Cucurbit[7]uril Host–Guest Complexes with Biguanidinium Cations in Aqueous Solution

Abstract

AbstractThe host–guest complexations of a series of biguanidinium cations (metformin and phenformin) and bis(biguanidinium) dications (chlorhexidine and alexidine) by the host molecule cucurbit[7]uril (CB[7]) in aqueous solution have been investigated by 1H NMR spectroscopy and ESI mass spectrometry. Metformin forms both 1:2 and 1:1 host–guest complexes, whereas phenformin forms only a 1:1 complex. The two bis(biguanidinium) dications both form sequential 1:1, 2:1 and 3:1 host–guest complexes, however, the mechanism is different in each case. With chlorhexidine, the first CB[7] binds to the central hexamethylene chain, followed by sequential complexation of the terminal 4‐chlorophenyl groups. With alexidine, the two terminal 2‐ethylhexyl groups are bound first, with the third CB[7] adding only with a concomitant relocation of one CB[7] to the central hexamethylene binding site. The stability constants of the various host–guest complexes were determined by 1H NMR titrations and competitive binding experiments, as well as by the application of a statistical binding model.