HOST-GUEST COMPLEXES OF CUCURBITURIL WITH 4-AMINO-4'-NITROAZOBENZENE AND 4,4'-DIAMINOAZOBENZENE IN ACIDIC AQUEOUS-SOLUTIONS

The thermodynamics and kinetics of the reaction of cucurbituril (Cuc) with 4-amino-4'-nitroazobenzene (DO3) and 4,4'-diaminoazobenzene (Diam ) have been studied in aqueous HCl, and it is shown that DO3 is well s uited as an indicator for uncomplexed Cuc (actual concentration of Cue ) and Diam is well suited as an indicator for the total concentration of Cuc (sequestering agent) in aqueous solutions. The formation of 1:1 guest-host complexes is observed. For proton concentrations up to 1 m ol dm(-3), DO3 reacts according to eqn. (a), with the values for the d issociation constant of DO3H(+) log(K-a/dm(3) mol(-1)) = CucH(+) + DO3 H(2)(2+) reversible arrow CucH(+) + DO3H(+) + H+ reversible arrow CucH (+) + DO3 + 2H(+) reversible arrow CucDO3H(+) + 2H(+) (a) -2.42, DO3H( 2)(2+) log(K-b/dm(3) mol(-1)) = 0.53, for the stability constant of Cu cDO3H(+) log (K-c/mol dm(3)) = 8.05 and for the rate constant of compl ex formation log (k(f)/dm(3) mol(-1) s(-1)) = 5.25. Reaction enthalpie s and activation energies have also been determined. The complexation of Cuc with Diam has been studied in 0.1-1 mol dm(-3) HCl where a very strong complexation has been observed.