Breaking Aggregation and Driving the Keto-to-gem-Diol Equilibrium of the N,N'-Dimethyl-2,6-diaza-9,10-anthraquinonediium Dication to the Keto Form by Intercalation in Cucurbituril
1H NMR, ESI-MS, and DFT calculations with the M062X/6-31G* method show that, in water, the bistetrafluoroborate salt of N,N'-dimethyl-2,6-diaza-9,10-anthraquinonediium dication (DAAQ·2BF4-) exists in equilibrium with both its gem-diol and several aggregates (from dimers to at least octamers). with high concentrations of HCl (e.g., 1.2–1.5 M), all aggregates break up and the keto-to-gem-diol equilibrium is shifted quantitatively toward the quinone form. the same effect is observed with 1.5 mol equiv of cucurbituril, CB, with which all equilibria are shifted toward the quinone form, which undergoes slow exchange with the CB cavity as both the free and the CB-intercalated quinone (DAAQ@CB) are observed simultaneously by 1H NMR. the affinity of DAAQ for the CB cavity (Keq = 4 × 106 M–1) is in the range found for tricyclic dyes (0.4–5.4 × 106 M–1), and among the highest observed to date. a computational comparative study of the corresponding CB complex of the N,N'-dimethyl-4,4'-bipyridinium dication (N,N'-dimethyl viologen, MeV) suggests that the higher binding constant for intercalation of DAAQ may be partially attributed to a lesser distortion of CB (required to maximize favorable nonbonding interactions) as a result of the flat geometry of DAAQ.
A. Thangavel et al., "Breaking Aggregation and Driving the Keto-to-gem-Diol Equilibrium of the N,N'-Dimethyl-2,6-diaza-9,10-anthraquinonediium Dication to the Keto Form by Intercalation in Cucurbituril," Journal of Organic Chemistry, vol. 78, no. 17, pp. 8297-8304, American Chemical Society (ACS), Jan 2013.
The definitive version is available at http://dx.doi.org/10.1021/jo3027577
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