Resolving the amine-promoted hydrolysis mechanism of N2O5 under tropospheric conditions.

Hydrolysis of N2O5 under tropospheric conditions plays a critical role in assessing the fate of O3, OH, and NOx in the atmosphere. However, its removal mechanism has not been fully understood, and little is known about the role of entropy. Herein, we propose a removal path of N2O5 on the water clusters/droplet with the existence of amine, which entails a low free-energy barrier of 4.46 and 3.76 kcal/mol on a water trimer and droplet, respectively, at room temperature. The free-energy barrier exhibits strong temperature dependence; a barrierless hydrolysis process of N2O5 at low temperature (≤150 K) is observed. By coupling constrained ab initio molecular dynamics (constrained AIMD) simulations with thermodynamic integration methods, we quantitively evaluated the entropic contributions to the free energy and compared NH3-, methylamine (MA)-, and dimethylamine (DMA)-promoted hydrolysis of N2O5 on water clusters and droplet. Our results demonstrate that methylation of NH3 stabilizes the product state and promotes hydrolysis of N2O5 by reducing the free-energy barriers. Furthermore, a quantitative analysis of the internal coordinate distribution of the reaction center and the relative position of surrounding species reveals that the significant entropic contribution primarily results from the ensemble effect of configurations observed in the AIMD simulations. Such an ensemble effect becomes more significant with more water molecules included. Lowering the temperature effectively minimizes the entropic contribution, making the hydrolysis more exothermic and barrierless. This study sheds light on the importance of the promoting effect of amines and the entropic effect on gas-phase hydrolysis reactions, which may have far-reaching implications in atmospheric chemistry.

Tuning the Selectivity of Nitrate Reduction via Fine Composition Control of RuPdNP Catalysts.

Herein, aqueous nitrate (NO3 -) reduction is used to explore composition-selectivity relationships of randomly alloyed ruthenium-palladium nanoparticle catalysts to provide insights into the factors affecting selectivity during this and other industrially relevant catalytic reactions. NO3 - reduction proceeds through nitrite (NO2 -) and then nitric oxide (NO), before diverging to form either dinitrogen (N2) or ammonium (NH4 +) as final products, with N2 preferred in potable water treatment but NH4 + preferred for nitrogen recovery. It is shown that the NO3 - and NO starting feedstocks favor NH4 + formation using Ru-rich catalysts, while Pd-rich catalysts favor N2 formation. Conversely, a NO2 - starting feedstock favors NH4 + at ≈50 atomic-% Ru and selectivity decreases with higher Ru content. Mechanistic differences have been probed using density functional theory (DFT). Results show that, for NO3 - and NO feedstocks, the thermodynamics of the competing pathways for N-H and N-N formation lead to preferential NH4 + or N2 production, respectively, while Ru-rich surfaces are susceptible to poisoning by NO2 - feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N2 production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.

One-Step Pyrolysis Construction of Bimetallic Atom-Cluster Sites for Boosting Bifunctional Catalytic Activity in Zn-Air Batteries.

Due to their unique advantages, single atoms and clusters of transition metals are expected to achieve a breakthrough in catalytic activity, but large-scale production of active materials remains a challenge. In this work, a simple solvent-free one-step annealing method is developed and applied to construct diatomic and cluster active sites in activated carbon by utilizing the strong anchoring ability of phenanthroline to metal ions, which can be scaled for mass productions. Benefiting from the synergy between the different metals, the obtained sub-nano-bimetallic atom-cluster catalysts (FeNiAC -NC) exhibit high oxygen reduction reactions (ORR) activity (E1/2 = 0.936 V vs. RHE) and a small ORR/oxygen evolution reaction (OER) potential gap of only 0.594 V. An in-house pouch Zn-air battery is assembled using an FeNiAC -NC catalyst, which demonstrates a stability of 1000 h, outperforming previous reports. The existence of clusters and their effects on catalytic activity is analyzed by density functional theory calculations to reveal the chemistry of nano-bimetallic atom-cluster catalysts.

New Insights into Calcite Dissolution Mechanisms under Water, Proton, or Carbonic Acid-Dominated Conditions.

Carbonate minerals are ubiquitous in nature, and their dissolution impacts many environmentally relevant processes including preferential flow during geological carbon sequestration, pH buffering with climate-change induced ocean acidification, and organic carbon bioavailability in melting permafrost. In this study, we advance the atomic level understanding of calcite dissolution mechanisms to improve our ability to predict this complex process. We performed high pressure and temperature (1300 psi and 50 °C) batch experiments to measure transient dissolution of freshly cleaved calcite under H2O, H+, and H2CO3-dominated conditions, without and with an inhibitory anionic surfactant present. Before and after dissolution experiments, we measured dissolution etch-pit geometries using laser profilometry, and we used density functional theory to investigate relative adsorption energies of competing species that affect dissolution. Our results support the hypothesis that calcite dissolution is controlled by the ability of H2O to preferentially adsorb to surface Ca atoms over competing species, even when dissolution is dominated by H+ or H2CO3. More importantly, we identify for the first time that adsorbed H+ enhances the role of water by weakening surface Ca-O bonds. We also identify that H2CO3 undergoes dissociative adsorption resulting in adsorbed HCO3- and H+. Adsorbed HCO3- that competes with H2O for Ca acute edge sites inhibits dissolution, while adsorbed H+ at the neighboring surface of CO3 enhances dissolution. The net effect of the dissociative adsorption of H2CO3 is enhanced dissolution. These results will impact future efforts to more accurately model the impact of solutes in complex water matrices on carbonate mineral dissolution.

Local-environment-guided selection of atomic structures for the development of machine-learning potentials.

Machine learning potentials (MLPs) have attracted significant attention in computational chemistry and materials science due to their high accuracy and computational efficiency. The proper selection of atomic structures is crucial for developing reliable MLPs. Insufficient or redundant atomic structures can impede the training process and potentially result in a poor quality MLP. Here, we propose a local-environment-guided screening algorithm for efficient dataset selection in MLP development. The algorithm utilizes a local environment bank to store unique local environments of atoms. The dissimilarity between a particular local environment and those stored in the bank is evaluated using the Euclidean distance. A new structure is selected only if its local environment is significantly different from those already present in the bank. Consequently, the bank is then updated with all the new local environments found in the selected structure. To demonstrate the effectiveness of our algorithm, we applied it to select structures for a Ge system and a Pd13H2 particle system. The algorithm reduced the training data size by around 80% for both without compromising the performance of the MLP models. We verified that the results were independent of the selection and ordering of the initial structures. We also compared the performance of our method with the farthest point sampling algorithm, and the results show that our algorithm is superior in both robustness and computational efficiency. Furthermore, the generated local environment bank can be continuously updated and can potentially serve as a growing database of feature local environments, aiding in efficient dataset maintenance for constructing accurate MLPs.

Mass Transfer Limitation within Molecular Crowding Electrolyte Reorienting (100) and (101) Texture for Dendrite-Free Zinc Metal Batteries.

Aqueous zinc metal batteries are emerging as a promising alternative for energy storage due to their high safety and low cost. However, their development is hindered by the formation of Zn dendrites and side reactions. Herein, a macromolecular crowding electrolyte (MCE40) is prepared by incorporating polyvinylpyrrolidone (PVP) into the aqueous solutions, exhibiting an enlarged electrochemical stability window and anti-freezing properties. Notably, through electrochemical measurements and characterizations, it is discovered that the mass transfer limitation near the electrode surface within the MCE40 electrolyte inhibits the (002) facets. This leads to the crystallographic reorientation of Zn deposition to expose the (100) and (101) textures, which undergo a "nucleation-merge-growth" process to form a uniform and compact Zn deposition. Consequently, the MCE40 enables highly reversible and stable Zn plating/stripping in Zn/Cu half cells over 600 cycles and in Zn/Zn symmetric cells for over 3000 hours at 1.0 mA cm-2. Furthermore, Na0.33V2O5/Zn and α-MnO2/Zn full cells display promising capacity and sustained stability over 500 cycles at room and sub-zero temperatures. This study highlights a novel electrochemical mechanism for achieving preferential Zn deposition, introducing a unique strategy for fabricating dendrite-free zinc metal batteries.

Enabling High-Performance All-Solid-State Batteries via Guest Wrench in Zeolite Strategy.

All-solid-state batteries with a high energy density and safety are desirable candidates for next-generation energy storage applications. However, conventional solid electrolytes for all-solid-state batteries encounter limitations such as poor ionic conduction, interfacial compatibility, instability, and high cost. Herein, taking advantage of the ingenious capability of zeolite to incorporate functional guests in its void space, we present an innovative ionic activation strategy based on the "guest wrench" mechanism, by introducing a pair of cation and anion of LiTFSI-based guest species (GS) into the supercage of the LiX zeolite, to fabricate a zeolite membrane (ZM)-based solid electrolyte (GS-ZM) with high Li ionic conduction and interfacial compatibility. The restriction of zeolite frameworks toward the framework-associated Li ions is significantly reduced through the dynamic coordination of Li ions with the "oxygen wrench" of TFSI- at room temperature as shown by experiments and Car-Parrinello molecular dynamics simulations. Consequently, the GS-ZM shows an ∼100% increase in ionic conductivity compared with ZM and an outstanding Li+ transference number of 0.97. Remarkably, leveraging the superior ionic conduction of GS-ZM with the favorable interface structure between GS-ZM and electrodes, the assembled all-solid-state Li-ion and Li-air batteries based on GS-ZM exhibit the best-level electrochemical performance much superior to batteries based on liquid electrolytes: a capacity retention of 99.3% after 800 cycles at 1 C for all-solid-state Li-ion batteries and a cycle life of 909 cycles at 500 mA g-1 for all-solid-state Li-air batteries. The mechanistic discovery of a "guest wrench" in zeolite will significantly enhance the adaptability of zeolite-based electrolytes in a variety of all-solid-state energy storage systems with high performance, high safety, and low cost.

Well-Defined Nanoparticle Electrocatalysts for the Refinement of Theory.

The relationship between experiment and theory in electrocatalysis is one of profound importance. Until fairly recently, the principal role of theory in this field was interpreting experimental results. Over the course of the past decade (roughly the period covered by this review), however, that has begun to change, with theory now frequently leading the design of electrocatalytic materials. Though rewarding, this has not been a particularly easy union. For one thing, experimentalists and theorists have to come to grips with the fact that they rely on different models. Theorists make predictions based on individual, perfect structural models, while experimentalists work with more complex and heterogeneous ensembles of electrocatalysts. As discussed in this review, computational capabilities have improved in recent years, so that theory is better represented by the structures that experimentalists are able to prepare. Likewise, synthetic chemists are able to make ever more complex electrocatalysts with high levels of control, which provide a more extensive palette of materials for testing theory. The goal of this review is to highlight research from the last ∼10 years that focuses on carefully controlled electrocatalytic experiments which, in combination with theoretical predictions, bring us closer to bridging the gap between real catalysts and computational models.

Controlling the Shape Anisotropy of Monoclinic Nb12O29 Nanocrystals Enables Tunable Electrochromic Spectral Range.

Electrochromic smart windows that modulate the solar transmittance in a wide and selective spectral range can optimize building energy efficiency. However, for conventional materials such as bulk transition metal oxides, the electrochromic spectral range is constrained by their crystal structure with limited tunability. Herein, we report a method to control the shape anisotropy of monoclinic Nb12O29 nanocrystals and obtain a tunable electrochromic spectral range. We demonstrate the synthesis of monoclinic Nb12O29 nanorods (NRs), extending one-dimensionally along the b direction, and monoclinic Nb12O29 nanoplatelets (NPLs), extending two-dimensionally along the b and c directions. Upon electrochemical reduction accompanied by Li insertion, the NR films show increasing absorbance mostly in the near infrared region. In contrast, the NPL films show increasing absorbance in the near infrared region first followed by increasing absorbance in both visible and near infrared regions. To elucidate the influence of shape anisotropy, we used density functional theory to construct the lithiated structures of monoclinic Nb12O29 and in these structures we identified the presence of square planar sites and crystallographic shear sites for Li insertion. By calculating the theoretical spectra of the lithiated structures, we demonstrate that the Li insertion into the square planar sites results in absorption in the near infrared region in both NRs and NPLs due to their extension in the b direction, while the subsequent insertion of Li into the crystallographic shear sites leads to absorption in both visible and near infrared regions, which only occurs in NPLs due to their extension in the c direction.

Low-Valent Metal Ions as MOF Pillars: A New Route Toward Stable and Multifunctional MOFs.

PCM-102 is a new organophosphine metal-organic framework (MOF) featuring diphosphine pockets that consist of pairs of offset trans-oriented P(III) donors. Postsynthetic addition of M(I) salts (M = Cu, Ag, Au) to PCM-102 induces single-crystal to single-crystal transformations and the formation of trans-[P2M]+ solid-state complexes (where P = framework-based triarylphosphines). While the unmetalated PCM-102 has low porosity, the addition of secondary Lewis acids to install rigid P-M-P pillars is shown to dramatically increase both stability and selective gas uptake properties, with N2 Brunauer-Emmett-Teller surface areas >1500 m2 g-1. The Ag(I) analogue can also be obtained via a simple, one-pot peri-synthetic route and is an ideal sacrificial precursor for materials with mixed bimetallic MA/MB pillars via postsynthetic, solvent-assisted metal exchange. Notably, the M-PCM-102 family of MOFs contain periodic trans-[P2M]+ sites that are free of counter anions, unlike traditional analogous molecular complexes, since the precursor PCM-102 MOF is monoanionic, enabling access to charge-neutral metal-pillared materials. Four M-PCM-102 materials were evaluated for the separation of C2 hydrocarbons. The separation performance was found to be tunable based on the metal(s) incorporated, and density functional theory was employed to elucidate the nature of the unusual observed sorption preference, C2H2 > C2H6 > C2H4.

Black Tungsten Oxide Nanofiber as a Robust Support for Metal Catalysts: High Catalyst Loading for Electrochemical Oxygen Reduction.

Black valve metal oxides with low oxygen vacancies are identified to be promising for various industrial applications, such as in gas sensing, photocatalysis, and rechargeable batteries, owing to their high reducibility and stability, as well as considerable fractions of low-valent metal species and oxygen vacancies in their lattices. Herein, the nanofiber (NF) of black oxygen-deficient tungsten trioxide (WO3- x ) is presented as a versatile and robust support for the direct growth of a platinum catalyst for oxygen reduction reaction (ORR). The nonstoichiometric, poorly crystallized black WO3- x NFs are prepared by electrospinning the W precursor into NFs followed by their low-temperature (650 °C) reductive calcination. The black WO3- x NFs have adequate electrical conductivity owing to their decreased bandgap and amorphous structure. Remarkably, the oxygen-deficient surface (surface O/W = 2.44) facilitates the growth of small Pt nanoparticles, which resist aggregation, as confirmed by structural characterization and computational analysis. The Pt-loaded black WO3- x NFs outperform the Pt-loaded crystalline white WO3- x NFs in both the electrochemical ORR activity and the accelerated durability test. This study can inspire the use of oxygen-deficient metal oxides as supports for other electrocatalysts, and can further increase the versatility of oxygen-deficient metal oxides.

3d Transition-Metal-Mediated Columbite Nanocatalysts for Decentralized Electrosynthesis of Hydrogen Peroxide.

Decentralized electrosynthesis of hydrogen peroxide (H2 O2 ) via oxygen reduction reaction (ORR) can enable applications in disinfection control, pulping and textile bleaching, wastewater treatment, and renewable energy storage. Transition metal oxides are usually not efficient catalysts because they are more selective to produce H2 O. Here, it is shown that divalent 3d transition metal cations (Mn, Fe, Co, Ni, and Cu) can control the catalytic activity and selectivity of columbite nanoparticles. They are synthesized using polyoxoniobate (K7 HNb6 O19 ·13H2 O) and divalent metal cations by a hydrothermal method. The optimal NiNb2 O6 holds an H2 O2 selectivity of 96% with the corresponding H2 O2 Faradaic efficiency of 92% in a wide potential window from 0.2 to 0.6 V in alkaline electrolyte, superior to other transition metal oxide catalysts. Ex situ X-ray photoelectron and operando Fourier-transformed infrared spectroscopic studies, together with density functional theory calculations, reveal that 3d transition metals shift the d-band center of catalytically active surface Nb atoms and change their interactions with ORR intermediates. In an application demonstration, NiNb2 O6 delivers H2 O2 productivity up to 1 molH2O2 gcat -1 h-1 in an H-shaped electrolyzer and can yield catholytes containing 300 × 10-3 m H2 O2 to efficiently decomposing several organic dyes. The low-cost 3d transition-metal-mediated columbite catalysts show excellent application potentials.

PTCDA Molecular Monolayer on Pb Thin Films: An Unusual π-Electron Kondo System and Its Interplay with a Quantum-Confined Superconductor.

The hybridization of magnetism and superconductivity has been an intriguing playground for correlated electron systems, hosting various novel physical phenomena. Usually, localized d or f electrons are central to magnetism. In this study, by placing a PTCDA (3,4,9,10-perylene tetracarboxylic dianhydride) molecular monolayer on ultrathin Pb films, we built a hybrid magnetism/superconductivity (M/SC) system consisting of only sp electronic levels. The magnetic moments reside in the unpaired molecular orbital originating from interfacial charge transfers. We reported distinctive tunneling spectroscopic features of such a Kondo screened π electron impurity lattice on a superconductor in the regime of T_{K}≫Δ, suggesting the formation of a two-dimensional bound states band. Moreover, moiré superlattices with tunable twist angle and the quantum confinement in the ultrathin Pb films provide easy and flexible implementations to tune the interplay between the Kondo physics and the superconductivity, which are rarely present in M/SC hybrid systems.

Unveiling the Role of Sulfur in Rapid Defluorination of Florfenicol by Sulfidized Nanoscale Zero-Valent Iron in Water under Ambient Conditions.

Groundwater contamination by halogenated organic compounds, especially fluorinated ones, threatens freshwater sources globally. Sulfidized nanoscale zero-valent iron (SNZVI), which is demonstrably effective for dechlorination of groundwater contaminants, has not been well explored for defluorination. Here, we show that SNZVI nanoparticles synthesized via a modified post-sulfidation method provide rapid dechlorination (∼1100 µmol m-2 day-1) and relatively fast defluorination (∼6 µmol m-2 day-1) of a halogenated emerging contaminant (florfenicol) under ambient conditions, the fastest rates that have ever been reported for Fe0-based technologies. Batch reactivity experiments, material characterizations, and theoretical calculations indicate that coating S onto the metallic Fe surface provides a highly chemically reactive surface and changes the primary dechlorination pathway from atomic H for nanoscale zero-valent iron (NZVI) to electron transfer for SNZVI. S and Fe sites are responsible for the direct electron transfer and atomic H-mediated reaction, respectively, and ß-elimination is the primary defluorination pathway. Notably, the Cl atoms in florfenicol make the surface more chemically reactive for defluorination, either by increasing florfenicol adsorption or by electronic effects. The defluorination rate by SNZVI is ∼132-222 times higher with chlorine attached compared to the absence of chlorine in the molecule. These mechanistic insights could lead to new SNZVI materials for in situ groundwater remediation of fluorinated contaminants.

Factors that influence hydrogen binding at metal-atop sites.

The d-band model has proven to be effective for understanding trends in the chemisorption of various adsorbates on transition metal surfaces. However, hydrogen adsorption at the atop site of transition metals and their bimetallic alloy surfaces do not always correlate well with the d-band center of the adsorption site. Additionally, the d-band model cannot explain the disappearance of the local minima for H adsorption at the hollow site on the potential energy surface of 5d single-atom element doped Au and Ag(111) surfaces. Here, we use a simple model with factors, including the d-band center, filling of the d-band, renormalized adsorbate states, coupling matrix elements, and surface-adsorbate bond lengths, to correlate with the density functional theory calculated H binding energies on both mono- and bimetallic (111) surfaces. Our results suggest that H adsorption at metal-atop sites is determined by all these factors, not only by the d-band center. The strong adsorption of H at the atop sites of 5d metal surfaces can be explained by their lower repulsive contribution.

Formation of HONO from the NH3-promoted hydrolysis of NO2 dimers in the atmosphere.

One challenging issue in atmospheric chemistry is identifying the source of nitrous acid (HONO), which is believed to be a primary source of atmospheric "detergent" OH radicals. Herein, we show a reaction route for the formation of HONO species from the NH3-promoted hydrolysis of a NO2 dimer (ONONO2), which entails a low free-energy barrier of 0.5 kcal/mol at room temperature. Our systematic study of HONO formation based on NH3 + ONONO2 + nH2O and water droplet systems with the metadynamics simulation method and a reaction pathway searching method reveals two distinct mechanisms: (i) In monohydrates (n = 1), tetrahydrates (n = 4), and water droplets, only one water molecule is directly involved in the reaction (denoted the single-water mechanism); and (ii) the splitting of two neighboring water molecules is seen in the dihydrates (n = 2) and trihydrates (n = 3) (denoted the dual-water mechanism). A comparison of the computed free-energy surface for NH3-free and NH3-containing systems indicates that gaseous NH3 can markedly lower the free-energy barrier to HONO formation while stabilizing the product state, producing a more exergonic reaction, in contrast to the endergonic reaction for the NH3-free system. More importantly, the water droplet reduces the free-energy barrier for HONO formation to 0.5 kcal/mol, which is negligible at room temperature. We show that the entropic contribution is important in the mechanism by which NH3 promotes HONO formation. This study provides insight into the importance of fundamental HONO chemistry and its broader implication to aerosol and cloud processing chemistry at the air-water interface.

Intrinsic Activity of Metal Centers in Metal-Nitrogen-Carbon Single-Atom Catalysts for Hydrogen Peroxide Synthesis.

Metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) show high catalytic activity for many important chemical reactions. However, an understanding of their intrinsic catalytic activity remains ambiguous because of the lack of well-defined atomic structure control in current M-N-C SACs. Here, we use covalent organic framework SACs with an identical metal coordination environment as model catalysts to elucidate the intrinsic catalytic activity of various metal centers in M-N-C SACs. A pH-universal activity trend is discovered among six 3d transition metals for hydrogen peroxide (H2O2) synthesis, with Co having the highest catalytic activity. Using density functional calculations to access a total of 18 metal species, we demonstrate that the difference in the binding energy of O2* and HOOH* intermediates (EO2* - EHOOH*) on single metal centers is a reliable thermodynamic descriptor to predict the catalytic activity of the metal centers. The predicted high activity of Ir centers from the descriptor is further validated experimentally. This work suggests a class of structurally defined model catalysts and clear mechanistic principles for metal centers of M-N-C SACs in H2O2 synthesis, which may be further extendable to other reactions.

Reversible Solid-State Isomerism of Azobenzene-Loaded Large-Pore Isoreticular Mg-CUK-1.

A large-pore version of Mg-CUK-1, a water-stable metal-organic framework (MOF) with 1-D channels, was synthesized in basic water. Mg-CUK-1L has a BET surface area of 2896 m2 g-1 and shows stark selectivity for CO2 sorption over N2, O2, H2, and CH4. It displays reversible, multistep gated sorption of CO2 below 0.33 atm. The dehydrated single-crystal structure of Mg-CUK-1L confirms retention of the open-channel structure. The MOF can be loaded with organic molecules by immersion in hot melts, providing single crystals suitable for X-ray diffraction. trans-Azobenzene fills the channels in a 2 × 2 arrangement. Solid-state UV-vis spectroscopy reveals that azobenzene molecules undergo reversible trans-cis isomerization, despite being close-packed; this surprising result is confirmed by DFT-simulated UV-vis spectra.

Electrical and Structural Dual Function of Oxygen Vacancies for Promoting Electrochemical Capacitance in Tungsten Oxide.

Intrinsic defects, including oxygen vacancies, can efficiently modify the electrochemical performance of metal oxides. There is, however, a limited understanding of how vacancies influence charge storage properties. Here, using tungsten oxide as a model system, an extensive study of the effects of structure, electrical properties, and charge storage properties of oxygen vacancies is carried out using both experimental and computational techniques. The results provide direct evidence that oxygen vacancies increase the interlayer spacing in the oxide, which suppress the structural pulverization of the material during electrolyte ion insertion and removal in prolonged stability tests. Specifically, no capacitive decay is detected after 30 000 cycles. The medium states and charge storage mechanism of oxygen-deficient tungsten oxide throughout electrochemical charging/discharging processes is studied. The enhanced rate capability of the oxygen-deficient WO3- x is attributed to improved charge storage kinetics in the bulk material. The WO3- x electrode exhibits the highest capacitance in reported tungsten-oxide based electrodes with comparable mass loadings. The capability to improve electrochemical capacitance performance of redox-active materials is expected to open up new opportunities for ultrafast supercapacitive electrodes.

Bioinspired CNP Iron(II) Pincers Relevant to [Fe]-Hydrogenase (Hmd): Effect of Dicarbonyl versus Monocarbonyl Motifs in H2 Activation and Transfer Hydrogenation.

A set of bioinspired carbamoyl CNP pincer complexes are reported that are relevant to [Fe]-hydrogenase (Hmd). The dicarbonyl species [(CNHNNHPR2)Fe(CO)2I] [R = Ph, 1; R = iPr, 2] undergoes ligand deprotonation, resulting in the dearomatized complexes of formulas [(CNHNN=PR2)Fe(CO)2] (5 and 6). The crystal structure and 1H{31P} NMR spectroscopy of the iodide-bound dearomatized species [Na(18-crown-6)][(CNHNN=PPh2)Fe(CO)2I] (7) showed that the deprotonated moiety was the phosphoramine N(H) linkage. Separately, the monocarbonyl complexes [(CNHNNHPR2)Fe(CO)(MeCN)2](BF4) (8 and 9) synthesized, as well as deprotonated and dearomatized in similar fashion. Reactivity studies revealed that the parent dicarbonyl complexes require more forceful conditions for H2 activation, compared with the monocarbonyl complexes. The ligand backbone was not found to participate in H2 activation and H2 → hydride transfer to an organic substrate was not observed in either case. Density functional theory calculations revealed that the higher reactivity of the monocarbonyl complex in H2 splitting could be attributed to its higher affinity for H2. This behavior is attributed to two key points related to the requisite dπ(Fe) → σ*(H2) back-bonding interaction in a conventional M-H2 Kubas interaction: (i) generally, the weaker π donor capacity of the dicarbonyls, and (ii) specifically, the detrimental effect of a strongly π acidic CO ligand (versus weakly π acidic MeCN ligand) trans to the H2 activation site. The higher reactivity of the monocarbonyl complex is also evidenced by the catalytic transfer hydrogenation by monocarbonyl 8, whereas dicarbonyl 1 was ineffective. Overall, the results suggest that Nature uses the dicarbonyl motif in [Fe]-hydrogenase to diminish the interaction between the Fe center and dihydrogen, thereby preventing premature H2 activation prior to substrate (H4MPT+) binding and any resulting nonspecific hydride transfer reactivity.