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 Cucurbit[7]uril

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 cucurbit[7]­uril, CB­[7], with which all equilibria are shifted toward the quinone form, which undergoes slow exchange with the CB­[7] cavity as both the free and the CB­[7]-intercalated quinone (DAAQ@CB­[7]) are observed simultaneously by ¹H NMR. The affinity of DAAQ for the CB­[7] cavity (K_eq = 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[7] 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[7] (required to maximize favorable nonbonding interactions) as a result of the flat geometry of DAAQ.