Isolable f-element diphosphene complexes by phosphinidene group transfer and coupling at uranium

Response to NMETH-{type:entrez-nucleotide,attrs:{text:C14399,term_id:1569106,term_text:C14399}}C14399: A battery of new mechanistic investigations will be required to provide a full explanation of the observation that individual, small-molecule compounds, referred to here as 'protective agents', have the capacity to promote dramatic enhancements to overall fluorophore performance1. As Tinnefeld and Cordes suggest, cycles of reduction (red) and oxidation (ox) - and/or the reverse -may confer a 'self-healing' property to the fluorogenic center. This could indeed be achieved by a single neighboring molecule, like Trolox, facilitating the fluorophore's rapid return to the ground state from relatively long-lived, non-fluorescent, radical intermediates through 'ping-pong' red-ox chemistry. However, this model requires that the rates of red-ox cycling be properly matched for benefits to be achieved. Further insights into the prevalence and lifetimes of charged intermediates for each molecule will need to be quantified to validate this model. Alternative mechanisms for photostabilization, which may play a significant or even dominant role, will also need to be carefully considered and quantified. Previous investigations, largely motivated by the once vigorous dye-laser industry, suggest that meaningful contributions to fluorophore performance may arise from changes in the rates of internal conversion and/or the introduction of mechanisms for triplet-triplet energy transfer and exciplex-type relaxation pathways2. Triplet-triplet energy transfer, a mechanism that would also fall into the 'self-healing' category, could provide substantial benefits to fluorophore performance by reducing the lifetime of non-fluorescent triplet states from which photobleaching and/or radical states can occur. Given our estimates of the triplet energy and red-ox potential of cyclooctatetraene (unpublished data), this mechanism may potentially be the dominant pathway of cyanine fluorophore photostabilization. Triplet-triplet energy transfer can be differentiated from red-ox-type chemistries by quantifying changes in triplet state lifetimes and charged, intermediate species. Other pathways may also exist that are distinct from the 'self-healing' variety. For example, a 'self-protecting' mechanism that reduces the probability of transitions into damaged states in the first place would also give rise to enhancements in fluorophore performance. Single-molecule imaging efforts can play an important role in the characterization of fluorophore performance by giving direct access to the frequency and duration of fluorescent and non-fluorescent states as well as a fluorophore’s total photon output prior to blinking or photobleaching. However, a quantitative understanding of the distinct mechanisms for enhancing fluorophore performance can only be delineated with confidence when married with bulk electrochemical and spectroscopic investigations. These combined approaches, while requiring large quantities of material, could provide grounded insights into the impact and relative weighting of 'self-healing' and 'self-protecting' mechanisms for distinct fluorophore types. Progress on this front will enable both further improvements in the performances of known protective agents and the search for new compounds with similar or improved properties. Developments of this kind may ultimately facilitate the design and synthesis of new classes of next-generation fluorophores spanning the visible spectrum that exhibit enhancements in performance even greater than those observed for the cyanine class (see Correspondence by Altman et al) and tailored properties for distinct experimental demands.

New luminescent complexes of Os(II) that contain either 2,2{prime}-bipyridine (bpy) or 1,10-phenanthroline (phen) as the chromophorib acceptor ligand have been prepared by a combination of established and new synthetic methods. Extensive use of Os(IV) and Os(III) precursors, e.g., Os{sup IV}(bpy)Cl{sub 4} and mer-Os{sup III}(PMe{sub 2}Ph){sub 3}Cl{sub 3} has led to the preparation of materials with ancillary ligands such as tertiary phosphines as preparative intermediates, including Os{sup III}(bpy)(PMe{sub 2}Ph)Cl{sub 3} and cis-Os{sup II}(phen)(diphosphine)Cl{sub 2}. Further substitution of chloro ligands into complexes such as these results in the formation of emissive complexes of Os(II). Another new synthetic route utilizes the versatile Os(II) precursor Os(bpy){sub 2}CO{sub 3}, which allows the facile preparation of dicationic, disubstituted species such as (Os(bpy){sub 2}(norbornadiene)){sup 2+}. Another general procedure, based on the control of solvent and temperature in the substitution chemistry of cis-Os(bpy){sub 2}Cl{sub 2}, has been further developed to produce a variety of new complexes of the types cis-(Os(bpy){sub 2}(L)Cl){sup +} and cis(Os(bpy){sub 2}(L){sub 2}){sup 2+}, where L is a phosphine, arsine, nitrogen, or olefin donor ligand. In a few cases, phosphine entering groups cause the cis geometry to be unfavorable and new trans-(Os(bpy){sub 2}(L){sub 2}){sup 2+} complexes have also been isolated. The resultant complexes comprise themore » largest family of transition-metal-based excited-state reagents with tunable photophysical and redox properties available. When possible, the new complexes have been characterized by UV-visible spectroscopy, emission spectroscopy, cyclic voltammetry, and {sup 31}P and/or {sup 1}H NMR spectroscopy.« less

Lewis acid-base pair chemistry has been placed on a new level with the discovery that adduct formation between an electron donor (Lewis base) and acceptor (Lewis acid) can be inhibited by the introduction of steric demand, thus preserving the reactivity of both Lewis centers, resulting in highly unusual chemistry. Some of these highly versatile frustrated Lewis pairs (FLP) are capable of splitting a variety of small molecules, such as dihydrogen, in a heterolytic and even catalytic manner. This is in sharp contrast to classical reactions where the inert substrate must be activated by a metal-based catalyst. Very recently, research has emerged combining the two concepts, namely the formation of FLPs in which a metal compound represents the Lewis base, allowing for novel chemistry by using the heterolytic splitting power of both together with the redox reactivity of the metal. Such reactivity is not restricted to the metal center itself being a Lewis acid or base, also ancillary ligands can be used as part of the Lewis pair, still with the benefit of the redox-active metal center nearby. This Minireview is designed to highlight the novel reactions arising from the combination of metal oxido transition-metal or rare-earth-metal compounds with the Lewis acid B(C6 F5 )3 . It covers a wide area of chemistry including small molecule activation, hydrogenation and hydrosilylation catalysis, and olefin metathesis, substantiating the broad influence of the novel concept. Future goals of this young and exciting area are briefly discussed.

A series of Ni(0) compounds supported by electronically similar N-heterocyclic carbene (NHC) ancillary ligands with a range of %Vbur were used as catalysts for aryl C–H bond silylation, germylation, and stannylation. The NHC steric bulk strongly influenced the selectivity of C–H functionalization to give new carbon–heteroatom bonds versus alkene hydroarylation, despite little structural change in the resting state of the catalysts. Studies were performed by reacting C6F5H and H2C═CHER3 (ER3 = SnBu3, GePh3, SiMe3) using catalytic amounts of Ni(COD)2 and NHC ligands IPr, IMes, IBn, and iPr2Im. Catalytic C–H stannylation to give C6F5SnBu3 was facile with all ligands. The catalytic C–H germylation reaction was more difficult than stannylation but was demonstrated using H2C═CHGePh3 to give C6F5GePh3 for all but the largest NHC. The bulkiest NHC, IPr, gave a 96:4 ratio of the hydroarylation product C6F5CH2CH2GePh3 versus C6F5GePh3. The C–H silylation reactions required the highest temperatures and gave selective silylation product C6F5SiMe3 only for the smallest IBn and iPr2Im NHC ligands. Using the larger IMes carbene resulted in a 66:34 mixture of silylation and hydroarylation products, and the largest NHC, IPr, gave exclusive conversion to the hydroarylation product, C6F5CH2CH2SiMe3. DFT calculations are provided that give insight into the mechanism and key reaction steps, such as the relative difficulty of the critical β-Sn, Ge, and Si elimination steps.

Systematically theoretical investigation the effect of nitrogen and iron-doped graphdiyne on the oxygen reduction reaction mechanism in proton exchange membrane fuel cells

The reduction reaction of diatomic oxygen molecules (O2(g)) to split into monatomic oxide ions (O2-) is attracting attention for the oxide cathode reactivity to reduce cathodic over-potential in solid oxide fuel cells and other electrochemical applications. Various experimental and theoretical approaches have been reported to understand the rate determining step (RDS) of the oxygen reduction reaction (ORR) on the surface of oxide cathodes, mainly composed of 3d transition metal like (La, Sr)CoO3. Although it is generally accepted that RDS of ORR on oxide cathodes is the process for a surface adsorbate of to meet with surface oxygen vacancy (Vö(s)) [i], an atomistic analysis on the adsorption of oxygen molecule and the splitting reaction is still under debated; how the electrons are transferred and how the splitting of oxygen molecule proceeds. On the other hand, the most stable form of O2(g) is so-called “triplet” oxygen, which has a pair of up -spin electrons in two orbitals and the highest molecular orbital of is completely unoccupied. This electron configuration indicates that, the oxygen molecule, during ionization process, initially accept only two down -spin electrons to form bonded to surface Ni ions, and subsequently a pair of up - and down -spin electrons to dissociated into two O2-on surface. One may, then, come up with a simple question whether or not oxygen molecules have such spin preference during ORR. If it is true, ORR on oxide is, to a certain extent, influenced by the spin polarization of donated electrons, and therefore, the spin configuration of the oxide surface. We employed ab initio molecular dynamics (ai-MD) simulation and a nudged elastic band (NEB) density function theory simulation to make precise analysis on the electron transfer process. The material we chose was nickelous oxide (NiO), which is well known to exhibit an anti-ferromagnetic (af) order below Néel temperature around 523 K and becomes the disordered paramagnetic (pm) state above the temperature. The ai-MD simulation was carried out using Ni24O24 and Ni23O23 supercells with and without Vö(s), respectively, connected to a vacuum cavity placed with 8 O2(g) molecules above NiO slab. In the latter case, Vö(s) was introduced as a Schottky pair to maintain charge neutrality. The ai-MD simulation was made with and without spin polarization corresponding to af- and pm-NiO. The simulation results show that, when a O2(g) molecule approaches to the NiO surface, O2(g) is immediately charged with electrons, primarily owing to the increase of the coordination number of Ni2+/3+ ions, from monodentate to bidentate regardless of the presence of Vö(s) and spin polarization [ii]. No splitting reaction was observed on surface without Vö. The adsorbates further dissociated on the surface of pm-NiO with Vö, but no splitting reaction proceeds on the surface of af-NiO even with the presence of Vö. The NEB simulation suggests that almost monotonous decrease of total energy with the increase of coordination number for pm-NiO, but a thermally activated process with its activation energy of 0.7 eV is necessary for the af-NiO case, because of additional spin dipole moment for the latter. Also observed was spin reversal during the electron transfer. The results indicate significant influence of spin polarization on ORR. A strong effect of af-order on the oxygen chemisorption on undoped and Li-doped NiO was discovered in late 60’s, but no reasonable explanation has been given [iii]. A similar strong influence of spin order of oxide cathode is anticipated in other 3d transition metal oxide systems with ferromagnetic and antiferromagnetic configuration, and the extent of “spin buffering capacity” seems a quite important factor that govern the reactivity of ORR on oxide surface. [i] M. Kukja et al, PCCP, 15(2013), 5443–5471: S. Miyoshi et al., Solid State Ionics, 285(2016), 202–208. [ii] S. Sugiura et al., Solid State Ionics, 285(2016), 209–214: S. Sugiura et al., To be submitted. [iii] E.R.S. Winter, J. of Catalysis 6(2016), 35–49: B. De Rosa et al., Reactivity of Solids 4(1987), 53-72.

A puzzling observation during the oxygen reduction reaction (ORR) on weak-binding electrodes such as Au is the preference to form hydrogen peroxide (H2O2) instead of the thermodynamically favored water product. This selectivity cannot be explained on the basis of thermodynamic reaction models that simply assume a series of proton-coupled electron transfers (PCETs). Here, we use ab initio molecular dynamics along with umbrella sampling to obtain free energy profiles for competing key ORR steps on Au(111). Our comparison includes not only PCETs but also “chemical” reaction steps that do not include an explicit faradaic charge transfer, such as desorption or surface dissociation. This allows one to explore favorable reaction paths while varying the capacitive charging to represent realistic ORR potentials. Our results show that all reaction steps competing with H2O2 formation have sizable kinetic barriers and are thus prohibited, even though they may be thermodynamically favored. We find that this situation does not change under more reducing conditions and specifically determines the “nobleness” of Au as playing a decisive role in preventing O–O bond scission. It is thus not the applied potential but the underlying chemistry that drives the ORR selectivity. Our study overall further highlights the kinetic competition between PCET and non-PCET steps that cannot be resolved via simple Bro̷nsted–Evans–Polanyi scaling relations.

Despite tremendous progress in developing doped carbocatalysts for the oxygen reduction reaction (ORR), the ORR activity of current metal-free carbocatalysts is still inferior to that of conventional Pt/C catalysts, especially in acidic media and neutral solution. Moreover, it also remains a challenge to develop an effective and scalable method for the synthesis of metal-free carbocatalysts. Herein, we have developed nitrogen and phosphorus dual-doped hierarchical porous carbon foams (HP-NPCs) as efficient metal-free electrocatalysts for ORR. The HP-NPCs were prepared for the first time by copyrolyzing nitrogen- and phosphorus-containing precursors and poly(vinyl alcohol)/polystyrene (PVA/PS) hydrogel composites as in situ templates. Remarkably, the resulting HP-NPCs possess controllable nitrogen and phosphorus content, high surface area, and a hierarchical interconnected macro-/mesoporous structure. In studying the effects of the HP-NPCs on the ORR, we found that the as-prepared HP-NPC materials exhibited not only excellent catalytic activity for ORR in basic, neutral, and acidic media, but also much better tolerance for methanol oxidation and much higher stability than the commercial, state-of-the-art Pt/C catalysts. Because of all these outstanding features, it is expected that the HP-NPC material will be a very suitable catalyst for next-generation fuel cells and lithium-air batteries. In addition, the novel synthetic method described here might be extended to the preparation of many other kinds of hierarchical porous carbon materials or porous carbon that contains metal oxide for wide applications including energy storage, catalysis, and electrocatalysis.

3D Nitrogen-doped graphene prepared by pyrolysis of graphene oxide with polypyrrole for electrocatalysis of oxygen reduction reaction

Perovskites (ABO3) with transition metals in active Bsites are considered alternative catalysts for the water oxidation tooxygen through the oxygen evolution reaction (OER) and for theoxygenreduction through the oxygen reduction reaction (ORR) back to water. We have synthesized a double perovskite (A2BB'O6) with different cations in A, B, and B' sites, namely, (La1.5Sr0.5)A(Ni0.5Mn0.5)B(Ni0.5Ru0.5)B'O6 (LSNMR), which displays an outstanding OER/ORR bifunctional performance. The composition and structure of the oxide has been determined by powder X-ray diffraction, powder neutron diffraction, and transmission electron microscopy to be monoclinic with the space group P21/ n and with cationic ordering between the ions in the B and B' sites. X-ray absorption near-edge spectroscopy suggests that LSNMR presents a configuration of ∼Ni2+, ∼Mn4+, and ∼Ru5+. This bifunctional catalyst is endowed with high ORR and OER activities in alkaline media, with a remarkable bifunctional index value of ∼0.83 V (the difference between the potentials measured at -1 mA cm-2 for the ORR and +10 mA cm-2 for the OER). The ORR onset potential ( Eonset) of 0.94 V is among the best reported to date in alkaline media for ORR-active perovskites. The ORR mass activity of LSNMR is 1.1 A g-1 at 0.9 V and 7.3 A g-1 at 0.8 V. Furthermore, LSNMR is stable in a wide potential window down to 0.05 V. The OER potential to achieve a current density of 10 mA cm-2 is 1.66 V. Density functional theory calculations demonstrate that the high ORR/OER activity of LSNMR is related to the presence of active Mn sites for the ORR- and Ru-active sites for the OER by virtue of the high symmetry of the respective reaction steps on those sites. In addition, the material is stable to ORR cycling and also considerably stable to OER cycling.

Advancing energy conversion technologies requires cost‐efficient electrocatalysts for the oxygen reduction reaction (ORR). Iron phthalocyanine (FePc) emerges as a scalable and economical ORR electrocatalyst. However, the Fe–N4 configuration in FePc still falls short of the satisfied ORR activity and stability under electrocatalytic conditions. Here, an effective f‐p‐d (Eu–O–Fe) gradient orbital coupling strategy is introduced by integrating FePc with Eu2O3 (FePc/Eu2O3) to enhance the spin state and ORR performance of the Fe center through a precisely designed, scalable synthetic approach. The Eu─O bond promotes electron delocalization and shifts the spin state of Fe center from low‐spin to intermediate‐spin, increasing the eg​ orbital occupancy. This modification optimizes the adsorption of oxygen‐containing intermediates and lowers the ORR energy barrier. Notably, the increased spin state of Fe accelerates charge transfer by releasing more unpaired electrons, improving reaction kinetics. Furthermore, the f‐band serves as a buffer layer for electron compensation during ORR, further stabilizing the covalency and electronic configuration of atomic Fe and boosting durability. The one‐batch synthesis produces exceeding 300 g of FePc/Eu2O3, achieving a half‐wave potential of 0.931 V (vs RHE) at a cost less than 1/15 of commercial Pt/C. It demonstrates exceptional ORR performance in aluminum–air batteries, highlighting its significant application potential.

O2 binding to metal ions is an essential step in the oxygen reduction reaction (ORR). Tailoring O2 binding is anticipated to realize highly active and selective four-electron ORR. Herein, we report on fine-tuning the molecular pocket of Co porphyrins to get a trade-off between thermodynamics and kinetics of O2 binding for ORR with high activity and selectivity. Three Co tetra(2-amidophenyl)porphyrins with an αααα structure but different steric hindrance at the pocket bottleneck are synthesized. The pocket thermodynamically favors O2 binding by stabilizing O2 adducts and thus improves the 4e selectivity, but the sterically hindered pocket entrance may obstruct the access of O2 to Co. By tuning the pocket entrance, we tailored the rate constant and equilibrium constant of O2 binding, and consequently, we achieved efficient ORR with a half-wave potential of 0.83V versus RHE and a transferred electron number of 3.78. This performance is remarkable among reported mononuclear Co porphyrins.

The research activity carried out during the Ph.D aimed at the development of new synthetic routes for the production of acrylic and methacrylic acid using bio-alcohols as raw materials, respectively glycerol and propylene glycol. The first chapter of the thesis concerns the one-pot transformation of glycerol into acrylic acid, performed by using multifunctional catalysts. The overall process formally consists in two reaction steps: i) glycerol dehydration to acrolein, promoted by acid catalysis, and ii) acrolein oxidation to acrylic acid, promoted by redox catalysis. The design of suitable multifunctional catalysts is a complex matter and, so far, fundamental understanding behind the catalytic phenomenon remains unclear. In this context, the research work here reported aimed to shed light on the molecular-level relations that lie behind the catalytic results shown by several types of V-containing catalysts. The second chapter of the thesis concerns the study of a new synthetic route for the production of methacrylic acid starting from bio-propylene glycol. The overall process formally consists in three reaction steps: i) propylene glycol dehydration to propanal, ii) propanal oxidation to propionic acid, and iii) propionic acid condensation with formaldehyde, generated in-situ from methanol. Referring to reactions i) and ii), the research activity focussed on the possibility to perform the single-step gas-phase transformation of propylene glycol into propionic acid, by means of multifunctional catalysis (as previously done for glycerol one-pot transformation to acrylic acid). Finally, the study of the latter stage of the overall process to produce methacrylic acid was started, that is the condensation reaction between propionic acid and formaldehyde, generated in-situ from methanol. In particular, the catalytic activity of aluminium phosphate was fully investigated, so as to define the reactions that may occur when feeding propionic acid and methanol on pure acid catalysts.

In order to facilitate widespread commercialization of proton‐exchange membrane fuel cells (PEMFCs), the synthesis of advanced catalysts toward oxygen reduction reaction with high activity and stability is essential. Here, we report the synthesis of strongly coupled porous Pt nanospheres /three‐dimensional graphene (P−Pt @3DG) hybrid materials through a facile strategy. The P−Pt @3DG displays an excellent oxygen reduction reaction (ORR) performance with the mass activity of 0.542 A/mg Pt and specific activity of 0.976 mA/cm 2 at 0.9 V vs. RHE, respectively, which are 4.0‐ and 3.7‐ times higher than those of commercial Pt/C catalysts. More importantly, high stability with only 5.35 % loss after 10,000 cycles is achieved. The superior performance of P−Pt @3DG is mainly attributed to the open structure of porous Pt spheres, which exposes the internal Pt atoms with an ultrahigh Pt utilization. Moreover, the 3D graphenes possess an open and interconnected porous structure and allow efficient charge and mass transfer. Besides, this synthetic method is suitable for large scale production, which is highly applicable to the commercialization of PEMFCs.

Because of their essential roles on determining pathways of the oxygen reduction reaction (ORR), the adsorption behavior and decomposition of the radical OOH and hydrogen peroxide on Pt clusters (Ptn, n = 3, 6, and 10) are extensively investigated using density functional theory. Two types of adsorption of the radical OOH on Pt clusters are found. One-end adsorbed hydrogen peroxide H2O2, arising from reduction of adsorbed OOH, is also located on Pt3, with an adsorption energy of -0.63 eV, suggesting that the ORR may proceed via a series pathway generating H2O2 as an intermediate. However, since OOH readily decomposes on Pt3 into atop adsorbed atomic oxygen and hydroxyl with an activation energy of only ∼0.25 eV, the OOR may take place preferentially via a direct pathway without H2O2 produced. A potential energy surface profile for the ORR is proposed, and the adsorption properties of other involved oxide species are characterized.