Hansgeorg Schnöckel

Metalloid Aluminum and Gallium Clusters: Element Modifications on the Molecular Scale?

As members of the same group in the periodic table, the industrially significant elements aluminum and gallium exhibit strong similarities in the majority of their compounds. In contrast there are significant differences in the structures of the two elemental forms: Aluminum forms a typical closest-packed metallic structure whereas gallium demonstrates a diversity of molecular bonding principles in its seven structural modifications. It can therefore be expected that differences between Al and Ga compounds will arise when, as for the elemental forms, many metal-metal bonds are formed. To synthesize such cluster compounds, we have developed the following synthesis procedure: Starting from gaseous monohalides at around 1000 degrees C, metastable solutions are generated from which the elements ultimately precipitate by means of a disproportionation reaction at room temperature. On the way to the elemental forms, molecular Al and Ga cluster compounds can be obtained by selection of suitable ligands (protecting groups), in which a core of Al or Ga atoms are protected from the formation of the solid element by a ligand shell. Since the arrangement of atoms in such clusters corresponds to that in the elements, we have designated these clusters as metalloid or elementoid. In accordance with the Greek word [see text] (ideal, prototype), the atomic arrangement in metalloid clusters represents the prototypic or ideal atomic arrangement in the elements at the molecular level. The largest clusters of this type contain 77 Al or 84 Ga atoms and have diameters of up to two nanometers. They hold the world record with respect to the naked metal-atom core for structurally characterized metalloid clusters.

Metalloide Aluminium‐ und Galliumcluster: Elementmodifikationen im molekularen Maßstab?

Die technisch wichtigen Elemente Aluminium und Gallium weisen als Nachbarn im Periodensystem in den meisten ihrer Verbindungen starke Ahnlichkeiten auf. Dagegen gibt es gravierende Unterschiede zwischen den Strukturen der beiden Elementformen: Aluminium hat als typisches Metall eine dichtgepackte Struktur, fur Gallium findet man hingegen sieben Modifikationen mit vielfaltigen molekularen Verknupfungsprinzipien. Unterschiede zwischen molekularen Al- und Ga-Verbindungen sind folglich immer dann zu erwarten, wenn wie in den Elementformen viele Metall-Metall-Bindungen geknupft werden. Fur die Synthese solcher Clusterverbindungen haben wir folgendes Syntheseprinzip entwickelt: Ausgehend von gasformigen Monohalogeniden bei ca. 1000 °C werden metastabile Losungen erzeugt, aus denen sich letztendlich durch eine Disproportionierung bei Raumtemperatur die Elemente abscheiden. Auf dem Weg zu den Elementen lassen sich durch eine geeignete Wahl der Liganden (Schutzgruppen) molekulare Al- und Ga-Clusterverbindungen erhalten, in denen ein Kern aus Al- bzw. Ga-Atomen durch eine Ligandenhulle vor einer Weiterreaktion geschutzt wird. Da die Anordnung der Atome in solchen Clustern derjenigen in den Elementen entspricht, haben wir diese Cluster als metalloid oder elementoid bezeichnet. Im Sinne des eιδoς-Begriffs sind die Atomanordnungen in metalloiden Clustern gewissermasen das „Urbild“ oder „Ideal“ der Atomanordnung der Elemente im molekularen Masstab. Die grosten Cluster dieser Art enthalten 77 Al- oder 84 Ga-Atome bei Durchmessern von bis zu zwei Nanometern. Sie halten hinsichtlich des „nackten“ Metallatomkerns den Grosenweltrekord unter den strukturell untersuchten metalloiden Clustern.

Spin Conservation Accounts for Aluminum Cluster Anion Reactivity Pattern with O2

The reactivity pattern of small (∼10 to 20 atoms) anionic aluminum clusters with oxygen has posed a long-standing puzzle. Those clusters with an odd number of atoms tend to react much more slowly than their even-numbered counterparts. We used Fourier transform ion cyclotron resonance mass spectrometry to show that spin conservation straightforwardly accounts for this trend. The reaction rate of odd-numbered clusters increased appreciably when singlet oxygen was used in place of ground-state (triplet) oxygen. Conversely, monohydride clusters AlnH–, in which addition of the hydrogen atom shifts the spin state by converting formerly open-shell structures to closed-shell ones (and vice versa), exhibited an opposing trend: The odd-n hydride clusters reacted more rapidly with triplet oxygen. These findings are supported by theoretical simulations and highlight the general importance of spin selection rules in mediating cluster reactivity.

[Al7{N(SiMe3)2}6]−: A First Step towards Aluminum Metal Formation by Disproportionation

A “naked” aluminum atom links two aluminum tetrahedra in the [Al7{N(SiMe3)2}6]− ion (see picture), which results from the reaction of a metastable AlCl solution with LiN(SiMe3)2 and crystallizes with [Li(OEt2)3]+ as cation. This unique structure among molecular metal atom clusters represents a small but characteristic section of cubic close-packed aluminum.

THE FIRST STRUCTURALLY CHARACTERIZED COORDINATION COMPOUND CONTAINING DIRECT AL-CR BONDING : CP*AL-CR(CO)5

Abstract In order to investigate the bonding of the carbene-like ligand AlCp* towards a transition metal, the compound Cp*AlCr(CO)5 has been prepared starting from Cr(CO)5COT and AlCp*. With the help of its crystal structure, some IR spectroscopic data and ab initio calculations, the Al–Cr bonding is discussed. The strong reducing ability of AlCp* leads to a shift in electron density towards the chromium atom and finally to the CO ligands.

[{(η5-C5Me5)2Sm}4P8]: A Molecular Polyphosphide of the Rare-Earth Elements

[{(eta(5)-C(5)Me(5))(2)Sm}(4)P(8)], a molecular polyphosphide of the rare-earth elements having a realgar core structure, was synthesized by a one-electron redox reaction of divalent samarocen and white phosphorus.

(Cyclopentadienyl)-Gallium(I)-Verbindungen

Abstract A metastable solution of GaCl reacts with LiCp or MgCP 2 compounds to a number of new cyclopentadienylgallium(I) species: GaCp★, GaCp t Bu , GaCp (SiMe 3 ) 3 and GaCp( Benzyl ) 5 . They are characterized by their mass and NMR spectra ( 1 H, 13 C and 71 Ga). By comparison with analogous Al and Mg compounds η 5 -bonding has to be concluded for all GaCp derivatives.

Al22Br20⋅12 THF: das erste polyedrische Aluminiumsubhalogenid – ein Schritt auf dem Weg zu einer neuen Aluminiummodifikation?

Das erste polyedrische Aluminiumsubhalogenid Al22Br20⋅12 L entsteht bei der Disproportionierung von AlI-Bromid-Losungen. Der Cluster ist aus einem Al12-Ikosaeder mit zehn exohedral gebundenen (formal zweiwertigen) Al-Atomen aufgebaut (siehe Struktur). Ein derartiges homoatomares Gerust ist bei molekularen Verbindungen unbekannt. Selbst beim Element Bor findet man dieses Gerust lediglich als Ausschnitt aus der β-rhomboedrischen Elementmodifikation.

Heats of Hydrogenation of Compounds Featuring Main Group Elements and with the Potential for Multiply Bonding

Reaction enthalpies are calculated for the hydrogenation reactions of main group hydrides with the potential for multiple bonding, and thus the unsaturated character of these species is determined. In addition to the global minimum structures, which leave in some cases no hope for even a single E-E bond (E=Group 13, 14, or 15 element), calculations are also performed for geometries with maximum potential for multiple bonding. The trends down the groups and the periods are established. Interpretations have to take several factors into account. These factors sometimes work hand in hand but also against each other. We also include in our survey the species [HGaGaH]2- as a free anion and Na2[HGaGaH] as well as their hydrogenation products [H2GaGaH2]2- and Na2[H2GaGaH2]2-. The results show that the presence of the Na+ ions has a significant impact on their chemistry, and thus suggests that they are involved to a large extent in the bonding. Our results indicate that the compounds should be described as cluster compounds.

Synthesis and structure of metalloid aluminum clusters—intermediates on the way to the elements

Abstract Metastable Aluminum(I) halide solutions proved to have a high potential for the synthesis of novel subvalent Al compounds, such as Al n X m species (X=Cl, Br, I; n m , average oxidation state of Al below +3 or n > m , average oxidation state of Al below +1) or Al n R m species (R=bulky ligand; n > m ). There are two principal reaction types, which are essential for the formation of the compounds discussed herein. The disproportionation, which finally results in Al(III) halides and Al metal and the metathesis which leads to a substitution of X atoms against R groups. By this way the metalloid cluster compounds [Al 7 {N(SiMe 3 ) 2 } 6 ] − , [Al 12 {N(SiMe 3 ) 2 } 8 ] − , [Al 14 I 6 {N(SiMe 3 ) 2 } 6 ] 2− , [Al 69 {N(SiMe 3 ) 2 } 18 ] 3− , and [Al 77 {N(SiMe 3 ) 2 } 20 ] 2− could be isolated. The characteristic feature of these metalloid Al clusters is the number of AlAl contacts being larger than the number of Alligand bonds, i.e. there are more ‘naked’ than ligand-bonded Al atoms. Furthermore, the topology of the closest packing in Al metal is already pre-formed in these compounds.