The impact of modular substitution on crystal packing: the tale of two ureas?

CrystEngComm Pub Date: 2016-08-04 DOI: 10.1039/C6CE01324D

Abstract

A small library of 13 (3a–m) compounds with modular positioning of iodide and urea/thiourea groups was synthesized in excellent yields in a single step synthetic protocol. The existence of the anticipated (thio)urea derivatives was unambiguously established using a combination of 1–2D NMR spectroscopy, ESI-MS and single crystal X-ray diffraction studies. These (thio)urea compounds were classified into four classes as follows: a) mono-substituted urea, b) di-substituted urea, c) di-substituted thiourea and d) electronically tailored di-substituted thiourea. The changes in molecular conformation and crystal packing due to the change in the relative positioning of the iodo group at the phenyl ring attached to one of the N atoms and different substituents at other N atoms in urea and thiourea derivatives have been discussed. The urea derivatives, in general, display the chain association through one-dimensional three-centered N–H?O hydrogen bonding interactions due to the trans–trans orientation of both NH protons with respect to the carbonyl group, whereas the thiourea compounds exhibit a centrosymmetric dimeric assembly via the complementary N–H?S interactions because of the trans–cis arrangement of the NH protons leading to either a helical or a sheet pattern. The presence of a nitro group at the para position of the thiourea derivative leads to trans–trans arrangement of both NH protons with respect to the thiocarbonyl groups, thus yielding a chain assembly through one-dimensional three-centered N–H?S interactions similar to urea derivatives. Associations of these chains or sheets in urea/thiourea derivatives through halogen bonding interactions (hal?hal, hal?π, hal?S etc.) generate a 2D assembly.

Graphical abstract: The impact of modular substitution on crystal packing: the tale of two ureas
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