Cobalt Porphyrin Complexes Research Articles

Cyclopropanation with D 2-Symmetrical Chiral Porphyrin

A cobalt porphyrin system was designed for efficient cyclopropanation. This protocol is attractive in that the alkene is the limiting reagent and large excess of substrate is not needed. The cobalt-porphyrin complex substantially outperforms the similar iron-porphyrin complex. A variety of styrene derivatives are successfully cyclopropanated with high diastereoselectivity and enantioselectivity.

Electrocatalytic Reduction of Dioxygen on the Surface of Glassy Carbon Electrodes Modified with Cobalt Porphyrin Complexes

AbstractThe preparation and electrocatalytic behavior of glassy carbon electrodes modified with three different cobalt porphyrin complexes were investigated. The electrocatalytic ability of the modified electrodes for the reduction of dioxygen to hydrogen peroxide and water in air‐saturated aqueous solutions was examined by cyclic voltammetry and chronoamperometry techniques. The porphyrin‐adsorbed glassy carbon electrodes possess excellent electrocatalytic abilities for dioxygen reduction with overpotential about 0.5 V lower than that at a plain glassy carbon electrode. The experimental parameters were optimized and the mechanism of the catalytic process was discussed. The possible effects of the electron‐donating properties of groups in the meso‐position of the porphyrin ring were investigated.

Dissociation kinetics of cobalt and zinc β-octabromo-meso-tetraphenyl- and β-octaethyl-meso-tetraphenylporphyrin complexes

Kinetics of solvoprotolytic dissociation of zinc and cobalt porphyrin complexes with a porphyrin macrocycle distorted to different extents (β-octaethyl-meso-tetraphenylporphyrin and β-octabromo-meso-tetraphenylporphyrin) have been studied in binary proton-donating solvents. The main factor decreasing the stability of the porphyrin complexes is the distortion of the planar structure of the macrocycle. The reactivity of the cobalt porphyrins is governed by both structural and electronic effects.

Autoxidation of 2,6-di- tert-butylphenol catalyzed by 5,10,15,20-tetrakis[4-(diethylmethylammonio)phenyl]porphyrinatocobalt(II) tetraiodide in water

The cationic cobalt(II) porphyrin complex: 5,10,15,20-tetrakis[4-(diethylmethylammonio)phenyl]porphyrinatocobalt(II) tetraiodide has been found to be an efficient catalyst for the oxidation of 2,6-di- tert-butylphenol by dioxygen in aqueous medium. The oxidation reaction afforded mainly the corresponding diphenoquinone and some of the 2,6-di- tert-butyl-1,4-benzoquinone. The initial rate constant of autoxidation reaction was found to increase with increasing pH from 7.0 to 10.0. The rate of autoxidation reaction showed linear dependence on catalyst concentration and dioxygen pressure. The rate of autoxidation reaction was found to fit a Michealis–Menten kinetic model for saturation of catalyst sites with increasing 2,6-di- tert-butylphenol concentration. The catalyst showed reduced activity in repeat oxidation reactions.

Structure and Dynamics of Dioxygen Bound to Cobalt and Iron Heme

In this study we use ab initio molecular dynamics simulations to analyze the structure and dynamics of the oxygen ligand in models of the oxymyoglobin active site and its cobalt-substituted analog. Our calculations are performed for iron-porphyrin and cobalt-porphyrin complexes with imidazole and oxygen as axial ligands, and we investigate the effect of the distal histidine in the structure and dynamics of the metal-oxygen unit (MeO 2, Me = Fe, Co). We find that the interaction between the distal histidine and the oxygen ligand is stronger for the cobalt complex than for the iron one, consistent with the superoxide ion character of the bound O 2. The dynamics of the O 2 ligand can be described as oscillations of the O-O axis projection on the porphyrin plane within a porphyrin quadrant combined with frequent jumps from one quadrant to another. However, the ligand motion is significantly faster for CoO 2 compared to FeO 2. As a result, the iron complex shows localized ligand sites, whereas for cobalt several configurations are possible. This gives support to the highly dynamic motion of the oxygen ligand found in several experiments on cobalt oxymyoglobin and model complexes and underlines the higher mobility of the CoO 2 fragment compared to FeO 2.

Oxidation of cyclohexene with molecular oxygen catalyzed by cobalt porphyrin complexes immobilized on montmorillonite

Composite materials were prepared by intercalating cationic porphyrinato cobalt complexes with the substituents of the quaternary ammonium salt of heterocyclic amine into a montmorillonite interlayer. Using these clay interlayer-fixed porphyrinato cobalt complexes as catalyst, the epoxidation of cyclohexene by oxygen molecules was examined. The prepared intercalation compounds have mesopores with an average diameter of about 12 nm, and their specific surface area increased in proportion to the amount of the intercalated porphyrinato cobalt complexes. It was proven that the porphyrinato cobalt complex was intercalated between montmorillonite layers functioned as a pillaring agent. A pillared clay catalyst, which was prepared by intercalating a [meso-tetrakis(1-ethyl-3-pyridinio)porphyrinato] cobalt complex into the montmorillonite interlayer, showed the highest catalytic activity forming 1,2-epoxycyclohexane preferentially in the presence of isobutyraldehyde. It is suggested that montmorillonite contributes to the stabilization of the porphyrinato cobalt complex, and also plays a role in accelerating oxidation by activating oxygen molecules by way of constructing a reaction field that is regulated three-dimensionally through electrostatic interaction with guest molecules.

Synthesis, Structure, and Unusual Reactivity of β-Halovinyl Cobalt Porphyrin Complexes

The preparation, structures, and reactivity of tetraphenylporphyrin (TPP) cobalt halovinyl complexes are reported. Beta-halovinyl complexes of (TPP)Co(E-CHCHX) (X = Br and I) were prepared from the insertion of acetylene into the cobalt halide bonds of the corresponding halide complexes. The reactivity of these compounds and of the previously reported (TPP)Co(E-CHCHCl), was studied in depth, and it was found that complex reactivity increased with the leaving group ability of the halide. A trans-dichlorovinyl cobalt porphyrin complex, (TPP)Co(Z-CClCHCl), was also prepared through the reaction of (TPP)CoNa and TCE. The structures of (TPP)Co(E-CHCHBr), (TPP)Co(Z-CClCHCl), and (TPP)Co(C(2)H) are reported. The C-C bond length of the vinyl group was found to vary for the beta-halovinyl complexes (TPP)Co(E-CHCHX) from 1.211 A for X = Br to 1.234 A for X = Cl and 1.320 A for (TPP)Co(Z-CClCHCl). A comparison of these structures to many chlorovinyl cobalt complexes shows that trans-2-halo substitution results in a dramatically decreased vinyl C-C bond length. The mechanism of halide substitution for the beta-halovinyl complexes was investigated with kinetic experiments that indicated a dissociative mechanism and supported the intermediacy of a cobalt acetylene complex.

Electropolymerizable iron (III) and cobalt (II) dicyanophenoxy tetraphenylporphyrin complexes: Potential electrocatalysts

Solution and solid phase electrochemical features of 5-[4-(3,4-dicyanophenoxy)phenyl],10,15,20-triphenylporphyrin complexes of iron(III) (FeCNOTPP(Cl)) and cobalt(II) (CoCNOTPP) have been described. These novel asymmetric dicyanophenoxy-derivatised cobalt and iron porphyrin complexes were electropolymerised onto glassy carbon electrodes, which in aqueous solutions, gave surface concentrations (ca. 10 −10 mol cm −2) typical of monolayer coverages. The films also exhibited excellent stability and electrocatalysis towards the direct detection of important analytes as nitrite, nitric oxide, and hydrogen peroxide in aqueous solutions.

Tight-Binding Quantum Chemical Molecular Dynamics Study on First Proton Transfer Process of ORR Catalyzed by Cobalt-Porphyrin Complex

We investigated oxygen reduction reaction (ORR) dynamics catalyzed by cobalt-porphyrin using our original novel tight-binding quantum chemical molecular dynamics method. First, we determined parameters for tight-binding calculation based on first-principles parameterization. We have successfully simulated a first proton transfer process of the oxygen reduction reaction with the explicit consideration of surrounding water molecules. The transferred proton formed hydrogen bonding with a water molecule and conformational change of the OOH species was observed. It was proved that our tight-binding quantum chemical molecular dynamics method is effective for the investigation of oxygen reduction reaction dynamics in large complex system.

Dioxygen Binding to a Cobalt(II) Porphycene Complex and Its Auto-Oxidized Cobalt(III) Complex

Abstract The reaction between a cobalt porphycene complex, [CoII(TPrPc)] (TPrPc is a dianion of 2,7,12,17-tetrapropylporphycene), and dioxygen was investigated by means of UV–vis, ESR, and resonance Raman (RR) spectroscopies. These spectra indicate the formation of [CoIII–O2−•]. In the presence of pyridine as an axial ligand, the formation of a six-coordinated superoxide species was confirmed by its characteristic ESR spectrum. The six-coordinated dioxygen adduct exhibits an RR band at 1144 cm−1, which is characteristic of O–O stretching of superoxide. ν(Co–O2) stretching is also observed at 540 cm−1. The ν(O–O) and ν(Co–O2) wavenumbers in the complex were assigned by means of 18O2 substitution. ν(Co–O2) of the O2 adduct shows a higher frequency relative to those of cobalt porphyrin complexes. On standing for one day at room temperature, the compound was auto-oxidized to a CoIII complex, which was structurally determined by X-ray analysis. The complex crystallized in the monoclinic space group P21/n, a = 9.6266(4) Å, b = 12.6343(5) Å, c = 15.6566(6) Å, β = 99.1200(10)°, V = 1880.17(13) Å3, and Z = 2. The CoIII ion lies on the plane of the porphycenato core, and coordinates four porphycenato nitrogen atoms and two pyridine nitrogen atoms. The complex cation, [CoIII(TPrPc)(Py)2]+, exhibits a slightly distorted octahedral coordination geometry. The average Co–N (porphycenato pyrrole) distance and the Co–N (pyridine) distance are 1.929 and 1.952 Å, respectively.

Catalytic activity and stability of anionic and cationic water soluble cobalt(II) tetraarylporphyrin complexes in the oxidation of 2-mercaptoethanol by molecular oxygen

The catalytic activity and stability of anionic cobalt(II) porphyrin complexes: 5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrinatocobalt(II), 5,10,15,20-tetrakis(2,4,6-trimethyl-3,5disulfonatophenyl)porphyrinatocobalt(II) and the cationic cobalt(II) porphyrin: 5,10,15,20-tetrakis[4-(diethylmethylammonio)phenyl]porphyrinatocobalt(II) tertraiodide have been investigated in the oxidation of 2-mercaptoethanol by dioxygen. All complexes were efficient catalysts for the auto-oxidation of 2-mercaptoethanol. The cationic cobalt(II) porphyrin has been found to be the most reactive catalyst. The rate of auto-oxidation of 2-mercaptoethanol catalysed by 5,10,15,20-tetrakis(2,4,6-trimethyl-3,5disulfonatophenyl)porphyrinatocobalt(II) has been found to increase with increasing the pH from 7 to 9 then decreased at higher pH. The rate constants of auto-oxidation reaction showed linear dependence on catalyst concentration and saturation kinetics in both 2-mercaptoethanol concentrations and dioxygen pressure. Anionic cobalt(II) porphyrin complexes showed higher stability than the cationic catalyst in repeat oxidation reactions. Immobilizing the anionic catalysts on ion exchange resin and supporting the cationic catalyst on clay mineral montmorillonite improved their stabilities towards oxidation.

Gas separation membranes containing Re6Se8(MeCN)6 2+ cluster-supported cobalt-porphyrin complexes

Cellulose nitrate (CN) composite membranes, containing cobalt porphyrin (CoP) complexes self-assembled within nanometer-sized rhenium clusters (ReCoP), have been prepared and their oxygen and nitrogen gas permeabilities were analyzed. The solubility of ReCoP and the characteristics of the corresponding composite membranes were analyzed using a Cahn microbalance, FT-IR spectroscopy, wide-angle X-ray scattering, and differential scanning calorimetry. The nitrogen permeability through the CN composite membranes decreased upon addition of ReCoP and CoP, which implies that the presence of these oxygen carrier complexes affects the structure of the polymer matrix. The oxygen permeability through the composite membranes containing small quantities of ReCoP decreased, but it increased upon increasing the concentration. The oxygen gas transport was affected by the matrix at low ReCoP concentrations, but higher concentrations of ReCoP increased the oxygen permeability as a result of its reversible and specific interactions with oxygen, effectively realizing ReCoP carrier-mediated oxygen transport.

General and Selective Olefination of Aldehydes and Ketones Catalyzed by a Cobalt(II) Porphyrin Complex.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electroreduction of Oxygen at a Pt/C-Modified Electrode with a Cobaltporphyrin Complex

Abstract Cobalt complexes that reversibly bind and release O2 functioned as O2 carriers, or O2-enriching media, at the surface of a cathode to enhance the current for the reduction of O2. The effect of the cobalt complex to accumulate O2 was evidenced by the higher open-circuit potential, which reflected the equilibrated concentration of O2 at the electrode surface. The combination of the cobalt complex and the conventional Pt/C catalyst resulted in a significant increase in the steady-state current for the four-electron reduction of O2, particularly at small overpotentials where typical fuel cells operate, based on the facilitated transport of O2 from the atmosphere to the catalyst at the electrode surface.

Diastereoselective and Enantioselective Cyclopropanation of Alkenes Catalyzed by Cobalt Porphyrins.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electrochemical and Spectroelectrochemical Investigations of Mono‐ and Binuclear Cobalt(II) Complexes of “Figure‐Eight” Octapyrrolic Macrocycles

AbstractIn this paper, electrochemical investigations of a set of three cyclooctaphyrin complexes containing divalent cobalt are reported. These complexes are the homodinuclear cobalt‐cobalt complex Co2L1 with the ligand H4L1, the mononuclear cobalt complex CoH2L2 and the heterodinuclear copper(II)‐cobalt complex CoCuL2 with the ligand H4L2. All of these species undergo two reduction and three oxidation processes, all of which are reversible one‐electron transfers. The two reduction and first two oxidation steps are ligand‐centered, as confirmed by spectroelectrochemical investigations. In contrast, the third oxidation corresponds to a metal‐centered reversible one‐electron transfer. The metal‐centered oxidation steps occur at lower potentials than the third oxidation steps of the free bases. UV/Visible spectral changes observed during oxidation confirm a metal‐centered oxidation generating a cobalt(III) cation, which is rather unusual when compared with cobalt porphyrin complexes, in which the oxidation of the metal usually occurs before the oxidation of the ligand. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Cobalt(II) porphyrin complex immobilized on the binary oxide SiO 2/Sb 2O 3: electrochemical properties and dissolved oxygen reduction study

In this work, SiO 2/Sb 2O 3 prepared by the sol–gel processing method, having a specific surface area, S BET, of 790 m 2 g −1, an average pore diameter of 1.9 nm and 4.7 wt.% of Sb, was used as substrate base for immobilization of the 5,10,15,20-tetrakis(1-methyl-4-pyridyl)-21 H,23 H-porphine ion. Cobalt(II) ion was inserted into the porphyrin ring with a yield of complex bonded to the substrate surface of 59.4 μ mol g −1. A carbon paste electrode of this material was used to study, by linear sweeping voltammetric and chronoamperometric techniques, the electrocatalytic reduction of dissolved oxygen. The reduction, at the electrode solid–solution interface, occurred at −0.25 V versus SCE in 1.0 mol l −1 KCl solution, pH 5.5, by a four electron mechanism. The electrode response was invariant under various oxidation–reduction cycles showing that the system is chemically very stable. Such characteristics allowed the study of the electrode response towards various dissolved oxygen concentrations using the chronoamperometry technique. The cathodic peak current intensities plotted against O 2 concentrations, between 1.0 and 12.8 mg l −1, showed a linear correlation. The electrode response time was very fast, i.e. about 1 s. This study was extended using the electrode to determine the concentration of dissolved oxygen in sea water samples.

General and Selective Olefination of Aldehydes and Ketones Catalyzed by a Cobalt(II) Porphyrin Complex

The Co(II) porphyrin complex Co(TPP) is an efficient and versatile catalyst for selective olefination of carbonyl compounds, including aldehydes and activated and unactivated ketones, with ethyl diazoacetate (EDA) in the presence of triphenylphosphine. The catalytic reactions of aldehydes and activated ketones can be carried out in a one-pot fashion with the use of a stoichiometric amount of diazo reagents, affording olefins in high yields and high E selectivity. In the case of unactivated ketones, a substoichiometric amount of benzoic acid is required to promote the catalytic reactions.

Diastereoselective and enantioselective cyclopropanation of alkenes catalyzed by cobalt porphyrins.

Cobalt(II) porphyrin complexes were shown to be general and efficient catalysts for selective cyclopropanation of alkenes with ethyl diazoacetate (EDA). The catalytic system can operate with alkenes as limiting reagents, requiring only stoichiometric amounts of EDA. The protocol is performed in one-pot fashion without the need of slow addition of EDA. The diastereoselectivity of the current system can be tuned by using different porphyrin ligands or additives, giving either trans- or cis-dominant cyclopropanes. The asymmetric cyclopropanation was also demonstrated with the use of chiral cobalt porphyrin complexes.

Mechanistic studies of nitric oxide reactions with water soluble iron(II), cobalt(II), and iron(III) porphyrin complexes in aqueous solutions: implications for biological activity.

The reactions of nitric oxide and carbon monoxide with water soluble iron and cobalt porphyrin complexes were investigated over the temperature range 298-318 K and the hydrostatic pressure range 0.1-250 MPa [porphyrin ligands: TPPS = tetra-meso-(4-sulfonatophenyl)porphinate and TMPS = tetra-meso-(sulfonatomesityl)porphinate]. Large and positive DeltaS(double dagger) and DeltaV(double dagger) values were observed for NO binding to and release from iron(III) complexes Fe(III)(TPPS) and Fe(III)(TMPS) consistent with a dissociative ligand exchange mechanism where the lability of coordinated water dominates the reactivity with NO. Small positive values for Delta and Delta for the fast reactions of NO with the iron(II) and cobalt(II) analogues (k(on) = 1.5 x 10(9) and 1.9 x 10(9) M(-1) s(-1) for Fe(II)(TPPS) and Co(II)(TPPS), respectively) indicate a mechanism dominated by diffusion processes in these cases. However, reaction of CO with the Fe(II) complexes (k(on) = 3.6 x 10(7) M(-1) s(-1) for Fe(II)(TPPS)) displays negative Delta and Delta values, consistent with a mechanism dominated by activation rather than diffusion terms. Measurements of NO dissociation rates from Fe(II)(TPPS)(NO) and Co(II)(TPPS)(NO) by trapping free NO gave k(off) values of 6.3 x 10(-4) s(-1) and 1.5 x 10(-4) s(-1). The respective M(II)(TPPS)(NO) formation constants calculated from k(on)/k(off) ratios were 2.4 x 10(12) and 1.3 x 10(13) M(-1), many orders of magnitude larger than that (1.1 x 10(3) M(-1)) for the reaction of Fe(III)(TPPS) with NO.