Transition Metal Clusters Research Articles

Automated, Consistent, and Even-Handed Selection of Active Orbital Spaces for Quantum Embedding.

A widely used strategy to reduce the computational cost of quantum-chemical calculations is to partition the system into an active subsystem, which is the focus of the computational efforts, and an environment that is treated at a lower computational level. The system partitioning is mostly based on localized molecular orbitals. When reaction paths or energy differences are to be calculated, it is crucial to keep the orbital space consistent for all structures. Inconsistencies in orbital space can lead to unpredictable errors on the potential energy surface. While successful strategies to ensure this consistency have been established for organic and even metal-organic systems, these methods often fail for metal clusters or nanoparticles with a high density of near-degenerate and delocalized molecular orbitals. However, such systems are highly relevant for catalysis. Accurate yet feasible quantum-mechanical ab initio calculations are therefore highly desired. In this work, we present an approach based on the subsystem projected atomic orbital decomposition algorithm that allows us to ensure automated and consistent partitioning even for systems with delocalized and near-degenerate molecular orbitals and demonstrate the validity of this method for the binding energies of small molecules on transition-metal clusters.

First principle study of enhanced CO adsorption on divacancy graphene-supported TM7 (TM = Fe, Co, Ni, Cu, Ag, and Au) clusters

The adsorption of CO on transition metal clusters TM7 (TM = Fe, Co, Ni, Cu, Ag, and Au) and their corresponding divacancy graphene-supported clusters has been investigated using plane-wave density functional theory with van der Waals D3 correction. The results reveal that all divacancy graphene-supported TM7 clusters show a stronger adsorption capacity for CO compared to the pure TM7 clusters. Orbital-resolved bonding interactions indicate, while the d-p orbital coupling between the substrate and CO dominates the magnitude of the absolute adsorption energy, the enhanced s-p orbital coupling between the substrate and CO determines the increase in the adsorption energy of CO on the divacancy graphene-supported TM7 clusters. On the other hand, the enhanced CO chemisorption energies on the divacancy graphene-supported TM7 clusters are caused by the induced electrostatic interaction between the positively charged TM7 on divacancy graphene and CO.

Chemical Diversity of Mo5S5 Clusters with Pyrazole: Synthesis, Redox and UV-vis-NIR Absorption Properties.

The chemistry of transition metal clusters has been intensively developed in the last decades, leading to the preparation of a number of compounds with promising and practically useful properties. In this context, the present work demonstrates the preparation and study of the reactivity, i.e., the possibility of varying the ligand environment, of new square pyramidal molybdenum chalcogenide clusters [{Mo5(μ3-S)i4(μ4-S)i(μ-pz)i4}(pzH)t5]1+/2+ (pzH = pyrazole, i = inner, t = terminal). The one-step synthesis starting from the octahedral Mo6Br12 cluster as well as the substitution of the apical pyrazole ligand or the selective bromination of the inner pyrazolate ligands were demonstrated. All the obtained compounds were characterized in detail using a series of physicochemical methods both in solid state (X-ray diffraction analysis, etc.) and in solution (nuclear magnetic resonance spectroscopy, mass spectrometry, etc.). In this work, redox properties and absorption in the ultraviolet-visible and near-infrared region of the obtained compounds were studied.

Enhanced HER activity of transition metal cluster decorated ReS2 monolayer

Two-dimensional material rhenium disulfide (ReS2) has attracted much attention recently as a potential electrochemical catalyst for the Hydrogen Evolution Reaction (HER). The ReS2 monolayer is found to be catalytically inert. The challenge lies in activating the ReS2 monolayer to increase HER output. One of the approaches to this lies in depositing small metal clusters on the monolayer. This paper examines the formation and stability of small metal clusters on 1T distorted ReS2 monolayer using dispersion-corrected Density Functional Theory. Four different kinds of transition metal clusters, viz. Au, Pt, Mo, and Cr are investigated for geometry and binding on the ReS2 monolayer. HER activity of each type of ReS2-supported metal cluster is investigated based on computed free energy. The HER mechanism was investigated for clusters with optimal activity, and it was found that while the ReS2 monolayer follows the Volmer-Heyrovsky route, for the transition metal cluster decorated ReS2, the Volmer-Tafel route is more probable.

Rh19-: A high-spin super-octahedron cluster.

Probing atomic clusters with magic numbers is of supreme importance but challenging in cluster science. Pronounced stability of a metal cluster often arises from coincident geometric and electronic shell closures. However, transition metal clusters do not simply abide by this constraint. Here, we report the finding of a magic-number cluster Rh19- with prominent inertness in the sufficient gas-collision reactions. Photoelectron spectroscopy experiments and global-minimum structure search have determined the geometry of Rh19- to be a regular Oh‑[Rh@Rh12@Rh6]- with unusual high-spin electronic configuration. The distinct stability of such a strongly magnetic cluster Rh19- consisting of a nonmagnetic element is fully unveiled on the basis of its unique bonding nature and superatomic states. The 1-nanometer-sized Oh-Rh19- cluster corresponds to a fragment of the face-centered cubic lattice of bulk rhodium but with altered magnetism and electronic property. This cluster features exceptional electron-spin state isomers confirmed in photoelectron spectra and suggests potential applications in atomically precise manufacturing involving spintronics and quantum computing.

Half-metallic ferromagnetism and thermoelectric effect in spinel chalcogenides SrX2S4 (X = Mn, Fe, Co) for spintronics and energy harvesting

Spintronics is an emerging technology in advanced quantum electronics, which has multidirectional applications. In this study, the electronic, magnetic, and thermoelectric characteristics of SrX2S4 (X = Mn, Fe, Co) are computed systematically. The stability in ferromagnetic states is verified by energy released from optimized structures and enthalpy of formation. The Heisenberg model is applied for Curie temperature. The band structures are plotted for both spin ↑ and ↓ configurations, which ensure 100% spin polarization and integer magnetic moments. Furthermore, the metallic behavior in spin ↑ and insulating behavior in spin ↓ confirm half-metallic ferromagnetism. The crystal field energy, direct exchange energy, indirect exchange energy, and exchange constants indicate that ferromagnetism is due to spin of electrons rather than any clusters of transition metals. The effect of thermoelectric parameters on spin functionality of electrons is addressed to enhance the device reliability. The thermoelectric performance is reported by power factor in the temperature span 100–500 K, which is important for energy harvesting.

Bond dissociation energies for Fe2+, Fe2O+, and Fe2O2+ clusters determined through threshold photodissociation in a cryogenic ion trap.

Understanding and controlling the chemical behavior of iron and iron oxide clusters requires accurate thermochemical data, which, because of the complex electronic structure of transition metal clusters, can be difficult to calculate reliably. Here, dissociation energies for Fe2+, Fe2O+, and Fe2O2+ are measured using resonance enhanced photodissociation of clusters contained in a cryogenically cooled ion trap. The photodissociation action spectrum of each species exhibits an abrupt onset for the production of Fe+ photofragments from which bond dissociation energies are deduced for Fe2+ (2.529 ± 0.006eV), Fe2O+ (3.503 ± 0.006eV), and Fe2O2+ (4.104 ± 0.006eV). Using previously measured ionization potentials and electron affinities for Fe and Fe2, bond dissociation energies are determined for Fe2 (0.93 ± 0.01eV) and Fe2- (1.68 ± 0.01eV). Measured dissociation energies are used to derive heats of formation ΔfH0(Fe2+) = 1344 ± 2 kJ/mol, ΔfH0(Fe2) = 737 ± 2 kJ/mol, ΔfH0(Fe2-) = 649 ± 2 kJ/mol, ΔfH0(Fe2O+) = 1094 ± 2 kJ/mol, and ΔfH0(Fe2O2+) = 853 ± 21 kJ/mol. The Fe2O2+ ions studied here are determined to have a ring structure based on drift tube ion mobility measurements prior to their confinement in the cryogenic ion trap. The photodissociation measurements significantly improve the accuracy of basic thermochemical data for these small, fundamental iron and iron oxide clusters.

Biomimetic Transition Metal Cluster Complexes: Synthetic Studies and Applications in the Reduction of Inert Small Molecules

Abstract Transition metal cluster complexes, particularly those containing iron and sulfur, are used as catalysts for the biological reduction of inert small molecules such as N2 and CO2. The structures of these biological clusters are complicated and the protein backbones around the clusters often play important roles in catalysis, hence reproducing or mimicking the enzymatic functions with synthetic cluster complexes remains a challenge. Appropriate assumptions and hypotheses on the relationships between the structures and functions of biological clusters are needed to develop synthetic molecular catalysts inspired by enzymes. This account reviews recent studies by the author and his coworkers on iron-containing biomimetic cluster complexes. Cubic Mo-Fe-S clusters supported by bulky cyclopentadienyl ligands on molybdenum were designed and synthesized, and their Fe sites captured and catalytically converted N2 under reducing conditions. Iron-hydride clusters, which are relevant to the active species of biological and industrial nitrogen fixation, also served as catalysts for the reduction of N2. Furthermore, various metal-sulfur clusters, ranging from a structural mimic of the complex active site of the N2-reducing enzyme to a simple and cubic [Fe4S4] cluster, were found to catalyze the biologically inaccessible direct conversion of CO2 to short-chain hydrocarbons. These studies have demonstrated the potential utility of biomimetic approaches to the catalytic reduction of inert small molecules, through the rational design and synthesis of simple yet appropriate iron-containing cluster complexes.

Correction to “Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In-Situ DRIFT and XAS Spectroscopy Assisted by DFT”

Correction to “Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with <i>In-Situ</i> DRIFT and XAS Spectroscopy Assisted by DFT”

A first-principles study of transition metal clusters supported on graphene as electrocatalysts for N2 to NH3 reaction

NH3 is an important raw material for a variety of chemicals, but also an important carbon-free energy carrier. Currently, NH3 synthesis mainly relies on Habor–Bosch method with high energy consumption, high pollution and low efficiency. So it is urgent to develop a green process for NH3 synthesis. The electrochemical NH3 synthesis can be carried out under ambient conditions. Therefore, it is very important to find electrocatalysts with good stability, high activity and excellent selectivity. Here, we have designed homonuclear transition metal (TM = Fe, Co, Ni) clusters catalyst supported on graphene (TM3 @Gra) as electrocatalysts for nitrogen reduction reaction (NRR). The mechanism of electrocatalytic NRR on TM3 @Gra was investigated. The Fe3 @Gra, Co3 @Gra, and Ni3 @Gra showed high catalytic activity with corresponding limiting potential of –0.60, –0.64 and −0.45 V, respectively. Finally, the selectivity and thermal stability analysis confirmed that TM3 @Gra were the ideal NRR electrocatalysts. We hope that the results of this study will facilitate the design and development of NRR catalysts and provide theoretical reference for the rational design of cluster catalysts.

Intrinsic vs Reaction-Driven Fluxionality in Transition-Metal Oxide Clusters: What Does Probing the Dynamics of M3O6– (M = Mo and W) Reacting with H2O Teach Us?

Fluxionality is an important concept in cluster science, with far reaching implications in the area of catalysis. The interplay between intrinsic structural fluxionality and reaction-driven fluxionality though is underexplored in the literature and is a topic of contemporary interest in physical chemistry. In this work, we present an easy-to-use computational protocol combining ab initio molecular dynamics simulations with static electronic structure computations to ascertain the role of intrinsic structural fluxionality in the course of fluxionality occurring due to a chemical reaction. The reactions of structurally well-defined M3O6- (M = Mo and W) with water─which were originally used in the literature to illustrate the significance of reaction-driven fluxionality in transition-metal oxide (TMO) clusters, were chosen for this study. Besides probing the nature of fluxionality, this work provides the timescale for the key proton-hop step in the fluxionality pathway and further attests to the significance of hydrogen bonding in both stabilizing the key intermediates as well as driving forward the reactions of M3O6- (M = Mo and W) with water. The approach presented in this work becomes valuable given that the use of molecular dynamics alone may not help us in accessing some metastable states whose formation involves an appreciable energy barrier. Similarly, merely obtaining a slice of the potential energy surface via static electronic structure calculations will not be helpful in probing the different types of fluxionality. Hence, there is the need for a combined approach to study fluxionality in structurally well-defined TMO clusters. Our protocol may also serve as a starting point in the analysis of much more complicated fluxional chemistry happening on surfaces wherein the recently developed "ensemble of metastable states" approach to catalysis is deemed to be particularly promising.

Magnetic Interactions in a [Co(II)3Er(III)(OR)4] Model Cubane through Forefront Multiconfigurational Methods

Strong electron correlation effects are one of the major challenges in modern quantum chemistry. Polynuclear transition metal clusters are peculiar examples of systems featuring such forms of electron correlation. Multireference strategies, often based on but not limited to the concept of complete active space, are adopted to accurately account for strong electron correlation and to resolve their complex electronic structures. However, transition metal clusters already containing four magnetic centers with multiple unpaired electrons make conventional active space based strategies prohibitively expensive, due to their unfavorable scaling with the size of the active space. In this work, forefront techniques, such as density matrix renormalization group (DMRG), full configuration interaction quantum Monte Carlo (FCIQMC), and multiconfiguration pair-density functional theory (MCPDFT), are employed to overcome the computational limitation of conventional multireference approaches and to accurately investigate the magnetic interactions taking place in a [Co(II)3Er(III)(OR)4] (chemical formula [Co(II)3Er(III)(hmp)4(μ2-OAc)2(OH)3(H2O)], hmp = 2-(hydroxymethyl)-pyridine) model cubane water oxidation catalyst. Complete active spaces with up to 56 electrons in 56 orbitals have been constructed for the seven energetically lowest different spin states. Relative energies, local spin, and spin-spin correlation values are reported and provide crucial insights on the spin interactions for this model system, pivotal in the rationalization of the catalytic activity of this system in the water-splitting reaction. A ferromagnetic ground state is found with a very small, ∼50 cm-1, highest-to-lowest spin gap. Moreover, for the energetically lowest states, S = 3-6, the three Co(II) sites exhibit parallel aligned spins, and for the lower states, S = 0-2, two Co(II) sites retain strong parallel spin alignment.

Adsorption and Sensing Performances of MoTe2 Monolayers Doped with Pd, Ni, and Pt for SO2 and NH3: A DFT Investigation

Density functional theory (DFT) calculation was used to study the adsorption and sensing performances of a transition metal atom (TMA) doped MoTe2 monolayer for two industrial toxic and harmful gases, SO2 and NH3, in this study. The adsorption structure, molecular orbital, density of state, charge transfer, and energy band structure were applied to investigate the interaction between the gas and MoTe2 monolayer substrate. The conductivity of the MoTe2 monolayer film doped with TMA (Ni, Pt, Pd) is significantly improved. The original MoTe2 monolayer has poor adsorptive ability for SO2 and NH3, which is physisorption, while for the TMA-doped MoTe2 monolayer, it is significantly enhanced and the adsorption process is chemisorption. All results provide a trustworthy theoretical basis for sensors based on MoTe2 to detect toxic and harmful gases SO2 and NH3. Additionally, it also provides guidance for further research on the transition metal cluster doped MoTe2 monolayer for gas detection.

Supramolecular Host–Guest Assemblies of [M6Cl14]2–, M = Mo, W, Clusters with γ-Cyclodextrin for the Development of CLUSPOMs

Host–guest assemblies open up opportunities for developing novel functional CLUSPOM multicomponent systems based on transition metal clusters (CLUS), polyoxometalates (POMs) and macrocyclic organic ligands. In water–ethanol solution γ-cyclodextrin (γ-CD) interacts with halide metal clusters [M6Cl14]2– (M = Mo, W) to form sandwich-type structures. The supramolecular association between the clusters and CDs, however, remains weak in solution, and the interactions are not strong enough to prevent the hydrolysis of the inorganic guest. Although analysis of the resulting crystal structures reveals inclusion complexation, 1H NMR experiments in solution show no specific affinity between the two components. The luminescent properties of the host–guest compounds in comparison with the initial cluster complexes are also studied to evaluate the influence of CD.

A dimer of the negatively charged carbonyl cluster {Ir4(CO)11−}2 bonded by a single Ir−Ir bond, and comparison with the singly bonded (C60−)2 dimer.

AbstractReduction of the Ir4(CO)12 cluster by two equivalents of Cp*2Cr (Cp* is η5‐Me5C5) produces the (Cp*2Cr+)2{Ir4(CO)11−}2 ⋅ 4 C6H4Cl2 complex. Two Ir4(CO)11− anions are bonded by a single Ir−Ir bond in the anionic {Ir4(CO)11−}2 dimer. The bond length is 2.8339(5) Å at 100 K. Therefore, clusters can form metal‐metal bonded dimers under reduction. The {Ir4(CO)11−}2 dimers do not dissociate to paramagnetic species up to 340 K and even higher temperatures that indicates their high stability. DFT calculations show that dimerization of clusters can occur due to the loss of one CO group in neutral or anionic clusters. Dimerization of anionic transition metal cluster is to a certain extend similar to dimerization of negatively charged carbon cluster – fullerene C60⋅− and here we compare both types of dimers.

Resolution of Electronic States in Heisenberg Cluster Models within the Unitary Group Approach.

In this work ground and excited electronic states of Heisenberg cluster models, in the form of configuration interaction many-body wave functions, are characterized within the spin-adapted Graphical Unitary Group Approach framework, and relying on a novel combined unitary and symmetric group approach. Finite-size cluster models of well-defined point-group symmetry and of general local-spin are presented, including J1-J2 triangular and tetrahedral clusters, which are often used to describe magnetic interactions in biological and biomimetic polynuclear transition metal clusters with unique catalytic activity, such as nitrogen fixation and photosynthesis. We show that a unique block-diagonal structure of the underlying Hamiltonian matrix in the spin-adapted basis emerges when an optimal lattice site ordering is chosen that reflects the internal symmetries of the model investigated. The block-diagonal structure is bound to the commutation relations between cumulative spin operators and the Hamiltonian operator, that in turn depend on the geometry of the cluster investigated. The many-body basis transformation, in the form of the orbital/site reordering, exposes such commutation relations. These commutation relations represent a rigorous and formal demonstration of the block-diagonal structure in Hamiltonian matrices and the compression of the corresponding spin-adapted many-body wave functions. As a direct consequence of the block-diagonal structure of the Hamiltonian matrix, it is possible to selectively optimize electronic excited states without the overhead of calculating the lower-energy states by simply relying on the initial ansatz for the targeted wave function. Additionally, more compact many-body wave functions are obtained. In extreme cases, electronic states are precisely described by a single configuration state function, despite the curse of dimensionality of the corresponding Hilbert space. These findings are crucial in the electronic structure theory framework, for they offer a conceptual route toward wave functions of reduced multireference character, that can be optimized more easily by approximated eigensolvers and are of more facile physical interpretation. They open the way to study larger ab initio and model Hamiltonians of increasingly larger number of correlated electrons, while keeping the computational costs at their lowest. In particular, these elements will expand the potential of electronic structure methods in understanding magnetic interactions in exchange-coupled polynuclear transition metal clusters.

Chiral Magnetic Interactions in Small Fe Clusters Triggered by Symmetry-Breaking Adatoms

The chirality of the interaction between the local magnetic moments in small transition-metal alloy clusters is investigated in the framework of density-functional theory. The Dzyaloshinskii–Moriya (DM) coupling vectors Dij between the Fe atoms in Fe2X and Fe3X with X = Cu, Pd, Pt, and Ir are derived from independent ground-state energy calculations for different noncollinear orientations of the local magnetic moments. The local-environment dependence of Dij and the resulting relative stability of different chiral magnetic orders are analyzed by contrasting the results for different adatoms X and by systematically varying the distance between the adatom X and the Fe clusters. One observes that the adatoms trigger most significant DM couplings in Fe2X, often in the range of 10–30 meV. Thus, the consequences of breaking the inversion symmetry of the Fe dimer are quantified. Comparison between the symmetric and antisymmetric Fe-Fe couplings shows that the DM couplings are about two orders of magnitude weaker than the isotropic Heisenberg interactions. However, they are in general stronger than the anisotropy of the symmetric couplings. In Fe3X, alloying induces interesting changes in both the direction and strength of the DM couplings, which are the consequence of breaking the reflection symmetry of the Fe trimer and which depend significantly on the adatom-trimer distance. A local analysis of the chirality of the electronic energy shows that the DM interactions are dominated by the spin-orbit coupling at the adatoms and that the contribution of the Fe atoms is small but not negligible.

Tracking and Understanding Dynamics of Atoms and Clusters of Late Transition Metals with In-Situ DRIFT and XAS Spectroscopy Assisted by DFT

Dynamic structural changes of single-atom catalysts (SACs) are key to many reactions that were reported to be catalyzed by supported single atoms. To understand these changes, systematic in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and X-ray absorption spectroscopy (XAS) experiments were performed under reducing (1% CO) and oxidizing (1% CO + 1% O2) reaction atmospheres at room temperature over CeO2-supported late transition metals (Ru, Rh, Pd, Ir, and Pt) synthesized via two different methods (wet impregnation and precipitation) to account for the influence of surface properties. As a general trend, the CO vibrational frequencies downshifted under the CO atmosphere, which we assigned to the formation of clusters. Upon changing the gas mixture to more oxidizing (1% CO + 1% O2), single sites are retained as evidenced by the CO vibrational frequencies at higher wavenumbers. Among the investigated metals, Pt2+ and Pd2+ are more prone to cluster formation, and Rh3+ and Ru4+ are found to be stable as single sites following the order Rh > Ru > Ir > Pt > Pd. In combination with the density functional theory (DFT) calculations of CO vibrational frequencies, we were able to assign shifts to changes in the oxidation state of the metals. These findings thus serve as a benchmark for ceria-supported Pd, Pt, Ru, Ir, and Rh SACs.

Studying Na criticality in NaxMn2−xO2 (x = 1.05–1.3) type Na-rich disordered rocksalt cathode for higher capacity

Disordered rocksalt (DRX) can achieve far better capacity than layered oxide cathodes, with the potential to use as cathodes for next-generation batteries, but are limited by their poor ion diffusion kinetics. Initial assessments on Li-rich disordered cathode suggest that Li percolation is heavily influenced by transition metal (TM) polyhedral surrounding Li diffusion path by a repulsive force. However, the kinetics and influence of TM polyhedral structures are not investigated for sodium rich DRX cathodes. Herein, we study the influence of Na stoichiometry in Na-rich DRX cathode—NaxMn2-xO2 (x = 1.05–1.3) and its influence on Na percolation. Increasing the Na stoichiometry systematically gives better insight into Na percolation network, TM cluster formation, and its effect on sodium diffusivity on Na-DRX cathodes. The cathode material is analyzed by transmission electron microscopy and X-ray absorption spectroscopy analysis, giving better understanding on influence of Na stoichiometry on its neighboring polyhedral structures. The Na-rich samples were initially assessed by electrochemical studies, which showed about 110 mAhg−1 of capacity for 0.5 Ag−1 current density. The study provides an insight into crystal structure of Na-rich disordered cathode and ways to improve its capacity and rate performance.

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy.

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO2 lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.