Functionalization of Nine-Atom Germanium Clusters with More Than Two Substituents

The primary goal of this research project is to develop methodologies for further functionalization of Zintl clusters in order to attach more than two substituents. Progress was made in a step by step manner as follows: (a) develop methodology for the synthesis of Zintl clusters functionalized with more than two substituents, (b) carry out subsequent studies for the properties of the functionalized clusters and their reactivity, and (c) introduce more organic functionalities to the cluster core. Each of these segments is discussed in detail in this dissertation.

After introducing the background of Zintl ions and the recent developments on the study of nine–atom germanium clusters (Chapter 1), the rational synthesis of tri–silylated Ge₉ clusters is discussed (Chapter 2). This approach allows for Ge₉ clusters with more than two substituents to be synthesized by reacting 'hypersilyl' chloride (Me₃Si)₃SiCl with acetonitrile suspension of the intermetallic precursor K₄Ge₉ to form [Ge₉(Si(SiMe₃)₃)₃]^–. Our investigations show this route to be general and applicable to a variety of R₃SiCl, with R = Et, ⁱPr, ⁿBu and Ph, etc.

Following the rational synthesis of [Ge₉(Si(SiMe₃)₃)₃]^–, the preparation of tetra–substituted Ge₉ clusters was studied next, and this is discussed in detail in Chapter 3. Reactions between K[Ge₉(Si(SiMe₃)₃)₃] and R₃SnCl (R = Me, ⁿBu and Ph) result in the addition of the –SnR₃ fragment to the tri–silylated Ge₉ clusters and the formation of neutral tetra–substituted species [Ge₉(Si(SiMe₃)₃)₃(SnR₃)]⁰. Dynamic behavior of the fourth substituent was observed in solution, and this is interpreted as the –SnR₃ fragment migrating along the three germanium atoms within the triangular base.

Studying the similarity in the reactivity of functionalized Ge₉ clusters and the naked parent Ge₉^4– anions has both experimental and theoretical implications. The results presented in Chapter 4 demonstrate that like the naked cluster Ge₉^4–, Tl atom could be added to one of the square faces of [Ge₉(Si(SiMe₃)₃)₃]^– to form a tri–substituted ten–atom closo–cluster [Ge₉Tl(Si(SiMe₃)₃)₃]⁰. Intermolecular dynamic behavior was observed and was studied by NMR investigations. It can be rationalized by a 'Tl⁺ dissociation–association' mechanism.

Chapter 5 describes the addition of a variety of alkyl groups to the already tri–functionalized species by the reactions between [Ge₉(Si(SiMe₃)₃)₃]^– and R–X undergoing S_N2 mechanism. [Ge₉(Si(SiMe₃)₃)₃Et]⁰ was crystallized and structurally characterized. An intramolecular dynamic behavior was observed in solution and theoretical study was carried out for clarifying its pathway.

Chapter 6 investigates the reactivity of [Ge₉(Si(SiMe₃)₃)₃]^– towards transition metal complexes. It turns out that naked transition metal atoms tend to interact with the triangular prismatic bases of the tri–silylated monoanion. [(Ge₉R₃)Cu^I(Ge₉R₃)(Cu^IPPh₃)]⁰ and [(Ge₉R₃)Pd⁰(Ge₉R₃)]^2– (R = –Si(SiMe₃)₃) were synthesized and structurally characterized through such investigations.

Chapter 7 presents further studies of the nucleophilic reactivity of organo–Zintl clusters. A variety of main group and alkyl halides are shown to react with organo–Zintl clusters such as [Ge₉(CH=CH–Ph)₂]^2–, [Ge₉(Adm)₂]^2–, [Ge₉(^tBu)]^3–, etc. We believe this can be developed into a general tool for synthesizing fully organically functionalized Zintl clusters.

The dissertation ends with a summary and a discussion of future prospects, primarily focused on the fields of embedding clusters with useful functionalities and expanding the synthetic methodology into other intermetallic systems.