On the mechanism of action of gated molecular baskets: The synchronicity of the revolving motion of gates and in/out trafficking of guests

Abstract

SummaryWe used dynamic 1H NMR spectroscopic methods to examine the kinetics and thermodynamics of CH3CCl3 (2) entering and leaving the gated molecular basket 1. We found that the encapsulation is first-order in basket 1 and guest 2, while the decomplexation is zeroth-order in the guest. Importantly, the interchange mechanism in which a molecule of CH3CCl3 directly displaces the entrapped CH3CCl3 was not observed. Furthermore, the examination of the additivity of free energies characterizing the encapsulation process led to us to deduce that the revolving motion of the gates and in/out trafficking of guests is synchronized, yet still a function of the affinity of the guest for occupying the basket: Specifically, the greater the affinity of the guest for occupying the basket, the less effective the gates are in “sweeping” the guest as the gates undergo their revolving motion.

On the Nature of the Transition State Characterizing Gated Molecular Encapsulations

“…On the basis of the reaction's stoichiometry, however, the reaction's rate could be v out = k out [1][2] (Figure 2A) and it follows that k out * = k out as long as the proposed model is valid. Since greater concentrations of guest [2] in solution (3.0-22.0 molar equivalents) had no measurable effects on the experimental rate constant k out * ( Figure 2B) we conclude that the process of decomplexation is zeroth-order in 1,1,1-tribromoethane 2 [20]. This, in turn, corroborates the absence of interchange mechanism ( Figure 2C) [19], whereby free guests substitute the one of the same kind trapped in the host.…”

“…To investigate the nature of the transition state of gated molecular encapsulations in greater details, we hereby used dynamic 1 H-NMR spectroscopy to measure rate coefficients (k out ) by which 1,1,1-tribromoethane 2 (V = 107 Å 3 ) departs from the interior of basket 1 in four aromatic solvents 3-6 ( Figure 1). In earlier studies [20], we surmised that the egress of guest molecule should be followed with the entrapment of solvent ( Figure 1). It follows that the rate of such guest swap should change with increasingly bigger solvent molecules (99-154 Å 3 , Figure 1) via a steric imposition of the transition state of the transformation [21,30].…”

“…Furthermore, the "opening" of some dynamic hosts, hemicarcerans [15,16] or self-folding capsules [17], were argued to constitute the rate-determining step of encapsulation. In other gating situations [18,19], however, the guest entrance generates enough van der Waals strain so that the encapsulation rate law exhibits a kinetic dependence in the concentration of both host and guest [20]. To our knowledge, there has been no particular study [21] about the nature of transition states characterizing gated encapsulations, and this is the objective of our paper.…”

“…Three aminopyridine rings [24], containing hydrogen-bonding donor and acceptor sites, are attached to the rim of the platform to, via CH 2 "hinges", act as gates that, by controlling the portal size, regulate the in/out encapsulation of small haloalkanes (V ~ 88-120 Å 3 ) [25]. The formation of basket/guest complex was, in solvophobic CD 2 Cl 2 solvent (V = 61 Å 3 ) [26], found to be first-while the dissociation is zeroth-order in guests ( Figure 1) [20]. In essence, the basket undergoes a rapid thermal racemization (∆G ‡ rac < 11 kcal/mol) to, occasionally, permit the entrance/departure of guest molecules [27].…”

Gated molecular baskets

“…7 We reasoned that in line with the encapsulation stoichiometry (Fig. 7B), 51,56 one CD 2 Cl 2 is too small (PC = 0.28), while two are incompatible in shape with the interior of 5 to populate it. When more sizeable solvents (C 6 D 6 (99 Å 3 ), C 6 D 5 CD 3 (117 Å 3 ), m-C 6 D 3 (CD 3 ) 2 (136 Å 3 ) and 1,3,5-C 6 D 3 (CD 3 ) 3 (154 Å 3 ) were probed as a medium for the encapsulation of haloalkanes, 57 the solubility of ''free'' basket 5 dropped, albeit it improved considerably in the presence of guests (i.e.…”