Study of Palladium Complexes with Chitosan and Its Derivatives as Potential Catalysts for Terminal Olefin Oxidation

FTIR spectroscopy and XPS are used to study palladium complexes with various forms of chitosan: initial and modified by glutaric aldehyde. IR bands are identified that are related to the Pd–N and Pd–O bonds. A difference between the spectra of complexes obtained by the methods of adsorption and coprecipitation of chitosanium hydrochloride with the tetrachloropalladate ion is found. It is shown that the systems studied can be used as catalysts for terminal olefin oxidation by oxygen in the presence of H₂O₂. In the course of the reaction, the complexes undergo redox transformations without breaking the structure of the complex or metal transfer to the reaction mixture. The use of heterogenized Pd(II) complexes as catalysts in terminal olefin oxidation leads to the formation of oxidation and isomerization products, similarly to the case of homogeneous oxidation.     

Zeolite-encapsulated vanadium picolinate peroxo complexes active for catalytic hydrocarbon oxidations

Zeolite-encapsulated vanadium (IV) picolinate complexes were prepared by treatment of dehydrated VO(2+)–NaY zeolite with molten picolinic acids. Treatment of the NaY-encapsulated VO(pic)₂ complex with urea hydrogen peroxide adduct in acetonitrile allowed to generate peroxovanadium species. The structure of vanadium peroxo species was studied by UV–vis, Raman and XAFS spectroscopies which suggested the formation of monoperoxo monopicolinate complex which could be active intermediate for various oxidation reactions with the catalysts. To elucidate effect of the encapsulation on catalytic performance, the catalytic properties of the encapsulated complexes were compared with that of corresponding homogeneous catalyst H[VO(O₂)(pic)₂]·H₂O. The novel `ship-in-a-bottle' catalysts retain solution-like activities in aliphatic and aromatic hydrocarbon oxidations as well as in alcohol oxidation. In addition, the encapsulated vanadium picolinate catalysts showed a number of distinct features such as preferable oxidation of smaller substrates in competitive oxidations, increased selectivity of the oxidation of terminal CH_3⁻ group in isomeric octanes and preferable (sometimes exclusive) formation of alkyl hydroperoxides in alkane oxidations. The distinct features were explained in terms of intrazeolitic location of the active complexes that imposed transport discrimination and substrate orientation. On the basis of the experimental data, a possible mechanism was discussed. Stability of the vanadium complexes during the liquid phase oxidations and leaching from the NaY zeolite matrix were also examined.     

Role of nitrogen dioxide in the oxidation of diesel soot on promoted mixed catalysts with fluorite and perovskite structures

The oxidation of soot on catalysts with the perovskite and fluorite structures (including platinum-promoted catalysts) in the presence and in the absence of NO₂ was studied using in situ IR spectroscopy and temperature-programmed techniques (TPR, TPD, and TPO). It was found that, as a rule, the temperature of the onset of soot oxidation considerably decreased upon the addition of NO₂ to a flow of O₂/N₂, whereas the amount of oxygen consumed in soot oxidation considerably increased. To explain these facts, we hypothesized that the initiation of soot combustion in the presence of NO₂ was related to the activation of the NO₂ molecule through the formation (at a low temperature) and decomposition (at a high temperature) of nitrate structures on the catalyst. Superequilibrium amounts of NO₂ resulted from the decomposition of nitrate complexes immediately on the catalyst for soot combustion. Based on a comparison between catalyst activities and data obtained by TPR and the TPD of oxygen, a conclusion was drawn that the presence of labile oxygen in the catalyst is a necessary but insufficient condition for the efficient occurrence of a soot oxidation reaction in the presence of NO₂. The introduction of platinum as a constituent of the catalyst increased the amount of labile oxygen and, as a consequence, increased the amount of highly reactive nitrate complexes. As a result, this caused a decrease in the temperature of the onset of soot combustion.     

Electroreduction of CO2 to Formate with Low Overpotential using Cobalt Pyridine Thiolate Complexes

Electrocatalytic CO₂ reduction to value-added products provides a viable alternative to the use of carbon sources derived from fossil fuels. Carrying out these transformations at reasonable energetic costs, for example, with low overpotential, remains a challenge. Molecular catalysts allow fine control of activity and selectivity via tuning of their coordination sphere and ligand set. Herein we investigate a series of cobalt(III) pyridine-thiolate complexes as electrocatalysts for CO₂ reduction. The effect of the ligands and proton sources on activity was examined. We identified bipyridine bis(2-pyridinethiolato) cobalt(III) hexaflurophosphate as a highly selective catalyst for formate production operating at a low overpotential of 110 mV with a turnover frequency (TOF) of 10 s⁻¹. Electrokinetic analysis coupled with density functional theory (DFT) computations established the mechanistic pathway, highlighting the role of metal hydride intermediates. The catalysts deactivate via the formation of stable cobalt carbonyl complexes, but the active species could be regenerated upon oxidation and release of coordinated CO ligands.     

Electroreduction of CO2 to Formate with Low Overpotential using Cobalt Pyridine Thiolate Complexes

Electrocatalytic CO₂ reduction to value‐added products provides a viable alternative to the use of carbon sources derived from fossil fuels. Carrying out these transformations at reasonable energetic costs, for example, with low overpotential, remains a challenge. Molecular catalysts allow fine control of activity and selectivity via tuning of their coordination sphere and ligand set. Herein we investigate a series of cobalt(III) pyridine‐thiolate complexes as electrocatalysts for CO₂ reduction. The effect of the ligands and proton sources on activity was examined. We identified bipyridine bis(2‐pyridinethiolato) cobalt(III) hexaflurophosphate as a highly selective catalyst for formate production operating at a low overpotential of 110 mV with a turnover frequency (TOF) of 10 s^?1. Electrokinetic analysis coupled with density functional theory (DFT) computations established the mechanistic pathway, highlighting the role of metal hydride intermediates. The catalysts deactivate via the formation of stable cobalt carbonyl complexes, but the active species could be regenerated upon oxidation and release of coordinated CO ligands.     

Homogeneous Oxidation of Water by Iron Complexes with Macrocyclic Ligands

The activity of eleven separated iron complexes and nine in situ‐generated iron complexes towards catalytic water oxidation have been examined in aqueous solutions with Ce(NH₄)₂(NO₃)₆ as the oxidant. Two iron complexes bearing tridentate and tetradentate macrocyclic ligands were found to be novel water oxidation catalysts. The one with tetradentate ligand exhibited a promising activity with a turnover number of 65 for oxygen evolution.     

Study of reduced Mo/Al2O3 metathesis catalysts prepared via metal complexes

The Mo/Al₂O₃ catalysts for propene metathesis were prepared both via anchoring Mo complexes of various nuclearities and by conventional method of impregnation. The catalysts from metal complexes were found to be active in metathesis at ambient temperature after reduction with H₂ or CO at 400–500°C. The average oxidation state of Mo in the activated catalysts was determined with regard to oxygen consumption needed for oxidation of the reduced Mo species to Mo⁶⁺.     

Formation and nature of catalysts based on nickel(0) phosphine complexes active in lower alkene dimerization and oligomerization

The catalytic properties and formation mechanism of alkene dimerization-active complexes in systems based on Ni(PPh₃)₄ and boron trifluoride etherate are considered. The nature of the modifying action of Brønsted acids on the properties of metal complex catalysts for propylene dimerization is reported. The interaction between Ni(PPh₃)₄ and BF₃ · OEt₂ is influenced by water. Depending on the water concentration, the reaction can proceed via formally one-electron oxidation to yield cationic Ni(I) complexes or via two-electron oxidation to yield Ni(II) hydrides. The catalytically active species in alkene dimerization and oligomerization in these systems are Ni(II) hydrido complexes.     

The formation of an active component in V-Mo-Nb-O catalysts of ethane oxidation and ammoxidation

Summary Genesis of an active component in V-Mo-Nb-O catalysts for the oxidation and ammoxidation of ethane was studied. It was shown that the mixing of aqueous solutions of initial reactants leads to the formation of MoV complexes in the solution and to sedimentation of a binary Nb-Mo compound. The latter serves as the basis for the formation of the catalyst active component phase (a triple V-Mo-Nb compound with a variable composition of Mo₅O₁₄-like structure) during the subsequent treatment in air at 400°C.     

Synthesis,Characterization, and Catalysis of Water-Soluble Trimeric and Monomeric Palladium Complexes of 8-Aminoquinolines

New water soluble neutral and cationic palladium complexes were synthesized using 8-aminoquinoline (8-AQ) and 2-methyl 8-aminoquinoline (2-Me 8-AQ) ligands and their catalytic properties were evaluated. The neutral trimeric complexes having a Pd₃N₃ core were found to form when Pd(OAc)₂ was reacted with 8-AQ or 2-Me 8-AQ irrespective of the stoichiometry between the 2 reagents. Controlled addition of triflic acid to the neutral trimeric complex resulted in the formation of a trimeric cationic palladium complex as well as a monomeric cationic complex. A cationic palladium complex having two units of 2-Me-8AQ ligand was also synthesized from the cationic monomeric complex. Crystal structures of the new palladium complexes are reported in this study. The water-soluble neutral palladium complex showed catalytic activity for the oxidation of benzyl alcohols to benzaldehydes, while the cationic palladium complexes were found to be efficient catalysts for the oxidation of styrenes to methyl ketones.     

Physical and Chemical Properties and Activity of Fe–Co–Cu Oxide Catalysts in Oxidation of CO

We have studied the physical and chemical properties and activity of Fe–Co–Cu oxide catalysts in oxidation of CO. We have shown that the high activity of these catalysts is promoted by formation of the Cu₂(OH)₃NO₃ structure, which is modified by hematite clusters. The presence of OH groups is favorable for the formation of active sites for CO oxidation on the surface of the oxide catalysts.     

In situ‐generated Ru(III)‐mediated ATRP from the polymeric Ru(III) complex in the absence of activator generation agents

Past research has examined the atom transfer radical polymerization (ATRP) with high oxidation state metal complexes and without the need for any additives such as reducing agent or free radical initiator. To extend this research, half‐metallocene ruthenium(III) (Ru(III)) catalysts were used for the polymerization of methyl methacrylate (MMA) for the first time. These catalysts were generated in situ simply by mixing phosphorus‐containing ligand and pentamethylcyclopentadienyl (Cp*) Ru(III) polymer ((Cp*RuCl₂)_n). The complexes in their higher oxidation state such as Cp*RuCl₂(PPh₃) were air‐stable, highly active, and removable catalysts for the ATRPs of MMA with both precision control of molecular weight and narrow polydispersity index. The addition of ppm amount of metal catalyst contributed to the formation of very well‐defined homopolymers and copolymers. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Discovery of Novel Catalysts for Alkene Epoxidation from Metal-Binding Combinatorial Libraries

New lead structures for olefin oxidation catalysts have been obtained from a combinatorial library of 5760 metal–ligand complexes (see the microscopy picture). Iron complexes led to clean epoxide product formation using H₂O₂ as the terminal oxidant. Parallel libraries were used to determine ligand features important for high catalytic activity and to identify enantioselective catalyst structures (see the Equation).     

Bio-inspired iron catalysts for degradation of aromatic pollutants and alkane hydroxylation

Today, more and more metalloenzymes are understood at the molecular level. The accumulated knowledge is a very rich source of inspiration for chemists to prepare new catalysts with iron, manganese or copper, which could be cheaper and lead to processes more friendily with environment. We report here two examples. First, the preparation and study with Elf of iron catalysts efficient in the degradation of aromatics by H₂O₂. In particular, we completely characterised iron hydroperoxo and peroxo intermediates that formed upon reaction of H₂O₂ with these iron catalysts. The same complexes activate catechols toward ring cleavage by dioxygen. Overall, these complexes mimic degradation of aromatics by bacteria. The second example of bio-inspired catalyst is a di-iron system able to catalyse the oxidation of alkanes by dioxygen in the presence of hydroquinone at ambient temperature and pressure. This research was done in the framework of a research program sponsored by the Japanese New Energy Development Organisation. The ultimate goal is the biomimetic oxidation of natural gas into methanol for fuel cells. Such artificial systems are still in their infancy, but large potentiality is open to them, in particular if the separation of the Fe(III) to Fe(II) reduction and the Fe(II) oxidation by dioxygen steps is achieved.     

Mn(II) and VO(IV) Schiff base complexes encapsulated in nanocavities of zeolite-Y: catalytic activity toward oxidation of alkenes and reduction of aldehydes

Oxovanadium(IV) and manganese(II) complexes of two Schiff base ligands, bis(2,4-dihydroxyacetophenone)-1,2-propandiimine (H₂L₁) and bis(2,4-dihydroxyacetophenone)-ethylenediimine (H₂L₂) were synthesized and characterized. The encapsulation of these complexes in the nanocavities of zeolite-Y was achieved by a flexible ligand method. The prepared heterogeneous catalysts have been characterized by FTIR, NMR and atomic absorption spectroscopy, X-ray diffraction patterns, scanning electron microscopy and BET. The catalytic activities of the encapsulated complexes were studied in the oxidation of alkenes with H₂O₂ and the reduction of aldehydes with NaBH₄. In most cases, the manganese (II) complexes (MnL₁-Y, MnL₂-Y) showed better activity than the oxovanadium (IV) complexes (VOL₁-Y, VOL₂-Y) in both oxidation of alkenes and reduction of aldehydes. The catalytic activity of the recovered catalysts was compared with the fresh ones.     

Catalytic oxidation of olefins and alcohols with hydrogen peroxide in a two-phase system giving mono- and dicarboxylic acids

The present study considered the influence of various factors on the catalytic activity of systems based on a combination of tetrakis(oxodiperoxotungsto)phosphate(3−) with quaternary ammonium cations, for example, with methyltri-n-octylammonium [Me(n-C₈H₁₇)₃N]⁺. The catalysts were tested in oxidation of cycloolefins (cyclohexene and cyclooctene), alcohols (octan-1-ol and phenylmethanol), and unsaturated fatty acids (cis-9-octadecenoic and 12-hydroxy-9Z-octadecenoic acids) with a 30% hydrogen peroxide solution. These reactions proceed under mild conditions (atmospheric pressure, 80–90 °C) to give carboxylic acids. The catalytic systems were characterized by vibrational (IR and Raman) spectroscopy. The state of the systems formed from various precursors, viz., polyoxometallates and phase-transfer catalysts, was studied. It was demonstrated for the first time that the structure formation of peroxo complexes depends on the nature of the halide anion of the quaternary ammonium salt used. The melting points of individual catalytic complexes were determined. The optimal conditions for oxidation were found. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1794–1801, August, 2005.     

Catalytic Water Oxidation by Ruthenium(II) Quaterpyridine (qpy) Complexes: Evidence for Ruthenium(III) qpy‐N,N′′′‐dioxide as the Real Catalysts

Polypyridyl and related ligands have been widely used for the development of water oxidation catalysts. Supposedly these ligands are oxidation‐resistant and can stabilize high‐oxidation‐state intermediates. In this work a series of ruthenium(II) complexes [Ru(qpy)(L)₂]²⁺ (qpy=2,2′:6′,2′′:6′′,2′′′‐quaterpyridine; L=substituted pyridine) have been synthesized and found to catalyze Ce^IV‐driven water oxidation, with turnover numbers of up to 2100. However, these ruthenium complexes are found to function only as precatalysts; first, they have to be oxidized to the qpy‐N,N′′′‐dioxide (ONNO) complexes [Ru(ONNO)(L)₂]³⁺ which are the real catalysts for water oxidation.     

Use of immobilized transition metal complexes as recyclable catalysts for oxidation reactions with hydrogen peroxide as oxidant

A tetradentate Schiff base (teta), obtained from triethylenetetramine and salicylaldehyde, has been covalently bonded to divinylbenzene cross-linked chloromethylated polystyrene. This chelating ligand, abbreviated as PS-teta (PS = polymeric support), reacts with metal chlorides (Cu²⁺, Co²⁺, and Ni²⁺) in methanol to give polymer-bound transition metal complexes, PS-Cu(II)teta/(Cat-1), PS-Ni(II)teta/(Cat-2), and PS-Co(II)teta/(Cat-3), formation of which has been established by various physiochemical methods and spectroscopic techniques. The catalytic potential of these materials has been tested for the oxidation of various alkenes, alkanes, alcohols, and thioethers in the presence of 30% H₂O₂ as an oxidant. At the same time, these catalysts are very stable and could be reused in oxidation reactions for more than five times without noticeable loss of their catalytic activity.     

Four-coordinate trispyrazolylboratomanganese and -iron complexes with a pyrazolato co-ligand: syntheses and properties as oxidation catalysts

A series of complexes of the type [(Tp^R1,R2)M(X)] (Tp=trispyrazolylborato) with R¹/R² combinations Me/tBu, Ph/Me, iPr/iPr, Me/Me and for M=Mn or Fe coordinating [Pz^Me,tBu]^? (Pz=pyrazolato) or Cl^? as co‐ligand X has been synthesised. Although the chloride complexes were very unreactive and stable in air, the pyrazolato series was far more reactive in contact with oxidants like O₂ and tBuOOH. The [(Tp^R1,R2)M(Pz^Me,tBu)] complexes proved to be active pre‐catalysts for the oxidation of cyclohexene with tBuOOH, reaching turnover frequencies (TOFs) ranging between moderate and good in comparison to other manganese catalysts. Cyclohexene‐3‐one and cyclohexene‐3‐ol were always found to represent the main products, with cyclohexene oxide occasionally formed as a side product. The ratios of the different oxidation products varied with the reaction conditions: in the case of a peroxide/alkene ratio of 4:1, considerably more ketone than alcohol was obtained and cyclohexene oxide formation was almost negligible, whereas a ratio of 1:10 led to a significant increase of the alcohol proportion and to the formation of at least small amounts of the epoxide. Pre‐treatment of the dissolved [(Tp^R1,R2)M(Pz^Me,tBu)] pre‐catalysts with O₂ led to product distributions and TOFs that were very similar to those found in the absence of O₂, so that it may be argued that tBuOOH and O₂ both lead to the same active species. The results of EPR spectroscopy and ESI‐MS suggest that the initial product of the reaction of [(Tp^Me,Me)Mn(Pz^Me,tBu)] with O₂ contains a Mn^III(O)₂Mn^IV core. Prolonged exposure to O₂ leads to a different dinuclear complex containing three O‐bridges and resulting in different TOFs/product distributions. Analogous findings were made for other complexes and formation of these overoxidised products may explain the deviation of the catalytic performances if the reactions are carried out in an O₂ atmosphere.     

Enantioselective Allylic Oxidation of Olefins Using Chiral Quinolinyl‐oxazoline Copper Complex Catalysts

Copper complexes of chiral quinolinyl‐oxazoline have been studied as the catalysts for enantioselective allylic oxidation of cycloalkenes with tert‐butyl perbenzoate. Using 5 mol% of these chiral catalysts, optical active allylic benzoates were obtained in moderate enantiomeric excesses. CuOTf prepared in situ, CuClO₄ and CuPF₆ were found to be good precatalysts in acetone.