Transition Metal Cluster Compounds Research Articles

Controlling the Deposition Process of Nanoarchitectonic Nanocomposites Based on {Nb6-xTaxXi12}n+ Octahedral Cluster-Based Building Blocks (Xi = Cl, Br; 0 ≤ x ≤ 6, n = 2, 3, 4) for UV-NIR Blockers Coating Applications.

The antagonism between global energy needs and the obligation to slow global warming is a current challenge. In order to ensure sufficient thermal comfort, the automotive, housing and agricultural building sectors are major energy consumers. Solar control materials and more particularly, selective glazing are part of the solutions proposed to reduce global energy consumption and tackle global warming. In this context, these works are focused on developing new highly ultraviolet (UV) and near-infrared (NIR) absorbent nanocomposite coatings based on K4[{Nb6-xTaxXi12}Xa6]. (X = Cl, Br, 0 ≤ x ≤ 6) transition metal cluster compounds. These compounds contain cluster-based active species that are characterized by their strong absorption of UV and NIR radiations as well as their good transparency in the visible range, which makes them particularly attractive for window applications. Their integration, by solution processes, into a silica-polyethylene glycol or polyvinylpyrrolidone matrices is discussed. Of particular interest is the control and the tuning of their optical properties during the integration and shaping processes. The properties of the solutions and films were investigated by complementary techniques (UV-Vis-NIR spectrometry, ESI-MS, SEM, HRTEM, etc.). Results of these works have led to the development of versatile solar control coatings whose optical properties are competitive with commercialized material.

Высоковалентные кластерные соединения переходных металлов, содержащие внедренные гетероатомы: особенности геометрии, электронного строения и физико-химических свойств

Рассмотрено современное состояние химии высоковалентных кластерных соединений переходных металлов 3—7 групп периодической системы, содержащих внедренные гетероатомы. Обсуждены методы синтеза, реакционная способность, основные структурные типы и электронная структура образующихся соединений.

Strong Electronic and Magnetic Coupling in M4 (M = Ni, Cu) Clusters via Direct Orbital Interactions between Low-Coordinate Metal Centers.

We present an extensive study of tetranuclear transition-metal cluster compounds M4(NPtBu3)4 and [M4(NPtBu3)4][B(C6F5)4] (M = Ni, Cu; tBu = tert-butyl), which feature low-coordinate metal centers and direct metal-metal orbital overlap. X-ray diffraction, electrochemical, magnetic, spectroscopic, and computational analysis elucidate the nature of the bonding interactions in these clusters and the impact of these interactions on the electronic and magnetic properties. Direct orbital overlap results in strongly coupled, large-spin ground states in the [Ni4(NPtBu3)4]+/0 clusters and fully delocalized, spin-correlated electrons. Correlated electronic structure calculations confirm the presence of ferromagnetic ground states that arise from direct exchange between magnetic orbitals, and, in the case of the neutral cluster, itinerant electron magnetism similar to that in metallic ferromagnets. The cationic nickel cluster also possesses large magnetic anisotropy exemplified by a large, positive axial zero-field splitting parameter of D = +7.95 or +9.2 cm-1, as determined by magnetometry or electron paramagnetic resonance spectroscopy, respectively. The [Ni4(NPtBu3)4]+ cluster is also the first molecule with easy-plane magnetic anisotropy to exhibit zero-field slow magnetic relaxation, and under a small applied field, it exhibits relaxation exclusively through an Orbach mechanism with a spin relaxation barrier of 16 cm-1. The S = 1/2 complex [Cu4(NPtBu3)4]+ exhibits slow magnetic relaxation via a Raman process on the millisecond time scale, supporting the presence of slow relaxation via an Orbach process in the nickel analogue. Overall, this work highlights the unique electronic and magnetic properties that can be realized in metal clusters featuring direct metal-metal orbital interactions between low-coordinate metal centers.

Apical Cyanide Ligand Substitution in Heterometallic Clusters [Re3Mo3Q8(CN)6]n– (Q = S, Se)

A number of transition metal cluster compounds can be obtained only in the melt of inorganic cyanides and, therefore, contain terminal cyanide ligands. Substitution of these ligands, which is often necessary to change the physicochemical properties of the clusters, is an urgent problem because of their low reactivity in substitution reactions. In this work, a synthetic approach has been developed for the substitution of CN‐ligands in the heterometallic cluster anions [Re3Mo3Q8(CN)6]n– (Q = S, n = 6; Q = Se, n = 5) by the 4‐tert‐butylpyridine (TBP) molecules. Two new compounds, namely [Re3Mo3S8(TBP)6] (1) and [Re3Mo3Se8(TBP)6] (2), were obtained with high yields and crystallized under solvothermal conditions. It has been shown that compounds 1 and 2 are based on the paramagnetic cluster cores {Re3Mo3Q8}0 containing 23 cluster valence electrons (CVE). The geometry of the new compounds has been investigated using X‐ray structural analysis. The electronic structure has been analyzed using the DFT calculations showing large distortion of M6 cluster core.

Large Anisotropy Barrier in a Tetranuclear Single-Molecule Magnet Featuring Low-Coordinate Cobalt Centers.

The tetranuclear cobalt cluster compound [Co4(μ-NPtBu3)4][B(C6F5)4] (tBu = tert-butyl) was synthesized by chemical oxidation of Co4(NPtBu3)4 with [FeCp2][B(C6F5)4] and magnetically characterized to study the effect of electronic communication between low-coordinate metal centers on slow magnetic relaxation in a transition metal cluster. The dc magnetic susceptibility data reveal that the complex exhibits a well-isolated S = 9/2 ground state, which persists even to 300 K and is attributed to the existence of direct metal-metal orbital overlap. The ac magnetic susceptibility data further reveals that the complex exhibits slow magnetic relaxation in the absence of an applied field, and that the relaxation dynamics can be fit with a combination of Orbach, quantum tunneling, and Raman relaxation processes. The effective spin reversal barrier for this molecule is 87 cm-1, the largest reported to date for a transition metal cluster, and arises due to the presence of a large easy-axis magnetic anisotropy. The complex additionally exhibits waist-restricted magnetic hysteresis and magnetic blocking below 3.6 K. Taken together, these results indicate that coupling of low-coordinate metal centers is a promising strategy to enhance magnetic anisotropy and slow magnetic relaxation in transition metal cluster compounds.

Peculiar magnetism of transition metal cluster compounds

We discuss a class of solids which have in their structure fairly separated tetrahederal clusters of transition metals: vanadium and molybednum. With decreasing temperature these compounds undergo a structural transition from cubic to rhombohederal phase at a temperature Ts, followed by a transition to a ferromagnetic phase at a lower temperarure Tc. Magnetic measurements indicate the presence of moments of spin half per cluster. Between Tc and Ts susceptibility shows large departures from Curie-Weiss behavior and specific heat has an unusually large non-phononic contribution. We present a model in which each cluster acts as a unit associated with a magnetic moment of spin half and a structural degree of freedom related to distortions of the tetrahedron. Our model accounts for all the observed features including effects due to external pressure and magnetic field.

ChemInform Abstract: Electronic Structures of (M3(μ3-X)(μ2-S2)3)4+ (M: Mo, Ti) Cluster Compounds and the Effects of the μ3-X Cap to the Self- Assembly in Cluster Synthesis.

Abstract The electronic structures and bonding in one class of trinuclear transition-metal cluster compounds having the cores [Ti3(μ3-O)(μ2-S2)3]4+ (1), [Mo3(μ3-S2)3]4+ (2) and [MO3S4]4+ (3) are discussed in terms of the results of molecular orbital (MO) calculations using the CNDO/ 2-LMO method. There are certain strong bonding interactions between the metal atoms in clusters 2 and 3, but no significant metal-metal bonding is found in cluster 1. The existence of one four-centred bond consisting of three skeletal Mo atoms and the μ3 cap in cluster 2 is of particular importance for shedding light on the nature of the self-assembly syntheses. From the analysis of the localized MO (LMO) we conclude that the μ3-O cap and the μ3-S cap have an important effect on the self-assembly of the trinuclear clusters.

Transition Metal Cluster and Polygadolinium Compounds as a New Paradigm For High Attenuation and/or Relaxivity in Contrast Media Design: Crashing The Molecular r1 = 100 Barrier

We have been developing the supporting chemistry for new types of X-ray and MRI contrast agents by exploiting the unique aspects of high atomic number metal cluster compounds and highly paramagnetic, rigid polygadolinium complexes. Octahedral halide clusters of tantalum (Ta) and tungsten (W) should have multiplied attenuation, compared to iodinated contrast media, in X-ray imaging because of their closer absorption match to the emission spectra of radiological equipment. Their substantial absorption at energies >60 kev increases diagnostic S/N and can lower ancillary absorbed dose. Polygadolinium compounds with μ3-hydroxo ligands have substantial magnetic moments, and the lack of Gd antiferromagnetic coupling combined with inner-sphere water coordination and novel outer-sphere H2O H-bonding to the μ3-hydroxos can multiply relaxivity compared to monomeric Gd complexes. Major impediments to the development of these polynuclear compounds as contrast media are low synthetic yields and lack of multidentate ligand coordination chemistry. The objectives of the research are to develop high yield syntheses and the coordination chemistries of known and new polynuclear compounds, to design encapsulating chelates that will impart biocompatibility and facilitate bioconjugation for molecular targeting/imaging, and to measure attenuations and relaxivities.

Aerosol-based synthesis of carbon nanotubes

Solutions of transition metal cluster compounds were atomised by electrohydrodynamic means and the resultant aerosol was reacted with ethyne in the gas phase to catalyse the formation of carbon nanotubes. The use of an aerosol of iron pentacarbonyl resulted in the formation of multi-walled nanotubes, mostly 6–9 nm in diameter, whereas the use of iron dodecacarbonyl gave results that were concentration dependent. High concentrations resulted in a wide diameter range (30–200 nm) whereas lower concentrations gave multi-walled nanotubes with diameters of 19–23 nm.

X-ray attenuation coefficients of high-atomic-number, hexanuclear transition metal cluster compounds: A new paradigm for radiographic contrast agents

The purpose of this study was to examine the radiologic attenuation properties of the parent cluster compounds, particularly attenuation as a function of discrete photon energy, before investigating ligand substitutions, which are necessary to improve cluster biocompatibility and to impart desirable physicochemical properties. The linear attenuation coefficients for solutions of the cluster compounds Ta6Br14, K8Ta6O19, and (H3O)2W6Cl14 were determined at 60, 80, 103, 122, and 140 keV from gamma-ray transmission measurements with americium-241, xenon-133, gadolinium-153, cobalt-57, and technetium-99m radioactive sources. Transmission measurements were obtained for a fixed time interval that ensured a statistically accurate count distribution exceeding 20,000 counts through the sample for each trial. On a strictly mole per liter basis, a 0.075 mol/L aqueous solution of K8Ta6O19 showed 1.08 times the attenuation of 0.063 mol/L aqueous iohexol at 60 keV and 3.30 times the attenuation at 80 keV. Similarly, a 0.05 mol/L methanolic solution of (H3O)2W6Cl4 showed 0.96 times (96%) the attenuation of 0.063 mol/L aqueous iohexol at 60 keV but 3.09 times the attenuation of the iohexol solution at 80 keV. Attenuations of 0.063 mol/L aqueous iohexol and 0.0125 mol/L Ta6Br14 (ie, at approximately one-fifth the iohexol concentration) were comparable at greater than 60 keV. These results confirm the theoretic potential for use of early transition metal cluster compounds as radiographic contrast agents. At higher x-ray energies, cluster compounds demonstrate multiplied x-ray attenuation relative to iodinated contrast agents.

Synthesis, Characterization and Theoretical Investigation on the Optical Nonlinearity of WS4Co(dppe)(DMF) Heterometallic Cluster Compound

A novel hetero-metallic binuclear cluster WS4Co(dppe)(DMF) has been synthesized and characterized. The crystal has a structure of monoclinic space group P21. Ab initio CPHF calculations are performed on the static first hyperpolarizabilities of the molecular building blocks of the cluster. The microscopic structural origin of nonlinear optical properties are discussed in-depth. The charge-transfer between tungsten ion and cobalt ion in a molecular building block critically contributes to the optical nonlinearity, while the joint effect of the ligands makes sufficient enhancements. The study is helpful to search for novel IR nonlinear optical materials among the transition metal cluster compounds.

Synthesis, Structure, and Electrical Properties of Ta4FeTe4

AbstractTa4FeTe4 has been synthesized and its structure determined by single‐crystal X‐ray methods. It crystallizes in the ortho‐rhombic space group Pbam with Z = 4 and a = 10.514(5), b = 18.275(7), and c = 4.815(1) Å. Ta4FeTe4 has a chain structure built up by Fe‐centered Ta8‐square antiprisms sharing common square faces. The resistivity of Ta4FeTe4 has been measured by a two‐point method. Ta4FeTe4 shows metallic behavior. The structure of Ta4FeTe4 is discussed in relation to other early transition metal cluster compounds. A rational approach to the synthesis of related compounds is proposed.

The vibrations of bridging carbonyl groups as more sensitive indicators of molecular geometry than terminal carbonyl groups in transition metal carbonyl cluster compounds

The ν(CO) spectra of a series of transition metal cluster compounds are compared with their structures. Despite considerable structural differences, the spectra in the terminal region are surprisingly invariant. Rather more variation is shown in the bridging region spectra; it proves possible to understand these variations in terms of a qualitative model. A generalisation concerning bridging region spectra is presented.

Carbon-13 NMR Spectroscopy Study of L-Zeolite- and Silica-Supported Platinum Catalysts

NMR studies of CO adsorbed on small Pt particles show evidence of changes in the metallic nature of these particles with size. Large particles on silica or the exterior of zeolite crystallites have conduction-band electrons that cause a Knight shift for adsorbed CO. Small particles in zeolite cavities are diamagnetic clusters, and yield spectra for linear and bridging carbonyls similar to those of transition-metal cluster compounds. 13C NMR of CO offers a simple probe of metal dispersion and particle size for these Pt catalysts and other noble metal systems.

Novel quasi-aromatic transition metal cluster compounds

Abstract From the point of view of chemical reactivities, molecular configurations, and bonding characteristics, a series of novel quasi-aromatic transition metal cluster compounds has been studied in this article. Experimental evidences for the cluster compounds [M3(μ3-X)(μ-y)3]4+ (M = Mo, W; X, Y = O, S, Se, Te) containing a puckered {M3(μ-Y)3}six-membered rings reveal benzene-like structural characteristics and chemical behaviours in a series of ligand substitution, addition, and oxidation reactions. The energy localized CNDO/2 molecular orbital method is applied to elucidate the nature of quasi-aromaticity in this type of cluster compound. The localized molecular orbitals (LMOs) of benzene as an aromatic prototype and a series of planar monocyclic polyene hydrocarbons CnHq n as well as certain typical conjugated six-membered rings are investigated, leading a deep-rooted inherent relationship between their aromaticity and bonding characteristics. The LMO analyses for these cluster compounds with [Mo3(μ...

Electronic structures of [M 3(μ 3-X)(μ 2-S 2) 3] 4+(M = Mo, Ti) cluster compounds and the effects of the μ 3-X cap to the self-assembly in cluster synthesis

The electronic structures and bonding in one class of trinuclear transition-metal cluster compounds having the cores [Ti 3(μ 3-O)(μ 2-S 2) 3] 4+ ( 1), [Mo 3(μ 3-S 2) 3] 4+ ( 2) and [MO 3S 4] 4+ ( 3) are discussed in terms of the results of molecular orbital (MO) calculations using the CNDO/ 2-LMO method. There are certain strong bonding interactions between the metal atoms in clusters 2 and 3, but no significant metal-metal bonding is found in cluster 1. The existence of one four-centred bond consisting of three skeletal Mo atoms and the μ 3 cap in cluster 2 is of particular importance for shedding light on the nature of the self-assembly syntheses. From the analysis of the localized MO (LMO) we conclude that the μ 3-O cap and the μ 3-S cap have an important effect on the self-assembly of the trinuclear clusters.

A fast atom bombardment mass spectrometry study of transition metal carbonyl cluster compounds

The use of fast atom bombardment (FAB) mass spectrometry for the characterization of several transition metal cluster compounds is described and compared with that of electron impact (EI) mass spectrometry. The FAB mass spectra of the series [H 2Os 7(CO) 22− n ] ( n = 0, 1 and 2) afforded high abundance quasi-molecular ions [ MH] +, only when obtained in a CO-saturated 3-nitrobenzyl alcohol (NBA) matrix; loss of CO ligands ( n = 0, 1) to yield [H 2Os 7(CO) 20] occurred in the argon saturated matrix and under the EI mass spectrometry conditions. No molecular signals were detected in the EI mass spectra of the series [HIr 4(CO) 10(μ-PPh 2)], [HIr 4(CO) 9(PPh 3)(μ-PPh 2)] and [(PhMe 2)AuIr 4(CO) 10(μ-PPh 2)], but their FAB mass spectra in NBA matrix displayed [ MH] + ions and fragment ions that are easily correlated to the structures of these species. The FAB mass spectra of compounds [MIr 4(CO) 9L(η 5-C 5Me 5)(μ 4-PPh)] (M = Rh, Ir; L = CO, PPh 2Me) in NBA matrix exhibited [ MH] + ions and fragment ions corresponding to loss of CO and of HPh from the PPh ligand. Comparative studies of the reactions of [HIr 4(CO) 10(μ-PPh 2)] and [(Bu 4N)(Ir 4(CO) 11Br)] with PPh 3 to give [HIr 4(CO) 9(PPh 3)(μ-PPh 2)] and [Ir 4(CO) 11(PPh 3), respectively, on the FAB probe tip and in solution have established a correlation between species observed by FAB mass spectrometry and those present in the reaction mixture.

Metallclusterverbindungen  Chemie und Bedeutung; Homogene und heterogene Katalyse

AbstractThe interest in transition metal cluster catalysis has developed tremendously over the past decade because of the potential benefits metal cluster compounds may provide over traditional catalysts. Although these expectations have not been met yet transition metal cluster catalysis has offered many insights into the phenomenon of catalysis and still remains the most important driving force of the metal cluster chemistry. This part of the review on transition metal cluster compounds deals with the homogeneous and heterogeneous catalysis by such compounds.

Influence of d-state density and chemistry of transition metal cluster selenides on electrocatalysis

Catalytic studies were carried out on two important electrochemical reaction processes, hydrogen evolution and molecular oxygen reduction in acid medium comparing the transition metal cluster compounds Mo 4Ru 2Se 8, Mo 2Re 4Se 8, Mo 6Se 8, and the Mo metal. Our results show that during the hydrogen evolution reaction, in the course of catalytic charge transfer with the cluster, the hydrogen species solely interacts with individual metal atoms. Molecular oxygen, on the other hand, during oxygen reduction interacts with the cluster as a whole. We propose that the important factors involved in such a process are the high density of d states and the chemistry of the elements. The former factor can be significantly improved by going from the pure metal to transition metal cluster compounds and mixed transition metal compounds. The second factor can be considered by incorporating adequate transition metal atoms into the cluster. Since less catalytic elements for the reduction of oxygen in acid medium (e.g., Mo, Ru, Re) behave like highly catalytic elements (e.g., Pt, Ir) when incorporated into the cluster structure, these experimental results probably indicate a new route for tailoring efficient catalysts from more abundant transition elements.

Principles of bonding and reactivity in transition metal cluster compounds

The bonding in transition metal cluster compounds is examined by partitioning the system into the component parts suggested by expressions for the total energy. The nature of MM (metal-metal) interactions, ML (metal-ligand) interactions and LL (ligand-ligand) interactions are examined, and their effect on the stability and hence structure of the system considered. The processes by which one structure can rearrange into another are discussed. Some consideration is given to the partitioning of a cluster into ML j fragments, and the interactions between these fragments. Isolobal analogies are discussed in this context. The emphasis of this work is on the general principles behind the structure and reactivity of transition metal cluster compounds, rather than focusing on specific systems.