Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Redox-controlled chalcogen-bonding at tellurium: impact on Lewis acidity and chloride anion Cite this: Chem. Sci., 2020, 11, 7495 † All publication charges for this article transport properties have been paid for by the Royal Society of Chemistry Benyu Zhou and François P. Gabba¨ı * Our interests in the chemistry of atypical main group Lewis acids have led us to devise strategies that augment the affinity of chalcogen-bond donors for anionic guests. In this study, we describe the oxidative methylation of diaryltellurides as one such strategy along with its application to the synthesis of + + [Mes(C6F5)TeMe] and [(C6F5)2TeMe] starting from Mes(C6F5)Te and (C6F5)2Te, respectively. These new telluronium cations have been evaluated for their ability to complex and transport chloride anions across phospholipid bilayers. These studies show that, when compared to their neutral Te(II) precursors, these Received 21st May 2020 Te(IV) cations display both higher Lewis acidity and transport activity. The positive attributes of these Accepted 2nd July 2020 telluronium cations, which originate from a lowering of the tellurium-centered s* orbitals and DOI: 10.1039/d0sc02872j Creative Commons Attribution-NonCommercial 3.0 Unported Licence. a deepening of the associated s-holes, demonstrate that the redox state of the main group element rsc.li/chemical-science provides a convenient handle over its chalcogen-bonding properties. The eld of chalcogen (Ch) bonding is in rapid expansion owing (C6F5)2Te is a particularly noteworthy molecule that to applications in supramolecular chemistry, catalysis, and embodies the points discussed above. The chalcogen-bond anion transport.1–11 The formation of chalcogen-bonds, which donor properties of this derivative manifest in the short inter- are dened as secondary bonding interactions between a donor molecular Te–F contacts found in the crystal17 as well as in the ¼ 12 (D) and a chalcogenide (ChR2,R substituent), can be formation of adducts with donors such as tetramethylth- This article is licensed under a 18 rationalized based on the presence of regions of positive elec- iourea. Matile brought (C6F5)2Te back into the spotlight by trostatic potential on the chalcogen atom. These regions oen demonstrating its use in anion binding catalysis19 and chloride referred to as s-holes, sit at the termini of Ch–R polar bonds anion transport.20 The chalcogen-bond donor properties of Open Access Article. Published on 06 July 2020. Downloaded 9/28/2021 8:41:53 AM. and spatially coincide with the largest lobe of the Ch–R s* (C6F5)2Te can be correlated to the electron-withdrawing prop- orbitals (Fig. 1). Investigations into the origin of chalcogen- erties of the pentauorophenyl substituents, which deepen the bonds point to the interplay of electrostatic, dispersion, and s-hole and stabilize the Te–C s* orbital. orbital delocalization effects.13,14 The dispersion term is accen- Motivated by these recent developments, we are charting tuated in the case of soer, inherently polarizable, chalcogens, new strategies to predictably enhance the chalcogen-bond while the electrostatic term can be controlled by adjusting the polarity of Ch–R s-bond. The orbital delocalization term cor- responding to the D/ChR2 interaction responds favourably to a lowering of the Ch–R s* orbital energy, as observed with heavy chalcogens. An analogous dependence is seen with respect to the polarity of the Ch–R bond which, when increased, leads to higher chalcogen parentage of the s* orbital. All three bonding terms culminate at tellurium, the soest, heaviest, and most electropositive chalcogen.2,5,15,16 Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental and computational details. NMR spectra of novel compounds. Anion binding and Fig. 1 (Left) Orbital and electrostatic origin of the Lewis acidity in transport data. Crystallographic data in CIF format. CCDC 2004978–2004980. tellurium derivatives. The intensification of the blue color illustrates an For ESI and crystallographic data in CIF or other electronic format see DOI: energy lowering in the case of the LUMO and a greater positive charge 10.1039/d0sc02872j development at tellurium in the case of the s-hole. This journal is © The Royal Society of Chemistry 2020 Chem. Sci.,2020,11,7495–7500 | 7495 View Article Online Chemical Science Edge Article donor properties of tellurium compounds, with the ultimate The structures of these new telluronium salts have also been aim of reaching bona de Lewis acidity. Based on the knowledge conrmed by X-ray crystallography (Fig. 2).37 In the solid-state, that telluronium cations form partially covalent halide short Te–F contacts between the tellurium atom and the tetra- complexes21–23 and encouraged by recent contributions on the uoroborate counter anion are observed. The shortest contact, effect of oxidation on the Lewis acidity of phosphorus,24–26 which is much smaller than the sum of the van der Waals radii 27–29 30–32 33,34 ˚ 38 antimony, sulfur, and iodine species, we have now of Te and F (Svdw(Te–F) ¼ 3.45 A), is observed in the case of [4] ˚ decided to probe the effect of oxidative alkylation of simple [BF4] which displays a Te/F distance of 2.6902(7) A (Fig. 2). ˚ diaryltellurides as a way to stabilize the accepting Te–C s* This distance is approximately 0.3 A shorter than in [2][BF4] ˚ ˚ ˚ orbital while at the same time deepening the coincident s-hole (3.066(5) A) and 0.1 A shorter than in [3][BF4] (2.7908(15) A), as illustrated in Fig. 1. attesting to the higher Lewis acidity of the tellurium centre in Considering stability, ease of synthesis, and synthetic the bis(pentauorophenyl) cation [4]+. The value of the C11–Te– 3 – – 4 modularity, we decided to target a series of telluronium cations F6 angle in [ ][BF4] (168.56 ) and C8 Te F11 angle in [ ][BF4] obtained by direct methylation of the diaryltellurides shown in (167.14) shows that the uorine atom involved in the short 17,35 Scheme 1. While (C6F5)2Te and Mes2Te are known, we were contact is positioned trans from the pentauorophenyl ring. able to obtain Mes(C6F5)Te (1) by the reaction of a C6F5MgBr This position corresponds to that dened by the lowest-lying s* solution with a solution of MesTeBr generated in situ by the orbital or deepest s-hole in these two salts, underscoring the combination of bromine and dimesitylditelluride. This new importance of the molecular design principles presented in telluride has been characterized by traditional means, Fig. 1. The higher Lewis acidity of the tellurium atom in [4]+ is including multinuclear NMR spectroscopy and elemental further reected by the presence of a second Te/F short analysis. These neutral tellurides were next converted into the contact of 2.814(1) A˚ involving the tetrauoroborate anion of corresponding methyltelluronium species. While the more a neighboring, symmetry equivalent unit. The resulting C2–Te– 0 electron-rich derivatives Mes2Te and 1 could be efficiently F12 angle of 171.86 shows that this second contact occurs 4 + Creative Commons Attribution-NonCommercial 3.0 Unported Licence. methylated using MeI/AgBF4 in CH2Cl2, methylation of trans from the second penta uorophenyl substituent of [ ] .It + (C6F5)2Te only proceeded with Me3O$BF4 upon heating to 90 C follows that [4] could, in principle, support interactions with in a mixture of 1,2-dichloroethane and toluene (vol. 1 : 2). The two anionic guests. harsher conditions used for this last reaction reects the The telluronium cations have also been studied by Density electron-deciency of the tellurium centre induced by the two Functional Theory (M062X functional; mixed basis set: 6- pentauorophenyl groups. The three new telluronium cations 31G(d0)/6-31G+(d0)/cc-pVTZ-PP). To better compare the electro- + + + + + [Mes2TeMe] ([2] ), [Mes(C6F5)TeMe] ([3] ), and [(C6F5)2TeMe] static potential about the tellurium centre, ESP maps were ([4]+) have been isolated as their tetrauoroborate salts and fully plotted on the same scale (Fig. 2). All cations feature characterized. All three salts give well-resolved 1H and 19F-NMR pronounced s-holes which can be conveniently characterized by This article is licensed under a spectra. Installation of the methyl group gives rise to a singlet in their corresponding Vs,max values. The Vs,max values of the two s- the 1H-NMR spectra of these derivatives. This singlet, which holes are almost identical to one another in the case of [2]+ – ¼ À1 4 + ¼ appears in the 3.17 3.45 ppm window for all three salts, is (Vs,max 103.1 and 104.0 kcal mol ) and [ ] (Vs,max 136.0 125 2 À1 Open Access Article. Published on 06 July 2020. Downloaded 9/28/2021 8:41:53 AM. ¼ – anked by Te satellites separated by JTe–H 29.1 31.2 Hz. and 138.3 kcal mol ) which feature two identical aryl substit- 3 + s These coupling constants are consistent with those observed for uents. In the case of [ ] , the Vs max values of the two -holes À known methyltelluronium species.36 The 125Te-NMR resonance (116.3 and 122.3 kcal mol 1) show a larger differentiation, with of the telluronium cations appears at 563.7 ppm for [2]+, the largest value corresponding to the hole trans from the 637.4 ppm for [3]+, and 598.1 for [4]+, a set of values that are electron-withdrawing pentauorophenyl substituent. The distinctly downeld from those of the telluride precursors electron-withdrawing properties of this substituent are also (258.7 ppm for Mes2Te, 280.0 ppm for 1, and 287.1 ppm for responsible for the signicant deepening of the s-holes (or 2 + 4 + (C6F5)2Te).