Halogen Elimination Research Articles

Asymmetric Elimination Reaction on Chiral Metal Surfaces.

The production of enantiopure materials and molecules is of uttermost relevance in research and industry in numerous contexts, ranging from nonlinear optics to asymmetric synthesis. In the context of the latter, dehalogenation, which is an essential reaction step for a broad class of chemical reactions, is investigated; specifically, dehalogenation of prochiral 5-bromo-7-methylbenz(a)anthracene (BMA) on prototypical, chiral, intermetallic PdGa{111} surfaces under ultrahigh vacuum conditions. Asymmetric halogen elimination is demonstrated by combining temperature-programmed X-ray photoelectron spectroscopy, scanning probe microscopy, and density functional theory. On the PdGa{111} surfaces, the difference in debromination temperatures for the two BMA surface enantiomers amounts up to an unprecedented 46 K. The significant dependence of the dehalogenation temperature of the BMA surface enantiomers on the atomic termination of the PdGa{111} surfaces implies that the ensemble effect is pronounced in this reaction step. These findings evidence enantiospecific control and hence promote intrinsically chiral crystals for asymmetric on-surface synthesis.

Structural Dynamics of C2F4I2 in Cyclohexane Studied via Time-Resolved X-ray Liquidography.

The halogen elimination of 1,2-diiodoethane (C2H4I2) and 1,2-diiodotetrafluoroethane (C2F4I2) serves as a model reaction for investigating the influence of fluorination on reaction dynamics and solute–solvent interactions in solution-phase reactions. While the kinetics and reaction pathways of the halogen elimination reaction of C2H4I2 were reported to vary substantially depending on the solvent, the solvent effects on the photodissociation of C2F4I2 remain to be explored, as its reaction dynamics have only been studied in methanol. Here, to investigate the solvent dependence, we conducted a time-resolved X-ray liquidography (TRXL) experiment on C2F4I2 in cyclohexane. The data revealed that (ⅰ) the solvent dependence of the photoreaction of C2F4I2 is not as strong as that observed for C2H4I2, and (ⅱ) the nongeminate recombination leading to the formation of I2 is slower in cyclohexane than in methanol. We also show that the molecular structures of the relevant species determined from the structural analysis of TRXL data provide an excellent benchmark for DFT calculations, especially for investigating the relevance of exchange-correlation functionals used for the structural optimization of haloalkanes. This study demonstrates that TRXL is a powerful technique to study solvent dependence in the solution phase.

Reaction mechanism of halogenated rubber crosslinking using a novel environmentally friendly curing system

Iron (III) acetylacetonate (Fe (acac)) in the presence of triethanolamine (TEOA) was utilized as a novel crosslinking agent for halogenated diene rubber. Following the assumption that the mechanism of the crosslinking bases on the Heck-type reaction mechanism, which requires the presence of a halogen and an unsaturated carbon-carbon double bond, chloroprene rubber (CR) and brominated butyl rubber (BIIR) were utilized as rubber matrices. The results of FTIR spectra analysis confirm the proposed mechanism and indicate that a Heck-type reaction is feasible for performing a crosslinking of halogenated diene rubbers. The use of the Fe (acac)/TEOA curing system results in a significant torque increase during the vulcanization, which confirms the high activity of those compounds. The elimination of halogen from a rubber macromolecular structure or elimination of a basic environment of the crosslinking reaction results in a deactivation of the new curing system.

Synthesis and Application of a Formaldehyde-free Flame-retardant for Cotton Fabrics by Polymerization between Diammonium Hydrogen Phosphate and Citric Acid

ABSTRACT Elimination of halogen and formaldehyde from flame-retardants is the focus of the recent research due to their adverse effects on the human health and the environment. This research investigates the numerous halogen-free and formaldehyde-free flame retardant recipes for cotton fabrics through polymerization between diammonium hydrogen phosphate (DAHP) and citric acid considering the effects of catalyst, initiator, temperature, normal atmosphere, inert atmosphere, and nitrogen atmosphere. Then, this research applies these nontoxic recipes on the cotton fabric samples and determines the char-length and char-width. The further analysis included the limiting oxygen index (LOI), crease recovery angle (CRA), air permeability retention, shrinkage, smoldering cigarette test, fourier transform infrared spectroscopy (FTIR) analysis, scanning electron microscope (SEM) analysis, energy dispersive X-ray spectroscopy (EDX), phosphorous concentration, and formaldehyde content in fabric. The newly proposed recipes exhibited similar or superior performance compared with formaldehyde-based commercial flame retardant, Pyrovatex.

Regio-selective cyanation of (Z)-(1,2-dibromo-2-arylvinyl)triisopropylsilane with suppression of halogen elimination

Highly regio-selective cyanation of vicinal (Z)-dibromoalkenyl silanes was achieved by a vinylic Rosenmund-von Braun reaction, significantly suppressing side-production of alkyne. The alkyne was generated by a halogen elimination side-reaction that is an intrinsic problem in metal-activation of vicinal dihaloalkenes. We have studied to overcome the problem, and finally found the combination of CuCN and O=PPh3 in toluene solvent effectively controlled the production of byproducts. The resultant single isomer has significance in potentially application as a multi-tunable synthetic scaffold.

Photoreductive Elimination of Chlorine from Antimony in an [SbPd]VII Complex.

As part of our ongoing interest in main group species that support the photoreductive elimination of halogens, we have synthesized [Cl3SbVPdIICl2(o-dppp)2] (o-dppp = o-(Ph2P)C6H4), a palladium dichloride complex featuring a Lewis acidic trichlorostiborane moiety positioned within the ligand architecture. Upon UV irradiation, this complex undergoes a clean photoreductive chlorine elimination reaction which produces [Cl2SbIVPdICl(o-dppp)2], a complex with a covalent Sb-Pd bond. This transformation illustrates the viability of antimony as an element from which a halogen can be photoeliminated; it also shows that a metal halide can be photolytically coupled with a main group halide to generate a metal-element bond.

Stereoelectronic Effects in Cl2 Elimination from Binuclear Pt(III) Complexes.

Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt(III) complexes featuring meridionally coordinated Pt(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt-X bond strength. Conversely, when compared to an isomeric facial Pt(III) trichloride, a much more efficient photoelimination is observed for the fac isomer than would be predicted based on thermochemistry. This difference in the fac vs mer isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing.

Photochemistry of 5‐Halogenosalicylic Acids: Evidence of the Triplet Involvement in the Carbene Formation

AbstractThe heterolytic photodehalogenation of 4‐halogenophenols in water is known to yield the long‐lived triplet carbene 4‐oxo‐2,5‐dienylidene. However, the nature of the excited state from which dehalogenation takes place remains an open issue. To get further insight into the mechanism of carbene formation, we investigated the photolysis of 5‐chlorosalicylic acid and 5‐bromosalicylic acid from experimental and theoretical points of view. Our results showed that irradiation of monoanionic species in water leaded to the formation of the carbene species and its characteristic photoproducts. Moreover, triplet excited states could be detected by laser flash photolysis of aqueous solutions and their involvement in the photodehalogenation was firmly demonstrated. Identification by quantum calculations of two distinct triplet species supports the experimental results. The short‐ and long‐lived triplets are assigned respectively to the planar and non planar structures. Both were successfully detected experimentally. The non planar triplet either reacted with water to yield the dihydroxyderivative or gave the triplet carbene after heterolytic halogen elimination. These findings shed a new light on our understanding of the halogenophenols photochemistry and could be exploited in further studies.

Halogen Photoelimination from Monomeric Nickel(III) Complexes Enabled by the Secondary Coordination Sphere

Endothermic halogen elimination reactions, in which molecular halogen photoproducts are generated in the absence of chemical traps, are rare. Inspired by the proclivity of mononuclear Ni(III) complexes to participate in challenging bond-forming reactions in organometallic chemistry, we targeted Ni(III) trihalide complexes as platforms to explore halogen photoelimination. A suite of Ni(III) trihalide complexes supported by bidentate phosphine ligands has been synthesized and characterized. Multinuclear NMR, EPR, and electronic absorption spectroscopies, as well as single-crystal X-ray diffraction, have been utilized to characterize this suite of complexes as distorted square pyramidal, S = 1/2 mononuclear Ni(III) complexes. All complexes participate in clean halogen photoelimination in solution and in the solid state. Evolved halogen has been characterized by mass spectrometry and quantified chemically. Energy storage via halogen elimination was established by solution-phase calorimetry measurements; in al...

Trap-Free Halogen Photoelimination from Mononuclear Ni(III) Complexes.

Halogen photoelimination reactions constitute the oxidative half-reaction of closed HX-splitting energy storage cycles. Here, we report high-yielding, endothermic Cl2 photoelimination chemistry from mononuclear Ni(III) complexes. On the basis of time-resolved spectroscopy and steady-state photocrystallography experiments, a mechanism involving ligand-assisted halogen elimination is proposed. Employing ancillary ligands to promote elimination offers a strategy to circumvent the inherently short-lived excited states of 3d metal complexes for the activation of thermodynamically challenging bonds.

Photocrystallographic observation of halide-bridged intermediates in halogen photoeliminations.

Polynuclear transition metal complexes, which frequently constitute the active sites of both biological and chemical catalysts, provide access to unique chemical transformations that are derived from metal–metal cooperation. Reductive elimination via ligand-bridged binuclear intermediates from bimetallic cores is one mechanism by which metals may cooperate during catalysis. We have established families of Rh2 complexes that participate in HX-splitting photocatalysis in which metal–metal cooperation is credited with the ability to achieve multielectron photochemical reactions in preference to single-electron transformations. Nanosecond-resolved transient absorption spectroscopy, steady-state photocrystallography, and computational modeling have allowed direct observation and characterization of Cl-bridged intermediates (intramolecular analogues of classical ligand-bridged intermediates in binuclear eliminations) in halogen elimination reactions. On the basis of these observations, a new class of Rh2 complexes, supported by CO ligands, has been prepared, allowing for the isolation and independent characterization of the proposed halide-bridged intermediates. Direct observation of halide-bridged structures establishes binuclear reductive elimination as a viable mechanism for photogenerating energetic bonds.

Probing Chemical Catalysts – How Can Advanced Crystallography Help?

Crystallography can directly provide ultimate three-dimensional structural information in a way that other science difficult to approach. We will discuss how we have used photocrystallography studies and anomalous scattering analysis to probe chemical catalysts: 1) Photochemical HCl splitting provides an opportunity to store solar energy as a chemical fuel, and is mechanistically simpler than water splitting chemistry. Families of binuclear transition metal complexes have been developed for photochemical HX-splitting but the bottle-neck to development of second generation catalysts is the challenge of halogen elimination chemistry. Current photocrystallography experiments have allowed direct observation of metastable intermediates in the photochemical manifolds of both Rh2 and Pt2 complexes, which has led to the design on new families of halide-bridged Rh2 catalysts for HX splitting. 2) Metal atom substitution within homotrinuclear clusters of Co, Mn, and Fe can have dramatic effects in the local electronic structure, which results in the changes in observed reactivity, substrate binding affinities, and catalytic function. Anomalous scattering measurements are critical for the unambiguous assignment of the trinuclear core composition, which has allowed us to investigate changes in both ensemble electronic structures and reactivity profiles.

Molecular halogen elimination from halogen-containing compounds in the atmosphere

Atmospheric halogen chemistry has drawn much attention, because the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. Atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas direct elimination of the X2 product has been seldom discussed or remained a controversial issue. This account is intended to review the detection of X2 primary products using cavity ring-down absorption spectroscopy in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C2H4Br2 and 1,2-C2H4Br2) and dibromoethylenes (1,1-C2H2Br2 and 1,2-C2H2Br2), diiodomethane (CH2I2), thionyl chloride (SOCl2), and sulfuryl chloride (SO2Cl2), along with a brief discussion on acyl bromides (BrCOCOBr and CH2BrCOBr). The optical spectra, quantum yields, and vibrational population distributions of the X2 fragments have been characterized, especially for Br2 and I2. With the aid of ab initio calculations of potential energies and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to assess the halogen-related environmental variation.

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.

Halogen-free ionic liquids and their utilization as cellulose solvents

This work demonstrates a novel synthesis route to halogen-free ionic liquids. A one-pot synthetic reaction route avoiding the use of toxic and high-energetic alkyl halides was developed to reduce the environmental impact of the synthesis process of ionic liquids. However, the elimination of halogens and alkyl halides in the preparation of ionic liquids is not just an environmental issue: the aforementioned species are also among the most common and persistent contaminants in today’s Ionic Liquids (ILs). Thus, this paper introduces a range of quaternized nitrogen based ionic liquids, including both aromatic and non-aromatic components, all prepared without alkyl halides in any step of the process. Moreover, bio-renewable precursors such as (bio-)alcohols and carboxylic acids were employed as anion sources and alkylation media, thus avoiding halogen contamination or halogen-containing anions. The IL’s prepared were designed to dissolve cellulose, some of which was included in a cellulose dissolution study using a sulphite cellulose from the company Domsjö.

Template Synthesis of Subnanometer Gold Clusters in Interfacially Cross-Linked Reverse Micelles Mediated by Confined Counterions

A cationic surfactant with a triallylammonium headgroup was cross-linked photochemically in the presence of a hydrophilic dithiol in the reverse micelle (RM) configuration. The interfacially cross-linked reverse micelles (ICRMs) are unusual templates for nanomaterials synthesis. Our previous work indicated that the ICRMs could extract anionic metal salts such as tetracholoroaurate into the hydrophilic interior, and the entrapped aurate was reduced without externally added reducing agent to form subnanometer luminescent gold clusters [Zhang, S.; Zhao, Y. ACS Nano 2011, 5, 2637-2646]. In this work, the bromide counterions were established as the reducing agent in the template synthesis. The reduction of tetrachloroaurate was proposed to happen through ligand exchange on the aurate by the bromide ions, reductive elimination of halogen, and disproportionation of the Au(I) intermediate. The size of the gold clusters could be tuned rationally by the water-to-surfactant ratio (W(0)) and the reducing agent. Monodisperse Au(4) and Au(9-10) clusters as well as larger Au(18) and Au(23) clusters were obtained from the ICRM templates. The as-prepared, metastable gold clusters were subject to reconstruction triggered by ligand exchange on the surface but could be stabilized through proper surface protection using a chelating dithiol.

ChemInform Abstract: Iron‐Mediated One‐Pot Formal Nitrocyclization onto Unactivated Alkenes.

AbstractChloronitration of ω‐alkenylamine or alcohol derivatives and subsequent halogen elimination, followed by intramolecular nitro‐Michael addition under basic conditions provides access to heterocyclic compounds.

ChemInform Abstract: Ynamines Derived from Nucleic Acids Bases: Synthesis, Reactivity, and Biological Activity.

Ynamines 1a–d are prepared by alkylation of nucleic acids bases with tetrachloroethylene followed by elimination of halogens from the intermediates 4a–d; reaction of 1a with acetone and cyclohexanone gives carbinols 8a and b and in the case of 8b cyclic ketal 9a is also obtained; compound 1a is a substrate for adenosine deaminase.

Halogen Photoreductive Elimination from Metal−Metal Bonded Iridium(II)−Gold(II) Heterobimetallic Complexes

Halogen oxidation of [Ir(I)Au(I)(dcpm)(2)(CO)X](PF(6)) (dcpm = bis(dicyclohexylphosphino)methane, X = Cl, Br) and [Ir(I)Au(I)(dppm)(2)(CN(t)Bu)(2)](PF(6))(2) (dppm = bis(diphenylphosphino)methane) furnishes the heretofore unknown class of d(7)-d(9) compounds comprising an Ir(II)Au(II) heterobimetallic core. A direct metal-metal bond is evident from a 0.2 A contraction in the intermetallic distance, as determined by X-ray crystallography. The photophysical consequence of iridium-gold bond formation, as elucidated by experimental and computational investigations, is an electronic structure dominated by a sigma --> sigma* transition that possesses significant ligand-to-metal charge transfer (LMCT) character. Accordingly, these compounds are non-emissive but photoreactive. Excitation of Ir(II)Au(II) complexes in the presence of a halogen trap prompts a net photoreductive elimination of halogen and the production of the two-electron reduced Ir(I)Au(I) species with about 10% quantum efficiency. The Ir(II)Au(II) complexes add to a growing library of d(7)-d(9) heterobimetallic species from which halogen elimination may be driven by a photon.

Halogen Photoreductive Elimination from Gold(III) Centers

Monomeric complexes of the type Au(III)(PR(3))X(3) and bimetallic complexes of the type Au(2)(I,III)[mu-CH(2)(R(2)P)(2)]X(4) and Au(2)(III,III)[mu-CH(2)(R(2)P)(2)]X(6) (R = Ph, Cy, X = Cl(-), Br(-)) undergo facile photoelimination of halogen. M-X bond activation and halogen elimination is achieved upon LMCT excitation of solutions of Au(III) complexes in the presence of olefin chemical traps. As opposed to the typical one-electron redox transformations of LMCT photochemistry, the LMCT photochemistry of the Au(III) centers allows for the unprecedented (i) two-electron photoelimination of X(2) from a monomeric center and (ii) four-electron photoelimination of X(2) from a bimetalllic center. The quantum yields for X(2) photoproduction, in general, are between 10% and 20% for all species, showing only minimal dependence on the identity of the ligands about gold, or the nuclearity of the complex. Efficient X(2) photoelimination is observed in the absence of a chemical trap, providing a rare example of authentic, trap-free halogen elimination from a transition metal center.