Azide/thiocyanate incorporated cobalt(III)-Schiff base complexes: Characterizations and catalytic activity in aerobic epoxidation of olefins

Reaction of cobalt(II) perchlorate hexahydrate with a potentially tetradentate Schiff base ligand, HL (2-methoxy-6-(2-diethylaminoethylimino)methyl]phenol) in presence of sodium azide and sodium thiocyanate yields two complexes Co( L)( HL)(N-3)]center dot ClO4 ( 1) and Co( L)( HL)(NCS)] center dot ClO4 ( 2); both being characterized by different physicochemical methods. Crystal structure of 1 was determined by single crystal X-ray diffraction while that of 2 was reported earlier. In 1, the central cobalt(III) adopts slightly distorted octahedral geometry with same donor set to that of 2. Catalytic efficacy of the complexes towards epoxidation of different alkenes under aerobic condition were investigated in homogeneous medium which reveals that 1 is better catalyst than 2 with respect to alkene oxidation, reflected from the turn over frequencies (TOF) measured at an optimum temperature of 60 degrees C in acetonitrile. (C) 2014 Published by Elsevier B.V.

Optical detection of submicromolar levels of nitro explosives by a submicron sized metal-organic phosphor material

Two isomorphous submicron sized metal-organic network compounds, Y-2(PDA)(3)(H2O)1]center dot 2H(2)O (PDA = 1,4-phenylenediacetate), 1 and Y1.8Tb0.2(PDA)(3)(H2O)1]center dot 2H(2)O, Tb@1 have been synthesized by employing solvent assisted liquid grinding followed by heating at 180 degrees C for 1' min and washing with water. Single crystal X-ray data of bulk 1 confirmed a three dimensional porous structure. The structure and morphology of 1 and Tb@1 were systematically characterized by PXRD, TGA, DSC, IR, SEM and EDX analysis. Dehydrated Tb@1 Tb@1'] shows a high intense visible green emission upon exposure to UV light. The green emission of Tb@1' was used for the detection of nitro explosives, such as 2,4,6-trinitrophenol (TNP), 1,3-dinitro benzene (DNB), 2,4-dinitro toluene (DNT), nitro benzene (NB), and 4-nitro toluene (NT) in acetonitrile. The results show that the emission intensity of dehydrated Tb@1' can be quenched by all the nitro analytes used in the present work. Remarkably, Tb@1' exhibited a high efficiency for TNP, DNB and DNT detection with K-SV K-SV = quenching constant based on linear Stern-Volmer plot] values of 70 920, 44 000 and 35 430 M-1, respectively, which are the highest values amongst known metal-organic materials. Using this material submicromolar level (equivalent to 0.18 ppm), a detection of nitro explosives has been achieved.

Self-assembled gold nanofilms as a simple, recoverable and recyclable catalyst for nitro-reduction

Hexaazamacrocycle (L) stabilized gold nanoparticles (AuNPs) were prepared by combining L with HAuCl4 center dot 3H(2)O in a variety of alcohol-water (1 : 1) mixtures. The dual roles of L as a reducing and stabilizing agent were exploited for the synthesis of AuNPs under the optimized ratio of L to Au3+ (2 : 1). Self-assembled gold nanofilms (AuNFs) were constructed at liquid-liquid interfaces by adding equal volumes of hexane to the dispersions of AuNPs in the alcohol-water systems. The nanofilms were formed spontaneously by shaking the two-phase mixture for a minute followed by standing. The alcohols explored for the self-assembly phenomenon were methanol, ethanol, i-propanol and t-butanol. The systems containing methanol or t-butanol resulted in AuNFs at the interfaces, whereas the other two alcohols were found not suitable and the AuNPs remained dispersed in the corresponding alcohol-water medium. The AuNFs prepared under suitable conditions were coated on a variety of surfaces by the dip and lift-off method/solvent removal approach. The AuNFs were characterized by UV-vis, SEM, TEM, AFM and contact angle measurement techniques. A coated glass-vial or cuvette was used as a catalytic reservoir for nitro-reduction reactions under ambient and aqueous conditions using NaBH4 as the reducing agent. The reduced products (amines) were extracted by aqueous work-up using ethyl acetate followed by evaporation of the organic layer; the isolated products required no further purification. The catalyst was recovered by simply decanting the reaction mixture whereupon the isolated catalyst remained coated inside the vessel. The recovered catalyst was found to be equally efficient for further catalytic cycles.

Preparation and catalytic studies of palladium nanoparticles stabilized by dendritic phosphine ligand-functionalized silica

Silica is a prominently utilized heterogeneous metal catalyst support. Functionalization of the silica with poly(ether imine) based dendritic phosphine ligand was conducted, in order to assess the efficacy of the dendritic phosphine in reactions facilitated by a silica supported metal catalyst. The phosphinated poly(ether imine) (PETIM) dendritic ligand was bound covalently to the functionalized silica. For this purpose, the phosphinated dendritic ligand containing an amine at the focal point was synthesized initially. Complexation of the dendritic phosphine functionalized silica with Pd(COD)Cl-2 yielded Pd(II) complex, which was reduced subsequently to Pd(0), by conditioning with EtOH. The Pd metal nanoparticle thus formed was characterized by physical methods, and the spherical nanoparticles were found to have >85% size distribution between 2 nm and 4 nm. The metal nanoparticle was tested as a hydrogenation catalyst of olefins. The catalyst could be recovered and recycled more than 10 times, without a loss in the catalytic efficiency.

Triplet-state Photophysics and Transient Photochemistry of Cyclic Enethiones A Laser Flash Photolysis Study

The triplets of four cyclic enethiones, including thiocoumarin, have been investigated by nanosecond laser flash photolysis. Data are presented for transient spectra and kinetics associated with triplets, quantum yields of intersystem crossing and singlet oxygen photosensitization. The quenching of the thiocoumarin triplet (A:, = 485 nm, E:,, = 8.8 x lo3 dm3 mol-' cm-'in benzene) by several olefins, amines and hydrogen donors occurs with rate constants of 107-5 x lo9 dm3 mol-' s-'; the lower limits of quantum yields ( c#+~) for the related photoreactions, estimated from ground-state depletion, are generally small (0.0-0.1 1 in benzene, except for good hydrogen donors, namely, p-methoxythiophenol and tri-n-butylstannane) . The radical anion of thiocoumarin (A,,, = 405-435 nm) is formed in two stages upon triplet quenching by triethylamine in acetonitrile; the fast component is the result of direct electron transfer to the triplet and the slower component is assigned to secondary photoreduction of the thione ground state by the a-aminoalkyl radical derived from the triethylamine radical-cation.

Photochemical Reactions in Oriented Systems

Modification of chemical reactions through the use of constrained and/or organized media has attracted a great deal of attention recently. Results from our laboratory in this direction which include a study of photochemical reactions in solid state and in cyclodextrins are presented here. A study of solid state photochemical behavior of coumarins has provided information regarding subtler aspects of topochemical postulates of photodimerization. Results pertaining to geometrical criteria for photodimerization and "chloro" as a crystal engineering group are discussed. As a part of an attempt to correlate chemical reactivity with molecular packing in the solid state, photooxidation of diarylthioketones in the solid state has been investigated. The observed differences in the reactivity of these crystals are rationalized in terms of crystal packing. Though cyclodextrins have been extensively studied, very few photochemical reactions involving molecules complexed to cyclodextrins have been examined. In this connection, the utility of cyclodextrins in bringing about selectivity in photochemical reactions through the study of excited state behavior of olefins and aryl alkyl ketones has been demonstrated in our laboratory.

Synthesis, spectral and electrochemical properties of donor/acceptor substituted fluoroarylporphyrins

Spectroscopic and electrochemical redox properties of a series of fluorinated porphyrins bearing donor-acceptor groups and their Zn(II) and Cu(II) derivatives are presented. The magnitude of the ring reduction potentials and charge transfer properties derived from spectral data depend on the nature and position of the substituent(s), (nitro/dimethylamino) and the central metal ions.

Design, Development, Synthesis, and Docking Analysis of 2-substituted Triclosan Analogs as Inhibitors for Plasmodium falciparum Enoyl-ACP Reductase

A structure-based approach has been adopted to develop 2'substituted analogs of triclosan. The Cl at position 2' in ring B of triclosan was chemically substituted with other functional groups like NH2, NO2 and their inhibitory potencies against PfENR were determined. The binding energies of the 2' substituted analogs of triclosan for enoyl-acyl carrier protein reductase (ENR) of Plasmodium falciparum were determined using Autodock. Based on the autodock results, we synthesized the potential compounds. The IC50 and inhibition constant (K-i) of 2' substituted analogs of triclosan were determined against purified PfENR. Among them, two compounds,2-(2'-Amino-4'-chloro-phenoxy)-5-chloro-phenol (compound 4) and 5-chloro-2-(4'-chloro-2'-nitro-phenoxy)-phenol) (compound 5) exhibited good potencies. Compound 4 followed uncompetitive inhibition kinetics with crotonoyl CoA and competitive with NADH. It was shown to have an IC50 of 110 nM; inhibition constant was 104 nM with the substrate and 61 nM with the cofactor. IC50 Of compound 5 was determined to be 229 nM. Compounds 4 and 5 showed significant inhibition of the parasite growth in P. falciparum culture. (C) 2009 IUBMB IUBMB Life, 61(11):1083-1091, 2009.

Quinone studies—Istar, open: Evidence for a new type of substitution reaction of phenanthrene-9,10-quinone derivatives

The para orientation by the carbonyl groups in the bromination of phenanthrenequinone derivatives has been explained on the basis of an excited state resulting from thermal excitation of the quinone and/or from a n→π* transition of the nonbonding electrons of the oxygen atoms. A general preparative method for the syntheses of 3-bromophenanthrenequinone derivatives has been developed. The structure of 2-nitro-6-bromophenanthrenequinone has been established by degradation. Synthesis of 2-nitro-6-bromofluorenone is described. Direct bromination of phenanthrenequinone to 2-bromo and 2,7-dibromo derivatives has also been described.

A study of the purification and properties of tryptophan synthetase of Bengal gram (Cicer arietinum)

Active preparations of tryptophan synthetase were obtained from Bengal gram (Cicer arietinum) by the following procedure: (1) precipitation of inactive materials by manganous sulfate, (2) Adsorption of impurities on Alumina Cγ, (3) Adsorption of tryptophan synthetase on tricalcium phosphate gel, removal of inert protein from the gel by treatment with phosphate buffer (pH 7.2), and selective elution of the enzyme by 0.15 M phosphate buffer pH 7.2 containing 10% ammonium sulfate and 10−3 M serine. A 220-fold purification of the enzyme with 44% recovery of the activity was achieved. The pH optimum, effect of temperature, and substrate concentration and other properties of the purified enzyme have been studied in detail. Only the Image -isomer of serine takes part in the reaction. The Km values for indole, Image -serine, and Image -serine were calculated to be 0.66, 4.1, and 8.6 × 10−4 M, respectively. A kinetic study of the inhibition of tryptophan synthetase by indole-propionic acid has shown that it is of a competitive type. It has been demonstrated for the first time that 4-nitro-salicylaldehyde can replace pyridoxal phosphate as a coenzyme for the tryptophan synthetase reaction.

Evaluation of solute-solvent interactions from solvent blue-shifts of n → π* transitions of C=O, C=S, NO2 and N=N groups: hydrogen bond energies of various donor-acceptor systems

Effects of non-polar, polar and proton-donating solvents on the n → π* transitions of C=O, C=S, NO2 and N=N groups have been investigated. The shifts of the absorption maxima in non-polar and polar solvents have been related to the electrostatic interactions between solute and solvent molecules, by employing the theory of McRAE. In solvents which can donate protons the solvent shifts are mainly determined by solute-solvent hydrogen bonding. Isobestic points have been found in the n → π* bonds of ethylenetrithio-carbonate in heptane-alcohol and heptane-chloroform solvent systems, indicating the existence of equilibria between the hydrogen bonded and the free species of the solute. Among the different proton-donating solvents studied water produces the largest blue-shifts. The blue-shifts in alcohols decrease in the order 2,2,2-trifluoroethanol, methanol, ethanol, isopropanol and t-butanol, the blue-shift in trifluoroethanol being nearly equal to that in water. This trend is exactly opposite to that for the self-association of alcohols. It is suggested that electron-withdrawing groups not merely decrease the extent of self-association of alcohols, but also increase the ability to donate hydrogen bonds. The approximate hydrogen-bond energies for several donor-acceptor systems have been estimated. In a series of aliphatio ketones and nitro compounds studied, the blue-shifts and consequently the hydrogen bond energies decrease with the decrease in the electron-withdrawing power of the alkyl groups. It is felt that electron-withdrawing groups render the chromophores better proton acceptors, and the alcohols better donors. A linear relationship between n → π* transition frequency and the infrared frequency of ethylenetrithiocarbonate has been found. It is concluded that stabilization of the electronic ground states of solute molecules by electrostatic and/or hydrogen-bond interactions determines the solvent shifts.

Colorimetric determination of 2,4-dinitrophenylamino groups by sodium borohydride reduction

When sodium borohydride is added to aqueous solutions of 2,4-dinitrophenylamino acids and related derivatives, an intense red color is formed. Measurement of the red color, with a 420 filter, permits the determination of such compounds in concentrations of 0.01 to 0.06 μmole per ml. with a precision to 2%. The reaction is highly specific-while 2,4-dinitroaniline will react to the test, o-, m-, and p-nitroanilines, 2,4-dinitrophenyl aryl or alkyl ethers, and 2,4-dinitrophenyl-imidazole and pyrrolidine derivatives will not. Heretofore aromatic nitro groups have been considered resistant to attack by sodium borohydride. The method, as developed, is applicable to the evaluation of the degree of substitution of protein amino groups by fluorodinitrobenzene.

On the specificity of activation of mitochondrial succinate dehydrogenase by ubiquinol

Activation of succinate dehydrogenase on preincubation with mitochondria is not specific for ubiquinol-10. Other homologues of ubiquinol, plastoquinol, o-, m- and p-quinols, 2-nitro-, 4-nitro- and 2,4-dinitro- phenols showed different degrees of activation. The lipid quinols, however, showed activation greater than succinate, hitherto considered to give maximum activation.

Chemical approaches to penicillin allergy—IV. Binding of carrier receptor protein with penicillin and its analogues

The availability of electrophoretically homogeneous rabbit penicillin carrier receptor protein (CRP) by affinity chromatography afforded an idealin vitro system to calculate the thermodynamic parameters of binding of penicillin and analogues with CRP as well as competitive binding of such analogues with CRP in presence of14C-penicillin G. The kinetics of association of CRP with 7-deoxy penicillin which does not bind covalently with CRP have been studied through equilibrium dialysis with14C-7-deoxybenzyl penicillin and found to be K=2·79×106M−1.−ΔG=8·106 k cal/mole as well as fluorescence quenching studies with exciter λ 280 K=3·573×106M−1,−ΔG=8·239 k cal/mole. The fluorescence quenching studies have been extended to CRP-benzyl penicillin and CRP-6-aminopenicillanic acid (6APA) systems also. The fluorescence data with benzyl penicillin indicate two conformational changes in CRP—a fast change corresponding to the non-covalent binding to CRP with 7-deoxy penicillin and a slower change due to covalent bond formation. With 6-APA the first change is not observed but the conformational change corresponding to covalent binding is only seen. Competitive binding studies indicate that the order of binding of CRP with the analogues of penicillin is as follows: methicillin > 6APA > carbenicillin >o-nitrobenzyl penicillin > cloxacillin ≈ benzyl penicillin ≈ 6-phenyl acetamido penicillanyl alcohol ≈ 7 phenyl acetamido desacetoxy cephalosporanic acid ≈p-amino benzyl penicillin ≈p-nitro benzyl penicillin > ticarcillin >o-amino benzyl penicillin > amoxycillin > 7-deoxy benzyl penicillin > ampicillin.From these data it has been possible to delineate partially the topology of the penicillin binding cleft of the CRP as well as some of the functional groups in the cleft responsible for the binding process.