Metalloide Aluminium- und Galliumcluster: Elementmodifikationen im molekularen Maßstab?

Abstract

Die technisch wichtigen Elemente Aluminium und Gallium weisen als Nachbarn im Periodensystem in den meisten ihrer Verbindungen starke Ähnlichkeiten auf. Dagegen gibt es gravierende Unterschiede zwischen den Strukturen der beiden Elementformen: Aluminium hat als typisches Metall eine dichtgepackte Struktur, für Gallium findet man hingegen sieben Modifikationen mit vielfältigen molekularen Verknüpfungsprinzipien. Unterschiede zwischen molekularen Al‐ und Ga‐Verbindungen sind folglich immer dann zu erwarten, wenn wie in den Elementformen viele Metall‐Metall‐Bindungen geknüpft werden. Für die Synthese solcher Clusterverbindungen haben wir folgendes Syntheseprinzip entwickelt: Ausgehend von gasförmigen Monohalogeniden bei ca. 1000 °C werden metastabile Lösungen 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 Ligandenhülle vor einer Weiterreaktion geschützt 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 ειδoς‐Begriffs sind die Atomanordnungen in metalloiden Clustern gewissermaßen das „Urbild“ oder „Ideal“ der Atomanordnung der Elemente im molekularen Maßstab. Die größten Cluster dieser Art enthalten 77 Al‐ oder 84 Ga‐Atome bei Durchmessern von bis zu zwei Nanometern. Sie halten hinsichtlich des „nackten“ Metallatomkerns den Größenweltrekord unter den strukturell untersuchten metalloiden Clustern.

Electronic structure ofGa84cluster compounds

“…In the past years several ligand stabilized aluminium and gallium cluster compounds were synthesized in different sizes. 1 Both clusters are surrounded by a ligand shell of 20 N͑SiMe 3 ͒ 2 groups. Altogether the number of direct metalmetal contacts exceeds the number of metal-ligand bonds, they can be denoted as "metalloid" clusters.…”

[Ga51(PtBu2)14Br6]3−: An Elementoid Gallium Cluster with Metalloid and Nonmetalloid Structural Elements

“…[1,2] The R groups used so far (R = N(SiMe 3 ) 2 , Si(SiMe 3 ) 3 , C(SiMe 3 ) 3 , SitBu 3 ) were shown to bind to their Ga atoms exclusively through two-center, two-electron (2c2e) bonds (Scheme 1 a). In these clusters, the central cores of ligand-free Ga atoms were often determined to have topologies corresponding to one of the seven forms of elemental gallium, [1] meaning that coordination numbers typical of both metals (e.g., coordination number 12, as in Ga-III) and nonmetals (e.g., coordination number 1, as in a-Ga, since one short and six long GaÀGa distances are observed) are found. We previously introduced the concept of metalloid or elementoid clusters [1,3] which seems tailor-made for such Ga clusters.…”

“…In these clusters, the central cores of ligand-free Ga atoms were often determined to have topologies corresponding to one of the seven forms of elemental gallium, [1] meaning that coordination numbers typical of both metals (e.g., coordination number 12, as in Ga-III) and nonmetals (e.g., coordination number 1, as in a-Ga, since one short and six long GaÀGa distances are observed) are found. We previously introduced the concept of metalloid or elementoid clusters [1,3] which seems tailor-made for such Ga clusters. To vary the influence of the outer shell, we recently introduced PtBu 2 groups.…”

[Ga16(PtBu2)10]: A Gallium Phosphide Sheathed Core of Four Naked Ga Atoms?

“…Both of these compounds as well as a third species with an AlP 3 ring structure [6] are colorless or weakly colored (Figure 1 c).To obtain metal-rich Ga-P compounds, whose dark color and metallic luster are suggestive of different bonding interactions, a different synthetic route, starting from metastable AlX and GaX (X = Cl, Br, I) solutions, has been pursued, which has led to numerous metalloid cluster compounds. [7] For example, we have had some success in previous years with the synthesis of metalloid cluster compounds with terminal AlÀN bonds. We thus obtained, among others, the shiny cluster compound Al 77 R 20 , whose core of 57 naked aluminum atoms is surrounded by 20 AlNR 2 groups; that is it contains 20 terminal AlÀN bonds.The deciding factor in these syntheses is the interplay between disproportionation of the AlX or GaX compound to form metal-rich species and substitution of the halides by bulky groups, which shield the cluster core.…”

“…To obtain metal-rich Ga-P compounds, whose dark color and metallic luster are suggestive of different bonding interactions, a different synthetic route, starting from metastable AlX and GaX (X = Cl, Br, I) solutions, has been pursued, which has led to numerous metalloid cluster compounds. [7] For example, we have had some success in previous years with the synthesis of metalloid cluster compounds with terminal AlÀN bonds. We thus obtained, among others, the shiny cluster compound Al 77 R 20 , whose core of 57 naked aluminum atoms is surrounded by 20 AlNR 2 groups; that is it contains 20 terminal AlÀN bonds.…”