Strongly underestimated dispersion energy in cryptophanes and their complexes

Abstract

Cryptophanes, composed of two bowl-shaped cyclotriveratrylene subunits linked by three aliphatic linker groups, are prototypal organic host molecules which bind reversibly neutral small guest compounds via London forces. The binding constants for these complexes are usually measured in tetrachloroethane and are in the range of 10 2 -10 3 M À 1 . Here we show that tetrachloroethane is-in contrast to the scientific consensus-enclosed by the cryptophane-E cavity. By means of NMR spectroscopy we show that the binding constant for CHCl 3 @cryptophane-E is in larger solvents two orders of magnitudes higher than the one measured before. Ab initio calculations reveal that attractive dispersion energy is responsible for high binding constants and for the formation of imploded cryptophanes which seem to be more stable than cryptophanes with empty cavities.

Understanding a Host–Guest Model System through 129Xe NMR Spectroscopic Experiments and Theoretical Studies

“…These two terms govern the host-guest interactions. This result is compatible with the finding of Haberhauer et al, [26] who showed the important role of attractive dispersion energy in CHCl 3 -cryptophane complexes. Johnson and co-workers [27] recently introduced a method for studying noncovalent interactions (NCIs) on the basis of the electron density (1), the reduced gradient of the density (s ¼ r1 j j=ð2ð3 p 2 Þ 1=3 1 4=3 Þ), and the Laplacian of the density (r 2 1).…”

Fully Collapsed Imploded Cryptophanes in Solution and in the Solid State

“…The typical structure has a crown‐crown (CC) out‐out conformation in which the two upper rims the CTBs face one another, Scheme . There are a handful of reported examples of out – saddle conformation, which is observed in those cases in which one crown has undergone pseudo‐rotation to the twisted saddle form giving a partially “imploded” crown‐saddle (CS) structure, Scheme . The out ‐ saddle conformation of imploded cryptophanes was first unequivocally demonstrated by Holman and co‐workers with a crystal structure of a m ‐xylyl‐linked anti cryptophane, and there has been one further crystal structure of an imploded cryptophane with triallyl decoration from Dmochowski and co‐workers .…”

“…With the benefit of these well‐characterised examples of out ‐ saddle cryptophanes, early reports by Collet of out ‐ out cryptophanes with highly flexible O(CH 2 ) n O ( n =6–10) linkers being in equilibrium with their out‐in CC conformations (Scheme ) were subsequently reinterpreted as the out ‐ saddle forms, based on signature chemical shifts . Furthermore, gas‐phase computational studies of the imploded form of cryptophane‐E with (OCH 2 CH 2 O) bridges between CTG units concludes the saddle‐out conformer is of relatively similar energy to out‐out forms, and even being slightly lower in energy than guest‐free cryptophane . However, the out‐in CC form of cryptophane‐E is at least 29 kJ mol −1 higher in energy .…”

“…Furthermore, gas‐phase computational studies of the imploded form of cryptophane‐E with (OCH 2 CH 2 O) bridges between CTG units concludes the saddle‐out conformer is of relatively similar energy to out‐out forms, and even being slightly lower in energy than guest‐free cryptophane . However, the out‐in CC form of cryptophane‐E is at least 29 kJ mol −1 higher in energy . Formation of imploded cryptophanes is generally attributed to partial collapse of the initially synthesised out‐out cage structure on removal of included guest molecules, achieved by high temperature evacuation or heating the cage in a solvent that is too large to occupy the cage cavity .…”

“…However, the out‐in CC form of cryptophane‐E is at least 29 kJ mol −1 higher in energy . Formation of imploded cryptophanes is generally attributed to partial collapse of the initially synthesised out‐out cage structure on removal of included guest molecules, achieved by high temperature evacuation or heating the cage in a solvent that is too large to occupy the cage cavity . An exception to this was reported by Berthault and co‐workers, who observed spontaneous isomerisation of a water‐soluble cryptophane to the out ‐ saddle form in degassed aqueous solution .…”