Mechanistic Insights into Electrocatalytic Hydrogen Evolution by an Exceptionally Stable Cobalt Complex.

Co(aPPy) is one of the most stable and active molecular first-row transition-metal catalysts for proton reduction reported to date. Understanding the origin of its high performance via mechanistic studies could aid in developing even better catalysts. In this work, the catalytic mechanism of Co(aPPy) was electrochemically probed, in both organic solvents and water. We found that different mechanisms can occur depending on the solvent and the acidity of the medium. In organic solvent with a strong acid as the proton source, catalysis initiates directly after a single-electron reduction of CoII to CoI, whereas in the presence of a weaker acid, the cobalt center needs to be reduced twice before catalysis occurs. In the aqueous phase, we found drastically different electrochemical behavior, where the Co(aPPy) complex was found to be a precatalyst to a different electrocatalytic species. We propose that in this active catalyst, the pyridine ring has dissociated and acts as a proton relay at pH ≤ 5, which opens up a fast protonation pathway of the CoI intermediate and results in a high catalytic activity. Furthermore, we determined with constant potential bulk electrolysis that the catalyst is most stable at pH 3. The catalyst thus functions optimally at low pH in an aqueous environment, where the pyridine acts as a proton shuttle and where the high acidity also prevents catalyst deactivation.

Substrate scope driven optimization of an encapsulated hydroformylation catalyst.

Caged complexes can provide impressive selective catalysts. Due to the complex shapes of such caged catalysts, however, the level of selectivity control of a single substrate cannot be extrapolated to other substrates. Herein, the substrate scope using 41 terminal alkene substrates is investigated in the hydroformylation reaction with an encapsulated rhodium catalyst [Rh(H)(CO)3(P(mPy3(ZnTPP)3))] (CAT1). For all substrates, the amount of branched products formed was higher with CAT1 than with the unencapsulated reference catalyst [Rh(H)(CO)2(P(mPy3))2] (CAT2) (linear/branched ratio between 2.14 and 0.12 for CAT1 and linear/branched ratio between 6.22 and 0.59 for CAT2). Interestingly, the level of cage induced selectivity depends strongly on the substrate structure that is converted. Analysis of the substrate scope combined with DFT calculations suggests that noncovalent interactions between the substrate moieties and cage walls play a key role in controlling the regioselectivity. Consequently, these supramolecular interactions were further optimized by replacing the ZnTPP building block with a zinc porphyrin analog that contained OiPr substituents on the meta position of the aryl rings. The resulting caged catalyst, CAT4, converted substrates with even higher branched selectivity.

Increased solar-driven chemical transformations through surface-induced benzoperylene aggregation in dye-sensitized photoanodes.

The impact of benzo[ghi]perylenetriimide (BPTI) dye aggregation on the performance of photoelectrochemical devices was explored, through imide-substitution with either alkyl (BPTI-A, 2-ethylpropyl) or bulky aryl (BPTI-B, 2,6-diisopropylphenyl) moieties, to, respectively, enable or suppress aggregation. While both dyes demonstrated similar monomeric optoelectronic properties in solution, adsorption onto mesoporous SnO2 revealed different behavior, with BPTI-A forming aggregates via π-stacking and BPTI-B demonstrating reduced aggregation in the solid state. BPTI photoanodes were tested in dye-sensitized solar cells (DSSCs) before application to dye-sensitized photoelectrochemical cells (DSPECs) for Br2 production (a strong oxidant) coupled to H2 generation (a solar fuel). BPTI-A demonstrated a twofold higher dye loading of the SnO2 surface than BPTI-B, resulting in a fivefold enhancement to both photocurrent and Br2 production. The enhanced output of the photoelectrochemical systems (with respect to dye loading) was attributed to both J- and H- aggregation phenomena in BPTI-A photoanodes that lead to improved light harvesting. Our investigation provides a strategy to exploit self-assembly via aggregation to improve molecular light-harvesting and charge separation properties that can be directly applied to dye-sensitized photoelectrochemical devices.

Gas Evolution as a Tool to Study Reaction Kinetics Under Biomimetic Conditions.

The field of bioorthogonal chemistry is rapidly growing, presenting successful applications of organic and transition metal-catalysed reactions in cells and living systems (in vivo). The development of such reactions typically proceeds through many iterative steps focused on biocompatibility and fast reaction kinetics to ensure product formation. However, obtaining kinetic data, even under simulated biological (biomimetic) conditions, remains a challenge due to substantial concentrations of salts and biomolecules hampering the use of typically employed solution-phase analytical techniques. In this study, we explored the suitability of gas evolution as a probe to study kinetics under biomimetic conditions. As proof of concept, we show that the progress of two transition metal-catalysed bioorthogonal chemical reactions can be accurately monitored, regardless of the complexity of the medium. As such, we introduce a protocol to gain more insight into the performance of a catalytic system under biomimetic conditions to further progress iterative catalyst development for in vivo applications.

Iron oxide-promoted photochemical oxygen reduction to hydrogen peroxide (H2O2).

Hydrogen peroxide (H2O2) is a valuable green oxidant with a wide range of applications. Furthermore, it is recognized as a possible future energy carrier achieving safe operation, storage and transportation. The photochemical production of H2O2 serves as a promising alternative to the waste- and energy-intensive anthraquinone process. Following the 12 principles of Green Chemistry, we demonstrate a facile and general approach to sustainable catalyst development utilizing earth-abundant iron and biobased sources only. We developed several iron oxide (FeOx) nanoparticles (NPs) for successful photochemical oxygen reduction to H2O2 under visible light illumination (445 nm). Achieving a selectivity for H2O2 of >99%, the catalyst material could be recycled for up to four consecutive rounds. An apparent quantum yield (AQY) of 0.11% was achieved for the photochemical oxygen reduction to H2O2 with visible light (445 nm) at ambient temperatures and pressures (9.4-14.8 mmol g-1 L-1). Reaching productivities of H2O2 of at least 1.7 ± 0.3 mmol g-1 L-1 h-1, production of H2O2 was further possible via sunlight irradiation and in seawater. Finally, a detailed mechanism has been proposed on the basis of experimental investigation of the catalyst's properties and computational results.

PhenTAA: A Redox-Active N4-Macrocyclic Ligand Featuring Donor and Acceptor Moieties.

Here, we present the development and characterization of the novel PhenTAA macrocycle as well as a series of [Ni(R2PhenTAA)]n complexes featuring two sites for ligand-centered redox-activity. These differ in the substituent R (R = H, Me, or Ph) and overall charge of the complex n (n = -2, -1, 0, +1, or +2). Electrochemical and spectroscopic techniques (CV, UV/vis-SEC, X-band EPR) reveal that all redox events of the [Ni(R2PhenTAA)] complexes are ligand-based, with accessible ligand charges of -2, -1, 0, +1, and +2. The o-phenylenediamide (OPD) group functions as the electron donor, while the imine moieties act as electron acceptors. The flanking o-aminobenzaldimine groups delocalize spin density in both the oxidized and reduced ligand states. The reduced complexes have different stabilities depending on the substituent R. For R = H, dimerization occurs upon reduction, whereas for R = Me/Ph, the reduced imine groups are stabilized. This also gives electrochemical access to a [Ni(R2PhenTAA)]2- species. DFT and TD-DFT calculations corroborate these findings and further illustrate the unique donor-acceptor properties of the respective OPD and imine moieties. The novel [Ni(R2PhenTAA)] complexes exhibit up to five different ligand-based oxidation states and are electrochemically stable in a range from -2.4 to +1.8 V for the Me/Ph complexes (vs Fc/Fc+).

Rotaxane-Functionalized Dyes for Charge-Rectification in p-Type Photoelectrochemical Devices.

A supramolecular photovoltaic strategy is applied to enhance power conversion efficiencies (PCE) of photoelectrochemical devices by suppressing electron-hole recombination after photoinduced electron transfer (PET). Here, the author exploit supramolecular localization of the redox mediator-in close proximity to the dye-through a rotaxane topology, reducing electron-hole recombination in p-type dye-sensitized solar cells (p-DSSCs). Dye PRotaxane features 1,5-dioxynaphthalene recognition sites (DNP-arms) with a mechanically-interlocked macrocyclic redox mediator naphthalene diimide macrocycle (3-NDI-ring), stoppering synthetically via click chemistry. The control molecule PStopper has stoppered DNP-arms, preventing rotaxane formation with the 3-NDI-ring. Transient absorption and time-resolved fluorescence spectroscopy studies show ultrafast (211 ± 7 fs and 2.92 ± 0.05 ps) PET from the dye-moiety of PRotaxane to its mechanically interlocked 3-NDI-ring-acceptor, slowing down the electron-hole recombination on NiO surfaces compared to the analogue . p-DSSCs employing PRotaxane (PCE = 0.07%) demonstrate a 30% PCE increase compared to PStopper (PCE = 0.05%) devices, combining enhancements in both open-circuit voltages (VOC = 0.43 vs 0.36 V) and short-circuit photocurrent density (JSC = -0.39 vs -0.34 mA cm-2 ). Electrochemical impedance spectroscopy shows that PRotaxane devices exhibit hole lifetimes (τh ) approaching 1 s, a 16-fold improvement compared to traditional I- /I3 - -based systems (τh = 50 ms), demonstrating the benefits obtained upon nanoengineering of interfacial dye-regeneration at the photocathode.

Copper-Catalyzed Sulfimidation in Aqueous Media: a Fast, Chemoselective and Biomolecule-Compatible Reaction.

Performing transition metal-catalyzed reactions in cells and living systems has equipped scientists with a toolbox to study biological processes and release drugs on demand. Thus far, an impressive scope of reactions has been performed in these settings, but many are yet to be introduced. Nitrene transfer presents a rather unexplored new-to-nature reaction. The reaction products are frequently encountered motifs in pharmaceuticals, presenting opportunities for the controlled, intracellular synthesis of drugs. Hence, we explored the transition metal-catalyzed sulfimidation reaction in water for future in vivo application. Two Cu(I) complexes containing trispyrazolylborate ligands (Tpx ) were selected, and the catalytic system was evaluated with the aid of three fitness factors. The excellent nitrene transfer reactivity and high chemoselectivity of the catalysts, coupled with good biomolecule compatibility, successfully enabled the sulfimidation of thioethers in aqueous media. We envision that this copper-catalyzed sulfimidation reaction could be an interesting starting point to unlock the potential of nitrene transfer catalysis in vivo.

Noncovalent Grafting of Molecular Complexes to Solid Supports by Counterion Confinement.

Grafting molecular complexes on solid supports is a facile strategy to synthesize advanced materials. Here, we present a general and simple method for noncovalent grafting on charge-neutral surfaces. Our method is based on the generic principle of counterion confinement in surface micropores. We demonstrate the power of this approach using a set of three platinum complexes: Pt1 (Pt1L4(BF4)2, L = p-picoline), Pt2 (Pt2L4(BF4)4, L = 2,6-bis(pyridine-3-ylethynyl)pyridine), and Pt12 (Pt12L24(BF4)24, L = 4,4'-(5-methoxy-1,3-phenylene)dipyridine). These complexes share the same counterion (BF4-) but differ vastly in their size, charge, and structure. Imaging of the grafted materials by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) and energy-dispersive X-ray (EDX) showed that our method results in a homogeneous distribution of both complexes and counterions. Nitrogen sorption studies indicated a decrease in the available surface area and micropore volume, providing evidence for counterion confinement in the surface micropores. Following the adsorption of the complexes over time showed that this is a two-step process: fast surface adsorption by van der Waals forces was followed by migration over the surface and surface binding by counterion confinement. Regarding the binding of the complexes to the support, we found that the surface-adsorbate binding constant (KS) increases quadratically with the number of anions per complex up to KS = 1.6 × 106 M-1 equaling ΔG°ads = -35 kJ mol-1 for the surface binding of Pt12. Overall, our method has two important advantages: first, it is general, as you can anchor different complexes (with different charges, counterions, and/or sizes); second, it promotes the distribution of the complexes on the support surface, creating well-distributed sites that can be used in various applications across several areas of chemistry.

Accelerated Electrophotocatalytic C(sp3 )-H Heteroarylation Enabled by an Efficient Continuous-Flow Reactor.

Electrophotocatalytic transformations are garnering attention in organic synthesis, particularly for accessing reactive intermediates under mild conditions. Moving these methodologies to continuous-flow systems, or flow ElectroPhotoCatalysis (f-EPC), showcases potential for scalable processes due to enhanced irradiation, increased electrode surface, and improved mixing of the reaction mixture. Traditional methods sequentially link photochemical and electrochemical reactions, using flow reactors connected in series, yet struggle to accommodate reactive transient species. In this study, we introduce a new flow reactor concept for electrophotocatalysis (EPC) that simultaneously utilizes photons and electrons. The reactor is designed with a transparent electrode and employs cost-effective materials. We used this technology to develop an efficient process for electrophotocatalytic heteroarylation of C(sp3 )-H bonds. Importantly, the same setup can also facilitate purely electrochemical and photochemical transformations. This reactor represents a significant advancement in electrophotocatalysis, providing a framework for its application in flow for complex synthetic transformations.

A Co(TAML)-based artificial metalloenzyme for asymmetric radical-type oxygen atom transfer catalysis.

We show that the incorporation of a biotinylated Co(TAML) cofactor within streptavidin enables asymmetric radical-type oxygen atom transfer catalysis with improved activity and enantioselectivity.

Broadening the catalytic region from the cavity to windows by M6L12 nanospheres in cyclizations.

Supramolecular cages have received tremendous attention as they can contain catalysts that exhibit confinement effects in the cavity, leading to excellent performances. Herein, we report an example wherein the catalytic region is extended from the cage cavity to the windows, and investigate its confinement effect by utilizing the Pd6LAu12 cage that contains rigidly fixed and isolated gold complexes at the windows. Pd6LAu12 exhibit three features of particular interest while assessing their properties in gold-catalyzed cyclization reactions. First, the catalysts experience a cage effect as they display higher reactivity and selectivity compared to the monomeric analogue, as a result of substrate pre-organization at the windows. Second, the metal complexes are physically separated by the cage structure, preventing the formation of less active dinuclear gold complexes making it more stable under hydrous conditions. Third, the cage windows present the characteristics of enzymatic catalysis via Michaelis-Menten-type mechanism analysis. This contribution presents an alternative way to engineer supramolecular catalysts through extending the catalytic region.

Pd12MnL24 (for n = 6, 8, 12) nanospheres by post-assembly modification of Pd12L24 spheres.

In this contribution, we describe a post-assembly modification approach to selectively coordinate transition metals in Pd12L24 cuboctahedra. The herein reported approach involves the preparation of Pd12L24 nanospheres with protonated nitrogen donor ligands that are covalently linked at the interior. The so obtained Pd12(LH+)24 nanospheres are shown to be suitable for coordinative post-modification after deprotection by deprotonation. Selective formation of tetra-coordinated MB in Pd12MB6L24, tri-coordinated MB in Pd12MB8L24 nanospheres and two-coordinated MB in Pd12MB12L24 nanospheres is achieved as a result of different nitrogen donor ligands. A combination of pulsed EPR spectroscopy (DEER) to measure Cu-Cu distances in the different spheres, NMR studies and computational investigations, support the presence of the complexes at precise locations of the Pd12MB6L24 nanosphere. The general post-assembly modification methodology can be extended using other transition metal precursors or supramolecular systems and can guide precise formation and investigation of novel transition metal-complex containing nanospheres with well-defined composition.

Tailoring Secondary Coordination Sphere Effects in Single-metal-site Catalysts by Surface Immobilization of Supramolecular Cages.

Controlling the coordination sphere of heterogeneous single-metal-site catalysts is a powerful strategy for fine-tuning their catalytic properties but is fairly difficult to achieve. To address this problem, we immobilized supramolecular cages where the primary- and secondary coordination sphere are controlled by ligand design. The kinetics of these catalysts were studied in a model reaction, the hydrolysis of ammonia borane, over a temperature range using fast and precise online measurements generating high-precision Arrhenius plots. The results show how catalytic properties can be enhanced by placing a well-defined reaction pocket around the active site. Our fine-tuning yielded a catalyst with such performance that the reaction kinetics are diffusion-controlled rather than chemically controlled.

Urea-Functionalized Fe4 L6 Cages for Supramolecular Gold Catalyst Encapsulation to Control Substrate Activation Modes.

The excellent catalytic performances of enzymes in terms of activity and selectivity are an inspiration for synthetic chemists and this has resulted in the development of synthetic containers for supramolecular catalysis. In such containers the local environment and pre-organization of catalysts and substrates leads to control of the activity and selectivity of the catalyst. Herein we report a supramolecular strategy to encapsulate single catalysts in a urea-functionalized Fe4 L6 cage, which can co-encapsulate a functionalized urea substrate through hydrogen bonding. Distinguished selectivity is obtained, imposed by the cage as site isolation only allows catalysis through π activation of the substrate and as a result the selectivity is independent of catalyst concentration. The encapsulated catalyst is more active than the free analogue, an effect that can be ascribed to transitionstate stabilization rather than substrate pre-organization, as revealed by the MM kinetic data. The simple strategy reported here is expected to be of general use in many reactions, for which the catalyst can be functionalized with a sulfonate group required for encapsulation.

Thermal/Blue Light Induced Cross-Linking of Acrylic Coatings with Diazo Compounds.

The thermal curing of industrial coatings (e.g., car painting and metal coil coatings) is accompanied by a substantial energy consumption due to the intrinsically high temperatures required during the curing process. Therefore, the development of new photochemical curing processes-preferably using visible light-is in high demand. This work describes new diazo-based cross-linkers that can be used to photocure acrylic coatings using blue light. This work demonstrates that the structure of the tethered diazo compounds influences the cross-linking efficiency, finding that side reactions are suppressed upon engineering greater molecular flexibility. Importantly, this work shows that these diazo compounds can be employed as either thermal or photochemical cross-linkers, exhibiting identical crosslinking performances. The performance of diazo-cross-linked coatings is evaluated to reveal excellent water resistance and demonstrably similar material properties to UV-cured acrylates. These studies pave the way for further usage of diazo-functionalized cross-linkers in the curing of paints and coatings.

Understanding the Oxidative Properties of Nickel Oxyhydroxide in Alcohol Oxidation Reactions.

The NiOOH electrode is commonly used in electrochemical alcohol oxidations. Yet understanding the reaction mechanism is far from trivial. In many cases, the difficulty lies in the decoupling of the overlapping influence of chemical and electrochemical factors that not only govern the reaction pathway but also the crystal structure of the in situ formed oxyhydroxide. Here, we use a different approach to understand this system: we start with synthesizing pure forms of the two oxyhydroxides, ß-NiOOH and γ-NiOOH. Then, using the oxidative dehydrogenation of three typical alcohols as the model reactions, we examine the reactivity and selectivity of each oxyhydroxide. While solvent has a clear effect on the reaction rate of ß-NiOOH, the observed selectivity was found to be unaffected and remained over 95% for the dehydrogenation of both primary and secondary alcohols to aldehydes and ketones, respectively. Yet, high concentration of OH- in aqueous solvent promoted the preferential conversion of benzyl alcohol to benzoic acid. Thus, the formation of carboxylic compounds in the electrochemical oxidation without alkaline electrolyte is more likely to follow the direct electrochemical oxidation pathway. Overoxidation of NiOOH from the ß- to γ-phase will affect the selectivity but not the reactivity with a sustained >95% conversion. The mechanistic examinations comprising kinetic isotope effects, Hammett analysis, and spin trapping studies reveal that benzyl alcohol is oxidatively dehydrogenated to benzaldehyde via two consecutive hydrogen atom transfer steps. This work offers the unique oxidative and catalytic properties of NiOOH in alcohol oxidation reactions, shedding light on the mechanistic understanding of the electrochemical alcohol conversion using NiOOH-based electrodes.