Influence of a diimine ligand and an activator on the processes taking place in Brookhart-type nickel catalytic systems

Research on Chemical Intermediates - Brookhart-type catalytic systems based on zero-valent nickel complexes Ni(COD)2 and Ni(COD)L [where COD is cyclooctadiene and L is...     

Formation of paramagnetic intermediates under the conditions of Brookhart-type catalyst activation and operation

The formation of two types of paramagnetic species in the catalytic system NiBr₂(DAB)/ MAO under real catalytic conditions was found. These are Ni^II are Ni^I complexes with the diimine ligand and complexes with the radical anion ligand in the coordination sphere. Under catalysis conditions, these forms are in equilibrium with each other, the equilibrium being shifted towards the radical anion form upon substrate addition.     

Efficient catalytic systems based on paramagnetic closo-ruthenacarboranes for the controlled synthesis of polymers

Novel catalytic systems were proposed for controlled radical polymerization of vinyl monomers. These systems are based on the paramagnetic (17-electron) closo-complex, namely, 3,3-(dppb)-3-Cl-closo-3,1,2-RuC₂B₉H₁₁ (1, dppb is 1,4-bis(diphenylphosphino)butane) and its mono(P-phenylene)- and di(P,P-ortho-phenylene)cycloboronated derivatives, namely, (R = H, n = 10 (2); R = Me, n = 8 (3)) and (4). With the polymerization of methyl methacrylate as an example, the effect of the steric hindrances in complexes 1?C4 on the synthesis and molecular-weight characteristics of the resulting polymers was analyzed. In the presence of aliphatic amines, the polymerization rate increases substantially and the catalyst concentration can be lowered without losing control of the process. The structure of 17-electron complex 3 in the solid state and its paramagnetic nature were confirmed by X-ray diffraction and ESR spectroscopy.     

The nature of paramagnetic species in nitrogen doped TiO2 active in visible light photocatalysis

Nitrogen doped TiO2, a novel photocatalyst active in the decomposition of organic pollutants using visible light, contains two different types of paramagnetic centres (neutral NO radicals and NO2(2-) type radical ions respectively) which are likely related to specific properties of the solid.     

Ruthenium-catalyzed Heck-type olefination and Suzuki coupling reactions: studies on the nature of catalytic species

Ruthenium-catalyzed Heck olefination and Suzuki cross coupling reactions have been developed. When starting with a ruthenium complex [RuCl(2)(p-cymene)](2) as a homogeneous catalyst precursor, induction periods were observed and ruthenium colloids of zero oxidation state were generated under catalytic conditions. Isolated ruthenium colloids carried out the olefination, implying that active catalytic species are ruthenium nanoclusters. To support this hypothesis, ruthenium nanoparticles stabilized with dodecylamine were independently prepared via a hydride reduction procedure, and their catalytic activity was subsequently examined. Olefination of iodobenzene with ethyl acrylate was efficiently catalyzed by the ruthenium nanoparticles under the same conditions, which could be also reused for the next runs. In poisoning experiments, the conversion of the olefination was completely inhibited in the presence of mercury, thus supporting our assumption on the nature of catalytic species. No residual ruthenium was detected from the filtrate at the end of the reaction. On the basis of the postulation, a heterogeneous catalyst system of ruthenium supported on alumina was consequently developed for the Heck olefination and Suzuki cross coupling reactions for the first time. It turned out that substrate scope and selectivity were significantly improved with the external ligand-free catalyst even under milder reaction conditions when compared to results with the homogeneous precatalyst. It was also observed that the immobilized ruthenium catalyst was recovered and reused up to several runs with consistent efficiency. Especially in the Suzuki couplings, the reactions could be efficiently carried out with as low as 1 mol % of the supported catalyst over a wide range of substrates and were scaled up to a few grams without any practical problems, giving coupled products with high purity by a simple workup procedure.     

The rebound mechanism in catalytic C-H oxidation by MnO(tpp)Cl from DFT studies: electronic nature of the active species

DFT studies show that the rebound mechanism for MnO(tpp)(Cl)-catalyzed C-H hydroxylation is favored for spin states with oxyl character.     

On the triplet nature of paramagnetic centers in conducting polymers

Temperature dependences of the magnetic susceptibility of solutions and powders of polyaniline synthesized by oxidative polymerization using two methods were measured by ESR in the temperature range from 123 to 423 K. The dependences observed can be described by the integral of susceptibility of the polymer fragments in the triplet state over the singlet—triplet splitting from E ₁ to E ₂ with constant weight. The susceptibility of the fragments was accepted to obey the Bleaney—Bowers equation. The most part of the experimental dependences can be presented as the sum of the temperature-independent susceptibility and the susceptibility obeying the Curie law. The both susceptibilities are described in a single manner at E ₁ < 0. In some cases, the comparison of the calculated and experimental dependences makes it possible to determine the length of the fragments L. The conditions of polymer synthesis, heating, and water vapors affect the E ₁ and E ₂ values. A similar analysis can be applied to other conducting polymers. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 316–321, February, 2008.     

On the mechanism of vanillin formation in the catalytic oxidation of lignin with oxygen

The influence of catalyst (CuSO₄ solution), pH and temperature on the vanillin formation and oxygen consumption in oxidation of lignin in alkaline media have been studied. It was shown that the main role of catalyst in the lignin oxidation is to change the selectivity of hydroperoxide fragmentation rather than to accelerate oxidation.     

Effect of the nature of the solvent on the rate and mechanism of the catalytic hydrogenation of difurfurylideneacetone

It was established that the hydrogenation of difurfurylideneacetone on a skeletal Ni-Ti-Al catalyst at atmospheric pressure proceeds nonselectively. The rate of hydrogenation in both aprotic and protic solvents increases as their overall electrophilicity increases. The process is realized selectively under hydrogen pressure. By changing the type of solvent one can obtain a catalyzate enriched in various reaction products.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 889–893, July, 1981.     

Pulsed electron paramagnetic resonance experiments identify the paramagnetic intermediates in the pyruvate ferredoxin oxidoreductase catalytic cycle

Pyruvate ferredoxin oxidoreductase (PFOR) is central to the anaerobic metabolism of many bacteria and amitochondriate eukaryotes. PFOR contains thiamine pyrophosphate (TPP) and three [4Fe-4S] clusters, which link pyruvate oxidation to reduction of ferredoxin. In the PFOR reaction, TPP reacts with pyruvate to form lactyl-TPP, which undergoes decarboxylation to form a hydroxyethyl-TPP (HE-TPP) intermediate. One electron is then transferred from HE-TPP to one of the three [4Fe-4S] clusters to form an HE-TPP radical and a [4Fe-4S]1+ intermediate. Pulsed EPR methods have been used to measure the distance between the HE-TPP radical and the [4Fe-4S]1+ cluster to which it is coupled. Computational analysis including the PFOR crystal structure and the spin distribution in the HE-TPP radical and in the reduced [4Fe-4S] cluster demonstrates that the distance between the HE-TPP radical and the medial cluster B matches the experimentally determined dipolar interaction, while one of the other two clusters is too close and the other is too far away. These results clearly demonstrate that it is the medial cluster (cluster B) that is reduced. Thus, rapid electron transfer occurs through the electron-transfer chain, which leaves an oxidized proximal cluster poised to accept an electron from the HE-TPP radical in the subsequent reaction step.     

Regularities of diene copolymerization by coordination catalytic systems and the mechanism of stereoregulation

A study has been conducted of the relative reactivity of a series of dienes (butadiene, isoprene, 2,3-dimethylbutadiene, cyclohexadiene-1,3), as well as of butadiene and styrene, in copolymerization by various coordination catalyst systems based on transition metals: nickel, cobalt, titanium, chromium, molybdenum, and tungsten. The microstructure of homopolymers and copolymers of dienes has been investigated. Regularities have been established in the mutual influence of the comonomers on the microstructure of a polymer chain. Experimental data on the influence of the concentration of diene monomer, and electron-donating and electron-accepting compounds on the microstructure of polydiene chain have been used in discussing the alternative mechanisms of stereoregulation in diene polymerization by active centers of the π-allyl type.     

The nature of active species in catalytic systems based on non-heme iron complexes, hydrogen peroxide, and acetic acid for selective olefin epoxidation

The catalytic systems [(BPMEN)Fe^II(CH₃CN)₂](ClO₄)₂/H₂O₂/CH₃OOH and [(TPA)Fe^II(CH₃CN)₂](ClO₄)₂/H₂O₂/CH₃OOH, where BPMEN = N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane and TPA = tris(2-pyridylmethyl)amine, provide selective olefin epoxidation. Proton NMR studies showed that the mononuclear iron(IV) oxo complexes [(L)Fe^IV=O]²⁺, with L = BPMEN or TPA, are present in the cited catalytic systems. These intermediates are the decomposition products of the acylperoxo complexes [(L)Fe^III-O₃CCH₃]²⁺. Such a complex was observed by the ²H NMR technique at low temperatures. The [(L)Fe^IV=O]²⁺ and [(L)Fe^V=O]³⁺ oxo complexes are possible active species in the studied catalytic systems.     

超微量铂丶铑连续催化极谱测定及其催化波机理的探讨

For simultaneous determination of ultramicro amounts of platinum and rhodium the optimum condition has been described as 1.5N H2SO4-1.2% NH4Cl-0.0012M (CH2)6N4-0.003% N2H4XH2SO4. Both platinum and rhodium produce hydrogen catalytic waves with peak potential at-1.03 V and - 1.27 V (vs. S. C. E.) and the peak height of differential wave in single-sweep polarograph is directly proportional to the concentration of the metals in the range from 0.05 ppb to 1.0 ppb for platinum and from 0.0025 ppb to 0.1ppb for rhodium, respectively. The influence of other platinum group metals and some base metals on the height of catalytic waves has been examined. It has been shown that the method is very selective. It is applied even at 200:1 or at 1:10 (Pt:Rh). The mechanism of the catalytic waves has been discussed. The catalytic waves of both platinum and rhodium are due to "surface catalytic wave of hydrogen". The wave of rhodium can be attributed to catalytic discharge of hydrogen ion by the complex (Rh(CH2O)2Cl4)-. One of the ligands, formaldehyde, is the product of hydrolysis of hexamethylenetetramine. The wave of plainum can be attributed to catalytic discharge of hydrogen ion by the complex (PtACl5)-, where a denotes intermediate product (a substance containing CH2=N group) formed during the hydrolysis of hexamethylenetetramine. The role of hydrazine sulfate in catalytic system has been shown. Hydzazine can react with formaldehyde to from (CH2=N)2 which promotes the growth of platinum catalytic wave and in this way the concentration of formaldehyde in the system can be controlled.     

Intersystem crossing versus electron transfer in porphyrin-based donor-bridge-acceptor systems: influence of a paramagnetic species

We have investigated how the spin state of an acceptor influences the photophysical processes in a donor-bridge-acceptor (D-B-A) system. The system of choice has zinc porphyrin as the electron donor and high- or low-spin iron(III) porphyrin as the acceptor. The spin state of the acceptor porphyrin is switched simply by coordinating imidazole ligands to the metal center. The D-A center-center distance is 26 A, and the bridging chromophore varies from pi-conjugated to a sigma-bonded system. The presence of a high-spin iron(III) porphyrin in such systems has previously been shown to significantly enhance intersystem crossing in the remote zinc porphyrin donor, whereas no significant electron transfer to the iron porphyrin acceptor was observed, even though the thermodynamics would allow for photoinduced electron transfer. Here, we demonstrate that by switching the acceptor to a low-spin state, the dominating photophysical process is drastically changed; the low-spin system shows long-range electron transfer on the picosecond time-scale, and intersystem crossing occurs at its "normal" rate.     

The influence of the nature of surfactants and dispersion medium on the catalytic effect of reverse micellar systems

The catalytic effect of reverse micellar systems in alkaline hydrolysis of phosphonic acid esters is to a considerable extent determined by the structure of the substrate and surfactant and the nature of the oil phase but weakly depends on the surface potential of the aggregates. AOT-butanol-decane-water, sodium dodecyl sulfate-butanol-decane-water, and cetyltrimethylammonium bromide-butanol-decane-water microemulsions exhibit specific catalytic properties rather than catalytic properties intermediate between those of the surfactant-alkane-water and surfactant-alkanol-water three-component systems.     

Study of ionic catalytic systems by the electrodialysis method

A special method of electrodialysis has been developed for investigation of the kinetics and mechanism of polymerization reactions involving ions. The ionic composition of homogeneous systems such as Cp₂TiRClAlRCl₂, RTiCl₃ + AlRCl₂, Ti(OR)₄ + AlR₃, and (RLi)_n has been studied. The composition of catalytically active ions has been determined. Using the regularities of “open” systems, the methods of determining the carbocation reactivity under electrodialysis conditions havebeen worked out. The impulse electrodialysis method has been suggested for studying ionic polymerization kinetics and evaluating the rate constants ofion reactions with the monomer and gegenion.     

Action of salt formation on the bifunctional nature of 12-molybdophosphoric acid and their relationship to the catalytic activity

The acidic properties and the catalytic activity of 12-molybdophosphates (M _x/n ⁿ⁺ H_3-x PMo₁₂O₄₀;M = Cu²⁺, Bi³⁺, Cr³⁺ andX=1–3) have been studied. The results are discussed as the effect of these catalyst components on the partial charge on oxygen atom which is in a relation with the acidity. It is shown that the oxygen-hydrogen bond is weakened as the value of partial charge on oxygen (– ₀) becomes more negative, while the vapor-phase dehydration activity of 2-propanol was explained on the basis of the reacting zone wideness taking into account the pseudo liquid phase nature of the heteropoly compounds. A correlation of the percentage conversion of 2-propanol with the fractional charge on the molybdenum atom was successful to interpret the effect of the redox properties of these catalysts on their catalytic activity.

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Auswirkung der Salzbildung auf die bifunktionelle Natur der 12-Molybdänphosphorsäure und ihre Beziehung zur katalytischen AKtivität

Zusammenfassung Es wurden die sauren Eigenschaften und die katalytische Aktivität von 12-Molybdänphosphaten (M _x/n ⁿ⁺ H_3-x PMo₁₂O₄₀;M = Cu²⁺, Bi³⁺, Cr³⁺ andX=1–3) untersucht. Die Ergebnisse werden bezüglich der Partialladung am Sauerstoffatom diskutiert, wobei gezeigt wird, daß die Sauerstoff—Wasserstoff-Bindung mit negativerem Sauerstoff schwächer wird, während die katalytische Dampfphasen-Dehydrierungsaktivität an 2-Propanol mit der Reaktionszone der pseudoflüssigen Struktur der Heteropolyverbindungen in Zusammenhang gebracht werden kann. Die Redox-Eigenschaften der Katalysatoren beim prozentuellen Umsatz der Reaktion von 2-Propanol sind direkt mit der fraktionellen Ladung am Molybdänatom zu korrelieren.