Fluorescent Charge-Assisted Halogen-Bonding Macrocyclic Halo-Imidazolium Receptors for Anion Recognition and Sensing in Aqueous Media

Abstract

The synthesis and anion binding properties of a new family of fluorescent halogen bonding (XB) macrocyclic halo-imidazolium receptors are described. The receptors contain chloro-, bromo-, and iodo-imidazolium motifs incorporated into a cyclic structure using naphthalene spacer groups. The large size of the iodine atom substituents resulted in the isolation of anti and syn conformers of the iodo-imidazoliophane, whereas the chloro- and bromo-imidazoliophane analogues exhibit solution dynamic conformational behavior. The syn iodo-imidazoliophane isomer forms novel dimeric isostructural XB complexes of 2:2 stoichiometry with bromide and iodide anions in the solid state. Solution phase DOSY NMR experiments indicate iodide recognition takes place via cooperative convergent XB-iodide 1:1 stoichiometric binding in aqueous solvent mixtures. (1)H NMR and fluorescence spectroscopic titration experiments with a variety of anions in the competitive CD(3)OD/D(2)O (9:1) aqueous solvent mixture demonstrated the bromo- and syn iodo-imidazoliophane XB receptors to bind selectively iodide and bromide respectively, and sense these halide anions exclusively via a fluorescence response. The protic-, chloro-, and anti iodo-imidazoliophane receptors proved to be ineffectual anion complexants in this aqueous methanolic solvent mixture. Computational DFT and molecular dynamics simulations corroborate the experimental observations that bromo- and syn iodo-imidazoliophane XB receptors form stable cooperative convergent XB associations with bromide and iodide.

Iodide Recognition and Sensing in Water by a Halogen‐Bonding Ruthenium(II)‐Based Rotaxane

“…Addition of sodium iodide to a 1×10 −5   m solution of rotaxane  1 in water gave a three nm hypsochromic shift in the emission wavelength, and reaching a 6 % enhancement of the Ru‐centred MLCT emission intensity in the presence of excess (1 m m) iodide (Figure S15 in the Supporting Information). This enhancement of emission intensity upon iodide binding is particularly notable because it is more common to observe quenching behaviour of emissive organic15–17 or transition‐metal‐based anion receptors 47–49. Indeed, in the case of the acyclic HB Ru complex  5 , addition of excess iodide in water caused up to a 10 % decrease in the luminescence emission intensity (see the Supporting Information).…”

“…Amongst the plethora of biologically and medically relevant anions, the recognition and sensing of iodide in water is of interest both due to its crucial role in hormone biosynthesis by the thyroid gland11, 12 and its use in contrast media for radiographic imaging applications 13. Examples of anion hosts capable of sensing iodide by optical methods are scarce,14–17 and to the best of our knowledge, the use of molecular‐anion receptors for the host–guest recognition and sensing of iodide in water is unprecedented.…”

Characterization of Halogen Bonded Adducts in Solution by Advanced NMR Techniques

“…[64] Interestingly, the crystal structure critically depends on the anions: in the presence of two PF6 -, no XB can be observed between the iodine of the cation and the anion; on the contrary, in the presence of one PF6 -and an iodide, or bromide, a dimeric structure is found in the solid-state, with two cations interacting with two different halide in a pincer-like arrangement ( Figure 9). Figure 9.…”

“…Adapted from ref. [64]. (Top) Dimeric crystal structure of iodo-imidazolium (PF6 -and hydrogens are omitted for clarity) and (bottom) DFT structure of the monomeric structure present in solution.…”

Selective Nitrate Recognition by a Halogen‐Bonding Four‐Station [3]Rotaxane Molecular Shuttle

“…Of the relatively few examples of XB anion receptors reported to date,i ti sn oteworthy that all display promising,a nd significantly contrasting, anion recognition behaviour when compared to HB analogues,byvirtue of their comparable bond strengths and more strict linear geometry preference. [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Importantly in as ignificant step forward for highlighting the potential importance of halogen bonding in anion supramolecular chemistry,w eh ave recently demonstrated the first examples of solution phase halogen bonding being exploited to control and facilitate the anion-templated assembly of interlocked structures [43][44][45][46][47] and demonstrated that the incorporation of halogen bond donor atoms into a[ 2]rotaxane host cavity dramatically improves the anion recognition capabilities of the XB interlocked receptor. [48][49][50][51] Herein, we report the synthesis of the first halogen bonding [3]rotaxane host system, containing ab is-iodo triazolium-bis-naphthalene diimide four station axle component, which employs multiple cooperative XB and HB interactions to exhibit enhanced recognition of anions relative to an all-HB analogue and, impressively,i sf ound to be selective for nitrate over other oxoanions and chloride.…”