Water-soluble Cobalt Porphyrin Research Articles

Visible-light-driven reduction of CO2 to CO in fully aqueous media using a water-soluble cobalt porphyrin

Herein we report a water-soluble cobalt porphyrin that efficiently catalyzes the photoreduction of CO2 in fully aqueous media with a good selectivity over H2 production.

DFT and Empirical Considerations on Electrocatalytic Water/Carbon Dioxide Reduction by CoTMPyP in Neutral Aqueous Solutions*.

A combined experimental and density functional theory (DFT) investigation was employed in order to examine the mechanism of electrochemical CO2 reduction and H2 formation from water reduction in neutral aqueous solutions. A water soluble cobalt porphyrin, cobalt [5,10,15,20-(tetra-N-methyl-4-pyridyl)porphyrin], (CoTMPyP), was used as catalyst. The possible attachment of different axial ligands as well as their effect on the electrocatalytic cycles were examined. A cobalt porphyrin hydride is a key intermediate which is generated after the initial reduction of the catalyst. The hydride is involved in the formation of H2 and formate and acts as an indirect proton source for the formation of CO in these H+ -starving conditions. The experimental results are in agreement with the computations and give new insights into electrocatalytic mechanisms involving water soluble metalloporphyrins. We conclude that in addition to the porphyrin's structure and metal ion center, the electrolyte surroundings play a key role in dictating the products of CO2 /H2 O reduction.

Highly Efficient and Selective Photocatalytic CO2 Reduction to CO in Water by a Cobalt Porphyrin Molecular Catalyst

The performance of a water-soluble cobalt porphyrin ([{meso-tetra(4-sulfonatophenyl)porphyrinato}cobalt(III)], CoTPPS) as a catalyst for the photoreduction of CO2 in fully aqueous media has been in...

Cobalt porphyrins as homogeneous catalysts for water oxidation

Three water-soluble cobalt porphyrins have been investigated as water oxidation catalysts via photo-initiation using Ru(II)(bpy)3(2+)/Na2S2O8. The pH dependence of the turnover frequency revealed maximum activity at pH 11. Based on the second order dependence on catalyst concentration for the rate of water oxidation, we suggest a bimolecular radical coupling process as the rate determining step.

Hydration of Terminal Alkynes Catalyzed by Water-Soluble Cobalt Porphyrin Complexes

Water-soluble cobalt(III) porphyrin complexes were found to promote the hydration of terminal alkynes to give methyl ketones. The alkyne hydration proceeded in good to excellent yield with 0.1 to 2 mol % cobalt catalyst 1 and was compatible with the presence of acid/base- or redox-sensitive functional groups such as alkyl silyl ethers; allyl ethers; trityl ethers; benzyl ethers; carboxylic esters; boronic esters; carboxamides; nitriles; and nitro, iodo, and acetal groups. Some of the alkyne substrates tested here are otherwise difficult to hydrate. The alkyne hydration can be performed on a gram scale, and the catalyst can be recovered by aqueous workup.

Self-Assembled Water-Soluble Cobalt Porphyrin Monolayers at Gold Electrodes with High Electrocatalysis of 2-Mercaptoethanol Oxidation

Self-Assembled Water-Soluble Cobalt Porphyrin Monolayers at Gold Electrodes with High Electrocatalysis of 2-Mercaptoethanol Oxidation

Batch reactor kinetic studies on the reductive dechlorination of chlorinated ethylenes by tetrakis-(4-sulfonatophenyl)porphyrin cobalt

Tetrakis-(4-sulfonatophenyl)porphyrin cobalt was identified as a highly-active reductive dechlorination catalyst for chlorinated ethylenes. Through batch reactor kinetic studies, degradation of chlorinated ethylenes proceeded in a step-wise fashion with the sequential replacement of Cl by H. For perchloroethylene (PCE) and trichloroethylene (TCE), the dechlorination products were quantified and the C 2 mass was accounted for. Degradation of the chlorinated ethylenes was found to be first-order in substrate. Dechlorination trials with increasing catalyst concentration showed a linearly increasing pseudo first-order rate constant which yielded rate laws for PCE and TCE degradation that are first-order in catalyst. The dechlorination activity of this catalyst was compared to that of another water-soluble cobalt porphyrin under the same reaction conditions and found to be comparable for PCE and TCE.

Mechanistic Analysis of Reductive Nitrosylation on Water-Soluble Cobalt(III)-Porphyrins

The reactions of NO and/or NO2- with three water-soluble cobalt porphyrins [Co(III)(P)(H2O)2]n, where P = TPPS, TCPP, and TMPyP, were studied in detail. At pH < 3, the reaction with NO proceeds through a single reaction step. From the kinetic data and activation parameters, the [Co(III)(P)(NO)(H2O)]n complex is proposed to be the primary product of the reaction with NO. This complex reacts further with a second NO molecule through an inner-sphere electron-transfer reaction to generate the final product, [Co(III)(P)(NO-)](n-1). At pH > 3, although a single reaction step is also observed, a systematic study as a function of the NO and NO2- concentrations revealed that two reaction steps are operative. In the first, NO2- and NO compete to substitute coordinated water in [Co(III)(P)(H2O)2]n to yield [Co(III)(P)(NO)(H2O)]n and [Co(III)(P)(NO2-)(H2O)](n-1) as the primary reaction products. Only the nitrite complex could be detected and no final product formation was observed during the reaction. It is proposed that [Co(III)(P)(NO)(H2O)]n rapidly reacts with NO2- to form the nitrite complex, which in the second reaction step reacts with another NO molecule to generate the final product through an inner-sphere electron-transfer reaction. The reported results are relevant for the interaction of vitamin B(12a) with NO and NO2-.

Oxygen Binding of Water-Soluble Cobalt Porphyrins in Aqueous Solution

Water-soluble cobalt porphyrin 1Co and imidazole ligand 2 were synthesized. 1Co binds dioxygen in the presence of imidazole ligand 2 in aqueous solution. The formation of the oxygen adduct 2-1Co(O(2)) was studied using UV-vis and EPR spectroscopy. The impact of pH on the kinetic stability of the oxygen adduct was examined.

Interactions between water soluble porphyrin-based star polymer and amino acids: Spectroscopic evidence of molecular binding

Molecular interactions giving rise to stable complexes between an uncharged water soluble cobalt-porphyrin and amino acids are investigated by time-resolved fluorescence, uv-vis, and circular dichroism measurements. This metalloporphyrin seems to act, by means of the coordination site of the cobalt of the core, as a recognition host, preferentially, with amino acids possessing aromatic groups. The binding with aliphatic amino acids requires longer time scales to be efficient and likely involves a slow kinetic process. The experimental findings suggest that, besides the metal(host)-N(guest) coordination bond, which is the common requisite for all amino acids, a preferential interaction with aromatic groups exists there. The solubility in water of the molecule, guaranteed by the polyethylene glycol arms as peripheral substituents, in the absence of electric charges, allows for a more selective discrimination of the binding process with respect to other water-soluble charged porphyrins. The interest devoted to the porphyrin-based star polymer and its recognition properties is, therefore, founded on the potential use either in polymeric matrices for material science or in aqueous solution for bioscience.

The catalytic and photocatalytic autoxidation of S x2− to SO 42− by water-soluble cobalt porphyrin

The catalytic and photocatalytic transformation of S 4 2−, S 3 2−, S 2 2− and S 2− to SO 4 2− by water-soluble cobalt tetrakis ( N-methyl-4-pyridyl) porphyrin (Co(4-TMPyP)) was performed by a two-step process. The catalytic autoxidation of S x 2− ( x=2, 3 and 4) to S 2O 3 2− by Co(4-TMPyP) in the presence of oxygen was followed by the photocatalytic oxidation of S 2O 3 2− by Co(4-TMPyP) under illumination in presence of oxygen at a strong basic solution. The direct transformation of S x 2− to SO 4 2− was also performed. The catalytic activity of Co(4-TMPyP) was compared to Fe(2-TMPyP) and Mn(4-TMPyP). All three catalysts have activity towards autoxidation of S 4 2− to S 2O 3 2−. The trend in photocatalytic oxidation activity of the porphyrin complexes is Co(4-TMPyP)>Fe(2-TMPyP)>Mn(4-TMPyP). The electrocatalytic reactions of S x 2− by Co(4-TMPyP) were also observed.

Reversible electrocatalytic reaction of sulfur oxoanions and sulfides by one catalyst of water-soluble cobalt porphyrin

Abstract The water-soluble cobalt tetrakis( N -methyl-2-pyridyl)phorpyrin (Co(2-TMPyP)) catalyzed the reduction of tetrathionate (() 3 SSSSO 3 ) 2− ), the oxidized form of glutathione (GSSG), and L-crystine (CySSCy) after Co 11 (2-TMPyP) was reduced electrochemically to Co 1 (2-TMPyP). The eletrocatalytic oxidation of thiosulfate (SSO 3 2− , reduced form of glutathione (GSH) and L-cysteine (CySH) are performed through Co 111 (2-TMPyP). These reactions occur in a reversible electrocatalytic system using only one catalyst Co(2-TMPyP). An electrocatalytic process is proposed. Co(2-TMPyP) shows two redox couples and can perform both electrocatalytic oxidation and reduction of sulfite in a strong acidic aqueous solution.

Cobaltmeso-Tetrakis(N-methyl-4-pyridiniumyl)porphyrin Becomes a Catalyst for the Electroreduction of O2by Four Electrons When [(NH3)5Os]n+(n= 2, 3) Groups Are Coordinated to the Porphyrin Ring

The reaction between the water-soluble cobalt porphyrin 1 and [(NH3)5Os(OH2)]2+ produces a multiply-osmiumated product that spontaneously adsorbs on graphite electrodes and acts as a catalyst for the reduction of O2 by four electrons. The nonosmiumated cobalt porphyrin is also adsorbed but catalyzes only the two-electron reduction of O2.

Electrocatalysis of Nitric Oxide Reduction by Water-Soluble Cobalt Porphyrin. Spectral and Electrochemical Studies

Water-soluble cobalt tetrakis(N-methyl-2-pyridy1)porphine (Co(2-TMPyP)) has been investigated for their catalytic behavior toward nitric oxide reduction. The acid dissociation constants (p&) for [CoIn(2-TMPyP)(H2O)2l5+ are 5.5 and 9.6. Nitrite anion coordinates to the Co(II1) center in pH 4-12 buffer solutions. The nitrite complex [CO~~I (~ -TMP~P) (NO~) ]~+ exhibits IR bands at 1401, 1306 and 830 cm-', indicating a cobalt-nitro complex. The 5-coordinated cobaltous porphyrin, [CO~I(~-TMP~P)(H~O)]~+, has been identified by ESR and has pKa = 13.0. This is the first case that the pKa of a Co(I1) porphyrin is presented. Nitric oxide from the disproportionation of nitrite anion in acidic solution can coordinate to the Co(I1) center. Resonance Raman spectrum of [CoI1(2TMPyP)(N0)l4+ exhibits a peak at 1722 cm-', which disappears upon exposure to air. Electrocatalytic reduction of [Con(2-TMPyP)(N0)l4+ at -0.3 V in pH 4.0 buffer solution generates nitrous oxide in a slow rate. Bulk electrolysis was also conducted at -0.7 V, at which [Co1(2-TMPyP)I3+ reacts with nitrite anion and nitric oxide. The reduction products are mainly ammonia and hydroxylamine.

Reactivity toward dioxygen of insoluble cobalt porphyrins dispersed on colloidal silica

The reduction of O 2, by Ru(NH 3) 2+ 6 is catalyzed by the water soluble cobalt porphyrin CoTMPyP (TMPyP = tetrakis(4-N-methylpyridyl porphyrin) and by colloidal dispersions on silica of two insoluble cobalt porphyrins, CoTPP (TPP = tetraphenylporphyrin), and Co 2FTF4, a cofacial dicobalt porphyrin. The kinetic behavior exhibited by the soluble porphyrin and the finely divided colloidal dispersions is similar. Dispersion of the insoluble porphyrins on high area silica colloids effectively “solubilizes” them. Amperometric titration of both the dissolved and the colloidally dispersed cobalt porphyrins before and after they are exposed to O 2 allows the stable products of their reactions with O 2 to be established. The relevance of the results to the mechanisms by which these cobalt porphyrins serve as catalysts for the reduction of O 2 is discussed.

Electrocatalysis of metalloporphyrins: Part 9. Catalytic electroreduction of cystine using water-soluble cobalt porphyrins

The electrocatalytic reduction of cystine with water-soluble cobalt(II) tetrakis(4-trimethylammoniumphenyl)porphyrin (CoTTAPP) and cobalt(II) tetrakis(4-sulphophenyl)porphyrin (CoTSPP) in 0.2 M H 2SO 4 solution was investigated. The experimental results of cyclic voltammetry and the rotating disc electrode technique indicated that both CoTTAPP and CoTSPP obviously exhibit catalytic activity for cystine reduction in 0.2 M H 2SC 4 solution. Cystine reduction at a glassy carbon electrode in 0.2 M H 2SO 4 solution containing CoTTAPP or CoTSPP is an irreversible, simple charge-transfer reaction. The number of electrons involved in the rate-determining step is 1. The rate constants of cystine reduction in these systems were calculated to be 2.9 × 10 −3−7.3 × 10 −3 cm s −1 in the potential range of −0.80 to −0.90 V (vs. SCE). It is demonstrated that Co(II) complexing in the N 4 internal ring of porphyrin plays the main role in the electrocatalytic reduction of cystine. The reaction mechanism of cystine reduction electrocatalysed by CoTTAPP or CoTSPP in 0.2 M H 2SO 4 solution is discussed.

Exothermic isomerization of water-soluble quadricyclanes to norbornadienes by soluble and insoluble catalysts

Water-soluble quadricyclane derivatives (1) were stable in an aqueous sodium carbonate solution, but addition of a catalytic amount of water-soluble cobalt porphyrin complexes (3) induced rapid and quantitative isomerization of (1) to norbornadiene derivatives (2) under release of heat. Insoluble catalysts (4) prepared by adsorption of Co-TPP or Co-Pc on activated carbon were similarly active as the soluble analogues. They were stable enough to keep high activities for recycling use.

Reactions of a water soluble Cobalt Porphyrin with thiocyanate

Rate and equilibrium constants have been determined for the reactions of Cobalt(III)Tetra(N-methyltetrapyridyl)Porphine with thiocyanate as a function of pH. The porphyrin ligand increases the substitution rates at the Co(III) center by several orders of magnitude relative to many other Co(III) complexes. The rates of formation and dissociation of the thiocyanate complexes are also dependent on the nature of the axial ligand trans to the water molecule being removed.