Novel self-assembly of m-xylylene type dithioureas by head-to-tail hydrogen bonding

Dithiourea 1a self-assembles to form an orthogonal dimer structure both in solution and in the solid state, wherein the four thiourea groups establish a closed network of hydrogen bonds through a head-to-tail binding mode. This novel dimer structure was elucidated on the basis of H-1 NMR spectra, vapor pressure osmometry, and X-ray crystal structure analysis. Furthermore, a series of m-xylylene type dithioureas were synthesized and their dimerization constants (K-a) in CDCl3 were determined by dilution experiments using H-1 NMR spectroscopy. The magnitude of the K-a values are dependent on the steric bulk of the side chains, the acidity of the thiourea groups, and the weak intermolecular interaction between the benzene rings of the side chains and the m-xylylene spacer.