Dynamic molecular recognition in solids: A synoptic approach to structure determination in p-tert-butylcalix[4]arene-toluene

Abstract
Materials in which weak molecular recognition plays a part often are disordered to an extent that hampers the derivation of reliable structural data. Modelling the detailed structure then cannot be done in a critical fashion, so that in many cases the molecular recognition process itself is not understood very well. Using solid-state NMR methods along with newly obtained crystallo-graphic data, we show that p-tert-butylcalix[4]arene-toluene is a fluxional guest-host material at room temperature. The new structural model refines down to an R-value of 0.046 instead of the 0.092 value obtained for a model based only on crystallographic data. Problems with a difficult-to-understand disorder involving the tert-butyl groups are resolved, and a number of previously unknown structural and dynamical features regarding the nature of the inserted toluene become apparent. Crystal data for C12.75H16O at 290 K: tetragonal P4/n, a = 12.7725(2), c = 13.8068(5) Å, D c = 1.089 g cm−3, Z = 8, 1385 observed data I > 2.5σ(I), R = 0.046.