Fluoride anion complexation and transport using a stibonium cation stabilized by an intramolecular PO → Sb pnictogen bond

Anion transporters based on halogen, chalcogen, and pnictogen bonds: towards biological applications

Motivated by their potential biological applications, anion receptors are increasingly explored as transmembrane transporters for anions. The vast majority of the reported anion transporters rely on hydrogen bonding to interact with the anions. However, in recent decades, halogen, chalcogen, and pnictogen bonding, collectively referred to as sigma-hole interactions, have received increasing attention. Most research efforts on these interactions have focused on crystal engineering, anion sensing, and organocatalysis. In recent years, however, these sigma-hole interactions have also been explored more widely in synthetic anion transporters. This perspective shows why synthetic transporters are promising candidates for biological applications. We provide a comprehensive review of the compounds used to transport anions across membranes, with a particular focus on how the binding atoms and molecular design affect the anion transport activity and selectivity. Few cell studies have been reported for these transporters based on sigma-hole interactions and we highlight the critical need for further biological studies on the toxicity, stability, and deliverability of these compounds to explore their full potential in biological applications, such as the treatment of cystic fibrosis.

A geminal antimony(iii)/phosphorus(iii) frustrated Lewis pair

The geminal Lewis pair (F5C2)2SbCH2P(tBu)2 (1) was prepared by reacting (F5C2)2SbCl with LiCH2P(tBu)2. Despite its extremely electronegative pentafluoroethyl substituents, the neutral 1 exhibits a relatively soft acidic antimony function according to the HSAB concept (hard-soft acid-base). These properties lead to a reversibility in the binding of CS2 to 1, as observed by VT-NMR spectroscopy, while no reaction with CO2 is observed. The reaction behaviour towards heterocumulenes and the specific interaction situation in the CS2 adduct were analysed by quantum chemical calculations. The FLP-type reactivity of 1 has also been demonstrated by reaction with a variety of small molecules (SO2, PhNCO, PhNCS, (MePh2P)AuCl). The reactions of 1 with PhNCO and PhNCS led to different types of cyclic addition products: PhNCO adds with its N[double bond, length as m-dash]C bond and PhNCS adds preferentially with its C[double bond, length as m-dash]S bond. The reaction of 1 with (MePh2P)AuCl gave an adduct {[(F5C2)2SbCH2(tBu)2P]2Au}+ with a clamp-like structure binding a chloride anion by its two antimony atoms in chelate mode. Compound 1 and its adducts have been characterised by X-ray diffraction experiments, multinuclear NMR spectroscopy, elemental analyses and computational calculations (DFT, QTAIM, IQA).

Oxidation-dependent Lewis acidity in chalcogen adducts of Sb/P frustrated Lewis pairs

The reactions of the frustrated Lewis pair (F5C2)2SbCH2P(tBu)2 with oxygen, sulphur, selenium and tellurium led to the mono-oxidation products (F5C2)2SbCH2P(E)(tBu)2 (E = O, S, Se, Te). Further oxidation of these chalcogen adducts with tetrachloro-ortho-benzoquinone (o-chloranil) gave (F5C2)2Sb(CH2)(μ-E)P(tBu)2·CatCl (CatCl = o-O2C6Cl4) with a central four-membered ring heterocycle for E = O, S, and Se. For E = Te the elimination of elemental tellurium led to an oxidation product with two equivalents of o-chloranil, (F5C2)2SbCH2P(tBu)2·2CatCl, which is also accessible by reaction of (F5C2)2SbCH2P(tBu)2 with o-chloranil. The synthesised compounds were characterised by NMR spectroscopy and X-ray structure analyses, and the structural properties were analysed in the light of the altered Lewis acidity due to the oxidation of the antimony atoms.

Selectivity in Adduct Formation of a Bidentate Boron Lewis Acid

A bidentate boron Lewis acid based on 1,8-diethynylanthracene has been studied in detail with respect to its adduct formation with diamines and diphosphanes of different linker lengths between the donor functions. A clear correlation between the linker length of the bifunctional base and the formation of 1 : 1 adducts, 1 : 2 adducts or oligomers was found. The adducts were characterized in solution by NMR titration experiments and structurally by X-ray diffraction. In addition, adduct formation and competition experiments of the host system with ZR3 (Z=N, P; R=H, Me) demonstrated the generally higher stability of alkylphosphane adducts compared to alkylamine adducts with boron functions. The results provide a general insight into the adduct formation of bidentate Lewis acids with guests of different sizes as well as the differences in stability between borane-amine and borane-phosphane adducts.

Bidentate boron Lewis acids: synthesis by tin boron exchange reaction and host-guest complex formation

Four bidentate boron Lewis acids based on the 1,8-diethynylanthracene backbone have been synthesized by a tin-boron exchange reaction with various chloroboranes, yielding the products in good to excellent yields. Complexation experiments of the host compounds with pyridine, pyrimidine and TMEDA demonstrated striking differences in terms of formation and solubility of the supramolecular adducts. The host-guest complexes were investigated by multinuclear NMR spectroscopy and structurally characterized by X-ray diffraction experiments, illustrating the adaptation of the host system upon adduct formation with different neutral guest molecules.

Tunable Pnictogen Bonding at the Service of Hydroxide Transport across Phospholipid Bilayers

Our growing interest in the design of pnictogen-based strategies for anion transport has prompted an investigation into the properties of three simple triarylcatecholatostiboranes (1-3) of the general formula (o-C6Cl4O2)SbAr3 with Ar = Ph (1), o-tolyl (2), and o-xylyl (3) for the complexation and transport of hydroxide across phospholipid bilayers. A modified hydroxypyrene-1,3,6-trisulfonic acid (HPTS) assay carried out in artificial liposomes shows that 1 and 2 are potent hydroxide transporters while 3 is inactive. These results indicate that the steric hindrance imposed by the three o-xylyl groups prevents access by the hydroxide anion to the antimony center. Supporting this interpretation, 1 and 2 quickly react with TBAOH·30 H2O ([TBA]+ = [nBu4N]+) to form the corresponding hydroxoantimonate salts [nBu4N][1-OH] and [nBu4N][2-OH], whereas 3 resists hydroxide coordination and remains unperturbed. Moreover, the hydroxide transport activities of 1 and 2 are correlated to the +V oxidation state of the antimony atom as the parent trivalent stibines show no hydroxide transport activity.

Poly-pnictogen bonding: trapping halide ions by a tetradentate antimony(iii) Lewis acid

A highly halide affine, tetradentate pnictogen-bonding host-system based on the syn-photodimer of 1,8-diethynylanthracene was synthesized by a selective tin-antimony exchange reaction. The host carries four C[triple bond, length as m-dash]C-Sb(C2F5)2 units and has been investigated regarding its ability to act as a Lewis acidic host component for the cooperative trapping of halide ions (F-, Cl-, Br-, I-). The chelating effect makes this host-system superior to its bidentate derivative in competition experiments. It represents a charge-reversed crown-4 and has the ability to dissolve otherwise poorly soluble salts like tetra-methyl-ammonium chloride. Its NMR-spectroscopic properties make it a potential probe for halide ions in solution. Insights into the structural properties of the halide adducts by X-ray diffraction and computational methods (DFT, QTAIM, IQA) reveal a complex interplay of attractive pnictogen bonding interactions and Coulomb repulsion.

A Bidentate Antimony Pnictogen Bonding Host System

A bidentate pnictogen bonding host-system based on 1,8-diethynylanthracene was synthesized by a selective tin-antimony exchange reaction and investigated regarding its ability to act as a Lewis acidic host component for the complexation of Lewis basic or anionic guests. In this work, the novel C≡C-Sb(C2 F5 )2 unit was established to study the potential of antimony(III) sites as representatives for the scarcely explored pnictogen bonding donors. The capability of this partly fluorinated host system was investigated towards halide anions (Cl- , Br- , I- ), dimethyl chalcogenides Me2 Y (Y=O, S, Se, Te), and nitrogen heterocycles (pyridine, pyrimidine). Insights into the adduct formation behavior as well as the bonding situation of such E⋅⋅⋅Sb-CF moieties were obtained in solution by means of NMR spectroscopy, in the solid state by X-ray diffraction, by elemental analyses, and by computational methods (DFT, QTAIM, IQA), respectively.

Cross-Electrophile C-PIII Coupling of Chlorophosphines with Organic Halides: Photoinduced PIII and Aminoalkyl Radical Generation Enabled by Pnictogen Bonding

Pnictogen bonding (PnB) has gained recognition as an appealing strategy for constructing novel architectures and unlocking new properties. Within the synthetic community, the development of a straightforward and much simpler protocol for cross-electrophile C-PIII coupling remains an ongoing challenge with organic halides. In this study, we present a simple strategy for photoinduced PnB-enabled cross-electrophile C-PIII couplings using readily available chlorophosphines and organic halides via merging single electron transfer (SET) and halogen atom transfer (XAT) processes. In this photomediated transformation, the PnB formed between chlorophosphines and alkyl amines facilitates the photogeneration of PIII radicals and α-aminoalkyl radicals through SET. Subsequently, the resulting α-aminoalkyl radicals activate C-X bonds via XAT, leading to the formation of carbon radicals. This methodology offers operational simplicity and compatibility with both aliphatic and aromatic chlorophosphines and organic halides.

Binding, Sensing, And Transporting Anions with Pnictogen Bonds: The Case of Organoantimony Lewis Acids

Motivated by the discovery of main group Lewis acids that could compete or possibly outperform the ubiquitous organoboranes, several groups, including ours, have engaged in the chemistry of Lewis acidic organoantimony compounds as new platforms for anion capture, sensing, and transport. Principal to this approach are the intrinsically elevated Lewis acidic properties of antimony, which greatly favor the addition of halide anions to this group 15 element. The introduction of organic substituents to the antimony center and its oxidation from the + III to the + V state provide for tunable Lewis acidity and a breadth of applications in supramolecular chemistry and catalysis. The performances of these antimony-based Lewis acids in the domain of anion sensing in aqueous media illustrate the favorable attributes of antimony as a central element. At the same time, recent advances in anion binding catalysis and anion transport across phospholipid membranes speak to the numerous opportunities that lie ahead in the chemistry of these unique main group compounds.

Synthesis, Structure, Reactivity, and Intramolecular Donor-Acceptor Interactions in a Phosphinoferrocene Stibine and Its Corresponding Phosphine Chalcogenides and Stiboranes

Ferrocene-based phosphines equipped with additional functional groups are versatile ligands for coordination chemistry and catalysis. This contribution describes a new compound of this type, combining phosphine and stibine groups at the ferrocene backbone, viz. 1-(diphenylphosphino)-1'-(diphenylstibino)ferrocene (1). Phosphinostibine 1 and the corresponding P-chalcogenide derivatives Ph2P(E)fcSbPh2 (1E, fc = ferrocene-1,1'-diyl, E = O, S, Se) were synthesized and further converted to the corresponding stiboranes Ph2P(E)fcSb(O2C6Cl4)Ph2 (6 and 6E) by oxidation with o-chloranil. All compounds were characterized by spectroscopic methods, X-ray diffraction analysis, cyclic voltammetry, and theoretical methods. Both NMR spectroscopy and DFT calculations confirmed the presence of P → Sb and P═O → Sb donor-acceptor interactions in 6 and 6O, triggered by the oxidation of the stibine moiety into Lewis acidic stiborane. The corresponding interactions in 6S and 6Se were of the same type but significantly weaker. A coordination study with AuCl as the model metal fragment revealed that the phosphine group acts as the "primary" coordination site, in line with its higher basicity. The obtained Au(I) complexes were applied as catalysts in the Au-catalyzed cyclization of N-propargylbenzamide and in the oxidative [2 + 2 + 1] cyclization of ethynylbenzene with acetonitrile and pyridine N-oxides. The catalytic results showed that the stibine complexes had worse catalytic performance than their phosphine counterparts, most likely due to the formation of weaker coordination bonds and hence poorer stabilization of the active metal species. Nevertheless, the stibine moiety could be used to fine-tune the properties of the ligated metal center by changing the oxidation state or substituents at the "remote" Sb atom.

N-Heterocyclic Nitrenium-Catalyzed Photohomolysis of CF3SO2Cl for Alkene Trifluoromethylation

N-Heterocyclic nitreniums (NHNs) have been utilized as Lewis acid catalysts to activate substrates with lone pairs. Alternative to their conventional applications, we have discovered that NHNs can also serve as charge transfer complex catalysts. Herein, we present another potential of NHNs by utilizing a weak interaction between NHNs and CF3SO2Cl. The method promotes CF3SO2Cl to undergo photohomolysis, resulting in the CF3 radical. Mechanistic studies suggested that the weak interaction could be due to the π-hole effect of NHNs.

Analyzing Fluoride Binding by Group 15 Lewis Acids: Pnictogen Bonding in the Pentavalent State

We report the results of a computational investigation into fluoride binding by a series of pentavalent pnictogen Lewis acids: pnictogen pentahalides (PnX5), tetraphenyl pnictogeniums (PnPh4+), and triphenyl pnictogen tetrachlorocatecholates (PnPh3Cat). Activation strain and energy decomposition analyses of the Lewis adducts not only clearly delineate the electrostatic and orbital contributions to these acid-base interactions but also highlight the importance of Pauli repulsion and molecular flexibility in determining relative Lewis acidity among the pnictogens.

Transmembrane transport of fluoride studied by time-resolved emission spectroscopy

Here we present a new method to monitor fluoride transmembrane transport into liposomes using a europium(III) complex. We take advantage of the long emission lifetime of this probe to measure the transport activity of a fluorescent transporter. The high sensitivity, selectivity, and versatility of the assay allowed us to study different types of fluoride transporters and unravel their mechanisms of action.

Involvement of Arsenic Atom of AsF3 in Five Pnicogen Bonds: Differences between X-ray Structure and Theoretical Models

Bonding within the AsF3 crystal is analyzed via quantum chemical methods so as to identify and quantify the pnicogen bonds that are present. The structure of a finite crystal segment containing nine molecules is compared with that of a fully optimized cluster of the same size. The geometries are qualitatively different, with a much larger binding energy within the optimized nonamer. Although the total interaction energy of a central unit with the remaining peripheral molecules is comparable for the two structures, the binding of the peripherals with one another is far larger in the optimized cluster. This distinction of much stronger total binding within the optimized cluster is not limited to the nonamer but repeats itself for smaller aggregates as well. The average binding energy of the cluster rises quickly with size, asymptotically approaching a value nearly triple that of the dimer.

Halonium, chalconium, and pnictonium salts as noncovalent organocatalysts: a computational study on relative catalytic activity

This theoretical study sheds light on the relative catalytic activity of pnictonium, chalconium, and halonium salts in reactions involving elimination of chloride and electrophilic activation of a carbonyl group. DFT calculations indicate that for cationic aromatic onium salts, values of the electrostatic potential on heteroatom σ-holes gradually increase from pnictogen- to halogen-containing species. The higher values of the potential on the halogen atoms of halonium salts result in the overall higher catalytic activity of these species, but in the case of pnictonium and chalconium cations, weak interactions from the side groups provide an additional stabilization effect on the reaction transition states. Based upon quantum-chemical calculations, the catalytic activity of phosphonium(V) and arsenonium(V) salts is expected to be too low to obtain effective noncovalent organocatalytic compounds, whereas stibonium(V), telluronium(IV) and iodonium(III) salts exhibit higher potential in application as noncovalent organocatalysts.