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A 7-tungstabicyclo[4.3.0]nonane complex forms slowly upon addition of cyclohexene to the ethylene complex, W(NAr)(OSiPh 3 ) 2 (C 2 H 4 ), at 22 °C. A single-crystal X-ray study showed its structure to be closest to a square pyramid (τ = 0.23). At 22 °C, loss of cyclohexene or ring contraction of the 7-tungstabicyclo[4.3.0]nonane complex is slow. Above ∼80 °C, cyclohexene is ejected to give W(NAr)(OSiPh 3 ) 2 (C 2 H 4 ), but a sufficient amount of 7-tungstabicyclo[4.3.0]nonane complex remains in the presence of cyclohexene and the ring contracts to yield methylenecyclohexane and a methylidene complex or ethylene and a cyclohexylidene complex. Other complexes that have been observed include an 8-tungstabicyclo[4.3.0]nonane complex formed from 1,7-octadiene, a 7-tungstabicyclo[4.2.0]octane complex (formed from a methylidene complex and cyclohexene), and a methylenecyclohexane complex. 13 C-Labeling studies show that the exo-methylene group in methylenecyclohexane and the α positions in the 8-tungstabicyclo[4.3.0]nonane come from ethylene. An alternative ring contraction of a tungstacyclopentane made from two molecules of cyclohexene cannot be excluded when concentrations of ethylene are low. A cyclohexylidene complex could also form from two cyclohexenes via a newly proposed "alkyl/allyl" mechanism. The results reported here are the first experimental confirmations that a tungstacyclopentane can ring-contract thermally at a substituted WC α position to form a tungstacyclobutane and therefore metathesis-active alkylidenes.

The reaction between silylamido complexes of Cr(II), Fe(II), and Co(II) and IMes·2HF salt in the presence of IMes (IMes = 1,3-dimesitylimidazol-2-ylidene) led to isolation of Cr(IMes) 2 F 2 ( 2-Cr ), Fe(IMes) 2 F 2 ( 2-Fe ), and Co(IMes) 2 F 2 ( 2-Co ). X-ray structural studies revealed that 2-Cr adopts square planar geometry, while 2-Fe and 2-Co have distorted tetrahedral geometry. Magnetic susceptibility studies of 2-Cr , 2-Fe , and 2-Co were consistent with high-spin complexes, S = 2 for 2-Cr / 2-Fe and S = 3/2 for 2-Co . We demonstrated that fluoride can be successfully exchanged for cyanide and azide using trimethylsilyl cyanide and trimethylsilyl azide ( 3-Fe and 4-Fe ). DFT studies suggest that the preference of 2-Cr to adopt square planar geometry over tetrahedral is due to its d 4 metal center, where four electrons fill the lower-lying d-orbitals.

Cadmium selenide nanomaterials are very important materials in photonics, catalysis, and biomedical applications due to their optical properties that can be tuned through size, shape, and surface passivation. In this report, static and ab initio molecular dynamics density functional theory (DFT) simulations are used to characterize the effect of ligand adsorption on the electronic properties of the (110) surface of zinc blende and wurtzite CdSe and a (CdSe) 33 nanoparticle. Adsorption energies depend on ligand surface coverage and result from a balance between chemical affinity and ligand-surface and ligand-ligand dispersive interactions. In addition, while little structural reorganization occurs upon slab formation, Cd···Cd distances become shorter and the Se-Cd-Se angles become smaller in the bare nanoparticle model. This originates mid-gap states that strongly influence the absorption optical spectra of nonpassivated (CdSe) 33 . Ligand passivation on both zinc blende and wurtzite surfaces does not induce a surface reorganization, and thus, the band gap remains nonaffected with respect to bare surfaces. In contrast, structural reconstruction is more apparent for the nanoparticle, which significantly increases its highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap upon passivation. Solvent effects decrease the band gap difference between the passivated and nonpassivated nanoparticles, the maximum of the absorption spectra being blue-shifted around 20 nm by the effect of the ligands. Overall, calculations show that flexible surface cadmium sites are responsible for the appearance of mid-gap states that are partially localized on the most reconstructed regions of the nanoparticle that can be controlled through appropriate ligand adsorption., Competing Interests: The authors declare no competing financial interest., (© 2023 The Authors. Published by American Chemical Society.)

Electric-field-driven ion motion to tailor magnetic properties of materials (magneto-ionics) offers much promise in the pursuit of voltage-controlled magnetism for highly energy-efficient spintronic devices. Electrolyte gating is a relevant means to create intense electric fields at the interface between magneto-ionic materials and electrolytes through the so-called electric double layer (EDL). Here, improved magneto-ionic performance is achieved in electrolyte-gated cobalt oxide thin films with the addition of inorganic salts (potassium iodide, potassium chloride, and calcium tetrafluoroborate) to anhydrous propylene carbonate (PC) electrolyte. Ab initio molecular dynamics simulations of the EDL structure show that K + is preferentially located on the cobalt oxide surface and KI (when compared to KCl) favors the accumulation of positive charge close to the surface. It is demonstrated that room temperature magneto-ionics in cobalt oxide thin films is dramatically enhanced in KI-containing PC electrolyte at an optimum concentration, leading to 11-fold increase of generated magnetization and 35-fold increase of magneto-ionic rate compared to bare PC.

Controlling the formation of the desired product in the appropriate crystalline form is the fundamental breakthrough of crystal engineering. On that basis, the preferential formation between polymorphic forms, which are referred to as different assemblies achieved by changing the disposition or arrangement of the forming units within the crystalline structure, is one of the most challenging topics still to be understood. Herein, we have observed the formation of two concomitant polymorphs with general formula {[Hg(Pip) 2 (4,4'-bipy)]·DMF} n ( P1A, P1B ; Pip = piperonylic acid; 4,4'-bipy = 4,4'-bipyridine). Besides, [Hg(Pip) 2 (4,4'-bipy)] n ( 2 ) has been achieved during the attempts to isolate these polymorphs. The selective synthesis of P1A and P1B has been successfully achieved by changing the synthetic conditions. The formation of each polymorphic form has been ensured by unit cell measurements and decomposition temperature. The elucidation of their crystal structure revealed P1A and P1B as polymorphs, which originates from the Hg(II) cores and intermolecular associations, especially pinpointed by Hg···π and π···π interactions. Density functional theory (DFT) calculations suggest that P1B , which shows Hg(II) geometries that are further from ideality, is more stable than P1A by 13 kJ·mol -1 per [Hg(Pip) 2 (4,4'-bipy)]·DMF formula unit, and this larger stability of P1B arises mainly from metal···π and π···π interactions between chains. As a result, these structural modifications lead to significant variations of their solid-state photoluminescence.

Iridium oxide is a highly efficient catalyst for the oxygen evolution reaction, whose large-scale application requires decreasing the metal content. This is achieved using small nanoparticles. The knowledge of the water-IrO 2 nanoparticle interface is of high importance to understand the IrO 2 behavior as electrocatalyst in aqueous solutions. In this contribution, DFT (PBE-D2) calculations and AIMD simulations on IrO 2 nanoparticle models of different sizes ((IrO 2 ) 33 and (IrO 2 ) 115 ) are performed. Results show that two key factors determine the H 2 O adsorption energy and the preferred adsorption structure (molecular or dissociated water): metal coordination and hydrogen bonding with oxygen bridge atoms of the IrO 2 surface. Regarding metal coordination, and since the tetragonal distortion existing in IrO 2 is retained on the nanoparticle models, the adsorption at iridium axial vacant sites implies stronger Ir-H 2 O interactions, which favors water dissociation. In contrast, Ir-H 2 O interaction at equatorial vacant sites is weaker and thus the relative stability of molecular and dissociated forms becomes similar. Hydrogen bonding increases adsorption energy and favors water dissociation. Thus, tip and corner sites of the nanoparticle, with no oxygen bridge atoms nearby, exhibit the smallest adsorption energies and a preference for the molecular form. Overall, the presence of rather isolated tip and corner sites in the nanoparticle leads to lower adsorption energies and a smaller degree of water dissociation when compared with extended surfaces.

Understanding the structure and arrangement of hybrid metal halides and their contribution to the optoelectronic properties is, thus far, a challenging topic. In particular, new materials composed of d 10 metal halides and pyridinium cations are still largely unexplored. Therefore, we report the synthesis and characterization of six Hg(II) salts built up from (Hg 2 Cl 6 ) 2- or (HgX 4 ) 2- anions (X = Cl, Br, I) and 2,2'-bipyridium (2,2'-Hbipy) + , 2,2'-bipyridine-1,1'-diium (2,2'-H 2 bipy) 2+ , or 1,10-phenantrolinium (1,10-Hphen) + cations, using the same experimental conditions. All of them have been characterized by PXRD, EA, FTIR-ATR, and 1 H NMR spectroscopies; single-crystal X-ray diffraction; and TG/DTA determinations. The study of their packing via Hirshfeld surface analysis and 3D deformation density mapping revealed the contributions of the intermolecular interactions to the structural arrangement, notably, the effect of the cation planarity on them. Successively, periodic DFT calculations showed that (i) the valence and conducting bands are mainly composed of the p orbitals of the halide and the organic cation, respectively, and (ii) the corresponding band gap depends mainly on the halide., Competing Interests: The authors declare no competing financial interest., (© 2020 American Chemical Society.)

Heterogeneous catalysts in the form of atomically dispersed metals on a support provide the most efficient utilization of the active component, which is especially important for scarce and expensive late transition metals. These catalysts also enable unique opportunities to understand reaction pathways through detailed spectroscopic and computational studies. Here, we demonstrate that atomically dispersed iridium sites on indium tin oxide prepared via surface organometallic chemistry display exemplary catalytic activity in one of the most challenging electrochemical processes, the oxygen evolution reaction (OER). In situ X-ray absorption studies revealed the formation of Ir V =O intermediate under OER conditions with an Ir-O distance of 1.83 Å. Modeling of the reaction mechanism indicates that Ir V =O is likely a catalyst resting state, which is subsequently oxidized to Ir VI enabling fast water nucleophilic attack and oxygen evolution. We anticipate that the applied strategy can be instrumental in preparing and studying a broad range of atomically dispersed transition metal catalysts on conductive oxides for (photo)electrochemical applications., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

The electrochemical reduction of organic contaminants allows their removal from water. In this contribution, the electrocatalytic hydrogenation of nitrobenzene is studied using both oxidized carbon fibres and ruthenium nanoparticles supported on unmodified carbon fibres as catalysts. The two systems produce azoxynitrobenzene as the main product, while aniline is only observed in minor quantities. Although PhNO2 hydrogenation is the favoured reaction, the hydrogen evolution reaction (HER) competes in both systems under catalytic conditions. H2 formation occurs in larger amounts when using the Ru nanoparticle based catalyst. While similar reaction outputs were observed for both catalytic systems, DFT calculations revealed some significant differences related to distinct interactions between the catalytic material and the organic substrates or products, which could pave the way for the design of new catalytic materials.

Understanding metal oxide MO 2 (M = Ti, Ru, and Ir)-water interfaces is essential to assess the catalytic behavior of these materials. The present study analyzes the H 2 O-MO 2 interactions at the most abundant (110) and (011) surfaces, at two different water coverages: isolated water molecules and full monolayer, by means of Perdew-Burke-Ernzerhof-D2 static calculations and ab initio molecular dynamics (AIMD) simulations. Results indicate that adsorption preferably occurs in its molecular form on (110)-TiO 2 and in its dissociative form on (110)-RuO 2 and (110)-IrO 2 . The opposite trend is observed at the (011) facet. This different behavior is related to the kind of octahedral distortion observed in the bulk of these materials (tetragonal elongation for TiO 2 and tetragonal compression for RuO 2 and IrO 2 ) and to the different nature of the vacant sites created, axial on (110) and equatorial on (011). For the monolayer, additional effects such as cooperative H-bond interactions and cooperative adsorption come into play in determining the degree of deprotonation. For TiO 2 , AIMD indicates that the water monolayer is fully undissociated at both (110) and (011) surfaces, whereas for RuO 2 , water monolayer exhibits a 50% dissociation, the formation of H 3 O 2 - motifs being essential. Finally, on (110)-IrO 2 , the main monolayer configuration is the fully dissociated one, whereas on (011)-IrO 2 , it exhibits a degree of dissociation that ranges between 50 and 75%. Overall, the present study shows that the degree of water dissociation results from a delicate balance between the H 2 O-MO 2 intrinsic interaction and cooperative hydrogen bonding and adsorption effects., Competing Interests: The authors declare no competing financial interest.

Present work addresses the reactivity of several phenyl-substituted metal-carbene complexes with 4-methylstyrene by means of density functional theory OPBE simulations. Different paths that lead to cyclopropanation were explored and compared to the olefin metathesis mechanism. For this purpose, we chose four different catalysts: (i) the Grubbs second-generation olefin metathesis catalyst, (ii) a Grubs second-generation-like complex, in which ruthenium is replaced by iron, and (iii) two iron carbene complexes (a piano stool and a porphyrin iron carbene) that experimentally catalyze alkene cyclopropanation. Results suggest that the nature of the applying mechanism is very sensitive to the coordination around the metal center and the spin state of the metal-carbene complex. Cyclopropanation by open-shell metal-carbene complexes seems to preferentially proceed through a two-step radical mechanism, in which the two C-C bonds are sequentially formed (path C). Singlet-state carbenes proceed either through a direct attack of the olefin to the carbene (path D) when the formation of the metallacycle is not feasible or through a reductive elimination from the metallacyclobutane when this intermediate is accessible both kinetically and thermodynamically (path B).

Alzheimer's disease is a challenge of the utmost importance for contemporary society. An early diagnosis is essential for the development of treatments and for establishing a network of support for the patient. In this light, the deposition in the brain of amyloid-β fibrillar aggregates, which is a distinctive feature of Alzheimer, is key for an early detection of this disease. In this work we propose an atomistic study of the interaction of amyloid tracers with recently published polymorphic models of amyloid-β 1-40 and 1-42 fibrils, highlighting the relationship between marker architectures and binding affinity. This work uncovers the importance of quaternary structure, and in particular of junctions between amyloid-β protofilaments, as the key areas for marker binding.

Alkane metathesis transforms small alkanes into their higher and lower homologues. The reaction is catalyzed by either supported d 0 metal hydrides (M = Ta, W) or d 0 alkyl alkylidene complexes (M = Ta, Mo, W, Re). For the silica-supported tantalum hydrides, several reaction mechanisms have been proposed. We performed DFT-D3 calculations to analyze the viability of the proposed pathways and compare them with alkane hydrogenolysis, which is a competitive process observed at the early stages of the reaction. The results show that the reaction mechanisms for alkane metathesis and for alkane hydrogenolysis present similar energetics, and this is consistent with the fact that the process taking place depends on the concentrations of the initial reactants. Overall, a modified version of the so-called one-site mechanism that involves alkyl alkylidene intermediates appears to be more likely and consistent with experiments. According to this proposal, tantalum hydrides are precursors of the alkyl alkylidene active species. During precursor activation, H 2 is released and this allows alkane hydrogenolysis to occur. In contrast, the catalytic cycle implies only the reaction with alkane molecules in excess and does not form H 2 . Thus, the activity for alkane hydrogenolysis decreases. The catalytic cycle proposed here implies three stages: (i) β-H elimination from the alkyl ligand, liberating ethene, (ii) alkene cross-metathesis, allowing olefin substituent exchange, and (iii) formation of the final products and alkyl alkylidene regeneration by olefin insertion and three successive 1,2-CH insertions to the alkylidene followed by α abstraction. These results relate the reactivity of silica-supported hydrides with that of the alkyl alkylidene complexes, the other common catalyst for alkane metathesis.

Described herein is a new visible-light photocatalytic strategy for the synthesis of enantioenriched dihydrofurans and cyclopentenes by an intramolecular nitro cyclopropane ring expansion reaction. Mechanistic studies and DFT calculations are used to elucidate the key factors in this new ring expansion reaction, and the need for the nitro group on the cyclopropane., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Functionalization of gold nanoparticles with supramolecular hosts allows their plasmon-based photocatalytic activity to be enhanced. This is mainly ascribed to the formation of labile host-guest complexes with the reagent molecules on the metal surface, thus promoting nanoparticle-substrate approximation without interfering with the light-induced catalytic process.

According to the amyloid cascade hypothesis, amyloid-β (Aβ) deposition is a central event in the Alzheimer's disease and thus, detection of Aβ deposits is crucial to monitor the progression of the pathology. Despite its low tissue penetration, fluorescence imaging may become an alternative technique for identifying these deposits because it is less toxic and less costly than positron emission tomography. Suitable dyes, however, should emit in the near infrared (NIR) region, cross the blood-brain barrier and target Aβ aggregates. In this work, we use TD-DFT, AIMD simulations and protein energy landscape exploration (PELE) to analyze the photophysical properties of a family of donor-acceptor markers and their binding to amyloid fibrils. These markers are formed by N,N-dimethylaniline donor and propanedinitrile acceptor groups separated by a spacer consisting of one, two or three conjugated double bonds. The smallest compound has a low emission wavelength, can deactivate through a non-radiative process involving a conical intersection and binds weakly to Aβ fibrils. In contrast, the largest dye is a suitable compound as it shows an emission wavelength in the NIR region, does not seem to relax through conical intersection processes and binds to Aβ fibrils strongly entering hydrophobic voids. Analysis of electronic excitations shows that the transition has an important charge transfer character that increases with the length of the spacer, the π bridge being an active participant in the transition. Therefore, adding double bonds to the dye skeleton has two beneficial effects: (i) it increases the emission wavelength as it enlarges the π system and (ii) it increases the charge transfer character of the transition, which increases the red-shifting of the emission wavelength in polar solvents.

Hydrazine reacts with silica-supported tantalum-hydrides [(≡SiO)2TaHx] (x = 1, 3), 1, under mild conditions (100 °C). The IR in situ monitoring of the reaction with N2H4 or (15)N2H4, and the solid-state MAS NMR spectra of the fully (15)N labeled compounds (CP (15)N, (1)H-(15)N HETCOR, (1)H-(1)H double-quantum, and (1)H-(1)H triple-quantum spectra) were used to identify stable intermediates and products. DFT calculations were used for determining the reaction pathway and calculating the (15)N and (1)H NMR chemical shifts. Combining the experimental and computational studies led to the following results. At room temperature, only hydrazine adducts, 1-N2H4, are formed. Upon heating at 100 °C, the hydrazine adducts are converted to several species among which [(≡SiO)2Ta(═NH)(NH2)], 2, [(≡SiO)2TaH(NH2)2], 3, and [(≡SiO)2TaH2(NH-NH2)], 4, were identified. The final product 2 is also formed in the reaction of N2 with the same silica-supported tantalum-hydride complexes, and the species identified as 3 and 4 had been previously suggested by DFT studies as intermediates on the reaction pathway for N-N cleavage in N2. The present computational studies (cluster models with M06 functional complemented by selected calculations with periodic calculations) show that 2 is formed via 3 and 4, with either N2 or N2H4. This strengthens the previous proposal of the existence of 3 and 4 as intermediates in the reaction of N2 with the tantalum-hydrides. However, the reaction of N2 does not imply the formation of N2H4 or its hydrazido monoanionic or dianionic ligand as an intermediate. For this reason, this study informs both on the similarities and differences of the reaction pathways involving N2 and N2H4 with tantalum-hydrides.

The main hallmark of Alzheimer's disease is the deposition of amyloid-β (Aβ) aggregates in the brain. An early diagnosis of the disease requires a fast and accurate detection of such aggregates in vivo. NIAD-4 is one of the most promising in vivo markers developed due to its high emission at λ > 600 nm and its ability to rapidly cross the blood-brain barrier (BBB) and target Aβ deposits. Furthermore, it shows a dramatic fluorescence enhancement upon binding to amyloid fibrils, which is essential for attaining good imaging contrast. Aiming at establishing novel design concepts for the preparation of optimized optical probes, the current work rationalizes the excellent performance of NIAD-4 by using a pool of computational (TD-DFT and CASPT2 calculations, ab initio molecular dynamics and protein energy landscape exploration) and spectroscopic techniques. Unlike other markers operating as molecular rotors or polarity-sensitive dyes, we uncover herein that the high fluorescence imaging contrast observed upon NIAD-4 binding to amyloid fibrils results from reversible aggregation. NIAD-4 forms non-emissive assemblies in aqueous solution already at very low concentrations, which convert into the highly fluorescent monomeric species by diffusion into the hydrophobic voids of Aβ deposits. This result paves the way to exploit aggregation-induced processes as a new strategy towards advanced fluorescence markers for amyloid detection.