Concurrent insertion of cationic guest and solvent molecules in molecular receptors. Co-complexation of the sodium cation and acetonitrile by a calix[4]arene tetra-acetamide

Abstract

The compound 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis(N,N-diethylaminocarbonyl)methoxycalix[4]arene, 1, forms a 1 ∶ 1 complex with the sodium cation in a binary 1 ∶ 1 solution of chloroform-d and acetonitrile-d₃ with tetraphenylborate as the counteranion. Kinetic and mechanistic solution studies were analyzed using ¹H and ²³Na NMR. The formation of a 1 ∶ 1 complex of 1 with the sodium cation is quantitative (K_f > 10⁴ M⁻¹) at 238, 274, and 320 K. The exchange between complexed and solvated sodium cation is very slow on the ²³Na NMR time scale even at 320 K. Analysis of 2D EXSY ¹H NMR data for the interconversion of the methylene protons from the N(CH₂CH₃)₂ moiety of the (Na,1)⁺ complex shows slow exchange at 320 K. The complexes (Na,1,MeCN),(Na,1),(BPh₄)₂ and (MeCN,1) were crystallographically characterized. X-Ray crystallographic data of (Na,1,MeCN),(Na,1),(BPh₄)₂ confirm the NMR data such that the sodium cation is encapsulated by the hydrophilic pseudo-cavity of 1. The crystal structure shows two diastereomorphs of (Na,1)⁺ in the lattice differing by a twist in the OCH₂CO arms and only one diastereomorph contains a second guest, acetonitrile, in the hydrophobic pseudo-cavity of 1. The crystal structure of (MeCN,1) shows identical preorganization of the hydrophobic pseudo-cavity to the complex (Na,1)⁺.