Neutral bimetallic rhenium(I)-containing halogen and hydrogen bonding acyclic receptors for anion recognition

Halogen bonding and chalcogen bonding mediated sensing

Sigma-hole interactions, in particular halogen bonding (XB) and chalcogen bonding (ChB), have become indispensable tools in supramolecular chemistry, with wide-ranging applications in crystal engineering, catalysis and materials chemistry as well as anion recognition, transport and sensing. The latter has very rapidly developed in recent years and is becoming a mature research area in its own right. This can be attributed to the numerous advantages sigma-hole interactions imbue in sensor design, in particular high degrees of selectivity, sensitivity and the capability for sensing in aqueous media. Herein, we provide the first detailed overview of all developments in the field of XB and ChB mediated sensing, in particular the detection of anions but also neutral (gaseous) Lewis bases. This includes a wide range of optical colorimetric and luminescent sensors as well as an array of electrochemical sensors, most notably redox-active host systems. In addition, we discuss a range of other sensor designs, including capacitive sensors and chemiresistors, and provide a detailed overview and outlook for future fundamental developments in the field. Importantly the sensing concepts and methodologies described herein for the XB and ChB mediated sensing of anions, are generically applicable for the development of supramolecular receptors and sensors in general, including those for cations and neutral molecules employing a wide array of non-covalent interactions. As such we believe this review to be a useful guide to both the supramolecular and general chemistry community with interests in the fields of host-guest recognition and small molecule sensing. Moreover, we also highlight the need for a broader integration of supramolecular chemistry, analytical chemistry, synthetic chemistry and materials science in the development of the next generation of potent sensors.

Influence of halogen bonding on gold(i)-ligand bond components and DFT characterization of a gold-iodine halogen bond

A gold(i) complex bearing nitrogen acyclic carbene (NAC) and selenourea (SeU) has been used to verify whether the second-sphere SeI halogen bond (XB) is able to modify the Dewar-Chatt-Duncanson components of the Au-C and Au-Se bonds. The chosen system was found to be thermically unstable but it allowed an in-depth theoretical study by means of Energy Decomposition Analysis, Natural Bond Orbital and Natural Orbitals for Chemical Valence methods, coupled with Charge Displacement analysis. Indeed, in the presence of iodoperfluoroalkanes as XB donors, iodine interacts with the lone pair of the coordinated selenium, enhancing the Au ← C σ donation and depressing the Au → C π back-donation, as demonstrated also by the increase of the rotational barrier of the C-N bond of the NAC (see G. Ciancaleoni and others, Chem. - Eur. J., 2015, 21, 2467). On the other hand, in the presence of N-iodosuccinimide (NIS), the gold directly establishes a XB with the iodine by using its d lone pairs. This AuI XB is favored by the low steric hindrance of the ligands coordinated to the gold and the presence of the amino protons of SeU, which establish additional hydrogen bonds with the NIS. Also in this case, the effect is to increase the σ acidity and decrease the π basicity of the metal.

An integrated urea and halogen bond donor based receptor for superior and selective sensing of phosphates

A new RuII based bis-heteroleptic ditopic receptor, 1[PF6]2 (C44H33F12IN10OP2Ru), having integrated anion binding iodotriazole (halogen bond donor) and urea units (-NH bond donor) is employed for selective sensing of phosphates (e.g., H2PO4- and HP2O73-). 1[PF6]2 showed superiority in phosphate sensing in CH3CN as compared to its hydrogen bond donor analogue, 2[PF6]2 (C44H34F12N10OP2Ru), non-urea halogen bond analogue, 3[PF6]2 (C38H27F12IN8P2Ru) and non-urea hydrogen bond donor analogue, 4[PF6]2 (C38H28F12N8P2Ru) in terms of enhanced binding constant values, longer excited state lifetimes and lower detection limit values. 1H-NMR, Isothermal Titration Calorimetry (ITC) and photophysical studies revealed the implementation of the combined role of both the halogen bond donor iodotriazole unit and the -NH unit of the urea moiety for selective and enhanced binding of phosphates.

Iodide Discrimination by Tetra-Iodotriazole Halogen Bonding Interlocked Hosts

Whilst the exploitation of interlocked host frameworks for anion recognition is widely established, examples incorporating halogen bond donor groups are still relatively rare. Through the integration of a novel tetra(iodotriazole)-pyridinium motif into macrocycle and axle components, a family of halogen bonding catenane and rotaxanes are constructed for anion recognition studies in a competitive aqueous-organic solvent mixture. Importantly, the degree of anion selectivity displayed is dictated by the topological nature and charged state of the respective interlocked host cavity. All the interlocked hosts exhibit iodide anion selectivity over other halides and sulfate, with the level of discrimination being the greatest with the mono-cationic rotaxane. Arising from greater electrostatic interactions working in tandem with halogen bonding and hydrogen bonding, the di-cationic rotaxane displays stronger anion association at the expense of a relatively lower degree of iodide selectivity.

Experimental and theoretical investigation of a metalloreceptor bearing a [Re(CO)3]+ core incorporating a multifunctional ligand: selective reactivity towards Zn2+ and CN− ions

Novel Re(i) complex containing multifunctional ligand HL shows selective reactivity towards Zn²⁺ and CN⁻.

Tetrel Bonding as a Vehicle for Strong and Selective Anion Binding

Tetrel atoms T (T = Si, Ge, Sn, and Pb) can engage in very strong noncovalent interactions with nucleophiles, which are commonly referred to as tetrel bonds. The ability of such bonds to bind various anions is assessed with a goal of designing an optimal receptor. The Sn atom seems to form the strongest bonds within the tetrel family. It is most effective in the context of a -SnF₃ group and a further enhancement is observed when a positive charge is placed on the receptor. Connection of the -SnF₃ group to either an imidazolium or triazolium provides a strong halide receptor, which can be improved if its point of attachment is changed from the C to an N atom of either ring. Aromaticity of the ring offers no advantage nor is a cyclic system superior to a simple alkyl amine of any chain length. Placing a pair of -SnF₃ groups on a single molecule to form a bipodal dicationic receptor with two tetrel bonds enhances the binding, but falls short of a simple doubling. These two tetrel groups can be placed on opposite ends of an alkyl diamine chain of any length although SnF₃⁺NH₂(CH₂)nNH₂SnF₃⁺ with n between 2 and 4 seems to offer the strongest halide binding. Of the various anions tested, OH− binds most strongly: OH− > F− > Cl− > Br− > I−. The binding energy of the larger NO₃− and HCO₃− anions is more dependent upon the charge of the receptor. This pattern translates into very strong selectivity of binding one anion over another. The tetrel-bonding receptors bind far more strongly to each anion than an equivalent number of K⁺ counterions, which leads to equilibrium ratios in favor of the former of many orders of magnitude.

Neutral iodotriazoles as scaffolds for stable halogen-bonded assemblies in solution

The halogen bond (XB) donor properties of neutral 1,4-diaryl-5-iodo-1,2,3-triazoles are explored using a combination of computational and experimental results and are shown to be competitive in halogen bonding efficiency with the classic pentafluoroiodobenzene XB donor. The SNAr reactivity of these donors permits the facile assembly of an iodotriazole functionalised with a 3-oxypyridine XB acceptor, thus generating a molecular scaffold capable of undergoing dimerisation through the formation of two halogen bonds. The formation of this halogen-bonded dimer is demonstrated by 1H and DOSY NMR experiments and a plausible structure generated using DFT calculations.

Halogen bonding anion recognition

The development of solution-based anion receptor molecules which exploit halogen bonding interactions is an emerging area of research. This Feature Article reviews recent advances which have been made in this rapidly developing field, surveying the use of iodoperfluoroarene, haloimidazolium and halotriazole/triazolium halogen-bond-donor motifs in anion receptor design and describing the application of mechanically interlocked rotaxane and catenane frameworks as halogen bonding anion host systems.