, thereby demonstrating that the substituents have a profound effect on the crystal structures. For example, each of the N,N′-aryldiethoxyethylthiourea derivatives adopts a different hydrogen bonding pattern. Specifically, the hydrogen-bonding network in (i) H₂detu^ArCl₃ consists of chains of 9-membered rings, with an [R²₂(9)] motif, that feature one N–H⋯O and one N–H⋯S interaction, (ii) H₂detu^ArOMe,NO₂ consists of chains of 6-membered rings, with an [R¹₂(6)] motif, that feature two head-to-tail N–H⋯S interactions, (iii) H₂detu^Ar(CF₃)₂ consists of a dimer that features two pairs of N–H⋯O interactions, of which each pair is a component of an 8-membered ring with an [R²₂(8)] motif, and (iv) H₂detu^Ar(OMe)₃ consists of a chain of head-to-head dimeric rings with a basic [R²₂(16)] motif, a notable feature of which is that sulfur does not play a role as a hydrogen bond acceptor. Each of the 1-arylimidazole-2-thiones exists as a “head-to-head” hydrogen-bonded dimer in the solid state, with an [R²₂(8)] motif. However, while the hydrogen-bonded motifs for Hmim^ArCl₃ and Hmim^Ar(OMe)₃ are planar, those for Hmim^Ar(CF₃)₂ and Hmim^ArOMe,NO₂ are extremely puckered, with fold angles of 24.2° (mean value) and 45.7°, respectively.