Abstract
A series of chloride receptors has been synthesized containing an amide hydrogen bonding site and a hydroquinone motif. It was anticipated that oxidation of the hydroquinone unit to quinone would greatly the diminish chloride binding affinity of these receptors. A conformational switch is promoted in the quinone form through the formation of an intramolecular hydrogen bond between the amide and the quinone carbonyl, which blocks the amide binding site. The reversibility of this oxidation process highlighted the potential of these systems for use as redox-switchable receptors. 1H-NMR binding studies confirmed stronger binding capabilities of the hydroquinone form compared to the quinone; however, X-ray crystal structures of the free hydroquinone receptors revealed the presence of an analogous inhibiting intramolecular hydrogen bond in this state of the receptor. Binding studies also revealed interesting and contrasting trends in chloride affinity when comparing the two switch states, which is dictated by a secondary interaction in the binding mode between the amide carbonyl and the hydroquinone/quinone couple. Additionally, the electrochemical properties of the systems have been explored using cyclic voltammetry and it was observed that the reduction potential of the system was directly related to the expected strength of the internal hydrogen bond.
Highlights
Considerable effort has been devoted to the development of sophisticated and functional molecular machine architectures [1,2,3,4]
We have demonstrated that the hydroquinone/quinone redox couple can be employed in the creation of a redox-switchable chloride receptor
It was discovered that the intramolecular bond initially expected to only be present in the quinone form of the molecule was present in the hydroquinone receptor, and it is likely this competing interaction led to a reduction in binding
Summary
Considerable effort has been devoted to the development of sophisticated and functional molecular machine architectures [1,2,3,4]. An integral part of this endeavour has focused on constructing simple molecular switches that mimic elegant examples found in nature [5,6,7] and expanding the scope of their functional remit to new applications [8,9,10,11,12] This includes the field of anion recognition chemistry, with systems being applied in the detection and extraction of environmentally damaging and biologically important ions [13,14,15,16].
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