Acid‐Regulated Switching of Metal Cation and Anion Guest Binding in Halogen‐Bonding Rotaxanes

Abstract

The ability of natural enzymes to regulate their guest binding affinities and preferences through the use of co-ligands which alter the features of the binding site is fundamental to biological homeostatic control. Herein, the rarely exploited orthosteric control of guest binding is demonstrated using neutral halogen bonding [2]rotaxanes, in which a chemical stimulus (acid) interacting with the interlocked host binding site switches the host's native guest preference from metal cations to anions. When neutral, the rotaxanes exhibit pronounced transition metal cation affinity and comparatively weak anion binding properties. However, the addition of acid attenuates the rotaxanes' ability to coordinate cations, while concurrently enabling strong binding of halides through charge assisted halogen bonding and hydrogen bonding interactions in competitive aqueous solvent media. The appendage of a fluorescent anthracene reporter group to the rotaxane framework also enables diagnostic sensory responses to cation/anion binding.

Rigidity and Flexibility in Rotaxanes and Their Relatives; On Being Stubborn and Easy-Going

“…In 2018, Beer reported a rotaxane composed of a semi-rigid macrocycle and thread where the conformation of the binding units, in the form of halogen-bonding iodotriazoles, is regulated by the charge of the ionic guest ( Figures 8A,B ) ( Li et al, 2018 ). Both macrocycle and thread are composed of rigid iodotriazoles and phenyls together with flexible alkoxy linkers.…”

“…Presumably, ion binding would also restrict the rotational and translation motions of the macrocycle, further rigidifying the assembly. Rotaxane rigidification upon ion binding was examined using an anthracene reporter attached to the macrocycle as a reporter ( Li et al, 2018 ). When anions were bound to the neutral rotaxane, a drastic fluorescence enhancement was observed.…”

“…We will discuss how intermolecular and steric interactions regulate the conformational freedom of macrocycles and threads to form rigid or flexible rotaxane architectures ( Figure 4 ). Specifically, we will show that intermolecular interactions mediating host-guest recognition at the primary binding site ( Li et al, 2018 ) can rigidify the components and thus the overall assembly. In addition, secondary macrocycle-thread ( Clegg et al, 1999 ) and macrocycle-macrocycle ( Belowich et al, 2012 ) interactions have an impact.…”

Exploiting the mechanical bond for molecular recognition and sensing of charged species

“…21). 74 The last of these are particularly interesting since conformational flexibility of combined Lewis basic and Lewis acidic groups facilitates rotaxanes 43 and 44 to strongly bind transition metal cations via bis-tridentate N-donor coordination and to bind anions weakly through XB and HB interactions. Protonation of the central amine moiety, however, reverses binding properties allowing for much stronger anion binding and negating metal cation recognition.…”

Hydrosulfide (HS−) Recognition and Sensing in Water by Halogen Bonding Hosts

“…[14,15] A recent addition to the anion-binding supramolecular toolbox is halogen bonding (XB), a highly directional, attractive interaction between an electron-deficient halogen atom, and a Lewis base. [16,17] The few acyclic and macrocyclic XB receptors reported thus far commonly exhibit superior anion binding affinities, as well as enhanced sensory performance, compared to HB analogues, [18][19][20][21] with examples of XB host systems that function in aqueous media and especially pure water being notably rare. [20,22] Herein, we report the first XB receptors to recognise and most importantly sense HS À in pure water using an optical fluorescent chemosensing method.…”