Fluoride Anion Complexation by a Triptycene-Based Distiborane: Taking Advantage of a Weak but Observable C−H⋅⋅⋅F Interaction

Reactions of 1,1- and 1,2-bis-boranes with a protic diamine and hydrazine

The reactions of 1,1- and 1,2-bis-boranes, PhCH2CH(B(C6F5)2)21 and Me3SiCH(B(C6F5)2)CH2B(C6F5)22, with o-phenylenediamine and hydrazine are reported. The coordination of o-phenylenediamine gives the expected adducts, PhCH2CH(B(C6F5)2(H2N))2C6H44 and Me3SiCH(B(C6F5)2)CH2B(C6F5)2((H2N)2C6H4) 6. Under thermal duress, 4 undergoes retrohydroboration, resulting in the reaction of the diamine with HB(C6F5)2 to give (C6F5)2B(NH)(NH2)C6H45. In contrast, 6 undergoes protolytic loss of C6F5H, affording Me3SiCH(B(C6F5)2)CH2(B(C6F5)(NH2)(NH)C6H4) 7. In a related fashion, the bis-boranes react with hydrazine. In the case of bis-borane 1, the isolated adduct PhCH2CH(B(C6F5)2(NH2))2·THF 8 can be deprotonated to give species 9. In contrast, the reaction of 2 proceeds with rapid protonolysis, leading to the formation of Me3SiCH(B(C6F5)2)CHB(C6F5)(N2H3)·THF 10. This study illustrates both the reaction pathways accessible from the adducts of bis-boranes and the impact of coordination to boron on the acidity of NH groups.

Adducts of Lewis acidic stibanes with phosphane chalcogenides

Starting from ClSbRF2 (RF = C2F5, 3,5-(CF3)2C6H3) and H(E)P(tBu)2 (E = O, S), we prepared the oxy- and sulphanediyl-bridged adducts RF2Sb(Cl)-E-(H)P(tBu)2, which are stable against the elimination of HCl. The different electron-withdrawing substituents and chalcogen bridging units influence the size of the Sb-E-P angle. ClSb(C2F5)2 and nBu3SnH react to give HSb(C2F5)2, which seems to interact weakly with H(O)P(tBu)2 in solution as observed via NMR. All products were characterised by NMR spectroscopy and all the stable ones were additionally characterised by X-ray diffraction and elemental analyses.

Cone Angles Quantify and Predict the Affinity and Reactivity of Anion Complexes between Trifluoroborates and Rigid Macrocycles

Steric manipulation is a known concept in molecular recognition but there is currently no linear free energy relationship correlating sterics to the stability of receptor-anion complexes nor to the reactivity of the bound anion. By analogy to Tolman cone angles in cation coordination chemistry, we explore how to define and correlate cone angles of organo-trifluoroborates (R-BF3 -) to the affinities observed for cyanostar-anion binding. We extend the analogy to a rare investigation of the anion's reactivity and how it changes upon binding. The substituent on the anion is used to define the cone angle, θ. A series of 10 anions were studied including versions with ethynyl, ethylene, and ethyl substituents to tune steric bulk across the sp, sp2 and sp3 hybridized α-carbons bearing 0, 1 and 2 hydrogen atoms. A linear relationship between affinity and cone angle is observed for anions bearing substituents larger than the -BF3 - headgroup. This correlation predicted affinities of two new anions to within ±5 %. We explored how complexation affects the reactivity of fluoride exchange. The yield of fluoride transfer from R-BF3 - to Lewis acid triphenylborane is correlated with cone angle. We predict that other rigid macrocycles, like commercially available bambusuril, could follow these trends.

Facile Access to Organostibines via Selective Organic Superbase Catalyzed Antimony-Carbon Protonolysis

The selective formation of antimony-carbon bonds via organic superbase catalysis under metal- and salt-free conditions is reported. This novel approach utilizes electron-deficient stibine, Sb(C6F5)3, to give upon base-catalyzed reactions with weakly acidic aromatic and heteroaromatic hydrocarbons access to a range of new aromatic and heteroaromatic stibines, respectively, with loss of C6HF5. Also, the significantly less electron-deficient stibines, Ph2SbC6F5 and PhSb(C6F5)2 smoothly underwent base-catalyzed exchange reactions with a range of terminal alkynes to generate the stibines of formulae PhSb(C≡CPh)2, and Ph2SbC≡CR [R=C6H5, C6H4-NO2, COOEt, CH2Cl, CH2NEt2, CH2OSiMe3, Sb(C6H5)2], respectively. These formal substitution reactions proceed with high selectivity as only the C6F5 groups serve as a leaving group to be liberated as C6HF5 upon formal proton transfer from the alkyne. Kinetic studies of the base-catalyzed reaction of Ph2SbC6F5 with phenyl acetylene to form Ph2SbC≡CPh and C6HF5 suggested the empirical rate law to exhibit a first-order dependence with respect to the base catalyst, alkyne and stibine. DFT calculations support a pathway proceeding via a concerted σ-bond metathesis transition state, where the base catalyst activates the Sb-C6F5 bond sequence through secondary bond interactions.

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).

Host-guest chemistry of tridentate Lewis acids based on tribenzotriquinacene

Flexible poly-Lewis acids (PLA) based on the tribenzotriquinacene (TBTQ) scaffold have been synthesised. Hydrosilylation of 4b,8b,12b-triallyltribenzotriquinacene and subsequent exchange of the chlorine substituents with weaker coordinating triflate groups afforded a novel triple silyl-functionalised PLA. By regioselective hydroboration of triallyl-TBTQ with various organoboranes, PLAs with different Lewis acidities were obtained. The synthesised PLAs were combined with neutral bases in host-guest experiments. DOSY NMR spectroscopy was performed to elucidate the complexation process in solution. These experiments revealed a highly dynamic interaction between the boron-functionalised PLA and triazine. However, the addition of one equivalent of tris(dimethylphosphino(methyl))phenylsilane led to the formation of a 1 : 1 adduct, which was confirmed by diffusion experiments.

Tridentate Lewis Acids Based on Tribenzotriquinacene Chalices

Chalice-shaped tridentate poly-Lewis acids (PLA) based on the tribenzotriquinacene (TBTQ) scaffold have been synthesised. Stannylation of the alkyne units, attached via phenyl-spacers to the benzhydrylic positions to the TBTQ scaffold, with Me2NSnMe3 afforded the trimethyltin substituted TBTQ derivative. Replacement of these tin functions with other elements resulted in rigid boron- and aluminium-functionalised PLAs. More flexible PLAs were obtained by hydrometallation reactions of the terminal alkyne groups of 4b,8b,12b-tris((ethynyl)phenyl)tribenzotriquinacene. The resulting poly-Lewis acids were tested for their acceptor abilities in host-guest experiments with suitable bases. Preliminary tests with pyridine led to the synthesis of a large tridentate base with three pyridyl groups attached to a TBTQ backbone. Complexation of this Lewis base with the PLAs resulted in the formation of aggregates, which were studied in solution in more detail by 1H DOSY NMR experiments regarding their size. Further experiments were performed with the tridentate bases 1,4,7-trimethyl-1,4,7-triazacyclononane and tris((dimethylphosphino)methyl)phenylsilane.

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.

Pnictogen bonding at the service of gold catalysis: the case of a phosphinostiborane gold complex

The search for alternative gold catalyst activators has led us to consider the design of platforms in which a phosphine gold chloride moiety could be activated via formation of a pnictogen bond with a neighboring antimony unit. Here, we describe that such a system can be accessed from 4-(diphenylphosphino)-5-(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene, by oxidation of the stibine with 3,5-di-tert-butyl-o-benzoquinone and by coordination of an AuCl unit to the phosphine. This strategy affords a complex in which a Lewis acidic or pnictogen-bond donor catecholatostiborane unit flanks the adjacent gold chloride moiety. This design impacts the catalytic reactivity of the gold center, as reflected by the ability of this complex to catalyze propargyl amide cyclization reactions. Comparisons with a phosphinostiborane ferrocene analog and computations point to the formation of an intramolecular Au-Cl → Sb(V) interaction as responsible for the observed catalytic 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.

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.

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.

Solution Studies of a Water-Stable, Trivalent Antimony Pnictogen Bonding Anion Receptor with High Binding Affinities for CN-, OCN-, and OAc. Inorg Chem 2023. [PMID: 37499143 DOI: 10.1021/acs.inorgchem.3c01887] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The solution phase anion binding behavior of a water-stable bidentate pnictogen bond donor was studied. A modest change in the visible absorption spectrum allowed for the determination of the binding constants. High binding constants were observed with cyanide, cyanate, and acetate, and these were corroborated with density functional theory (DFT) calculations. The receptor could be recovered free from the anion following treatment with methyl triflate, confirming that it remains intact. The tight binding of cyanide and water stability were exploited to use this system as a supramolecular catalyst in a phase-transfer Strecker reaction, further demonstrating the utility of pnictogen bonding as a tool in noncovalent catalysis.

Four-Electron Reduction of O2 Using Distibines in the Presence of ortho-Quinones

This study, which aims to identify atypical platforms for the reduction of dioxygen, describes the reaction of O2 with two distibines, namely, 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene and 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethyldihydroacridine, in the presence of an ortho-quinone such as phenanthraquinone. The reaction proceeds by oxidation of the two antimony atoms to the + V state in concert with reductive cleavage of the O2 molecule. As confirmed by 18O labeling experiments, the two resulting oxo units combine with the ortho-quinone to form an α,α,β,β-tetraolate ligand that bridges the two antimony(V) centers. This process, which has been studied both experimentally and computationally, involves the formation of asymmetric, mixed-valent derivatives featuring a stibine as well as a catecholatostiborane formed by oxidative addition of the quinone to only one of the antimony centers. Under aerobic conditions, the catecholatostiborane moiety reacts with O2 to form a semiquinone/peroxoantimony intermediate, as supported by NMR spectroscopy in the case of the dimethyldihydroacridine derivative. These intermediates swiftly evolve into the symmetrical bis(antimony(V)) α,α,β,β-tetraolate complexes via low barrier processes. Finally, the controlled protonolysis and reduction of the bis(antimony(V)) α,α,β,β-tetraolate complex based on the 9,9-dimethylxanthene platform have been investigated and shown to regenerate the starting distibine and the ortho-quinone. More importantly, these last reactions also produce two equivalents of water as the product of O2 reduction.

Binuclear Triphenylantimony(V) Catecholates through N-Donor Linkers: Structural Features and Redox Properties

A series of binuclear triphenylantimony(V) bis-catecholato complexes 1–11 of the type (Cat)Ph₃Sb-linker-SbPh₃(Cat) was prepared by a reaction of the corresponding mononuclear catecholates (Cat)SbPh₃ with a neutral bidentate donor linker ligands pyrazine (Pyr), 4,4′-dipyridyl (Bipy), bis-(pyridine-4-yl)-disulfide (PySSPy), and diazobicyclo[2,2,2]octane (DABCO) in a dry toluene: Cat = 3,6-di-tert-butyl-catecholate (3,6-DBCat), linker = Pyr (1); PySSPy (2); Bipy (3); DABCO (4); Cat = 3,5-di-tert-butyl-catecholate (3,5-DBCat), linker = Bipy (5); DABCO (9); Cat = 4,5-(piperazine-1,4-diyl)-3,6-di-tert-butylcatecholate (pip-3,6-DBCat), linker = Bipy (6); DABCO (10); Cat = 4,5-dichloro-3,6-di-tert-butylcatecholate (4,5-Cl₂-3,6-DBCat), linker = Bipy (7); DABCO (11); and Cat = 4,5-dimethoxy-3,6-di-tert-butylcatecholate (4,5-(MeO)₂-3,6-DBCat), linker = Bipy (8). The same reaction of (4,5-Cl₂-3,6-DBCat)SbPh₃ with DABCO in an open atmosphere results in a formation of 1D coordination polymer {[(4,5-Cl₂-3,6-DBCat)SbPh₃·H₂O]·DABCO}_n (12). Bis-catecholate complex Ph₃Sb(Cat-Spiro-Cat)SbPh₃ reacts with Bipy as 1:1 yielding a rare macrocyclic tetranuclear compound {Ph₃Sb(Cat-Spiro-Cat)SbPh₃∙(Bipy)}₂ (13). The molecular structures of 1, 3, 4, 5, 8, 10, 12, and 13 in crystal state were established by single-crystal X-ray analysis. Complexes demonstrate different types of relative spatial positions of mononuclear moieties. The nature of chemical bonds, charges distribution, and the energy of Sb...N interaction were investigated in the example of complex 5. The electrochemical behavior of the complexes depends on the coordinated N-donor ligand. The coordination of pyrazine, Bipy, and PySSPy at the antimony atom changes their mechanism of electrooxidation: instead of two successive redox stages Cat/SQ and SQ/Cat, one multielectron stage was observed. The coordination of the DABCO ligand is accompanied by a significant shift in the oxidation potentials of the catecholate ligand to the cathodic region (by 0.4 V), compared to the initial complex.

Synthesis and Structure of Mono-, Di-, and Trinuclear Fluorotriarylbismuthonium Cations

A series of cationic fluorotriarylbismuthonium salts bearing differently substituted aryl groups (Ar = 9,9-Me₂-9H-xanthene, Ph, Mes, and 3,5-tBu-C₆H₃) have been synthesized and characterized. While the presence of simple phenyl substituents around the Bi center results in a polymeric structure with three Bi centers in the repeating monomer, substituents at the ortho- and meta-positions lead to cationic mono- and dinuclear fluorobismuthonium complexes, respectively. Preparation of all compounds is accomplished by fluoride abstraction from the parent triaryl Bi(V) difluorides using NaBAr^F (BAr^F– = B[C₆H₃-3,5-(CF₃)₂]₄^–). Structural parameters were obtained via single crystal X-ray diffraction (XRD), and their behavior in solution was studied by NMR spectroscopy. Trinuclear and binuclear complexes are held together through one bridging fluoride (μ-F) between two Bi(V) centers. In contrast, the presence of Me groups in both ortho-positions of the aryl ring provides the adequate steric encumbrance to isolate a unique mononuclear nonstabilized fluorotriarylbismuthonium cation. This compound features a distorted tetrahedral geometry and is remarkably stable at room temperature both in solution (toluene, benzene and THF) and in the solid state.

Triptycene Derivatives: From Their Synthesis to Their Unique Properties

Since the first preparation of triptycene, great progress has been made with respect to its synthesis and the understanding of its properties. Interest in triptycene-based systems is intense; in recent years, advances in the synthetic methodology and properties of new triptycenes have been reported by researchers from various fields of science. Here, an account of these new developments is given and placed in reference to earlier pivotal works that underpin the field. First, we discuss new approaches to the synthesis of new triptycenes. Progress in the regioselective synthesis of sterically demanding systems is discussed. The application of triptycenes in catalysis is also presented. Next, progress in the understanding of the relations between triptycene structures and their properties is discussed. The unique properties of triptycenes in the liquid and solid states are elaborated. Unique interactions, which involve triptycene molecular scaffolds, are presented. Molecular interactions within a triptycene unit, as well as between triptycenes or triptycenes and other molecules, are also evaluated. In particular, the summary of the synthesis and useful features will be helpful to researchers who are using triptycenes as building blocks in the chemical and materials sciences.

High-Efficiency Solution-Processable OLEDs by Employing Thermally Activated Delayed Fluorescence Emitters with Multiple Conversion Channels of Triplet Excitons

The state-of-the-art luminescent materials are gained widely by utilizing thermally activated delayed fluorescence (TADF) mechanism. However, the feasible molecular designing strategy of fully exploiting triplet excitons to enhance TADF properties is still in demand. Herein, TADF emitters with multiple conversion channels of triplet excitons are designed by concisely halogenating the electron acceptors containing carbonyl moiety. Compared with the chlorinated and brominated analogues, the fluorinated emitter exhibits distinguishing molecular stacking structures, participating in the formation of trimers through integrating CH···F and C═O···H hydrogen bonds together. It is also demonstrated that the multiple channels can be involved synergistically to accelerate the spin-flip of triplet excitons, and to take charge of the relatively superior reverse intersystem crossing constant rate of 6.20 × 105 s-1 , and thus excellent photoluminescence quantum yields over 90% can easily be achieved. Then the solution-processable organic light emitting diode based on fluorinated emitter can achieve a record-high external quantum efficiency value of 27.13% and relatively low efficiency roll-off with remaining 24.74% at 1000 cd m-2 . This result manifests the significance of enhancing photophysical properties through constructing multiple conversion channels of triplets excitons for high-efficiency TADF emitters and provides a guideline for the future study.

Tertiary Chiral Nanostructures from C-H⋅⋅⋅F Directed Assembly of Chiroptical Superatoms

We report the synthesis and structure of tertiary chiral nanostructures with 100 % optical purity. A novel synthetic strategy, using chiral reducing agent, R and S-BINAPCuBH₄ (BINAP is 2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl), is developed to access to atomically precise, intrinsically chiral [Au₇ Ag₆ Cu₂ (R- or S-BINAP)₃ (SCH₂ Ph)₆ ]SbF₆ nanoclusters in one-pot synthesis. The clusters represent the first tri-metallic superatoms with inherent chirality and fair stability. Both metal distribution (primary) and ligand arrangement (secondary) of the enantiomers exhibited perfect mirror images, and unprecedentedly, the self-assembly driven by the C-H⋅⋅⋅F interaction between the phenyl groups of the superatom moieties and SbF₆ ^- anions induced the formation of bio-mimic left- and right-handed helices, achieving the tertiary chiral nanostructures. DFT calculations revealed the connections between the molecular details and chiral optical activity.

Anion Chelation via Double Chalcogen Bonding: The Case of a Bis-telluronium Dication and Its Application in Electrophilic Catalysis via Metal-Chloride Bond Activation

Telluronium cations have long been known to engage their counteranions via secondary interactions. Yet, this property has rarely been exploited for anion binding. Motivated by such an application, we have now synthesized a bis-telluronium dication ([3]²⁺) that was obtained as a tetrafluoroborate salt by reaction of 2,7-di-tert-butyl-9,9-dimethylxanthene-4,5-diboronic acid with phenoxatellurine difluoride and BF₃·OEt₂. As confirmed by the formation of Te-(μ-BF₄)-Te bridges in the structure of [3][BF₄]₂, [3]²⁺ functions as a bidentate Lewis acid toward anions. [3][BF₄]₂ has also been converted into the more exposed [3][BArF₂₄]₂ ([BArF₂₄]^- = [B(3,5-(CF₃)₂C₆H₃)₄]^-). The latter, which readily ionizes Ph₃CCl, displays a chloride anion binding constant that exceeds that of a monofunctional model compound by almost 4 orders of magnitude. The unique properties of this new bis-telluronium dication are further highlighted by its ability to activate Ph₃PAuCl and cis-(Ph₃P)₂PtCl₂, leading to catalytic systems highly active in the cycloisomerization of propargylamide or enyne substrates.

Distiboranes based on ortho-phenylene backbones as bidentate Lewis acids for fluoride anion chelation

As part of our efforts in the chemistry of main group platforms that support anion sensing and transport, we are now reporting the synthesis of anitmony-based bidentate Lewis acids featuring the o-C6F4 backbone. These compounds can be easily accessed by reaction of the newly synthesized o-C6F4(SbPh2)2 (5) with o-chloranil or octafluorophenanthra-9,10-quinone, affording the corresponding distiboranes 6 and 7 of general formula o-C6F4(SbPh2(diolate))2 with diolate = tetrachlorocatecholate for 6 and octafluorophenanthrene-9,10-diolate for 7, respectively. While 6 is very poorly soluble, its octafluorophenanthrene-9,10-diolate analog 7 readily dissolves in CH2Cl2 and undergoes swift conversion into the corresponding fluoride chelate complex [7-μ2-F]- which has been isolated as a [nBu4N]+ salt. The o-C6H4 analog of 7, referred to as 8, has also been prepared. Although less Lewis acidic than 7, 8 also forms a very stable fluoride chelate complex ([8-μ2-F]-). Altogether, our experiental results, coupled with computational analyses and fluoride anion affinity calculations, show that 7 and 8 are some of the strongest antimony-based fluoride anion chelators prepared to date. Another notable aspect of this work concerns the use of the octafluorophenanthrene-9,10-diolate ligand and its ablity to impart advantageous solubility and Lewis acidity properties.

Dibismuthanes in catalysis: from synthesis and characterization to redox behavior towards oxidative cleavage of 1,2-diols

A family of aryl dinuclear bismuthane complexes has been successfully synthesized and characterized. The two bismuth centers are bonded to various xanthene-type backbones, which differ in ring-size and flexibility, resulting in complexes with different intramolecular BiBi distances. Moreover, their pentavalent Bi(v) analogues have also been prepared and structurally characterized. Finally, the synergy between bismuth centers in catalysis has been studied by applying dinuclear bismuthanes 5-8 to the catalytic oxidative cleavage of 1,2-diols. Unfortunately, no synergistic effects were observed and the catalytic activities of dinuclear bismuthanes and triphenylbismuth are comparable.

Redox-controlled chalcogen and pnictogen bonding: the case of a sulfonium/stibonium dication as a preanionophore for chloride anion transport

Our interest in the chemistry of tunable chalcogen and pnictogen bond donors as Lewis acidic platforms for the complexation and transport of anions has led us to investigate examples of such compounds that can be activated by redox events. Here, we describe the synthesis of [o-MePhS(C6H4)SbPh3]2+ ([3]2+) and [o-MePhS(C6H4)Sb(p-Tol)3]2+ ([4]2+), two dicationic stibonium/sulfonium bifunctional Lewis acids which were obtained by methylation of the phenylthioether derivatives [o-PhS(C6H4)SbPh3]+ ([1]+) and [o-PhS(C6H4)Sb(p-Tol)3]+ ([2]+), respectively. An evaluation of the chloride anion transport properties of these derivatives using chloride-loaded POPC unilamellar vesicles shows that the activity of the monocations [1]+ and [2]+ greatly exceeds that of the dications [3]2+ and [4]2+, a phenomenon that we assign to the higher lipophilicity of the monocationic compounds. Harnessing this large transport activity differential, we show that [4]2+ can be used as a prechloridophore that is readily activated by reduction of the sulfonium moiety. Indeed, [4]2+ reacts with GSH to afford [2]+ as an active transporter. This activation, which has been monitored in aqueous solution, can also be carried out in situ, in the presence of the chloride-loaded POPC unilamellar vesicles.

Polyfunctional Sterically Hindered Catechols with Additional Phenolic Group and Their Triphenylantimony(V) Catecholates: Synthesis, Structure, and Redox Properties

New polyfunctional sterically hindered 3,5-di-tert-butylcatechols with an additional phenolic group in the sixth position connected by a bridging sulfur atom—(6-(CH₂-S-tBu₂Phenol)-3,5-DBCat)H₂ (L₁), (6-(S-tBu₂Phenol)-3,5-DBCat)H₂ (L₂), and (6-(S-Phenol)-3,5-DBCat)H₂ (L₃) (3,5-DBCat is dianion 3,5-di-tert-butylcatecolate)—were synthesized and characterized in detail. The exchange reaction between catechols L₁ and L₃ with triphenylantimony(V) dibromide in the presence of triethylamine leads to the corresponding triphenylantimony(V) catecholates (6-(CH₂-S-tBu₂Phenol)-3,5-DBCat)SbPh₃ (1) and (6-(S-Phenol)-3,5-DBCat)SbPh₃ (2). The electrochemical properties of catechols L₁–L₃ and catecholates 1 and 2 were investigated using cyclic voltammetry. The electrochemical oxidation of L₁–L₃ at the first stage proceeds with the formation of the corresponding o-benzoquinones. The second process is the oxidation of the phenolic moiety. Complexes 1 and 2 significantly expand their redox capabilities, owing to the fact that they can act as the electron donors due to the catecholate metallocycle capable of sequential oxidations, and as donors of the hydrogen atoms, thus forming a stable phenoxyl radical. The molecular structures of the free ligand L₁ and complex 1 in the crystal state were determined by single-crystal X-ray analysis.

Highly Selective Fluoride Recognition by a Small Tris-Urea Covalent Cage

A highly selective recognition of fluoride was achieved through the design of a small hemicryptophane cage (3) presenting a southern tris-urea hosting moiety. The resulting host-guest complex has been characterized by electrospray ionization-high-resolution mass spectrometry, ¹H and ¹⁹F NMR, and X-ray diffraction techniques. In particular, X-ray diffraction analysis of [3·F^-] reveals that the encapsulation of one fluoride, within 3, occurs through NH···F^- H-bonding with the six NH residues of the tris-urea ligand. An association constant of 1200 M^-1 was extracted from ¹H NMR titration experiments, indicating that efficient fluoride binding also occurs in solution. Finally, in sharp contrast with previously reported urea-based hemicryptophane hosts, the small preorganized cavity found in 3 allows for an exclusive selectivity for fluoride over other competing halides.

Naked-Eye Heterogeneous Sensing of Fluoride Ions by Co-Polymeric Nanosponge Systems Comprising Aromatic-Imide-Functionalized Nanocellulose and Branched Polyethyleneimine

Heterogeneous colorimetric sensors for fluoride ions were obtained by cross-linking TEMPO-oxidized cellulose nanofibers (TOCNF) with chemically modified branched polyethyleneimine 25 kDa (bPEI). Functionalization of bPEI primary amino groups with aromatic anhydrides led to the formation of the corresponding mono- and bis-imides on the grafted polymers (f-bPEI). A microwave-assisted procedure allowed the optimization of the synthetic protocol by reducing reaction time from 17 h to 30 minutes. Hydrogels obtained by mixing different ratios of TOCNF, bPEI and f-bPEI were lyophilized and thermally treated at about 100 °C to promote the formation of amide bonds between the amino groups of poly-cationic polymers and the carboxylic groups of cellulose nanofibers. This approach generated a series of cellulose nanosponges S1-S3 which were characterized by FT-IR and by solid state ¹³ C CPMAS NMR. These sponge materials can act as colorimetric sensors for the selective naked-eye recognition of fluoride ions over chloride, phosphate and acetate ions at concentrations of up to 0.05 M in DMSO. Moreover, when the sponges were functionalized with perylene tetracarboxylic diimide, successful naked-eye detection was achieved with only 0.02 % w/w of chromophore units per gram of material.

Heavy Pnictogenium Cations as Transmembrane Anion Transporters in Vesicles and Erythrocytes

Our work on the complexation of fluoride anions using group 15 Lewis acids has led us to investigate the use of these main group compounds as anion transporters. In this paper, we report on the anion transport properties of tetraarylstibonium and tetraarylbismuthonium cations of the general formula [Ph₃PnAr]⁺ with Pn = Sb or Bi and with Ar = phenyl, naphthyl, anthryl, or pyrenyl. Using EYPC-based large unilamellar vesicles, we show that these main group cations transport hydroxide, fluoride and chloride anions across phospholipid bilayers. A comparison of the properties of [Ph₃SbAnt]⁺ and [Ph₃BiAnt]⁺ (Ant = 9-anthryl) illustrates the favorable role played by the Lewis acidity of the central pnictogen element with respect to the anion transport. Finally, we show that [Ph₃SbAnt]⁺ accelerates the fluoride-induced hemolysis of human red blood cells, an effect that we assign to the transporter-facilitated influx of toxic fluoride anions.

Trifunctional organometallic frameworks and cages based on all-cis-1,3,5-triethynyl-1,3,5-trisilacyclohexanes

All-cis-1,3,5-triethynyl-1,3,5-triorganyl-1,3,5-trisilacyclohexanes (organyl R = Me, Ph) offer flexible scaffolds with three concordantly oriented ethynyl groups. Lithiation with n-BuLi affords the dimeric {[CH2Si(R)(C2Li)]3(THF)3}2 with a drum-like structure in high yields. They can be transformed into other trinuclear organometallic derivatives like the trimercury cage species [CH2Si(R)C2HgC2(R)SiCH2]3.

Bidentate Boron Lewis Acids: Selectivity in Host-Guest Complex Formation

Bidentate boron Lewis acids based on 1,8-diethynylanthracene were synthesised in two steps by initial stannylation of the terminal alkynes and subsequent tin-boron exchange with different chloroboranes. The reactions were very selective, and the target compounds were obtained in high purity and good to excellent yields. Complexation experiments of 1,8-bis[(diphenylboranyl)ethynyl]anthracene with nitrogen bases (pyridine, pyrimidine, TMEDA) afforded three stable adducts, which were structurally characterised by X-ray diffraction. Competition experiments demonstrated the selective exchange of guests, and quantum-chemical calculations provided information on their energetics. NMR experiments at low temperature gave insight into the dynamic behaviour of the TMEDA adduct.

Phosphonium–stibonium and bis-stibonium cations as pnictogen-bonding catalysts for the transfer hydrogenation of quinolines

Bifunctional group 15 dications are effective transfer hydrogenation catalysts, the activity of which culminates in the case of the bis-stibonium dication [o-C₆H₄(SbMePh₂)₂]²⁺.