A doubly annelated triple calix[4]arene in the cone/1,2-alternate/cone conformation†

Abstract

The doubly annelated triple calix[4]arene 2a, substituted by four nitro groups was synthesised by fragment condensation of the known exo-calix[4]arene 3 and the bisbromomethylated dimer of p-nitrophenol 4. Single crystals of its calcium salt [({[Ca·(DMSO)₄·(H₂O)₂]²⁺(4DMSO)}(2a – 2H)^2–) 2DMSO] were obtained from DMSO–water containing Ca(HCO₃)₂. Their X-ray analysis, the first crystallographic proof of the constitution of an annelated calixarene, shows both monoprotonated endo-calix[4]arene structures in the expected cone conformation, while the exo-calix[4]arene in the middle of the molecule is fixed in the 1,2-alternate conformation. The Ca²⁺ ion is completely solvated by four DMSO and two water molecules in a slightly distorted octahedral fashion. Each water binds two further DMSO molecules, one of which is included in the cavity of the endo-calix[4]arene parts, leading in this way to infinite chains of anions and cations. No direct contact between the Ca²⁺-ion and the phenolate oxygens or the aromatic π-electron systems of 2a is present. The pK_a-values for the first deprotonation step of each endo-calix[4]arene part of 2a are similar to an analogous calix[4]arene 5 with two adjacent p-nitrophenol units, while in contrast to 5 no further deprotonation can be observed.