Host–Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

Abstract

Dynamics in complexes of porphyrin cage compounds and viologen-derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host-guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less-bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host. Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.

Unveiling the structural features of the host–guest complexes of carboxylated pillar[5]arene with viologen derivatives

“…Viologens are commonly used in the supramolecular chemistry to fabricate capsular or threaded host-guest assemblies both in solution and solid state. 9 Their cationic electron-deficient nature makes them perfect guests for electron-rich and/or anionic host molecules. Notably, the dimethyl viologen salt Please do not adjust margins Please do not adjust margins (paraquat) was the first guest to reveal the inclusion properties of deca-anionic form of CPA5 in an aqueous solution.…”

113Cd as a Probe in NMR Studies of Allosteric Host‐Guest‐Ligand Complexes of Porphyrin Cage Compounds

“…The negative activation entropies are likely associated with the solvation of Me 2 V once it exits the cavity [25] . Prior to Me 2 V dissociation, Cd1 in the absence of tbupy likely has an acetonitrile bound.…”

“…Previously, kinetic cooperativity has been described for natural systems, e. g. for changes in substrate conversion rates by enzymes [24] . We investigated the kinetic factors of the allosteric effect of the binding in Cd1 with the help of 1D Exchange Spectroscopy (EXSY) NMR experiments [25] …”

Internal and external catalysis in boronic ester networks

“…In the 1D EXSY experiment, the ortho-protons of PBA were irradiated, and the integrated peaks of the acid and the ester were converted to concentrations for each mixing time using an internal standard, following a literature procedure (Figure 2c). 66 The decrease of in acid concentration with respect to mixing time was fit to a first-order reversible rate law to obtain pseudo-first-order rate constants for the forward (esterification, kobs,f) and reverse (hydrolysis, kobs,r) reactions. These measurements were repeated at several temperatures to determine the activation energies for esterification and hydrolysis (Figure 2d, see ESI for details).…”