Activation of a Metal-Halogen Bond by Halogen Bonding

The Potential of Self-Activating Au(I) Complexes in Gold Catalysis

Gold catalysis has emerged as a groundbreaking field in synthetic chemistry, revolutionizing numerous organic transformations. Despite the significant achieved advancements, the mechanistic understanding behind many gold-catalyzed reactions remains elusive. This Concept article covers the so-called "self-activating" Au(I) complexes, sorting out their pivotal role in gold catalysis. We comment on how Au(I) complexes can undergo self-activation, triggering diverse catalytic transformations without the need for external additives. The most important examples reported so far that underlie the catalytic activity of these species are discussed. This intrinsic reactivity represents a paradigm shift in gold catalysis, offering new avenues for the design of efficient and sustainable catalytic systems. Furthermore, we explore the factors influencing the stability, reactivity, and selectivity of these Au(I) complexes, providing insights into their synthetic utility and potential applications. This area of research not only advances our fundamental understanding of gold catalysis but also paves the way for the development of novel catalytic strategies with broad implications in organic synthesis and the chemical industry.

Halogen, Chalcogen, Pnictogen, and Tetrel Bonding in Non-Covalent Organocatalysis: An Update

The use of noncovalent interactions based on electrophilic halogen, chalcogen, pnictogen, or tetrel centers in organocatalysis has gained noticeable attention. Herein, we provide an overview on the most important developments in the last years with a clear focus on experimental studies and on catalysts which act via such non-transient interactions.

Hydrogen-Bonding Activation of Gold(I) Chloride Complexes: Enantioselective Synthesis of 3(2H)-Furanones by a Cycloisomerization-Addition Cascade

Enantioselective synthesis of 3(2H)-furanones has been achieved using the intermolecular H-bonding activation of gold(I) chloride complexes. A DM-BINAP [(R)-(+)-2,2'-Bis[di(3,5-xylyl)phoshino]-1,1'-binaphthyl] digold(I) dichloride complex in combination with a sulfonyl squaramide (SO2Sq) has been identified as the optimal catalytic system. The process involves a 5-endo-dig oxa-cyclization followed by stereocontrolled addition of indoles. Interestingly, the soft L*Au-Cl activation by H-bonding allowed the recovery of both L*Au-Cl and the activator after chromatographic purification.

Synthesis and reactivity of N-heterocyclic carbene (NHC) gold-fluoroalkoxide complexes

We disclose a novel series of N-heterocyclic carbene (NHC) gold complexes with varied steric and electronic properties, bearing fluorinated alkoxide anions. Early reactivity studies involving these synthons, lead to the synthesis of various complexes of relevance to gold chemistry and catalysis.

Carbene-Catalyzed and Pnictogen Bond-Assisted Access to PIII-Stereogenic Compounds

Intermolecular pnictogen bonding (PnB) catalysis has received increased interest in non-covalent organocatalysis. It has been demonstrated that organic electron-deficient pnictogen atoms can act as prospective Lewis acids. Here, we present a catalytic approach for the asymmetric synthesis of chiral PIII compounds by combining intramolecular PnB interactions and carbene catalysis. Our design features a pre-chiral phosphorus molecule bearing two electron-withdrawing benzoyl groups, resulting in the formation of a σ-hole at the P atom. X-ray and non-covalent interaction (NCI) analysis indicate that the model substrates exhibit intrinsic PnB interaction between the oxygen atom of the formyl group and the phosphorus atom. This induces a conformational locking effect, leading to the crystallization of the phosphorus substrate in a preferred conformation (P212121 chiral group). Under the catalysis of N-heterocyclic carbene, the aldehyde moiety activated by the pnictogen bond selectively reacts with an alcohol to yield the corresponding chiral monoester/phosphorus product with excellent enantioselectivity. This Lewis acidic phosphorus center, aroused by the non-polarized intramolecular pnictogen bond interaction, assists in conformational and selective regulations, providing unique opportunities for catalysis and beyond.

A Fluorescent Organic Probe for Naked-Eye Detection of Chloromethanes

An organic fluorescent probe (OFP-TAR) with a propeller-like structure was designed and synthesized. The photoluminescence of OFP-TAR in solution exhibited a significant red shift with the increase of solvent polarity, enabling a transition of fluorescence emission from blue (445 nm) to yellow (540 nm). The organic thin-film materials based on OFP-TAR/PMMA exhibit significant color changes upon exposure to CH2Cl2, CHCl3, and CCl4, with their maximum fluorescence wavelengths measured at 445, 471, and 494 nm respectively. The device facilitates the visual detection of chloromethanes and is capable of enduring more than 7 cycles of testing. These materials can also be prepared as binary-coded microarray data storage devices or applied in the field of anti-counterfeiting. The quantum yields of guest-loaded crystals CH2Cl2@OFP-TAR, CHCl3@OFP-TAR and CCl4@OFP-TAR are observed as 19.13 %, 8.79 %, and 0.83 % respectively, which are consistent with the tendency of OFP-TAR in CH2Cl2 (47.30 %), CHCl3 (34.27 %) and CCl4 (3.10 %). The fluorescence properties of OFP-TAR, OFP-TAR/PMMA, guest-loaded and guest-free crystals provided insights into the special response mechanism of OFP-TAR towards different chloromethanes.

Weak Interaction Activates Esters: Reconciling Catalytic Activity and Turnover Contradiction by Tailored Chalcogen Bonding

The activation of esters by strong Lewis acids via the formation of covalent adducts is a classic strategy to give reactivity; however, this approach frequently incurs limited turnover due to the low efficiency in the dissociation of catalyst from a stable catalyst-product complex. While the use of some weak interaction catalysts that can easily dissociate from any bonding complexes in the reaction system would solve this catalyst turnover problem, the poor catalytic activity in the ester activation that can be provided by these noncovalent forces in turn sets up a formidable challenge. Herein, we describe the activation and catalytic transformation of esters by weak interactions, which provides a promising platform to reconcile the catalytic activity and turnover problems. Several tailored chalcogen-bonding catalysts were developed for the activation of esters, enabling achieving several inherently low reactive Diels-Alder reactions as well as the ring-opening polymerization of lactones through weak chalcogen bonding interactions. This supramolecular catalysis approach is particularly highlighted by its capability to promote some uncommon Diels-Alder reactions involving using dienes bearing electron-withdrawing groups coupled by α,β-unsaturated ester as dienophiles and substrate incorporating competitive Lewis basic sites, in which typical strong Lewis acids showed low catalytic efficiency, while representative hydrogen and halogen bonding catalysts were inactive.

Dioxygen-halogen bonding exemplified by crystalline peroxosolvates of N,N'-bis(haloacetyl) bispidines

The halogen bonding in molecular crystals and supramolecular assemblies has been widely investigated. Special attention is given to the molecular structures capable of simultaneously exhibiting different types of non-covalent interactions, including conventional hydrogen bonds and halogen bonds. This paper systematically analyzes crystalline peroxosolvates of bispidine-based bis-amide derivatives, containing haloacetic acid residues, namely previously reported 1,1'-(1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonane-3,7-diyl)bis(2-iodooethanone) peroxosolvate C13H20I2N2O2·H2O2 (1) and four new crystalline compounds, 1,1'-(1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonane-3,7-diyl)bis(2-bromoethanone) peroxosolvate C13H20Br2N2O2·H2O2 (2), 1,1'-(9-hydroperoxy-9-hydroxy-1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonane-3,7-diyl)bis(2-iodoethanone) peroxosolvate C13H20I2N2O5·0.5H2O2 (3), 1,1'-(9-hydroperoxy-9-hydroxy-1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonane-3,7-diyl)bis(2-bromoethanone) peroxosolvate C13H20Br2N2O5·H2O2 (4), and 1,1'-(9-hydroperoxy-9-hydroxy-1,5-dimethyl-3,7-diazabicyclo[3.3.1]nonane-3,7-diyl)bis(2-chloroethanone) peroxosolvate C13H20Cl2N2O5·H2O2 (5). Compounds 2-5 were synthesized for the first time and their crystal structures were determined by single-crystal X-ray diffractometry (SCXRD). To the best of our knowledge, 3-5 are unprecedented crystalline hydrogen peroxide adducts of organic hydroperoxides (R-OOH). Short intermolecular contacts between halogen and hydroperoxo oxygen atoms were found in 1-3. The halogen bonding of C-I(Br) fragments with dioxygen species in compounds 1-3 as well as in the previously reported cocrystal of diacetone diperoxide with triodotrinitrobenzene (6) was identified through reduced density gradient analysis, Hirshfeld surface analysis, and Bader analysis of crystalline electron density. The interactions were quantified using the electron density topological properties acquired from the periodic DFT calculations and evaluated to lie in the range of 9-19 kJ mol-1. A distinctive spectral feature was revealed for this type of interaction, involving a red shift of the characteristic O-O stretching vibration by about 6 cm-1, which appeared in IR spectra as a narrow low-intensity band in the region 837-872 cm-1.

Halogen Bond Catalysis: A Physical Chemistry Perspective

As important noncovalent interactions, halogen bonds have been widely used in material science, supramolecular chemistry, medicinal chemistry, organocatalysis, and other fields. In the past 15 years, halogen bond catalysis has become a developed field in organocatalysis for the catalysts' advantages of being environmentally friendly, inexpensive, and recyclable. Halogen bonds can induce various organic reactions, and halogen bond catalysis has become a powerful alternative to the fully explored hydrogen bond catalysis. From a physical chemistry view, this perspective provides an overview of the latest progress and key examples of halogen bond catalysis via activation of the lone pair systems of organic functional group, π systems, and metal complexes. The research progresses in halogen bond catalysis by our group were also introduced.

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.

Elucidating mechanochemical reactivity of a ternary halogen-bonded cocrystal system by computational and calorimetric studies

Discovery of a halogen-bonded ternary cocrystal of 1,3,5-trifluoro-2,4,6-triiodobenzene with pyrazine and triphenylphosphine sulfide has revealed a complex landscape of multicomponent phases, all achievable by mechanochemical interconversion. The observed solid-state reaction pathways were explained by periodic density-functional calculations and comprehensive intermolecular interaction analysis, supported by dissolution calorimetry measurements.

Carbon-Bonding Metal Catalysis (CBMC): A Supramolecular Complex Directs Structural-Isomer Selection in Gold-Catalyzed Reactions

Carbon is a primary element to constitute organic molecules, while metal catalysis is a basic tool in organic synthesis. The establishment of a link between the ubiquitous carbon bonding and metal catalysis is thus a fundamentally important problem. However, there is yet no experimental example to introduce the role of carbon bonding in a metal catalysis process. Herein, we merged the topics of carbon bonding and metal catalysis together and demonstrated that a supramolecular carbon-bonding metal complex can not only give rise to catalytic activity but, more remarkably, direct structural-isomer selection events in gold-catalyzed reactions. The experimental results unveil the fact that the imposing of weak carbon-bonding interactions on a gold complex can alter the carbene as well as the Lewis acid property of these catalysts. These results illustrate a non-negligible role of weak carbon-bonding interactions in the modulation of metal catalysis. As such, carbon-bonding metal catalysis is suggested to be used as a routine tool not only in the development of reactions but more frequently in analyzing reaction processes in metal catalysis.

Influence of Coordination to Silver(I) Centers on the Activity of Heterocyclic Iodonium Salts Serving as Halogen-Bond-Donating Catalysts

Kinetic data based on 1 H NMR monitoring and computational studies indicate that in solution, pyrazole-containing iodonium triflates and silver(I) triflate bind to each other, and such an interplay results in the decrease of the total catalytic activity of the mixture of these Lewis acids compared to the separate catalysis of the Schiff condensation, the imine-isocyanide coupling, or the nucleophilic attack on a triple carbon-carbon bond. Moreover, the kinetic data indicate that such a cooperation with the silver(I) triflate results in prevention of decomposition of the iodonium salts during the reaction progress. XRD study confirms that the pyrazole-containing iodonium triflate coordinates to the silver(I) center via the pyrazole N atom to produce a rare example of a pentacoordinated trigonal bipyramidal dinuclear silver(I) complex featuring cationic ligands.

Halogen Bond-Involving Self-Assembly of Iodonium Carboxylates: Adding a Dimension to Supramolecular Architecture

We designed 0D, 1D, and 2D supramolecular assemblies made of diaryliodonium salts (functioning as double σ-hole donors) and carboxylates (as σ-hole acceptors). The association was based on two charge-supported halogen bonds (XB), which occurred between IIII sites of the iodonium cations and the carboxylate anions. The sequential introduction of the carboxylic groups in the aryl ring of the benzoic acid added a dimension to the 0D supramolecular organization of the benzoate, which furnished 1D-chained and 2D-layered structures when terephthalate and trimesate anions, correspondingly, were applied as XB acceptors. The structure-directing XB were studied using DFT calculations under periodic boundary conditions and were followed by the one-electron-potential analysis and the Bader atoms-in-molecules topological analysis of electron density. These theoretical methods confirmed the existence of the XB and verified the philicities of the interaction partners in the designed solid-state structures.

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.

Exploring the role of halogen bonding in iodonium ylides: insights into unexpected reactivity and reaction control

Halogen bonding is commonly found with iodine-containing molecules, and it arises when Lewis bases interact with iodine's σ-holes. Halogen bonding and σ-holes have been encountered in numerous monovalent and hypervalent iodine-containing compounds, and in 2022 σ-holes were computationally confirmed and quantified in the iodonium ylide subset of hypervalent iodine compounds. In light of this new discovery, this article provides an overview of the reactions of iodonium ylides in which halogen bonding has been invoked. Herein, we summarize key discoveries and mechanistic proposals from the early iodonium ylide literature that invoked halogen bonding-type mechanisms, as well as recent reports of reactions between iodonium ylides and Lewis basic nucleophiles in which halogen bonding has been specifically invoked. The reactions discussed herein are organized to enable the reader to build an understanding of how halogen bonding might impact yield and chemoselectivity outcomes in reactions of iodonium ylides. Areas of focus include nucleophile σ-hole selectivity, and how ylide structural modifications and intramolecular halogen bonding (e.g., the ortho-effect) can improve ylide stability or solubility, and alter reaction outcomes.

Balancing Activity and Stability in Halogen-Bonding Catalysis: Iodopyridinium-Catalyzed One-Pot Synthesis of 2,3-Dihydropyridinones

A one-pot cascade reaction for 2,3-dihydropyridinone synthesis was accomplished with 3-fluoro-2-iodo-1-methylpyridinium triflate as the halogen bond catalyst. The desired [4+2] cycloaddition products, bearing aryl, heteroaryl, alkyl, and alicyclic substituents, were successfully furnished in 28-99% yields. Mechanistic investigations proved that a strong halogen-bonding interaction forged between the iodopyridinium catalyst and imine intermediate was essential to dynamically masking the vulnerable C-I bond on the catalyst and accelerating the following aza-Diels-Alder reaction.

Cyclic Homo- and Heterohalogen Di-λ3-diarylhalonium Structures

In the context of the ever-growing interest in the cyclic diaryliodonium salts, this work presents synthetic design principles for a new family of structures with two hypervalent halogens in the ring. The smallest bis-phenylene derivative, [(C₆H₄)₂I₂]²⁺, was prepared through oxidative dimerization of a precursor bearing the ortho-disposed iodine and trifluoroborate groups. We also report, for the first time, the formation of cycles containing two different halogen atoms. These present two phenylenes linked by hetero-(I/Br) or -(I/Cl) halogen pairs. This approach was also extended to the cyclic bis-naphthylene derivative [(C₁₀H₆)₂I₂]²⁺. The structures of these bis-halogen(III) rings were further assessed through X-ray analysis. The simplest cyclic phenylene bis-iodine(III) derivative features the interplanar angle of ∼120°, while a smaller angle of ∼103° was found for the analogous naphthylene-based salt. All dications form dimeric pairs through a combination of π-π and C-H/π interactions. As the largest member of the family, a bis-I(III)-macrocycle was also assembled using the quasi-planar xanthene backbone. Its geometry enables the two iodine(III) centers to be bridged intramolecularly by two bidentate triflate anions. In a preliminary manner, the interaction of the phenylene- and naphthalene-based bis-iodine(III) dications with a new family of rigid bidentate bis-pyridine ligands was studied in solution and the solid state, with an X-ray structure showing the chelating donor bonding to just one of the two iodine centers.

Exploring Au(i) involving halogen bonding with N-heterocyclic carbene Au(i) aryl complexes in crystalline media

Among the known types of non-covalent interactions with a Au(i) metal center, Au(i) involving halogen bonding (XB) remains a rare phenomenon that has not been studied systematically. Herein, using five N-heterocyclic carbene (NHC) Au(i) aryl complexes and two iodoperfluoroarenes as XB donors, we demonstrated that the XB involving the Au(i) metal center can be predictably obtained for neutral Au(i) complexes using the example of nine co-crystals. The presence of XB involving the Au(i) center was experimentally investigated by single-crystal X-ray diffraction and solid-state ¹³C CP-MAS NMR methods, and their nature was elucidated through DFT calculations, followed by electron density, electrostatic potential, and orbital analyses. The obtained results revealed a connection between the structure and HOMO localization of Au(i) complexes as XB acceptors, and the geometrical, electronic, and spectroscopic features of XB interactions, as well as the supramolecular structure of the co-crystals.

Noncovalent Chelation by Halogen Bonding in the Design of Metal-Containing Arrays: Assembly of Double σ-Hole Donating Halolium with CuI-Containing O,O-Donors

Five new copper(I) complexes─composed of the paired dibenzohalolium and [CuL₂]^- (L = 1,2,4-oxadiazolate) counterions in which O,O-atoms of the anion are simultaneously linked to the halogen atom─were generated and isolated as the solid via the three-component reaction between [Cu(MeCN)₄](BF₄), sodium 1,2,4-oxadiazolates, and dibenzohalolium triflates (or trifluoroacetates). This reaction is different from the previously reported Cu^I-catalyzed arylation of 1,2,4-oxadiazolones by diaryliodonium salts. Inspection of the solid-state X-ray structures of the complexes revealed the strong three-center X···O,O (X = Br, I) halogen bonding occurred between the oxadiazolate moieties and dibenzohalolium cation. According to performed theoretical calculations, this noncovalent interaction (or noncovalent chelation) was recognized as the main force in the stabilization of the copper(I) complexes. An explanation for the different behavior of complexes, which provide either chelate or nonchelate binding, is based on the occurrence of additional -CH₃···π interactions, which were also quantified.

Activation of metal-involved halogen bonds and classical halogen bonds in gold(I) catalysis

In gold(I) catalysis, the activation of Au(I) chloride catalysts via chloride abstraction and noncovalent interactions has become a research focus in organometallic catalysis. In this work, taking halogen bond donors (C₄H₂INO₂, C₆F₅I, C₈H₉O₂I) as activators for a Au(I) chloride catalyst (Ph₃PAuCl), the mechanism of the cyclization reaction of propargylic amide was investigated. It was found that there are two activation modes as design principles to obtain the catalytically active species Ph₃PAu⁺: the halogen bond donors activate the Cl atoms of Ph₃PAuCl to form X-I⋯Cl (X = C, N) classical halogen bonds and activate the Au atoms of Ph₃PAuCl to form X-I⋯Au (X = C, N) metal-involved halogen bonds. For the two activation modes, the mechanism of the cyclization reaction of propargylic amide has pathways: the chloride abstraction process of the first step and the 5-exo/6-endo cyclization process of the second step. Both activation modes show good activity for the cyclization reaction with the activation ability of classical halogen bonds being slightly stronger than that of the metal-involved halogen bonds, which is consistent with the strength of the X-I⋯Cl halogen bonds being slightly stronger than that of the X-I⋯Au halogen bonds. Therefore, both metal-involved halogen bonds and classical halogen bonds have important development prospects for the activation of catalysts in gold(I) catalysis.

Synthesis of cationic polymer decorated with halogen for highly efficient trapping 99TcO4-/ReO4. JOURNAL OF HAZARDOUS MATERIALS 2023;443:130325. [PMID: 36372023 DOI: 10.1016/j.jhazmat.2022.130325]

The elimination of anion is of great importance from radioactive nuclear waste containing ⁹⁹TcO₄^- by rationally designing anion-scavenging materials with high density of charge and more accessible adsorption sites. Herein, a tailor-made cationic organic polymer with donor-acceptor (D-A) structure, namely TrDCPN, was successfully synthesized by rationally modifying the benzimidazole unit for efficient trapping the perrhenate (ReO₄^-) as a ⁹⁹Tc surrogate. Systematic control of the skeleton affect enables the material to integrate a variety of features, surmounting the long-term challenge of ⁹⁹TcO₄^-/ReO₄^- remediation under extreme conditions of high acid/base and high ionic strength. Furthermore, the TrDCPN shows excellent affinity toward ReO₄^- in the existence of large excess of competitive anions (SO₄^2-, NO₃^- and PO₄^3-etc.) as well as promising reusability for trapping ReO₄^-. The excellent stability and separation were derived from the introduction of large conjugated modules, triazine core and hydrophobic. More importantly, the synthetic cationic organic polymer with D-A feature was first proved that the introduction of halogen can effectively enhance the backbone charge, and increase the adsorption capacity by synergy of ion exchange, electrostatic interaction and δ hole-anion interaction. The adsorption capacity of TrDCPN can be up to 420.3 mg/g and reach equilibrium within 20 min. It is noteworthy that TrDCPN successfully immobilizes ReO₄^- from simulated Hanford waste with a high separation efficiency of 93 %, providing a new paradigm for material design to dispose of the problem of radioactive pollutants in the environment.

Silver-Free Gold-Catalyzed Heterocyclizations through Intermolecular H-Bonding Activation

Modulable monosulfonyl squaramides have been shown to exert activation of gold(I) chloride complexes through H-bonding in an intermolecular way. Combinations of (PPh₃)AuCl or IPrAuCl complexes and an optimal sulfonyl squaramide cocatalyst bearing two 3,5-bis(trifluoromethyl)phenyl groups efficiently catalyzed diverse heterocyclizations and a cyclopropanation reaction, avoiding in all cases undesired side reactions. Computational studies indicate that the Au-Cl bond breaks by transligation to the triple bond in a ternary complex formed by the actual AuCl···HBD catalyst and the substrate.

Recent Progress in Cyclic Aryliodonium Chemistry: Syntheses and Applications

Hypervalent aryliodoumiums are intensively investigated as arylating agents. They are excellent surrogates to aryl halides, and moreover they exhibit better reactivity, which allows the corresponding arylation reactions to be performed under mild conditions. In the past decades, acyclic aryliodoniums are widely explored as arylation agents. However, the unmet need for acyclic aryliodoniums is the improvement of their notoriously low reaction economy because the coproduced aryl iodides during the arylation are often wasted. Cyclic aryliodoniums have their intrinsic advantage in terms of reaction economy, and they have started to receive considerable attention due to their valuable synthetic applications to initiate cascade reactions, which can enable the construction of complex structures, including polycycles with potential pharmaceutical and functional properties. Here, we are summarizing the recent advances made in the research field of cyclic aryliodoniums, including the nascent design of aryliodonium species and their synthetic applications. First, the general preparation of typical diphenyl iodoniums is described, followed by the construction of heterocyclic iodoniums and monoaryl iodoniums. Then, the initiated arylations coupled with subsequent domino reactions are summarized to construct polycycles. Meanwhile, the advances in cyclic aryliodoniums for building biaryls including axial atropisomers are discussed in a systematic manner. Finally, a very recent advance of cyclic aryliodoniums employed as halogen-bonding organocatalysts is described.

An alkali-resistant metal-organic framework as halogen bond donor for efficient and selective removing of ReO4-/TcO4. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH INTERNATIONAL 2022;29:86815-86824. [PMID: 35794336 DOI: 10.1007/s11356-022-21870-y] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

⁹⁹Tc is one of the most problematic nuclear fuel products due to its long half-life and high environmental mobility. Direct removal of TcO₄^- from the highly alkaline solution of nuclear fuel is a serious and challenging environmental issue. In this work, the first efficient synthetic approach introducing halogens into a two-dimensional metal-organic framework, named Mn-MOF, is established using MnCl₂·4H₂O coordinating with neutral nitrogen-donor ligand, showing ultrahigh stability in alkaline aqueous even under 1 M NaOH. The luxuriant Mn-Cl bonds and ordered hydrophobic pore channels enable the Mn-MOF to have an efficient adsorption capacity for ReO₄^- with a large capacity (403 mg g^-1), which is higher than most MOF adsorbents. More importantly, the Mn-MOF shows an excellent selectivity toward ReO₄^- in high-density competitive anions, such as NO₃^- and SO₄^2-. Moreover, the outstanding performance of Mn-MOF in removing ReO₄^- endowed it successfully separated ReO₄^- from the simulated Savannah River Site (SRS) high-level waste (HLW) stream with high removal of 66.84% at the phase ratio of 10. The adsorption mechanism is further demonstrated by FT-IR, XPS analysis, and DFT calculation, showing that the ReO₄^- can selectively interact with Mn-Cl bonds and imidazole groups, forming unique halogen bonds Cl-O-Re, and a series of hydrogen bonds, respectively. This work suggests a new approach to the removal of TcO₄^- from nuclear fuel.

Halogen Bonding Involving Gold Nucleophiles in Different Oxidation States

A single-crystal X-ray diffraction (XRD) study of diaryliodonium tetrachloroaurates (or, in the recent terminology, tetrachloridoaurates), [(p-XC₆H₄)₂I][AuCl₄] (X = Cl, 1; Br, 2), was performed for 1 (the structure is denoted as 1a to show similarity with the isomorphic structure 2a) and two polymorphs─2a (obtained from MeOH) and 2b (from 1,2-C₂H₄Cl₂). Examination of the XRD data for these three structures revealed 2-center C-X···Au^III (X = Cl and Br) and 3-center bifurcated C-Br···(Cl-Au) halogen bonding (abbreviated as XB) between the p-Cl or p-Br atoms of the diaryliodonium cations and the gold(III) atom of [AuCl₄]^-. The noncovalent nature of Au^III-involving interactions, the nucleophilicity of the gold(III) atoms, and the electrophilic role of p-X atoms of the diaryliodonium cations in the XBs were studied by a set of complementary computational methods. Combined experimental and theoretical studies allowed the recognition of the d-nucleophilicity of the [d⁸Au^III] atom which, regardless of its rather substantial formal 3+ charge, can function as a d-nucleophilic partner of XB. This conclusion was also supported by theoretical calculations performed for the structures' refcodes BINXOM and ICSD 62511; the obtained data verified the nucleophilicity of Au^III toward a K⁺ ions or a σ-(Cl)-hole, respectively. All our results, together with consideration of relevant literature, indicate that gold atoms in the three oxidation states (0, I, and even III) exhibit nucleophilicity in XBs.

Halogen Bond Motifs in Cocrystals of N,N,O and N,O,O Acceptors Derived from Diketones and Containing a Morpholine or Piperazine Moiety

In this study, we investigate the halogen bond acceptor potential of oxygen and nitrogen atoms of morpholine and piperazine fragments when they are peripherally located on N,O,O or N,N,O acceptor molecules. We synthesized four acceptor molecules derived from either acetylacetone or benzoylacetone and cocrystallized them with 1,4-diiodotetrafluorobenzene and 1,3,5-triiodotrifluorobenzene. This resulted in eight cocrystals featuring different topicities and geometric dispositions of donor atoms. In all cocrystals, halogen bonds are formed with either the morpholinyl oxygen atom or the terminal piperazine nitrogen atom. The I···O_morpholine halogen bonds feature lower relative shortening values than I···N_terminal, I···O_carbonyl, and I···N_proximal halogen bonds. The N and O halogen bond acceptor sites were evaluated through calculations of molecular electrostatic potential values.

Chalcogen bond-guided conformational isomerization enables catalytic dynamic kinetic resolution of sulfoxides

Conformational isomerization can be guided by weak interactions such as chalcogen bonding (ChB) interactions. Here we report a catalytic strategy for asymmetric access to chiral sulfoxides by employing conformational isomerization and chalcogen bonding interactions. The reaction involves a sulfoxide bearing two aldehyde moieties as the substrate that, according to structural analysis and DFT calculations, exists as a racemic mixture due to the presence of an intramolecular chalcogen bond. This chalcogen bond formed between aldehyde (oxygen atom) and sulfoxide (sulfur atom), induces a conformational locking effect, thus making the symmetric sulfoxide as a racemate. In the presence of N–heterocyclic carbene (NHC) as catalyst, the aldehyde moiety activated by the chalcogen bond selectively reacts with an alcohol to afford the corresponding chiral sulfoxide products with excellent optical purities. This reaction involves a dynamic kinetic resolution (DKR) process enabled by conformational locking and facile isomerization by chalcogen bonding interactions.

Conformational isomerization of organic molecules can be guided by noncovalent interactions. Here, the authors report the synthesis of chiral sulfoxides catalyzed by N-heterocyclic carbenes; intramolecular chalcogen bonding interactions are key for conformational locking.

Hydrogen bonding-enabled gold catalysis: ligand effects in gold-catalyzed cycloisomerizations in hexafluoroisopropanol (HFIP)

Gold catalysis has witnessed immense evolution in recent years, yet it still requires the use of activators to render the common [AuCl(L)] complexes catalytically active. Herein, the H-bonding donor properties of hexafluoroisopropanol (HFIP) are utilized for Au-Cl bond activation and the ancillary ligand and counteranion effects on cycloisomerization reactions are showcased in HFIP as solvent.

Iodolium salts as halogen-bond donor catalysts in the Nazarov cyclization: the molecular oxygen enigma

Nazarov cyclizations of activated precurosrs are achieved under iodolium catalysis, provided that oxygen is present for catalyst activation and turnover.

Ligand Exchange Among Iodine(I) Complexes

A detailed investigation of ligand exchange between iodine(I) ions in [N···I···N]+ halogen-bonded complexes is presented. Ligand exchange reactions were conducted to successfully confirm whether iodine(I) complex formation, via the classical...

Evaluation of Halogenopyridinium Cations as Halogen Bond Donors

We have performed a database survey and a structural and computational study of the potential and the limitations of halogenopyridinium cations as halogen bond donors. The database survey demonstrated that adding a positive charge on a halogenopyridine ring increases the probability that the halogen atom will participate in a halogen bond, although for chloropyridines it remains below 60%. Crystal structures of both protonated and N-methylated monohalogenated pyridinium cations revealed that the iodo- and bromopyridinium cations always form halogen-bonding contacts with the iodide anions shorter than the sum of the vdW radii, while chloropyridinium cations mostly participate in longer contacts or fail to form halogen bonds. Although a DFT study of the electrostatic potential has shown that both protonation and N-methylation of halogenopyridines leads to a considerable increase in the ESP of the halogen σ-hole, it is generally not the most positive site on the cation, allowing for alternate binding sites.

Halogen Bonding in N-Alkyl-3-halogenopyridinium Salts

We performed a structural study of N-alkylated halogenopyridinium cations to examine whether choice of the N-substituent has any considerable effect on the halogen bonding capability of the cations. For that purpose, we prepared a series of N-ethyl-3-halopyridinium iodides and compared them with their N-methyl-3-halopyridinium analogues. Structural analysis revealed that N-ethylated halogenopyridinium cations form slightly shorter C−X⋯I− halogen bonds with iodide anion. We have also attempted synthesis of ditopic symmetric bis-(3-iodopyridinium) dications. Although successful in only one case, the syntheses have afforded two novel ditopic asymmetric monocations with an iodine atom bonded to the pyridine ring and another on the aliphatic N-substituent. Here, the C−I⋯I− halogen bond lengths involving pyridine iodine atom were notably shorter than those involving an aliphatic iodine atom as a halogen bond donor. This trend in halogen bond lengths is in line with the charge distribution on the Hirshfeld surfaces of the cations—the positive charge is predominantly located in the pyridine ring making the pyridine iodine atom σ-hole more positive than the one on the alkyl chan.

N-Heterocyclic Iod(az)olium Salts - Potent Halogen-Bond Donors in Organocatalysis

This article describes the application of N-heterocyclic iod(az)olium salts (NHISs) as highly reactive organocatalysts. A variety of mono- and dicationic NHISs are described and utilized as potent XB-donors in halogen-bond catalysis. They were benchmarked in seven diverse test reactions in which the activation of carbon- and metal-chloride bonds as well as carbonyl and nitro groups was achieved. N-methylated dicationic NHISs rendered the highest reactivity in all investigated catalytic applications with reactivities even higher than all previously described monodentate XB-donors based on iodine(I) and (III) and the strong Lewis acid BF3 .

Predicting the catalytic activity of azolium-based halogen bond donors: an experimentally-verified theoretical study

This report demonstrates the successful application of electrostatic surface potential distribution analysis for evaluating the relative catalytic activity of a series of azolium-based halogen bond donors. A strong correlation (R² > 0.97) was observed between the positive electrostatic potential of the σ-hole on the halogen atom and the Gibbs free energy of activation of the model reactions (i.e., halogen abstraction and carbonyl activation). The predictive ability of the applied approach was confirmed experimentally. It was also determined that the catalytic activity of azolium-based halogen bond donors was generally governed by the structure of the azolium cycle, whereas the substituents on the heterocycle had a limited impact on the activity. Ultimately, this study highlighted four of the most promising azolium halogen bond donors, which are expected to exhibit high catalytic activity.

Synthesis and Properties of ortho-t-BuSO2C6H4-Substituted Iodonium Ylides

Iodonium ylides have recently attracted much attention on account of their synthetic applications. However, only a limited number of reports concerning the properties and reactivity of iodonium ylides exist, which is partly due to their instability. In this study, we synthesized several iodonium ylides that bear both an electron-withdrawing group and an aromatic ring with an ortho-t-BuSO2 group. Based on the crystal structures of the synthesized iodonium ylides in combination with natural-bond-orbital (NBO) calculations, we estimated the strength of the intra- and intermolecular halogen-bonding interactions. In addition, we investigated the reactivity of the iodonium ylides under photoirradiation.

Silver-Free Au(I) Catalysis Enabled by Bifunctional Urea- and Squaramide-Phosphine Ligands via H-Bonding

A library of gold(I) chloride complexes with phosphine ligands incorporating pendant (thio)urea and squaramide H-bond donors was prepared with the aim of promoting chloride abstraction from Au(I) via H-bonding. In the absence of silver additives, complexes bearing squaramides and trifluoromethylated aromatic ureas displayed good catalytic activity in the cyclization of N-propargyl benzamides, as well as in a 1,6-enyne cycloisomerization, a tandem cyclization-indole addition reaction and the hydrohydrazination of phenylacetylene. Kinetic studies and DFT calculations indicate that the energetic span of the reaction is accounted by both the chloride abstraction step, facilitated by the bidentate H-bond donor via an associative mechanism, and the subsequent cyclization step.

Iodine(III)-Based Halogen Bond Donors: Properties and Applications

Halogen bonding, the non-covalent interaction of Lewis bases with an electron-deficient region of halogen substituents, received increased attention recently. Consequently, the design and evaluation of numerous halogen-containing species as halogen bond donors have been subject to intense research. More recently, organoiodine compounds at the iodine(III) state have been receiving growing attention in the field. Due to their electronic and structural properties, they provide access to unique binding modes. For this reason, our groups have been involved in the development of such compounds, in the quantification of their halogen bonding strength (through the evaluation of their Lewis acidities), as well as in the evaluation of their activities as catalysts in several model reactions. This account will describe these contributions.

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.

Preparation and Synthetic Applicability of Imidazole-Containing Cyclic Iodonium Salts

A novel approach to the preparation of imidazole-substituted cyclic iodonium salts has been developed via the oxidative cyclization of 1-phenyl-5-iodoimidazole using a cheap and available Oxone/H₂SO₄ oxidative system. The structure of the new polycyclic heteroarenes has been confirmed by single-crystal X-ray diffractometry, revealing the characteristic structure features for cyclic iodonium salts. The newly produced imidazole-flanked cyclic iodonium compounds were found to readily engage in a heterocyclization reaction with elemental sulfur, affording benzo[5,1-b]imidazothiazoles in good yields.

Halogen Bonding Involving I2 and d8 Transition-Metal Pincer Complexes

We systematically investigated iodine–metal and iodine–iodine bonding in van Koten’s pincer complex and 19 modifications changing substituents and/or the transition metal with a PBE0–D3(BJ)/aug–cc–pVTZ/PP(M,I) model chemistry. As a novel tool for the quantitative assessment of the iodine–metal and iodine–iodine bond strength in these complexes we used the local mode analysis, originally introduced by Konkoli and Cremer, complemented with NBO and Bader’s QTAIM analyses. Our study reveals the major electronic effects in the catalytic activity of the M–I–I non-classical three-center bond of the pincer complex, which is involved in the oxidative addition of molecular iodine I2 to the metal center. According to our investigations the charge transfer from the metal to the σ* antibonding orbital of the I–I bond changes the 3c–4e character of the M–I–I three-center bond, which leads to weakening of the iodine I–I bond and strengthening of the metal–iodine M–I bond, facilitating in this way the oxidative addition of I2 to the metal. The charge transfer can be systematically modified by substitution at different places of the pincer complex and by different transition metals, changing the strength of both the M–I and the I2 bonds. We also modeled for the original pincer complex how solvents with different polarity influence the 3c–4e character of the M–I–I bond. Our results provide new guidelines for the design of pincer complexes with specific iodine–metal bond strengths and introduce the local vibrational mode analysis as an efficient tool to assess the bond strength in complexes.

P,N-Chelated Gold(III) Complexes: Structure and Reactivity

Gold(III) complexes are versatile catalysts offering a growing number of new synthetic transformations. Our current understanding of the mechanism of homogeneous gold(III) catalysis is, however, limited, with that of phosphorus-containing complexes being hitherto underexplored. The ease of phosphorus oxidation by gold(III) has so far hindered the use of phosphorus ligands in the context of gold(III) catalysis. We present a method for the generation of P,N-chelated gold(III) complexes that circumvents ligand oxidation and offers full counterion control, avoiding the unwanted formation of AuCl₄^-. On the basis of NMR spectroscopic, X-ray crystallographic, and density functional theory analyses, we assess the mechanism of formation of the active catalyst and of gold(III)-mediated styrene cyclopropanation with propargyl ester and intramolecular alkoxycyclization of 1,6-enyne. P,N-chelated gold(III) complexes are demonstrated to be straightforward to generate and be catalytically active in synthetically useful transformations of complex molecules.

A Bidentate Iodine(III)-Based Halogen-Bond Donor as a Powerful Organocatalyst*

In contrast to iodine(I)-based halogen bond donors, iodine(III)-derived ones have only been used as Lewis acidic organocatalysts in a handful of examples, and in all cases they acted in a monodentate fashion. Herein, we report the first application of a bidentate bis(iodolium) salt as organocatalyst in a Michael and a nitro-Michael addition reaction as well as in a Diels-Alder reaction that had not been activated by noncovalent organocatalysts before. In all cases, the performance of this bidentate XB donor distinctly surpassed the one of arguably the currently strongest iodine(I)-based organocatalyst. Bidentate coordination to the substrate was corroborated by a structural analysis and by DFT calculations of the transition states. Overall, the catalytic activity of the bis(iodolium) system approaches that of strong Lewis acids like BF3 .