Transition Metal Systems Research Articles

Quantum Yu-Shiba-Rusinov dimers

Magnetic adatoms on a superconducting substrate undergo a quantum phase transition as their exchange coupling to the conduction electrons increases. For quantum spins, this transition is accompanied by screening of the adatom spin. Here, we explore the consequences of this screening for the phase diagrams and subgap excitation spectra of dimers of magnetic adatoms coupled by hybridization of their Yu-Shiba-Rusinov states and spin-spin interactions. We specifically account for higher spins, single-ion anisotropy, Ruderman-Kittel-Kasuya-Yosida coupling, and Dzyaloshinsky-Moriya interactions relevant in transition-metal and rare-earth systems. Our flexible approach based on a zero-bandwidth approximation provides detailed physical insight and is in excellent qualitative agreement with available numerical-renormalization group calculations on monomers and dimers. Remarkably, we find that even in the limit of large impurity spins or strong single-ion anisotropy, the phase diagrams for dimers of quantum spins remain qualitatively distinct from phase diagrams based on classical spins, highlighting the need for a theory of quantum Yu-Shiba-Rusinov dimers.

3 + 2] Cycloadditions and Retrocycloadditions of Niobium Imido Complexes: An Experimental and Computational Mechanistic Study.

We demonstrate reactivity between a β-diketiminate-supported niobium(III) imido complex and alkyl azides to form niobatetrazene complexes (BDI)Nb(NtBu)(RNNNNR) (BDI = N,N-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate; R = cyclohexyl (1), benzyl (2)). Intriguingly, niobatetrazene complexes 1 and 2 can be interconverted via addition of an appropriate alkyl azide, likely through a series of concerted [3 + 2] cycloaddition and retrocycloaddition reactions in which π-loaded bis(imido) intermediates are formed. The bis(imido) intermediates were trapped upon addition of alkyl isocyanides to yield five-coordinate bis(imido) complexes (BDI)Nb(NtBu)(NCy)(CNR) (R = tert-butyl (4a), cyclohexyl (4b)). Two computational methods─density functional theory and density functional tight binding (DFTB)─were employed to calculate the lowest energy pathway across the potential energy surface for this multistep transformation. Reaction path calculations for individual cycloaddition or retrocycloaddition processes along the multistep reaction pathway showed that these transformations occur via a concerted, yet highly asynchronous mechanism, in which the two bond-breaking or -making events do not occur simultaneously. The use of the DFTB method in this work highlights its advantages and utility for studying transition metal systems.

A rate model approach for FEL pulse induced transmissions changes, saturable absorption, X-ray transparency and stimulated emission

As the use of free electron laser (FEL) sources increases, so do the findings mentioning non-linear phenomena occurring at these experiments, such as saturable absorption, induced transparency and scattering breakdowns. These are well known among the laser community, but are still rarely understood and expected among the X-ray community and to date lack tools and theories to accurately predict the respective experimental parameters and results. We present a simple theoretical framework to access short X-ray pulse induced light–matter interactions which occur at intense short X-ray pulses as available at FEL sources. Our approach allows to investigate effects such as saturable absorption, induced transparency and scattering suppression, stimulated emission, and transmission spectra, while including the density of state influence relevant to soft X-ray spectroscopy in, for example, transition metal complexes or functional materials. This computationally efficient rate model based approach is intuitively adaptable to most solid state sample systems in the soft X-ray spectrum with the potential to be extended for liquid and gas sample systems as well. The feasibility of the model to estimate the named effects and the influence of the density of state is demonstrated using the example of CoPd transition metal systems at the Co edge. We believe this work is an important contribution for the preparation, performance, and understanding of FEL based high intensity and short pulse experiments, especially on functional materials in the soft X-ray spectrum.

Ionization Energies and Redox Potentials of Hydrated Transition Metal Ions: Evaluation of Domain-Based Local Pair Natural Orbital Coupled Cluster Approaches.

Hydrated transition metal ions are prototypical systems that can be used to model properties of transition metals in complex chemical environments. These seemingly simple systems present challenges for computational chemistry and are thus crucial in evaluations of quantum chemical methods for spin-state and redox energetics. In this work, we explore the applicability of the domain-based pair natural orbital implementation of coupled cluster (DLPNO-CC) theory to the calculation of ionization energies and redox potentials for hydrated ions of all first transition row (3d) metals in the 2+/3+ oxidation states, in connection with various solvation approaches. In terms of model definition, we investigate the construction of a minimally explicitly hydrated quantum cluster with a first and second hydration layer. We report on the convergence with respect to the coupled cluster expansion and the PNO space, as well as on the role of perturbative triple excitations. A recent implementation of the conductor-like polarizable continuum model (CPCM) for the DLPNO-CC approach is employed to determine self-consistent redox potentials at the coupled cluster level. Our results establish conditions for the convergence of DLPNO-CCSD(T) energetics and stress the absolute necessity to explicitly consider the second solvation sphere even when CPCM is used. The achievable accuracy for redox potentials of a practical DLPNO-based approach is, on average, 0.13 V. Furthermore, multilayer approaches that combine a higher-level DLPNO-CCSD(T) description of the first solvation sphere with a lower-level description of the second solvation layer are investigated. The present work establishes optimal and transferable methodological choices for employing DLPNO-based coupled cluster theory, the associated CPCM implementation, and cost-efficient multilayer derivatives of the approach for open-shell transition metal systems in complex environments.

Thermodynamic calculation of the phase diagram of the Co–Nb–Ta system

Abstract A thermodynamic description of the ternary system Co–Nb–Ta is obtained by combining the descriptions of the lower-order systems according to the CALPHAD method. The thermodynamic data set for the binary Co–Nb system is taken from [1], where a new thermodynamic model for the μ-phase was introduced. The μ-phase appears in several transition metal systems, including Co–Ta. The model is applied to the Co–Ta system and extended to the ternary Co–Nb–Ta system. The thermodynamic descriptions of the systems Co–Ta, Nb–Ta and Co–Nb–Ta are reported in the present work.

Adsorption of metal ions on graphene sheet for applications in environmental sensing and wastewater treatment

Experimental and computational studies showed that metal ion binding to graphene nanosheets involves a new mechanism of the interaction, ion-graphene charge transfer. Surprisingly, highly acidic= ions showed the highest affinity and ion-induced dipole appears to play a very important role in this interaction. Aqueous suspensions of graphene nanosheets (not graphene oxide or reduced graphene) were prepared by bovine serum albumin-assisted exfoliation (biographene) and metal ion binding carried out by optical spectroscopy, zeta potential analysis and Raman data. In DFT computational studies, the interactions of metal ions with graphene sheets of 20 Å × 12 Å (containing 46 carbon atoms) were examined. Interactions of the alkali and alkali earth metals were examined by M026x theory with 6–31 g basis set and d3 dispersion, while those of the transition metal systems have been examined by B3LYP theory with LanL2DZ basis set. The basis set superimposition errors were avoided using counterpoise calculations. The computed complexation energies (∆EAdsorption) were compared with the experimental values. These studies showed strong correlation of binding energies with the metal ion wlwctronegativity and charge density wkwxr, involving charge transfer between the metal ion and graphene. These surprising results imply a significant role for ion and ion-induced dipole interactions as a possible guiding factor in predicting metal ion binding affinities. A new hypothesis that charge transfer from the graphene π-cloud to the metal ion contributes to thπe binding free energy is proposed. Graphene is a strong candidate, therefore, for detecting and adsorbing metal ions from wastewater streams for sensing and environmental applications.

Investigation of phosphine donor properties to vanadium(V) nitrides

Catalyst optimization has been greatly aided over the years by methods of characterizing donor and steric properties of ligands. Here, we investigated a vanadium model system, NV(NH2)2L, to determine if the enthalpic barriers associated with amide rotation are in a range that would make this a synthetic target for determining the donor ability of L experimentally, e.g., by spin saturation transfer of the amide protons in the 1H NMR. In addition to determining that the barriers to rotation should be amenable to this method, we investigated how various phosphine donor properties affected amide rotation for the d0 metal and contrasted this with a late transition metal system. The calculations suggest that some PX3 ligand features, such as lone pairs on the X substituent, may lead to increases in the high oxidation state system where they are electron withdrawing in the low oxidation state systems. The cause of this electronic difference was investigated using Natural Resonance Theory.

Slater-Pauling-like Behavior of Spin Hall Conductivity in Pt-based Superlattices

The intrinsic spin Hall effect in the bulk systems of late transition metals (Os, Ir, Pt, and Au) as well as the Pt-based superlattices were investigated by using first-principle calculations. By comparing the computed spin Hall conductivities of Pt−M superlattices (M=Os, Ir, and Au) with different compositions and those obtained from atomic bulk composition, we saw that the spin Hall conductivities (SHCs) follow the behavior described by the Slater-Pauling curve, the maximum of which is at pure Pt bulk. From the examination of the band structures of the considered systems, we found that the origin of this behavior comes from the variation of the band structures as a direct consequence of the change of the number of electrons and hybridization effects.

Ab Initio Study of the C–O Bond Dissociation in CO2 Reduction by Redox and Carboxyl Routes on 3d Transition Metal Systems

The C–O bond dissociation of the CO2 molecule via the reverse water gas shift reaction is crucial for several reactions used as renewable alternatives for fuel synthesis. However, our atomistic understanding of this process on transition metal (TM) clusters, where quantum-size effects might play a significant role, is far from complete. Here, we addressed the C–O bond dissociation by redox and carboxyl routes on 13-atom TM (Fe, Co, Ni, Cu) clusters using density functional theory calculations and the climbing image nudged elastic band algorithm. From the potential energy profiles, we found that CO2 is prone to dissociate into adsorbed CO via the redox route with lower activation energies, Ea’s, than the carboxyl route on all studied TM13 systems. Our results suggested that the smaller activation barrier found on the Co13 cluster is due to the stronger adsorption exhibited for both CO2 and O. By increasing the d-state occupation (from Fe to Cu), the Ea differences between CO2 dissociation and COOH formation decrease. We associated this behavior with a decrease in the (CO2 + H) adsorption energy from Fe13 to Cu13 that facilitates the COO–H bond formation and H–TM bond cleavage, i.e., favoring the carboxyl route. Also, our analyses indicate that the adsorption energies of the CO2 and trans-COOH species are the best descriptors for the C–O bond dissociation via the redox and carboxyl routes, respectively.

Substituent Effects on Aluminyl Anions and Derived Systems: A High-Level Theory.

Aluminyl anions are low-valent aluminum species bearing a lone pair of electrons and a negative charge. These systems have drawn recent synthetic interest for their nucleophilic nature, allowing for the activation of σ-bonds, and have been proposed as a pathway to hydrogen energy storage. In this research, we provide high-level ab initio geometries and energies for both the simplest aluminyl anion (AlH2-) and several substituted derivatives. Geometries are reported using the gold-standard CCSD(T)/aug-cc-pV(T+d)Z level of theory. Energies were extrapolated to the complete basis set limit through the focal point approach, utilizing coupled-cluster methods through perturbative quadruples and basis sets up to five-ζ quality. Geometries were rationalized using electrostatic, steric, and orbital donation effects. The donation from substituents to Al is accompanied by back-donation effects, a property traditionally thought of in transition-metal systems. Stereoelectronic effects through the secondary orbital interaction play a fundamental role in stabilizing these low-valent aluminum compounds and would likely also affect the feasibility of their use within several industrial applications. The energetic analysis of the formation of each substituted anion is rationalized as the result of three energetic schemes. The effectiveness of these schemes for determining the relative formation energies is discussed.

Role of Li2MnO3 in Li-Rich Composite Cathode through Defect Reaction, Doping and Electrochemical Testing

Li-rich layered oxide, 0.5Li2MnO3‧0.5Li(Ni1/3Mn1/3Co1/3)O2, exhibits interestingly high capacity among several cathode systems. In this study, the structure and electrochemical behaviors of Li-rich 0.5Li2MnO3‧0.5Li(Ni1/3Mn1/3Co1/3)O2 composite were investigated. To circumvent the complication of the combination of ternary transition metal and Li excess system, Li2MnO3 was fabricated and studied as cathode material alone.H2 treatment and tungsten doping on Li2MnO3 are adopted for the understanding of its electrochemical behavior and resulted defect structure. The objective of this study was to find a promising solution for improvement of Li-rich cathode materials based on the understanding of role of Li2MnO3 in Li-rich system. H2-treated oxygen-deficient Li2MnO3 initially shows high capacity resulted from oxygen oxidation (release) and the formation of VO ․․. On the other hand, such high concentration of oxygen vacancies is detrimental to its structural stability. Furthermore, the enhancement of electrochemical performance may be contributed to better electronic conductivity, expansion of lattice, and structure stability at surface influenced by tungsten doping. However, the poor cycling performance of Li2MnO3 was still observed due to the instability of surface structure resulted in the hindrance of lithium intercalation after first cycle. Such electrochemical behavior of Li2MnO3 is illustrated based on defect chemistry consideration. The electrochemical properties of Li2MnO3 were characterized by X-ray diffraction(XRD), X-ray photoemission Spectroscopy(XPS), DC conductivity measurement, Electrochemical impedance spectroscopy(EIS), scanning/transmission electron microscopy(STEM), and charge-discharge measurement.

Highly Efficient Spin-Filtering Transport in Chiral Hybrid Copper Halides.

Chiral Pb(Sn)-I hybrid organic-inorganic perovskites exhibit outstanding chiral-induced spin selectivity (CISS) performance, but the nontoxic lead-free hybrid materials with high stability are still greatly desired for spin filtering in spintronic applications. We synthesize chiral hybrid copper halides (R/S-MBA)2 CuX4 (MBA=methylbenzylammonium; X=Cl, Br) with characteristic 0D CuX4 tetrahedral structural motifs, combining the low toxicity of Cu2+ and air stability of halide ions (Cl- and Br- ). Despite similar structural and electronic features, (R/S-MBA)2 CuBr4 shows much smaller chiroptical activity than the chloride counterpart. Magnetically conductive atomic force microscopy measurements display a typical spin-polarized charge-transport property with high efficiency up to 90 % for both copper halides. Our work expands the CISS effect into eco-friendly and stable metal-organic halides, which is promising for applications in spintronics based on transition-metal hybrid systems.

Porous polymer‐derived ceramics: Flexible morphological and compositional controls through sol–gel chemistry

AbstractPorous nonoxide ceramics have exhibited impressive progress in terms of synthesis and applications over the past few decades because of their unique characteristics distinguished from the oxide counterparts. From the synthetic aspect, the preceramic polymer route, where nonoxide ceramics such as carbides and nitrides are produced from molecular precursors, offers exceptional opportunities to elaborate and control the material shape as well as the micro‐ and nanostructures in concert with various techniques. This review presents monolithic ceramic materials based on various reduced phases bearing hierarchical porosity with a focus on those obtained from macroporous preceramic monoliths prepared via the one‐pot sol–gel process accompanied by spinodal decomposition. Here, we highlight two classes of preceramic inorganic–organic hybrid gels: organometallic crosslinked polymers based on poly(silsesquioxane)s and nonorganometallic hybrid networks related to Ti without Ti–C bonds. The polymer‐to‐ceramic conversion processes are discussed with concern for the crystal transition behaviors and the variation of pore properties in different length scales upon heating. In addition, although out of the preceramic polymer category, some examples of inorganic–organic nanocomposite gels with a carbonizable polymer and/or urea for yielding porous metal carbides and nitrides in a monolithic form are introduced as well, which provides extended versatility toward a variety of transition metal systems.

Machine Learning-Assisted Selection of Active Spaces for Strongly Correlated Transition Metal Systems.

Active space quantum chemical methods could provide very accurate description of strongly correlated electronic systems, which is of tremendous value for natural sciences. The proper choice of the active space is crucial but a nontrivial task. In this article, we present a neural network-based approach for automatic selection of active spaces, focused on transition metal systems. The training set has been formed from artificial systems composed of one transition metal and various ligands, on which we have performed the density matrix renormalization group and calculated the single-site entropy. On the selected set of systems, ranging from small benchmark molecules up to larger challenging systems involving two metallic centers, we demonstrate that our machine learning models could predict the active space orbitals with reasonable accuracy. We also tested the transferability on out-of-the-model systems, including bimetallic complexes and complexes with ligands, which were not involved in the training set. Also, we tested the correctness of the automatically selected active spaces on a Fe(II)-porphyrin model, where we studied the lowest states at the DMRG level and compared the energy difference between spin states or the energy difference between conformations of ferrocene with recent studies.

On the Accuracy of Mean-Field Spin-Orbit Operators for 3d Transition-Metal Systems.

We present an extensive study of the performance of mean-field approximations to the spin-orbit operators on realistic molecular systems, as widely used in applications like single-molecule magnets, molecular quantum bits, and molecular spintronic devices. The test systems feature a 3d transition-metal center ion (V, Cr, Mn, Fe, Co, and Ni) in various coordinations and a multitude of energetically close-lying open-shell configurations that can couple via the spin-orbit operator. We performed complete active space spin-orbit configuration interaction calculations and compared the full two-electron Breit-Pauli spin-orbit operator to different approximations: the one-center approximation, the spin-orbit mean-field approach with electron densities from different state-averaging procedures, and the atomic mean-field integral approximation. We show that the mean-field approaches can lead to significant errors in the spin-orbital coupling matrix elements, which becomes particularly visible for the computed zero-field splittings. The one-center approximation, keeping all relevant two-electron terms, seems to be a significantly more accurate choice for the examples from our test set.

Redox Isomerism in the S3 State of the Oxygen-Evolving Complex Resolved by Coupled Cluster Theory.

The electronic and geometric structures of the water‐oxidizing complex of photosystem II in the steps of the catalytic cycle that precede dioxygen evolution remain hotly debated. Recent structural and spectroscopic investigations support contradictory redox formulations for the active‐site Mn4CaO x cofactor in the final metastable S3 state. These range from the widely accepted MnIV 4 oxo‐hydroxo model, which presumes that O−O bond formation occurs in the ultimate transient intermediate (S4) of the catalytic cycle, to a MnIII 2MnIV 2 peroxo model representative of the contrasting “early‐onset” O−O bond formation hypothesis. Density functional theory energetics of suggested S3 redox isomers are inconclusive because of extreme functional dependence. Here, we use the power of the domain‐based local pair natural orbital approach to coupled cluster theory, DLPNO‐CCSD(T), to present the first correlated wave function theory calculations of relative stabilities for distinct redox‐isomeric forms of the S3 state. Our results enabled us to evaluate conflicting models for the S3 state of the oxygen‐evolving complex (OEC) and to quantify the accuracy of lower‐level theoretical approaches. Our assessment of the relevance of distinct redox‐isomeric forms for the mechanism of biological water oxidation strongly disfavors the scenario of early‐onset O−O formation advanced by literal interpretations of certain crystallographic models. This work serves as a case study in the application of modern coupled cluster implementations to redox isomerism problems in oligonuclear transition metal systems.

Direct, Catalytic α-Alkylation of N-Heterocycles by Hydroaminoalkylation: Substrate Effects for Regiodivergent Product Formation.

Saturated N-heterocycles are prevalent in pharmaceutical and agrochemical industries, yet remain challenging to catalytically alkylate. Most strategies for C-H activation of these challenging substrates use protected amines or high loadings of precious metal catalysts. We report an early transition-metal system for the broad, robust, and direct alkylation of unprotected amine heterocycles with simple alkenes. Short reaction times are achieved using an in situ generated tantalum catalyst that avoids the use of bases, excess substrate, or additives. In most cases, this catalyst system is selective for the branched reaction product, including examples of products that are generated with excellent diastereoselectivity. Alkene electronic properties can be exploited for substrate-modified regioselectivity to access the alternative linear amine alkylation product with a group 5 catalyst. This method allows for the facile isolation of unprotected N-heterocyclic products, as useful substrates for further reactivity.

Evaluation of optical band gaps and dopant state energies in transition metal oxides using oxidation-state constrained density functional theory

We report the use of oxidation-state constrained density functional theory (OS-CDFT) to calculate the optical band gaps of transition metal oxides and dopant state energies of different doped anatase. OS-CDFT was used to control electron transfer from the valence band maximum of the transition metal system to the conduction band minimum or to the dopant state in order to calculate the band gap or the dopant state energies respectively. The calculation of the dopant state energies also allows identification of the transition responsible for the reduced band gap of the doped system in ambiguous cases. We applied this approach to the band gap calculation in TiO2 anatase and rutile, vanadium pentoxide (V2O5), chromium(III) oxide (Cr2O3), manganese(IV) oxide (MnO2), ferric oxide (Fe2O3), ferrous oxide (FeO) and cobalt(II) oxide (CoO). The dopant state energies calculations were carried out in the V-, Cr-, Mn-, and Fe-doped anatase.

Mechanistic Insights into Activation of Carbon Monoxide, Carbon Dioxide, and Nitrous Oxide by Acyclic Silylene.

Owing to an empty p orbital and a lone pair of electrons on the Si center, silylene exhibits reactivity similar to a transition-metal system capable of activating H2/C-H bonds and small molecules. In this work, with the aid of density functional theory calculations, we systematically investigated the reactions of an acyclic silylene with CO, CO2, and N2O. The detailed mechanisms obtained lead to an in-depth understanding of the silylene single-site ambiphilic reactivity.

Mechanism-Guided Development of Transition-Metal-Catalyzed C-N Bond-Forming Reactions Using Dioxazolones as the Versatile Amidating Source.

Catalytic reactions that construct carbon-nitrogen bonds are one of central themes in both synthetic and medicinal chemistry since the obtainable nitrogen-containing motifs are commonly encountered in natural products and have also seen a growing prominence as key structural features in marketed drugs and preclinical candidates. Pd-catalyzed cross-couplings, such as Buchwald-Hartwig amination, are at the forefront of such synthetic methods in practical settings. However, they require prefunctionalized substrates such as (hetero)aryl halides that must be prepared independently, often by multiple operations. One emerging way to circumvent these preparatory steps and directly convert ubiquitous C-H bonds into valuable C-N bonds is catalytic C-H amination, which allows synthetic chemists to devise shorter and more efficient retrosynthetic schemes. The past two decades have witnessed considerable progress in expanding the repertoire of this strategy, especially by identifying effective amino group precursors. In this context, dioxazolones have experienced a dramatic resurgence in recent years as a versatile nitrogen source in combination with transition-metal catalyst systems that facilitate decarboxylation to access key metal-acylnitrenoid intermediates. In addition to their high robustness and easy accessibility from abundant carboxylic acids, the unique reactivity of the transient intermediates in the amido group transfer has led to a fruitful journey for mild and efficient C-H amidation reactions.This Account summarizes our recent contributions to the development of C-N bond-forming reactions using dioxazolones as effective nitrenoid precursors, which are categorized into two subsets according to their mechanistic differences: inner- versus outer-sphere pathways. The first section describes how we could unveil the synthetic potential of dioxazolones in the realm of the inner-sphere C-H amidation, where we demonstrated that dioxazolones serve not only as manageable alternatives to acyl azides but also as highly efficient reagents to significantly reduce the catalyst loading and temperature. Taking advantage of the mild conditions in combination with group 9 Cp*M complexes (M = Rh, Ir, Co) or isoelectronic Ru species, we have dramatically expanded the accessible synthetic scope. Mechanistic investigations revealed that the putative metal-nitrenoid species is involved as a key intermediate during catalysis, which leads to facile C-N bond formation. On the basis of the mechanistic underpinning, we have succeeded in developing novel catalytic platforms that harness the intermediacy of metal-nitrenoids to explore C-H insertion chemistry via an outer-sphere pathway. Indeed, the tailored catalysts were capable of suppressing the competitive Curtius-type decomposition, thus granting access to versatile lactam products. We have further repurposed the catalytic systems upon modification of chelating ligands and also the identity of the transition metal to achieve three goals: (i) addressing selectivity issues to control the regio-, chemo-, and enantioselectivities, (ii) developing sustainable catalysis by first-low metals, and (iii) navigating chemical space for (di)functionalization of alkenes/alkynes. Together with our own research efforts, highlighted herein are some important relevant advances by other groups. We finally conclude with a brief overview with an eye toward further developments.