Cobalt Salen Complexes Research Articles

Asymmetric Hydrolytic and Aminolytic Kinetic Resolution of Racemic Epoxides using Recyclable Macrocyclic Chiral Cobalt(III) Salen Complexes

AbstractNew chiral macrocyclic cobalt(III) salen complexes were synthesized and used as catalyst for the asymmetric kinetic resolution (AKR) of terminal epoxides and glycidyl ethers with aromatic/aliphatic amines and water as nucleophiles. This is the first occasion where a Co(III) salen complex demonstrated its ability to catalyze AKR as well as hydrolytic kinetic resolution (HKR) reactions. Excellent enantiomeric excesses of the epoxides, the corresponding amino alcohols and diols (upto 99%) with quantitative yields were achieved by using the chiral Co(III) salen complexes in dichloromethane at room temperature. This protocol was further extended for the synthesis of two important drug molecules, i.e., (S)‐propranolol and (R)‐naftopidil. The catalytic system was also explored for the synthesis of chirally pure diols and chiral cyclic carbonates using carbon dioxide as a greener renewable C1 source. The catalyst was recycled for upto 5 catalytic cycles with retention of enantioselectivity.magnified image

Cooperative Activation of Cobalt-Salen Complexes for Epoxide Hydration Promoted on Flexible Porous Organic Frameworks.

Developing solid catalysts with multiple active sites working cooperatively is desirable for efficient chemical transformations. However, most solid catalysts are rigid and impede the cooperation between their spatially isolated active sites. Two flexible porous organic frameworks (POFs) with integrated Co(salen) as active sites have been successfully synthesized for mimicking the cooperative modes of enzymes. The POFs exhibit second-order rate dependence on Co(salen) concentration in the network and afford much higher TOF (3300 versus 2670 h-1 ) than the homogeneous counterpart in the hydration of propylene epoxide. POFs with a flexible network thus not only facilitate but also enhance the cooperation of nearby Co(salen). Moreover, POFs could catalyze oversized substrates, have a wide substrate scope, and exhibit high stability.

Superior Effect of Ultrasonic Homogenization to Mechanical Agitation on Accelerating Reaction Rates in Asymmetric Ring Opening of Epoxides

Double‐layered mesoporous silica was prepared for use as an efficient support for the encapsulation of chiral salen Co‐BF3 complexes in the pore channels. The heterogenized chiral salen complex was applied successfully to the asymmetric ring opening (ARO) of terminal epoxides using water, alcohols, and carboxylic acids as nucleophiles. Although the homogeneous salen catalyst exhibited higher catalytic activity than the heterogenized catalyst in ARO reactions as expected, the heterogeneous salen catalysts exhibited high catalytic activities that were sufficient for practical applications. This study evaluated the unique contribution of ultrasonic irradiation as a powerful mixing medium to improve the kinetics in comparison to conventional mechanical agitation during the ARO reactions. Both the homogeneous and heterogeneous chiral salen complexes catalyzed enantioselectively the racemic or chiral epoxides under ultrasonication. The adoption of ultrasonic irradiation in the catalysis resulted in a superior mixing effect to vigorous mechanical stirring. The reaction time was shortened to obtain the highest enantiomeric excess (%ee) when ultrasonic irradiation was applied to the reaction system. Through the ARO reaction of epoxide by phenol derivatives, the highly enantio‐enriched β‐blockers could be synthesized easily from racemic terminal epoxides or chiral ones using chiral cobalt–salen catalysts. The regioselective ARO reaction of (R)‐ECH with propionic or butyric acid proceeded smoothly at room temperature under ultrasonic exposure in the presence of the heterogenized catalyst. The corresponding glycidyl alkylates were produced with excellent optical purity of up to 98%ee. The chiral cobalt–salen complexes encapsulated in MCM‐41 retained its catalytic activity and enantioselectivity for three recycles. Consequently, the heterogeneous chiral catalysts anchored in MCM‐41 offered the practical advantages of facile reuse through simple recovery from the reactant and products.

Multinuclear cobalt-salen complexes with phenylene linker for epoxide polymerizations

Di- and trinuclear cobalt (Co)–salen complexes with a benzene ring as a rigid linker were explored for epoxide polymerizations. The dinuclear Co–salen complex with a 1,2-phenylene linker showed higher catalytic activity than the dinuclear Co–salen complex with a 1,3-phenylene linker and the trinuclear Co–salen complex with a 1,3,5-benzenetriyl linker for the copolymerization of propylene oxide (PO) with carbon dioxide. A combination of the absolute configuration of the two Co–salen moieties was found to affect its catalytic activity. The optimized dinuclear Co–salen complex with a heterochiral combination demonstrated highest activity and maintained its catalytic activity under a low catalyst concentration. The heterochiral dinuclear Co–salen complex also showed high activity for the copolymerization of PO with cyclic anhydride. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2150–2159

Copolymerization of Carbon Dioxide and Propylene Oxide by Binary Cobalt Salen Complexes with Various Anion Groups

A series of salen–Co(III)(X) complexes tethering quaternary ammonium salts are designed to investigate the influence of the axial group X in the complex and the anion Y of quaternary ammonium salt on the copolymerization of CO2 and PO. By copolymerization, the complex 9, where X and Y are both 2,4-dinitrophenolate, has the highest catalytic efficiency. When X is OAc and Y is BF4−/NO3−, the complexes 11/12 have lower catalytic efficiency. For the complex 10, where X and Y are both OAc, the catalytic efficiency is the lowest. At the same time, complex 9 can produce the copolymer with the highest carbonate fraction and Mn. And the best copolymerization conditions were as follows: reaction temperature 30°C, copolymerization time 24 h, and CO2 pressure 2 MPa with complex 9. The thermal properties of the copolymers are also studied by differential scanning calorimetry (DSC) and thermogravimetry (TG).

Asymmetric Ring Opening of Epoxides with a Supported Cobalt–Salen Complex

Asymmetric Ring Opening of Epoxides with a Supported Cobalt–Salen Complex

Co(salen)] derived Co/Co3O4 nanoparticle@carbon matrix as high-performance electrode for energy storage applications

Abstract Cobalt/cobalt oxide nanoparticle-embedded in a carbon matrix was synthesized by one spot pyrolysis of cobalt salen complex [Co(salen)] at 800 °C in an argon atmosphere. The X-ray diffraction studies confirmed the presence of Co and Co 3 O 4 in the carbon matrix. The SEM and TEM observations showed the homogeneous distribution of the Co/Co 3 O 4 grains on the carbon matrix. Cyclic voltammograms and galvanostatic charge-discharge studies done on the CR2032 coin cell confirmed that the Co/Co 3 O 4 @carbon matrix was electrochemically active. A stable specific capacity as high as 1000 mA h g −1 has been observed over 50 charge-discharge cycles at C/5 rate. It is believed that the carbon matrix acted both as a spacer to accommodate volume changes during Li intercalation-deintercalation process and also as conductive network leading to the excellent electrochemical performance of the Co/Co 3 O 4 @carbon matrix. Further, supercapacitor studies revealed that a specific capacitance of 615 F g −1 at 1 A g −1 has been exhibited by the Co/Co 3 O 4 @carbon matrix electrode in 1 M KOH with high Coulombic efficiency (92%) as well as excellent cycling stability for 5000 cycles.

Co(salen)を前駆体とするシングルサイト触媒の開発と酸化反応への応用

Co(salen)を前駆体とするシングルサイト触媒の開発と酸化反応への応用

Redox-Switchable Hydroelementation of a Cobalt Complex Supported by a Ferrocene-Based Ligand

The first crystallographically characterized tetrahedral cobalt salen (salen = N,N′-ethylenesalicylimine) complex was synthesized by using a 1,1′-ferrocene derivative, salfen (salfen = 1,1′- di(2,4-di-tert-butyl-6-salicylimine)ferrocene). The complex undergoes two oxidation events at low potentials, which were assigned as ligand centered by comparison to the corresponding zinc complex. The cobalt complex was found to catalyze the hydroalkoxylation of styrenes, similarly to related square planar cobalt salen complexes, likely due to its fluxional behavior in alcoholic solvents. Furthermore, the one-electron-oxidized species was found to be inactive toward hydroalkoxylation. Thus, the hydroalkoxylation reactivity could be turned on/off in situ by redox chemistry.

Spin modulation and electrochemical behavior of a five-coordinate cobalt(III) salen complex

We report the synthesis of a Co(III) complex with the five-coordinate salen-type ligand (N,N′-bis(3,5-di-tert-butyl-2-hydroxybenzyliden)-1,7-diamino-4-methyl-4-azaheptane). This complex is stable in air with a trigonal bipyramidal geometry and we show spectroscopically and computationally that a high-spin triplet ground state is preferred. This spin state is readily modulated by introduction of an exogenous ligand (pyridine, acetonitrile) to yield a six-coordinate complex with low-spin ground state. The five-coordinate complex exhibits solvent- and ligand-dependent electrochemical behavior in solution for the CoII/III transition and undergoes a one-electron ligand oxidation to generate a phenoxyl radical species that is relatively stable in the absence of oxygen. We show that this phenoxyl radical species is a Class I mixed-valence compound that can undergo photoinduced inner-sphere charge transfer with the neighboring phenoxide. This process is mediated by the Co(III) center which acts as a bridge. Understanding this behavior will lead to a better understanding of a dicobalt bis-salen analog previously reported by our group as a proton reduction catalyst.

Macroporous Helical Silica Immobilizing Cobalt-Salen Complex Catalyzed Asymmetric Hydrolytic Kinetic Resolution of Epoxides

Based on an entropy-driven principle, new macroporous helical silica rods were synthesized by using mild templates in sol–gel, then functionalized for immobilizing Co-salen complexes to catalyze enantioselective hydrolytic kinetic resolution of epoxides. Characterization revealed both silica supports and heterogeneous Co-salen complexes were macroporous helical materials with affirmative chirality. Catalysis revealed enantioselective adsorption of substrate on silica supports dominated major configuration of resolution products rather than configuration of Co-salen. The (S)-amino alcohol-doped silica and Co-(R,R)salen comprised the most enantioselective combination, which proved chiral synergy between support and Co-salen did exist. This work would contribute to the design of new support materials for asymmetric catalysis.

Phenol oxidation by air using a Co (II) Salen complex catalyst supported on nanoporous materials: synthesis, characterization and kinetic analysis

Cobalt (II) Salen complex catalysts were synthesized and immobilized through covalent bonds on synthetic silica (SBA-15) and commercial supports, including silica (SiO2), alumina (Al2O3) and granular activated carbon (C). Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV–vis), diffuse reflectance ultraviolet-visible spectroscopy (DR–UV–vis) and X-ray diffraction (XRD) were used to characterize and follow the synthesis of the cobalt (II) Salen complex The UV–vis absorption spectra confirmed the formation of the Salen ligand and its coordination with the Co (II). FTIR spectra showed the incorporation of the Co Salen complex and its anchoring to the support was confirmed by DR–UV–vis. The catalytic activity was evaluated measuring the oxidation of phenol by air in a buffer solution using micellar electrokinetic chromatography (MEKC). The effects of support, initial substrate concentration, pH, oxidant (O2 vs H2O2) and temperature were investigated. The best catalytic performance was obtained when the Co Salen complex was immobilized on SBA-15 (Co Salen/SBA-15). Conversely, the results suggest that the activated carbon promoted the irreversible adsorption of the substrate. The Co Salen support was also characterized by scanning electron microscopy (SEM) to elucidate the morphology. Elemental analysis using energy dispersive X-ray spectroscopy (EDX) exhibited better Co Salen dispersion on the SiO2 and SBA-15 supports. The cobalt loading calculated from atomic absorption spectroscopy (AAS) was 0.22, 0.20, 0.14 and 0.09mmol/g for the Co Salen supported on SiO2, SBA-15, Al2O3 and C, respectively. A specific surface area of 736.13m2/g and a nanoporosity of 3.62nm were obtained for SBA-15 using N2 physisorption analysis; this high specific surface area is characteristic of SBA-15 and plays a critical role in the reported activity. According to these results and the corresponding data analysis, the catalyst promoted the oxidation of phenol with a reversible first-order kinetic reaction in air, as observed from the data obtained at 3 different temperatures (25, 38 and 50°C). These results enabled the calculation of activation energy values of 63 and 57kJ/mol. In addition, the Co Salen/SBA-15 showed very good selectivity toward the production of catechol, which is one of the most important precursors in the chemical industry.

Cobalt–Salen Complexes as Catalyst Precursors for Electrocatalytic Water Oxidation at Low Overpotential

Water oxidation is an important half-reaction to achieve overall water splitting. In this present study, we show that a series of molecular cobalt–salen complexes can serve as catalyst precursors to form nanostructured and amorphous cobalt-based thin films during electrodeposition, which can catalyze the water oxidation reaction at low overpotentials. Cyclic voltammetry and bulk electrolysis using the cobalt-based film electrodes demonstrated obvious catalytic currents in 0.1 M KBi solution at pH 9.2. The onset catalytic potentials of the catalyst films are at ∼0.84 V (vs Ag/AgCl) with a film made by electrodeposition of cobalt–salen complex 2 on FTO and at ∼0.85 V for complex 4. Oxygen gas bubbles were clearly seen on the FTO electrode when the applied potential was above the onset potential. The Tafel plots using a catalyst film made of complex 4 showed that appreciable catalytic current was observed starting at η = 0.26 V for the film (a current density of 0.01 mA/cm2 required η = 290 mV), accompanied by a Faradaic efficiency >93% at 1.2 V. The catalyst film was further characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS).

Methanol electrooxidation on glassy carbon electrode modified with bimetallic Ni(II)Co(II)salen complexes encapsulated in mesoporous zeolite A

Nickel and cobalt salen complexes were simultaneously encapsulated in mesoporous zeolite A by flexible ligand method. The prepared catalyst NiCosalenA was investigated for its electrochemical behavior and electrocatalytic activity towards the oxidation of methanol on glassy carbon electrode in 0.1M NaOH solution. Upon encapsulation in zeolite A, the NiCosalen complex exhibits the typical cyclic voltammetric response, which can be anticipated for the Ni2+Co2+(Salen)(OH)2/Ni3+Co3+(Salen)O(OH) redox couple. It also showed much higher electrocatalytic activity towards the oxidation of methanol than pure NisalenA, due to a synergetic effect that may originate from the interaction of Ni(salen) and Co(salen) and/or the formation of dinuclear salen complexes in adjacent or appropriate sites in a nano-cage through the lattice oxygen of the zeolitic host. The mechanism of electrocatalytic oxidation of methanol on NiCosalenA modified electrode was proposed to be EC process. In both the chronoamperometric and chronocoulometric studies, the reaction of methanol electrooxidation followed a Cottrellian behavior and similar diffusion coefficients (D) of methanol were found to be 3.82×10−7cm2s−1 and 3.91×10−7cm2s−1, respectively. The catalytic rate constant (kcat) was also calculated to be 5.52×105cm3mol−1s−1 in the range of 0.005–2.0M of methanol.

Molecular cobalt–salen complexes as novel cocatalysts for highly efficient photocatalytic hydrogen production over a CdS nanorod photosensitizer under visible light

The present study reports an efficient photocatalytic system for hydrogen production based on CdS nanorods photosensitizer and a cobalt–salen complex as the cocatalyst.

ChemInform Abstract: A Salen—Co3+ Catalyst for the Hydration of Terminal Alkynes and in Tandem Catalysis with Ru—TsDPEN for the One‐Pot Transformation of Alkynes into Chiral Alcohols.

The cobalt–salen complex (C1:[(salen)Co3+(OAc)]; salen= N,N′-bis(salicylidene)ethylenediamine, OAc=acetate) was found to efficiently promote the hydration of terminal alkynes to give methyl ketones in the presence of the H2SO4 cocatalyst. In addition, the one-pot transformation of alkynes into chiral alcohols through tandem catalysis by catalyst C1 coupled with a ruthenium–TsDPEN complex (C3: [(R,R-TsDPEN)Ru2+(cymene)]; TsDPEN=(1R,2R)-N-(p-toluenesulfonyl)-1,2-diphenylethylenediamine, cymene=1-methyl-4-(1-methylethyl)benzene) catalyst was realized with excellent yields and enantioselectivities.

ChemInform Abstract: Chiral Cobalt(II)‐Salen‐Catalyzed Michael Addition of Amines to β‐Substituted Nitroalkenes.

Abstract Optically active cobalt(II) salen complexes were found to be effective Lewis acid catalysts for the enantioselective Michael addition of O -alkylhydroxylamines to nitroalkenes to afford the corresponding N -alkylhydroxyl-1,2-nitroamines in high yields and with good to high enantioselectivities. This study represents the first example of a transition-metal-catalyzed asymmetric Michael addition of amines to nitroalkenes.

A Salen–Co3+ Catalyst for the Hydration of Terminal Alkynes and in Tandem Catalysis with Ru–TsDPEN for the One‐Pot Transformation of Alkynes into Chiral Alcohols

AbstractThe cobalt–salen complex (C1:[(salen)Co3+(OAc)]; salen= N,N′‐bis(salicylidene)ethylenediamine, OAc=acetate) was found to efficiently promote the hydration of terminal alkynes to give methyl ketones in the presence of the H2SO4 cocatalyst. In addition, the one‐pot transformation of alkynes into chiral alcohols through tandem catalysis by catalyst C1 coupled with a ruthenium–TsDPEN complex (C3: [(R,R‐TsDPEN)Ru2+(cymene)]; TsDPEN=(1R,2R)‐N‐(p‐toluenesulfonyl)‐1,2‐diphenylethylenediamine, cymene=1‐methyl‐4‐(1‐methylethyl)benzene) catalyst was realized with excellent yields and enantioselectivities.

Chiral cobalt(II)-salen-catalyzed Michael addition of amines to β-substituted nitroalkenes

Optically active cobalt(II) salen complexes were found to be effective Lewis acid catalysts for the enantioselective Michael addition of O-alkylhydroxylamines to nitroalkenes to afford the corresponding N-alkylhydroxyl-1,2-nitroamines in high yields and with good to high enantioselectivities. This study represents the first example of a transition-metal-catalyzed asymmetric Michael addition of amines to nitroalkenes.

Tuning ligand electronics and peripheral substitution on cobalt salen complexes: structure and polymerisation activity

A series of cobalt salen complexes, where salen represents an N2O2 bis-Schiff-base bis-phenolate framework, are prepared, characterised and investigated for reversible-termination organometallic mediated radical polymerisation (RT-OMRP). The salen ligands contain a cyclohexane diimine bridge and systematically altered para-substituted phenoxide moieties as a method to examine the electronic impact of the ligand on complex structure and reactivity. The complexes are characterised by single crystal X-ray diffraction, cyclic voltammetry, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy and computational methods. Structural studies all support a tailorable metal centre reactivity altered by the electron-donating ability of the salen ligand. RT-OMRP of styrene, methyl methacrylate and vinyl acetate is reported and suggests that cobalt-carbon bond strength varies with the ligand substitution. Competing β-hydrogen abstraction affords long-chain olefin-terminated polymer chains and well controlled vinyl acetate polymerisations, contrasting with the lower temperature associative exchange mechanism of degenerative transfer OMRP.