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.
Highlights
Since the advent of supramolecular chemistry, host-guest complexes have been studied as models for receptor-substrate interactions in nature and as mimics of the action of enzymes.[1]
In this paper we describe our studies in this direction using selective exchange NMR spectroscopy, abbreviated 1D EXSY NMR
In 2018 our group reported a study on the unidirectional binding of viologen-derived guests to porphyrin cage compounds, and chemical exchange was observed from the 2D ROESY spectrum (Figure SI22).[16]
Summary
Since the advent of supramolecular chemistry, host-guest complexes have been studied as models for receptor-substrate interactions in nature and as mimics of the action of enzymes.[1]. 106 MÀ1).[12,13] these cage compounds display cooperative binding behavior and can act as biomimetic catalysts in the epoxidation of low molecular weight and polymeric alkenes.[14] More recently, we have started a program to write digital information onto single polymer chains in the form of chiral epoxides (R,R-epoxide = digit 1, S,Sepoxide = digit 2) with the help of chiral porphyrin cage compounds derived from glycoluril.[15] As part of this project we are studying the threading and binding of viologen-based guest molecules, which are blocked on one side with a bulky stopper, in these host molecules,[16,17] and we have observed unidirectional binding depending on the type of viologen guest These studies are a first important step in the design of porphyrin cage catalysts that can thread onto a polymer chain containing alkene double bonds and move along it, while converting the alkene functions into epoxides (digital printing process). Prompted follow-up 1D EXSY experiments to characterize this exchange
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