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 t etrakis(diphenylphosphino)calix[4]arene tetramethyl ether (1) elucidated th at in the solid-state 1 adopts a flattened partial cone conformation having C, symmetry. In solution, on the other hand, the H-1, C-13, and P-31 NMR s pectra measured at - 20 degreesC indicated that 1 has a partial cone confor mation with a mirror plane. Two-dimensional P-31 EXSY and variable-temperat ure P-31 NMR spectra reveal a fluxional behavior of 1 in solution on the NM R time scale due to annulus rotation from the methoxy side. Concerning the energetics for the four conformers of 1, an ab initio molecular orbital cal culation showed that the partial cone conformation is the most stable form, while the cone and the 1,3-alternate conformers are only slightly less sta ble.