Structure-reactivity relationship in interlocked molecular compounds and in their supramolecular model complexes

Examination of the pseudorotaxane-like geometries adopted in the solid state by a series of 1:1 complexes revealed significant differences in the hydrogen bonding interactions between oxygen atoms in some hydroquinone-based guests carrying polyether/polyester functions and the acidic hydrogen atoms on the bipyridinium units of the host-cyclobis(paraquat-p-phenylene). These differences are reflected directly in the stabilities of the complexes in solution and dramatic changes in the magnitudes of their association constants (K(a) values ranging from 130 to 4300 M^-1 in MeCN at 25 °C) are observed upon varying the location of the carbonyl ester function(s) along the polyether/ester chains. A similar effect (K(a) values ranging from 5 to 730 M^-1 in Me₂CO at 25 °C) was observed in the binding of paraquat as its bipyridinium bis(hexafluorophosphate) salt by analogous macrocyclic hydroquinone-based mono- and bis-lactones. Investigations of the kinetics of hydrolyses of the ester functions revealed that-while inert in their free forms-the macrocyclic mono- and bis-lactones undergo hydrolysis when incorporated within catenanes composed of one of these macrocyclic lactones and cyclobis(paraquat-p-phenylene). Presumably, the enhanced reactivity of the ester functions is a result of [C-H^...O] hydrogen bonding interactions involving the ester carbonyl oxygen atoms and the acidic hydrogen atoms on the bipyridinium units, as suggested by single-crystal X-ray crystallographic analyses. Thus, cyclobis(paraquat-p-phenylene) can act as a mechanically-interlocked 'catalyst'.