Chalcogen Bonds: How to Characterize Them in Solution ?

Chalcogen bonds (ChBs) occur between molecules containing Lewis acidic chalcogen substituents and Lewis bases. Recently, ChB emerged as a pivotal interaction in solution-based applications such as anion recognition, anion transport and catalysis. However, before moving to applications, the involvement of ChB must be established in solution. In this Concept article, we provide a brief review of the currently available experimental investigations of ChB in solution.


Introduction
According to IUPAC, a chalcogen bond (ChB) results from net attractive interaction between an electrophilic region associated with a chalcogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. [1]In other words, the term ChB describes the inter-or intra-molecular noncovalent interaction (NCI) occurring between a Lewis base (B L ) and a chalcogen atom (Ch = S, Se, Te) acting as Lewis acid (Figure 1).
Such NCI is very similar to the more common halogen bond (XB). [2]Both are now understood through the concept of σ-hole, resulting of anisotropic electronic densities within covalentlybonded atoms. [3]Regions of lower density lying on the extension of a σ bond are called σ-holes. [4]Associated with positive electrostatic surface potentials, σ-holes can induce attractive interactions with negative sites (lone pairs, anions, πelectrons).In sharp contrast to hydrogen bonds (HB), the high directionality of XB and ChB (Figure 1) and experimental and theoretical studies have also suggested that dispersion and orbital delocalization effects, especially n-σ* orbital interactions, make important contributions to σ-hole interactions. [5]hatever its exact nature, ChB possesses many similarities to XB, [6] but with a different interaction pattern.Due to the divalent nature of Ch atoms, two σ-holes can be identified and even three for chalcogenonium, [7] instead of one for halogen atom.More than one ChB can thus be established, and due to the lone pairs of Ch atoms, the latter can act both as Lewis base (B L ) and as Lewis acid through its σ-holes.
As for XB, but roughly a decade later, [8] ChB has been mostly investigated in solid state, [9] has been identified in biological systems [10] and is currently raising increasing interest in coordination chemistry [11] and organic chemistry, especially in anion recognition, anion transport and organocatalysis. [12]wever, the latter aspects require exploiting and controlling ChB in solution.Unfortunately, characterizing and measuring such interactions in solution remains a difficult task, due to their weakness but also to possible competition with solvent molecules.
As guidelines to do so, the present essay covers the so far applied and possible experimental investigations of ChB in solution.

Assessing ChB presence
The main tool to establish the presence of ChB remains X-ray diffraction (XRD), able to provide geometric information, especially bond distances and angles, although ChB weakness sometimes induce the formation of crystals assembled through other (stronger) NCI. [13]Unfortunately, ChB observed through XRD may or not be maintained in solution for the same reasons.In solution, ChB presence can be assessed by a handful set of techniques and their adequate combination.They are briefly surveyed here.
Besides using the techniques mentioned below, ChB assessment is often supported by theoretical calculations. [14]However, the correct evaluation of solvent effects in ChB systems through calculations is not an easy task. [15]In this respect, molecular balances represent an interesting alternative (see below). [16]ultinuclear NMR Although identified as early as 1998 through crystallographic studies on enzyme inhibitors, [17] S•••O close contacts, as it was named at that time, were analyzed by theoretical tools in 2011, [18] but it is only very recently that the use in solution of such Ch•••O interactions was reported (Figure 2). [19]n crystals, hexafluorophosphate salts 1-COR (Ch = S, Se) exhibit intramolecular ChB, but not the corresponding isourea (Ch = O) (Figure 2A). 13C NMR confirms in CDCl 3 its respective presence and absence with carbonyl chemical shifts deshielded in S and Se compounds (δ C = 174.5 and 174.0 ppm), compared to the oxygenated compound (δ C = 169.7 ppm).nOe enhancement was observed for the corresponding N-acetyl isothio-and seleno-uronium salts 1-COCH 3 but not for the isourea (Figure 2B).These results show that N-acyl isochalcogenuroniums 1-COR adopt a locked conformation due to ChB which activates the carbonyl group and thus allows enantioselective acyl transfer in catalytic reactions.
5b,16] Besides the common 1 H and 13 C NMR, 17 O, 77 Se and 125 Te NMR can be used to identify and study ChB.The main interest of using such Ch nuclei is their direct involvement in ChB, associated with their sensitivity and wide chemical shift range.In a seminal study, [20] δ O and δ Se values were collected for a series of specifically designed ortho-selenobenzaldehydes, alcohols and ethers.These values offer a good experimental measure for the strength of the Se•••O interaction (Figure 3).Indeed, the shorter the SeÀ O atomic distance, the larger the 17 O NMR upfield shift for 2 a-f relative to PhCHO (dashed line in Figure 3, right) and this shift became larger as the electronwithdrawing ability of the Se substituent increased.
In a preliminary study for applications in organocatalysis, a series of diphenylmethyltelluronium derivatives 3 carrying electron-rich or -poor substituents was prepared (Figure 4A) and their ChB intensity evaluated through a combination of techniques. [7]Crystal structures revealed three ChBs between the more nucleophilic anions and the telluronium centre (Figure 4B).DFT calculations confirmed the presence of three σholes and allowed to assess their deepness (Figure 4C).In CD 2 Cl 2 solution, addition of 17 OPPh 3 induced significant shifts of 17 O, 31 P and 125 Te NMR signals, which can be correlated to the substituents σ Hammett constants and to the coordination ability of the anions.Furthermore, nOe were also detected between the ortho-protons of Ph 3 PO and the methyl group of the telluronium (Figure 4D), and in situ mass spectrometry allowed to detect the mass peak (849.0448) with isotopic distribution typical of a 1 : 1 adduct between 3-CF 3 and Ph 3 PO (not shown).As expected, the largest NMR shifts and nOe were observed for the more electrodeficient telluronium [3-CF 3 ] + , the one having the largest σ holes with the less coordinating BAr 4 F anions.Nevertheless, this in solution study also revealed that the effect of Ph 3 PO addition is balanced by other parameters, related to competition with the anions for interaction with the telluronium σ holes and may be some modifications of the telluronium geometry depending on interaction strength of the anion and/or Ph 3 PO.The strategy of combining XRD, theoretical calculations, 1 H and 125 Te NMR, and 2D NMR also allowed detection of Te•••O ChB in solution between diaryl ditellurides and phosphine oxides. [13]hB between selenonium salts and dimethylacetamide could also be detected through carbonyl and selenium shifts in 13 C and 77 Se NMR. [21]he heteronuclear multiple quantum coherence (HMQC) NMR involving selenium and tellurium atoms was applied for the detection of ChB in supramolecular polymers between chalcogen-containing macrocyclic receptors 4 and bispyridine N-oxide 5 (Figure 5A). [22]In the 1 H-125 Te HMQC NMR spectrum, the two cross-correlation resonances corresponding to 3 J Teα-H1 and 3 J Teβ-H3 couplings in 4-Te were altered upon addition of 5 (red signals compared to blacks ones in Figure 5B) because of the enhanced shielding effect caused by the N-oxide moiety trapped into the macrocyclic core through ChB with the four Te atoms (Figure 5C).

Isothermal Titration Calorimetry (ITC)
ITC is a technique measuring the thermodynamic data occurring upon interactions in solution, which leads to accurate estimation of binding constants.Mostly applied to supramolecular and biological systems, this method has been used since 2012 to investigate XB, especially solvent and temperature effects as well as couteranion effects in the case of charged molecules. [23]Regarding ChB study, the use of ITC is scarce probably because of the Ch ability to interact with more than one base.
Te-derivative 6-Te was shown to catalyze the addition of methoxyindole to trans-β-nitrostyrene while the S-and Seanalogs were almost inactive (Figure 6A). [24]Moreover, the BAr F 4 salt 6-Te(BAr F 4 ) proved to be more active than 6-Te(BF 4 ).To explain it, their association constants K a to nBu 4 N + Cl À in CH 2 Cl 2 were determined by ITC and those to trans-β-nitrostyrene by 1 H NMR titrations.Expectedly, K a (Cl À ) proved much higher compared to K a (substrate), and in agreement with the catalysis experiments, the more active catalyst 6-Te(BAr F 4 ) binds slightly more strongly than 6-Te(BF 4 ).
ITC is also a powerful tool for anion binding studies of ChBbased anion receptors in water. [25]The 7-Te receptor resulted in very high affinity and selectivity for I À over other anions (Br À , Cl À , ReO 4 À , NCS À and ClO 4 À ).ITC experiments attested the formation of a 2 : 1 7-Te-I À complex and molecular dynamics simulations revealed that the I À guest is actually held by an average of five ChBs (Figure 6B).
The ITC technique was also used to determine the binding affinity of a few supramolecular assemblies, [26] including the supramolecular polymers between species 4 and 5 (see Figure 5). [22]

UV-Visible, Fluorescence and IR Spectroscopies
In a few cases, UV-vis, fluorescence or IR spectroscopies were applied to investigate ChB strength.
In an interesting approach, benzochalcogenadiazoles 8 have been designed to monitor changes in their UV-vis absorbance or emission spectra upon ChB formations in solution. [27]Addition of increasing amount of nBu 4 N + Cl À to 8 in THF solution was accompanied by a chromophore red-shift (Figure 7A).This change could be fitted to a 1 : 1 binding  isotherm by nonlinear regression analysis and allowed to determine a K a of 970 � 10 M À 1 .Such titrations were performed with different substituents and in various solvents.The soobtained K a varied here again according to the solvent coordination ability.
This method, well-suited for the detection of ChB involving π-extended ChB donors, was employed for measuring dissociation constants K D involving dithienothiophene-S,S-dioxides 9 as ChB-based anion transporters [12c] and benzodiselenazoles 10 as ChB catalysts (Figure 7B). [28]Chloride binding to these compounds in THF caused appreciable hypo-and bathochromism in their absorption spectra.For S-compounds 9, the stronger binding was observed with 9 a bearing two strong electronacceptor cyano groups.Substitution of cyano by sulfones (9 b) or aldehydes (9 c) moderately affected their K D .Coherently, removing one cyano acceptor (9 d) resulted in weaker binding.Se-compounds 10 with deeper σ-holes delivered much higher K D , in the micromolar range.In this series, deepening the σ-hole of Se atoms by increasing the oxidation state of the side-sulfur substituents and thus their electron-withdrawing power, induced higher binding to Cl À .
In some cases, the detection of ChB in solution could be achieved by fluorescence spectroscopy.12c] In contrast, fluorescence quenching arose from the ChB formation between benzoselenadiazole-based derivative 11 and trimethylarsine (Me 3 As) (Figure 7C). [30]vertheless, the use of IR spectroscopy is very limited because of the complexity of the recorded spectra at room temperature.It was however successfully used in order to confirm the intramolecular ChB in 1-COCH 3 (Figure 2).Indeed, IR stretching frequencies of the C=O bond in S-and Sederivatives (1709 cm À 1 and 1713 cm À 1 ) were significantly shifted compared to the O-derivative (1748 cm À 1 ), as expected from the weakening of the carbonyl bond arising from n O !σ* S/Se-C delocalization.

Summary and Outlook
The presence of 2 or 3 σ-holes confers to chalcogens more possibilities of binding, while σ-hole deepness can be finetuned according to substituent, anion and solvent nature.In term of strength, ChB is comparable to HB, but its high directionality allows greater precision in 3D control.These interesting properties have initiated over the last five years the increasing ChB exploitation for solution-based applications such as anion recognition, anion transport and catalysis.Nevertheless, it is important to ascertain the real involvement of ChB when moving to applications, especially for (enantioselective) catalyst design.
Although significant progress has been achieved in probing σ-hole based interactions, ChB detection and characterization in solution is still challenging due to its weakness.As shown in this Concept article, multinuclear NMR spectroscopy is currently the method of choice, allowing to access binding constants K by titration and to detect ChB through 1D and 2D techniques.In a few cases, ITC as well as UV-vis, fluorescence or IR spectroscopies have also been used to determine K values and energies of association.