Selective recognition and extraction of iodide from pure water by a tripodal selenoimidazol(ium)-based chalcogen bonding receptor

A selenium-based tripodal chalcogen bond (ChB) donor TPI-3Se is demonstrated for the recognition and extraction of I- from 100% water medium. NMR and ITC studies with the halides reveal that the ChB donor selectively binds with the large, weakly hydrated I-. Interestingly, I- crystallizes out selectively in the presence of other halides supporting the superiority of the selective recognition of I-. The X-ray structure of the ChB-iodide complex manifests both the μ1 and μ2 coordinated interactions, which is rare in the C-Se···I chalcogen bonding. Furthermore, to validate the selective I- binding potency of TPI-3Se in pure water, comparisons are made with its hydrogen and halogen bond donor analogs. The computational analysis also provides the mode of I- recognition by TPI-3Se. Importantly, this receptor is capable of extracting I- from pure water through selenium sigma-hole and I- interaction with a high degree of efficiency (∼70%).

Influence of Internal Angular Arrangement on Pnicogen Bond Strength

The three Z-X covalent bonds of a ZX3 unit (Z = P, As, Sb, Bi) are normally arranged in a pyramidal structure. Quantum chemical calculations show that pnicogen bonds (ZBs) to the central Z are weakened if ZX3 is flattened, as in the opening of an umbrella. The partial closing of the umbrella has the opposite effect of substantially strengthening these ZBs, even amounting to a 2- or 3-fold magnification in certain cases. The strongest such bonds, wherein Sb and Bi are in a strained configuration within a ZO3CH model system, have interaction energies of 20 kcal/mol with an NH3 base. Most of these systems, whether flattened or more pyramidal, are capable of engaging in three ZBs simultaneously, despite a certain amount of negative cooperativity.

Transition between the Noncovalency and Covalency of σ-Hole Bonds

The properties of the bond between a N-ligand and a Lewis acid containing a σ-hole are studied by quantum chemical methods. Interactions considered include pnicogen bonds involving SbX5, PX5, and PX3, where X represents any of the halogen atoms F, Cl, Br, or I. Also studied are the tetrel bonds of PbX4 and SiX4, as well as the chalcogen bond involving TeOX4. Both NH3 and NCH are applied as two possible bases of differing potency. Some of the bonds are very strong with interaction energies easily exceeding 25 kcal/mol and with AIM bond critical point densities much higher than 0.04 au, suggesting their classification as coordinate covalent bonds. The pentavalent SbX5 and PX5 fall into this category when combined with NH3, as does TeOX4. Although the tetrel bonds involving PbX4 are only slightly weaker, they are probably better viewed as a strong noncovalent bond on the cusp of covalency. Changing the internal bonding of hypervalent SbX5 to the more conventional SbX3 weakens the interaction to a classical noncovalent pnicogen bond. Reducing the base nucleophilicity from NH3 to NCH weakens the bonds so that they are clearly noncovalent.

Relation between Halogen Bond Strength and IR and NMR Spectroscopic Markers

The relationship between the strength of a halogen bond (XB) and various IR and NMR spectroscopic quantities is assessed through DFT calculations. Three different Lewis acids place a Br or I atom on a phenyl ring; each is paired with a collection of N and O bases of varying electron donor power. The weakest of the XBs display a C-X bond contraction coupled with a blue shift in the associated frequency, whereas the reverse trends occur for the stronger bonds. The best correlations with the XB interaction energy are observed with the NMR shielding of the C atom directly bonded to X and the coupling constants involving the C-X bond and the C-H/F bond that lies ortho to the X substituent, but these correlations are not accurate enough for the quantitative assessment of energy. These correlations tend to improve as the Lewis acid becomes more potent, which makes for a wider range of XB strengths.

Competition between Binding to Various Sites of Substituted Imidazoliums

The imidazolium cation has a number of different sites that can interact with a nucleophile. Adding a halogen atom (X) or a chalcogen (YH) group introduces the possibility of an NX···nuc halogen or NY···nuc chalcogen bond, which competes against the various H-bonds (NH and CH donors) as well as the lone pair···π interaction wherein the nucleophile lies above the plane of the cation. Substituted imidazoliums are paired with the NH₃ base, and the various different complexes are evaluated by density functional theory (DFT) calculations. The strength of XB and YB increases quickly along with the size and polarizability of the X/Y atom, and this sort of bond is the strongest for the heavier Br, I, Se, and Te atoms, followed by the NH···N H-bond, but this order reverses for Cl and S. The various CH···N H-bonds are comparable to one another and to the lone pair···π bond, all with interaction energies of 10-13 kcal/mol, values which show very little dependence upon the substituent placed on the imidazolium.

Superiority of the Supramolecular Halogen Bond Receptor over Its H-Bond Analogue toward the Efficient Extraction of Perrhenate from Water

A halogen bond-based water-soluble tetrapodal iodoimidazolium receptor, (L-I)(4Br), exhibited a high degree of efficiency (∼96%) in extracting ReO₄^- from 100% aqueous medium within a wide range of concentrations and of pH values along with excellent reusability. The solid-state X-ray diffraction study showed the trapping of ReO₄^- by (L-I)(4Br) via the Re-O····I halogen bonding interaction. XPS studies also suggested the interaction between I and ReO₄^- through polarization of the electron density of I atoms by ReO₄^-. (L-I)(4Br) is found to be capable of retaining its high extraction efficiency in the presence of competing anions such as F^-, Cl^-, I^-, SO₄^2-, H₂PO₄^-, CO₃^2-, NO₃^-, BF₄^-, ClO₄^-, Cr₂O₇^2-, and a mixture of these anions. Interestingly, (L-I)(4Br) was found to be superior in ReO₄^- extraction as compared to its hydrogen-bond donor analogue, (L-H)(4Br), as confirmed by a series of control experiments and theoretical calculations. Our synthesized dipodal and tripodal halogen bond donor receptors and their H-analogues validated the superiority of these classes of supramolecular halogen bond donor receptors over their hydrogen-bond analogues. (L-I)(4Br) also showed superior practical applicability in terms of the removal of ReO₄^- at anion concentrations as low as ∼100 ppm, which was a major shortcoming of (L-H)(4Br).

Halogen bonding effect on electrochemical anion oxidation in ionic liquids

Three Ionic liquids (ILs) based on an imidazolium core have been compared and used as solvents for the oxidation of various anions. Electrochemical experiments as well as NMR titrations and X-ray diffraction analyses unambiguously confirm the crucial role of non-covalent halogen bonding on the oxidation potentials and consequently the electrochemical window of the respective ILs.

How Many Pnicogen Bonds can be Formed to a Central Atom Simultaneously?

A central ZF₃ molecule (Z = P, As, Sb, Bi) is allowed to interact with a number of nucleophiles exemplified by NCH, NH₃, and NC^– anion. The Z···N pnicogen bond (ZB) to a single base grows stronger for heavier Z atom: P < A < Sb < Bi and follows the NCH < NH₃ < NC^– order for the three bases. The maximum number of ZBs depends on both the nature of the base and pnicogen atom. PF₃ and AsF₃ can pnicogen bond with only a single CN^–; SbF₃ and BiF₃ can interact with two anions but only weakly. The weak NCH nucleophile can engage in a maximum of two ZBs, while three ZBs occur for NH₃. The latter NH₃ maximum can be extended to four ZBs but only for BiF₃. The fourth ZB is somewhat longer and weaker than the others, and the entire (H₃N)₄···BiF₃ complex relies partially on secondary interactions for its stability.