Cyclodextrin and modified cyclodextrin complexes of E-4-tert-butylphenyl-4′-oxyazobenzene: UV-visible, 1H NMR and ab initio studies

Abstract

α-Cyclodextrin, β-cyclodextrin, 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 (αCD, βCD, 1 and 2) form inclusion complexes with E-4-tert-butylphenyl-4′-oxyazobenzene, E-3⁻. In aqueous solution at pH 10.0, 298.2 K and I = 0.10 mol dm⁻³ (NaClO₄) spectrophotometric UV-visible studies yield the sequential formation constants: K₁₁ = (2.83 ± 0.28) × 10⁵ dm³ mol⁻¹ for αCD·E-3⁻, K₂₁ = (6.93 ± 0.06) × 10³ dm³ mol⁻¹ for (αCD)₂·E-3⁻, K₁₁ = (1.24 ± 0.12) × 10⁵ dm³ mol⁻¹ for βCD·E-3⁻, K₂₁ = (1.22 ± 0.06) × 10⁴ dm³ mol⁻¹ for (βCD)₂·E-3⁻, K₁₁ = (3.08 ± 0.03) × 10⁵ dm³ mol⁻¹ for 1·E-3⁻, K₁₁ = (8.05 ± 0.63) × 10⁴ dm³ mol⁻¹ for 2·E-3⁻ and K₁₂ = (2.42 ± 0.53) × 10⁴ dm³ mol⁻¹ for 2·(E-3⁻)₂. ¹H ROESY NMR studies show that complexation of E-3⁻ in the annuli of αCD, βCD, 1 and 2 occurs. A variable-temperature ¹H NMR study yields k(298 K) = 6.7 ± 0.5 and 5.7 ± 0.5 s⁻¹, ΔH^‡ = 61.7 ± 2.7 and 88.1 ± 4.2 kJ mol⁻¹ and ΔS^‡ = −22.2 ± 8.7 and 65 ± 13 J K⁻¹ mol⁻¹ for the interconversion of the dominant includomers (complexes with different orientations of αCD) of αCD·E-3⁻ and (αCD)₂·E-3⁻, respectively. The existence of E-3⁻ as the sole isomer was investigated through an ab initio study.