Muhammad Naseem Akhtar

Synthesis of Functional Polyolefins using Metallocenes: A Comprehensive Review

The copolymerization mainly of ethylene and propylene with various polar functional comonomers, using various metallocenes and the methylaluminoxane (MAO) cocatalyst, has been primarily reviewed from the perspective of the following two subjects. One is the influence of the various functional groups on the copolymerization reactions, and the properties of the resulting products; the other is the areas of future research. The functional groups have been classified into oxygen-, nitrogen-, and halogen-containing moieties; borane-containing α-olefins; silanes; dienes; cyclic olefins; as well as styrene and its various derivatives. The following areas—synthesis of easily soluble functional cooligomer/copolymer; products with uniform distribution of the comonomer; establishment of relation among the catalyst structure and the various steps of copolymerization (initiation, propagation, and chain termination); the degradation and stabilization study of the functional copolymers and the correlation of the same with the catalyst structure; the application of supported metallocenes to synthesize the resulting polymer; the minimization of multi-step synthesis; and the development MAO cocatalyst formulation (to improve the activity and comonomer incorporation)—have been identified to be the subjects of future research, which will be of special importance to the researchers working in functional polyolefins.

Solvent-free iridium-catalyzed CO2 hydrosilylation: experiments and kinetic modeling

The iridium(III) complex [Ir(H)(CF3SO3)(NSiN)(coe)] (NSiN = bis(pyridine-2-yloxy)methylsilyl, coe = cyclooctene) has been demonstrated to be an active catalyst for the solvent-free hydrosilylation of CO2 with 1,1,1,3,5,5,5-heptamethyltrisiloxane (HMTS) under mild reaction conditions (3 bar). The activity of this catalytic system depends on the reaction temperature. The best catalytic performance has been achieved at 75 °C. A kinetic study at variable temperature (from 25 °C to 75 °C) and constant pressure (3 bar) together with kinetic modeling has been carried out. The results from such a study show an activation energy of 73.8 kJ mol−1 for the process.

XPS investigation of the electronic environment in selected heterogenized zirconocene catalysts

Ethylbisindenyl zirconium dichloride (Et(Ind)2ZrCl2) and the MAO methylalumoxane (MAO) co-catalyst were heterogenized on Davision silica 955 partially dehydroxylated at 275 °C, following the concept of equilibrium adsorption. The influence of MAO on the electronic environment resulting from the heterogenization was investigated using XPS. Heterogenization of Et(Ind)2ZrCl2 and MAO on the above silica generated two types of zirconocenium cations (Cation 1 and Cation 2), independent of the heterogenization methods. Based on the postulated surface chemistry, Cation 1 is presumed to be in the form of an ion-pair [SiO]−[Et(Ind)2ZrCl]+, whereas Cation 2 is presumed to be a trapped multi-coordinated crown complex of MAO. In the absence of MAO, only Cation 1 is formed. The present study provides some support for the postulated surface chemistry regarding heterogenization of Et(Ind)2ZrCl2 and MAO on silica. Copyright

The effect of coke deposition on the activity and selectivity of the HZSM-5 zeolite during ethylbenzene alkylation reaction in the presence of ethanol

The alkylation of ethylbenzene (EB) with ethanol over HZSM-5 zeolite catalysts was carried out using a riser simulator reactor at different reaction temperatures and contact times. It was observed that the amount of coke deposited over the zeolite has a great influence on the reaction pathway. At higher temperature (400 °C), the spent catalyst was found to exhibit a much higher activity towards the alkylation products (DEB) as compared to the fresh catalyst. This difference between spent and fresh samples became less significant at lower temperature (250 °C). The highest yield of DEB products over the spent catalyst was obtained for intermediate temperatures (300–350 °C). The coke deposition was further analyzed using terahertz time-domain spectroscopy (THz-TDS), 27Al MAS NMR spectroscopy, TPO measurements and pyridine-FTIR. THz-TDS and TPO results revealed that the structure of coke formed on all catalysts is essentially the same, while pyridine-FTIR studies revealed that coke leads to a reduction in the acidity of the catalyst. 27Al MAS NMR results of spent samples suggested a relationship between alkylation activity and extra-framework aluminium species, which is possibly associated with the formation of very active Lewis sites. This work shows that an understanding and control of different types of catalytic sites over zeolite surfaces may improve and optimize reaction performances during alkylation of aromatics.

Phytochemical composition and pharmacological prospectus of Ficus bengalensis Linn. (Moraceae)- A review

Ficus bengalensis Linn (Moraceae) is a plant that is widely distributed in India. To the Hindus it is sacred and worshipped with special prayers on Vata Sawitri day. In traditional medicines it is used for healing obstruction of urine flow, diarrhea, dysentery, conjunctivitis, scabies and diabetes. So it has been a subject of chemical, biological and pharmacological interest since a long time. Its chemical investigation shows that it contains Bengalenosides that is, glycosides or flavonoids, ketones, flavonols, pentacyclic triterpenes and triterpenoids, coumarin, sterols, tiglic acid esters, alpha-Dglucose and meso-inositol. The aqueous or alcoholic extracts of various parts of this plant were found to have various pharmacological activities for example, antidiabetic, hypocholesterolemic, hypolipidemic, anti-inflammatory, anthelmintic, antibacterial, antiallergic and anti-tumor activity. In this review, we have investigated the pharmacological activities of F. bengalensis Linn (Moraceae) and discussed its various chemical constituents that may be responsible for these multi-aspect activity spectrums of this plant.

Thermal Decomposition of Trialkyl/Arylphosphine Gold(I) Cyanide Complexes

Combined TG/DTA techniques have been used to study the thermal decomposition of R3PAuCN (where Ris ethyl, cyclohexyl, o-tolyl, m-tolyl, p-tolyl, allyl, cyanoethyl,1-naphthyl and phenyl) complexes. It was observed that all of these complexes underwant decomposition cum redox reactions in the range of 200–600oC with evolution of both transligands, which are phosphine and cyanide, leaving metallic gold as a residue. The thermal decomposition of o-Tol3PAuCN has revealed that this is a stepwise process. In the first step decomposition takes place with evolution of phosphine and generation of AuCN, which in second step undergoes a redox reaction to produce metallic gold. The DTA curves have also confirmed these results.

13 C and 15 N NMR Studies of the Interaction of Gold(I) Thiolates with Thiourea ( 13 C and 15 N Labelled)

The interaction of gold(I) thiomalate [(Autm) n ] and gold(I) thioglucose [(Autg) n ] with thiourea (Tu) has been studied using 13 C and 15 N NMR spectroscopy. It is observed that thiourea binds to (Autm) n to form a ternary complex, Tu-Au-tm. An upfield shift of 2.6 ppm in the >C=S resonance of Tu in 13 C NMR and a downfield shift of 1.0 ppm in 15 N NMR are indicative of gold(I) binding with Tu through sulfur only. In the case of (Autg) n , the >C=S resonance of Tu remains unchanged throughout the titration, suggesting that Tu could not break the (Autg) n polymer completely. Imidazolidine-2-thione (Imt) also shows a weak interaction towards (Autg) n , similar to that of Tu.

Antifungal and antispasmodic activities of the extracts of Euphorbia granulata

The dichloromethane and methanolic extracts of the plant Euphorbia granulata were investigated for their antifungal, antibacterial, phytotoxic, brine-shrimp cytotoxic, antioxidant, spasmolytic (antispasmodic) and acetylcholinestrase inhibitory activities. The dichloromethane extract showed strong inhibition against Microsporum canis (90%) and against Aspergillus flavus (50%). Both the extracts inhibited the spontaneous contractions in rabbit jejunum preparations with EC50 value of 0.17 and 1.3 mg/mL, respectively and also relaxed the K + -induced contractions with EC50 0.2 and 2.8 mg/mL, respectively, suggesting a calcium channel blocking activity. However, the extracts did not show antibacterial, phytotoxic, brine-shrimp cytotoxic, antioxidant and acetylcholinestrase inhibitory activities.

Nonisothermal, uncontrolled homo- and copolymerization of ethylene using selected zirconocenes

Nonisothermal, uncontrolled polymerization, conducted in varying mixing regimes, offered a facile methodology to evaluate the influence of several important process development factors such as mixing, reaction exotherm, and thermal perturbations on the catalytic activity and kinetic stability, polymerization performance, and properties of the resulting polymers. Ethylene was homo- and copolymerized with hexene-1 under varying impeller speeds (hence, thermal perturbations), using Ind2ZrCl2 and Et(Ind)2ZrCl2 and the MAO cocatalyst. With respect to the effects of the above process development factors, the following was observed: The reaction exotherm profiles, tracing the polymerization history, qualitatively represented the kinetic profile and the catalytic stability. The unbridged Ind2ZrCl2 was shown to be more stable than the bridged Et(Ind)2ZrCl2. With change in the level of stirring from a diffusion-controlled regime to a nondiffusion-controlled, external gas–liquid mass-transfer resistance-free one, the reaction exotherm and the run time-average catalytic activity increased. So far as the influence of the chiral versus the achiral zirconocene structure is concerned, the copolymer composition distribution and soluble fraction generated by chiral Et(Ind)2ZrCl2 were more sensitive to the mixing conditions and thermal perturbations than were those produced by achiral Ind2ZrCl2. Et(Ind)2ZrCl2 produced higher molecular weight backbones, incorporated more hexene-1 and chain branching, and introduced less crystallinity in the copolymers than did Ind2ZrCl2. The influence of Ind2ZrCl2 on higher-weight homopolymer backbones was opposite to that of Et(Ind)2ZrCl2. Incorporation of hexene-1 significantly decreased the average molecular weights and density and increased the run-time-dependent average catalyst activity. A positive comonomer effect took place. The bulk polymer properties did not critically depend on the mixing state. Thermal perturbations broadened the polydispersity index.

Crystal structure and theoretical investigation of bis(cis-1,2-diaminocyclohexane)zinc(II) tetrachloridozincate(II)

Abstract A zinc(II) complex of cis-1,2-diaminocyclohexane (Dach), [Zn(Dach)2][ZnCl4] (1), was prepared and its structure was determined by X-ray crystallography. Theoretical (density functional theory) studies were performed for the two model compounds, [Zn(Dach)2][ZnCl4] (1) and {[Zn(Dach)2][ZnCl4]}3 (13). The structure of complex 1 is composed of [Zn(Dach)2]2+ cations and [ZnCl4]2− anions. The Zn1 atom in the cationic complex adopts a severely distorted tetrahedral geometry, while in the anionic part, Zn2 displays only a slight distortion from tetrahedral coordination. The adjacent cations and anions are associated with each other through hydrogen bonding interactions to form a two-dimensional network in the solid state.