Photophysical and Electrochemical Properties of Pyrimidopteridine‐Based Organic Photoredox Catalysts

Herein we describe the synthesis and photophysical and electrochemical characterization of pyrimidopteridine-based photoredox catalysts. The pyrimidopteridines can be obtained from the corresponding N-oxides through photo-mediated oxygen atom transfer to a sacrificial acceptor molecule on gramscale. The presence of a triplet excited state was evidenced by means of transient absorption spectroscopy. Pyrimidopteridines are potent excited state oxidants with excited state reduction potentials exceeding +2.10 V vs. SCE in MeCN. The catalytic activity is illustrated in the photo-mediated oxidative annulation of 2-phenylbenzoic acid

Effects of Imidazole-Type Ligands in Cu^I/TEMPO-Mediated Aerobic Alcohol Oxidation

Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)­Cu^I(IM)/TEMPO catalyst (IM represents differently substituted imidazoles) has been studied by simultaneous operando electron paramagnetic resonance/UV–vis/attentuated total reflectance infrared spectroscopy in combination with cyclic voltammetry to explore the particular role of imidazole in terms of ligand and/or base as well as of its substitution pattern on the catalytic performance. For molar ratios of IM/Cu ≥ 2, a (bpy)­Cu^I/II(IM)_a(IM)_b complex is formed, in which the Cu–N distances and/or angles for the two IM ligands a and b are different. The coordination of a second IM molecule boosts the oxidation of Cu^I to Cu^II and, thus, helps to activate O₂ by electron transfer from Cu^I to O₂. The rates of Cu^I oxidation and Cu^II reduction and, thus, the rates of benzaldehyde formation depend on R of the R–N moiety in the IM ligand. Oxidation is fastest for R = H and alkyl, while reduction is slowest for R = H. The Cu^I/Cu^II interplay leads to decreasing total benzaldehyde formation rates in the order R (I+ effect) > R (conjugated system) > R = H

Direct electrochemistry of horseradish peroxidase immobilized in a chitosan–[C4mim][BF4] film: Determination of electrode kinetic parameters

The direct electrochemistry of a HRP–chi–[C4mim][BF4] film (where HRP = horseradish peroxidase, chi = chitosan, and [C4mim][BF4] = the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium tetrafluoroborate) has been studied by cyclic voltammetry on a glassy carbon electrode. The mechanism for the electrochemical reaction of HRP is suggested to be EC for the reduction, and CE for the following re-oxidation, as the oxidative peak potential remained approximately unchanged across the scan rate range. The half wave potential of HRP reduction was found to be pH dependent, suggesting that a concomitant proton and electron transfer is occurring. Using theoretical simulations of the experimentally obtained peak positions, the standard electron transfer rate constant, k0, was found to be 98 (± 16) s- 1 at 295 K in pH 7 phosphate buffer solution, which is very close to the value reported in the absence of ionic liquid. This suggests that the ionic liquid used here in the HRP–chi–[C4mim][BF4]/GC electrode does not enhance the rate of electron transfer. k0 was found to increase systematically with increasing temperature and followed a linear Arrhenius relation, giving an activation energy of 14.20 kJ mol- 1. The electrode kinetics and activation energies obtained are identical to those reported for HRP films in aqueous media. This leads us to question if the use of RTIL films provide any unique benefits for enzyme/protein voltammetry. Rather the films may likely contain aqueous zones in which the enzymes are located and undergo electron transfer

Highly Scalable Conversion of Blood Protoporphyrin to Efficient Electrocatalyst for CO2_2 ‐to‐CO Conversion

Electrochemical CO$_2$ reduction to valuable chemicals represents a green and sustainable approach to close the anthropogenic carbon cycle, but has been impeded by low efficiency and high cost of electrocatalysts. Here, a cost-effective hybrid catalyst consisting of hemin (chloroprotoporphyrin IX iron(III)), a product recovered from bovine blood, adsorbed onto commercial Vulcan carbon is reported. Upon heat treatment, this material shows significantly improved activity and selectivity for CO$_2$ reduction in water while exhibiting good stability for more than 10 h. The heat treatment leads to consecutive removal of the axial chlorine atom and decomposition of the iron porphyrin ring, restructuring to form atomic Fe sites. The optimized hybrid catalyst obtained at 900 °C shows near-unity selectivity for reduction of CO$_2$ to CO at a small overpotential of 310 mV. The insight into transformation of adsorbed Fe complexes into single Fe atoms upon heat treatment provides guidance for development of single atom catalysts

Highly Scalable Conversion of Blood Protoporphyrin to Efficient Electrocatalyst for CO 2

Electrochemical CO$_2$ reduction to valuable chemicals represents a green and sustainable approach to close the anthropogenic carbon cycle, but has been impeded by low efficiency and high cost of electrocatalysts. Here, a cost-effective hybrid catalyst consisting of hemin (chloroprotoporphyrin IX iron(III)), a product recovered from bovine blood, adsorbed onto commercial Vulcan carbon is reported. Upon heat treatment, this material shows significantly improved activity and selectivity for CO$_2$ reduction in water while exhibiting good stability for more than 10 h. The heat treatment leads to consecutive removal of the axial chlorine atom and decomposition of the iron porphyrin ring, restructuring to form atomic Fe sites. The optimized hybrid catalyst obtained at 900 °C shows near-unity selectivity for reduction of CO$_2$ to CO at a small overpotential of 310 mV. The insight into transformation of adsorbed Fe complexes into single Fe atoms upon heat treatment provides guidance for development of single atom catalysts

Electron- and Energy-Transfer Processes in a Photocatalytic System Based on an Ir(III)-Photosensitizer and an Iron Catalyst

The reaction pathways of bis-(2-phenylpyridinato-)­(2,2′-bipyridine)­iridium­(III)­hexafluorophosphate [Ir­(ppy)₂(bpy)]­PF₆ within a photocatalytic water reduction system for hydrogen generation based on an iron-catalyst were investigated by employing time-resolved photoluminescence spectroscopy and time-dependent density functional theory. Electron transfer (ET) from the sacrificial reagent to the photoexcited Ir complex has a surprisingly low probability of 0.4% per collision. Hence, this step limits the efficiency of the overall system. The calculations show that ET takes place only for specific encounter geometries. At the same time, the presence of the iron-catalyst represents an energy loss channel due to a triplet–triplet energy transfer of Dexter type. This loss channel is kept small by the employed concentration ratios, thus favoring the reductive ET necessary for the water reduction. The elucidated reaction mechanisms underline the further need to improve the sun light’s energy pathway to the catalyst to increase the efficiency of the photocatalytic system