Ion Pairs (ion + pair)

Distribution by Scientific Domains
Distribution within Chemistry

Kinds of Ion Pairs

  • contact ion pair


  • Selected Abstracts


    ChemInform Abstract: Preparation and Characterization of Cobalt-Containing P,N-Ligands and an Unusual Palladium Complex Ion Pair: Their Applications in Amination Reactions.

    CHEMINFORM, Issue 7 2009
    You-Chen Hsiao
    Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]


    Self-Aggregation Tendency of Zirconocenium Ion Pairs Which Model Polymer-Chain-Carrying Species in Aromatic and Aliphatic Solvents with Low Polarity

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2008
    Luca Rocchigiani
    From pairs to double pairs: Zirconocene ion pairs bearing an aliphatic chain of variable length were synthesized and investigated by means of NOE and diffusion NMR spectroscopy experiments. The presence of long aliphatic chains allowed an unprecedented investigation of their self-aggregation tendency in cyclohexane (see figure), which has a dielectric constant similar to that of isoparaffins used in industrial plants. [source]


    The Role of Ion Pairs in the Second-Order NLO Response of 4-X-1-Methylpiridinium Salts,

    CHEMPHYSCHEM, Issue 2 2010
    Francesca Tessore Dr.
    Abstract A series of 4-X-1-methylpyridinium cationic nonlinear optical (NLO) chromophores (X=(E)-CHCHC6H5; (E)-CHCHC6H4 -4,-C(CH3)3; (E)-CHCHC6H4 -4,-N(CH3)2; (E)-CHCHC6H4 -4,-N(C4H9)2; (E,E)-(CHCH)2C6H4 -4,-N(CH3)2) with various organic (CF3SO3,, p -CH3C6H4SO3,), inorganic (I,, ClO4,, SCN,, [Hg2I6]2,) and organometallic (cis -[Ir(CO)2I2],) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second-order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second-order NLO response, investigated by the electric-field-induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10,3,M, and their dipole moments were determined. Generally, below 5×10,4,M, ion pairs start to dissociate into ions with parallel increase of the second-order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10,3,M, some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second-order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self-aggregate. [source]


    Amphiphilic Organic Ion Pairs in Solution: A Theoretical Study

    CHEMPHYSCHEM, Issue 10 2007
    Vincent Pradines Dr.
    Abstract The macroscopic manifestation of hydrophobic interactions for amphiphilic organic ion pairs (tetraalkylammonium,anion) has been shown experimentally by measuring their association constants and their affinity with the organic phase. Beyond a certain size, there is a direct relation between association constants and chain lengths in tetraalkylammonium ions. We propose to cast a bridge between these results and geometrical properties considered at the level of a single ion pair by means of quantum chemistry calculations performed on model systems: trimethylalkylammonium,pentyl sulfate instead of tetraalkylammonium,dodecyl sulfate. Two limiting cases are considered: head-to-head configurations, which yield an optimal electrostatic interaction between polar heads, and parallel configurations with a balance between electrostatic and hydrophobic interactions. All properties (geometries, complexation energies, and atomic charges) were obtained at the MP2 level of calculation, with water described by a continuum model (CPCM). Dispersion forces link hydrocarbon chains of tetraalkylammonium ions and pentyl sulfate, thus yielding (for the largest ion pairs) parallel configurations favored with respect to head-to-head geometries by solute,solvent electrostatic interactions. Given the small experimental association energies, we probe the accuracy limit of the MP2 and CPCM methods. However, clear trends are obtained as a function of chain length, which agree with the experimental observations. The calculated monotonic stabilization of ion pairs when the hydrocarbon chain increases in length is discussed in terms of electrostatic interactions (between ions and between ion pairs and water), dispersion forces, and cavitation energies. [source]


    Ferrocene Conjugates Containing Diarginine and Aspartic Acid: Salt Bridge Interactions in Water

    EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 29-30 2009
    Anas Lataifeh
    Abstract The ferrocene peptide conjugates of diarginine (MeO-Fc-Arg-Arg-NH2) (1) and aspartic acid [Boc-Fca-Asp(OH)-OH] (2) were found to form a stable 1:1 associate in aqueous solution. The molecular recognition was achieved through a combination of multipoint hydrogen bonding (H-bonding) sites and a guanidinium-carboxylate ion pair. The associate stoichiometry was confirmed by using ESI-MS and NMR experiments; the NMR titration curve shows multiple equilibria with stepwise interconversion from 1:2,,,1:1 binding ratios, and the electrochemical behaviour of the ferrocenyl groups (Fc, Fca) confirm the formation of an ion pair. The CD spectra in the peptide region exhibit a characteristic absorption of a more ordered structure, while the ferrocene helical chirality remains intact. The solid-state IR measurements exclude the involvement of the amide backbone in the interaction.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]


    The Role of Amine,B(C6F5)3 Adducts in the Catalytic Reduction of Imines with H2: A Computational Study

    EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 15 2009
    Timofei Privalov
    Abstract This study thoroughly examines the potential energy surfaces (PESs) of two possible mechanisms for reduction of imines by B(C6F5)3 and H2. The key reaction steps of the first catalytic mechanism, which is the focus of our study, are: (i) the uptake of H2 by a thermally activated amine,B(C6F5)3 species; (ii) proton transfer from the NH2+ moiety of [RNH2CH2R,]+[HB(C6F5)3], to the imine; (iii) nucleophillic attack of the C-center of the iminium ion by the BH, group. The potential energy barriers of the latter, as determined by calculating the evolution of the H-bonded complex of an imine and [RNH2CH2R,]+[HB(C6F5)3], in toluene, are around 10 kcal,mol,1 each. In the second mechanism, only imines serve as basic partners of B(C6F5)3 in the H2 activation, which affords an [RN(H)CHR,]+[HB(C6F5)3], ion pair; direct reduction then proceeds via nucleophilic attack of the C-center by the BH, in [RN(H)CHR,]+[HB(C6F5)3],. This route becomes catalytic when the product amine is released into the solvent and B(C6F5)3 is re-used for H2 activation. Upon taking into account the association energy of an amine,B(C6F5)3 adduct [,9.5 kcal,mol,1 for tBuN(H)CH2Ph and B(C6F5)3 in toluene], the potential energy barrier for H2 uptake by an imine and B(C6F5)3 increases to 14.5 kcal,mol,1. We report a somewhat lower potential energy barrier for H2 uptake by thermally activated amine,B(C6F5)3 adducts [12.7 kcal,mol,1 for the B-N adduct of tBuN(H)CH2Ph and B(C6F5)3 in toluene], although the difference between the two H2 activationbarriers is within the expected error of the computational method. Two catalytic routes are compared based on B3LYP-computed PESs in solvent (toluene).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]


    Crystal structure of highly thermostable glycerol kinase from a hyperthermophilic archaeon in a dimeric form

    FEBS JOURNAL, Issue 10 2008
    Yuichi Koga
    The crystal structure of glycerol kinase from the hyperthermophilic archaeon Thermococcus kodakaraensis (Tk-GK) in a dimeric form was determined at a resolution of 2.4 Å. This is the first crystal structure of a hyperthermophilic glycerol kinase. The overall structure of the Tk-GK dimer is very similar to that of the Escherichia coli glycerol kinase (Ec-GK) dimer. However, two dimers of Ec-GK can associate into a tetramer with a twofold axis, whereas those of Tk-GK cannot. This may be the reason why Tk-GK is not inhibited by fructose 1,6-bisphosphate, because the fructose 1,6-bisphosphate binding site is produced only when a tetrameric structure is formed. Differential scanning calorimetry analyses indicate that Tk-GK is a highly thermostable protein with a melting temperature (Tm) of 105.4 °C for the major transition. This value is higher than that of Ec-GK by 34.1 °C. Comparison of the crystal structures of Tk-GK and Ec-GK indicate that there is a marked difference in the number of ion pairs in the ,16 helix. Four ion pairs, termed IP1,IP4, are formed in this helix in the Tk-GK structure. To examine whether these ion pairs contribute to the stabilization of Tk-GK, four Tk-GK and four Ec-GK derivatives with reciprocal mutations at the IP1,IP4 sites were constructed. The determination of their stabilities indicates that the removal of each ion pair does not affect the stability of Tk-GK significantly, whereas the mutations designed to introduce one of these ion pairs stabilize or destabilize Ec-GK considerably. These results suggest that the ion pairs in the ,16 helix contribute to the stabilization of Tk-GK in a cooperative manner. [source]


    Molecular Design of Superabsorbent Polymers for Organic Solvents by Crosslinked Lipophilic Polyelectrolytes,

    ADVANCED FUNCTIONAL MATERIALS, Issue 24 2008
    Toshikazu Ono
    Abstract Molecular design of lipophilic polyelectrolyte gels as superabsorbent polymers that exhibit a high degree of swelling in less-polar and nonpolar organic solvents is demonstrated. A small amount of tetraalkylammonium tetraphenylborate with long alkyl chains as a lipopholic ion pair is incorporated into crosslinked polyacrylates with variable alkyl chain lengths to provide novel lipophilic polyelectrolyte gels. Their swelling degree becomes more than 100 times as much as their dried weights in various organic solvents. The high effectiveness of the swellable solvents shifts to the polar ones by decreasing the length of the alkyl chain. Swelling or collapsing of the lipophilic polyelectrolyte gels originates from both incompatibility of the polymer chains in the media and dissociation of ionic groups. Thus, a unique superabsorbency is observed when the polymer chains have good compatibility with the solvents and the solvents have relatively high polarities enough to dissociate the ionic groups. By varying the polarity of the neutral monomer in these polyelectrolyte gels, the design of gels that can absorb solvents of nearly any polarity is demonstrated. [source]


    Mechanism of lidocaine release from carbomer,lidocaine hydrogels

    JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 1 2002
    Alvaro Jimenez-Kairuz
    Abstract Rheology, acid-base behavior, and kinetics of lidocaine release of carbomer,lidocaine (CL) hydrogels are reported. A series of (CL)x (x,=,mol% of L,=,25, 50, 75, 100) that covers a pH range between 5.33 and 7.96 was used. Concentrations of ion pair ([R-COO,LH+]) and free species (L) and (LH+) were determined by the selective extraction of (L) with cyclohexane (CH) together with pH measurements, i.e., CH in a ratio CH/hydrogel 2:1 extracted 48% of the whole concentration of lidocaine [LT] of a (CL)100, {[LT],=,([R-COO,LH+]),+,(L),+,(LH+)}. The remaining species in the aqueous phase were distributed as: (L) 3.82%, (LH+) 14.5%, and [R-COO, LH+] 81.7%. Rheology and pH as a function of (CL) concentration are also reported. Delivery rates of free base L were measured in a Franz-type bicompartmental device using water and NaCl 0.9% solution as receptor media. (CL) hydrogels behave as a reservoir that releases the drug at a slow rate. pH effects on rate suggest that, under the main conditions assayed, dissociation of [R-COO,LH+] is the slow step that controls releasing rates. Accordingly, release rate was increased upon addition of a second counterion (i.e., Na+), or through the diffusion of neutral salts such as NaCl, into the matrix of the gel. © 2002 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 91:267,272, 2002 [source]


    Tetraalkylammonium salt as photoinitiator of vinyl polymerization in organic and aqueous media: A mechanistic and laser flash photolysis study

    JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 7 2002
    María L. Gómez
    Abstract N -Dimethyl- N -[2-(N,N -dimethylamino)ethyl]- N -(1-methylnaphthyl)ammonium tetrafluoroborate (I) was synthesized with the aim of obtaining a versatile photoinitiator for vinyl polymerization in organic solvents and water. Salt I was able to trigger the polymerization of acrylamide, 2-hydroxyethylmethacrylate and styrene even at very low concentrations of the salt (,1.0 × 10,5 M). Using laser flash photolysis and fluorescence techniques and analyzing the photoproduct distribution, we were able to postulate a mechanism for the photodecomposition of the salt. With irradiation, I undergoes an intramolecular electron-transfer reaction to form a radical ion pair (RIP). The RIP intermediate decomposes into free radicals. The RIP and the free radicals are active species for initiating the polymerization. Depending on the concentration of the vinyl monomers studied, the initiation mechanism of the polymerization reaction changes. At large monomer concentrations, the RIP state is postulated to trigger the reaction by generating the anion radical of the olefin substrate. At a low monomer concentration, the free radicals produced by the decomposition of I are believed to start the chain reaction. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 901,913, 2002; DOI 10.1002/pola.10166 [source]


    Structural determination of hexadecanoic lysophosphatidylcholine regioisomers by fast atom bombardment tandem mass spectrometry

    RAPID COMMUNICATIONS IN MASS SPECTROMETRY, Issue 22 2002
    Jongki Hong
    The structural determination of sn -1 and sn -2 hexadecanoic lysophosphatidylcholine (LPC) regioisomers was carried out using fast atom bombardment tandem mass spectrometry (FAB-MS/MS). The collision-induced dissociation (CID) of protonated and sodiated molecules produced diverse product ions due mainly to charge remote fragmentations. Based on the information obtained from the CID spectra of protonated and sodiated molecules, sn-1 and sn-2 hexadecanoic LPC isomers could be discriminated. Especially, the abundance ratio of the diagnostic ion pair [m/z 224/226] in the CID spectra of [M,+,H]+ ions was shown to be greatly different. Moreover, the CID-MS/MS spectra of sodium-adducted molecules for hexadecanoic LPC isomers showed characteristic product ions such as [M,+,Na,,,103]+, [M,+,Na,,,85]+, and [M,+,Na,,,59]+, by which their regio-specificity can be differentiated. Copyright © 2002 John Wiley & Sons, Ltd. [source]


    On the accurate bond-valence parameters for the Sb3+/O2, ion pair

    ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2010
    Vasyl Sidey
    The improved values of the bond-valence parameters for the Sb3+/O2, ion pair, r0 = 1.927,Å and b = 0.446,Å, have been deduced from the crystal structures of the , and , polymorphs of Sb2O3 and from the set of precisely determined complex structures containing [Sb3+On] coordination shells. [source]


    Accurate bond-valence parameters for the Bi3+/Br, ion pair

    ACTA CRYSTALLOGRAPHICA SECTION B, Issue 5 2006
    Vasyl Sidey
    Using a new calculation scheme, reliable and physically meaningful values of the bond-valence parameters (r0 = 2.567,Å and b = 0.421,Å) have been deduced for the Bi3+/Br, ion pair from the molecular geometry and from the crystal structure of BiBr3. [source]


    2,3-Dihydro-1,3-benzothiazol-2-iminium monohydrogen sulfate and 2-iminio-2,3-dihydro-1,3-benzothiazole-6-sulfonate: a combined structural and theoretical study

    ACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2009
    Rafal Kruszynski
    The 2-aminobenzothiazole sulfonation intermediate 2,3-dihydro-1,3-benzothiazol-2-iminium monohydrogen sulfate, C7H7N2S+·HSO4,, (I), and the final product 2-iminio-2,3-dihydro-1,3-benzothiazole-6-sulfonate, C7H6N2O3S2, (II), both have the endocyclic N atom protonated; compound (I) exists as an ion pair and (II) forms a zwitterion. Intermolecular N,H...O and O,H...O hydrogen bonds are seen in both structures, with bonding energy (calculated on the basis of density functional theory) ranging from 1.06 to 14.15,kcal,mol,1. Hydrogen bonding in (I) and (II) creates DDDD and C(8)C(9)C(9) first-level graph sets, respectively. Face-to-face stacking interactions are observed in both (I) and (II), but they are extremely weak. [source]


    Mechanistic Insights into Water Activation in SAM Hydroxide Adenosyltransferase (duf-62)

    CHEMBIOCHEM, Issue 15 2009
    Hai Deng Dr.
    Breaking waters: The substrate analogue S -adenosyl- L -homocysteine (SAH) was co-crystallised with SAM hydroxide adenosyltransferase from Pyrococcus horikoshii. Of the two active site water molecules one appears to be structural and the other is a candidate for nucleophilic attack, to become the C5, adenosyl hydroxyl group. The data support a mechanism in which the Arg,Asp ion pair is important for positioning both water molecules. [source]


    Mechanism of Charge Separation in DNA by Hole Transfer through Consecutive Adenines

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2008
    Kiyohiko Kawai Prof.
    Abstract To investigate the mechanism of charge separation in DNA with consecutive adenines adjacent to a photosensitizer (Sens), a series of naphthalimide (NI) and 5-bromouracil (brU)-modified DNAs were prepared, and the quantum yields of formation of the charge-separated states (,) upon photo-excitation of the Sens NI in DNA were measured. The , was modulated by the incorporation site of brU, which changes the oxidation potential of its complementary A through hydrogen bonding and the hole-transfer rates between adenines. The results were interpreted as charge separation by means of the initial charge transfer between NI in the singlet excited state and the second- and third-nearest adenine to the NI. In addition, the oxidation of the A nearest to NI leads to the rapid charge recombination within a contact ion pair. This suggests that the charge-separation process can be refined to maximize the , by putting a redox-inactive spacer base pair between a photosensitizer and an A,T stretch. [source]


    Organocatalytic Asymmetric Mannich Reactions with N -Boc and N -Cbz Protected ,-Amido Sulfones (Boc: tert -Butoxycarbonyl, Cbz: Benzyloxycarbonyl)

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 29 2007
    Olindo Marianacci
    Abstract Different malonates and ,-ketoesters can react with N - tert -butoxycarbonyl- (N -Boc) and N -benzyloxycarbonyl- (N -Cbz) protected ,-amido sulfones in an organocatalytic asymmetric Mannich-type reaction. The reaction makes use of a simple and easily obtained phase-transfer catalyst and proceeds under very mild and user-friendly conditions. The optimised protocol avoids the preparation and the isolation of the relatively unstable N -Boc and N- Cbz imines that are generated in situ from the bench-stable ,-amido sulfones. The corresponding Mannich bases are generally obtained in good yields and enantioselectivities, and can be readily transformed into key compounds, such as optically active ,3 -amino acids in one easy step. Enantioenriched N -Boc and N -Cbz protected ,-amino acids that are suitable for peptide synthesis are also available from the Mannich adducts through simple manipulations. Control experiments showed the dual role of the enolate,catalyst ion pair in this reaction, as well as the crucial role of the presence of water to achieve high enantioselectivities. [source]


    Ab Initio Molecular Dynamics Studies of Ionic Dissolution and Precipitation of Sodium Chloride and Silver Chloride in Water Clusters, NaCl(H2O)n and AgCl(H2O)n, n = 6, 10, and 14

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2006
    Chi-Kit Siu Dr.
    Abstract An ab initio molecular dynamics method was used to compare the ionic dissolution of soluble sodium chloride (NaCl) in water clusters with the highly insoluble silver chloride (AgCl). The investigations focused on the solvation structures, dynamics, and energetics of the contact ion pair (CIP) and of the solvent-separated ion pair (SSIP) in NaCl(H2O)n and AgCl(H2O)n with cluster sizes of n = 6, 10 and 14. We found that the minimum cluster size required to stabilize the SSIP configuration in NaCl(H2O)n is temperature-dependent. For n = 6, both configurations are present as two distinct local minima on the free-energy profile at 100 K, whereas SSIP is unstable at 300 K. Both configurations, separated by a low barrier (<10 kJ,mol,1), are identifiable on the free energy profiles of NaCl(H2O)n for n = 10 and 14 at 300 K, with the Na+/Cl, pairs being internally solvated in the water cluster and the SSIP configuration being slightly higher in energy (<5 kJ,mol,1). In agreement with the low bulk solubility of AgCl, no SSIP minimum is observed on the free-energy profiles of finite AgCl(H2O)n clusters. The AgCl interaction is more covalent in nature, and is less affected by the water solvent. Unlike NaCl, AgCl is mainly solvated on the surface in finite water clusters, and ionic dissolution requires a significant reorganization of the solvent structure. [source]


    The DMAP-Catalyzed Acetylation of Alcohols,A Mechanistic Study (DMAP=4-(Dimethylamino)pyridine)

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 16 2005
    Shangjie Xu Dr.
    Abstract The acetylation of tert -butanol with acetic anhydride catalyzed by 4-(dimethylamino)pyridine (DMAP) has been studied at the Becke3,LYP/6-311+G(d,p)//Becke3,LYP/6-31G(d) level of theory. Solvent effects have been estimated through single-point calculations with the PCM/UAHF solvation model. The energetically most favorable pathway proceeds through nucleophilic attack of DMAP at the anhydride carbonyl group and subsequent formation of the corresponding acetylpyridinium/acetate ion pair. Reaction of this ion pair with the alcohol substrate yields the final product, tert -butylacetate. The competing base-catalyzed reaction pathway can either proceed in a concerted or in a stepwise manner. In both cases the reaction barrier far exceeds that of the nucleophilic catalysis mechanism. The reaction mechanism has also been studied experimentally in dichloromethane through analysis of the reaction kinetics for the acetylation of cyclohexanol with acetic anhydride, in the presence of DMAP as catalyst and triethylamine as the auxiliary base. The reaction is found to be first-order with respect to acetic anhydride, cyclohexanol, and DMAP, and zero-order with respect to triethyl amine. Both the theoretical as well as the experimental studies strongly support the nucleophilic catalysis pathway. [source]


    7Li, 31P, and 1H Pulsed Gradient Spin-Echo (PGSE) Diffusion NMR Spectroscopy and Ion Pairing: On the Temperature Dependence of the Ion Pairing in Li(CPh3), Fluorenyllithium, and Li[N(SiMe3)2] amongst Other Salts

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 5 2005
    Ignacio Fernández
    Abstract 7Li, 31P, and 1H variable-temperature pulsed gradient spin-echo (PGSE) diffusion methods have been used to study ion pairing and aggregation states for a range of lithium salts such as lithium halides, lithium carbanions, and a lithium amide in THF solutions. For trityllithium (2) and fluorenyllithium (9), it is shown that ion pairing is favored at 299 K but the ions are well separated at 155 K. For 2-lithio-1,3-dithiane (13) and lithium hexamethyldisilazane (LiHMDS 16), low-temperature data show that the ions remain together. For the dithio anion 13, a mononuclear species has been established, whereas for the lithium amide 16, the PGSE results allow two different aggregation states to be readily recognized. For the lithium halides LiX (X = Br, Cl, I) in THF, the 7Li PGSE data show that all three salts can be described as well-separated ions at ambient temperature. The solid state structure of trityllithium (2) is described and reveals a solvent-separated ion pair formed by a [Li(thf)4]+ ion and a bare triphenylmethide anion. [source]


    Synthesis and Reactivity of the Monomeric Late-Transition-Metal Parent Amido Complex [Ir(Cp*)(PMe3)(Ph)(NH2)]

    CHEMISTRY - A EUROPEAN JOURNAL, Issue 16 2004
    Daniela Rais Dr.
    Abstract The late-transition-metal parent amido compound [Ir(Cp*)(PMe3)(Ph)(NH2)] (2) has been synthesized by deprotonation of the corresponding ammine complex [Ir(Cp*)(PMe3)(Ph)(NH3)][OTf] (6) with KN(SiMe3)2. An X-ray structure determination has ascertained its monomeric nature. Proton-transfer studies indicate that 2 can successfully deprotonate p -nitrophenylacetonitrile, aniline, and phenol. Crystallographic analysis has revealed that the ion pair [Ir(Cp*)(PMe3)(Ph)(NH3)][OPh] (8) exists as a hydrogen-bonded dimer in the solid state. Reactions of 2 with isocyanates and carbodiimides lead to overall insertion of the heterocumulenes into the NH bond of the Ir-bonded amido group, demonstrating the ability of 2 to act as an efficient nucleophile. Intriguing reactivity is observed when amide 2 reacts with CO or 2,6-dimethylphenyl isocyanide. ,4 -Tetramethylfulvene complexes [Ir(,4 -C5Me4CH2)(PMe3)(Ph)(L)] (L=CO (15), CNC6H3 -2,6-(CH3)2 (16)) are formed in solution through displacement of the amido group by the incoming ligand followed by deprotonation of a methyl group on the Cp* ring and liberation of ammonia. Conclusive evidence for the presence of the Ir-bonded ,4 -tetramethylfulvene moiety in the solid state has been provided by an X-ray diffraction study of complex 16. [source]


    Cationic Scandium Allyl Complexes Bearing Mono(cyclopentadienyl) Ligands: Synthesis, Novel Structural Variety, and Olefin-Polymerization Catalysis

    CHEMISTRY - AN ASIAN JOURNAL, Issue 8-9 2008
    Nan Yu
    Abstract The one-pot salt-metathesis reaction of ScCl3, cyclopentadienyl lithium salts, and allylmagnesium chlorides afforded with ease the corresponding base-free half-sandwich scandium di(,3 -allyl) complexes [(C5Me4SiMe3)Sc(C3H5)2] (1,a), [(C5Me5)Sc(C3H5)2] (1,b), and [(C5Me5)Sc(2-MeC3H4)2] (1,c) in high yields. Reaction of 1,a with 1,equivalent of [PhNMe2H][B(C6F5)4] in toluene gave rapidly the N,N -dimethylaniline-coordinated cationic mono(,3 -allyl) complex [(C5Me4SiMe3)Sc(,3 -C3H5)(,6 -PhNMe2)][B(C6F5)4] (2). The similar reaction of 1,a with [Ph3C][B(C6F5)4] yielded the analogous toluene-separated ion pair [(C5Me4SiMe3)Sc(,3 -C3H5)(,6 -PhMe)][B(C6F5)4] (3). When [PhNMe2H][BPh4] was treated with 1,a, the contact ion pair [(C5Me4SiMe3)Sc(,3 -C3H5)( ,,,6 -Ph)BPh3] (4) was obtained. Recrystallization of 2, 3, and 4 in THF yielded the corresponding thf-separated ion pair complexes [(C5Me4SiMe3)Sc(,3 -C3H5)(thf)2][B(C6F5)4] (5) and [(C5Me4SiMe3)Sc(,3 -C3H5)(thf)2][BPh4] (6). The N,N -dimethylaniline-coordinated cationic scandium allyl complex 2 and the toluene-coordinated analogue 3 showed high activity (activity: 3>2) toward the polymerization and copolymerization of isoprene and norbornene to afford random copolymers with a broad range of isoprene content (33,86,mol,%). The tight ion pair 4 and the thf-coordinated complexes 5 and 6 showed no activity under the same conditions. These results offer unprecedented insight into the structure,activity relationship of a cationic metal polymerization-catalyst system. [source]


    Excited-State Double Proton Transfer in Model Base Pairs: The Stepwise Reaction on the Heterodimer of 7-Azaindole Analogues

    CHEMPHYSCHEM, Issue 2 2008
    Wan-Ting Hsieh
    Abstract A four fused-ring system 11-propyl-6H-indolo[2,3-b]quinoline (6,HIQ) is strategically designed and synthesized; it possesses a central moiety of 7-azaindole (7AI) and undergoes excited-state double proton transfer (ESDPT). Despite a barrierless type of ESDPT in the 6,HIQ dimer, femtosecond dynamics and a kinetic isotope effect provide indications for a stepwise ESDPT process in the 6,HIQ/7AI heterodimer, in which 6,HIQ (deuterated 6,HIQ) delivers the pyrrolyl proton (deuteron) to 7AI (deuterated 7AI) in less than 150 fs, forming an intermediate with a charge-transfer-like ion pair, followed by the transfer of a pyrrolyl proton (deuteron) from cation-like 7AI (deuterated 7AI) to the pyridinyl nitrogen of the anion-like 6,HIQ (deuterated 6,HIQ) in ,1.5±0.3 ps (3.5±0.3 ps). The barrier of second proton transfer is estimated to be 2.86 kcal,mol,1 for the 6,HIQ/7AI heterodimer. [source]


    Amphiphilic Organic Ion Pairs in Solution: A Theoretical Study

    CHEMPHYSCHEM, Issue 10 2007
    Vincent Pradines Dr.
    Abstract The macroscopic manifestation of hydrophobic interactions for amphiphilic organic ion pairs (tetraalkylammonium,anion) has been shown experimentally by measuring their association constants and their affinity with the organic phase. Beyond a certain size, there is a direct relation between association constants and chain lengths in tetraalkylammonium ions. We propose to cast a bridge between these results and geometrical properties considered at the level of a single ion pair by means of quantum chemistry calculations performed on model systems: trimethylalkylammonium,pentyl sulfate instead of tetraalkylammonium,dodecyl sulfate. Two limiting cases are considered: head-to-head configurations, which yield an optimal electrostatic interaction between polar heads, and parallel configurations with a balance between electrostatic and hydrophobic interactions. All properties (geometries, complexation energies, and atomic charges) were obtained at the MP2 level of calculation, with water described by a continuum model (CPCM). Dispersion forces link hydrocarbon chains of tetraalkylammonium ions and pentyl sulfate, thus yielding (for the largest ion pairs) parallel configurations favored with respect to head-to-head geometries by solute,solvent electrostatic interactions. Given the small experimental association energies, we probe the accuracy limit of the MP2 and CPCM methods. However, clear trends are obtained as a function of chain length, which agree with the experimental observations. The calculated monotonic stabilization of ion pairs when the hydrocarbon chain increases in length is discussed in terms of electrostatic interactions (between ions and between ion pairs and water), dispersion forces, and cavitation energies. [source]


    Separation of amino alcohols using divalent dipeptides as counter ions in aqueous CE

    ELECTROPHORESIS, Issue 10 2010
    Jakob Haglöf
    Abstract Divalent dipeptides have been introduced as counter ions in aqueous CZE. The dipeptides form ion pairs with amino alcohols in the BGE and facilitate the separation of amino alcohols. High concentrations of dipeptide caused reversed effective mobility for the analytes. The net charge of the dipeptide can be controlled using a buffer or a strong base, and regulates the interaction between the dipeptide and the amino alcohol. A stronger interaction and higher selectivity of amino alcohols was observed when the dipeptides were used as divalent counter ions, than in monovalent or uncharged form. Association constants for ion pairs between divalent dipeptides and amino alcohols can be used to enhance selectivity for amino alcohols in CZE. No chiral separation of amino alcohols was observed when using the dipeptides as ion-pairing chiral selectors in aqueous BGE, but addition of methanol to the BGE promoted enantioselectivity. [source]


    EuIII -Doping of Lamellar Bilayer and Amorphous Mono-Amide Cross-Linked Alkyl/Siloxane Hybrids

    EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 18 2010
    Silvia C. Nunes
    Abstract Two structurally different but chemically similar families of alkyl/siloxane mono-amidosil hosts, represented by m-A(x) [where x = 14 or 8 represents the number of CH2 groups of the pendant alkyl chains directly bonded to the carbonyl group of the amide cross-link] have been doped with a wide range of concentrations of Eu(CF3SO3)3. Mono-amidosils m-A(x)nEu(CF3SO3)3 with n,,,10 (where n is the molar ratio of carbonyl groups per Eu3+ ion) have been analyzed. The m-A(8)nEu(CF3SO3)3 mono-amidosils are transparent and amorphous films, in which the alkyl chains adopt gauche conformations. In contrast, the m-A(14)nEu(CF3SO3)3 mono-amidosils are solid powders; here the lamellar bilayer hierarchical structure of m-A(14) coexists with a new lamellar phase in which the Eu3+ ions are bonded to carbonyl oxygen atoms of the amide groups. At n = 10 the hydrogen-bonded associations formed are highly ordered and considerably stronger than those found in the less concentrated hybrids and in the nondoped matrices. "Free" and weakly coordinated triflate ions occur in all the mono-amidosil samples. The hybrids are white light emitters (maximum quantum yield: 0.08,±,0.01), presenting a broad emission band in the blue/purplish-blue spectral region (ascribed to the hybrid host) superimposed on the 5D0,7F0,4 Eu3+ intra-4f6 transitions. Two Eu3+ local coordination sites (named A and B) have been discerned in both systems. Site A is attributed to weakly coordinated Eu3+/CF3SO3, ion pairs, whereas site B involves Eu3+ coordination to the oxygen atoms of the C=O groups, of the CF3SO3, ions and of the water molecules. For site B, the long-range order of the hybrid host induces distinct features in the energy of the 5D0,7F0,4 transitions, the 5D0 lifetime and the degree of covalency of the Eu3+,first-ligand bonds. [source]


    An Acyclic Aminonaphthyridine-Based Receptor for Carbohydrate Recognition: Binding Studies in Competitive Solvents

    EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2007
    Monika Mazik
    Abstract 1H NMR spectroscopic and microcalorimetric titrations revealed that receptor 3b, consisting of three protonated 2-amino-1,8-naphthyridine units, binds N -acetylneuraminic acid (Neu5Ac), the most commonly occurring sialic acid, with high affinity in competitive solvents such as water/dimethyl sulfoxide. Receptor 3b is able to form neutral/charge-reinforced hydrogen bonds and ion pairs with Neu5Ac, similar to sialic acid-binding proteins. Furthermore, indications for weak binding of neutral sugars, such as methyl ,- D -glucopyranoside, D -maltose and D -cellobiose were provided by NMR spectroscopy. Molecular modelling calculations, synthesis and binding studies in aqueous media are described. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]


    1,4-Diazabicyclo[2.2.2]octane (DABCO) as an Efficient Reagent for the Synthesis of Isoxazole Derivatives from Primary Nitro Compounds and Dipolarophiles: The Role of the Base

    EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2006
    Luca Cecchi
    Abstract The dehydration of primary nitro compounds can be performed by bases in the presence of dipolarophiles. The reactivity of several tertiary amines or azaheteroaromatic compounds containing one or two basic centres is shown to be related to the ability of the protonated base to establish H-bonded ion pairs with the adduct that is formed from the nitronate and the dipolarophile in chloroform. Among the organic bases examined, caged tertiary diamine 1,4-diazabicyclo[2.2.2]octane (DABCO) gave the best results. The reaction is applicable to activated nitro compounds and to phenylnitromethane and affords isoxazoline derivatives in higher yields compared with those of other methods. The reaction, however, is not compatible with nitroalkanes. The proposed mechanism of the reaction is based on the collapse of the H-bonded ion pair. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]


    SN2 Displacement by Bromide Ions in Dichloromethane , The Role of Reverse Micelles

    EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2006
    Lucia Brinchi
    Abstract Reverse micellar systems are of interest as reaction media because they are powerful models for biological compartmentalization, enzymatic catalysis and separation of biomolecules. Solutions of ionic surfactants in apolar solvents may contain reverse micelles, but they may also contain ion pairs, or small clusters, with waters of hydration. We studied the bimolecular reaction in CH2Cl2 solutions of cationic tetraalkylammonium bromide salts (onium salts), such as cetyltrimethylammonium bromide (CTABr), cetyltripropylammonium bromide (CTPABr) and tetra- n -butylammonium bromide (TBABr). Methylnaphthalene-2-sulfonate (,-MeONs), its 6-sulfonate derivative (,-MeONsS,) as the 2,6-lutidinium salt and methyl-5- N,N,N,trimethylammonium naphthalene-1-sulfonate (,-MeONsNT+) as the trifluoromethanesulfonate salt react with Br, in CH2Cl2. First-order rate constants, kobs, increase linearly and similarly for the three substrates with increasing concentrations of the onium salts. Reactions are faster with TBABr than they are with CTPABr and CTABr, and the reactivity of the three substrates is in the order: ,-MeONsNT+ >> ,-MeONsS, > ,-MeONs. The reactions are inhibited by the addition of H2O, but CTABr tolerates H2O in large excess. At [H2O]/[CTABr] = w0 , 6, "water-pool" reverse micelles form, and kobs for all three substrates is then independent of w0. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]


    Calcite and gypsum solubility products in water-saturated salt-affected soil samples at 25°C and at least up to 14 dS m,1

    EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 2 2010
    F. Visconti
    Calcite and gypsum are salts of major ions characterized by poor solubility compared with other salts that may precipitate in soils. Knowledge of calcite and gypsum solubility products in water-saturated soil samples substantially contributes to a better assessment of processes involved in soil salinity. The new SALSOLCHEMIS code for chemical equilibrium assessment was parameterized with published analytical data for aqueous synthetic calcite and gypsum-saturated solutions. Once parameterized, SALSOLCHEMIS was applied to calculations of the ionic activity products of calcium carbonate and calcium sulphate in 133 water-saturated soil samples from an irrigated salt-affected agricultural area in a semi-arid Mediterranean climate. During parameterization, sufficiently constant values for the ionic activity products of calcium carbonate and calcium sulphate were obtained only when the following were used in SALSOLCHEMIS: (i) the equations of Sposito & Traina for the free ion activity coefficient calculation, (ii) the assumption of the non-existence of the Ca (HCO 3)+ and CaCO3o ion pairs and (iii) a paradigm of total ion activity coefficients. The value of 4.62 can be assumed to be a reliable gypsum solubility product (pKs) in simple aqueous and soil solutions, while a value of 8.43 can only be assumed as a reliable calcite solubility product (pKs) in simple aqueous solutions. The saturated pastes and saturation extracts were found to be calcite over-saturated, with the former significantly being less so (p IAP = 8.29) than the latter (p IAP = 8.22). The calcite over-saturation of saturated pastes increased with the soil organic matter content. Nevertheless, the inhibition of calcite precipitation is caused by the soluble organic matter from a dissolved organic carbon threshold value that lies between 7 and 12 mm. The hypothesis of thermodynamic equilibrium is more adequate for the saturated pastes than for the saturation extracts. [source]