Home About us Contact
|
Home About us Contact | ||
|
|
||
Lewis Acidity (lewi + acidity)
Selected AbstractsChemInform Abstract: Lewis Acidities and Hydride, Fluoride, and X - Affinities of the BH3-nXn Compounds for (X: F, Cl, Br, I, NH2, OH, and SH) from Coupled Cluster Theory.CHEMINFORM, Issue 47 2009Daniel J. Grant 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] Enhancement of Lewis Acidity by Ligand-Defined Metal Geometry: A Catalytic Allylation of Aldehydes with Allyltrimethylsilane.CHEMINFORM, Issue 7 2003Motomu Kanai No abstract is available for this article. [source] New Bis(silyl)cyclopentadienidoniobium and -tantalum Complexes:X-ray Crystal Structures of [NbCp,Cl4] and [NbCp,Cl4(CNAr)][Cp, = ,5 -C5H3(SiClMe2)(SiMe3); Ar = 2,6-Me2C6H3]EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 24 2006Manuel Gómez Abstract The [bis(silyl)cyclopentadienido]tetrachloroniobium and-tantalum complexes [MCp,Cl4] [Cp, = ,5 -C5H3(SiClMe2)(SiMe3); M = Nb 3, Ta 4] were synthesized by reaction of the pentachlorides MCl5 with C5H3(SiClMe2)(SiMe3)2 (1). Although the Lewis acidity of tetrachloro complexes 3 and 4 is lower than that of the pentahalides, two adducts [M{,5 -C5H3(SiClMe2)(SiMe3)}Cl4(CNAr)] (Ar = 2,6-Me2C6H3; M = Nb 5, Ta 6) have been isolated by reaction with ArNC. Complexes 3 and 4 react with tert -butylamine or lithium amides to afford the dichloroimido and amidochloroimido complexes [M{,5 -C5H3(SiClMe2)(SiMe3)}Cl2(NR)] (R = tBu, M = Nb 7, Ta 8; R = Me, M = Nb 9) and [Ta{,5 -C5H3(SiClMe2)(SiMe3)}Cl(NHtBu)(NtBu)] (10), respectively. In addition, 7 and 8 can be prepared by treatment of the pentachlorides with [C5H3(SiMe2NHtBu)(SiMe3)2] (2) by elimination of 1. The tetrachloro compound 3 reacts with four equivalents of tBuNH2 to give the constrained-geometry derivative [Nb{,5 -C5H3(SiMe2NtBu-,N)(SiMe3)}Cl(NtBu)] (11), whereas the treatment of toluene solutions of 3 and 4 with H2NCH2CH2NH2 in the presence of triethylamine leads to the trichloro complexes [M{,5 -C5H3(SiMe2NCH2CH2NH2 -,2N,N)(SiMe3)}Cl3] (M = Nb 12, Ta 13). All the reported complexes were studied by IR and NMR spectroscopy and the molecular structures of complexes 3 and 5 were determined by X-ray diffraction methods. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Cobalt(II) Complexes with Substituted Salen-Type Ligands and Their Dioxygen Affinity in N,N -Dimethylformamide at Various TemperaturesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2005Andreas Huber Abstract Several unsymmetrically substituted salen-type cobalt(II) Schiff-base complexes CoL [H2L = 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-hexadiene (1); 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (2); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-(2-hydroxyphen-yl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (3); 1-(2-hydroxyphenyl)-6-methyl-2,5-diaza-1,5-nonadien-8-one (4); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-methyl-2,5-di-aza-1,5-nonadien-8-one (5); 1-(2-hydroxyphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (6); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (7)] were prepared and characterized by their UV/Vis absorption spectra, magnetic moments, and oxidation potentials. Except for complex 4 (irreversible oxidation with t½ , 3 h), complexes 1,3 and 5,7 are remarkably resistant against irreversible auto-oxidation in air-saturated N,N -dimethylformamide (DMF) at ambient temperature. To characterize the Lewis acidity of the cobalt center in 1,7, the equilibrium constant Kpy was determined for monoadduct formation with pyridine (CoL + pyCoL·py). An O2 -sensitive optode was used to determine the Henry constant, KH, for the system O2/DMF in the temperature range 298,228 K. The formation of 1:1 adducts of complexes 1,7 with O2 in DMF, as characterized by the equilibrium constant K, was followed spectrophotometrically in the temperature range 298,228 K. The parameters ,Ho, ,So, and K are reported. At 298 K, K ranges from 21.9 M,1 (5) to 155 M,1 (7). The overall spectroscopic information, including EPR spectra obtained with frozen solutions of 3 and 7 in O2 -saturated DMF, confirm that the 1:1 adducts CoL·O2 are cobalt(III) superoxo compounds. The symmetrically substituted salen complex8 [H2L = 1,6-bis(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,4,4-tetramethyl-2,5-diaza-1,5-hexadiene in 8] is shown to catalyze the oxidation of triphenylphosphane and 2,6-di- tert -butylphenol by O2 in DMF at ambient temperature. The correlation of the data obtained for K, Kpy, and the oxidation potential E½ is discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Carboxy Ester Hydrolysis Promoted by a Dicopper(II) Macrocyclic Polyamine Complex with Hydroxypropyl Pendant GroupsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 9 2004Jin Huang Abstract A dinuclear CuII complex containing a hexaaza macrocyclic ligand bearing two 2-hydroxypropyl pendants, 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxypropyl)tricyclo[22.2.2.211,14]triaconta-1,11,13,24,27,29-hexaene (L), was synthesized. The title complex [Cu2(H,2L)Cl2]·6.5H2O was isolated as a blue crystal, orthorhombic, space group Fddd, with a = 16.4581(12), b = 32.248(2), c = 35.830(2) Å, V = 19017(2) Å3, Z = 16, R1 = 0.0690, and wR2 = 0.1546 [I > 2,(I)]. The protonation constants of Cu2L were determined by potentiometric titration, and it was found that the alcoholic hydroxypropyl group of the complex Cu2L exhibits low pKa values of pKa1 = 7.31, pKa2 = 7.83 at 25 °C. The hydrolysis kinetics of 4-nitrophenyl acetate (NA) promoted by the title complex have also been studied. The pH-rate profile for Cu2L gave a sigmoidal curve and showed a second-order rate constant of 0.39 ± 0.02 M,1 s,1 in 10% CH3CN/H2O(v/v), which is greater than that of the dinuclear CuII complex formed by a hexaaza macrocycle without pendants. The reason for the higher catalytic activity of the title complex is discussed. We found that the volume of nucleophile RO, can effect the hydrolysis of the carboxy ester, the nucleophilicity of RO, and the Lewis acidity of the metal macrocycle also affect the carboxy ester hydrolysis. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Lanthanide Formamidinates as Improved Catalysts for the Tishchenko ReactionEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2008Agustino Zuyls Abstract The tris(formamidinato)lanthanum(III) complexes [La(o -TolForm)3(thf)2] [1; o -TolForm = N,N, -bis(o -tolyl)formamidinate], [La(XylForm)3(thf)] [2; XylForm = N,N, -bis(2,6-dimethylphenyl)formamidinate], and [La(EtForm)3] [3, EtForm = N,N, -bis(2,6-diethylphenyl)formamidinate] are a new class of precatalysts for the Tishchenko reaction. Their catalytic activity is a result of their high Lewis acidity and the ease with which the ligand spheres can be interchanged. For the dimerization of benzaldehyde to give benzyl benzoate, which is a benchmark reaction, compound 1 is, to the best of our knowledge, the most active catalyst ever reported. On a preparative scale, the reaction can be performed in the absence of solvent. A range of aromatic, heteroaromatic, and aliphatic aldehydes was rapidly converted into the corresponding esters by using catalysts 1,3.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Catalysis in electron transfer reactions: facts and mechanistic insights,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2002Shunichi Fukuzumi Abstract Catalysis in electron transfer reactions between electron donors and acceptors is described and the important mechanistic insight is provided by showing a number of examples of both thermal and photochemical reactions that involve metal ion-catalyzed electron transfer processes as the rate-determining steps. The quantitative measure of Lewis acidity of metal ions was obtained from the gzz values of ESR spectra of superoxide,metal ion complexes which vary significantly depending on the type of metal ions. Copyright © 2002 John Wiley & Sons, Ltd. [source] Mechanistic investigations of antimony-catalyzed polycondensation in the synthesis of poly(ethylene terephthalate)JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 3 2006Faissal-Ali El-Toufaili Abstract The chemical aspects of poly(ethylene terephthalate) synthesis via the antimony-catalyzed polycondensation of hydroxy ethylene terephthalate end groups were studied to elucidate its mechanism. A polycondensation mechanism was proposed in which activation occurs by the formation of a chelate ligand on antimony composed of the hydroxyl end group and alcoholic oxygen of the ester of the same end group. The rate-determining step of the polycondensation reaction was concluded to be the coordination of a second chain end to antimony. The low activity of antimony at a high concentration of hydroxyl end groups was proposed to result from the competition between hydroxyl end groups and the chelate structure leading to the transition state. The high selectivity of antimony is probably due to its relatively low Lewis acidity. Moreover, antimony was found to stabilize hydroxyl end groups against degradation by preventing their complexation to carbonyl functionalities. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1049-1059, 2006 [source] Supported (nBuCp)2ZrCl2 Catalysts: Effects of Selected Lewis Acid Organotin Silica Surface Modifiers on Ethylene PolymerizationMACROMOLECULAR REACTION ENGINEERING, Issue 4 2008Muhammad N. Akhtar Abstract This study investigated the effects of several organotin silica surface modifiers on the ethylene polymerization performance of (nBuCp)2ZrCl2 -based supported catalysts in which MAO and metallocene were sequentially loaded. Each organotin compound acted as a spacer, increasing the catalyst activity. However, the catalyst activity and of the resulting polyethylenes varied as follows: Activity and fractional Sn+ charge: nBuSn(OH)2Cl,>,MeSnCl3,>,nBuSnCl3,>,Reference catalyst; and, : Reference catalyst,>,nBuSnCl3,>,MeSnCl3,>,nBuSn(OH)2Cl. The above catalyst activity rating was explained considering the influence of the Lewis acidity, that is, the fractional Sn+ charge of the organotin modifiers on the generation, concentration, and electron density at the active [(nBuCp)2ZrMe]+ cation. All the catalysts showed fairly stable kinetic profiles and produced narrow molecular weight distribution resins; 2.8,,,PDI,,,3. [source] The effects of alumina and silica nanoparticles on the cure kinetics of bisphenol E cyanate esterPOLYMER ENGINEERING & SCIENCE, Issue 6 2010Xia Sheng Nanoparticles can be used as fillers to reinforce polymers, forming nanocomposites with better thermomechanical properties than composites with macrosized fillers. Furthermore, the addition of nanoparticles may influence the curing behavior of the polymer matrix during processing. In this study, the effect of various loadings of alumina or silica nanoparticles on the cure kinetics of bisphenol E cyanate ester (BECy) is investigated by differential scanning calorimetry (DSC). Alumina nanoparticles are shown to have a catalytic effect on the cure of BECy. The greater catalytic effect of alumina nanoparticles, compared with silica, is attributed to the increased number of hydroxyl groups on the surface and the Lewis acidity of ,-phase alumina. Kinetic parameters were obtained from dynamic DSC experiments. For an autocatalytic model of the cure process, the kinetic parameters obtained from the model suggest that the addition of alumina nanoparticles changed the cure reaction mechanism of BECy. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers [source] Analysis of a Pentacoordinate Iron Dicarbonyl as Synthetic Analogue of the Hmd or Mono-Iron Hydrogenase Active Site,CHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2010Tianbiao Liu Abstract Pentacoordinate iron dicarbonyls, (NS)Fe(CO)2P (NS=2-amidothiophenylate, P=PCy3 (4), PPh3, (5), and P(OEt)3 (6)) were prepared as potential biomimetics of the active site of the mono-iron hydrogenase, [Fe]-H2ase. Full characterization including X-ray diffraction, density functional theory (DFT) computations, and Mössbauer studies for complexes 5 and 6 find that, despite similar infrared v(CO) pattern and absorption frequencies as the active site of the [Fe]-H2ase, the geometrical distortions towards trigonal bipyramidal, the negative isomer shift parameters, and the differences in CO-uptake reactivity are due to the "non-innocence" of the NS ligand. Ligand-based protonation with a strong acid, HBF4,Et2O, interrupted the extensive ,-delocalization over Fe and NS ligand of complex 4 and switched on CO uptake (1,bar) to form a CO adduct, mer -[(H-NS)Fe(CO)3(PCy3)]+ or 4(CO)-H+. The extrinsic CO is reversibly removed on deprotonation with Et3N to regenerate complex 4. In a 13CO atmosphere, concomitant CO uptake by 4 -H+ and exchange with intrinsic CO groups provide a facile route to 13C-labeled 4(CO)-H+ and, upon deprotonation, 13C-labeled complex 4. DFT calculations show substantial Fe character in the LUMO of 4 -H+ typical of the d6 FeII in a regular square-pyramidal geometry. Thus, the Lewis acidity of 4 -H+ makes it amenable for CO binding, whereas the dianionic NS ligand renders the iron center of 4 insufficiently electrophilic and largely of FeI character. [source] Reactivity of Molecular Dioxygen towards a Series of Isostructural Dichloroiron(III) Complexes with Tripodal Tetraamine Ligands: General Access to ,-Oxodiiron(III) Complexes and Effect of ,-Fluorination on the Reaction KineticsCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2008Nasser Abstract We have synthesized the mono, di-, and tri-,-fluoro ligands in the tris(2-pyridylmethyl)amine (TPA) series, namely, FTPA, F2TPA and F3TPA, respectively. Fluorination at the ,-position of these nitrogen-containing tripods shifts the oxidation potential of the ligand by 45,70,mV per added fluorine atom. The crystal structures of the dichloroiron(II) complexes with FTPA and F2TPA reveal that the iron center lies in a distorted octahedral geometry comparable to that already found in TPAFeCl2. All spectroscopic data indicate that the geometry is retained in solution. These three isostructural complexes all react with molecular dioxygen to yield stable ,-oxodiiron(III) complexes. Crystal structure analyses are reported for each of these three ,-oxo compounds. With TPA, a symmetrical structure is obtained for a dicationic compound with the tripod coordinated in the ,4N coordination mode. With FTPA, the compound is a neutral ,-oxodiiron(III) complex with a ,3N coordination mode of the ligand. Oxygenation of the F2TPA complex gave a neutral unsymmetrical compound, the structure of which is reminiscent of that already found with the trifluorinated ligand. On reduction, all ,-oxodiiron(III) complexes revert to the starting iron(II) species. The oxygenation reaction parallels the well-known formation of ,-oxo derivatives from dioxygen in the chemistry of porphyrins reported almost three decades ago. The striking feature of the series of iron(II) precursors is the effect of the ligand on the kinetics of oxygenation of the complexes. Whereas the parent complex undergoes 90,% conversion over 40,h, the monofluorinated ligand provides a complex that has fully reacted after 30,h, whereas the reaction time for the complex with the difluorinated ligand is only 10,h. Analysis of the spectroscopic data reveals that formation of the ,-oxo complexes proceeds in two distinct reversible kinetic steps with k1,10,k2. For TPAFeCl2 and FTPAFeCl2 only small variations in the k1 and k2 values are observed. By contrast, F2TPAFeCl2 exhibits k1 and k2 values that are ten times higher. These differences in kinetics are interpreted in the light of structural and electronic effects, especially the Lewis acidity at the metal center. Our results suggest coordination of dioxygen as an initial step in the process leading to formation of ,-oxodiiron(III) compounds, by contrast with an unlikely outer-sphere reduction of dioxygen, which generally occurs at negative potentials. [source] Understanding the Reactivity and Basicity of Zeolites: A Periodic DFT Study of the Disproportionation of N2O4 on Alkali-Cation-Exchanged Zeolite YCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2008Pierre Mignon Dr. Abstract The disproportionation of N2O4 into NO3, and NO+ on Y zeolites has been studied through periodic DFT calculations to unravel 1),the role of metal cations and the framework oxygen atoms and 2),the relationship between the NO+ stretching frequency and the basicity of zeolites. We have considered three situations: adsorption on site,II cations with and without a cation at site,III and adsorption on a site III cation. We observed that cations at sites II and III cooperate to stabilize N2O4 and that the presence of a cation at site,III is necessary to allow the disproportionation reaction. The strength of the stabilization is due to the number of stabilizing interactions increasing with the size of the cation and to the Lewis acidity of the alkali cations, which increases as the size of the cations decreases. In the product, NO3, interacts mainly with the cations and NO+ with the basic oxygen atoms of the tetrahedral aluminium through its nitrogen atom. As the cation size increases, the NO3,,,,cation interaction increases. As a result, the negative charge of the framework is less well screened by the larger cations and the interaction between NO+ and the basic oxygen atoms becomes stronger. NO+ appears to be a good probe of zeolite basicity, in agreement with experimental observations. [source] Glycerol Etherification over Highly Active CaO-Based Materials: New Mechanistic Aspects and Related Colloidal Particle FormationCHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2008Agnieszka Abstract Glycerol is an attractive renewable building block for the synthesis of di- and triglycerols, which have numerous applications in the cosmetic and pharmaceutical industries. In this work, the selective etherification of glycerol to di- and triglycerol was studied in the presence of alkaline earth metal oxides and the data are compared with those obtained with Na2CO3 as a homogeneous catalyst. It was found that glycerol conversion increased with increasing catalyst basicity; that is, the conversion increases in the order: MgO Structural, Electronic, and Bonding Properties of Zeolite Sn-Beta: A Periodic Density Functional Theory StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2006Sharan Shetty Abstract The structural, electronic, and the bonding properties of the zeolite Sn-beta (Sn-BEA) have been investigated by using the periodic density functional theory. Each of the nine different T-sites in BEA were substituted by Sn atoms and all the nine geometries were completely optimized by using the plane-wave basis set in conjunction with the ultra-soft pseudopotential. On the basis of the structural and the electronic properties, it has been demonstrated that the substitution of Sn atoms in the BEA framework is an endothermic process and hence the incorporation of Sn in the BEA is limited. The lowest unoccupied molecular orbitals (LUMO) energies have been used to characterize the Lewis acidity of each T-site. On the basis of the relative cohesive energy and the LUMO energy, the T2 site is shown to be the most favorable site for the substitution Sn atoms in the BEA framework. [source] The Zirconium Alkoxide-Catalyzed Aldol-Tishchenko Reaction of Ketone AldolsCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2005Christoph Schneider Prof. Dr. Abstract The aldol-Tishchenko reaction of ketone aldols as enol equivalents has been developed as an efficient strategy to furnish differentiated 1,3- anti -diol monoesters in one step. The thermodynamically unstable ketone aldols undergo a facile retro-aldolization to yield a presumed zirconium enolate in situ, which then undergoes the aldol-Tishchenko reaction in typically high yields and with complete 1,3- anti diastereocontrol. Evaluation of a broad range of metal alkoxides as catalysts and optimization of the reaction protocol led to a modified zirconium alkoxide catalyst with attenuated Lewis acidity and dichloromethane as solvent, which resulted in suppression of the undesired acyl migration to a large extent. Various ketone aldols have been prepared and subjected to the general process, giving rise to a broad range of differently substituted 1,3- anti -diol monoesters, which may be hydrolyzed to the corresponding 1,3- anti -diols. [source] Solvation of Uranyl(II), Europium(III) and Europium(II) Cations in "Basic" Room-Temperature Ionic Liquids: A Theoretical StudyCHEMISTRY - A EUROPEAN JOURNAL, Issue 16 2004Alain Chaumont Abstract We report a molecular dynamics study of the solvation of UO22+, Eu3+ and Eu2+ ions in two "basic" (Lewis acidity) room-temperature ionic liquids (IL) composed of the 1-ethyl-3-methylimidazolium cation (EMI+) and a mixture of AlCl4, and Cl, anions, in which the Cl,/AlCl4, ratio is about 1 and 3, respectively. The study reveals the importance of the [UO2Cl4]2, species, which spontaneously form during most simulations, and that the first solvation shell of europium is filled with Cl, and AlCl4, ions embedded in a cationic EMI+ shell. The stability of the [UO2Cl4]2, and [EuIIICl6]3, complexes is supported by quantum mechanical calculations, according to which the uranyl and europium cations intrinsically prefer Cl, to the AlCl4, ion. In the gas phase, however, [EuIIICl6]3, and [EuIICl6]4, complexes are predicted to be metastable and to lose two to three Cl, ions. This contrasts with the results of simulations of complexes in ILs, in which the "solvation" of the europium complexes increases with the number of coordinated chlorides, leading to an equilibrium between different chloro species. The behavior of the hydrated [Eu(OH2)8]3+ complex is considered in the basic liquids; the complex exchanges H2O molecules with Cl, ions to form mixed [EuCl3(OH2)4] and [EuCl4(OH2)3], complexes. The results of the simulations allow us to better understand the microscopic nature and solvation of lanthanide and actinide complexes in "basic" ionic liquids. [source]
| | ||||||||||||