Torsional motion in (tert-butyl)ammonium hemispheraplexes: Rotational barriers and energy of binding

The ADPs (ADPs = atomic anisotropic displacement parameters) from the single-crystal X-ray studies of nine related TBA⁺ (TBA⁺ = (tert-butyl)ammonium) hemispheraplexes are analyzed, and the results compared to the free energy of binding of this guest by the nine hosts. The lipophilic hosts (Fig. 1) were synthesized over a number of years, with increasing pre-organization for and specificity of binding. Structural studies for six of the complexes have been published, but the remaining three structures including those of the strongest binders of TBA⁺, are disordered and have only now been completed. New area-detector data has been analyzed for the TBA⁺ClO₄^- complexes of 5 and of 8 at two temperatures, while the original data for 9-TBA⁺SCN^- has been treated with a disorder model. In addition, improved models are presented for the complexes of 6 and 7. Methods for assessing the precision of the ADP analyses are discussed. Although most of the structures are imprecise, the TBA⁺ groups do demonstrate some of the characteristics of independent motion. The general trend in calculated libration amplitudes for the TBA⁺ group suggests that the guests with the greatest free energy of binding, and the shortest distances from N⁺ to the ligand plane, are those with the highest barriers to internal rotation.