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

Abstract

alpha-Cyclodextrin, beta-cyclodextrin, N-(6(A)-deoxy-alpha-cyclodextrin-6(A)-yl)-N'-(6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea and N,N-bis(6(A)-deoxy-beta-cyclodextrin-6(A)-yl)urea (alpha CD, beta CD, 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: K-11 = (2.83 +/- 0.28) x 10(5) dm(3) mol(-1) for alpha CD.E-3(-), K-21 = (6.93 +/- 0.06) x 10(3) dm(3) mol(-1) for (alpha CD)(2).E-3(-), K-11 = (1.24 +/- 0.12) x 10(5) dm(3) mol(-1) for beta CD.E-3(-), K-21 = (1.22 +/- 0.06) x 10(4) dm(3) mol(-1) for (beta CD)(2).E-3(-), K-11 = (3.08 +/- 0.03) x 10(5) dm(3) mol(-1) for 1.E-3(-), K-11 = (8.05 +/- 0.63) x 10(4) dm(3) mol(-1) for 2.E-3(-) and K-12 = (2.42 +/- 0.53) x 10(4) dm(3) mol(-1) for 2.(E-3(-))(2). H-1 ROESY NMR studies show that complexation of E-3(-) in the annuli of alpha CD, beta CD, 1 and 2 occurs. A variable-temperature H-1 NMR study yields k(298 K) = 6.7 +/- 0.5 and 5.7 +/- 0.5 s(-1), Delta H double dagger = 61.7 +/- 2.7 and 88.1 +/- 4.2 kJ mol(-1) and Delta S double dagger = -22.2 +/- 8.7 and 65 +/- 13 J K-1 mol(-1) for the interconversion of the dominant includomers (complexes with different orientations of alpha CD) of alpha CD.E-3(-) and (alpha CD)(2).E-3(-), respectively. The existence of E-3(-) as the sole isomer was investigated through an ab initio study.