Showing papers on "Pyridine published in 1997"

Comprehensive study of surface chemistry of MCM-41 using 29Si CP/MAS NMR, FTIR, pyridine-TPD, and TGA

Abstract: A comprehensive study was conducted on mesoporous MCM-41. Spectroscopic examinations demonstrated that three types of silanol groups, i.e., single, (SiO)3Si−OH, hydrogen-bonded, (SiO)3Si−OH-OH−Si(SiO)3, and geminal, (SiO)2Si(OH)2, can be observed. The number of silanol groups/nm2, αOH, as determined by NMR, varies between 2.5 and 3.0 depending on the template-removal methods. All these silanol groups were found to be the active sites for adsorption of pyridine with desorption energies of 91.4 and 52.2 kJ mol-1, respectively. However, only free silanol groups (involving single and geminal silanols) are highly accessible to the silylating agent, chlorotrimethylsilane. Silylation can modify both the physical and chemical properties of MCM-41.

DRFT-IR Studies of the Surface of γ-Alumina

Abstract: γ-Alumina surfaces dehydrated at 450 and 700 °C were studied by using the conventional pyridine adsorption-DRFTIR technique. Three types of Lewis acid sites are present on the surface, and in each case pyridine adsorbed on them is concomitantly interacting with their adjacent OH groups. Because only three types of Lewis acid sites are present, only three hydrogen bonding bands were observed upon pyridine adsorption. From correlation of OH bands with hydrogen bonding bands, it is found that weak Lewis acid sites have type II66 or II64 OH groups as their neighbors, medium strong Lewis acid sites have type III OH groups near by, and strong Lewis acid sites have type I6 OH groups next to them. Dehydration at 700 °C generates more types of OH groups that also interact with pyridine. The reactivity of OH groups is not determined by their acidity and space restrictions, but rather by their proximity to Lewis acid sites. We assigned the three types of Lewis acid sites to five-, four-, and three-coordinate Al3+ ions.

Acidic properties of H-beta zeolite as probed by bases with proton affinity in the 118-204 kcal mol(-1) range: A FTIR investigation

Abstract: The interaction of Bronsted sites of H−β (AH) with bases B with proton affinity (PA) ranging in the 118 (N2) to 204 (NH3) kcal mol-1 interval leads to the formation of 1:1 adducts. The vibrational manifestations of these adducts change considerably as a function of the proton potential. In 1:1 neutral adducts characterized by weak-medium hydrogen bonds, the shift of the ν(AH) mode (Δν) is proportional to the shift caused by the same bases on the Si−OH (weak) Bronsted acid used as a standard. As far as the 1:1 adducts formed with ethers and tetrahydrofuran (THF; PA = 196 kcal mol-1), the situation is more complex owing to high ionicity acquired by the hydrogen bond. When stronger bases are used (pyridine, Py), the true proton transfer is observed. The ν(AH···B) modes of the medium-strong 1:1 adducts are modulated by Fermi resonances with 2δ and 2γ overtones and with internal modes of A and B moieties. A general review of the phenomenon is given. Also the spectra of the ionic 1:1 adducts formed with Py and...

Nitrogen-containing carbon nanotubes

Abstract: Carbon nanotubes containing small amounts of nitrogen are produced by the pyrolysis of aza-aromatics such as pyridine, methylpyrimidine and triazine over cobalt nanoparticles in an Ar atmosphere; good yields of such nanotubes are obtained by carrying out the pyrolysis of a mixture of pyridine and Fe(CO)(5) in flowing Ar + H-2.

Ru3(CO)12-Catalyzed Reaction of Pyridylbenzenes with Carbon Monoxide and Olefins. Carbonylation at a C−H Bond in the Benzene Ring

Abstract: A ruthenium-catalyzed carbonylation at a C−H bond in a benzene ring is described. The reaction of pyridylbenzenes with CO (20 atm) and ethylene in toluene at 160 °C in the presence of a catalytic amount of Ru3(CO)12 results in propionylation at an ortho C−H bond in the benzene ring. Carbonylation does not occur at the pyridine ring, although this is necessary as a directing group to promote the reaction. Olefins such as trimethylvinylsilane and tert-butylethylene in place of ethylene can also be used in this reaction, however 1-hexene, cyclohexene, allyltrimethylsilane, styrene, methyl methacrylate, vinyl acetate, triethoxyvinylsilane, and isopropenyltrimethylsilane do not afford the coupling products. Transition metal complexes, other than ruthenium carbonyl, examined thus far, do not show catalytic activity. In the reaction of meta-substituted pyridylbenzenes, such as those having Me, OMe, CF3, and COOMe group at the meta position in the benzene ring, carbonylation takes place at the less hindered C−H b...

Acid properties of dealuminated beta zeolites studied by IRspectroscopy

Abstract: The concentration of acid sites in non-dealuminated (Si/Al=10), and dealuminated (Si/Al=20–90) H-beta zeolites has been studied by quantitative IR measurements of pyridine sorption The number of Bronsted sites was found to be less than that of Al atoms Most probably this was due to the presence of Al in extraframework positions (Al–OH groups and Lewis acid sites were found) Some positively charged extraframework Al species may neutralize the charge of AlO 4 - and lower the concentration of protonic sites Dealumination decreased the amount of extraframework Al species and, hence, the difference between the amounts of Al and Bronsted sites Dealumination also decreased the number of Lewis acid sites IR studies of pyridine desorption evidenced that, as well as strong Bronsted sites (Si–OH–Al), there were also weak Bronsted sites, not Si–OH–Al; their nature is not clear The contribution of such weak Bronsted sites decreased with the extent of dealumination IR studies of pyridine desorption and benzene sorption, evidenced that Si–OH–Al groups were heterogeneous in H-beta zeolites Dealumination removed the less acidic hydroxy groups first

Mechanism of Carbon−Fluorine Bond Activation by (C5Me5)Rh(PMe3)H2

Abstract: The complex Cp*Rh(PMe3)H2 (Cp* = C5Me5) reacts with C6F6, C6F5H, C12F10, or C10F8 in pyridine or 1:1 pyridine/benzene to give the C−F cleavage products Cp*Rh(PMe3)(arylF)H in high yield. Kinetic studies reveal that the reaction has autocatalytic character, and fluoride ion is shown to be responsible for the catalysis. The anion [Cp*Rh(PMe3)H]- reacts rapidly with C12F10 or C10F8 to give the same C−F cleavage products as Cp*Rh(PMe3)H2. A mechanism initiated by deprotonation of Cp*Rh(PMe3)H2 followed by nucleophilic attack of the resulting anion on the polyfluoroaromatic with subsequent loss of fluoride is proposed. The fluoride ion continues the cycle by deprotonating Cp*Rh(PMe3)H2.

A novel class of phenol–pyridine co-crystals for secondharmonic generation

Abstract: To test the approach of combining both ionic and hydrogen-bonding interactions for the design of non-linear optical (NLO) materials, a number of phenol–pyridine co-crystals have been synthesized and their NLO properties investigated. The co-crystals are characterized by second harmonic generation measurements as well as the more conventional methods of melting point measurements, infrared and nuclear magnetic resonance spectroscopy. To investigate whether the phenol co-crystals are organic salts, the 2-methoxy-4-nitrophenol–4-(dimethylamino)pyridine (2:1) co-crystal 6 and the 2-methoxy-4-nitrophenol–4-pyrrolidinylpyridine–water (1:1:1) co-crystal 8 are further characterized by X-ray single-crystal diffraction. Crystal structure analyses reveal that both 6 and 8 are ionic co-crystals (or organic salts) composed of a phenoxide anion, a pyridinium cation and a neutral molecule. In the two co-crystals, the phenoxide, pyridinium and neutral molecules are held together by ionic attractions as well as hydrogen-bonding interactions. Both 6 and 8 crystallize in non-centrosymmetric structures [Pna2 1 (orthorhombic), a=6.880(4), b=38.40(1), c=8.454(3) A, Z=4, D c =1.369 g cm -3 and R=0.051 for 6 and Cc (monoclinic), a=7.302(3), b=23.518(2), c=9.940(1) A, β=107.12(2)°, Z=4, D c =1.365 g cm -3 and R=0.036 for 8]. In addition to X-ray structure determination, it is possible to predict whether phenol–pyridine co-crystals are organic salts based on the ΔpK a [pK a (pyridine)-pK a (phenol)] and stoichiometric ratio of the co-crystals. Preliminary results suggest that this type of co-crystals, particularly for the ionic co-crystals, may have a higher chance of forming non-centrosymmetric structures than the normal achiral organic compounds.

Synthesis, Reactivity, and Catalytic Behavior of Iron/Zinc-Containing Species Involved in Oxidation of Hydrocarbons under Gif-Type Conditions

Abstract: The present study explores the nature and reactivity of iron- and zinc-containing species generated in hydrocarbon-oxidizing GifIV-type solutions (Fe catalyst/Zn/O2 in pyridine/acetic acid (10:1 v/v). The ultimate goal of this investigation is to unravel the role of metal sites in mediating dioxygen-dependent C−H activation, which in the case of Gif chemistry demonstrates an enhanced selectivity for the ketonization of secondary carbons. Reaction of [Fe3O(O2CCH3)6(py)3]·py (1) with zinc powder in CH3CN/CH3COOH or CH2Cl2/CH3COOH affords the trinuclear compound [Zn2FeII(O2CCH3)6(py)2] (2). Single-crystal X-ray analysis confirms that one monodentate and two bidentate acetate groups bridge adjacent pairs of metals with the iron atom occupying a centrosymmetric position. The analogous reduction of 1 in py/CH3COOH (10:1, 5:1, 2:1 v/v) yields [FeII(O2CCH3)2(py)4] (3), [FeII2(O2CCH3)4(py)3]n (4), and [Zn(O2CCH3)2(py)2] (5) depending on the isolation procedure employed. Compound 3 possesses a distorted octahedral ...

Properties and Catalytic Activity of Acid-Modified Montmorillonite and Vermiculite

Abstract: The acidic properties of acid-modified montmorillonite and vermiculite were determined by pyridine and ammonia adsorption to correlate with the alkylating and dehydrating activity of the activated samples. Treatment of the minerals with different concentrations of hydrochloric acid results in the variation of overall acidity and density of acidic sites. Infrared (IR) spectral and differential scanning calorimetric analyses have revealed the presence of Bronsted and Lewis acid sites on the activated samples. The catalytic activity towards the above reaction has been correlated to the acid strength and density of Lewis acid sites. Treatment of montmorillonite with hydrochloric acid in the range of 0.1 and 0.3 M and vermiculite with 0.2 to 0.3 M seemed to be suitable for the conversion of methanol into olefin-rich hydrocarbons. Acid-activated montmorillonite catalyzed the isopropylation of benzene to a maximum extent of 16%, whereas acid-activated vermiculite gave a maximum conversion of only 4%.

Highly Stabilized Low-Spin Iron(III) and Cobalt(III) Complexes of a Tridentate Bis-Amide Ligand 2,6-Bis(N-phenylcarbamoyl)pyridine. Novel Nonmacrocyclic Tetraamido-N Coordination and Two Unusually Short Metal−Pyridine Bonds

Abstract: Two novel low-spin complexes, [Et4N][FeL2]·1.5H2O (1) and [Et4N][CoL2]·2H2O (2) (L is a deprotonated bis-amide ligand), have been synthesized and characterized. Four relatively longer equatorial M−Namide bonds and two significantly shorter axial M−Npy bonds are the noteworthy features of their X-ray structures. EPR and (34−300 K) magnetic studies of 1 confirm its low−spin character. Cyclic voltammetric studies reveal a highly stabilized M(III) state. For 1 a linear correlation between the Fe(III)−Fe(II) reduction potentials and the reciprocal of solvent dielectric constants is obtained.

Brønsted and Lewis acid catalysis with ion- exchanged clays

Abstract: An acid-treated montmorillonite clay has been ion-exchanged with Al3+, Fe3+, Cu2+, Zn2+, Ni2+, Co2+ and Na+. The catalytic activities of these materials have been measured in the Bronsted acid catalysed rearrangement of α-pinene to camphene, and the Lewis acid catalysed rearrangement of camphene hydrochloride to isobornyl chloride, following thermal activation at temperatures from 75 to 350°C. The surface acidities of the ion-exchanged clays have been measured using a microcalorimetric method involving ammonia adsorption, and through the infrared spectra of adsorbed pyridine. The results show that maximum Bronsted acidity is generated on thermal activation at approximately 150°C and maximum Lewis acidity at 250- -300°C. A good correlation has been found between the surface acidities and the catalytic activities of the ion- exchanged clays in both reactions. A significant result is the relatively low surface Lewis acid strength of Al3+- exchanged clays, for which a possible explanation is proposed.

Toward an understanding of the high enantioselectivity in the osmium-catalyzed asymmetric dihydroxylation. 4. Electronic effects in amine-accelerated osmylations

Abstract: Electronic effects in osmylation reactions accelerated by pyridine and quinuclidine derivatives were investigated by varying the substituents on the amine ligand as well as on the alkene substrate. Ligand substituent effects were gauged by determination of the equilibrium constants for coordination of the amines to OsO4, evaluation of structural properties and reduction potentials of the amine−OsO4 complexes, and analysis of the kinetics of osmylations in the presence of the amines. Substrate substituent effects were gauged by kinetic Hammett studies using several different amine/alkene combinations. Nonlinear Hammett relationships resulting from alkene substituent effects were observed, and the deviation from a linear free energy relationship was found to depend on the structure, binding capacity, and concentration of the amine. The results were evaluated in terms of the contending “[3 + 2]” and “[2 + 2]” mechanisms currently under consideration. A change in mechanism that depends on the structural and e...

Synthesis and NMR Studies of Three Pyridine-Containing Triaza Macrocyclic Triacetate Ligands and Their Complexes with Lanthanide Ions.

Abstract: The synthesis of three triazamacrocycles containing the pyridine moiety and three acetate pendant arms (PCTA) is reported. The three systems differ due to the number of carbon atoms in the macrocyclic ring forming ligands PCTA-[12], -[13], and -[14], endowed with different coordination capabilities toward lanthanide(III) ions. Microscopic protonation sequences for the three ligands have been investigated by 1H NMR spectroscopy. Complexes of PCTA-[12], -[13], and -[14] with La(III), Gd(III), and Lu(III) have been prepared. Relaxometric measurements on the aqueous solutions of the paramagnetic Gd(III) complexes in the presence of competitive ligands gave the following stability constants: log Kf = 20.8 for Gd-PCTA-[12], log Kf = 19.3 for GdPCTA-[13], and log Kf = 12.5 for GdPCTA-[14]. The measurement of water relaxation rates indicated a tendency to decrease the degree of hydration upon increasing the ring size. The VT 1H and 13C-NMR spectra of the diamagnetic La(III) and Lu(III) complexes exhibit a large ...

XPS of nitrogen-containing functional groups formed during the C-NO reaction

Abstract: The reaction between nitrogen oxides and carbon generates nitrogen-containing complexes on the carbon surface. The use of X-ray photoelectron spectroscopy allowed the characterization of these surface complexes. Five distinct structures have been distinguished, corresponding to nitrogen incorporated in five- and six-member ring as well as pyridine N-oxide and nitrate. Temperature has been found to be the determinant in the repartition of surface species. The reaction with NO−O2 at 600 °C has produced both pyrrolic and pyridinic nitrogens, while the reaction with NO at 950 °C mainly produced a pyridinic one. Quaternary nitrogen increased upon the heat treatment in He at 950 °C after the char was reacted with NO−O2 at 600 °C.

The electrochemical reduction of indigo dissolved in organic solvents and as a solid mechanically attached to a basal plane pyrolytic graphite electrode immersed in aqueous electrolyte solution

Abstract: The electrochemical properties of solid materials such as organic compounds, irrespective of their electrical conductivity, can be studied via the use of submicron sized particles mechanically attached to electrode surfaces immersed in aqueous media containing different electrolytes. In this study the reduction of solid indigo in buffered and non-buffered aqueous media has been investigated. Data are compared to those obtained from a voltammetric study of indigo dissolved in dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF) and in pyridine and the interpretation of results is facilitated by in situ EPR, in situ UV–VIS spectroscopy and atomic force microscopy (AFM) studies. For the reduction of solid indigo two distinct types of reduction processes, ‘surface-type’ and ‘bulk-type’, have been observed. The latter process has been found to be associated with the reductive dissolution of indigo. The ‘surface-type’ responses which occur in a microphase at the electrode/solid/solution interface are reversible 2H+-2e- reduction and oxidation processes with a corresponding 60 mV shift in half wave potential per pH unit in buffered electrolyte media over the pH range 4 to 11. The ‘bulk-type’ reduction process is proposed to be associated with the electroinsertion of cations into the solid indigo particles, thereby producing a reduced material and causing the loss of material from the electrode surface under convective flow conditions. The electrochemical solubilization of indigo occurred in non-buffered aqueous media even at neutral pH values.

Palladium(0)-Catalyzed Heteroarylation of 2- and 3-Indolylzinc Derivatives. An Efficient General Method for the Preparation of (2-Pyridyl)indoles and Their Application to Indole Alkaloid Synthesis.

Abstract: Palladium(0)-catalyzed coupling of (1-(benzenesulfonyl)-2-indolyl)zinc chloride (1) and (1-(tert-butyldimethylsilyl)-3-indolyl)zinc chloride (6) with diversely substituted (alkyl, methoxy, methoxycarbonyl, nitro, hydroxy) 2-halopyridines gives the corresponding 2- and 3-(2-pyridyl)indoles [4 and 7 (or 8), respectively] in excellent yields. A series of other 3-(heteroaryl)indoles (pyrazinyl, furyl, thienyl, indolyl) have been similarly prepared from 6. The potential of some of these (2-pyridyl)indoles in alkaloid synthesis is demonstrated. Thus, from 2-(2-pyridyl)indole 4b, a new synthetic entry to the indolo[2,3-a]quinolizidine system, involving stereoselective hydrogenation of the pyridine ring with subsequent electrophilic cyclization upon the indole 3-position from an appropriately Nb-substituted 2-(2-piperidyl)indole, is reported. For this purpose, Pummerer cyclizations have been extensively studied. Whereas the indole-unprotected sulfoxide 17 gives the corresponding indoloquinolizidine 19 in low yiel...

Poly(aryleneethynylene) Type Polymers Containing a Ferrocene Unit in the π-Conjugated Main Chain. Preparation, Optical Properties, Redox Behavior, and Mössbauer Spectroscopic Analysis

Abstract: Palladium-catalyzed polycondensation between diiodoferrocenes (1,1‘-diiodoferrocene and 1,6-diiodo-1‘,6‘-biferrocene) and diethynyl aromatic compounds HC⋮CArC⋮CH (e.g., 2,5-diethynylpyridine and 2,5-diethynyl-3-hexylthiophene) gives poly(aryleneethynylene) (PAE) type polymers containing the ferrocene unit in the π-conjugated main chain. The prepared polymers include (FcC⋮CPhC⋮C)n, PAE-Fc-1, (FcC⋮CPyC⋮C)n, PAE-Fc-2, and (FcC⋮ChexThC ⋮C)n, PAE-Fc-3 (Fc = 1,1‘-ferrocenylene; Ph = p-phenylene; Py = pyridine-2,5-diyl; hexTh = 3-hexylthiophene-2,5-diyl). 1H-NMR and IR spectra of the polymers are reasonable for their structures. The PAE type polymer containing the pyridine unit (PAE-Fc-2) is soluble in formic acid, and the polymer containing the hexylthiophene unit (PAE-Fc-3) is soluble in common organic solvents such as CHCl3, THF, and benzene. UV−visible spectra of the polymers exhibit a main π−π* absorption peak at about 330 nm and a d−d absorption peak at about 450 nm. The cyclic voltammogram of the polymers...

A Solid Acid Catalyst at the Threshold of Superacid Strength: NMR, Calorimetry, and Density Functional Theory Studies of Silica-Supported Aluminum Chloride

Abstract: Solid state NMR, calorimetry, and density functional theory (DFT) all provide a consistent interpretation of the acidity of the solid acid catalyst (SG)nAlCl2, which is prepared by reacting aluminum chloride with conditioned silica gel. These studies firmly establish that the acid sites are Bronsted in nature and that their strength is significantly greater than those in zeolites. Proton NMR results, including experiments exploiting 1H−27Al dipolar couplings, demonstrate that the Bronsted acid sites have an isotropic 1H chemical shift of 5.7 ppm and a concentration of 0.58 mmol/g. The strongest sites on this solid acid, present at 0.03 mmol/g, have −ΔHav values of 52 kcal/mol for reaction with pyridine. A value of 44 kcal/mol is maintained for incremental addition of pyridine up to 0.1 mmol/g. In comparison, −ΔHav for the strongest sites in zeolite HZSM-5 is only 42 kcal/mol. 15N magic angle spinning (MAS) NMR studies of adsorbed pyridine and 31P MAS NMR of trimethylphosphine confirm the Bronsted nature o...