The Foundations of Organo-Zintl Chemistry

The primary focus of this dissertation is the development of the chemistry of organo-Zintl clusters. The goal of the project was to put into place the necessary building blocks so as to bring germanium deltahedral clusters inline with mainstream organic chemistry. In order to accomplish this, it was necessary to achieve a number of phases of progression, namely: (a) develop techniques for synthesizing organo-Zintl clusters, (b) carry out subsequent mechanistic investigations, (c) modify the solubility properties of the clusters, (d) increase the clusters's compatibility with reaction conditions for organic transformations, and (e) carry out broad investigations into their reactivity. Each of these segments have been dealt with in turn. After introducing the foundations of Zintl chemistry (Chapter 1), the first method for synthesizing organo-Zintl clusters is discussed, namely the 'halide' route (Chapter 2). This technique allows organo-Zintl clusters to be synthesized by reacting naked Ge94- clusters with R-X substrates to form [Ge9-R]3-, [R-Ge9-R]2-, and [R-Ge9-Ge9-R]4-. Investigations show this route to be general, and applicable to primary, secondary, and tertiary carbon centers. Moreover, it is independent of steric bulk, as evidenced by the characterization of [Ge9-C10H15]3- from reactions with the bulky, tertiary bromoadamantane. The second method for synthesizing organo-Zintl clusters, the 'alkyne' route, is discussed in Chapter 3. Reactions between Ge94- and both terminal and internal alkynes (R-CÌ¢'¡åÁC-H and R-CÌ¢'¡åÁC-R') result in the addition to the clusters and hydrogenation of the parent alkyne to form alkenylated clusters, [Ge9-CR'=CH-R]3- and [Ge9-(CR'=CH-R)2]2- (where R' = H or any R group). The organo-Zintl and organometallic-organo-Zintl species, [Ge9-CH=CH-Ph-OMe]3- and [Ge9-(CH=CH-Fc)2]2- (Fc = ferrocenyl) respectively, where structurally characterized by X-ray diffraction, along with reactions with a variety of internal and terminal alkynes by mass spectrometry. Zintl clusters in general are only soluble in a limited number of non-traditional solvents such as ethylenediamine and liquid ammonia, and occasionally dimethylformamide. The results presented in Chapter 4 demonstrate that the alkali metal cation (normally K+) of organo-Zintl clusters can be exchanged for quaternary ammonium cations, R4N+. Three cation exchanged organo-Zintl salts were structurally characterized by X-ray diffraction: [Me4N]2[Ge9-(CH=CH2)2]Ì¢‰âÂå¢0.5en, [Et4N]2[Ge9-(CH=CH2)2], and [Pr4N]2[Ge9-(CH=CH2)2], where [Ge9-(CH=CH2)2]2- is synthesized by the reaction of Ge94- with TMS-CÌ¢'¡åÁC-TMS. The vinyl disubstituted cluster is also structurally characterized with [K-18-crown-6]+ in [K-18-crown-6]2[Ge9-(CH=CH2)2] Ì¢‰âÂå¢en. By exchanging the alkali metal cation for tetraoctylammonium cations, [Oc4N]+, the organo-Zintl clusters can be solubilized in a variety of conventional organic solvents such as THF, dioxane, DMSO, acetonitrile, and diethyl ether, and even solvents as non-polar as benzene and toluene. The 'alkyne' route was further developed by showing that it is compatible with a large range of alkyne substrates with various secondary functional groups. Chapter 5 presents the results of a systematic investigation of the reactions of nine-atom deltahedral clusters of germanium (Zintl ions, Ge9n-) with alkynes and alkyl halides. The reaction pathways were probed in depth using various, appropriately substituted alkynes and organic halides, including some typical mechanistic probes and radical clocks. The regioselectivity and stereoselectivity of the reaction with alkynes was examined by systematically varying the steric and electronic nature of the substituents. The studies showed that the Zintl clusters act as strong, anionic nucleophiles towards the alkynes and primary and secondary alkyl halides but, most likely, as electron donors in reactions with tertiary alkyl halides and halogenated olefins. The pentenyl and methylcyclopropyl functionalized clusters, [Ge9-(C5H9)]3- and [Ge9-(CH2CH(CH2)2)2)]2-, respectively, were also crystallographically characterized in compounds with [K-crypt]+ countercations. Chapter 6 further develops the nucleophilic reactivity of Zintl clusters. Specifically, their reactivity towards substrates with two different electrophilic centers is probed, namely: (a) organostannyl center + alkyne, (b) halide carbon center + alkyne, and (c) halide carbon center plus carbonyl group. These reactions show the preference of the clusters for organostannyl centers over alkynes, and halide carbon center over either of the other two. Additionally, initial investigations into functionalizing Zintl clusters with an sp carbon are discussed, completing the sequence of primary, secondary, and tertiary sp3 carbons from alkyl halides, and sp2 carbons from alkenyl halides or alkynes. Organo-Zintl clusters are also unreactive towards tertiary alkyl halides, ketones, and aldehydes. Chapter 7 discusses the stability towards H2O that organo-Zintl clusters exhibit. This compatibility with H2O is coupled with their non-reactivity towards aldehydes to demonstrate that subsequent organic transformations are possible with organo-Zintl clusters. Proof of concept comes from the aldehyde condensation reaction between [Ge9-(CH=CH-CH2NH2)2]2- and ferrocene carboxaldehyde. The former anion is structurally characterized by X-ray diffraction in [K-(2,2,2-crypt)]2[Ge9-(CH=CH-CHNH2)2] from a reaction between Ge94- and propargylamine (H-CÌ¢'¡åÁC-CH2NH2). In addition to water, organo-Zintl clusters are stable towards dicationic transition metal complexes, as evidenced by isolation of [Fe(en)3][Ge9-(CH=CH-Fc)2]Ì¢‰âÂå¢3en. Chapter 8 involves a break from germanium-based clusters, and discusses the results of some investigations into the reactivity of bismuth-based Zintl anions towards Ni(CO)2(PPh3)2. Two heterometallic species were isolated and crystallographically characterized; namely, the open cluster [Bi3Ni6(CO)9]3-, and the intermetalloid and hypoelectronic closo-[Ni@{Bi6Ni6(CO)8}]3-. The species are also characterized in the gas phase by electrospray mass spectrometry, and their full structural and electronic descriptions are discussed. The dissertation closes with a discussion of prospects for future work, primarily in the area of organo-Zintl clusters.