Solid-State and Solution Structures of Calix[4]arene Tetramethyl Ether Containing Four Diphenylphosphino Moieties at the Upper Rim

X-ray crystallographic analysis and CPMAS solid-state NMR measurements of tetrakis(diphenylphosphino)calix[4]arene tetramethyl ether (1) elucidated that in the solid-state 1 adopts a flattened partial cone conformation having C1 symmetry. In solution, on the other hand, the 1H, 13C, and 31P NMR spectra measured at −20 °C indicated that 1 has a partial cone conformation with a mirror plane. Two-dimensional 31P EXSY and variable-temperature 31P NMR spectra reveal a fluxional behavior of 1 in solution on the NMR time scale due to annulus rotation from the methoxy side. Concerning the energetics for the four conformers of 1, an ab initio molecular orbital calculation showed that the partial cone conformation is the most stable form, while the cone and the 1,3-alternate conformers are only slightly less stable.