One ring to bind them all: shape-selective complexation of phenylenediamine isomers with cucurbit[6]uril in the gas phase.

We examined complexes between cucurbit[6]uril and each of ortho-, meta-, and para-phenylenediamine using computational methods, Fourier transform ion cyclotron resonance mass spectrometry, and ion mobility spectrometry. These fundamental gas phase studies show that the lowest energy binding sites for ortho- and meta-phenylenediamine are on the exterior of cucurbit[6]uril, whereas para-phenylenediamine preferentially binds in the interior, in a pseudorotaxane fashion. This conclusion is based on reactivity of each of the complexes with tert-butylamine, where the ortho- and meta-phenylenediamine complexes exchange with tert-butylamine, whereas the para-phenylenediamine complex undergoes two slow additions without displacement. Further, under sustained off-resonance irradiation conditions, the ortho- and meta-phenylenediamine complexes fragment easily via losses of neutral phenylenediamine, whereas the para-phenylenediamine complex fragments at higher energies primarily via cleavage of covalent bonds in the cucurbituril. Finally, ion mobility studies show ion populations for the ortho- and meta-phenylenediamine complexes that primarily have collision cross sections consistent with external complexation, whereas the para-phenylenediamine complex has a collision cross section that is smaller, the same as that of protonated cucurbit[6]uril within experimental error. In agreement with experiment, computational studies indicate that at the HF/6-31G* and B3LYP/6-31G*//HF/6-31G* levels of theory external complexation is favored for ortho- and meta-phenylenediamine, whereas internal complexation is lower in energy for para-phenylenediamine. In contrast, MP2/6-31G*//HF-6-31G* calculations predict internal complexation for all three isomers.