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

The doubly annelated triple calix[4]arene 2a, substituted by four nitro gro ups 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-)) 2DM SO] were obtained from DMSO water containing Ca(HCO3)(2). Their X-ray analy sis, the first crystallographic proof of the constitution of an annelated c alixarene, shows both monoprotonated cndo-calix[4]arene structures in the e xpected 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 compl etely solvated by four DMSO and two water molecules in a slightly distorted octahedral fashion. Each water binds two further DMSO molecules, one of wh ich is included ire the cavity of the exo-calix[4]arene parts, leading in t his way to infinite chains of anions and cations. No direct contact between the Ca2+-ion and the phenolate oxygens or the aromatic pi-electron systems of 2a is present. The pK(a)-values for the first deprotonation step of eac h 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 d eprotonation can be observed.