Fluoride Counterions Research Articles

Fluoride counterions boost gold(i) catalysis: case studies for hydrodefluorination and CO2 hydrosilylation

We reported a prominent counterion effect, where bifluoride synergistically activates hydrosilane to promote the formation of an active gold hydride intermediate, on hydrodefluorination and hydrosilylation of CO2.

In Situ Generated DBU⋅HF Acts as a Fluorinating Agent in a Hexafluoroisobutylation Tandem Reaction: An Effective Route to 5,5,5,5’,5’,5’‐Hexafluoroleucine

AbstractWe report the direct incorporation of the hexafluoroisobutyl group on a chiral glycine Schiff base complex mediated by 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU). The fluoroalkylation involves 2‐(bromomethyl)‐1,1,1,3,3,3‐hexafluoropropane reagent, which generates in situ hexafluoroisobutylene (HFIB), and reacts then with the enolate through a tandem allylic shift/hydrofluorination process. We showed that the use of neutral organic base DBU generates in situ an original DBU⋅HF salt, which preserves the fluoride nucleophilicity and acts as a fluorinating agent. This fluoride salt promotes the hydrofluorination of the pentafluorinated alkene overcoming the usual fluoride β‐elimination observed with α‐CF3‐vinyl reagents. With alkali metal bases, by contrast, the hydrofluorination is disfavored and the pentafluorinated alkene intermediate is obtained predominantly. This study highlights the critical role of the fluoride counter ion to preserve its nucleophilicity. The protocol is amenable to multidecagram scale synthesis of enantiopure (S)‐ and (R)‐5,5,5,5’,5’,5’‐hexafluoroleucine and their N‐Fmoc or N‐Boc derivatives in good overall yield.

Inside-protonated 1-azaadamantane: computational studies on the structure, stability, and generation

Post Hartree–Fock and density functional theory methods have been employed to study inside-protonated 1-azaadamantane 7 and its complexes with the fluoride counterion (contact ion pairs) 10 and 11. The study also involved 1-azaadamantane 4, its outside-protonated form 8, and 1-azaadamantane radical cation 17. Inside-protonated 1-azaadamantane 7 is more than 82 kcal mol−1less stable than out-isomer 8. The repulsive interaction between the internal N+–H group and the azaadamantane cage and a substantial deformation of this cage greatly weaken the C–N and C–C bonds and, consequently, lead to a low kinetic stability of in-ion 7 in the studied unimolecular and bimolecular reactions involving the removal of the encapsulated proton from the cage. Among these reactions, a 7 → 8 rearrangement through a reversible cage opening at the C–N bond was found to be the main transformation channel ([Formula: see text] < 16 kcal mol−1) for in-ion 7. This rearrangement can be catalyzed by an external base, e.g., the fluoride anion. A 1,4-hydrogen migration in 1-azaadamantane radical cation 17 as a possible pathway to the inside-protonated 1-azaadamantane 7 was explored. It was found that this process has a prohibitively high activation barrier, [Formula: see text] > 104 kcal mol−1, and is not able to compete with the α-C–C cleavage of the azaadamantane cage ([Formula: see text] < 26 kcal mol−1).

DNA-Polyelectrolyte Complexation Study: The Effect of Polyion Charge Density and Chemical Nature of the Counterions.

Complexes of polycations and DNA, also known as polyplexes, have been extensively studied in the past decade, as potential gene delivery systems. Their stability depends strongly on the characteristics of the polycations, as well as the nature of the added salt. We present here a study of the DNA ionene complexation in which we used fluorescence, UV, and CD spectroscopy, combined with molecular dynamics computer simuations, to systematically examine the influence of the polycation charge density, as well as the influence of the nature of the counterion, on the stability of these systems. Ionenes as polycations, depending on their structural characteristics, have previously been found to possess low cytotoxicity, and are therefore particularly interesting as potential gene delivery agents. The results show that the DNA solutions in the presence of the polycation are more stable in the case of very large or very small ionene charge density, suggesting different mechanism of complexation. The computer simulations show that the ionenes with high charge density bind to the minor groove of the DNA molecules, while the ionenes with lower charge density bind to the major groove of the DNA. The nature of the counterions play only a minor role: precipitation of the DNA molecules occurs at slightly lower ionene concentration when fluoride counterion are present, compared to the bromide counterions.

Quantification of counterion binding to and its effects on aqueous dispersions of dialkyl cationic surfactants

Dialkyl cationic surfactants, such as dioctadecyl dimethyl ammonium bromide (DODAB) form vesicles upon dispersion in water. In order to study the influence of the surfactant's counterion, the same dialkyl surfactant was prepared with a fluoride counterion (DODAF). Visually, quite transparent dispersions were prepared when fluoride was selected as counterion, whereas more turbid dispersions were obtained using the bromide surfactant. LR-NMR revealed that the permeability of the DODAF dispersions for water was much larger, which indicated that these rather contained uni- or oligolamellar vesicles, whereas the DODAB dispersions were multilamellar. These observations seemed to indicate an increasing degree of surface charge neutralization by the heavier anions, in line with the Hofmeister series, which was indeed confirmed by electrophoretic light scattering measurements. In order to further quantify the surfactant-counterion interaction, HR-NMR diffusometry measurements were performed on dispersions of a dialkyl cationic surfactant with methyl sulphate counterion in deuterated water. The diffusion data showed that the mobility of this organic anion increased when adding different sodium halides, with a stronger effect from fluoride over chloride and bromide to iodide, which clearly indicated the gradual displacement of the methyl sulphate counterion from the vesicular surface by the stronger binding anions.

Assessment of the various ionization methods in the analysis of metal salen complexes by mass spectrometry.

Metal salen complexes are one of the most frequently used catalysts in enantioselective organic synthesis. In the present work, we compare a series of ionization methods that can be used for the mass spectral analysis of two types of metalosalens: ionic complexes (abbreviated as Com(+)X(-)) and neutral complexes (NCom). These methods include electron ionization and field desorption (FD) which can be applied to pure samples and atmospheric pressure ionization techniques: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) which are suitable for solutions. We found that FD is a method of choice for recording molecular ions of the complexes containing even loosely bonded ligands. The results obtained using atmospheric pressure ionization methods show that the results depend mainly on the structure of metal salen complex and the ionization method. In ESI spectra, Com(+) ions were observed, while in APCI and APPI spectra both Com(+) and [Com + H](+) ions are observed in the ratio depending on the structure of the metal salen complex and the solvent used in the analysis. For complexes with tetrafluoroborate counterion, an elimination of BF3 took place, and ions corresponding to complexes with fluoride counterion were observed. Experiments comparing the relative sensitivity of ESI, APCI and APPI (with and without a dopant) methods showed that for the majority of the studied complexes ESI is the most sensitive one; however, the sensitivity of APCI is usually less than two times lower and for some compounds is even higher than the sensitivity of ESI. Both methods show very high linearity of the calibration curve in a range of about 3 orders of magnitude of the sample concentration.

Thermodynamic analysis of the interaction of partially hydrophobic cationic polyelectrolytes with sodium halide salts in water

Isothermal titration calorimetry was used to determine the temperature and concentration dependence of the enthalpy of mixing of 3,3- and 6,6-ionene fluorides, bromides, and iodides with low molecular weight salts (NaF, NaCl, NaBr, and NaI) in water. The magnitudes of the enthalpies, measured in the temperature range from 273 to 318 K, depended on the number of methylene groups on the ionene polyion (hydrophobicity), and on the anion of the added salt (ion-specificity). All enthalpies of mixing of 3,3- and 6,6-ionene fluorides with low molecular weight salts (NaCl, NaBr, and NaI) were negative, which is in contrast to the predictions of standard theories of polyelectrolyte solutions. This fact was interpreted in the light of the ion–water short-range interactions that are not accounted for in those theories. In contrast, the enthalpies of mixing of 3,3- and 6,6-ionene bromides and iodides with NaF were positive, being in accord with theory. Using the calorimetric data, we performed a model thermodynamic analysis of the polyelectrolyte–salt mixing process to obtain changes in the apparent standard Gibbs free energy, enthalpy, entropy, and heat capacity relative to the pure ionene fluorides in water. The results prove that halide ions replace fluoride counterions with a strength increasing in the order chloride < bromide < iodide. The process is enthalpy governed, accompanied by a positive change in the heat capacity.

Aqueous solutions of ionenes: interactions and counterion specific effects as seen by neutron scattering

Aqueous solutions of ionenes with bromide and fluoride counterions have been investigated using small angle neutron scattering for the first time. Ionenes are a class of cationic polyelectrolytes based on quaternary ammonium atoms and, considering the very low solubility of their uncharged part (hydrocarbon chain), would be formally classified as hydrophobic. Ionenes present important structural differences over previously studied polyelectrolytes: (a) charge is located on the polyelectrolyte backbone, (b) the distance between charges is regular and tunable by synthesis, (c) hydrophobicity comes from methylene groups of the backbone and not from bulky side groups. Results for Br ionenes feature a disappearance of the well-known polyelectrolyte peak beyond a given monomer concentration. Below this concentration, the position of the peak depends on the chain charge density, f(chem), and scales as f(chem)(0.30±0.04). This is an indication of a hydrophilic character of the ionene backbone. In addition, osmotic coefficients of ionene solutions resemble again other hydrophilic polyelectrolytes, featuring no unusual increase in the water activity (or a significant counterion condensation). We conclude that despite the hydrophobicity of the hydrocarbon chain separating charged centers on ionenes, these chains behave as hydrophilic. In contrast to Br ionenes, the polyelectrolyte peak remains at all concentrations studied for the single F ionene investigated. This strong counterion effect is rationalized in terms of the different hydrating properties and ion pairing in the case of bromide and fluoride ions.

ChemInform Abstract: Charge Transport and Optical Properties of the First Highly Conductive One-Dimensional Molecular Metal Containing Fluoride Counterions: (TSeT) F0.25Cl0.50.

ChemInform Abstract: Charge Transport and Optical Properties of the First Highly Conductive One-Dimensional Molecular Metal Containing Fluoride Counterions: (TSeT) F0.25Cl0.50.

Interplay of Ion-Specific and Charge-Density Effects in Aqueous Solutions of Weakly Charged Ionenes as Revealed by Electric-Transport Measurements

In 3,3-ionenes, one quaternary nitrogen is bonded to a chain of three methylene groups on each side, and in 6,9-ionene, it is bonded to a chain of six on one side and nine on the other. We examined how the solution properties of several ionenes changed with increased hydrophobicity of the polyion, depending on the nature of the counterion. We determined the electrical conductivities of aqueous solutions of 3,3-, 4,5-, 6,6-, and 6,9-ionene fluorides and bromides in the range of concentrations from 5 x 10(-3) to 1 x 10(-1) M and for the temperature interval 5-35 degrees C. Over these ranges, the conductivities of the ionenes were found to decrease with increasing concentration and increase with increasing temperature. The conductivity of 3,3-ionene bromide was lower than that of its fluoride analogue throughout the whole range of concentrations, whereas for the 6,9-ionenes, the trend was reversed. For 4,5- and 6,6-ionene, we observed a crossover in the concentration dependence of conductivity. The conductivity data were compared with the predictions of Manning's theory and scaling theory. Separately determined transport-number values were combined with the conductivity data to obtain the fractions of so-called "free" counterions, f. For bromide samples, f increased from 3,3- to 6,9-ionene. In the case of fluoride counterions, the fraction of free counterions was the lowest for 3,3-ionene and, within the experimental uncertainty, approximately constant for the other less charged ionenes.

Highly Selective Biaryl Cross-Coupling Reactions between Aryl Halides and Aryl Grignard Reagents: A New Catalyst Combination ofN-Heterocyclic Carbenes and Iron, Cobalt, and Nickel Fluorides

Combinations of N-heterocyclic carbenes (NHCs) and fluoride salts of the iron-group metals (Fe, Co, and Ni) have been shown to be excellent catalysts for the cross-coupling reactions of aryl Grignard reagents (Ar(1)MgBr) with aryl and heteroaryl halides (Ar(2)X) to give unsymmetrical biaryls (Ar(1)-Ar(2)). Iron fluorides in combination with SIPr, a saturated NHC ligand, catalyze the biaryl cross-coupling between various aryl chlorides and aryl Grignard reagents in high yield and high selectivity. On the other hand, cobalt and nickel fluorides in combination with IPr, an unsaturated NHC ligand, exhibit interesting complementary reactivity in the coupling of aryl bromides or iodides; in contrast, with these substrates the iron catalysts show a lower selectivity. The formation of homocoupling byproducts is suppressed markedly to less than 5% in most cases by choosing the appropriate metal fluoride/NHC combination. The present catalyst combinations offer several synthetic advantages over existing methods: practical synthesis of a broad range of unsymmetrical biaryls without the use of palladium catalysts and phosphine ligands. On the basis of stoichiometric control experiments and theoretical studies, the origin of the unique catalytic effect of the fluoride counterion can be ascribed to the formation of a higher-valent heteroleptic metalate [Ar(1)MF(2)]MgBr as the key intermediate in our proposed catalytic cycle. First, stoichiometric control experiments revealed the stark differences in chemical reactivity between the metal fluorides and metal chlorides. Second, DFT calculations indicate that the initial reduction of di- or trivalent metal fluoride in the wake of transmetalation with PhMgCl is energetically unfavorable and that formation of a divalent heteroleptic metalate complex, [PhMF(2)]MgCl (M = Fe, Co, Ni), is dominant in the metal fluoride system. The heteroleptic ate-complex serves as a key reactive intermediate, which undergoes oxidative addition with PhCl and releases the biaryl cross-coupling product Ph-Ph with reasonable energy barriers. The present cross-coupling reaction catalyzed by iron-group metal fluorides and an NHC ligand provides a highly selective and practical method for the synthesis of unsymmetrical biaryls as well as the opportunity to gain new mechanistic insights into the metal-catalyzed cross-coupling reactions.

Dielectric Relaxation Spectroscopy of Aliphatic Ionene Bromides and Fluorides in Water: The Role of the Polyion’s Charge Density and the Nature of the Counterions

Dielec. relaxation spectroscopy (DRS) has been chosen to study the dielec. properties of ionene bromides and fluorides in water. The best model for describing the exptl. data in the range of frequencies 0.05 < v/GHz < 89 was the one that considered two relaxing species: (i) counterions and (ii) the bulk water. From the redn. of the water amplitude, S2, effective hydration nos. could be derived. Relaxation times of counterions, τ1, decreased with the increasing concn., which was a consequence of stronger electrostatic screening in more concd. solns. Fluoride counterions were to a lesser extent affected by the polyion than bromide counterions. It seemed that charged sites of the polyions had a much greater influence on the F- counterions than the chains of -CH2- units between them. For ionene bromides, however, the influence of polyion's charges and of hydrophobic parts of the backbone manifested itself in a more complex manner. High hydration nos. in the case of Br- counterions as well as the insensitivity of τ1 on the charge d. of the polyion indicated that the role of the solvent could not be disregarded in these systems.

Modelling fast mode dielectric relaxation of counterions in aqueous solutions of ionene bromides and fluorides

The parameters derived from previously published (M. Luksicet al., Macromolecules, 2009, 42, 4337-4342) dielectric spectra of aqueous solutions of aliphatic 3,3-, 4,5-, 6,6-, and 6,9-ionene bromides and fluorides were analyzed within the existing models for the fast counterion-relaxation process. By fitting the Poisson-Boltzmann results to the experimental data for the osmotic coefficients of the ionenes in question, we were able to estimate the self diffusion coefficients of counterions. The coefficients were then used to calculate the correlation lengths as proposed by the Ito model. The corresponding correlation lengths obtained by the Mandel-Manning model were of similar magnitude. The observed trends in correlation lengths were consistent with the model-independent values derived from the experimental relaxation times. Combination of the dielectric data and Poisson-Boltzmann model analysis yields the conclusion that the fluoride counterions prefer longitudinal, whereas the bromide counterions favour transverse, fluctuation with respect to the polyelectrolyte backbone.

Description of the metal–ligand bonding in f‐element complexes: A DFT study including scalar relativistic effects

AbstractA comparative density functional study on metal–ligand (M–L) interaction has been performed on X3–M–L (X = F, I; M = U, La, Nd: L = NH3, NCCH3, CO) and X3–M–(CO)2 species including scalar relativistic effects by means of the zero‐order regular approximation (ZORA) hamiltonian. The role of the halogen atoms in modeling the M–L interactions is discussed for π‐acceptor ligands CO and NCCH3 and for the σ‐donor NH3. The fluoride counterions, compared with previous calculations on the iodide complexes, stabilize more tedraedral structure with longer M–L distances. Finally, the addition of a second ligand to the previous species has permitted a more complete discussion on the backbonding effects on the uranium complexes. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Location of Fluoride Counterion in As-Synthesized Silicalite-1 by Single Crystal X-ray Diffraction

This paper presents results from single-crystal X-ray diffraction at room temperature on as-synthesized tetrapropylammonium fluoride silicalite. The fluoride counteranion lies in a [415262] cage on a general position at an interacting distance from a silicon framework atom (Si−F = 1.915(3) A). This silicon atom is close to the center of a slightly distorted trigonal bypyramid consisting of four oxygen framework atoms and a fluoride anion. The Si−O apical bond distance (1.667(2) A) is clearly longer than the Si−O basal bond distances (1.62 ± 0.01 A). The occluded TPA+ cation is described using a disordered model at the channels intersection.

Charge transport and optical properties of the first highly conductive one-dimensional molecular metal containing tetraselenotetracene fluoride counterions, (TSeT)F0.25Cl0.50

Charge transport and optical properties of the first highly conductive one-dimensional molecular metal containing tetraselenotetracene fluoride counterions, (TSeT)F0.25Cl0.50

The first molecular metal containing fluoride counterions

Anhydrous, anaerobic electrocrystallization techniques have afforded (TSeT)F 0.25Cl 0.50, the first organic molecular metal to contain off-axis fluoride counterions. This material crystallizes in a unique tetragonal motif with stacks of canted TSeT +0.75 molecules. Disordered chloride and fluoride counterions are found in two different types of channels extending parallel to the stacking axis. Interchain Se…Se interactions are shorter than in the analogous (TSeT +y) (X −) y salts (X − = Cl −, Br −, I −). These structural changes are reflected in the electrical [ σ (300K) = 1700 S/cm]; σ (40K) = 7000 S/cm, thermoelectrical, and optical properties of this new organic metal.

Micellar catalysis of organic reactions. 28. A comparison of the catalytic effects of micelles containing bromide and fluoride counterions on some SNAr AND BAc2 reactions

AbstractThe catalytic effects of micelles of cetyltrimethylammonium bromide (CTAB) and fluoride (CTAF) on several SNAr and BAC2 reactions were studied. Rate versus detergent concentration curves predicted by the pseudophase models PPIE (ion exchange) for CTAB studies and PPMA (mass action) for the CTAF studies were fitted to the experimental results to determine the best values of the substrate binding constant (Ks), the second‐order rate constant for reaction in the micelles (K) and the exchange constant KOH/X (PPIE) or binding constant (PPMA) for the counterions. Better catalysis by micelles of CTAF is shown to be due to the combined effects of a more favourable exchange constant in CTAF than in CTAB, slightly different rates of reaction within the two micelles and different values of β, the fraction of micellar head groups neutralized in CTAF and in CTAB.In both micelles the magnitude of catalysis depends on the charge produced in the intermediate complex and the presence of electron‐withdrawing groups to stabilize that charge. Strongest catalysis was found for a reaction leading to a monoanionic intermediate complex in which the charge was stabilized by two nitro groups, and the weakest catalysis was found for a reaction leading to a dianionic intermediate in which no electron‐withdrawing groups were present.