Engineering the weak N-H⋯π hydrogen bond in 4-tritylbenzamide host and controlling the interaction through guest selection

Abstract

The title amide host 1 crystallizes in the wheel-and-axle framework via amide N-H⋯O dimer and includes several aromatic and aliphatic guest molecules in cavities of 40 Å2 size between supramolecular axles. Bulky triphenylmethyl groups make it impossible for the second NH donor to engage in strong hydrogen bonding and this promotes a weak intermolecular N-H⋯π interaction in inclusion adducts of aromatic and hydrophobic guests (structure type 1, guest = xylenes, chloro/bromo-toluene). On the other hand, the N-H⋯π interaction is absent for guests with C=O groups because of stronger N-H⋯Oguest hydrogen bonding (type 2, guest = EtOAc, MeNO2). Crystal structures of both types are virtually identical except for the rotation of CONH2 group that transforms the N-H⋯π interaction to the N-H⋯O hydrogen bond. In addition to controlling the occurrence of the weak N-H⋯π hydrogen bond through host⋯ guest recognition, a third structure type with N-H⋯O host and N-H⋯π hydrogen bonds is present in the anisole adduct. Amide group conformations, strong and weak hydrogen bonds, and close packing of aromatic residues determine the three structure types of composition 1·(guest)0.5 in space group P1̄. The CH 2Cl2 solvate, 1·(CH2Cl2) 1.5, has a different crystal packing in space group C2/c with guest molecules included in channels between amide dimers and also between Ph 3C groups. The design and control of the weak N-H⋯π hydrogen bond are shown for the first time in a family of isomorphous crystal structures. Infrared spectroscopy and variable temperature X-ray diffraction are consistent with the hydrogen bond nature of the N-H⋯π interaction. Differential scanning calorimetry and thermal gravimetric analysis confirm the functional behavior of inclusion host 1 and show differences in the release of CH2Cl2 molecules from the two types of channels. Crystal latttice energies follow the order structure type 3 < type 2 < type 1 in the range of -90 to -76 kcal mol-1 per host molecule. © The Royal Society of Chemistry 2005.