Complexation by α- and β-cyclodextrin C(6) linked homo- and hetero-dimers of Brilliant Yellow tetraanion: a study of host–guest size relationships

Abstract

Complexation by α- and β-cyclodextrin (αCD and βCD) homo- and heterodimers linked at C(6) by a urea linker (N,N′-bis(6^A-deoxy-α-cyclodextrin-6^A-yl)urea, N-(6^A-deoxy-α-cyclodextrin-6^A-yl)-N′-(6^A-deoxy-β-cyclodextrin-6^A-yl)urea and N,N′-bis(6^A-deoxy-β-cyclodextrin-6^A-yl)urea 1–3) of the tetraanion of the dye Brilliant Yellow (4) has been studied. In aqueous solution at 298.2 K, pH 10.0 (borate) and I = 0.10 mol dm⁻³ (NaClO₄) the spectrophotometrically determined complexation constants K = (1.40 ± 0.08) × 10⁴, (9.05 ± 0.16) × 10⁴ and (3.92 ± 0.06) × 10⁴ dm³ mol⁻¹, for the complexes 1···4, 2··4 and 3··4, respectively, that compare with K = (1.05 ± 0.08) × 10⁴ and (2.20 ± 0.05) × 10³ dm³ mol⁻¹ for the αCD·4 and βCD·4 complexes, respectively. Thus, the complexation of 4 in 1···4 shows little cooperativity consistent with the annulus of each αCD component of 1 being too small to pass over the phenylsulfonate component of 4. It is probable that two complexes are formed when 2 complexes 4: one in which 4 is complexed by the αCD component alone and which has a similar stability to 1···4 and a second complex where 4 is complexed by both the αCD and βCD components of 2 to form a complex 8.6 and 41 times more stable than αCD·4 and βCD·4, respectively. The cooperativity between the two βCD components of 3 causes 3··4 to be 18 times more stable than βCD·4. These conclusions are supported by ¹H NMR spectroscopic studies.