Comparison of immobilized Box and azaBox–Cu(II) complexes as catalysts for enantioselective Mukaiyama aldol reactions

Enantioselective Mukaiyama aldol reactions of different α-ketoesters was tested with copper complexes of chiral Box and azaBox ligands in both homogeneous and heterogeneous systems. Results in the homogeneous reactions were greatly influenced by the nature of the ketoester, the chiral ligand, and the reaction solvent. In the case of supported catalysts, the use of strongly coordinating azaBox ligands prevented the leaching of metal but reduced the Lewis acidity, and thus the catalytic activity, and did not solve the problem of poisoning by strong coordination of products, byproducts, or solvents. The counter-ion effect also was very significant, and electrostatic immobilization was efficient only with Box ligands (up to 86% ee at room temperature), whereas covalent immobilization allowed the use of azaBox ligands (up to 85% ee at room temperature) in the heterogeneous phase. Recovered deactivated solids could be reused in a reaction with a completely different mechanism that does not require acid centers, such as cyclopropanation.     

Chiral manganese(III) Schiff base complexes anchored onto activated carbon as enantioselective heterogeneous catalysts for alkene epoxidation

Two chiral manganese(III) salen catalysts, bearing different chiral diamine bridges, were anchored by direct axial coordination of the metal centre onto the phenolate groups of a modified commercial activated carbon. The modification of the activated carbon was achieved by reaction between sodium hydroxide and surface phenol groups giving rise to phenolate groups (CoxONa), which were characterised by XPS, TG and TG-IR. Characterisation of immobilised manganese(III) salen catalysts onto CoxONa material by XPS, ICP-AES and TG-IR clearly point to reaction between carbon surface phenolate groups and the manganese(III) complexes through axial coordination of the metal centre to these groups.These materials were active and enantioselective in the epoxidation of styrene and α-methylstyrene in dichloromethane at 0 °C using, respectively, m-CPBA/NMO and NaOCl. Only for α-methylstyrene comparable asymmetric inductions were found in the epoxide as the homogeneous phase reactions and catalyst reuse led to no significant loss of catalytic activity and enantioselectivity.     

Syntheses, structures and catalytic activities of ruthenium (II) carbonyl iodide complexes with CNC-pincer bis (carbene) ligand

Two new pincer bis (carbene) ligands and corresponding ruthenium (II) carbonyl iodide complexes have been synthesized and characterized. The structures of complexes 3 and 4 have been determined by X-ray crystallography. Complexes 3 and 4 have been proved to be efficient catalysts in the transfer hydrogenation of acetophenone.     

Homopolymerization of n‐butyl methacrylate using bis(salicylaldiminate)copper(II)/ methylaluminoxane catalysts

Polymerization catalysts based on copper precursors appear particularly interesting due to the low metal cost, limited toxicity and modest sensitivity to deactivation by polar species. To date, α‐olefin and polar monomer coordination polymerization catalysed using copper catalysts has been scarcely investigated, and a good part of the literature is represented by patents. Here this research has been expanded to the study of the performances of bis(salicylaldiminate)copper(II)/methylaluminoxane (MAO) catalysts in the polymerization of n‐butyl methacrylate. The study of the catalytic activity of bis(salicylaldiminate)copper(II)/MAO systems in n‐butyl methacrylate polymerization was focused on the relationship between the catalytic behaviour and the main reaction conditions and ligand structures. The electronic and steric characteristics of the chelate ligands play an important role in the catalytic performances. The presence of electron‐withdrawing nitro groups on the chelate ligands increased the catalytic activity which reached 36 kg_polymer mol^?1 h^?1, the highest value up to now reported for copper systems in methacrylic or acrylic monomer polymerization. These performances were ascribed to copper catalysts activated by MAO: without copper precursor, working in the presence of MAO and free salicylaldimine ligand, complete inactivity was ascertained. Copyright © 2010 Society of Chemical Industry     

Spectroscopic studies on bis(1-amidino-O-alkylurea) copper(II) sulfate complexes where alkyl = methyl,ethyl, n-propyl or n-butyl: EPR evidence for binuclear complexes

Four new Cu(II) Complexes viz. [Cu(II)(1-amidino-O-methylurea)₂]SO₄·2H₂O (1), [Cu(II)(1-amidino-O-ethylurea)₂](SO₄)₂·2H₂O (2), [Cu(II)(1-amidino-O-n-propylurea)₂] SO₄·2H₂O (3), [Cu(II)(1-amidino-O-n-butylurea)₂]₂(SO₄)₂·2H₂O (4) have been synthesized and characterized. EPR spectrum of complex 4 at RT consisted of fine-structure transitions (ΔMs = ±1) with zero-field splitting (ZFS) of 0.0485 cm^?1 and a half-field signal (ΔMs = ±2) at ca. 1600 G, revealed formation of binuclear complex (S = 1). Whereas EPR spectrum of complex 2 in DMSO showed a mixture of mononuclear and binuclear complex in the ratio 0.75:1.0 with ZFS of 0.0385 cm^?1. From the temperature dependence of the EPR signal intensity, the isotropic exchange-interaction constant J was evaluated.     

Novel functionalized bis(imino)pyridine cobalt(II) catalysts for ethylene polymerization

This report describes synthesis and ethylene polymerization in the various conditions by two novel 2,6-bis(imino)pyridine (BIMP) catalysts B and C based on cobalt activated by methylaluminoxane (MAO) in a slurry semi-batch reactor. The catalyst activities as well as polymer properties were affected dramatically by electronic effects of the attached substitutions on the para-position of the pyridine ring. Theoretical study exhibited more positive charge on the central metal of the catalyst B resulted in higher activity at the expense of lower thermal stability and lifetime. The polymer obtained using the catalysts exhibited high molecular weight and almost narrow molecular weight distribution (MWD) ranging from 2.35 to 4.10 at the employed polymerization conditions. The highest and lowest molecular weight of the obtained polymers were produced by the catalyst A and C respectively. Hydrogen could slightly increase the catalyst activities with the exception of the catalyst B. The catalyst C bearing electron-donor OMe substitution at the para-position of the pyridine ring, produced PE with narrower PDI relative to the polymer resulted by catalysts A and B.     

Application of new unsymmetrical chiral Mn(III), Co(II,III) and Ti(IV) salen complexes in enantioselective catalytic reactions

New unsymmetrical chiral salen complexes were synthesized and the efficiency of Mn(III), Ti(IV), Co(II) and Co(III) type catalysts were examined in the enantioselective epoxidation of styrene and α‐methylstyrene, the trimethylsilylcyanation of benzaldehyde, the borohydride reduction of aromatic ketones and asymmetric hydrolysis of epoxides to diols, respectively. A very high level of enantioselectivity was attainable over the unsymmetrical chiral salen complexes prepared mainly from salicylaldehyde and 2‐formyl‐4,6‐di‐tert‐butylphenol derivatives. Enantiomeric excess of the corresponding reaction product obtained using unsymmetrical chiral salen catalysts was generally higher than that over conventional symmetric chiral salen catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

A bithiazole-containing calix[4]arene podand as versatile ligand for copper(I) and copper(II)

A new calix[4]arene-based bithiazole podand designed to complex copper(I) or copper(II), and the corresponding complexes, have been synthesised and characterised. Structural information was notably obtained for the copper(I) complex by a ¹H¹⁵N HMBC NMR experiment.     

Synthesis, structure and catalytic activities for the hydrogen transfer reactions of the ruthenium(II) carbonyl chloride complexes with pyridine-2,6-diimine

Two Ru(II) carbonyl chloride complexes with pyridine-2,6-diimine, (L)Ru(CO)Cl₂ (L = 2,6-diacetylpyridinebis(2,4,6-trimethylanil), 1; L = 2,6-diacetylpyridinebis(2,6-diisopropylanil), 2) have been synthesized and characterized. The structure of compound 1 has been determined by X-ray crystallography, which show the distorted octahedral geometry around Ru(II). Complexes 1 and 2 have been proved to be active in the transfer hydrogenation of acetophenone by propan-2-ol.     

Synthesis and investigation of anchored copper (II) complexes within MCM-41 as selective catalysts for epoxidation of olefins

In the present study, preparation, characterization and catalytic activity of encapsulated copper (II) complexes within MCM-41 were investigated. The mesoporous molecular sieve MCM-41 was synthesized and grafted with 3-(trimethoxysilyl)-1-propanethiol to give a thiol modified material (MCM-41-S). The prepared material successively was reacted with acrylic acid and acrylonitrile to give AA-MCM-41 and AN-MCM-41, respectively. Using hexamethyldisilazane and copper (II) chloride, copper (II) complexes in the cavities of MCM-41 (Cu-AA-MCM-41 and Cu-AN-MCM-41) were prepared. The presence of copper (II) complex on the surfaces of host matrix and the structure of prepared catalyst is investigated by FT-IR, inductively coupled plasma-optical emission spectroscopy (ICP-OES), powder X-ray diffraction (XRD) and BET nitrogen adsorption–desorption methods. The catalytic activities of the catalysts were studied in epoxidation of olefins. The results showed that activities of the prepared catalysts (Cu-AA-MCM-41 and Cu-AN-MCM-41) and their selectivity to corresponding epoxides were more than those of MCM-41. In addition, the synthesized heterogeneous catalysts were truly reusable.     

Synthesis,crystal structures and spectral characterization of trans-bisaquabis(o-vanillinato)copper(II), cis-aquabis(o-vanillinato)copper(II) and aqua[bis(o-vanillinato)-1,2-ethylenediimin]copper(II)

trans-Bisaquabis(o-vanillinato)copper(II) (I), cis-aquabis(o-vanillinato)copper(II) (II) and aqua[bis(o-vanillinato)-1,2-ethylenediimin]copper(II) (III) complexes were synthesized and characterized by means of elemental analysis, IR and UV–vis spectroscopy, thermal analysis and X-ray diffraction techniques. The coordination geometry around Cu(II) is a octahedral with coordination number of six for I and is a square-pyramidal with coordination number of five for both II and III. In all three compounds, a three-dimensional structure is formed via C–H⋯O hydrogen-bond interactions and intermolecular π–π and π-ring interactions. The compounds I and III have two-dimensional hydrogen-bonded step-chain structure in xz-plane, while compound II has a zigzag chain structure in xy-plane. On the basis of the first DTA_max of the anhydrous complexes of I and II, the thermal stability sequence is trans-isomer > cis-isomer. This fact should be related with the binding of the ligand.     

Synthesis,spectral and structural study of sulfur-containing copper(II) complexes

A series of four new copper(II) complexes [Cu(H₂L)(L¹)] 1, [Cu(H₂L)(PMDT)] 2, [Cu(H₂L)(Dien)] 3 and [Cu(H₂L)(L²)] 4 have been synthesized by template condensation (H₂L=thiodiglycolic acid, L¹=N-[(1)-1-(4-methylphenyl)ethylidene]benzohydrazide, PMDT=N,N,N′,N′,N ^′′-pentamethyldiethylene- triamine, Dien=diethylenetriamine L²=N-[(1)-pyridin-2-ylmethylidene]benzohydrazide). The bonding and stereochemistry of the complexes have been characterized by molar conductance, elemental analysis, magnetic susceptibility, infrared, UV–visible, electron paramagnetic resonance structural studies and electrochemical studies. g-Values were calculated for all complexes in polycrystalline form as well as in DMSO solution. The magnetic and spectral data indicate square pyramidal geometry for 1 and octahedral geometry for 2–4 complexes. Cyclic voltammograms for all the complexes are similar and involve two irreversible redox processes. Their biological properties have also been studied. The thio complexes show more antibacterial activity than the controlled one. The antibacterial activities of the compounds have also been tested against Escherichia coli with different concentrations.     

Synthesis and characterization of alumina-supported Mn(II), Co(II), Ni(II) and Cu(II) complexes of bis(salicylaldiminato)hydrazone as catalysts for oxidation of cyclohexene with tert-buthylhydroperoxide

Complexes of type [M(SAH)(OH₂)], where M is Mn(II),Co(II),Ni(II) and Cu(II), and SAH is the Schiff-base formed by condensation of salicylaldehyde (2 equiv.) and hydrazine (1 equiv.), bis(salicylaldiminato)hydrazone, or “2-({(z)-2-[(E)-1-(2-hydroxyphenyl)methylidene]hydrazono}methyl)phenol” have been prepared and characterized by elemental analysis, IR, UV–vis spectroscopy, conductometric, small area X-ray photoelectron spectroscopy and magnetic measurements. Elemental analysis suggests the stoichiometry to be 1:1 (metal:ligand). The results indicate that the Schiff-base ligand coordinates through one azomethine nitrogen and two phenolic oxygen to the metal ions. Conductance measurements suggest the non-electrolytic nature of the complexes. The atomic concentration of the complexes showed the ratio of M:N:O = 1:2:3, that indicates that a water molecule was in the complex. Alumina-supported complexes “[M(SAH)OH₂]-Al₂O₃” catalyze the oxidation of cyclohexene with tert-butylhydroperoxide (TBHP). The major products of the reaction were 2-cyclohexene-1-ol, 2-cyclohexene-1-one and 2-cyclohexene-1-(tert-butylperoxy). The influence of solvent on the oxidation reaction has been studied. [M(SAH)OH₂]-Al₂O₃ shows significantly higher catalytic activity than other alumina-supported complexes. These catalysts can also be reused in the oxidation of cyclohexene several times. The new materials “[M(SAH)OH₂]-Al₂O₃” were characterized by several techniques: chemical analysis and spectroscopic methods (FT-IR, UV–vis, XRD, DRS).     

Synthesis,crystal structure and electrocatalytic activity of discrete and polymeric copper(II) complexes with bitopic bis(pyrazol-1-yl)methane ligands

New coordination copper(II) compounds containing bitopic bis(pyrazol-1-yl)methane ligands, namely 1,1,2,2-tetrakis(pyrazol-1-yl)ethane, 1,4-bis[bis(pyrazol-1-yl)methyl]benzene and 1,4-bis[bis(3,5-dimethylpyrazol-1-yl)methyl]benzene were prepared. Reactions of ligands with Cu^2 + ions in 1:2 ratio gave discrete homodinuclear complexes, while 1:1 ligand-to-metal ratio lead to the formation of coordination polymers. Electrocatalytic activity of compounds in oxygen electroreduction reaction on the surface of modified carbon paste electrode was evaluated.     

Homogeneous catalysts for ethylene polymerization based on bis(imino)pyridine complexes of iron, cobalt, vanadium and chromium

Results of a comparative study of ethylene polymerization activity and the structure of polyethylene (PE) produced over homogeneous catalysts based on bis(imino)pyridine complexes with close ligand frameworks and different transition metal centers (Fe(II), Co(II), Cr(III) and V(III)) are reported. The effects of the activator nature and polymerization conditions on the activity of these complexes and the resulting PE structure (molecular weight, molecular weight distribution, content of methyl and vinyl groups) have been studied. The experimental data obtained under comparable conditions demonstrate a pronounced effect of transition metal center on the catalytic properties of bis(imino)pyridine complexes (polymerization activity, copolymerization reactivity, thermal stability, PE structure, composition of optimal activator, formation of single-site or multiple-site catalytic system).     

Dimerization reactions with iron and cobalt bis(imino)pyridine catalysts: A substrate based approach

Herein, we present a substrate based approach to gain insight into effects on reactivity as well as selectivity of dimerization reactions using bis(imino)pyridine catalysts. Common iron and cobalt systems show several limitations applying sterically demanding alkenes. Systematic substrate alterations reveal that structural properties are able to influence reactivity and dimer selectivity whereas electronic properties have an impact on insertion selectivity. Finally, successful conversion of the simplest vinylidene compound isobutene is achieved by catalyst variation.     

Crystal structures of palladium(II) and copper(II) complexes of meso-phenyl tripyrrinone

Meso-phenyl tripyrrinone (2), which is derived directly from N-confused porphyrin by Cu(II)-assisted oxygenolysis, can serve as a metal coordination ligand using three nitrogens and one oxygen in the core. The crystal structures of Pd(II) and Cu(II) complexes show the similar square-planar N₃O-coordination but the mode of oligomeric chains formed in the crystals by π–π stacking interaction is different: zig–zag for Cu(II) and straight for Pd(II) complex.     

Synthesis and crystal structures of copper(I) iodide complexes chelating with bis(ethylamidophosphine)

Two copper(I) iodide complexes with polydentate bis(ethylamidophosphine) ligands were synthesized, characterized with crystal structures. They include a dimeric complex [Cu(μ-I)(CH₂NHCOC₂H₄PPh₂)₂]₂ 1 containing a planar Cu₂I₂ rhombohedron with two doubly bridged ligands and a tetrameric complex {Cu₄(μ-I)₄[(CH₂NHCOC₂H₄PPh₂)₂]₂} 3 with all the coppers and iodines forming a highly distorted cubane geometry.     

Metal displacement (cementation) reactions: The mercury(II)/copper system

The kinetics of the metal displacement reaction between mercury(II) ions and a rotating copper disc are reported. Since the smooth film of mercury deposited does not interfere with the rate of the reaction, the system behaves more ideally than any reported so far. With a constant volume of reactant solution, the reaction rate is found to be accurately first order in mercury(II) ions, the first order rate constant being proportional to the square root of the rotation speed and having an activation energy of 16 ± 2 kJ mol^?1. It is concluded that the reaction is diffusion controlled, the calculated diffusion coefficient being 6.4 ± 0.6 × 10^?10 m²s^?1. The results of polarization measurements for the reduction of mercury(II) ions to mercury metal on a rotating platinum disc and the oxidation of copper metal to copper(II) on a partially cemented copper disc are also presented. These results confirm that the displacement reaction is diffusion controlled, and verify the magnitude of the diffusion coefficient.