Transition Metal Clusters Research Articles

Activation of Methane and Carbon Dioxide Mediated by Transition-Metal Doped Magnesium Oxide Clusters [MMgO](+/0/-) (M=Sc-Zn).

Mission: impossible? DFT calculations show that the trends in the thermochemistry are very different for the activation of CO2 and CH4 mediated by transition-metal doped magnesium oxide clusters [MMgO](+/0/-) (M=Sc-Zn). Thus, seeking a "simple" reagent to simultaneously mediate activation and coupling of CH4 and CO2 with high efficiency seems extremely daunting, if not impossible.

Surface immobilization of Mo6I8 octahedral cluster cores on functionalized amorphous carbon using a pyridine complexation strategy

Tetrahedral amorphous carbon (a-C) films have been grown by pulsed laser deposition to investigate a liquid phase process for surface immobilization of electroactive [Mo6Ii8]4+ transition metal cluster cores using a complexation reaction with a pyridine-terminated alkyl monolayer covalently bonded to the a-C surface (PyS–alkyl/a-C). These films are stable against thermally-assisted grafting of alkene molecules and the covalent CC interface provides a robust monolayer/a-C assembly. Octahedral [Mo6Ii8]4+ cluster cores with iodine inner ligands and labile triflate apical ligands [Mo6Ii8(CF3SO3)a6]2− have been immobilized through partial complexation in apical positions by surface pyridine groups (PyS). The remaining CF3SO3− apical ligands of [Mo6Ii8 (PyS)ay(CF3SO3)a6−y] cluster units were further substituted with bromopyridine (Py-Br) to obtain air stable surface with expected final composition [Mo6Ii8 (PyS)ay(Py-Br)a6−y]. The yield of the different reaction steps is followed by X-ray photoelectron spectroscopy, providing cluster coverage ΣMo6I8=9×1012cm−2. Each [Mo6I8]4+ cluster is bound to the carbon surface through multiple anchoring metal sites (NPYR=3 or 4), indicating that pyridine-terminated alkyl chains are flexible enough to accommodate four bonds. Electrical transport through Hg//Mo6I8–PyS–alkyl/a-C/p-Si(111) junctions shows rectifying current–voltage characteristics but does not reveal any signature of cluster immobilization.

Application of vibrational correlation formalism to internal conversion rate: case study of Cu(n) (n = 3, 6, and 9) and H2/Cu3.

This work reports non-radiative internal conversion (IC) rate constants obtained for Cun with n = 3, 6, and 9 and H2 on Cu3. The Time-Dependent Density Functional Theory (TDDFT) method was employed with three different functionals in order to investigate the electronic structures and the absorption spectra. The performance of the generalized gradient approximation of Perdew, Burke and Ernzerhof (PBE) and the hybrid B3LYP and PBE0 exchange correlation functionals in combination with the SVP and the def2-TZVP basis sets was examined. TDDFT results were used as input data to compute internal conversion rate constants. For this purpose, we have developed a program package. A description of the theoretical background used in our numerical implementation and the program input file is presented. In view of future applications of this program package in photoinduced catalysis, we present the analysis of the IC rate processes for the photodissociation of H2 on Cu3. These results showed the applicability of the method and the computational program to identify the vibrational modes in transition metal clusters giving rise to the largest IC rate constant due to their interactions with the excited electronic states occurring in the hot-electron induced dissociation phenomena.

Electron counting approach to the structure and bonding of sandwiched low dimensional palladium clusters

One dimensional transition metal clusters are of theoretical and practical interest due to their special electronic and magnetic properties. One class of such clusters consists of a linear Pd moiety sandwiched between two olefin chains which act as a π-sheath. In this work, we provide a simple model, based on electron counting and basic background knowledge on coordination chemistry, to elucidate the structure and bonding properties of this kind of clusters. We also apply what we have learned from these simple examples to some other low-dimensional non-linear Pd clusters.

Macrocyclic Platforms for the Construction of Tetranuclear Oxo and Hydroxo Zinc Clusters

The design of ligands that can act as platforms for the controlled, “bottom-up” synthesis of transition-metal clusters is a promising approach to accessing enzymatic mimics and new small-molecule reaction chemistry. This approach is exemplified here through the coordination chemistry of two compartmental Schiff-base calixpyrroles (H4L) that usually act as dinucleating ligands for transition metals. While reactions between H4L and Zn{N(SiMe3)2}2 form the expected dinuclear Zn “Pacman” complexes Zn2(L), reactions with ZnEt2 result in the tetranuclear Zn alkyl complexes Zn4Et4(THF)4(L), in which open, “bowl-shaped” structures are adopted due to the flexibility of the macrocyclic platform. The outcome of hydrolysis reactions of these tetranuclear complexes is found to depend on the macrocyclic cavity size, with the smaller macrocycle favoring oxo formation in Zn4(μ4-O)Et2(L) and the larger macrocycle favoring complete hydrolysis to form the hydroxide-bridged cluster Zn4(μ2-OH)4(L). This latter complex reacts ...

Influence of the Rotational Domain in the Growth of Transition Metal Clusters on Graphene

The influence of the relative orientation between graphene monolayers and the underlying substrate on the growth of transition metal clusters over epitaxial graphene on Ir(111) surfaces is investigated by scanning tunneling microscopy (STM) in ultrahigh vacuum (UHV). This experimental study has been carried out for W and Ir clusters and, in both cases, the results revealed the existence of noticeable differences in the size and distribution of the clusters formed over areas where graphene and substrate lattices are aligned with respect to those grown on regions where both lattices present a relative rotation angle. In particular, while over aligned domains, in a consistent way with previous findings, the formation of ordered arrays of monodisperse clusters exhibiting a great structural perfection is observed, on the rotated ones it takes place the formation of larger size isolated clusters scattered around the surface. Moreover, the boundaries between different rotational domains are found to be decorated by these larger clusters. This disparity observed in the growth of clusters is explained in terms of the differences in the graphene–substrate interaction existing on aligned and rotated domains.

Is NICS a reliable aromaticity index for transition metal clusters?

In the present account the nature of aromaticity/antiaromaticity of fourteen metallic complexes/clusters are reexamined. These species were classified as aromatic by means of different nucleus independent chemical shift (NICS) based approaches, previously. Visualization of the current density and magnetizability of atomic basins reveals that none of the studied systems are magnetic aromatic, i.e. sustain diamagnetic ring current. It is demonstrated that negative NICS values near the ring plane of the studied molecules originates from remarkably strong local paramagnetic current around their transition metal atom nuclei. This phenomenon has been observed only for Sc3 − all-metal cluster but current study demonstrates that the influence of the local paramagnetic currents around transition metal atoms on NICS is a general phenomenon that must be carefully considered prior to classification of the metallic systems as aromatic. Furthermore, this study suggests that NICS is not a reliable aromaticity index for transition-metal clusters/molecules.

Main Source of Microwave Loss in Transition‐Metal‐Doped Ba(Zn1/3Ta2/3)O3 and Ba(Zn1/3Nb2/3)O3 at Cryogenic Temperatures

Microwave resonator measurements were performed on high‐performance microwave ceramics Ba(Zn1/3Ta2/3)O3 (BZT) and Ba(Zn1/3Nb2/3)O3 (BZN) containing additives commonly used by commercial manufacturers (i.e., Co, Mn, and Ni). We find that the loss tangent, even in ambient magnetic fields, is dominated by electron paramagnetic resonance (EPR) absorption by exchange‐coupled 3d electrons in transition metal clusters at cryogenic temperatures. The large orbital angular momentum in Co2+ and Ni2+ ions of L = 3 causes strong anisotropic‐broadened dipolar interactions that extend EPR losses to zero applied field. This effect is greatest in BZN with Co concentrations greater than 0.5 mol%, dominating the losses at liquid nitrogen temperatures (77 K) and below. In samples containing Mn2+ ions with L = 0, the dipolar interactions and associated EPR losses in ambient fields are smaller. We show the magnetic‐field‐dependent changes in the EPR losses (i.e., tan δ) and magnetic reactive response (i.e., μr) are from the same mechanism, as they follow the Kramers–Kronig relation. Finally, we note that these materials can make ultra‐high Q passive microwave devices with externally controlled transfer functions, as the quality factor (Q) of the composition Ba(Co1/15Zn4/15Nb2/3)O3 at 77 K can be tuned from 1 100 to 12 000 at 10 GHz by applying practical magnetic fields.

Recent progress in polyoxoniobates decorated and stabilized via transition metal cations or clusters

Transition metal (TM) cations and clusters have been widely used to functionalize polyoxometalates (POMs) of tungsten, molybdenum and vanadium due to their acidic nature, which made them compatible with addendum TM cations. Together with the good understanding of their solution behavior, a number of TM-based POMs have been achieved, which exhibited potential applications in many fields. Comparably, fewer polyoxoniobates (PONs) were obtained due to the poor understanding of their solution behavior, and they could only exist in strong alkaline aqueous solutions, where most of the TM cations precipitate because of the formation of hydroxide or oxide. Recently, TM cations were introduced into PONs by protection of chelate organic ligands or via selection of alkaline-soluble TM cations. Further, lacunary POMs, composed of early TM cations, were used as polydentate ligands to isolate PONs, and they make the PON clusters stably exist in acidic solutions. The acidic nature of tungstoniobate (W–Nb) clusters allowed acid-soluble TM cations to directly functionalize the Nb–W clusters. This highlight focuses on discussing the roles of TM centers in constructing PON-based materials, including TM-cluster-containing PONs, TM–organic group-decorating PONs and niobate clusters isolated by lacunary POMs. This systematic compilation of TM-PON studies will reveal a promising direction for further functionalization of PONs, and it will disclose a richer and more diverse set of structures in PON chemistry.

The role of charge transfer in the oxidation state change of Ce atoms in the TM13-CeO2(111) systems (TM = Pd, Ag, Pt, Au): a DFT + U investigation.

Despite extensive studies of transition metal (TM) clusters supported on ceria (CeO2), fundamental issues such as the role of the TM atoms in the change in the oxidation state of Ce atoms are still not well understood. In this work, we report a theoretical investigation based on static and ab initio molecular dynamics density functional theory calculations of the interaction of 13-atom TM clusters (TM = Pd, Ag, Pt, Au) with the unreduced CeO2(111) surface represented by a large surface unit cell and employing Hubbard corrections for the strong on-site Coulomb correlation in the Ce f-electrons. We found that the TM13 clusters form pyramidal-like structures on CeO2(111) in the lowest energy configurations with the following stacking sequence, TM/TM4/TM8/CeO2(111), while TM13 adopts two-dimensional structures at high energy structures. TM13 induces a change in the oxidation state of few Ce atoms (3 of 16) located in the topmost Ce layer from Ce(IV) (itinerant Ce f-states) to Ce(III) (localized Ce f-states). There is a charge flow from the TM atoms to the CeO2(111) surface, which can be explained by the electronegativity difference between the TM (Pd, Ag, Pt, Au) and O atoms, however, the charge is not uniformly distributed on the topmost O layer due to the pressure induced by the TM13 clusters on the underlying O ions, which yields a decrease in the ionic charge of the O ions located below the cluster and an increase in the remaining O ions. Due to the charge flow mainly from the TM8-layer to the topmost O-layer, the charge cannot flow from the Ce(IV) atoms to the O atoms with the same magnitude as in the clean CeO2(111) surface. Consequently, the effective cationic charge decreases mainly for the Ce atoms that have a bond with the O atoms not located below the cluster, and hence, those Ce atoms change their oxidation state from IV to III. This increases the size of the Ce(III) compared with the Ce(IV) cations, which builds-in a strain within the topmost Ce layer, and hence, also affecting the location of the Ce(III) cations and the structure of the TM13 clusters.

Size-dependent properties of transition metal clusters: from molecules to crystals and surfaces--computational studies with the program ParaGauss.

In the so-called scalable regime the size-dependent behavior of the physical and chemical properties of transition metal clusters is described by scaling relationships. For most quantities this scalable regime is reached for cluster sizes between a few tens and a few hundreds of atoms, hence for systems for which an accurate treatment by density functional theory is still feasible. Thus, by invoking scaling relations one is able to obtain properties of very large nanoparticles and even the bulk limit from the results of a series of smaller cluster models. In this invited review we illustrate this strategy by exploiting results from computational studies that mostly were carried out with the density functional theory software ParaGauss. We address mainly the size-dependent behavior of the properties of transition metal clusters. To this end, we first present benchmark studies probing various approximations that are used in such density functional calculations. Subsequently we show how physical insight may be gained by exploring less understood types of systems. These applications range from bare clusters to nanoislands and nanoalloys to adsorption complexes.

Structural formation of binary PtCu clusters: A density functional theory investigation

Binary transition-metal clusters have attracted great attention and a large number of studies have been reported, however, our atomistic understanding of the structural formation mechanisms of those clusters is still far from satisfactory. In this paper, we report a systematic study of the PtnCum−n clusters (m=2,3,…,14,n=0,1,…,m) employing Ab-Initio density functional theory calculations within the Perdew–Burke–Ernzerhof functional. Using a set of structural design principles, we obtained a hierarchical set of atomic configurations from which the structural formation mechanisms are discussed in details. We found a negative excess energy for m>2, providing strong evidence favouring the formation of binary PtCu clusters. In general, the Cu atoms tend to form agglomerates located near the center of gravity of the clusters while the Pt atoms tend to lie further away from the center of gravity and separated from each other. Therefore, this behaviour tends to reduce the number of Pt–Pt bonds whereas the number of Cu–Cu and Pt–Cu bonds tends to be maximized. These mechanisms help to release strain energy by means of relaxation of the gas-phase clusters, and the locations of the Cu and Pt atoms suggest that the formation of core–shell like structures starts already in this small size regime. Furthermore, the formation of the PtCu alloy does not lead to changes in the magnetic properties of the clusters in comparison with the parent Pt and Cu clusters. The systematic work presented here provides a basis to understand and tailor the properties of binary cluster.

Binary Alloy Clusters: Structures and Electronic Properties

Synthesis, characterization, physics, and chemistry of nanoclusters have been the focus of intense attention for the last two decades. Also, their unique statues that they are between molecule and bulk material is the central reason for the theoretical researcher to understand the transition from atom to cluster/molecule and then to solid state. In this review we restrict ourselves to the recent advances in the structural and electronic properties of binary alloy clusters. This type of clusters is particularly of importance thanks to their very availability for being tuned, by changing their sizes, according to any special needs. A brief summary of the classification of clusters as well as the experimental techniques and theoretical methods to study them, and the possible applications of alloy clusters are given in the introduction part. Several binary alloy cluster types, including semiconductor, alkali and transition metal, oxide, and ionic binary alloy clusters, are widely reviewed considering extensively the most recent articles.

Nanoclusters synthesized by synchrotron radiolysis in concert with wet chemistry.

Wet chemical reduction of metal ions, a common strategy for synthesizing metal nanoparticles, strongly depends on the electric potential of the metal, and its applications to late transition metal clusters have been limited to special cases. Here, we describe copper nanoclusters grown by synchrotron radiolysis in concert with wet chemistry. The local structure of copper aggregates grown by reducing Cu(II) pentanedionate using synchrotron x-ray beam was studied in situ by x-ray absorption spectroscopy. A detailed analysis of the XANES and EXAFS spectra, compared with DFT calculations and full-potential non-muffin-tin multiple scattering calculations, identified the nanocluster as Cu13 with icosahedral symmetry. The novel “charged” nanoclusters tightly bound to electron-donating amido molecules, which formed as a result of photo-induced deprotonation of ligand amines, were stabilized by irradiation. Monodispersive deposition of nanoclusters was enabled by controlling the type and density of “monomers”, in remarkable contrast to the conventional growth of metallic nanoparticles.

Hydroxyl migration in heterotrimetallic clusters: an assessment of fluxionality pathways.

Water splitting at the unsaturated metal center and subsequent hydroxyl migration are key steps toward successful H2 liberation from cheap and abundant water using transition metal cluster anions. In this report we initiate a theoretical study (DFT) to assess the efficacy of heterometallic cores instead of the widely studied and well established homometallic cores. To accomplish this goal, one tungsten center in W3O6(-) core has been replaced by different transition metals such as titanium, technetium, and osmium. Introduction of the heterometal makes the core asymmetric and electronically anisotropic. To evaluate the efficiency of these heterometallic cores, fluxionality pathways for hydroxyl migration have been studied in detail. We show that the cores W2TcO6(-) (2) and W2OsO6(-) (3) can exhibit fluxionality for hydroxyl migration and thus can potentially facilitate H2 liberation from H2O. Notably, a new class of low-energy structures generated upon oxide bridge opening process and subsequent structural rearrangement facilitates the hydroxyl migration event. To illustrate the heterometallic effect further, we show that previously inaccessible energy barriers for hydroxyl migration in a homometallic trimolybdenum core become energetically achievable when one of the metals is replaced by a 5d element osmium.

Hydrothermal combination of trilacunary Dawson phosphotungstates and hexanickel clusters: from an isolated cluster to a 3D framework.

Three novel hexa-Ni-substituted Dawson phosphortungstates [Ni6 (en)3 (H2O)6 (μ3-OH)3 (H3 P2 W15 O56 )]⋅14 H2O (1), [Ni(enMe)2 (H2O)][Ni6 (enMe)3 (μ3-OH)3 (H2O)6 (HP2 W15 O56)]⋅ 10 H2O (2), and [Ni(enMe)2 ]3 [Ni(enMe)2 (H2O)][Ni(enMe)(H2O)2][Ni6 (enMe)3 (μ3-OH)3 (Ac)2 (H2O)(P2 W15 O56)]2 ⋅6 H2O (3) (en=ethylenediamine, enMe=1, 2-diaminopropane, Ac=CH3 COO(-)) have been made under hydrothermal conditions and were characterized by IR spectroscopy, elemental analysis, diffuse reflectance spectroscopy, thermogravimetric analysis, powder X-ray diffraction, and single-crystal X-ray diffraction. The common structural features of compounds 1-3 contain the similar hexa-Ni-substituted Dawson polyoxometalate (POM) units that can be viewed as a [Ni6 (μ3-OH)3](9+) cluster capping on a [P2 W15 O56](12-) fragment. Compounds 1 and 2 are two isolated clusters, whereas compound 3 is the first 3D POM framework constructed from hexa-Ni-substituted Dawson POM units and Ni(enMe) complex bridges. The preparations of compounds 1-3 not only indicate that triangle coplanar Ni6 clusters are very stable fragments in both trivacant Keggin and trivacant Dawson POM systems, but also offer that the hydrothermal technique can act as an effective strategy for making novel Dawson-type high-nuclear transition-metal cluster substituted POMs by combination of lacunary Dawson precusors with transition-metal cations in the tunable role of organic ligands. In addition, magnetic measurements illustrate that there exist overall ferromagnetic interactions in compound 3.

Surface-based supramolecular chemistry using hydrogen bonds.

CONSPECTUS: The arrangement of molecular species into extended structures remains the focus of much current chemical science. The organization of molecules on surfaces using intermolecular interactions has been studied to a lesser degree than solution or solid-state systems, and unanticipated observations still lie in store. Intermolecular hydrogen bonds are an attractive tool that can be used to facilitate the self-assembly of an extended structure through the careful design of target building blocks. Our studies have focused on the use of 3,4,9,10-perylene tetracarboxylic acid diimides (PTCDIs), and related functionalized analogues, to prepare extended arrays on surfaces. These molecules are ideal for such studies because they are specifically designed to interact with appropriate diaminopyridine-functionalized molecules, and related species, through complementary hydrogen bonds. Additionally, PTCDI species can be functionalized in the bay region of the molecule, facilitating modification of the self-assembled structures that can be prepared. Through a combination of PTCDI derivatives, sometimes in combination with melamine, porous two-dimensional arrays can be formed that can entrap guest molecules. The factors that govern the self-assembly processes of PTCDI derivatives are discussed, and the ability to construct suitable target arrays and host-specific molecular species, including fullerenes and transition metal clusters, is demonstrated.

ChemInform Abstract: Gas‐Phase Perspective on the Thermodynamics and Kinetics of Heterogeneous Catalysis

AbstractReview: studies of small gas‐phase transition metal cluster cations; 144 refs.

Multi-reference Ab initio calculations of 3d transition-metal dimers: Sc2

The problem of calculation of the electronic structure of transition-metal clusters (even dimers) still presents a challenge for computational chemistry. The reason is that the expansion of the ground state wave function on electronic configurations does not contain a principal configuration and a large number of reference configurations must be treated equally. Thus, the multi-reference (MR) approaches are, in general, mandatory. A short description of the MRCI methods is presented. The calculation of Sc2 at the MRCISD(+Q)/cc-pV5Z level, showed that the its ground state corresponds to a quintet, 5Σu−, in agreement with experiment and previous precise calculations. The triplet 3Σu− state is located about 1.1 kcal/mol above. We calculated the ground state, X5Σu−, of the Sc2 dimer by the MRCISD(+Q) method at the complete basis set (CBS) limit. This is the first MRCISD(+Q) calculation of 3d transition-metal clusters at the CBS limit. From the Mulliken population analysis and comparison with atomic energies follows that in the ground state Sc2 dissociates on one Sc in the ground state and the other in the second excited quartet state, 4Fu. The spectroscopic parameters of the ground potential curve, obtained by the Dunham analysis at the valence MRCISD(+Q)/CBS level, are: Re = 5.20 bohr, De = 50.37 kcal/mol, and e = 234.5 cm−1. The obtained value for the harmonic frequency agrees very well with the experimental one, e = 239.9 cm−1. From the indistinguishability of identical atoms in quantum mechanics follows that in spite of an asymmetric dissociation limit for Sc2, both Sc atoms have the same electron population and it is incorrect to consider them as different. Therefore, the accepted classification of the dimer states should be revised, since some of them are forbidden by the Pauli Exclusion Principle.

Oxidation at the Subnanometer Scale

Metals are commonly oxidized under ambient conditions. Although bulk oxidation has received considerable attention, far less is known about oxidation at the subnanometer scale. This is unfortunate, as metal particles used in heterogeneous catalysis typically range from subnanometer to some nanometers. Here, density functional theory calculations are used to explore oxidation of gas-phase transition metal clusters in the range from the dimer to the dodecamer. Comparisons with the corresponding bulk systems uncover that the decomposition temperature of stoichiometrically oxidized clusters may be lower than for the bulk. Despite pronounced variations in ground state geometries, oxidized clusters closely mimic energetic trends across the periodic table valid for bulk systems.