Modeling Electrochemical Vacancy Regeneration in Single-Walled Carbon Nanotubes.

Synthesis-induced defects in single-walled carbon nanotubes (SWCNTs) enable diverse catalytic reactions, but the nature of catalytic intermediates and how active species regeneration occurs are unclear. Using a quantum mechanics/molecular mechanics (QM/MM) hybrid methodology based on density functional theory (DFT) and a classical force-field, we explore the reactivity and electrochemical regeneration of a vacancy defect in a zigzag SWCNT. Our findings indicate that hydrolysis of the defect forms a ketone group on one carbon atom and C-H bonds on two adjacent carbons. Applying an electrochemical potential of ESHE = -0.740 V triggers a proton-coupled electron transfer (PCET), converting the ketone to a hydroxyl group. Further reduction at ESHE = -1.08 V induces another PCET, expelling the hydroxyl as water and forming an active carbon with carbene character that can react with hydrogen peroxide and perchlorate. The hydrogen atoms on neighboring carbons prevent further water dissociation, maintaining the catalytic vacancy.

BODIPY Chemisorbed on SnO2 and TiO2 Surfaces for Photoelectrochemical Applications.

Advancement toward dye-sensitized photoelectrochemical cells to produce solar fuels by solar-driven water splitting requires a photosensitizer that is firmly attached to the semiconducting photoelectrodes. Covalent binding enhances the efficiency of electron injection from the photoexcited dye into the metal oxide. Optimization of charge transfer, efficient electron injection, and minimal electron-hole recombination are mandatory for achieving high efficiencies. Here, a BODIPY-based dye exploiting a novel surface-anchoring mode via boron is compared to a similar dye bound by a traditional carboxylic acid anchoring group. Through terahertz and transient absorption spectroscopic studies, along with interfacial electron transfer simulations, we find that, when compared to the traditional carboxylic acid anchoring group, electron injection of boron-bound BODIPY is faster into both TiO2 and SnO2. Although the surface coverage is low compared with carboxylic acids, the binding stability is improved over a wide range of pH. Subsequent photoelectrochemical studies using a sacrificial electron donor showed that this combined dye and anchoring group maintained photocurrent with good stability over long-time irradiation. This recently discovered binding mode of BODIPY shows excellent electron injection and good stability over time, making it promising for future investigations.

Feed-Forward Neural Network for Predicting Enantioselectivity of the Asymmetric Negishi Reaction.

Density functional theory (DFT) is a powerful tool to model transition state (TS) energies to predict selectivity in chemical synthesis. However, a successful multistep synthesis campaign must navigate energetically narrow differences in pathways that create some limits to rapid and unambiguous application of DFT to these problems. While powerful data science techniques may provide a complementary approach to overcome this problem, doing so with the relatively small data sets that are widespread in organic synthesis presents a significant challenge. Herein, we show that a small data set can be labeled with features from DFT TS calculations to train a feed-forward neural network for predicting enantioselectivity of a Negishi cross-coupling reaction with P-chiral hindered phosphines. This approach to modeling enantioselectivity is compared with conventional approaches, including exclusive use of DFT energies and data science approaches, using features from ligands or ground states with neural network architectures.

O-Allylhydroxyamine: A Bifunctional Olefin for Construction of Axially and Centrally Chiral Amino Alcohols via Asymmetric Carboamidation.

Difunctionalization of olefins offers an attractive approach to access complex chiral structures. Reported herein is the design of N-protected O-allylhydroxyamines as bifunctional olefins that undergo catalytic asymmetric 1,2-carboamidation with three classes of (hetero)arenes to afford chiral amino alcohols via C-H activation. The C═C bond in O-allylhydroxyamine is activated by the intramolecular electrophilic amidating moiety as well as a migrating directing group. The asymmetric carboamidation reaction pattern depends on the nature of the (hetero)arene reagent. Simple achiral (hetero)arenes reacted to give centrally chiral ß-amino alcohols in excellent enantioselectivity. The employment of axially prochiral or axially racemic heteroarenes afforded amino alcohols with both axial and central chirality in excellent enantio- and diastereoselectivity. In the case of axially racemic heteroarenes, the coupling follows a kinetic resolution pattern with an s-factor of up to >600. A nitrene-based reaction mechanism has been suggested based on experimental studies, and a unique mode of induction of enantio- and diastereoselectivity has been proposed. Applications of the amino alcohol products have been demonstrated.

Sigma-Bond Metathesis as an Unusual Asymmetric Induction Step in Rhodium-Catalyzed Enantiodivergent Synthesis of C-N Axially Chiral Biaryls.

Mechanistic understanding of asymmetric induction plays a crucial role in designing new catalytic asymmetric reactions. Reported herein is atroposelective access to C-N axially chiral isoquinolones via rhodium-catalyzed C-H activation of N-alkoxy benzamides and annulation with imidoyl sulfoxonium ylides. The coupling system proceeded with excellent functional group tolerance, and different conditions were identified to afford one or the other enantiomeric product each in excellent enantioselectivity for a representative class of the sulfoxonium ylide reagent, thus making both enantiomers readily available using the same catalyst. Experimental and computational studies revealed a pathway of C-H alkylation and enantio-determining formal nucleophilic substitution-C-N cyclization that is mediated by the rhodium catalyst via σ-bond metathesis as the asymmetric induction mechanism. Computational studies indicated that the solvent-dependent enatiodivergence originated from different levels of σ-bond metathesis mediated by neutral versus cationic rhodium species.

BODIPY and dipyrrin as unexpected robust anchoring groups on TiO2 nanoparticles.

Covalent attachment of molecules to metal oxide surfaces typically demands the presence of an anchoring group that in turn requires synthetic steps to introduce. BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) chromophores have long been used in dye-sensitized solar cells, but carboxylic acid groups typically had to be installed to act as surface anchors. We now find that even without the introduction of such anchors, the unmodified BODIPY can bind to TiO2 surfaces via its BF2 group through boron-oxygen surface bonds. Dipyrrin, the parent molecule of BODIPY, is also capable of binding directly to TiO2 surfaces, likely through its chelating nitrogen atoms. These binding modes prove to be even more robust than that of an installed carboxylate and offer a new way to attach molecular complexes to surfaces for (photo)catalytic applications since, once bound, we show that surface bound BODIPY and dipyrrin derivatives exhibit ultrafast photoinjection of electrons into the conduction band of TiO2.

Electrocatalytic, Homogeneous Ammonia Oxidation in Water to Nitrate and Nitrite with a Copper Complex.

Electrocatalytic ammonia oxidation at room temperature and pressure allows energy-economical and environmentally friendly production of nitrites and nitrates. Few molecular catalysts, however, have been developed for this six- or eight-electron oxidation process. We now report [Cu(bipyalk)]+, a homogeneous electrocatalyst that realizes the title reaction in water at 94% Faradaic efficiency. The catalyst exhibits high selectivity against water oxidation in aqueous media, as [Cu(bipyalk)]+ is not competent for water oxidation.

Optimization of Surface Loading of the Silatrane Anchoring Group on TiO2.

Anchoring groups are usually needed for the attachment of small molecules to metal oxide surfaces such as in water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs). Here, we optimize the surface loading onto titanium dioxide surfaces of the silatrane anchoring group, a triethanolamine-protected trialkoxysilane. This anchoring group is not yet widely used because prior protocols afforded low surface coverage, but it has the advantage of high stability over a wide pH range and at both oxidizing and reducing potentials when bound. A new and improved method for estimating surface coverage is described here and used to determine that loading using previously reported binding protocols is very low. However, we were able to uncover several factors contributing to this low loading, which has allowed us to develop methods to greatly improve surface coverage for a variety of silatranes. Most notably, we were able to increase the loading of a model arylsilatrane by 145% through use of a benzoic acid additive. This is not general acid catalysis because alkylsilatranes are not similarly affected and 4-t-butylbenzoic acid, having a similar pKa to benzoic acid, is not effective. Because the bulky t-butyl group of the latter additive is not expected to pi-stack with our arylsilatrane, we have tentatively assigned this enhancement to aromatic stacking between the aromatic additive and the arylsilatrane.

Distorted Copper(II) Complex with Unusually Short CF···Cu Distances.

We find a Cu(II)-(L-CF3)2 complex (L-CF3 = 2,2,2-trifluoro-N-[2-(pyridin-2-yl)propan-2-yl]acetamide) with a distorted "seesaw" geometry. It has the shortest crystallographic CF···Cu distances yet reported, to the best of our knowledge (<2.6 Å), for which computational and experimental data indicate a secondary bonding interaction. A comparison with a CCl3 version and one without ligand backbone gem-dimethyl groups suggests a steric origin for the distorted geometry, resulting from the specific ligand interactions.

Accessing Molecular Dimeric Ir Water Oxidation Catalysts from Coordination Precursors.

One ongoing challenge in the field of iridium-based water oxidation catalysts is to develop a molecular precatalyst affording well-defined homogeneous active species for catalysis. Our previous work by using organometallic precatalysts Cp*Ir(pyalk)OH and Ir(pyalk)(CO)2 (pyalk = (2-pyridyl)-2-propanolate) suggested a µ-oxo-bridged Ir dimer as the probable resting state, although the structure of the active species remained elusive. During the activation, the ligands Cp* and CO were found to oxidatively degrade into acetic acid or other products, which coordinate to Ir centers and affect the catalytic reaction. Two related dimers bearing two pyalk ligands on each iridium were crystallized for structural analysis. However, preliminary results indicated that these crystallographically characterized dimers are not active catalysts. In this work, we accessed a mixture of dinuclear iridium species from a coordination precursor, Na[Ir(pyalk)Cl4], and assayed their catalytic activity for oxygen evolution by using NaIO4 as the oxidant. This catalyst showed comparable oxygen-evolution activity to the ones previously reported from organometallic precursors without demanding oxidative activation to remove sacrificial ligands. Future research along this direction is expected to provide insights and design principles toward a well-defined active species.

Electronic and Spin-State Effects on Dinitrogen Splitting to Nitrides in a Rhenium Pincer System.

Bimetallic nitrogen (N2) splitting to form metal nitrides is an attractive method for N2 fixation. Although a growing number of pincer-supported systems can bind and split N2, the precise relationship between the ligand properties and N2 binding/splitting remains elusive. Here we report the first example of an N2-bridged rhenium(III) complex, [(trans-P2tBuPyrr)ReCl2]2(µ-η1:η1-N2) (P2tBuPyrr = [2,5-(CH2PtBu2)2C4H2N]-). In this case, N2 binding occurs at a higher oxidation level than that in other reported pincer analogues. Analysis of the electronic structure through computational studies shows that the weakly π-donor pincer ligand stabilizes an open-shell electronic configuration that leads to enhanced binding of N2 in the bridged complex. Utilizing SQUID magnetometry, we demonstrate a singlet ground state for this Re-N-N-Re complex, and we offer tentative explanations for antiferromagnetic coupling of the two local S = 1 sites. Reduction and subsequent heating of the rhenium(III)-dinitrogen complex leads to chloride loss and cleavage of the N-N bond with isolation of the terminal rhenium(V) nitride complex (P2tBuPyrr)ReNCl.

Surprisingly big linker-dependence of activity and selectivity in CO2 reduction by an iridium(i) pincer complex.

Here, we report the quantitative electroreduction of CO2 to CO by a PNP-pincer iridium(i) complex bearing amino linkers in DMF/water. The electrocatalytic properties greatly depend on the choice of linker within the ligand. The complex 3-N is far superior to the analogues with methylene and oxygen linkers, showing higher activity and better selectivity for CO2 over proton reduction.

Diazo coupling for surface attachment of small molecules to TiO2 nanoparticles.

Robust surface attachment of molecular species to metal oxide semiconductors is desirable for many applications. Here, we report the interfacial diazo coupling of surface-bound amines with aromatics to bind them to the surface of TiO2 nanoparticles via a siloxane anchor and a diazo linker. The technique shows potential for the inexpensive, stable, modular and tunable attachment of molecules to metal oxide surfaces.

Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex.

The high-valent nickel(iii) complex Ni(pyalk)2 + (2) was prepared by oxidation of a nickel(ii) complex, Ni(pyalk)2 (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and X-ray crystallography. Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between the measured rate constant and the substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ∼91 kcal mol-1 for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.

Strongly Coupled Phenazine-Porphyrin Dyads: Light-Harvesting Molecular Assemblies with Broad Absorption Coverage.

The development of light-harvesting architectures with broad absorption coverage in the visible region continues to be an important research area in the field of artificial photosynthesis. Here, we introduce a new class of ethynyl-linked panchromatic dyads composed of dibenzophenazines coupled ortho and meta to tetrapyrroles with an anchoring group that can be grafted onto metal oxide surfaces. Quantum chemical calculations and photophysical measurements of the synthesized materials reveal that both of the dibenzophenazine dyads absorb broadly from 300 to 636 nm and exhibit absorption bands different from those of the constituent chromophore units. Moreover, the different points of attachment of dibenzophenazines to tetrapyrroles give different absorption profiles which computations suggest result from differences in the planarity of the two dyads. Applicability of the dyads in artificial photosynthesis systems was assessed by their incorporation and characterization of their performance in dye-sensitized solar cells.

Silatrane Anchors for Metal Oxide Surfaces: Optimization for Potential Photocatalytic and Electrocatalytic Applications.

Silatrane surface anchors are protected siloxanes that are known to bond firmly (from pH 2-11) to metal oxide electrodes under heating. However, these conditions are not always compatible with the other functionality present. A silatrane-containing porphyrin molecule and a silatrane-containing ruthenium complex have now been designed, synthesized and optimized conditions have been identified for surface binding. Two mild, room-temperature surface binding methods were explored: binding with or without an acidic pretreatment; these methods were compared to the traditional, harsher binding conditions involving strong heating. We find that a preacidified electrode gave comparable surface loadings at room temperature compared to sensitization by using the previous strong heating method. This was also true on TiO2, SnO2, and nanoITO electrodes and thus may be generalizable. The new, milder binding methods also resulted in excellent aqueous and electrochemical stability from pH 2-11. Using a water-insoluble porphyrin with a silatrane anchor further increased the aqueous stability of the deposit, aided by the insolubility of the porphyrin. Finally, X-ray photoelectron spectroscopy (XPS) data confirmed for the first time that the triethanolamine released from the silatrane on deprotection/binding in turn binds to TiO2, SnO2, and nanoITO electrodes. This undesired triethanolamine deposit was easily removed from the surface by electrochemical voltage cycling or with an aqueous acidic wash for 1 h.

On the damage done to the structure of the Thermoplasma acidophilum proteasome by electron radiation.

It has long been known that proteins are damaged when they are exposed to the electron beam in an electron microscope. Here we show that exposure to electrons under cryo-EM conditions leads to a small change in the quaternary structure of the Thermoplasma acidophilum proteasome, and that backbones atoms belonging to the α-helices in this molecule appear to be particular prone to chemical damage. A chemical mechanism is proposed for this damage. Both this local chemical effect and the more global quaternary structure effect appear to heterogenize samples leading to a radiation dose-dependent degradation of the resolution of the EM maps obtained from this molecule.

A Dinuclear Iridium(V,V) Oxo-Bridged Complex Characterized Using a Bulk Electrolysis Technique for Crystallizing Highly Oxidizing Compounds.

We report a general method for the preparation and crystallization of highly oxidized metal complexes that are difficult to prepare and handle by more conventional means. This method improves typical bulk electrolysis and crystallization conditions for these reactive species by substituting oxidation-prone organic electrolytes and precipitants with oxidation-resistant compounds. Specifically, we find that CsPF6 is an effective inert electrolyte in acetonitrile, and appears to have general applicability to electrochemical studies in this solvent. Likewise, CCl4 is not only an oxidation-resistant precipitant for crystallization from MeCN but it also enters the lattice. In this way, we synthesized and characterized an Ir(V,V) mono-µ-oxo dimer which only forms at a very high potential (1.9 V vs NHE). This compound, having the highest isolated oxidation state in this redox-active system, cannot be formed chemically. DFT calculations show that the oxidation is centered on the Ir-O-Ir core and facilitated by strong electron-donation from the pyalk (2-(2-pyridinyl)-2-propanolate) ligand. TD-DFT simulations of the UV-visible spectrum reveal that its royal blue color arises from electron excitations with mixed LMCT and Laporte-allowed d-d character. We have also crystallographically characterized a related monomeric Ir(V) complex, similarly prepared by oxidizing a previously reported Ir(IV) compound at 1.7 V, underscoring the general applicability of this method.

Key factors in pincer ligand design.

Pincers, tridentate ligands that prefer a meridional geometry, are a rising class because of their distinctive combination of properties. They permit a good level of control on the nature of the coordination sphere by holding the donor groups in a predictable arrangement. Some groups, such as an aryl or a pyridine, that would normally be easily lost as monodentate ligands, become reliably coordinated, especially if they form the central donor unit of the three. Many pincer complexes show exceptional thermal stability, a property that is particularly prized in homogeneous catalysis where they can permit high temperature operation. The connectors between the three donor groups are often rigid, enforcing a strict mer geometry but flexible linkers permit fac binding and even fluxionality between the two forms. Rigid pincers can make good ligands for asymmetric catalysis-if the wingtip groups cannot easily rotate they may instead maintain a geometry in which suitable substituents project into the active site area of the catalyst where they help enantio-differentiation of the relevant transition states. Examples have been selected to illustrate these and other properties of this promising ligand class.

Optimization of Photoanodes for Photocatalytic Water Oxidation by Combining a Heterogenized Iridium Water-Oxidation Catalyst with a High-Potential Porphyrin Photosensitizer.

The development of water-splitting dye-sensitized photoelectrochemical cells has gained interest owing to their ability to generate renewable fuels from solar energy. In this study, photoanodes were assembled from a SnO2 film sensitized with a combination of a high-potential CF3 -substituted porphyrin dye with a tetrahydropyranyl-protected hydroxamic acid surface-anchoring group and a Cp*Ir (Cp*=pentamethylcyclopentadienyl) water-oxidation catalyst containing a silatrane anchoring group. The dye/catalyst ratios were varied from 2:1 to 32:1 to optimize the photocatalytic water oxidation. Photoelectrochemical measurements showed not only more stable and reproducible photocurrents for lower dye/catalyst ratios but also improved photostability. O2 production was confirmed in real time over a 20 h period with a Clark electrode. Photoanodes prepared from 2:1 and 8:1 dye/catalyst sensitization solutions provided the most active electrodes for photocatalytic water oxidation and performed approximately 30-35 turnovers in 20 h.