Dynamics in host-guest complexes of molecular tweezers and clips
The dynamics in the host-guest complexes of the molecular tweezers 1 a,b and clips 2 a,b with 1,2,4,5-tetracyanobenzene (TCNB, 3) and tropylium tetrafluoroborate (4) as guest molecules were analyzed by temperature-dependent 1H NMR spectroscopy. The TCNB complexes of tweezers 1 a,b were found to be particularly stable (dissociation barrier: DeltaG(++)=16.8 and 15.7 kcal mol(-1), respectively), more stable than the TCNB complexes of clips 2 a,b and the tropylium complex of tweezer 1 b (dissociation barrier: DeltaG(++)=12.4, 11.2, and 12.3 kcal mol(-1), respectively). A detailed analysis of the kinetic and thermodynamic data (especially the negative entropies of activation found for complex dissociation) suggests that in the transition state of dissociation the guest molecule is still clipped between the aromatic tips of the host molecule. The 1H NMR analysis of the TCNB complexes 3@1 b and 3@2 a at low temperatures (T<-80 degrees C) showed that 3 undergoes fast rotation inside the cavity of tweezer 1 b or clip 2 a (rotational barrier: DeltaG( not equal)=11.7 and 8.3 kcal mol(-1), respectively). This rotation of a guest molecule inside the host cavity can be considered to be the dynamic equilibration of noncovalent conformers. In the case of clip complex 3@2 a the association and rotational barriers are smaller by DeltaDeltaG(++)=3-4 kcal mol(-1) than those in tweezer complexes 3@1 a,b. This can be explained by the more open topology of the trimethylene-bridged clips compared to the tetramethylene-bridged tweezers. Finally, the bromo substituents in the newly prepared clip 2 b have a substantial effect on the kinetics and thermodynamics of complex formation. Clip 2 b forms weaker complexes with (TCNB, 3) and tetracyanoquinodimethane (TCNQ, 12) and a more stable complex with 2,4,7-trinitrofluoren-9-ylidene (TNF, 13) than the parent clip 2 a. These results can be explained by a less negative electrostatic potential surface (EPS) inside the cavity and a larger van der Waals contact surface of 2 b compared to 2 a. In the case of the highly electron-deficient guest molecules TCNB and TCNQ the attractive electrostatic interaction is predominant and hence responsible for the thermodynamic complex stability, whereas in the case of TNF with its extended pi system, dispersion forces are more important for host-guest binding.
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