Cobalt Metalloporphyrins Research Articles

Oxygen selective iron and cobalt–metalloporphyrin polymers – Extraordinary selectivity at low temperature

Oxygen binding properties of a recent class of porphyrin-based porous organic polymers (POPs) were investigated. POPs containing iron (Fe-POP) and cobalt (Co-POP) were synthesized, characterized and evaluated. Adsorption studies under ambient and cryogenic conditions (91–303 K) were studied. Interestingly, these POPs showed selective adsorption of O2 over N2 of up to 1.3 at 273 K, and most significantly, the oxygen uptake was found to be stable and reversible over the range of temperatures studied. Furthermore, the polymers also exhibited a molecular sieving effect, which showed increasing nitrogen exclusion between 110 K and 160 K and led to an extraordinary O2:N2 selectivity of up to 14.7 at 91 K. The high oxygen loading (13 wt% at 1 atm) together with the high selectivity suggests great potential for use of these materials in hybrid adsorption–cryogenic air separation systems.

Heterometal‐Embedded Organic Conjugate Frameworks from Alternating Monomeric Iron and Cobalt Metalloporphyrins and Their Application in Design of Porous Carbon Catalysts

Heterometal‐Embedded Organic Conjugate Frameworks from Alternating Monomeric Iron and Cobalt Metalloporphyrins and Their Application in Design of Porous Carbon Catalysts

Synthesis and catalytic properties of trans-A2B2-type metalloporphyrins in cyclohexane oxidation

A new efficient and mild protocol for synthesizing a series of trans-A2B2-porphyrins through a TFA-catalyzed condensation reaction between various aldehydes and dipyrromethanes has been developed. Several trans-A2B2-metalloporphyrins (cobalt, nickel) were synthesized and used to catalyze cyclohexane oxidation C–H bonds with dioxygen in the absence of additives and solvents. The results show that the catalytic activities were relative to the nature of the substituted groups and the central metal ions of trans-A2B2-metalloporphyrin. Cobalt metalloporphyrin presents better catalytic performance in the conversion of cyclohexane than the nickel metalloporphyrin under the same reaction conditions.

Sensitization of TiO2 by the MLCT Excited State of CoI Coordination Compounds

CoI coordination compounds, based on porphyrin or glyoxime ligands, anchored to TiO2 nanocrystallites interconnected in a mesoporous thin film were found to efficiently inject electrons into TiO2 when excited with visible or near-infrared light. When a cobalt metalloporphyrin sensitizer was employed, electron injection was rapid and led to a long-lived (millisecond time scale) charge-separated state whose recombination kinetics were nonexponential but were first order in the concentration of TiO2 electrons. The MLCT excited state was found to inject electrons more efficiently than the π → π* excited state of the porphyrin, even though unfilled d orbitals were present. These findings provide an earth-abundant metal to use in coordination compounds for sensitization of wide-band-gap semiconductor nanoparticles, especially those whose conduction band edges are generally closer to the vacuum level.

Plasma modification of self-assembled structures of CoTMPP molecules

Plasma-treated cobalt metalloporphyrins have recently been proposed as electrocatalysts for the oxygen and oxygen peroxide reduction reaction. Whereas the effects of plasma treatment on the elemental composition of the surface of catalysts have been investigated, the effects of plasma treatment on the morphology of catalysts have not yet been studied. In this study, plasma modified nanosized structures of cobalt tetramethoxyphenylporphyrin (CoTMPP) molecules arising from the deposition of a porphyrin solution on an a-C:H film are investigated using an atomic force microscope (AFM). Additionally, the effects of plasma treatment on the structure of porphyrin molecules are studied by using ultra violet visible (UV–vis) absorption analysis. The investigations reveal the morphological changes which accompany the transformation of CoTMPP into the final catalytic material. First, the large CoTMPP aggregates are split into smaller ones. Second, the CoTMPP layer appears to be sublimated after plasma treatment. Sublimated CoTMPP molecules can be decomposed by plasma and after redeposition can form catalytic active fragments.

Direct Observation of Photodriven Intermolecular Hole Transfer across TiO2 Nanocrystallites: Lateral Self-Exchange Reactions and Catalyst Oxidation

Photoselection of Ru(II)-polypyridyl sensitizers with polarized pulsed-light excitation, when anchored to TiO(2) nanocrystallites interconnected in a mesoporous thin film, results in an anisotropic distribution of excited sensitizers. Under conditions where excited-state sensitizers efficiently inject electrons into TiO(2), the resulting oxidized sensitizers exhibit an initial anisotropy in their absorption difference spectra. Over the course of the charge-separated lifetime for many sensitizers, the transient absorption anisotropy signal decays to nearly zero indicative of lateral self-exchange hole-transfer reactions at the interface. When a cobalt metalloporphyrin catalyst was coanchored to the sensitized nanocrystalline TiO(2) film, excited-state injection was followed by lateral hole transfer to oxidize the surface-bound catalyst.

Poisoning Influence of Cyanide on the Catalytic Oxygen Reduction by Cobalt(III) Tetra(3‐methoxy‐4‐hydroxylphenyl) Porphyrin Modified Electrode

Cobalt(III) tetra(3‐methoxy‐4‐hydroxylphenyl) porphyrin can be irreversibly bound to a platinum electrode through a sol‐gel network. Based on our finding that cobalt metalloporphyrin catalyzes dioxygen reduction near 0 mV (vs. Ag/AgCl), the electrocatalytic kinetics of oxygen reduction has been studied by cyclic voltammetry and rotating disk voltammetric methods. A possible reaction mechanism is suggested for the observed catalysis. In the presence of cyanide, the electrocatalytic reductive current for dioxygen is decreased due to the strong binding of cyanide ions to the cobalt metal center. A simplified model is proposed to describe this poisoning effect based on the Koutecký‐Levich theory and coordination equilibrium between cyanide and the electrocatalyst. The model is well supported by the experimental results. The poisoning effect can be used for highly sensitive cyanide detection. © 2000 The Electrochemical Society. All rights reserved.

Regulation of heme-controlled eukaryotic polypeptide chain initiation factor 2 alpha-subunit kinase of reticulocyte lysates.

We have obtained highly purified preparations of the heme-controlled eukaryotic initiation factor 2 alpha-subunit (eIF-2 alpha) kinase (HCI) from rabbit reticulocyte lysates containing five different polypeptides. One of these is a 87-kDa (p87) phosphopeptide which appears to show an autokinase activity. The controlled digestion with trypsin of HCI preparations leads to the suggestion that phosphorylation of p87 is not needed for kinase activity and, furthermore, that another 89-kDa polypeptide could be the kinase catalytic subunit. In agreement with this, monoclonal antibodies directed against p87 do not interfere with eIF-2 alpha kinase activity. Moreover, the anti-p87 antibodies and those directed against the mammalian 90-kDa heat shock protein recognize the same p87 polypeptide from rabbit reticulocyte lysates. Upon incubation of the HCI preparation with hemin (5-10 microM), the eIF-2 alpha kinase is converted into an inactive form and appears to become associated with related peptides forming high molecular weight complexes which can be reversibly activated by 2-mercaptoethanol. The maintenance of the integrity of the porphyrin ring is absolutely required for kinase inactivation and although the presence of metal ion is not essential, the iron and cobalt metalloporphyrins are more effective than protoporphyrin IX. The formation of the inactive form of HCI by hemin is prevented by either N-ethylmaleimide, monoclonal antibodies directed against p87, or phosphorylation of p87. The data strongly suggest that hemin regulates eIF-2 alpha kinase activity by promoting formation of the inactive dimer HCI.p87 via disulfide bonds and direct binding of hemin. A model of HCI regulation is discussed.

Application of a polymer solid electrolyte for the vapor-phase electrocatalysis of dioxygen reduction by some cobalt porphyrins

An electrochemical cell suitable for the study of dioxygen reduction electrocatalysis by films of cobalt metalloporphyrins on electrodes in the absence of liquid electrolytes is described. The glassy carbon disk of a (stationary) ring-disk electrode is coated with either an electropolymerized film of cobalt tetra( o-aminophenyl)porphyrin or an adsorbed multimolecular layer of a dicobalt cofacial porphyrin, Co 2 FTF4. Then the disk, the ring (serving as an auxiliary electrode), and a peripheral silver wire (serving as a pseudo-reference electrode) are coated with a thin film of an ionically conducting polymer: poly(ethyleneoxide) containing a 16:1 ether oxygen/Li + concentration of lithium triflate. Bathing this solid state cell in moist, acidic or neutral bathing gases containing dioxygen produces electrocatalytic currents for dioxygen reduction. The fraction of the reduction leading to H 2O 2 was determined with a twin electrode sandwich modification of the above cell in which a Au film electrode atop a Nucleopore membrane is contacted to the top surface of the PEO 16·Li(CF 3SO 3) film. No H 2O 2 was produced in the case of the Co 2FTF4 catalyst in a neutral bathing gas; this electrocatalysis was also quite stable.

Structural requirements for porphyrin inhibition of the hemin-controlled protein kinase and maintenance of protein synthesis in reticulocytes.

The role of hemin in the maintenance of protein synthesis in reticulocyte lysates was examined by comparing the effects of various porphyrins and metalloporphyrins on the protein kinase activity of the hemin-controlled repressor and on protein synthesis. The porphyrin requirements for maintenance of protein synthesis were relatively specific. Iron and cobalt metalloporphyrins sustained protein synthesis whereas other metalloporphyrins, metal-deficient porphyrins, and non-porphyrin precursor and degradation products of protoporphyrin IX were ineffective. These same compounds were examined for their effectiveness in inhibiting the protein kinase activity of the hemin-controlled repressor with initiation factor 2 (eIF-2). Most of the metalloporphyrins and porphyrins tested were inhibitory. The presence of the iron atom in the porphyrin was not essential for inhibition, but the maintenance of the integrity of the porphyrin ring was imperative. The porphyrins which inhibited the hemin-regulated protein kinase contained vinyl groups or ethyl groups, or were protonated in the 2- and 4-positions of the porphyrin ring, whereas those with bulky or acidic groups in these positions were ineffective. Precursor and degradation products of protoporphyrin IX and synthetic porphyrins modified at other positions had no effect on the enzyme. Both hemin and protoporphyrin IX inhibited phosphorylation of eIF-2 exogenously added to a reticulocyte lysate; however, hemin sustained protein synthesis in the lysate, whereas protoporphyrin IX did not. These results suggest that regulation of the protein kinase phosphorylating the alpha subunit of eIF-2 is not the only point at which hemin modulates protein synthesis in reticulocytes and reticulocyte lysates, since a correlation between inhibition of protein synthesis, inhibition of protein kinase activity, and phosphorylation of eIF-2 is not observed with all porphyrins.