Lewis Acid Character Research Articles

Vibrational spectroscopy of adsorbed sulfate on Pt(111)

The absorption of sulfate species was studied on well ordered Pt(111) surfaces at pH 2.8 and 0.23 using in situ external reflectance ir spectroscopy. Independently of the pH in the solution the spectra present two overlapped vibrational bands. A strong one is located at 1220–1280 cm−1 and is potential dependent. The second one is observed as a shoulder at 1190 cm−1. The analysis using group theory and the surface selection rule shows that only sulfate ions are adsorbed at both pH, suggesting a strong Lewis acid character of the Pt(111) surface. The randomly distributed sulfate species are relatively tightly adsorbed and present weak lateral interaction. At increasing coverage the sulfate adlayer becomes more organized, as attested by the observed band narrowing. The maximum adsorption is observed at 0.8 V vs. Pd/H2ie at potentials higher than that of the so called anomalous states in the voltammogram. A quadratic dependence of the applied electric field on the SO vibrational band of adsorbed sulfate is observed, suggesting a second order Stark effect.

Macrotricyclic Borane—Amine Adducts: The First Uncharged Synthetic Host Compounds Without Lewis Acid Character, for Anionic Guests

Macrotricyclic Borane—Amine Adducts: The First Uncharged Synthetic Host Compounds Without Lewis Acid Character, for Anionic Guests

The Surface Thermodynamic Properties of Talc Treated with Octadecylamine

The surface of talc is nearly apolar (γ LW = 31.5, γ ⊙ 2.4, and γ ⊙ = 2.7 mJ/m 2). The surface properties of talc can be modified by progressive coverage of the surface with an organic material. The present experiments used varying quantities (0-3% by weight) of (primarily) octadecylamine deposited on a talc powder. The surface properties were determined by thin layer wicking; the deposition of the octadecylamine resulted in the rapid loss of the Lewis acid (γ ⊙) and base (γ ⊙) character producing a completely apolar material. For the amine-treated samples, hydrocarbon liquids of varying surface tension (γL = 18.4 to 27.47 mJ/m 2) continued to interact with the silicate surface (γ LW = 28.5 mJ/m 2) and spread, while the nonhydrocarbon liquids interacted with a lower energy surface, presumably the hydrocarbon tails of the octadecylamine, giving γ LW S values as low as 15.7 mJ/m 2. These observations are consistent with a surface having the octadecylamine chains oriented with the nitrogen toward the surface and with the alkyl chain perpendicular, or nearly so, to the talc surface.

A mechanistic study on the polymerization of n-butyl isocyanate by pruitt-baggett adduct

The mechanism of the polymerization of n-butyl isocyanate by the Pruitt-Baggett adduct (PBA) was studied; on the basis of the experimental evidences, a coordinated mechanism was proposed and verified. This mechanism considers a monatomic active site, which is the Fe atom of PBA with Lewis acid character. The N atom of the isocyanate monomer is coordinated by the Fe atom and the resultant positive charge is transmitted to another N atom which covalently binds the growing chain to the active centre. Thus the formed ammonium ion is added to the incoming monomer yielding Nylon-1 structure and the active site is regenerated.

A new iodine-free initiating system for the living polymerization of isobutyl vinyl ether

A new iodine-free initiating system, MeCH(Oi-Bu)Cl/n-Bu4NTiCl5, was found to induce the living carbocationic polymerization (LC⊕Pzn) of isobutyl vinyl ether (IBuVE) at-20 °C in CH2Cl2. Using this system, poly(isobutyl vinyl ether) (PIBuVE) with the theoretical molecular weights (up toca. 45,000) calculated from the initiator/monomer input and narrow molecular weight distribution (Mw/Mn < 1.1) can be readily obtained. The polymerization is first order in the monomer. The coinitiating function ofn-Bu4NTiCl5 may be due to a salt effect or to its Lewis acid character. An attempt for the synthesis of the polyisobutylene (PIB)-PIBuVE block copolymer has failed most likely becausen-Bu4NTiCl5 is unable to activate the ∼C(CH3)2-Cl bond of the PIB termini.

Chemistry on fluorinated surfaces. Brønsted and lewis acid sites

The fluorination of high surface area metal oxides is an important step in the production of heterogeneous catalysts for halogen exchange, e.g. fluorinated γ-alumina or silica. Two situations have been encountered experimentally when the defect spinel γ-alumina is fluorinated at room temperature using a variety of volatile inorganic fluorides : (i) formation of new Brønsted acid surface sites, or (ii) formation of new Brønsted and Lewis sites. Models for both types of behaviour have been developed from an idealised γ-alumina surface by combining the results obtained from several experimental methods. I.r. spectroscopy and [ l8F]-labelling were used to follow the course and extent of the gas-solid reactions, to obtain kinetic data and to determine the lability of surface fluoride. Adsorption-desorption phenomena were studied using [ 14C]- and [ 35S]-labelled molecules and reactions involving but-1-ene or CH 3CCl 3 were used to probe Brønsted and Lewis acid character respectively. Polymerization of a C 2 chloroolefin in the presence of Lewis acid promoted, fluorinated γ-alumina leads to the formation of an organic layer on the surface. The nature of these surfaces, which are also catalytically active, will be discussed in the light of 1H, 13C and 27Al solid state MAS n.m.r. measurements. Support of this work from the SERC and I.C.I. plc, is gratefully acknowledged.

Cobalt(II)-catalyzed reaction between polycyclic aromatic aldehydes and acetic anhydride. Formation of acylals, not 1,2-diketones

Several polycyclic aromatic aldehydes were found to react with acetic anhydride in acetonitrile in the presence of excess CoCl 2 to afford the corresponding acylals [ArCH(OAc) 2 ]. The results are not consistent with the literature report (Ahmad, S.; Iqbal, J. J. Chem. Soc., Chem. Commun. 1987, 692) that substituted benzaldehydes afford 1,2-diketones under similar conditions. The cobalt(II)chloride probably promotes acylal formation through its weakly Lewis acid character

Preparation of n-Butyl(Isopropyl)Tin Dichloride and its Lewis Acid Character

Preparation of n-Butyl(Isopropyl)Tin Dichloride and its Lewis Acid Character

Physicochemical studies of the solvolysed products from various Lewis acids in tribromoacetic acid as solvent

Lewis acids when refluxed with excess of tribromoacetic acid as solvent form the solid compounds SnCl 2(CBr 3COO) 2, Ti(CBr 3COO) 4, Zr(CBr 3COO) 4, ZrO(CBr 3COO) 2CBr 3COOH, Th(CBr 3COO) 4, U(CBr 3COO) 4, Si 2O(CBr 3COO) 6, SbCl 2(CBr 3COO) 3, Al(CBr 3COO) 3, Fe(CBr 3COO) 3 and FeCl(CBr 3COO) 2. The structures of these new compounds have been established by their elemental analysis, conductance, infrared, reflectance, NMR and FAB mass spectrometry. In solution the tribromoacetate groups are readily lost. The Lewis acid character of these newly isolated compounds was established by preparing and characterising their complexes with organic bases.

Novel Strategy for Control of Synthetic Reactions by Metalloporphyrins

The present paper describes novel strategy for control of synthetic reactions by metalloporphyrins. Aluminum porphyrins have been found to be very effective initiators for living polymerizations of a wide variety of monomers to give polymers of uniform, controlled molecular weight. A novel concept, “immortal polymerization” has been established, which is of much use for elaborate molecular design of polymer materials. Photochemical fixations of carbon dioxide as a CI source for organic syntheses have been also achieved by using aluminum porphyrins. A highly stereoselective reaction has been observed in the reduction of ketones with alcohols catalyzed by a chloroaluminum porphyrin with Lewis acid character. Zinc N-substituted porphyrins as initiators have provided the first example of photo-induced living polymerization of epoxides. Novel chiral porphyrins and metalloporphyrins with molecular asymmetry have been synthesized and resolved.

Isomerization of 1-Butene on High-Temperature-Chlorinated Alumina

Abstract Regarding aluminas chlorinated at 973–1223 K, which have only a Lewis acidic character without hydroxyl groups and basic sites, the IR spectra of the adsorbed 1-butene were measured and the isomerization of 1-butene and cis-2-butene was performed at 273–323 K. The obtained results were compared with those for aluminas dehydrated at the same temperatures. The IR spectra of the adsorbed 1-butene well resembled that of cis-2-butene, including the absence of peaks due to double-bond and π-allyl carbanion species, observed even after outgassing at 473 K. During isomerization at 273 K, the initial isomerization reaction rates and cis/trans ratios were greater than those for dehydrated alumina. The ratio decreased rapidly with increasing reaction temperature. The specific reaction rates and the ratio were independent of the chlorination temperature. In the isomerization of cis-2-butene the most thermodynamically unstable molecule, 1-butene, was formed preferentially. The obtained results could be interpreted by an isomerization mechanism through a doubly bonded intermediate, i.e., the allylic butenyl cation.

NMR studies of some η 4-diene-rhodium(I) and main group metal derivatives of funtionally substituted cyclopentadienes

The carbon-13 NMR spectra of twenty four acyclic and cyclic η 4-1,3-, 1,4- and 1,5-diene-rhodium(I) complexes of type η 5-C 5H 4XRh (η 4-diene), (X =H, Cl, Ph, CO 2CH 3 or CHO) are described. Each diene type can be easily distinguished from the values of its 103Rh 13C coupling constants and olefincarbon coordination shifts. The effects of methyl substituents on the relative chemical shifts of the acyclic η 4-1,3-diene carbon atom are broadly similar to that in analogous η 4-dienetricarbonyliron systems, but the metalcarbon coupling constants imply a greater degree of retrodative bonding in the case of the rhodium complexes. The largest coordination shifts are found for the β olefin carbons of the 1,4-diene complexes. The unusual metalcarbon coupling in these complexes are a reflection of the strained conformation adopted on bonding to rhodium, and do not suggest conjugative interaction between the two alkene functions. The spectra of the 1,5-diene complexes show the expected trend resulting from a reduction in strain energy relative to that for the previous case. The spectra of the η 5-cyclopentadienyl groups in these complexes as discussed in terms of a small contribution from either an η 3-allyl-η 2-ene or a η 4-diolefin-η 1-alkyl rotamer to the metalring bonding scheme; the rotamer type is governed by the cyclopentadienyl ring substituent and counter diene ligand. Several functionally-substituted cyclopentadienide salts have been examined by variable temperature hydrogen-1 and carbon-13 NMR spectroscopy. The fluxional behaviour of Tl 1(C 5H 4NO 2 and K(C 5H 4CHO) demonstrate how the Lewis acid character of the metal ion and the electron-accepting nature of the ring substituent control the structural preference for a form in which the substituent is either co-planar or orthogonal with respect to the cyclopentadienide group. Two dimensional 1H 13C NMR spectra and simulation techniques peritted unambiguous assignment of ring nuclei. The chemical ionisation and fast atom bombardment mass spectra of several of the thallium(I) complexes reveal considerable association in the vapour state.

Stationary phase for gas chromatography VII. phenol alkylated with half equivalently chlorinated to carbon number of n-Alkanes separated from kerosene. (PHECAK) phase

PHECAK phase was prepared by alkylation of phenol with n-alkanes mixture, which vas separated from kerosene and chlorînated in a nıanner that eme of each tu o carbons is substitu- ted. This phase contained less chlorine but unsubstituted methylene groups in the straight al- kane chain. PHECAK phase exhibits moderate.ly polar phase properties by some additional Lewis acid character and better coataing property of the support material than other kerosene based chlorinated stationary phases. This packing has been utilized for the separation of a number of compounds carrying dif- ferent functional groups. McReynolds' conctants were given.

Preparation and spectroscopic study of functionally substituted cyclopentadienides of thallium(I), potassium, rhodium(I), and iridium(I)

A 13C and 1H n.m.r. study has been carried out on a series of monosubstituted cyclopentadienide compounds of type MI(C5H4X)[M = K or Tl; X = Cl, COMe, CO2Me, CHO, Ph, COCO2Et, or C(CN)C(CN)2]. The different patterns observed for the ring nuclei reflect both the Lewis-acid character of the metal and the electronic effect of the substituent. Fluxional behaviour is observed for K(C5H4COMe). Pseudo-first-order rate constants have been evaluated for hydrogen–deuterium exchange in several of the potassium compounds. The thallium(I) compounds generally exhibit greater synthetic utility than their potassium analogues and several have been used in the synthesis of new [M(η5-C5H4X)(η2-C2H4)2](M = Rh or Ir) complexes. Activation barriers for alkene rotation and mass spectral fragmentation patterns are discussed for these compounds. The 1H n.m.r. spectra found for the cyclopentadienyl ring indicate that the M–ring bonding is not fully delocalised. Analysis of the products resulting from reaction between methyl chloroformate and cyclopentadienide ion confirms the strongly electrophilic character of the former reagent.

Recent advances in the chemistry of tungsten—carbene complexes

High oxidation state tungsten—carbene complexes similar to W(CHt-Bu)(OCH 2t-Bu) 2Br 2 were synthesized and their physical and chemical properties analysed and compared. In particular, we discuss in this review (i) the Lewis base character of the halide ligands governing the amount of cationic complex formed in the presence of Lewis acids, (ii) the Lewis acid character of the tungsten center illustrating the availability of a vacant site in these molecules, (iii) the nucleophilicity of the carbene carbon, (iv) the stereochemical non-rigidity of some of the compounds and the barrier to rotation about the tungsten—carbon double bond, and (v) the intermolecular ligand transfer reactions. The activity of these compounds as olefin metathesis catalysts was investigated, in the presence as well as in the absence of Lewis acids. The analysis of its dependence on the steric and electronic features of the ligands allows in particular definition of the relative importance of the properties described above in the catalytic process.

Solvolytic reactions of Lewis acids in dibromoacetic acid and the characterization of the solvolysed products

Solid compounds, SnCl 2(CHBr 2COO) 2, Ti(CHBr 2COO) 4, Zr(CHBr 2COO) 4, Th(CHBr 2COO) 4, U(CHBr 2COO) 4 and Si 2O(CHBr 2COO) 6 have been isolated when the respective tetrachlorides are refluxed with excess of dibromoacetic acid. Zirconyl chloride forms ZrO(CHBr 2COO) 2·CHBr 2COOH. These compounds have been characterized by their elemental analysis, molar conductance, and infra-red. NMR and FAB mass spectra. In solution, the bromoacetate group is readily lost. Their Lewis acid character was established by isolating and characterizing their adducts with organic tertiary bases. Antimony pentachloride, aluminium trichloride and ferric chloride form compounds of composition SbCl 2(CHBr 2COO) 3, Al(CHBr 2COO) 3, FeCl(CHBr 2COO) 2 and Fe(CHBr 2COO) 3 when refluxed with excess of the solvent and these have been characterized by infra-red studies.

Solvolytic reactions of Lewis acids in monobromoacetic acid and the nature of the solvolysed products

Solid compounds of composition SnCl 2(CH 2BrCOO) 2, Ti(CH 2BrCOO) 4, Zr(CH 2BrCOO) 4, Th(CH 2BrCOO) 4, Si 2O(CH 2BrCOO) 6 have been isolated when the respective tetrachlorides are refluxed with excess of monobromoacetic acid. Zirconylchloride forms the compound ZrO(CH 2BrCOO) 2. CH 2BrCOOH in a similar manner. These compounds have been characterized by their elemental analysis, molar conductance and by infrared studies, NMR and FAB mass spectrometry. In solution, the bromoacetate groups readily lost. Their Lewis acid character was established by isolating and characterizing their adducts with organic tertiary bases. Antimony pentachloride, aluminium trichloride and ferric chloride form compounds of composition SbCl 2(CH 2BrCOO) 3, Al(CH 2BrCOO) 3, FeCl(CH 2BrCOO) 2 and Fe(CH 2BrCOO) 3 when refluxed with excess of the solvent and these have been characterized by infrared studies.

Essai de stabilisation du polychlorure de vinyle par un système totalement organique

Owing to its Lewis acid character, dichloromaleic anhydride (DCAM) has been tried as a thermal stabilizer of polyvinylchloride (PVC), in association with nucleophilic oxiranes; comparative experiments have been carried out using maleic anhydride and maleic acid. Chlorohexene was used as model compound for labile allylic chloride in dichlorobutane solution at 80°. Experiments with plasticized PVC were carried out at 180°. DCAM in an efficient catalyst for the production of the intermediate carbocation, although it is less efficient than organostannic chlorides; it causes oxirane polymerization and other secondary reactions more than nucleophilic substiution on the carbocation. Moreover it is not an efficient dienophilic agent for conjugated polyene, although it gives with them a strongly coloured complex. For these reasons, it cannot be used as a major component of a stabilizer recipe for PVC.

Solvolytic reactions of Lewis acids in dichloroacetic acid and the nature of the solvolysed products

Tetrachlorides of tin, titanium, zirconium, thorium and silicon form compounds of composition SnCl2(CHCl2COO)2, Ti(CHCl2COO)4, Zr(CHCl2COO)4, Th(CHCl2COO)4 and Si2O(CHCl2COO)6 when refluxed with excess of dichloroacetic acid. From i.r., molecular weight and conductance studies, their structures have been elucidated. Their Lewis acid character is established by isolating and characterizing their adducts with organic tertiary bases. Antimony pentachloride, aluminium trichloride and iron(III) trichloride form compounds of composition SbCl2(CHCl2COO)3, Al(CHCl2COO)3 and FeCl(CHCl2COO)2 when refluxed with excess of the solvent and these have been characterized by i.r. studies.

A new method for the preparation of n-Propyl(n-Butyl)-Tin dichloride and its lewis acid character

A new method for the preparation of n-Pr(n-Bu)SnCl 2 is described. It has been found to form 1: 2 complexes with unidentate nitrogen-containing ligands such as pyridine and pyrrolidine and 1:1 complexes with bidentate chelating ligands such as 2,2′-bipyridyl and 1,10-phenanthroline. An attempt has been made to propose structures for these complexes on the basis of IR spectra.