Porphyrin Catalyst Research Articles

CO2 reduction by electropolymerized catalyst of triphenylamine‐substituted iron porphyrin

AbstractA new catalyst, triphenylamine‐substituted iron porphyrin (FeCTP), for CO2 reduction has been synthesized, and its polymer (FeCTP‐film) directly deposited on glassy carbon electrode was made by means of electropolymerization employing cyclic voltammetry technique. This strategy allows us to prepare nondestructive porphyrin catalysts and reduce the amount of the catalyst required for CO2 reduction. The Faradaic efficiencies of CO for FeCTP and FeCTP‐film were estimated to be 48 and 66%, respectively, after the controlled electrolysis for an hour, which were higher than that of FeTPP, 40%.

A PEGylated Tin Porphyrin Complex for Electrocatalytic Proton Reduction: Mechanistic Insights into Main-Group-Element Catalysis.

Electrocatalytic proton reduction to form dihydrogen (H2 ) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main-group-element-based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (-4.6 mA cm-2 at -1.7 V vs. Fc/Fc+ ) and high selectivity (H2 Faradaic efficiency of 94 % at -1.7 V vs. Fc/Fc+ ) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOFmax ) for H2 production was obtained as 1099 s-1 . Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron-chemical-electron-chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

A PEGylated Tin Porphyrin Complex for Electrocatalytic Proton Reduction: Mechanistic Insights into Main‐Group‐Element Catalysis

AbstractElectrocatalytic proton reduction to form dihydrogen (H2) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main‐group‐element‐based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (−4.6 mA cm−2 at −1.7 V vs. Fc/Fc+) and high selectivity (H2 Faradaic efficiency of 94 % at −1.7 V vs. Fc/Fc+) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOFmax) for H2 production was obtained as 1099 s−1. Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron‐chemical‐electron‐chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

(Digital Presentation) OER Catalysts Supported on Layered Zirconium Phosphate Nanomaterials

Learning from nature, instead of using expensive catalysts such as platinum for splitting water, cheaper alternatives (such as cobalt, iron, or nickel) could be used for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Hydrogen solar fuel produced in artificial photosynthesis schemes for water splitting could be used in fuel cells. However, obtaining efficient catalysts based on earth-abundant materials is still difficult, particularly for the OER. Recent theory has shown that nanoscopic confinement of catalysts increases the stability and efficiency of catalysts for OER. We have designed and develop earth-abundant electrocatalysts for oxygen reduction and water splitting using nanostructured layered inorganic materials of zirconium phosphate (ZrP). We have demonstrated improved electrocatalytic activity of ZrP nanomaterials loaded with metal ions suitable for the OER of water splitting. Adsorbing Co or Ni catalysts on the ZrP nanoparticles surface proved to improved OER activity compared to intercalated catalysts. A comparison between adsorbed Co or Ni catalysts and those catalysts on exfoliated ZrP hexagonal nanoplatelets proved that those on exfoliated nanoplatelets were more active, with diminished overpotentials and reduced Tafel plot slopes as well as higher mass activities. More recently, comparison between Co and Ni catalyst on ZrP particles with different morphologies (hexagonal platelets, rods, cubes, and spheres) revealed that the more active Co catalysts are those on hexagonal ZrP platelets, whereas the best Ni catalysts are those on ZrP spheres. Finally, encouraging recent results with a cobalt porphyrin catalyst and bimetal catalysts will be presented, as well as recent XAS operando results. These results demonstrate that ZrP nanoparticles are suitable supports for electrocatalysis of the OER and water splitting.Sponsored by the NSF-PREM Center for Interfacial Electrochemistry for Energy Materials (CIE2M) grant DMR-1827622.

Electrocatalytic oxygen reduction reaction with metalloporphyrins

<p indent="0mm">The development of H₂-O₂ fuel cell technology is an important strategy to solve the current energy problem. The catalytic efficiency of oxygen reduction reaction (ORR) on the cathode of fuel cell is a key factor that affects the overall energy conversion efficiency. Inspired by cytochrome c oxidases (<italic>Cc</italic>Os) in nature, the researchers have studied many metal macrocyclic porphyrin complexes as ORR catalysts which show excellent electrocatalytic activity for ORR. This review addresses the current status of metal porphyrins in ORR, including the effects of <italic>meso</italic>- and β-substituents, axial ligands, second coordination spheres, binuclear metal, different metal centers, and carbon-based materials on the catalytic activity and selectivity of ORR. Furthermore, the future research direction of metal porphyrin catalysts is also predicted.

Templating Bicarbonate in the Second Coordination Sphere Enhances Electrochemical CO2 Reduction Catalyzed by Iron Porphyrins.

Bicarbonate-based electrolytes are ubiquitous in aqueous electrochemical CO2 reduction, particularly in heterogenous catalysis, where they demonstrate improved catalytic performance relative to other buffers. In contrast, the presence of bicarbonate in organic electrolytes and its roles in homogeneous electrocatalysis remain underexplored. Here, we investigate the influence of bicarbonate on iron porphyrin-catalyzed electrochemical CO2 reduction. We show that bicarbonate is a viable proton donor in organic electrolyte (pKa = 20.8 in dimethyl sulfoxide) and that urea pendants in the second coordination sphere can be used to template bicarbonate in the vicinity of a molecular iron porphyrin catalyst. The templated binding of bicarbonate increases its acidity, resulting in a 1500-fold enhancement in catalytic rates relative to unmodified parent iron porphyrin. This work emphasizes the importance of bicarbonate speciation in wet organic electrolytes and establishes second-sphere bicarbonate templating as a design strategy to harness this adventitious acid and enhance CO2 reduction catalysis.

Lowering the Symmetry of Cofacial Porphyrin Prisms for Selective Oxygen Reduction Electrocatalysis.

Cofacial porphyrin catalysts for the Oxygen Reduction Reaction (ORR) formed via coordination-driven self-assembly have so far been limited to designs with fourfold symmetry, where four molecular clips bridge two porphyrin sites. We have synthesized six PynPhm (Py = pyridyl, Ph = phenyl) metalloporphyrin prisms (Co2+, Zn2+) bridged by molecular clips containing two Rh3+ centers. Four of these structures are lower symmetry, with the Py3Ph and Py2Ph2 prisms containing three and two molecular clips, respectively. The Co2+ species were evaluated for their ORR activity. Cyclic and hydrodynamic voltammetry studies of heterogeneous catalyst inks in aqueous media revealed marked differences in selectivity from ∼5% (Py3Ph) to ∼37% (Py2Ph2) for the formation of H2O2. The single-crystal X-ray structure of the Zn2 Py2Ph2 prism shows an offset between the porphyrin faces. This structural feature may be responsible for the change in selectivity, consistent with previous studies of covalently tethered cofacial porphyrins that have shown that geometry is a critical determinant of two-electron/two-proton versus four-electron/four-proton pathways. Extraction of standard rate constants ks for the ORR revealed a cofacial enhancement of ∼2 orders of magnitude over mononuclear Co2+ tetrapyridyl porphyrin. Even though all the prisms described here use the same molecular clip, the resultant structures, and thus the reactivity for the ORR, differ significantly based on the number and orientation of pyridyl donor groups on the porphyrins, highlighting how coordination-driven self-assembly can be used to rapidly tune dinuclear catalysts.

Mechanistic Studies on the Epoxidation of Alkenes by Macrocyclic Manganese Porphyrin Catalysts.

Macrocyclic metal porphyrin complexes can act as shape‐selective catalysts mimicking the action of enzymes. To achieve enzyme‐like reactivity, a mechanistic understanding of the reaction at the molecular level is needed. We report a mechanistic study of alkene epoxidation by the oxidant iodosylbenzene, mediated by an achiral and a chiral manganese(V)oxo porphyrin cage complex. Both complexes convert a great variety of alkenes into epoxides in yields varying between 20–88 %. We monitored the process of the formation of the manganese(V)oxo complexes by oxygen transfer from iodosylbenzene to manganese(III) complexes and their reactivity by ion mobility mass spectrometry. The results show that in the case of the achiral cage complex the initial iodosylbenzene adduct is formed on the inside of the cage and in the case of the chiral one on the outside of the cage. Its decomposition leads to a manganese complex with the oxo ligand on either the inside or outside of the cage. These experimental results are confirmed by DFT calculations. The oxo ligand on the outside of the cage reacts faster with a substrate molecule than the oxo ligand on the inside. The results indicate how the catalytic activity of the macrocyclic porphyrin complex can be tuned and explain why the chiral porphyrin complex does not catalyze the enantioselective epoxidation of alkenes.

Self-Assembled Liposomes Enhance Electron Transfer for Efficient Photocatalytic CO2 Reduction.

Light-driven conversion of CO2 to chemicals provides a sustainable alternative to fossil fuels, but homogeneous systems are typically limited by cross reactivity between different redox half reactions and inefficient charge separation. Herein, we present the bioinspired development of amphiphilic photosensitizer and catalyst pairs that self-assemble in lipid membranes to overcome some of these limitations and enable photocatalytic CO2 reduction in liposomes using precious metal-free catalysts. Using sodium ascorbate as a sacrificial electron source, a membrane-anchored alkylated cobalt porphyrin demonstrates higher catalytic CO production (1456 vs 312 turnovers) and selectivity (77 vs 11%) compared to its water-soluble nonalkylated counterpart. Time-resolved and steady-state spectroscopy revealed that self-assembly facilitates this performance enhancement by enabling a charge-separation state lifetime increase of up to two orders of magnitude in the dye while allowing for a ninefold faster electron transfer to the catalyst. Spectroelectrochemistry and density functional theory calculations of the alkylated Co porphyrin catalyst support a four-electron-charging mechanism that activates the catalyst prior to catalysis, together with key catalytic intermediates. Our molecular liposome system therefore benefits from membrane immobilization and provides a versatile and efficient platform for photocatalysis.

Unity Makes Strength: Constructing Polymeric Catalyst for Selective Synthesis of CO 2 /Epoxide Copolymer

Unity Makes Strength: Constructing Polymeric Catalyst for Selective Synthesis of CO ₂ /Epoxide Copolymer

Phenoxazine‐Sensitized CO2‐to‐CO Reduction with an Iron Porphyrin Catalyst: A Redox Properties‐Catalytic Performance Study

AbstractWe have evaluated six phenoxazine derivatives as visible light photosensitizers for the photochemical reduction of CO2 to CO with an iron porphyrin catalyst in organic media. The phenoxazine core was functionalized with electron‐donating or ‐withdrawing groups to modify the photophysical properties. Both singlet and triplet excited state potentials of the sensitizers spanned several hundred mV range and the ground state oxidation potentials spanned 250 mV. We observed that no correlation can be established between the production of CO and the excited state potential of the phenoxazine, which determines the driving force for electron transfer to the catalyst. On the contrary, a clear correlation can be made between the oxidation potentials of the phenoxazine and the production of CO. This observation indicates that the process was limited by photosensitizer regeneration and highlights the fact that electron transfers not directly related to the catalyst activation could play a key role in homogeneous photocatalytic systems.

Dissection of Light-Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction.

Iron porphyrins are among the best molecular catalysts for the electrocatalytic CO2 reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO2 substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h-1 , respectively. While the Fe0 redox state has been widely accepted as the catalytically active species, we show here that the FeI species is already involved in the CO2 activation, which represents the rate-determining step in the photocatalytic cycle. The urea functions help to dock the CO2 upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO2 .

Dissection of Light‐Induced Charge Accumulation at a Highly Active Iron Porphyrin: Insights in the Photocatalytic CO2 Reduction

AbstractIron porphyrins are among the best molecular catalysts for the electrocatalytic CO2 reduction reaction. Powering these catalysts with the help of photosensitizers comes along with a couple of unsolved challenges that need to be addressed with much vigor. We have designed an iron porphyrin catalyst decorated with urea functions (UrFe) acting as a multipoint hydrogen bonding scaffold towards the CO2 substrate. We found a spectacular photocatalytic activity reaching unreported TONs and TOFs as high as 7270 and 3720 h−1, respectively. While the Fe0 redox state has been widely accepted as the catalytically active species, we show here that the FeI species is already involved in the CO2 activation, which represents the rate‐determining step in the photocatalytic cycle. The urea functions help to dock the CO2 upon photocatalysis. DFT calculations bring support to our experimental findings that constitute a new paradigm in the catalytic reduction of CO2.

An iron-porphyrin grafted metal–organic framework as a heterogeneous catalyst for the photochemical reduction of CO2

Photocatalytic reduction of carbon dioxide (CO2) into value-added carbon products is a promising strategy for dealing with carbon emission. Selected metal–organic frameworks (MOFs) have been shown promise in photocatalytic CO2 reduction owing to their photochemical activity and structural modularity. Here, we take advantage of displaceable nonstructural ligands on the nodes of a pyrene-based MOF, NU-1000, to graft a molecular catalyst, tetra(4-carboxyphenyl) porphyrin iron (III) chloride (Fe-TCPP). The pyrene linkers in the MOF absorb photons and transfer electrons to the porphyrin catalyst, resulting in a self-contained photocatalyst. The obtained Fe-TCPP@NU-1000 heterogeneous catalysts exhibit activity for CO2 photoreduction under illumination at 390 nm, with a TON for CO of 22 in 2 h. Molecular hydrogen is co-evolved. Leaching of the grafted catalyst from the MOF ultimately limits performance.

Facile Aluminum Porphyrin Complexes Enable Flexible Terminal Epoxides to Boost Properties of CO2‐Polycarbonate

AbstractIncorporation of flexible monomer is regarded as a valid way to the chemical modification of the highly brittle poly(cyclohexene carbonate) (PCHC), which derived from the copolymerization of CO2 and cyclohexene oxide (CHO). However, toxic metal catalyst such as cobalt dominates these terpolymerizations which against the green development philosophy. Here, a series of environmentally benign aluminum porphyrin catalysts with different steric and electronic effects for the terpolymerization of CO2, CHO, and terminal epoxides with various flexible pendant alkyl groups to adjust the properties of PCHC is reported. The resulting terpolymers display a random distribution of cyclohexene carbonate along the backbone with a regulated terminal epoxide insertion from 30% to 70%. Moreover, the mechanical properties can be well regulated in a wide range indicating the feasibility of the present approach. The present study provides a green way to chemical modification of rigid PCHC.

Imbroglio at a photoredox-iron-porphyrin catalyst dyad for the photocatalytic CO 2 reduction

We have covalently connected the ruthenium trisbipyridine complex (bpyRu) as a photoredox module to an iron porphyrin catalyst (porFe) through an amido function for investigating the synergistic action to power the photocatalytic CO2 reduction. The electrochemical studies of the porFe-bpyRu dyad did not show any marked effect on the redox properties of the constitutive units. However, the photophysical properties of the porFe-bpyRu dyad point to the complete extinction of the photoredox module that undergoes ultrafast quenching processes with the porFe acolyte, the unavoidable dilemma in this type of molecular assemblies. Nevertheless, when exogenous bpyRu and a sacrificial electron donor were added to this dyad, we found that it exhibits much higher turnover number and selectivity towards CO2 photocatalytic reduction to CO than with the iron porphyrin analogue (porFe). Comprehensive analyses of the data suggest that this catalytic enhancement displayed by the dyad can be attributed to an interesting electron relay role played by the appended bpyRu moiety.

A biomimetic SH2 cross-coupling mechanism for quaternary sp3-carbon formation.

Bimolecular homolytic substitution (SH2) is an open-shell mechanism that is implicated across a host of biochemical alkylation pathways. Surprisingly, however, this radical substitution manifold has not been generally deployed as a design element in synthetic C–C bond formation. We found that the SH2 mechanism can be leveraged to enable a biomimetic sp3-sp3 cross-coupling platform that furnishes quaternary sp3-carbon centers, a long-standing challenge in organic molecule construction. This heteroselective radical-radical coupling uses the capacity of iron porphyrin to readily distinguish between the SH2 bond-forming roles of open-shell primary and tertiary carbons, combined with photocatalysis to generate both radical classes simultaneously from widely abundant functional groups. Mechanistic studies confirm the intermediacy of a primary alkyl–Fe(III) species prior to coupling and provide evidence for the SH2 displacement pathway in the critical quaternary sp3-carbon bond formation step.

Mechanistic and selectivity investigations into Fe-catalyzed 2, 3-disubstituted azaindole formation from β,β-disubstituted tetrazole

Computational studies at the B3LYP-D3(BJ) level were performed to explore the mechanism and migratorial selectivity of Fe-catalyzed 2,3-disubstituted azaindole formation from β, β-disubstituted tetrazole and mechanistic details of key steps in this reaction are compared to those in Rh2-catalyzed indole formation. The calculated results show: Fe-catalyzed spirocyclization proceeds via a radical pathway, which is contrary to Rh2-catalyzed spirocyclization that occurs via a carbocation pathway; the migration of C→C significantly prefers to that of C→N due to far lower breakage extents of Fe-N and CC bonds. In addition, the comparisons of uncatalyzed vs. catalyzed N2 extrusion and C→C migration show that iron porphyrin catalyst can lower activation energies of these two steps.

Catalytic Synthesis of 1H-2-Benzoxocins: Cobalt(III)-Carbene Radical Approach to 8-Membered Heterocyclic Enol Ethers.

The metallo-radical activation of ortho-allylcarbonyl-aryl N-arylsulfonylhydrazones with the paramagnetic cobalt(II) porphyrin catalyst [CoII(TPP)] (TPP = tetraphenylporphyrin) provides an efficient and powerful method for the synthesis of novel 8-membered heterocyclic enol ethers. The synthetic protocol is versatile and practical and enables the synthesis of a wide range of unique 1H-2-benzoxocins in high yields. The catalytic cyclization reactions proceed with excellent chemoselectivities, have a high functional group tolerance, and provide several opportunities for the synthesis of new bioactive compounds. The reactions are shown to proceed via cobalt(III)-carbene radical intermediates, which are involved in intramolecular hydrogen transfer (HAT) from the allylic position to the carbene radical, followed by a near-barrierless radical rebound step in the coordination sphere of cobalt. The proposed mechanism is supported by experimental observations, density functional theory (DFT) calculations, and spin trapping experiments.

Aerobic Direct Dioxygenation of Terminal/Internal Alkynes to α-Hydroxyketones by an Fe Porphyrin Catalyst.

We herein report a new synthetic method for the preparation of α-hydroxyketones by the dioxygenation of alkynes. The reaction proceeds at room temperature under the action of Fe porphyrin and pinacolborane under air as a green oxidant to produce α-hydroxyketones. The mild reaction conditions allow chemoselective oxidation with functional group tolerance. Terminal alkynes in addition to internal alkynes are applicable, affording unsymmetrical α-hydroxyketones that are difficult to obtain by any reported dioxygenation of unsaturated C-C bonds.