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[(α-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 1 H 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 .