Cluster chemistry

In chemistry, a cluster is an ensemble of bound atoms intermediate in size between a molecule and a bulk solid. Clusters exist of diverse stoichiometries and nuclearities. For example, carbon and boron atoms form fullerene and borane clusters, respectively. Transition metals and main group elements form especially robust clusters.^[1].

The phrase cluster was coined by F.A. Cotton in the early 1960s as compounds containing metal-metal bonds. In another definition a cluster compound contains a group of two or more metal atoms where direct and substantial metal metal bonding is present ^[2].

The main cluster types are naked clusters without stabilizing ligands or those with ligands. Typical ligands that stabilize clusters include carbon monoxide, halides, isocyanides, alkenes, and hydrides.

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Applications of clusters in catalysis

Synthetic cluster compounds were once proposed to be useful as catalysts for a wide range of industrial reactions, especially related to carbon monoxide utilization^[3], but few industrial exist. The clusters Ru₃(CO)₁₂ and Ir₄(CO)₁₂ are catalysts for the Water gas shift reaction, also catalyzed by iron oxide, and Rh₆(CO)₁₆ catalyzes the Fischer-Tropsch process, although again iron-based heterogeneous catalysts are used industrially.

The major role of clusters in catalysis are found in Nature. Nitrogen is converted to ammonia via the Fe-Mo-S cluster at the heart of the nitrogenase. CO is oxidized to CO₂ by carbon monoxide dehydrogenase. Hydrogenases rely on Fe₂ and NiFe clusters.^[4]

Electronic structure

Metal cluster are prominently found with refractory metals. In general metal centers with large d-orbitals form stable clusters because of favorable overlap of valence orbitals. Thus, metals with a low oxidation state and therefore small effective charges tend to form stable clusters. Polynuclear metal carbonyls are generally found in late transition metals with low formal oxidation states. Polynuclear halides and oxides are found with early transition metals.

The polyhedral skeletal electron pair theory or Wade's electron counting rules predict trends in the stability and structures of clusters.

History

The oldest identified metal cluster was probably calomel which was known in India already in the 12th century. The existence of a mercury to mercury bond in this compound was established in beginning of the 20th century.

The development of metal carbonyl compounds led quickly to the isolation of Fe₂(CO)₉ and Fe₃(CO)₁₂. Linus Pauling characterized MoCl₂ to contain Mo₆ octahedra. Rundle and Dahl discovered that Mn₂(CO)₁₀ featured an “unsupported” Mn-Mn bond, thereby verifying the ability of metals to bond to one another in molecules. F. Albert Cotton established that ReCl₃ was in fact the cluster Re₃Cl₉, which could be converted to a host of adducts without breaking the Re-Re bonds. Contemporaneously with the development of metal cluster compounds, numerous boron hydrides were discovered by Alfred Stock and his successors who popularized the use of vacuum-lines for the manipulation of these often volatile, air-sensitive materials. In the 1970s, ferredoxin was demonstrated to contain Fe₄S₄ clusters and later nitrogenase was shown to contain a distinctive MoFe₇S₉ active site.^[5]

The development of metal carbonyl compounds led quickly to the isolation of Fe₂(CO)₉ and Fe₃(CO)₁₂. Rundle and Dahl discovered that Mn₂(CO)₁₀ featured an “unsupported” Mn-Mn bond, thereby verifying the ability of metals to bond to one another in molecules.

The CO ligand can add to the polynuclear complex in three different ways:

• two-center or terminal carbonyl (μ₁ CO) in which the CO molecule is bonded to one vertex only. The M-CO bond can be linear or bent.
• three-center or edge-bridging carbonyl (μ₂ CO) in which CO bonds to two metal centers.
• four-center or triangular face-bridging (μ₃ CO) in which CO bonds to three metal centers sharing a face.

An often studied metal cluster compounds is dinuclear potassium octachlorodirhenate(III) or K₂Re₂Cl₈ whose peculiar molecular structure is explained by Quadruple bonding. Another dinuclear compound is di-tungsten tetra(hpp), the currect record holder low ionization energy.

In trinuclear rhenium trichloride or (ReCl₃)₃ the metal centers bond directly and through chlorine bridges. Albert Cotton established that ReCl₃ was in fact the cluster Re₃Cl₉, which could be converted to a host of adducts without breaking the Re-Re bonds.

Because this compound is diamagnetic and not paramagnetic the rhenium bonds are double bonds and not single bonds. In the solid state further bridging occurs between neighbours and when this compound is dissolved in hydrochloric acid a Re₃Cl₁₂^3- complex forms. An example of a tetranuclear complex is hexadecamethoxytetratungsten W₄(OCH₃)₁₂ with tungsten single bonds and molybdenum chloride (Mo₆Cl₈)Cl₄ is a hexanuclear molybdenum compound and an example of an octahedral cluster. A special group of clusters with the general structure M_xMo₆X₈ such as PbMo₆S₈ form a Chevrel phase which exhibit superconductivity at low temperatures.

Zintl clusters

Zintl compounds represent a separate class of metal clusters. Historically, they were generated by reduction of metalloids with a solution of sodium in liquid ammonia. Examples of Zintl anions are [[Bi₃]]^3-, [[Sn₉]]^4-, [Pb₇]^4- and [Sb₇]^3-. These anions do not require ligands and are called naked clusters but are unstable and their isolation requires the use of cryptate complexes of the alkali metal cation.

The structure of the Pb₁₀^2- anion is that of a capped square antiprism ^[6]. According to Wade's rules (2n+2) the number of cluster electrons is 22 and therefore a closo cluster. The compound is prepared from oxidation of K₄Pb₉ ^[7] by Au⁺ in PPh₃AuCl (by reaction of Hydrogen tetrachloroaurate and triphenylphosphine) in ethylene diamine with 2.2.2-crypt. This type of cluster was already known as is the endohedral Ni@Pb₁₀^2- (the cage contains one nickel atom).

The icosahedral tin cluster Sn₁₂^2- or stannaspherene anion is another closed shell structure observed (but not isolated) with photoelectron spectroscopy ^{[8] [9]}. With an internal diameter of 6.1 Angstrom it is of comparable size to fullerene and should be capable of containing small atoms as in endohedral fullerenes.

Bioinorganic clusters

In bioinorganic chemistry in the 1970s, ferredoxin was demonstrated to contain Fe₄S₄ clusters and later nitrogenase was shown to contain a strikingly distinctive MoFe₇S₉ active site.

Gas-phase clusters

Unstable clusters can also be observed in the gas-phase by means of mass spectroscopy even though they may be thermodynamically unstable and aggregate easily upon condensation. Such naked clusters, i.e. those that are not stabilized by ligands, are often produced by laser induced evaporation - or ablation - of a bulk metal or metal-containing compound. Typically, this approach produces a broad distribution of size distributions. Their electronic structures can be interrogated by techniques such as photoelectron spectroscopy. Their properties (Reactivity, Ionization potential, HOMO-LUMO-gap) often show a pronounced size dependence. Examples of such clusters are certain aluminium clusters as superatoms and certain gold clusters. Certain metal clusters are considered to exhibit metal aromaticity.

Carbon and Boron clusters

Contemporaneously with the development of metal cluster compounds, numerous boron hydrides were discovered by Alfred Stock and his successors who popularized the use of vacuum-lines for the manipulation of volatile, air-sensitive materials. Clusters of boron are boranes such as pentaborane and decaborane.

Clusters of carbon are fullerenes and nanotubes. The fullerene sphere can be filled with small molecules in Endohedral fullerenes.

Composite clusters of carbon and boron are carboranes.

Extended metal atom chains

Extended metal atom chain complexes (EMAC) are a novel topic in academic research. They are comprised of linear chains of metal atoms stabilized with ligands. EMACS are known based on nickel (with 9 atoms), chromium and cobalt (7 atoms) and ruthenium (5 atoms). In theory it should be possible to obtain infinite one-dimensional molecules and research is oriented towards this goal. In one study ^[10] a EMAC was obtained that consisted of 9 chromium atoms in a linear array with 4 ligands (based on an oligo pyridine) wrapped around it. In it the chromium chain contains 4 quadruple bonds.

References

1. ^ Inorganic Chemistry Huyee, JE , 3rd ed. Harper and Row, New York
2. ^ Introduction to cluster chemistry by D. M. P. Mingos, David J Wales 1990 ISBN 0-13-479049-9
3. ^ Cluster Chemistry: Introduction to the Chemistry of Transition Metal and Main Group Element Molecular Clusters Guillermo Gonzalez-Moraga 1993 ISBN 0-387-56470-5
4. ^ Bioorganometallics: Biomolecules, Labeling, Medicine; Jaouen, G., Ed. Wiley-VCH: Weinheim, 2006.3-527-30990-X.
5. ^ "Metal Clusters in Chemistry" P. Braunstein, L. A. Oro, P. R. Raithby, eds Wiley-VCH, Weinheim, 1999. ISBN 3-527-29549-6.
6. ^ The Zintl Ion [Pb₁₀]^2-: A Rare Example of a Homoatomic closo Cluster A. Spiekermann, S. D. Hoffmann, T. F. Fässler Angewandte Chemie International Edition Volume 45, Issue 21 , Pages 3459 - 3462 2006 Abstract
7. ^ itself made by heating elemental potassium and lead at 350°C
8. ^ Tin particles are generated as K⁺Sn₁₂^2- by laser evaporation from solid tin containing 15% potassium and isolated by mass spectrometer before analysis
9. ^ Sn₁₂^2-: Stannaspherene Li-Feng Cui, Xin Huang, Lei-Ming Wang, Dmitry Yu. Zubarev, Alexander I. Boldyrev, Jun Li, and Lai-Sheng Wang J. Am. Chem. Soc.; 2006; 128(26) pp 8390 - 8391; (Communication) doi:10.1021/ja062052f 10.1021/ja062052f.
10. ^ Four quadruple metal–metal bonds lined up: linear nonachromium(II) metal string complexes Rayyat H. Ismayilov, Wen-Zhen Wang, Rui-Ren Wang, Chen-Yu Yeh, Gene-Hsiang Lee and Shie-Ming Peng Chem. Commun., 2007, 1121 - 1123, doi:10.1039/b614597c

See also

• Cluster (physics)
• Water molecules form clusters as well: see water clusters