ExBox: A Polycyclic Aromatic Hydrocarbon Scavenger

A template-directed protocol, which capitalizes on donor–acceptor interactions, is employed to synthesize a semi-rigid cyclophane (ExBox⁴⁺) that adopts a box-like geometry and is comprised of π-electron-poor 1,4-phenylene-bridged (“extended”) bipyridinium units (ExBIPY²⁺). ExBox⁴⁺ functions as a high-affinity scavenger of an array of different polycyclic aromatic hydrocarbons (PAHs), ranging from two to seven fused rings, as a result of its large, accommodating cavity (approximately 3.5 Å in width and 11.2 Å in length when considering the van der Waals radii) and its ability to form strong non-covalent bonding interactions with π-electron-rich PAHs in either organic or aqueous media. In all, 11 PAH guests were observed to form inclusion complexes with ExBox⁴⁺, with coronene being the largest included guest. Single-crystal X-ray diffraction data for the 11 inclusion complexes ExBox⁴⁺⊂PAH as well as UV/vis spectroscopic data for 10 of the complexes provide evidence of the promiscuity of ExBox⁴⁺ for the various PAHs. Nuclear magnetic resonance spectroscopy and isothermal titration calorimetric analyses of 10 of the inclusion complexes are employed to further characterize the host–guest interactions in solution and determine the degree with which ExBox⁴⁺ binds each PAH compound. As a proof-of-concept, a batch of crude oil from Saudi Arabia was subjected to extraction with the water-soluble form of the PAH receptor, ExBox·4Cl, resulting in the isolation of different aromatic compounds after ExBox·4Cl was regenerated.