Encapsulation of Ferrocene and Peripheral Electrostatic Attachment of Viologens to Dimeric Molecular Capsules Formed by an Octaacid, Deep‐Cavity Cavitand

Abstract

In aqueous media the deep-cavity cavitand octaacid 1 forms stable dimeric molecular capsules 1(2), which are stabilized by hydrophobic effects. In this work we investigate the binding interactions in aqueous solution between these capsules and the redox active guests, ferrocene (Fc) and three 4,4'-bipyridinium (viologen) dications: methyl viologen (MV(2+)), ethyl viologen (EV(2+)), and butyl viologen (BV(2+)). Using NMR spectroscopic and electrochemical techniques we clearly show that the hydrophobic Fc guest is encapsulated inside 1(2). An interesting effect of this encapsulation is that the reversible voltammetric response of Fc is completely eliminated when it resides inside the 1(2) capsular assembly, a finding that is attributed to very slow electrochemical kinetics for the oxidation of Fc@1(2). Diffusion coefficient measurements (PGSE NMR spectroscopy) reveal that all three viologen guests are strongly bound to the dimeric capsules. However, the (1)H NMR spectroscopic data are not consistent with encapsulation and the measured diffusion coefficients indicate that two viologen guests can strongly associate with a single dimeric capsule. Furthermore, the (V(2+))(2)*1(2) complex is capable of encapsulating ferrocene, clearly suggesting that the viologen guests are bound externally, via coulombic interactions, to the anionic polar ends of the capsule. The electrochemical kinetic rate constants for the reduction of the viologen residue in the V(2+)*1(2) complexes were measured and found to be substantially lower than those for the free viologen guests.

Van Der Waals Interactions and the Hydrophobic Effect

“…However, as the polarity of this head group is lowered, dimerization becomes more thermodynamically favored. Held together by the hydrophobic effect, these complexes possess considerable thermodynamic and kinetic stability [137], allowing the capsule to act as a yocto liter reaction flask [138][139][140][141], affect the separation of hydrocarbon gases [142], and modulate the conformational [143] and electrochemical [144] properties of entrapped guests. Although the subtleties of how the structure of either host or guest influences assembly are as yet undetermined, it is apparent that careful design with "nondirectional" supramolecular motifs can lead to welldefined supramolecular structure.…”

Binding of cyclic carboxylates to octa-acid deep-cavity cavitand

“…These supramolecular capsules can entrap a guest or guests in the dry nano-space defined by the pockets of the two cavitand subunit ‘hemispheres’ [6]. Consequently, they have been used as yoctoliter reaction vessels,[7–9] separation devices [10,11], for modulating the properties of redox-active[12] or fluorescent guests [13,14], as well as controlling electron transfer [15] and electron–electron communication [16]. …”

Simulation optimization of spherical non-polar guest recognition by deep-cavity cavitands

“…3 Second, the pocket entrance is rimmed with aromatic rings that bestows OA a predisposition to assemble into dimeric capsules. 4 These capsules have been used as yoctoliter reaction vessels, [5][6][7] separation devices, 8,9 for modulating the properties of redox-active 10 or fluorescence guests, 11,12 as well as controlling electron transfer 13 and electron-electron communication. 14 In addition to this capsule chemistry, three classes of guests bind to OA without triggering assembly and consequently allow the study of a range of 1:1 complexes.…”