Thiourea-Based Tritopic Halogen-Bonding Acceptors

Abstract

A series of thiourea-based tritopic receptor molecules were synthesized to be used as building blocks for halogen-bonded assemblies. Here, 16 new receptor molecules were synthesized from two different 2,4,6-trialkyl-1,3,5-tris(bromomethyl)benzene starting materials via tris(isothiocyanatomethyl)benzene intermediates. The alkyl substituents in the benzene ring proved to be important for isothiocyanate group formation instead of competing thiocyanate group. The synthesis route allowed us to synthesize the isothiocyanate intermediates and further the receptor molecules without the typically used and highly toxic thiophosgene. The synthesized receptor molecules were used to study their halogen-bond acceptor properties with diiodotetrafluorobenzene donors by single-crystal X-ray diffraction method. We were able to obtain five new crystal structures of halogen-bonded complexes, in which all receptors showed two to four accepted C-I⋅⋅⋅S halogen bonds. The observed halogen bonds were highly directional and showed large variation in C=S⋅⋅⋅I acceptor angles, indicating flexible acceptor properties of sulfur.

Keywords: X-ray diffraction; halogen bonding; isothiocyanates and thiocyanates; noncovalent interactions; thiourea-based receptors.