Absence Of Metal Salts Research Articles

Influence of Lewis acidity and CaCl2 on the direct transformation of glucose to 5-hydroxymethylfurfural

Nb-Zr oxides were evaluated for glucose dehydration to 5-hydroxymethylfurfural. A linear relationship was found between the amount of acid sites and niobium content. Pure Nb2O5 showed the highest acid concentration and surface area and both Brönsted and Lewis acid sites, whereas Zr-containing catalysts mostly showed Lewis acid sites. Thus, a correlation between the amount of acid sites and catalytic performance was demonstrated, in the absence of metal salt, being Nb2O5 the most active catalyst. However, the catalytic results obtained by the combination of CaCl2 and Zr-containing catalysts, which only showed Lewis acid sites, suggested the direct formation of HMF, without fructose formation, unlike that observed for Nb2O5, which possesses both Brönsted and Lewis acid sites. A HMF yield of 44% was attained by using Nb3Zr7 as acid catalyst and CaCl2 after 90 min at 175 ºC. These catalysts can be reused at least 3 catalytic runs without any pretreatment.

Lanthanide(III) and yttrium(III) coordination compounds of diastereomeric (2+2) macrocyclic imines derived from 2,6-diformylpyridine and trans-1,2-diaminocyclopentane

The enantiopure mononuclear coordination compounds [Ln(L1)]Cl3·nH2O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III), Yb(III)) of the chiral macrocyclic (2+2) imine L1 (L1RRRR and L1SSSS), derived from (1R,2R) and (1S,2S)-1,2-diaminocyclopentane (DACP) and 2,6-diformylpyridine (DFP), have been synthesized in the templated condensation of the precursors in the presence of the appropriate metal salts. From the racemic trans-DACP and DFP in the templated condensation, a series of racemic mononuclear coordination compounds [Ln(L1rac)]Cl3·nH2O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III), Yb(III)) have been obtained. In the absence of metal salt, the condensation of the achiral precursors leads to a meso type Schiff base L2, which in the presence of lanthanide(III) salts produced the mononuclear coordination compounds [Ln(L2)]Cl3·nH2O (Ln = Y(III), Nd(III), Sm(III), Eu(III), Tb(III)). The mononuclear coordination compounds with the L1 and L2 ligands have been characterised by NMR spectroscopy, mass spectrometry, elemental analyses and/or circular dichroism (CD). 1H NMR signals of CDCl3/CD3OD solutions of the Y(III), Nd(III), Sm(III) and Eu(III) coordination compounds have been assigned on the basis of their COSY and HMQC spectra, and for the remaining lanthanide coordination compounds (Tb(III) and Yb(III)) the signals were tentatively assigned on the basis of linewidths analyses. The properties and stability of the lanthanide(III) and yttrium(II) coordination compounds of the racemic macrocycle L1rac have been compared with those of the isomeric meso type macrocycle L2 in organic solvents and water. Axial ligand exchange for several coordination compounds was investigated by 1H NMR titration experiments in organic solvents. The X-ray crystal structures of the representative mononuclear coordination compounds [Nd(L1RRRR)Cl(H2O)2]Cl2·0.5MeOH·H2O, [Nd(L1rac)Cl(H2O)2]Cl2·1.6MeOH·0.2H2O, [Y(L1rac)Cl(H2O)2]Cl2·3.5H2O, [Y(L1rac)Cl(H2O)2][Y(L1rac)(H2O)3]Cl5·6.8H2O and [Nd(L2)Cl2(H2O)]Cl with the diastereomeric ligands L1 and L2 have been determined.

Notable Effects of the Metal Salts on the Quenching Reaction of Singlet Oxygen by α-Tocopherol in Ethanol Solution

Abstract Measurement of the quenching rate constant (kQ) of singlet oxygen (1O2) by α-tocopherol was performed in ethanol solution including four kinds of alkali and alkaline earth metal salts (NaClO4, Mg(ClO4)2, MgCl2, and CaCl2) by varying concentrations of metal salts. A remarkable effect of metal salts on the kQ value of α-tocopherol was observed. The 1O2-quenching rate constant (kQ) decreased notably with increasing concentrations of metal salts, and reached a constant value for each salt at high concentration of the salt. For example, the kQ value (9.48 × 107 M−1 s−1) obtained in the presence of 20 mM of Mg(ClO4)2 was 46% of that (2.06 × 108 M−1 s−1) in the absence of metal salt. The kQ value decreased in the order of no metal salt > NaClO4 > CaCl2 > MgCl2 ≈ Mg(ClO4)2 at the same concentration of the metal salts. The metal salts having a smaller ionic radius and a larger charge of the cation gave a smaller kQ value. Effects of anions were negligible. These metal cations coexist with α-tocopherol in foods and biological systems, suggesting that the metal cations may give a notable effect to the 1O2-quenching rate constant (kQ).

Control of reaction pathways in the photochemical reaction of a quinone with tetramethylethylene by metal binding.

The present study reports a novel supramolecular photochemical reaction that focuses on the direct electronic interactions between a host reaction substrate and guest metal salts. The reaction pathways in the photochemical reactions of quinone derivatives bearing a methoxy group and a long oligoether sidearm QEn (n = 0 and 3) with tetramethylethylene (TME) are changed upon noncovalent complexations of the host reactant with alkali and alkaline earth metal ions and a transition metal salt. The photochemical reaction of QEn with TME provides a mixture of [2 + 2] cycloadducts 1aEn and 1bEn, hydroquinone H2QEn, and monoallyl ether adducts of hydroquinones 2aEn and 2bEn. The photochemical reaction proceeds by the photoinduced electron transfer mechanism, where photoirradiation brings about formation of a radical ion pair [QEn˙(-), TME˙(+)] as the primary intermediate. We found that the yields and selectivity of these photoproducts are changed upon electronic interactions of QEn˙(-) with the metal salts. The photochemical reaction in the absence of metal salts provides H2QEn as its major product, whereas QE3, having the long sidearm, dominantly produces 2aE3 at the expense of 1aE3, 1bE3, and H2QE3 when it forms a size-favorable host-guest complex with divalent Ca(2+). In contrast, QEn selectively provides oxetanes 1aEn and 1bEn in the presence of Pd(OAc)2, which can form complexes with the quinone through metal-olefin and coordination interactions in the ground and photoexcited states of the quinone.

Mutual Separation of Actinides from Middle Lanthanides by the Combination of Two Neutral Donors, N,N,N′,N′-Tetraoctyl-3,6-dioxaoctanediamide and N,N,N′,N′-Tetraethyldiglycolamide

Abstract The separation of Am and Cm from Sm, Eu, and Gd was examined by the combination of new extractant, DOODA (N,N,N′,N′-tetraoctyl-3,6-dioxaoctanediamide) and masking agent, TEDGA (N,N,N′,N′-tetraethyldiglycolamide). DOODA extracts light lanthanides with high D values into the organic phase, while TEDGA shows strong complexing ability with middle and heavy lanthanides in the aqueous phase. Americium and Cm behave as light lanthanides in this extraction system, therefore Am and Cm show high separation from middle lanthanides. The separation factors for D(Am)/D(Sm), D(Am)/D(Eu), and D(Am)/D(Gd) are 14.4, 40, and 111, respectively, for 0.3 M DOODA/n-dodecane and 10 mM TEDGA/5 M HNO3. The high separation of Am and Cm from these lanthanides, having high neutron cross section, can be seen in HNO3–n-dodecane in the absence of metal-salt.

Kinetic Study of the Aroxyl Radical-Scavenging Reaction of α-Tocopherol in Methanol Solution: Notable Effect of the Alkali and Alkaline Earth Metal Salts on the Reaction Rates

A kinetic study of the aroxyl (ArO*) radical-scavenging reaction of alpha-tocopherol (alpha-TocH) has been performed in the presence of six kinds of alkali and alkaline earth metal salts (LiI, LiClO(4), NaI, NaClO(4), KI, and Mg(ClO(4))(2)) in methanol solution, using stopped-flow spectrophotometry. The decay rate of the ArO* for the reaction of alpha-TocH with ArO* increased linearly with increasing concentration of metal salts. The second-order rate constants (k(s)) for the reaction of alpha-TocH with ArO* increased in the order of no metal salt < KI approximately NaClO(4) approximately NaI <or= LiClO(4) < Mg(ClO(4))(2) < LiI at the same concentration of metal salts. For example, the k(s) values in methanol solution including 4.00 x 10(-1) M of LiI and Mg(ClO(4))(2) were 3.04 and 1.30 times larger than that in the absence of metal salts, respectively. The alkali and alkaline earth metal salts having smaller ionic radius of cation and anion and larger charge of cation gave larger rate constants (k(s)). Effects of metal cations on the UV-vis absorption spectra of the alpha-Toc* (and ArO*) radical were negligible in methanol solution, suggesting that the complex formation between the alpha-Toc* (and ArO*) radical molecule and metal cations is hindered by the hydrogen bond between radical and methanol molecules. The results indicate that the hydrogen transfer reaction of alpha-TocH proceeds via an electron transfer intermediate from alpha-TocH to ArO* radicals followed by proton transfer. Both the coordinations of metal cations to the one-electron reduced anions of ArO* (ArO: (-)) and of counteranions to the one-electron oxidized cations of alpha-TocH (alpha-TocH(+)*) may stabilize the intermediate, resulting in the acceleration of electron transfer. On the other hand, the effect of metal salts on the rate of bimolecular self-reaction (2k(d)) of the alpha-Toc* radical was not observed. The result suggests that the hydrogen transfer reaction between two alpha-Toc* radical molecules proceeds via a one-step hydrogen atom transfer mechanism rather than via an electron-transfer intermediate. High concentrations of alkali and alkaline earth metal salts coexist with alpha-TocH in plasma, blood, and many tissues, suggesting the contribution of the metal salts to the antioxidant actions of alpha-TocH.

An Unusual, His-dependent Family I Pyrophosphatase from Mycobacterium tuberculosis

Soluble inorganic pyrophosphatases (PPases) comprise two evolutionarily unrelated families (I and II). These two families have different specificities for metal cofactors, which is thought to be because of the fact that family II PPases have three active site histidines, whereas family I PPases have none. Here, we report the structural and functional characterization of a unique family I PPase from Mycobacterium tuberculosis (mtPPase) that has two His residues (His21 and His86) in the active site. The 1.3-A three-dimensional structure of mtPPase shows that His86 directly interacts with bound sulfate, which mimics the product phosphate. Otherwise, mtPPase is structurally very similar to the well studied family I hexameric PPase from Escherichia coli, although mtPPase lacks the intersubunit metal binding site found in E. coli PPase. The cofactor specificity of mtPPase resembles that of E. coli PPase in that it has high activity in the presence of Mg2+, but it differs from the E. coli enzyme and family II PPases because it has much lower activity in the presence of Mn2+ or Zn2+. Replacements of His21 and His86 in mtPPase with the residues found in the corresponding positions of E. coli PPase had either no effect on the Mg2+- and Mn2+-supported reactions (H86K) or reduced Mg2+-supported activity (H21K). However, both replacements markedly increased the Zn2+-supported activity of mtPPase (up to 11-fold). In the double mutant, Zn2+ was a 2.5-fold better cofactor than Mg2+. These results show that the His residues in mtPPase are not essential for catalysis, although they determine cofactor specificity.

Nucleophilic ring opening of 1,2-epoxides in aqueous medium

Nucleophilic ring opening of 1,2-epoxides in aqueous medium in the presence and absence of metal salts is reviewed. Azidolysis, hydrolysis, iodolysis and thiolysis are the reactions mainly investigated. The pH of the reaction medium controls the reactivity and regioselectivity of the process. By working at suitable pH values, even salts such as AlCl3, SnCl4 and TiCl4 are active catalysts.

Crystallization and preliminary X-ray diffraction data of mouse L-chain apoferritin crystals.

Crystals of recombinant mouse L-chain apoferritin were obtained by the hanging-drop technique using ammonium sulfate as precipitant. Two crystal forms were observed in the same drop. The crystals belong to either the P2 monoclinic or to the P42(1)2 tetragonal space group. The monoclinic crystals diffracted to beyond 2.4 A resolution but were systematically twinned, while the tetragonal crystals diffracted to beyond 2.9 A. These crystallization conditions in the absence of metal salts should facilitate the study of the interaction between L-chain ferritins and heavy metals, particularly the iron core.

Synthesis, spectroscopic and cyclic voltammetry studies of copper(II) complexes with open chain, cyclic and a new macrocyclic thiosemicarbazones

New copper complexes have been prepared from benzilbisthiosemicarbazone (L 1H 6) and from the cyclic 6-methoxi-1,6-diphenyl-4-thio-3,4,5,6-tetrahydro-2,3,5-triazine (L 2H 2). The complexes were characterized by mass spectrometry, IR, electronic and electron paramagnetic resonance spectra. Complexes from L 1H 6 ( 1– 5) present 1:1 stoichiometry and show different characteristics with a variable grade of deprotonation in the ligand, depending on the salt used (chloride, nitrate or sulfate) and the presence of acid in the medium. A macrocyclic Schiff base, 3,4,9,10-tetraphenyl-1,2,5,6,8,11-hexaazacyclododeca-7,12-dithione-3,4,9,10-tetraene (L 3H 2), containing thiosemicarbazone moieties is readily prepared and characterized for the first time, with fairly good yield, from the mesocycle (L 2H 2) in methanol with copper chloride as template. Near quantitative synthesis of the precursor, L 2H 2, which is a potential chemotherapeutic agent against cancer, has been achieved by reacting benzil and thiosemicarbazide in the absence of metal salt and using a high dilution technique. Macrocyclic complexes 6 and 7 containing thiosemicarbazone moieties were obtained from L 2H 2 and copper nitrate and sulfate. Direct reaction between the copper salt and the macrocyclic thiosemicarbazone (L 3H 2) gave similar complexes but with a lower grade of purity. Macrocyclic complexes 6 and 7 present metal–ligand ratios of 1:2 and 1:1 and the ligand is in a deprotonated form. The redox behaviour was explored by cyclic voltammetry. L 1H 6 complexes show Cu(II)/Cu(I) couples and quasireversible waves associated with the Cu(III)/Cu(II) process. The reduction/oxidation potential depends on the structure and conformation of the central atom in the coordination compounds.

Novel Silica Structures Which Are Prepared by Transcription of Various Superstructures Formed in Organogels

Three cholesterol-based gelators were synthesized which have a monobenzo-18-crown-6 (2), monoaza-18-crown-6 (3), and 1,10-diaza-18-crown-6 (4), respectively. These gelators could gelatinize 8 of 14 organic solvents tested herein, indicating that they possess a versatile gelation ability. Scanning electron microscopy observations of these xerogels showed that in cyclohexane 2, 3, and 4 assemble into a fibrous network structure, a curved lamellar structure, and a cylindrical tubular structure, respectively. Sol−gel polymerization of tetraethoxysilane was carried out in these gel systems. The silica obtained from 2 in the absence of metal salt had a conventional granular structure whereas that in the presence of KClO4 had a hollow fiber structure featuring the rough surface and the thick tube wall. This structure is created by the adsorption of anion-charged silica particles onto the cation-charged organogel fibers. On the other hand, the silica obtained from 3 and 4 had a hollow fiber structure featuring the smooth surface and the thin tube wall both in the absence and the presence of metal salt. In the absence of metal salt, the cationic charge generated by protonation of azacrown ethers plays a crucial role in the creation of such a hollow fiber structure. In the presence of KClO4 or CsClO4, sol−gel polymerization resulted in the tubular silica with a multilayer structure like a roll of paper. The findings suggest that sol−gel polymerization proceeds along the surface of the curved lamellar surface of 3 or 4 and the silica eventually grows up as a tubular structure. These results indicate that various novel silica structures can be prepared by transcription using various superstructures in organogels as a template.

Conformational preference and diastereoselectivity of ( S)- N-(α,β-unsaturated carbonyl)-γ-trityloxymethyl-γ-butyrolactam

The conformational dependence of (S)-N-(α,β-unsaturated carbonyl)-γ- trityloxymethyl-γ-butyrolactam 1 on metal salts was studied by means of theoretical calculations and NMR spectroscopy. The anti-conformation is dominant in the absence of metal salt, whereas the syn-form is predominant in the presence of magnesium perchlorate. Diastereofacial selection exhibited by 1 is reasonably correlated with its conformational dependency on metal salts.

ChemInform Abstract: METHYLATION OF QUINONES BY METHYLCOBALT COMPLEXES IN THE PRESENCE OF PALLADIUM SALT

Die Methylierung von 1,4-Benzo- und 1,4-Na-phthochinon (I) mit Methylkoba1t- Komplexen (Methyl-bis-(diacetylmonoxim-imino)-1,3-propan-kobalt(III), Methylbis-(dimethylglyoximato)-koba1t(III) und Methylcobalamin) in Gegenwart und Abwesenheit vonMetallsalzen (LiPdCl3, Li2PdCl4, Ni(II)-, Fe(III)-, Ag(I)- und Cu= (II)-Salzen) in Methanol/Acetonitril oder Äthanol wurde untersucht.

Methylation of quinones by methylcobalt complexes in the presence of palladium salt.

Methylation of 1, 4-quinones by methylcobalt complexes has been studied in the presence or absence of some metal salts. Methylcobaloximes were found to afford monomethylated quinones though the yield was very small in the absence of metal salts. The transmethylation was, however, remarkably promoted in the presence of transition metal salt such as Pd (II) and reached nearly 70% at the highest. Methylcobalamins were also found to methylate 1, 4-quinones to a slight extent but no promoting effect was recognized with Pd (II) probably due to complexing side reactions. Nevertheless, the observations may indicate that methylcobalt complexes act as methylating reagents not only for mercuric salts but also for organic compound such as 1, 4-quinones. Reaction mechanisms are briefly discussed.

Increased rates of initiation of polymerization by 4‐4′‐dicyano‐4‐4′‐azopentanoic acid in the presence of ferric salts

AbstractThe initiation of the polymerization of acrylamide by 4‐4′‐dicyano‐4‐4′‐azopentanoic acid in aqueous solution has been studied kinetically at 25°C. Ferric chloride and ferric sulfate were used to terminate polymerization so that rates of initiation could be calculated from the rates of production of ferrous iron. Velocity coefficients at 25°C. for the initiation reaction were found to be (25.7 ± 2.8) × 10−7 sec.−1 for the ferric chloride terminated reaction and (73.6 ± 0.6) × 10−7 sec.−1 for the ferric sulfate‐terminated polymerization. The value reported for the initiation reaction when acrylamide is polymerized in the absence of metal salts is 1.29 × 10−7 sec.−1. Velocity coefficients for the termination reaction have been calculated from the overall rates of polymerization obtained with ferric salts present. In the case of the ferric chloride‐terminated reaction, it has been shown that the rate of polymerization is reduced by increasing the total concentration of chloride ions. Termination velocity coefficients at 25°C. for the inner sphere complexes FeCl2+·5H2O and FeSO4+·4H2O have been calculated to be 18.9 × 104 and 7.98 × 104 l./mole‐sec., respectively. The dependence on the concentration of ferric chloride of the molecular weights of the polymers produced has also been considered.