Ligand-Free Metal Clusters

Recent advances in the synthesis and spectroscopic characterization of ligand-free metal clusters immobilized in cryogenic rare gas matrices have contributed greatly to the understanding of electronic, geometric, dynamic, and chemical bonding properties of a wide range of uni- and bimetallic clusters as a function of nuclearity and metal type. The knowledge accumulated on molecular metal aggregates devoid of ligands and isolated on various supports will form an important data base for gauging the reliability of quantum chemical calculations on metal clusters, as well as for comprehending certain aspects of chemisorption on, and catalysis by, supported metal clusters. It can be envisaged that information on ligand-free metal clusters entrapped in a wide range of matrix environments in conjunction with the data for these same metal clusters in the gas phase and in molecular beams will probably contribute towards understanding metal-support interactions and to the designed synthesis of a new breed of high-technology heterogeneous catalysts in the not too distant future.     

Trinuclear Clusters of the Early Transition Elements

Trinuclear clusters of the early transition elements, i.e. those elements situated on the left-hand side of the transition series, represent the simplest types of clusters. They characteristically have a pronounced formation tendency and high stability; they are therefore produced under a wide variety of conditions and their triangular M₃ skeleton is conserved in ligand exchange reactions. These clusters play a very important role in the chemistry of the respective elements, and especially those of the 4 d and 5 d series. Biochemical implications are of interest in connection with Mo^IV. This progress report presents a systematic account of the chemistry and the molecular and electronic structure of such compounds. A connection is also established with the crystal field treatment of mononuclear metal complexes.     

Catalysis by Molecular Metal Clusters

Molecular metal clusters form a very large and diverse family. They present the opportunity of modeling the intermediates involved in surface mediated catalytic reactions, of providing a source of very reactive mononuclear metal fragments, and of effecting catalytic cycles in which the cluster remains intact. The last mentioned aspect is the subject of this review article. The state-of-the-art of cluster catalysis is critically analyzed. The possibilities offered by molecular metal catalysts of performing catalytic reactions at multimetal atom sites are also discussed.     

Oxoniobates Containing Metal Clusters

Metal clusters, discrete or condensed, are characteristic of the structures of many compounds which contain transition metals in low oxidation states. The highly reduced oxoniobates support the concept of condensed clusters. They contain Nb₆O₁₂ clusters which are either isolated or linked at the apices of the Nb₆ octahedra to form oligomeric chains or networks. The analysis of the bonding relationships allows the identification of different types of Nb atoms and thus the quantitative prediction of valence electron concentrations for finite or infinite structures composed of these condensed M₆X₁₂ clusters.     

Synthesis and Characterization of Intercluster Compounds Consisting of Various Gold Clusters and Differently Charged Keggin Anions

The crystal structures of five intercluster compounds consisting of gold clusters and Keggin anions have been determined by single‐crystal x‐ray diffraction: [Au₉(PPh₃)₈][PMo₁₂O₄₀] (P4/n, Z = 2, a = 24.0195(13), c = 13.6818(10) Å), [Au₉(PPh₃)₈][HSiMo₁₂O₄₀] (P4/n, Z = 2, a = 24.270(3), c = 13.752(2) Å), [Au₉(PPh₃)₈][H₃CoW₁₂O₄₀] (P4/n, Z = 2, a = 24.4776(16), c = 13.7759(13) Å), [Au₈(PPh₃)₈]₂[SiMo₁₂O₄₀] (Pbca, Z = 4, a = 36.269(4), b = 24.488(3), c = 38.612(4) Å) and (nBu₄N)[Au₉(Ptol₃)₈][SiMo₁₂O₄₀] (Cc, Z = 4, a = 24.832(5), b = 24.955(5), c = 40.096(8) Å, β = 106.744(3)°). In these compounds, the charges of the building blocks were varied. Altering the charge on the Keggin anion in combination with the [Au₉(PPh₃)₈]³⁺ cluster gave rise to isostructural compounds as the charge is compensated by protonation of the polyoxometalate anion. Varying the charge or the ligand type of the gold cluster led to compounds with a completely different packing principle.     

Ionothermal Synthesis of a NaCl‐type Topological Network Based on Trinuclear Cobalt(II) Clusters as Nodes

A new metal‐organic network [Co₃(tbip)₃(H₂O)₄] · 2H₂O ( 1 ) (H₂tbip = 5‐tert‐butyl‐isophthalic acid) was synthesized through the ionothermal reaction of H₂tbip, cobalt nitrate, and [bmim]Br ionic liquid ([bmim]Br = 1‐butyl‐3‐methylimidazolium bromide). It exhibits a three‐dimensional (3D) framework with NaCl topology based on trinuclear cobalt(II) clusters as nodes. The magnetic studies show that there exist antiferromagnetic interactions between the Co^II ions.     

Synthesis and Characterization of a Novel Binuclear Cobalt(II) Complex with Discrete Water Clusters (H2O)8

A new octameric water cluster was observed in the complex Co₂(dptc)(bipy)₂(H₂O)₆ · 4H₂O ( 1 ) (H₄dptc = diphenyl‐3,3′,4,4′‐tetracarboxylic acid; bipy = 2,2′‐bipyridine), which was characterized by single‐crystal X‐ray diffraction, elemental analysis and IR spectroscopy. The centrosymmetric octamer consists of a water hexamer in the chair form and two water molecules and brings to light a novel mode of the cooperative association of water molecules. Those complex units are connected into a 2D infinite layer framework through hydrogen bonding. Consequently, the 2D layers are further aggregated by hydrogen bonding with octameric subunits and π ··· π stacking interactions to form a 3D supramolecular architecture.     

(SiO2)n团簇的基态能量和结构

应用遗传算法对二氧化硅团簇（SiO2）n（n≤20）的结构进行了优化计算。分析讨论了结构和结合能随团簇尺寸的变化规律,发现（SiO2）n团簇系列不存在明显的幻数,并在n≤20的范围内,不呈现出相应大块物质的结构特征,     

Investigation on Oxidation-Reduction Properties of Some Fe-S and Mo-Fe-S Clusters

Oxidation-reduction properties of some Fe-S and Mo-Fe-S clusters have been studied by cyclic voltammetric measurement. It is indicated that there are some factors which affect the oxidation- reduction properties of clusters, of which are charge density of metal atom in cluster core, electron effect of ligands, charge and geometric configurations of the reactant particle, etc.     

Protection of Ag Clusters by Metal-Oxo Modules

Monodisperse and atomically precise Ag nanoclusters have attracted considerable recent research interest. A conventional silver cluster usually consists of a silver metallic kernel and an organic peripheral ligand shell. Nevertheless, the present inevitable problem is the unsatisfied stability of such nanoclusters. In this concept, we will give an introduction to Ag clusters protected by metal-oxo modules, which exhibit enhanced stability and unique properties. Accordingly, three different types of clusters are summarized: (1) Ag clusters protected by mononuclear oxometallates; (2) Ag clusters protected by block-like metal-oxo clusters; (3) Ag clusters protected by hollow-like metal-oxo clusters. The aim of this concept is to offer possible general guidance and insight into future rational design of more metal-oxo clusters protected silver clusters or even other coinage metal nanoclusters.     

Tetrahedral chalcogenide clusters and open frameworks

By integrating porosity with electrical or optical properties, microporous chalcogenides may have unique applications. Here we review recent advances and discuss concepts in the synthesis and crystal structure of tetrahedral clusters and their frameworks. These chalcogenides can be viewed as trivalent metal chalcogenides doped with tetra-, di-, or monovalent metal cations. Low-valent cations help to increase the cluster size, while high-valent cations have the opposite effect.     

Facile Synthesis of High Nuclearity Silver–Ferrocenyldiselenolate Clusters

1,1′-Bis(trimethylsilylseleno)ferrocene has been used to prepare the silver–ferrocenyldiselenolate clusters Ag₈(Se₂fc)₄(PnPr₃)₄ 1 and Ag₁₆(Se₂fc)₈(PPh₂Et)₆ 2 in good yield. The structures of the frameworks in 1 and 2 have been determined by single crystal X-ray diffraction analysis. This work is dedicated to Prof. Dr. Günter Schmid in celebration of the 70th anniversary of his birthday and in recognition of his pioneering and inspirational work in the arena of metal cluster chemistry.     

Cobalt Chalcogenide Clusters. Synthesis,Characterization and DFT Calculations of [Co4Se2(CO)10] and [Co4Te2(CO)11]. Crystal Structure of [Co8Se4(CO)16(μ‐dppa)2]

The reactions of [Co₂(CO)₈] with E(SiMe₃)₂ (E = Se, Te) in CH₂Cl₂ result in the formation of the compounds [Co₄Se₂(CO)₁₀]> ( 1 ) and [Co₄Te₂(CO)₁₁] ( 2 ), respectively. Both cluster complexes have similar molecular structures in which the cobalt atoms form four‐membered rings with μ₄‐bridging chalcogen atoms (Se and Te) above and below the plane of the metal atoms and the carbonyl ligands as either terminal or μ₂‐bridging ligands. DFT‐calculations for both compounds have been carried out in order to obtain some more information about their electronic distribution. In the presence of the phosphine Ph₂PC≡CPPh₂ (dppa), the reaction of [Co₂(CO)₈] with Se(SiMe₃)₂ leads to the formation of [Co₈Se₄(CO)₁₆(μ‐dppa)₂] ( 3 ). During the reaction two molecules of [Co₂(CO)₈] have been added to the acetylene groups of the dppa ligands, whilst the remaining cobalt atoms coordinate to the phosphorus atoms of the phosphine. In this compounds the selenium atoms act as μ₃‐ligands, bridging the metal atoms bonded to the phosphorus with those bonded to the acetylene groups.     

Condensed Metal Clusters

The chemistry of metals in low valence states is marked by the frequent occurrence of metal clusters, which are easily recognizable when they occur as molecular units. Many metal-rich compounds of transition metals with p-elements (3rd to the 6th main groups) are closely related to the corresponding halides, since they are built up from metal clusters of the same type. The clusters are however, linked together (condensed) by metal-metal bonds. This principle of construction holds particularly well in the case of the novel reduced halides of the lanthanoids.     

Synthesen und Strukturen neuer amido- und imidoverbrückter Cobaltcluster: [Li(THF)2]3[Co3(μ2-NHMes)3Cl6] (1), [Li(DME)3]2[Co18(μ4-NPh)3(μ3-NPh)12Cl3] (2), [Li(DME)3]2[Co6(μ4-NPh)(μ2-NPh)6(PPh2Et)2] (3) und [Li(THF)4][Co8(μ3-NPh)6(μ2-NPh)3(PPh3)2] (4)

Syntheses and Crystal Structures of new Amido- und Imidobridged Cobalt Clusters: [Li(THF)₂]₃[Co₃(μ₂-NHMes)₃Cl₆] (1), [Li(DME)₃]₂[Co₁₈(μ₄-NPh)₃(μ₃-NPh)₁₂Cl₃] (2), [Li(DME)₃]₂[Co₆(μ₄-NPh)(μ₂-NPh)₆(PPh₂Et)₂] (3), and [Li(THF)₄][Co₈(μ₃-NPh)₆(μ₂-NPh)₃(PPh₃)₂] (4) The reactions of cobalt(II)-chloride with the lithium-amides LiNHMes and Li₂NPh leads to an amido-bridged multinuclear complex [Li(THF)₂]₃[Co₃(μ₂-NHMes)₃Cl₆] ( 1 ) as well as to the imido-bridged cobalt cluster [Li(DME)₃]₂[Co₁₈(μ₄-NPh)₃(μ₃-NPh)₁₂Cl₃] ( 2 ). In the presence of tertiary phosphines two imido-bridged cobalt clusters [Li(DME)₃]₂[Co₆(μ₄-NPh)(μ₂-NPh)₆(PPh₂Et)₂] ( 3 ) and [Li(THF)₄][Co₈(μ₃-NPh)₆(μ₂-NPh)₃(PPh₃)₂] ( 4 ) result. The structures of 1 – 4 were characterized by X-ray single crystal structure analysis.     

Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters

The homoleptic group 5 carbonylates [M(CO)₆]⁻ (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Ag_mM_n clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag₆{Nb(CO)₆}₄]²⁺ ( 4 a ²⁺) precedent, we established access to such Ag_mM_n clusters of the composition [Ag_m{M(CO)₆}_n]^x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1−, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt₄][M(CO)₆] and Ag[Al(OR^F)₄] (R^F=C(CF₃)₃) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at −35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3 ⁺ (m=6, n=5) and 4 ²⁺ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag₆ superatom cores. The [M(CO)₆]⁻ ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX₄ ( 4 ²⁺) or trigonal bipyramidal AX₅ molecules ( 3 ⁺).