Bismuth-Based Metal Clusters─From Molecular Aesthetics to Contemporary Materials Science

ConspectusBismuth-based research has become a highly topical field in recent years, yielding remarkable prospects for new fundamental insights and new materials applications, ranging from innovative catalysts to novel pharmaceuticals, due to this heavy metal's virtually nonradioactive and nontoxic properties. Given that the 6s² electron pair can be stereochemically active under certain circumstances, bismuth atoms adopt a variety of coordination modes and bonding environments with oxidation states ranging from (formally) +V to -III. As a consequence, bismuth-based compounds cover the entire spectrum from simple coordination compounds to much more unusual cluster cations and cluster anions exhibiting metal-metal bonding in a homoatomic manner, or in concert with other s-, d-, p-, or f-block metal atoms. Such bismuth clusters show high potential for the development of new bismuth-based materials, but they are also interesting objects by themselves. Given the relatively recent development of bismuth-rich cluster molecules, a deep understanding of their properties─including unprecedented structural features, complex electronic structures, substantial heavy metal aromaticity, as well as their formation pathways─is still in its infancy. The topic thus spans a broad range from highly sophisticated synthetic chemistry through interdisciplinary experimental and theoretical analyses to materials science.Based on our recent work and several notable reports from other groups, this article will highlight the successful access to a number of novel bismuth-rich cluster ions emerging from both solution-based approaches and solid-state chemistry. It will shed light on the unique structural and electronic properties that cause chemical and physical peculiarities of such compounds. Selected examples include, but are not limited to, (1) the first encapsulation of actinide ions in intermetalloid clusters which additionally served to manifest substantial all-metal π-aromaticity with a (calculated) record ring current per electron; (2) a large metalloid {Zn₁₂} unit stabilized in a porphine-related {Zn₈Bi₁₆} moiety in [K₂Zn₂₀Bi₁₆]^6-; (3) the largest assembly of bismuth atoms within one molecule, observed in [{Ru(cod)}₄Bi₁₈]^4- that consists of two Bi-Bi-linked "[{Ru(cod)}₂Bi₉]^2-" subunits.Notably, cluster growth has remained largely a black box, which is starting to be revealed, however. We discuss possible formation pathways of such (multi)metallic nanoarchitectures on the basis of smaller subunits that were detected by mass spectrometric analyses and could also be captured upon reaction with organometallic complexes. In addition to the intrinsic structural and electronic properties of the cluster anions and cluster cations reviewed herein, we will briefly introduce the emerging usage of bismuth-based compounds in material science and give an outlook to future developments.