Porphyrin Catalyst Research Articles

Iron Strapped Porphyrins on Carbon Nanotubes As Hybrid Materials for the ORR Reaction

One of the most important challenge of nanosciences is the fabrication of functional hybrid materials, preserving and combining the properties of their building blocks. Among the different classes of nanomaterials, carbon nanotubes are promising for electronics, opto-electronics, catalysis and composite applications. Within the context of sustainable development and renewable energy, we and others have been envisioning the use of functionalized or doped carbon nanotubes in electrocatalytic systems.1,2 In such systems, catalytic sites need to be supported on conducting materials. Carbon nanotubes, thanks to their electrical conductivity and their high surface, area appear to be the ideal material for that purpose.In actual Proton Exchange Membrane Fuel Cell Devices (PEMFC), the reactions at the electrodes and in particular the reduction of oxygen are performed by platinum nanoparticles. The cost of platinum is an incentive to developing new catalysts-based on non-noble metals. Thus, whereas in nature oxygen reduction is performed by iron porphyrins in the active center of enzymes, bio-inspired catalysts based on cobalt or iron macrocycles have been extensively studied for Oxygen Reduction Reaction (ORR).3,4 Here, we describe the synergic effect on catalytic activity of carbon nanotubes and strapped iron porphyrin hybrids for ORR. In particular, we demonstrate that combining both MWNTs and porphyrins leads to better catalytic activity compared to isolated nanotubes or porphyrins on their own. This study hence highlights the importance of carbon support for the catalysis: nanotubes ensure the availability of electrons to porphyrin catalysts and allow ORR to occur via the 4-electron pathway, avoiding the production of hydrogen peroxide.For this study, two series of iron-strapped porphyrins have been explored.5-6 In the first one, the porphyrins were adsorbed at the surface on nanotubes whereas, for the second one, porphyrins were synthesized with propargylic ether groups at their periphery, allowing covalent linkage by Hay coupling at the surface of the nanotubes. Each of the nanotube hybrids was characterized and tested for ORR in a series of electrochemical measurements.

Catalytic Synthesis of 8-Membered Ring Compounds via Cobalt(III)-Carbene Radicals.

The metalloradical activation of o‐aryl aldehydes with tosylhydrazide and a cobalt(II) porphyrin catalyst produces cobalt(III)‐carbene radical intermediates, providing a new and powerful strategy for the synthesis of medium‐sized ring structures. Herein we make use of the intrinsic radical‐type reactivity of cobalt(III)‐carbene radical intermediates in the [CoII(TPP)]‐catalyzed (TPP=tetraphenylporphyrin) synthesis of two types of 8‐membered ring compounds; novel dibenzocyclooctenes and unprecedented monobenzocyclooctadienes. The method was successfully applied to afford a variety of 8‐membered ring compounds in good yields and with excellent substituent tolerance. Density functional theory (DFT) calculations and experimental results suggest that the reactions proceed via hydrogen atom transfer from the bis‐allylic/benzallylic C−H bond to the carbene radical, followed by two divergent processes for ring‐closure to the two different types of 8‐membered ring products. While the dibenzocyclooctenes are most likely formed by dissociation of o‐quinodimethanes (o‐QDMs) which undergo a non‐catalyzed 8π‐cyclization, DFT calculations suggest that ring‐closure to the monobenzocyclooctadienes involves a radical‐rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring‐closure reactions to chiral monobenzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt‐porphyrin catalyst.

Catalytic Synthesis of 8‐Membered Ring Compounds via Cobalt(III)‐Carbene Radicals

AbstractThe metalloradical activation of o‐aryl aldehydes with tosylhydrazide and a cobalt(II) porphyrin catalyst produces cobalt(III)‐carbene radical intermediates, providing a new and powerful strategy for the synthesis of medium‐sized ring structures. Herein we make use of the intrinsic radical‐type reactivity of cobalt(III)‐carbene radical intermediates in the [CoII(TPP)]‐catalyzed (TPP=tetraphenylporphyrin) synthesis of two types of 8‐membered ring compounds; novel dibenzocyclooctenes and unprecedented monobenzocyclooctadienes. The method was successfully applied to afford a variety of 8‐membered ring compounds in good yields and with excellent substituent tolerance. Density functional theory (DFT) calculations and experimental results suggest that the reactions proceed via hydrogen atom transfer from the bis‐allylic/benzallylic C−H bond to the carbene radical, followed by two divergent processes for ring‐closure to the two different types of 8‐membered ring products. While the dibenzocyclooctenes are most likely formed by dissociation of o‐quinodimethanes (o‐QDMs) which undergo a non‐catalyzed 8π‐cyclization, DFT calculations suggest that ring‐closure to the monobenzocyclooctadienes involves a radical‐rebound step in the coordination sphere of cobalt. The latter mechanism implies that unprecedented enantioselective ring‐closure reactions to chiral monobenzocyclooctadienes should be possible, as was confirmed for reactions mediated by a chiral cobalt‐porphyrin catalyst.

Electrochemical properties and biomimetic activity of water-soluble meso-substituted Mn(III) porphyrin complexes in the electrocatalytic reduction of hydrogen peroxide

The aim of this study was to investigate biomimetic activity of water-soluble manganese porphyrin complexes with a series of meso-substituents of the porphyrin macrocycle in the electrocatalytic reduction of hydrogen peroxide in aqueous solutions and to obtain information on possible intermediates, processes, and mechanisms. Mn porphyrins were compared in the process of the electrocatalytic reduction of hydrogen peroxide at pH 4, 7.4, and 10 in the deoxygenated solutions and in the presence of oxygen. The highest sensitivity, defined as the reduction current increase in relation to the concentration of hydrogen peroxide, was found in the case of Mn(III) meso-tetra(N-methyl-4-pyridyl) porphyrin, MnTMPyP, in alkaline 2.9·10−2 A M−1 and acidic 1.6·10−2 A M−1 solutions in the presence of oxygen. The reduction currents at pH 7.4, 10, and 4 in the presence of H2O2 were about 4, 7, and 12 times higher, respectively, in the solutions with the MnTMPyP complex than those at a GCE without a porphyrin complex in the solution. The electrocatalytic reduction of hydrogen peroxide occurs in parallel with an oxidative degradation of the porphyrin catalyst depending on the conditions of the experiment and was most significant in the presence of oxygen. The effect of the functional substituents at the meso-positions of a porphyrin ligand on the electrocatalytic activity of the water-soluble Mn(III) porphyrins complexes is discussed and reaction mechanisms are proposed.

Selectivity-Determining Steps in O2 Reduction Catalyzed by Iron(tetramesitylporphyrin).

The oxygen reduction reaction (ORR) is the cathode reaction in fuel cells and its selectivity for water over hydrogen peroxide production is important for these technologies. Iron porphyrin catalysts have long been studied for the ORR, but the origins of their selectivity are not well understood because the selectivity-determining step(s) usually occur after the rate-determining step. We report here the effects of acid concentration, as well as other solution conditions such as acid pKa, on the H2O2/H2O selectivity in electrocatalytic ORR by iron(tetramesitylporphyrin) (Fe(TMP)) in DMF. The results show that selectivity reflects a kinetic competition in which the dependence on [HX] is one order greater for the production of H2O than H2O2. Based on such experimental results and computational studies, we propose that the selectivity is governed by competition between protonation of the hydroperoxo intermediate, FeIII(TMP)(OOH), to produce water versus dissociation of the HOO- ligand to yield H2O2. The data rule out a bifurcation based on the regioselectivity of protonation of the hydroperoxide, as suggested in the enzymatic systems. Furthermore, the analysis developed in this report should be generally valuable to the study of selectivity in other multi-proton/multi-electron electrocatalytic reactions.

Post-functionalization of dibenzothiophene to functionalized biphenyls via a photoinduced thia-Baeyer-Villiger oxidation

The Baeyer-Villiger reaction is used extensively in organic chemistry. Sila- and bora-variants have also been documented widely, with these processes underpinning, for example, the Fleming-Tamao oxidation and hydroborative alkene hydration, respectively. By contrast, the development of thia-Baeyer-Villiger reactions involving sulfoxides has long been considered unlikely because competitive oxidation to the sulfone occurs exclusively. Here, we disclose a photoinduced thia-Baeyer-Villiger-type oxidations; specifically, we find that exposure of dibenzothiophene (DBT) derivatives to an iron porphyrin catalyst under Ultraviolet irradiation in the presence of t-BuOOH generates sulfinic esters in up to 87% yield. The produced sulfinic esters are transformed to a variety of biphenyl substrates including biphenyl sulfoxides, sulfones and sulfonamides in 1-2 steps. These results provide a mild process for the selective functionalization of sulfur compounds, and offer a biomimetic approach to convert DBT into 2-hydroxybiphenyl under controllable stepwise pathway. Based upon experimental evidences and DFT calculation, a mechanism is proposed.

Thermally Gated Bio-orthogonal Nanozymes with Supramolecularly Confined Porphyrin Catalysts for Antimicrobial Uses

Summary Bio-orthogonal catalysis has the capability of localized generation of imaging and therapeutic molecules in vitro and in vivo. The integration of these catalysts into thermoresponsive nanoparticle platforms would generate bio-orthogonal “nanozymes” that could be controlled through endogenous or exogenous thermal control. We have fabricated thermoresponsive nanozymes by confining supramolecular assemblies of porphyrins into the monolayer of gold nanoparticles. The resulting nanodevices feature an on-off gated thermal response occurring over a 3°C range with commensurate tunability of activation temperature from 25°C to 37°C. Reversible activation of catalysis was demonstrated in complex biological environments, and the efficacy of bi-stable thermoresponsive nanozymes demonstrated through thermal activation of antibiotic-based prodrugs to effectively treat bacterial biofilms.

Understanding the Origin of Highly Selective CO2 Electroreduction to CO on Ni,N‐doped Carbon Catalysts

AbstractNi,N‐doped carbon catalysts have shown promising catalytic performance for CO2 electroreduction (CO2R) to CO; this activity has often been attributed to the presence of nitrogen‐coordinated, single Ni atom active sites. However, experimentally confirming Ni−N bonding and correlating CO2 reduction (CO2R) activity to these species has remained a fundamental challenge. We synthesized polyacrylonitrile‐derived Ni,N‐doped carbon electrocatalysts (Ni‐PACN) with a range of pyrolysis temperatures and Ni loadings and correlated their electrochemical activity with extensive physiochemical characterization to rigorously address the origin of activity in these materials. We found that the CO2R to CO partial current density increased with increased Ni content before plateauing at 2 wt % which suggests a dispersed Ni active site. These dispersed active sites were investigated by hard and soft X‐ray spectroscopy, which revealed that pyrrolic nitrogen ligands selectively bind Ni atoms in a distorted square‐planar geometry that strongly resembles the active sites of molecular metal–porphyrin catalysts.

From 3D to 4D printing: a reactor for photochemical experiments using hybrid polyurethane acrylates for vat-based polymerization and surface functionalization.

The vat-based 3D printing of a chemical reactor with flow-meter geometry from an isocyanate-functionalized acrylate monomer followed by post-processing with amino-functionalized photocatalysts is described. This approach results in solvent- and air-stable flow photochemical reactors with UV-A transparent windows that can be applied for photooxygenation and photo redox catalysis.

Highly ordered mesoporous carbon/iron porphyrin nanoreactor for the electrochemical reduction of CO2

A highly ordered mesoporous carbon having a large surface area is utilized as a conductive substrate to immobilize iron porphyrin catalysts for electrochemical CO2 reduction, resulting in the selective conversion of aqueous CO2 into CO with 92.1% faradaic efficiency.

Enantioselective oxygenation of exocyclic methylene groups by a manganese porphyrin catalyst with a chiral recognition site.

The natural enzyme cytochrome P450 is widely recognised for its unique ability to catalyse highly selective oxygen insertion reactions into unactivated C–H bonds under mild conditions. Its exceptional potential for organic synthesis served as an inspiration for the presented biomimetic hydroxylation approach. Via a remote hydrogen bonding motif a high enantioselectivity in the manganese-catalysed oxygenation of quinolone analogues (27 examples, 18–64% yield, 80–99% ee) was achieved. The site-selectivity was completely altered in favour of a less reactive but more accessible position.

Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite-BiVO4 tandems.

The photoelectrochemical (PEC) production of syngas from water and CO2 represents an attractive technology towards a circular carbon economy. However, the high overpotential, low selectivity and cost of commonly employed catalysts pose challenges for this sustainable energy-conversion process. Here we demonstrate highly tunable PEC syngas production by integrating a cobalt porphyrin catalyst immobilized on carbon nanotubes with triple-cation mixed halide perovskite and BiVO4 photoabsorbers. Empirical data analysis is used to clarify the optimal electrode selectivity at low catalyst loadings. The perovskite photocathodes maintain selective aqueous CO2 reduction for one day at light intensities as low as 0.1 sun, which provides pathways to maximize daylight utilization by operating even under low solar irradiance. Under 1 sun irradiation, the perovskite-BiVO4 PEC tandems sustain bias-free syngas production coupled to water oxidation for three days. The devices present solar-to-H2 and solar-to-CO conversion efficiencies of 0.06 and 0.02%, respectively, and are able to operate as standalone artificial leaves in neutral pH solution.

A photocatalytic comparison study between tin complex and carboxylic acid derivatives of porphyrin/TiO2 composites

We synthesized a novel dihydroxotin(IV) porphyrin complex with full structural confirmation. The tin complex further converted to TiO2 composite (PR-2–TiO2) and photodegradation of methylene blue property compared with carboxylic-anchored porphyrin photocatalyst (PR-3–TiO2), basic porphyrin photocatalyst (PR-1–TiO2) and bare TiO2. PR-1–TiO2 showed slight photocatalytic activity, which was better than that of bare TiO2. The photocatalytic performances of tin-porphyrin (PR-2–TiO2) and carboxylic anchor porphyrin (PR-3–TiO2) were comparable, and tin-porphyrin catalyst showed similar stability in their reusability test result in comparison with carboxylic-anchored porphyrin catalyst. Finally, we showed a plausible structure for the tin porphyrin complex between the axial –OH ligand and TiO2 and a suitable mechanism with theoretical comparison for the photocatalytic process.

Influence of Mesityl and Thiophene Peripheral Substituents on Surface Attachment, Redox Chemistry, and ORR Activity of Molecular Iron Porphyrin Catalysts on Electrodes.

Two iron porphyrin complexes with either mesityl (FeTMP) or thiophene (FeT3ThP) peripheral substituents were attached to basal pyrolytic graphite and Ag electrodes via different immobilization methods. By combining cyclic voltammetry and in-operando surface-enhanced Raman spectroscopy along with MD simulations and DFT calculations, their respective surface attachment, redox chemistry and activity toward electrocatalytic oxygen reduction was investigated. For both porphyrin complexes, it could be shown that catalytic activity is restricted to the first (few) molecular layer(s), although electrodes covered with thiophene-substituted complexes showed a better capability to consume the oxygen at a given overpotential even in thicker films. The spectroscopic data and simulations suggest that both porphyrin complexes attach to a Ag electrode surface in a way that maximum planarity and minimum distance between the catalytic iron site and the electrode is achieved. However, due to the distinctive design of the FeT3ThP complex, the thiophene rings are capable of occupying a conformation, via rotation around the bonding axis to the porphyrin, in which all four sulfur atoms can coordinate to the Ag surface. This effect creates a dense and planar surface coverage of the porphyrin on the electrode facilitating a fast (multi) electron transfer via several covalent Ag-S bonds. In contrast, bulky mesityl groups as peripheral substituents, which have been initially introduced to prevent aggregation and improve catalytic behavior in solution, exert a negative effect on the overall electrocatalytic performance in the immobilized state as a less dense coverageand less stable interactions with the surface are formed. Our results underline the importance of rationally designed heterogenized molecular catalysts to achieve optimal turnover, which not onlystrictly applies to the here discussed oxygen reduction reaction but eventually holds also true for other energy conversion reactions such as carbon dioxide reduction.

Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O2 Reduction with a Mononuclear Cobalt Porphyrin Catalyst.

The selective reduction of O2, typically with the goal of forming H2O, represents a long-standing challenge in the field of catalysis. Macrocyclic transition-metal complexes, and cobalt porphyrins in particular, have been the focus of extensive study as catalysts for this reaction. Here, we show that the mononuclear Co-tetraarylporphyrin complex, Co(porOMe) (porOMe = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e–/2H+ or 4e–/4H+ reduction of O2 with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O2 reduction to H2O2 or H2O in DMF are determined and exhibit a Nernstian dependence on the acid pKa, while the CoIII/II redox potential is independent of the acid pKa. The reaction product, H2O or H2O2, is defined by the relationship between the thermodynamic potential for O2 reduction to H2O2 and the CoIII/II redox potential: selective H2O2 formation is observed when the CoIII/II potential is below the O2/H2O2 potential, while H2O formation is observed when the CoIII/II potential is above the O2/H2O2 potential. Mechanistic studies reveal that the reactions generating H2O2 and H2O exhibit different rate laws and catalyst resting states, and these differences are manifested as different slopes in linear free energy correlations between the log(rate) versus pKa and log(rate) versus effective overpotential for the reactions. This work shows how scaling relationships may be used to control product selectivity, and it provides a mechanistic basis for the pursuit of molecular catalysts that achieve low overpotential reduction of O2 to H2O.

Non-covalent conjugation of sulfonated porphyrins to polyethylenimine-grafted multiwalled carbon nanotubes as efficient recyclable heterogeneous catalysts for dihydroxynaphthalenes photooxidation

Biomimetic heterogeneous porphyrin catalysts were prepared by inducing the multiwalled carbon nanotubes (MWCNT) and polyethylenimine (PEI). The MWCNTs have been grafted with PEI for further non-covalent conjugation to water-soluble 5, 10, 15, 20-tetrakis (4-sulfonatophenyl) porphyrins (TPPS). These novel MWCNT-PEI-TPPS composites can be easily and efficiently prepared and characterized by SEM, TEM, FT-IR, UV–vis, fluorescence spectrum and TGA. Both PEI and TPPS with grafting amounts 9.3% and 4% respectively have been successfully supported on nanotubes and the MWCNT-PEI-TPPS composites have indeed been obtained. They have been used as heterogeneous photocatalysts for the oxidation of 1,5- and 2,3-dihydroxynaphthalene, using environment-friendly H2O2 as oxidant. Conversions of the two dihydroxynaphthalenes substrates reached respectively 88 and 86% within 120 min. Through centrifugation, the catalysts could be conveniently separated and reused over several times without significant loss of activity. As a result, this study proves that these MWCNT-PEI-TPPS composites can be considered as efficient, robust and recyclable catalysts for photooxidation reactions.

Electron Transfer Control of Reductase versus Monooxygenase: Catalytic C-H Bond Hydroxylation and Alkene Epoxidation by Molecular Oxygen.

Catalytic oxidation of organic substrates, using a green oxidant like O2, has been a long-term goal of the scientific community. In nature, these oxidations are performed by metalloenzymes that generate highly oxidizing species from O2, which, in turn, can oxidize very stable organic substrates, e.g., mono-/dioxygenases. The same oxidants are produced during O2 reduction/respiration in the mitochondria but are reduced by electron transfer, i.e., reductases. Iron porphyrin mimics of the active site of cytochrome P450 (Cyt P450) are created atop a self-assembled monolayer covered electrode. The rate of electron transfer from the electrode to the iron porphyrin site is attenuated to derive monooxygenase reactivity from these constructs that otherwise show O2 reductase activity. Catalytic hydroxylation of strong C–H bonds to alcohol and epoxidation of alkenes, using molecular O2 (with 18O2 incorporation), is demonstrated with turnover numbers >104. Uniquely, one of the two iron porphyrin catalysts used shows preferential oxidation of 2° C–H bonds of cycloalkanes to alcohols over 3° C-H bonds without overoxidation to ketones. Mechanistic investigations with labeled substrates indicate that a compound I (FeIV=O bound to a porphyrin cation radical) analogue, formed during O2 reduction, is the primary oxidant. The selectivity is determined by the shape of the distal pocket of the catalyst, which, in turn, is determined by the substituents on the periphery of the porphyrin macrocycle.

Reduction of CO2 to CO by an Iron Porphyrin Catalyst in the Presence of Oxygen

Reduction of CO2 to value-added chemicals is a logical way of fixing the rising levels of CO2. Activation and reduction of CO2 requires low-valent transition metals as catalysts. A major challenge in this chemistry is sensitivity of these low-valent metal sites to more abundant O2. Since O2 is a stronger oxidant than CO2 and isolated from the obvious competitive inhibition of CO2, partial reduction of O2 leads to formation of reactive oxygen species like O2– and H2O2, which are deleterious to the catalyst itself. An iron porphyrin complex appended with four ferrocene groups in its distal site is demonstrated to reduce CO2 unabated in the presence of O2 as it can reduce O2 to benign H2O under the same conditions. Further investigations reveal that iron porphyrins, in general, reduce CO2 selectively in the presence of O2. The aforementioned selectivity is derived from a 500 times faster rate of reaction of CO2 with Fe(0) porphyrin relative to O2 despite a higher driving force for the latter.

Aerobic Epoxidation of Low-Molecular-Weight and Polymeric Olefins by a Supramolecular Manganese Porphyrin Catalyst

We report on the highly efficient epoxidation of low-molecular-weight and polymeric olefins catalyzed by a supramolecular manganese porphyrin complex using molecular oxygen as an oxidant and an aldehyde as a co-reductant. At ambient temperature and under optimized reaction conditions, the catalyst showed high activity and stereoselectivity. The efficiency of the supramolecular manganese porphyrin was higher than that of a reference porphyrin catalyst, possibly because it was more stable under the applied reaction conditions. Mechanistic studies suggest that a manganese oxo porphyrin complex may be an intermediate in the epoxidation reaction.

Endings and New Beginnings

Endings and New Beginnings