Complexation of the caesium cation by the host p-tert-butylcalix[6]arene hexaacetamide

Abstract

The complexation of the caesium cation by a p-tert-butylcalix[6]arene hexaacetamide derivative (5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40-hexakis(N,N-diaethylaminocarbonyl)methoxycalix[6]arene) (I) in a binary mixture of deuterated chloroform and acetonitrile was studied by ¹H, ¹³³Cs NMR spectroscopy and X-ray crystallography. Different complexes between Cs⁺ and I with Cs⁺ ∶ I stoichiometries ranging from 1 to 3 are formed in solution. The 3 ∶ 1 complex is only observed below 250 K, in the presence of excess of Cs⁺. The structure of the 2 ∶ 1 complex in solution is similar to the distorted partial cone crystal structure in the solid state. The two caesium cations are coordinated to carbonyl and phenolic oxygens, with a short distance of 4.16 Å between them. The dissociation of the 2 ∶ 1 complex follows a dissociative mechanism with ΔH^≠ = 58 kJ mol⁻¹ and ΔS^≠ = −2 J K⁻¹ mol⁻¹. The activation parameters indicate that the dissociative process is kinetically governed by the successive flipping of two aromatic rings leading to a 1,2,3 alternate conformation of a 1 ∶ 1 complex, similar to the crystal structure of the uncomplexed calix[6]arene hexaacetamide.