Synthesis, structural characterization, and alkali-metal complexation of the six possible (1,3)- and (1,2)-bridged p-tert-butylcalix[4]crown-5 conformers bearing α-picolyl pendant groups

Complementary synthetic strategies leading to the six possible regioisomers and conformational isomers of bis[(alpha-picolyl)oxy]-p-tert-butylcalix[4]crown-5 (1-6) are reported. The overall conformation of 1-6 was deduced by NMR spectroscopy and further proven by single-crystal X-ray analysis for (1,3)-bridged cone and (1,2)-bridged 1,2-alternate conformers 1 and 6. Compounds 1-6 form 1:1 complexes with alkali-metal cations. The complexation sites and solution structures for Na+ and K+ complexes have been determined by H-1 NMR titration experiments. The binding affinities of ligands 1-6 for alkali-metal ions have been assessed by phase transfer and stability constant measurements using spectrophotometric, potentiometric, and calorimetric techniques. All 1,3-bridged calixcrowns 1-3 show a strong affinity for K+ (partial cone 2 and 1,3-alternate 3 also for Rb+), conformer 3 being slightly more selective than the naturally occurring ionophore valinomycin. Conversely, 1,2-bridged calixcrowns 4-6 are less efficient and their selectivities vary from K+ for cone 4 to both K+ and Rb+ for partial cone 5 and to Cs+ for 1,2-alternate conformer 6.