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 Ų 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 CO groups because of stronger N–H⋯O_guest 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 P. The CH₂Cl₂ 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 Ph₃C 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.