Complexation of the p-t-butyl-calix[4]arene anion with alkali metal cations in polar, non-aqueous solvents: experimental and theoretical studies

Complexation of the p-t-butyl-calix[4]arene monoanion (L(-)) with alkali metal cations (M(+)) in acetonitrile has been studied experimentally using NMR spectroscopy and UV spectrophotometry, and computationally using molecular dynamics and free energy perturbation simulations. Experiment indicates that in polar solvents such as acetone and acetonitrile, all the cations form 1:1 complexes which have an exo (non-inclusive) structure, with stability constant values falling in the order Li+ > Na+ > K+ > (Rb+) > Cs+. This is in full accord with the calculations, which indicate that an endo structure, as found for CsL in the solid state and chloroform solution should only result when cation solvation energies are small. For Lif, there is experimental evidence for the formation of a 2:1 (metal/ligand) species but its exact nature is obscure. The cone conformation adopted by the monoanion both when free and complexed (as indicated by room temperature NMR spectroscopy) has been characterised by a single crystal X-ray structure determination of a salt containing a large, 'non-coordinating' cation, [N((CH2)(3)CH3)(4)][p-t-butyl-calix [4] arene] CH3CN.