Dynamics in host-guest complexes of molecular tweezers and clips.
The dynamics in the host-guest complexes of the mol. tweezers 1a,b and clips 2a,b with 1,2,4,5-tetracyanobenzene (TCNB, 3) and tropylium tetrafluoroborate (4) as guest mols. were analyzed by temp.-dependent 1H NMR spectroscopy. The TCNB complexes of tweezers 1a,b were found to be particularly stable (dissocn. barrier: DG!= = 16.8 and 15.7 kcal mol-1, resp.), more stable than the TCNB complexes of clips 2a,b and the tropylium complex of tweezer 1b (dissocn. barrier: DG!= = 12.4, 11.2, and 12.3 kcal mol-1, resp.). A detailed anal. of the kinetic and thermodn. data (esp. the neg. entropies of activation found for complex dissocn.) suggests that in the transition state of dissocn. the guest mol. is still clipped between the arom. tips of the host mol. The 1H NMR anal. of the TCNB complexes 3@1b and 3@2a at low temps. (T < -80 DegC) showed that 3 undergoes fast rotation inside the cavity of tweezer 1b or clip 2a (rotational barrier: DG!= = 11.7 and 8.3 kcal mol-1, resp.). This rotation of a guest mol. inside the host cavity can be considered to be the dynamic equilibration of noncovalent conformers. In the case of clip complex 3@2a the assocn. and rotational barriers are smaller by DDG!= = 3-4 kcal mol-1 than those in tweezer complexes 3@1a,b. This can be explained by the more open topol. of the trimethylene-bridged clips compared to the tetramethylene-bridged tweezers. Finally, the bromo substituents in the newly prepd. clip 2b have a substantial effect on the kinetics and thermodn. of complex formation. Clip 2b 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 2a. These results can be explained by a less neg. electrostatic potential surface (EPS) inside the cavity and a larger van der Waals contact surface of 2b compared to 2a. In the case of the highly electron-deficient guest mols. TCNB and TCNQ the attractive electrostatic interaction is predominant and hence responsible for the thermodn. complex stability, whereas in the case of TNF with its extended p system, dispersion forces are more important for host-guest binding.
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