Fluoride ion chelation by a bidentate phosphonium/borane Lewis acid

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.

The Effect of Carborane Substituents on the Lewis Acidity of Boranes

The Lewis acidity of primary, secondary, and tertiary boranes with phenyl, pentafluorophenyl, and all three isomers of the C-substituted icosahedral carboranes (ortho, meta, and para) was investigated by computing their fluoride, hydride, and ammonia affinities as well as their global electrophilicity indices and LUMO energies. From these calculations, it was determined that the substituent effects on the Lewis acidity of these boranes follow the trend of ortho-carborane > meta-carborane > para-carborane > C6F5 > C6H5.

Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid

The synthesis of tris(ortho-carboranyl)borane (BoCb₃ ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb₃ is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C₆ F₅ )₃ ].

Triarylborane-triphenylamine based luminophore for the mitochondria targeted live cell imaging and colorimetric detection of aqueous fluoride

Bioimaging of subcellular organelles such as mitochondria is crucial for detecting physiological abnormalities induced by fluctuations in the levels of various analytes. Herein, we report the design and synthesis of two novel water-soluble cationic Lewis acid triarylborane-triarylamine conjugates 1 and 2. The optical characteristics of 1 and 2 and their precursor compounds BTPA-NMe2 and BTPA-2NMe2 were evaluated, which show similar absorption and fluorescence spectra, with 1 and 2 exhibiting higher quantum yields of 0.73 and 0.64, respectively, than those of the precursors BTPA-NMe2 and BTPA-2NMe2, indicating the partial disruption of the ICT process and the activation of alternative emission bands in 1 and 2. The live cell imaging ability of compound 2 was examined in HeLa cells using a confocal microscope. Moreover, mitochondrial internalisation using compound 2 was effective and it was found to have high photostability under UV light conditions. Furthermore, compound 2 demonstrated an evident colorimetric response with a colour change to dark yellow in aqueous environments, indicating that it could be used for anion sensing. The spectral changes were observed in UV-visible and fluorescence titration experiments, which were strongly supported by DFT calculations. In short, compound 2 synthesized by us can be exclusively utilized for the selective localization of mitochondria with less cytotoxicity and shows excellent colorimetric response to aqueous inorganic fluoride at levels as low as 0.1 ppm with high selectivity.

Fluoride ion sensing with an acridinium borane

With our continuing interest in the chemistry of cationic boranes, we have synthesizedthe tetrafluoroborate salt of 1-dimesitylboron-4-(N-methyl-9-acridinium)-phenylene which acts as a turn-on fluoride anion sensor, visibly changing from yellow to orange upon binding fluoride. To understand this reactivity, we spectroscopically and computationally analyzed the cation and triarylfluoroborate adduct. UV-vis spectroscopy and TD-DFT revealed the basis of the color change to be a red shift in a low-energy absorption band resulting from intramolecular charge-transfer. Electrochemical studies were undertaken to further probe this system. Cyclic voltammetry indicated a reversible one-electron reduction for the cation and a cathodic shift of -0.12 V in the first reduction wave upon fluoride binding. Chemical reduction of the cation yielded the acridine borane radical which was verified by EPR spectroscopy.

Relative rates of alkylation for B-substituted triarylphosphines: an ortho-Boron group enhances reactivity on phosphorus

Advancements in main-group catalysis are contingent on our ability to quantify effects that enhance reactivity in these systems. Herein we report the rates of alkylation for several substituted phosphines. We report that by incorporating a single pinacol boronic ester group in the ortho-position on triphenylphosphine, the rate of substitution with benzyl bromide is approximately 4.7 times faster than the parent compound as measured by initial rates. The corresponding meta- and para-isomers are only 1.3 and 1.5 times as fast, respectively. Using X-ray crystallographic data and quantum chemical calculations, we propose this rate acceleration occurs from an O to P electrostatic interaction that stabilizes the transition state.

Carbon Dioxide Reduction by an Al–O–P Frustrated Lewis Pair

The reaction of tBu₂P(O)H with Bis₂AlH (Bis = CH(SiMe₃)₂) afforded the adduct tBu₂P(H)–O–Al(H)Bis₂ (3). It slowly releases H₂ to form the first oxygen-bridged geminal Al/P frustrated Lewis pair tBu₂P–O–AlBis₂. It is capable of reversibly binding molecular hydrogen to afford 3, shown by NMR and H/D scrambling experiments, and forms a 1,2-adduct with CO₂. Importantly, the H₂ adduct 3 reduces CO₂ in a stoichiometric reaction leading to the formic acid adduct tBu₂P(H)–O–Al(CO₂H)Bis₂. The formation of the different species was explored by density functional theory calculations which provide support for the experimental results. All products were characterized by NMR spectroscopy as well as X-ray diffraction experiments and elemental analyses.

Addition vs. reduction: the geminal FLP Bis₂Al–O–PtBu₂ can reversibly bind molecular hydrogen, it reacts with CO₂ to give an adduct, and its hydrogen adduct reduces CO₂ to an adduct of formic acid.

A free boratriptycene-type Lewis superacid

An exceptionally strong ferrocene-containing, cationic boratriptycene-type Lewis acid is stabilized by a weak Fe⋯B through-space interaction.

Proton to hydride umpolung at a phosphonium center via electron relay: a new strategy for main-group based water reduction

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H-OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH-) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.

Polymeric Membrane Fluoride-Selective Electrodes Using Lewis Acidic Organo-Antimony(V) Compounds as Ionophores

Four Lewis acidic organo-antimony(V) compounds with strong binding affinity to fluoride were used for the first time as ionophores to fabricate polymeric membrane fluoride-selective electrodes. Improved detection limits and significant anti-Hofmeister selectivity could be achieved by optimizing ionophores, lipophilic additives, and plasticizers. Membrane electrodes fabricated with tetrakis-(pentafluorophenyl)stibonium (ionophore 2) performed best in detection limit, sensitivity, and selectivity. Optimal performance was obtained by fluoride with a slope of -59.5 mV/decade in the linear range of 1 × 10^-5 to 4 × 10^-2 M and a detection limit of 5 × 10^-6 M. Studies on the influence of sample solution pH demonstrate that the best pH for fluoride determination is pH 3.0. All of the electrodes studied respond rapidly (in 1 min) in different concentrations of fluoride solutions. The anion-ionophore complex constants in the membrane phase determined using the segmented sandwich membrane method correlate well with the solution-phase binding data and determined selectivity sequence of the ion-selective electrodes. The possibility of real life application of the optimized electrodes was assessed by determination of fluoride concentrations in tap water.

Understanding the Diverse Reactivity of Pentaphenylborole toward Epoxides

Density functional theory calculations have been performed to study the diverse reactivity of pentaphenylborole toward different epoxides. We systematically investigated the effect of substituents on epoxides for the preference/competition of three experimentally observed pathways, that is, intramolecular proton transfer, direct ring expansion via insertion of one epoxide molecule, and ring expansion via insertion of two epoxide molecules. Our calculations also predicted a high competitivity between the proton transfer and direct ring expansion pathways for the epoxide containing both alkyl and aryl substituents.

Arylisoquinoline-derived organoboron dyes with a triaryl skeleton show dual fluorescence

Four new dyes that derive from borylated arylisoquinolines were prepared, containing a third aryl residue (naphthyl, 4-methoxynaphthyl, pyrenyl or anthryl) that is linked via an additional stereogenic axis. The triaryl cores were synthesized by Suzuki couplings and then transformed into boronic acid esters by employing an Ir(I)-catalyzed reaction. The chromophores show dual emission behavior, where the long-wavelength emission band can reach maxima close to 600 nm in polar solvents. The fluorescence quantum yields of the dyes are generally in the range of 0.2-0.4, reaching in some cases values as high as 0.5-0.6. Laser-flash photolysis provided evidence for the existence of excited triplet states. The dyes form fluoroboronate complexes with fluoride anions, leading to the observation of the quenching of the long-wavelength emission band and ratiometric response by the build-up of a hypsochromically shifted emission signal.

β-carboline-based turn-on fluorescence chemosensor for quantitative detection of fluoride at PPB level

A novel β-carboline-based chemosensor, having an acidic NH proton that leads to fluoride-induced deprotonation involving a vivid color change from colorless to yellow is described. The absorption spectrum of the chemosensor in acetonitrile has a peak at 375 nm, which changes to 428 nm with the gradual addition of only fluoride in the solution with a clear isosbestic points at 357 nm and 392 nm. More interestingly, the chemosensor gives a turn-on type of fluorescence at 554 nm in the presence of fluoride. Further it was found that the sensor is highly selective towards fluoride over other anions including chloride, bromide, iodide, nitrate, borate, perchlorate and can quantitatively detect fluoride at ppb level with a limit of detection of 0.02 mg/ L or 20 ppb. The chemosensor was successfully demonstrated to assess the fluoride concentration in the tap water.

Room temperature phosphorescent triarylborane functionalized iridium complexes

We report a series of room temperature phosphorescent compounds 1-6 composed of triarylborane (TAB) and cyclometallated iridium complexes. The optical characteristics such as energy of transition and luminescence quantum yield of these compounds can be conveniently fine-tuned by judiciously varying the cyclometallating ligand and the spacer between boron and iridium centers. Compounds 1-6 exhibit bright phosphorescence with the emission color ranging from green to red under a N2 atmosphere. A maximum quantum yield of 0.95 was observed for complex 2. Complexes 1-6 exhibit a rare type of dual emission from singlet and triplet excited states under ambient conditions. The experimental observations are well supported by the theoretical calculations.

DNA-templated copper nanoclusters as a fluorescent probe for fluoride by using aluminum ions as a bridge

A selective fluorescent on-off-on probe has have designed for the detection of fluoride (F^-) ions based on DNA-templated copper nanocluster (CuNCs) and by using aluminum(III) ions as a bridge. A 40-mer polythymine acts as a template for the reduction of Cu(II) to Cu(0) by ascorbic acid. This result is the formation of red fluorescent CuNCs, with excitation/emission peaks at 340/640 nm. After addition of Al³⁺ ions, the fluorescence of CuNCs is quenched because the interaction of Al³⁺ and DNA disturbs the formation of DNA-templated CuNCs. Fluorescence is restored on addition of fluoride to the system. This is due to the desorption of Al³⁺ from the DNA and the formation of the Al(OH)₃F^- complex. This system displays a fast fluorometric response to fluoride, with high selectivity over other anions. Fluorescence increases linearly in the 2 to 150 μM F^- concentration range, and the detection limit is 1.0 μM. This probe has been successfully used for the detection of F^- ions in four brands of toothpaste. The method is rapid, cost-effective, selective, and does not require toxic solvents and reagents. Graphical abstract Schematic presentation of a method for fluorometric determination of fluoride by using DNA-templated copper nanoclusters (CuNCs) and using aluminum(III) as a bridge. The red fluorescence of the CuNCs is quenched in the presence of Al(III) ions but restored after addition of fluoride.

Supramolecular Assembly of Phosphole Oxide Based Alkynylplatinum(II) 2,6-Bis(N-alkylbenzimidazol-2'-yl)pyridine Complexes-An Interplay of Hydrophobicity and Aromatic π-Surfaces

A new class of phosphole oxide based alkynylplatinum(II) 2,6-bis(N-alkylbenzimidazol-2'-yl)pyridine (bzimpy) complexes were synthesized and characterized. Their self-assembly was driven by hydrophobic-hydrophobic and π-π stacking interactions. The self-assembly properties were also investigated by UV/Vis absorption spectroscopy, which revealed that the alkyl-chain length of the bzimpy moiety and the π-surface area of the alkynyl ligand have significant influence on the overall self-assembly process. The alkyl-chain length also affected the morphological structures of the aggregates, which were studied by transmission electron microscopy and scanning electron microscopy.

Implementation of a logic gate by chemically induced nitrogen and oxygen rich C-dots for the selective detection of fluoride ions

The widespread pollution of fluoride ions in the environment badly affects the ecological system due to their high toxicity, mobility and the difficulty of their degradation.

Anion Sensing with a Blue Fluorescent Triarylboron-Functionalized Bisbenzimidazole and Its Bisbenzimidazolium Salt

A blue fluorescent p-dimesitylboryl-phenyl-functionalized 1,3-bisbenzimidazolyl benzene molecule (1) has been synthesized in high yield by Stille coupling of bisbenzimidazolyl bromobenzene with p-BMes₂-SnBu₃-benzene. Methylation of 1 led to the formation of the bisbenzimidazolium salt (2). The utility of both 1 and 2 in sensing CN^- and halide (F^-, Cl^-, Br^-, and I^-) was examined, and it was found that only the small fluoride and cyanide anions were able to bind to the boron atom with binding constants in the range of 2.9 × 10⁴ to 5 × 10⁵ M^-1. Computational studies provided insight into the photophysical properties of the molecules and verified that a charge-transfer process is quenched in these "turn-off" molecular sensors.

Desymmetrization of Bisallylic Amides through Catalytic Enantioselective Bromocyclization with BINAP Monoxide

We report the first desymmetrization of bisallylic amides by enantioselective bromocyclization with BINAP monoxide as a catalyst. Depending upon the substitution pattern of the alkene moieties, densely functionalized, optically active oxazoline or dihydrooxazine compounds were obtained in a highly stereoselective manner. The remaining alkene moiety was subjected to various functional group manipulations to afford a diverse array of chiral molecules with multiple stereogenic centers.

Radiofluorination of a NHC-PF5 adduct: toward new probes for 18F PET imaging

The radiofluorination of N-heterocyclic carbene (NHC) phosphorus(v) fluoride adducts has been investigated. The results show that the IMe-PF₅ derivative (IMe = 1,3-dimethylimidazol-2-ylidene) undergoes a Lewis acid promoted ¹⁸F-¹⁹F isotopic exchange. The resulting radiofluorinated probe is remarkably resistant to hydrolysis both in vitro as well as in vivo.

Green, Water-Dispersible Photoluminescent On-Off-On Probe for Selective Detection of Fluoride Ions

Considering the high toxicity and widespread availability of fluoride ions in different environmental matrices, it is imperative to design a probe for its detection. In view of this, a selective fluorescent on-off-on probe based on carbon quantum dots (CQDs) and Eu³⁺ has been designed. We have synthesized water-soluble carboxylic acid-functionalized CQDs and monitored their interaction with Eu³⁺. Luminescence quenching in the CQD emission was observed (switch-off) on adding Eu³⁺ ions. We investigate the reason for this luminescence quenching using time-resolved emission and high-resolution transmission electron microscopy (HRTEM) studies and observed that both electron transfer from CQDs to Eu³⁺ and aggregation of CQDs are responsible for the luminescence quenching. ζ-Potential and X-ray photoelectron spectroscopy studies confirm Eu³⁺ binding with the COOH groups on CQD surface. Interestingly, luminescence regains after the addition of fluoride ions to the CQDs/Eu³⁺ system (switch-on). This has been assigned to the removal of Eu³⁺ from the CQD surface due to the formation of EuF₃ and is confirmed by X-ray diffraction and HRTEM measurements. The sensitivity of the probe was tested by carrying out experiments with other competing ions and was found to be selective for fluoride ions. Experiments with variable concentrations of fluoride ions suggest that the working range of the probe is 1-25 ppm. The probe has been successfully tested for the detection of fluoride ions in a toothpaste sample and the results were compared to those of ion chromatography. To the best of our knowledge, this is the first report based on CQDs and Eu³⁺ for the detection of fluoride ions, wherein a clear mechanism of the detection has been demonstrated, which, in turn, will help to develop better detection methods. The suggested probe is green, economical, rapid, efficient, and, most importantly, selective and can be used for the detection of fluoride ions in real environmental samples.

A model for C–F activation by electrophilic phosphonium cations

The electrophilic phosphonium cation (EPC) salt [C₁₀H₆(CF₃)PF(C₆F₅)₂][B(C₆F₅)₄] 4 exhibited structural and spectroscopic features evidencing an interaction between the CF₃ and fluorophosphonium units. It thus models a key step in the proposed mechanism of main group C–F activation.

Synthesis and oxidation of phosphine cations

Cationic phosphines of the form [(L)PPh₂]⁺ are prepared from Ph₂PCl and carbenes (L), including a chiral bis(oxazoline)-based carbene, a cyclic(alkyl)(amino) carbene, and a 1,2,3-triazolium-derived carbene. A related dication was prepared from PhPCl₂ and a bis-carbene. The monocations, but not the dication, can be oxidized with XeF₂.

A thermochromic europium(iii) room temperature ionic liquid with thermally activated anion–cation interactions

An observable and reversible case of thermochromism due to an unusual interaction between the countercation and the ligand of an europium(iii) tetrakis-β-diketonate room temperature ionic liquid is reported.

[18F]-Fluoride capture and release: azeotropic drying free nucleophilic aromatic radiofluorination assisted by a phosphonium borane

[¹⁸F]-Fluoride ready for SNAr was prepared according to a simple process including trapping of aqueous [¹⁸F]-fluoride on a cartridge pre-loaded with the phosphonium borane [(Ph₂MeP)C₆H₄(BMes₂)]⁺, then releasing by elution of TBACN in dry acetonitrile.

The Chemistry of a Non-Interacting Vicinal Frustrated Phosphane/Borane Lewis Pair

The dimesitylphosphinocyclopentene/HB(C₆ F₅ )₂ -derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane-borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO₂ . It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C₆ F₅ )₂ ] to generate a FLP-η² -formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.