Molecular recognition of pyrazineN,N′-dioxide using aryl extended calix[4]pyrroles

Abstract

Calix[4]pyrrole (C4P)-based systems have been extensively explored as binding agents for anions and ion pairs. However, their capacity to act as molecular containers for neutral species remains underexplored. We report here the molecular recognition of pyrazineN,N′-dioxide (PZDO) using a series of aryl extended C4Ps including three α,α-diaryl substituted C4Ps (receptors1-3), an α,β-diaryl substituted C4P (receptor4) and an α,α,α,α-tetraaryl substituted C4P (receptor5). Single crystal structural analyses of the 2 : 1 host-guest complexes between receptors1-3andPZDOrevealed that the C4P subunits exist in an unusual partial cone conformation and that thePZDOguest is held within electron-rich cavities formed by the lower rims of the individual C4P macrocycle. In contrast, receptor5was seen to adopt the cone conformation in the solid state, allowing onePZDOmolecule to be accommodated inside the upper-rim cavity. Evidence for guest-directed self-assembly is also seen in the solid state. Evidence for C4P-PZDOinteractions in CD3CN/CD3OD solution came from1H NMR spectroscopic titrations. Electrostatic potential maps created by means of density functional theory calculations were constructed. Density functional theory calculations were also performed to analyse the energetics of various limiting binding modes. On the basis of these studies, it is inferred that interactions between the ‘two-wall’ C4P derivatives (i.e.receptors1-4) andPZDOinvolve a complex binding mode that differs from what has been seen in previous host-guest complexes formed between C4Ps andN-oxides. The present study thus paves the way for the further design of C4P-based receptors with novel recognition features.