Diazacoronand linked β-cyclodextrin dimer complexes of Brilliant Yellow tetraanion and their sodium(I) analogues

Abstract

Complexation of the Brilliant Yellow tetraanion, 3⁴⁻, by two new diazacoronand linked β-cyclodextrin (βCD) dimers 4,13-bis(2-(6A-deoxy-β-cyclodextrin-6A-yl)aminoethylamidomethyl- and 4,13-bis(8-(6A-deoxy-β-cyclodextrin-6Ayl) aminooctylamidomethyl)-4,13-diaza-1,7,10-trioxacyclopentadecane, 1 and 2, respectively, has been studied in aqueous solution. UV-visible spectrophotometric studies at 298.2 K, pH 10.0 and I = 0.10 mol dm⁻³ (NEt₄ClO₄) yielded complexation constants for the complexes 1•3⁴⁻ and 2•3⁴⁻, K1 = (1.08 ± 0.01) × 105 and (6.21 ± 0.08) × 10³ dm³ mol⁻¹, respectively. Similar studies at 298.2 K, pH 10.0 and I = 0.10 mol dm⁻³ (NaClO₄) yielded K₃ = (4.63 ± 0.09) × 10⁵ and (3.38 ± 0.05) × 10⁴ dm³ mol⁻¹ for the complexation of 3⁴⁻ by Na⁺•1 and Na⁺•2 to give Na⁺•1•3⁴⁻ and Na⁺•2•3⁴⁻, respectively. Potentiometric studies of the complexation of Na⁺ by 1 and 2 by the diazacoronand component of the linkers to give Na⁺•1 and Na⁺•2 yielded K₂ = (2.00 ± 0.05) × 10² and (1.8 ± 0.05) × 10³ dm³ mol⁻¹, respectively, at 298.2 K and I = 0.10 mol dm⁻³ (NEt₄ClO₄). For complexation of Na⁺ by 1•3⁴⁻ and 2•3⁴⁻ to give Na⁺• 1•3⁴⁻ and Na⁺•2•3⁴⁻ K₂K₃/K₁ = K₄ = 8.6 × 10² and 9.8 × 10³ dm³ mol⁻¹, respectively. The pKas of 1H₄⁴⁺ are 7.63 ± 0.01, 6.84 ± 0.02, 5.51 ± 0.04 and 4.98 ± 0.03, and those of 2H₄⁴⁺ are 8.67 ± 0.02, 8.11 ± 0.02, 6.06 ± 0.02 and 5.14 ± 0.05. The larger magnitude of K₁ for 1 by comparison with K₁ for 2 is attributed to the octamethylene linkers of 2 competing with 3⁴⁻ for occupancy of the annuli of the βCD entities while the competitive ability of the dimethylene linkers of 1 is less. A similar argument applies to the relative magnitudes of K₃ for Na⁺•1 and Na⁺•2. Increased electrostatic attraction probably accounts for K₃ > K1 for Na⁺•1•3⁴⁻ and 1•3⁴⁻ and for Na⁺•2•3⁴⁻ and 2•3⁴⁻. The lesser magnitudes of K₂ and K₄ for Na⁺•1 and Na⁺•1•3⁴⁻ compared with those for Na⁺•2 and Na⁺•2•3⁴⁻ are attributed to the octamethylene linkers of 2 producing a more hydrophobic environment for the diazacoronand than that produced by the dimethylene linkers of 1. ¹H NMR spectroscopic studies and the syntheses of 1 and 2 are described.