The efficient synthesis of complex molecules often relies on accessing simpler building blocks. Single atoms represent the ultimate simplicity, yet for many elements, their direct use in synthesis remains impractical, leading to complex, low-yield processes. This is particularly true for group 14 elements, where a lack of readily available atomic synthons hinders progress. While some advances have been made with molecular sources of elemental forms like metallylones (formally E(O), E = Si, Ge, Sn, Pb) coordinated by Lewis bases, or dinuclear compounds, the full potential of these as single-atom sources is still under investigation. This research focuses on aromatic compounds containing heavy group 14 elements (Si, Ge, Sn), known as heavy benzenoids, which are highly reactive due to their propensity for auto-oligomerization. By employing bulky protecting groups like Tbt (2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), researchers have succeeded in synthesizing and isolating thermally stable heavy benzenoids. Recently, the generation of heavy analogs of phenyl anions (germa- and stannabenzenyl anions) by treating Tbt-substituted germa- or stannabenzene with reducing agents has been reported. These anions, despite lacking steric protection, exhibit surprising thermal stability and act as nucleophiles. This study explores the reactivity of a germabenzenyl anion with dibromodimetallenes, leading to the discovery of an unusual germanium atom transfer reaction.

The literature review highlights the challenges in utilizing single atoms of group 14 elements for organic synthesis due to the lack of suitable atomic transfer reagents. Previous research has focused on metallylones and dinuclear compounds as potential single-atom sources, though their reactivity and bonding characteristics are still under investigation. The use of bulky protecting groups to stabilize heavy benzenoids, and the subsequent generation of heavy phenyl anion analogs, is also discussed as crucial background for this work. The authors also draw a parallel with oxygen atom replacement reactions in pyrylium salts, highlighting the potential for similar reactivity with heavier group 14 elements. The scarcity of reports detailing aromatic-to-aromatic nuclear exchange reactions for group 14 elements emphasizes the novelty of this research.

The researchers investigated the reaction of potassium germabenzenide (1) with 1,2-dibromodigermene (2-Ge) in THF. The initial reaction yielded a mixture of products, including germabenzene (3-Ge), a digermabenzenylgermyl anion (4), and a Ge/C cluster (5-1). Heating the mixture promoted the conversion of (4) to (3-Ge) and (5-1). Exposure to ambient light subsequently transformed (5-1) into (5-2). Independent synthesis of (3-Ge), (4), and (5-1) confirmed their identities. The authors analyzed the structure of clusters (5-1) and (5-2) using X-ray crystallography, showing their similarity except for the connecting positions between the Ge2 atom and the GeC5 ring. The isomerization from (5-1) to (5-2) was attributed to a [1,3]-sigmatropic rearrangement. The reactivity of the germyl anion (4) was studied further by reacting it with (2-Ge) leading to (3-Ge) and (5-1), with light exposure facilitating the transformation of (5-1) into (5-2). In the presence of 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (Im iPr2Me2), the reaction yielded a germabenzenylgermylene NHC adduct (6-NHC). Independent synthesis of (6-NHC) was achieved by reacting the NHC complex of [Tbb(Br)Ge:] with (1). Thermolysis of (6-NHC) at elevated temperatures resulted in the formation of (3-Ge) and an NHC complex of zero-valent germanium, although its structure remains undetermined. DFT calculations were employed to investigate the potential energy surface for the formation of (3-Ge) using a model compound (Gebzl(H)Ge:). Calculations indicated a four-step isomerization pathway via several intermediates (INT1a-INT4a) involving Ge atom exchange on the germabenzenyl ring. To confirm the Ge atom exchange, the reaction of (1) with 1,2-dibromodisilene (2-Si) was conducted, resulting in the formation of silabenzene (3-Si). The reaction of potassium germabenzenide (1) with 1,2-dibromodigermene (2-Ge) in hexane produced 2,4,5-tribromo pentagerma[1.1.1]propellane (7), further supporting the proposed mechanism. Specific experimental procedures, including reaction conditions and characterization data for all compounds (NMR spectra, X-ray crystallography, elemental analysis) are detailed in the methods section and supplementary information.

The key finding is the discovery of an unprecedented germanium atom transfer reaction involving a germabenzenyl anion and a dibromodigermene. This reaction proceeds via a novel aromatic-to-aromatic nuclear germanium replacement mechanism. Computational studies using DFT calculations support a four-step isomerization pathway involving germabenzenylgermylene (6) as a key intermediate, leading to the formation of germabenzene (3-Ge) and Ge/C clusters (5-1 and 5-2). The intermediate (6) was isolated as its NHC adduct (6-NHC), which served as a source of Ge(0) atoms, reacting with diimines to form an N-heterocyclic germylene. Replacing the dibromodigermene with a dibromodisilene resulted in a similar atom-transfer reaction, yielding silabenzene (3-Si), providing strong evidence for the exchange mechanism. The reaction in hexane yielded a different product, 2,4,5-tribromo pentagerma[1.1.1]propellane (7), which indicates that the reaction pathway can be tuned by modifying reaction conditions. The study uncovered strong homoconjugative interactions in the intermediates, influencing the reaction pathway and the unusual atom transfer process. The formation of germanium clusters (5 and 7) with naked Ge atoms is also a significant finding.

This work demonstrates a novel germanium atom transfer reaction, providing a new approach to synthesizing germanium-containing compounds. The unusual aromatic-to-aromatic nuclear exchange reaction, supported by DFT calculations, offers valuable insights into the reactivity of heavy benzenoids. The ability to control the reaction pathway by altering the solvent and the successful isolation of germabenzenylgermylene (6) as an NHC adduct significantly advance the understanding of this transformation. The observation that analogous reactions occur with silicon species showcases the versatility of this approach. The formation of germanium clusters with naked Ge atoms opens up new avenues for exploring the synthesis of molecular germanium clusters, analogous to siliconoids. The findings challenge traditional understanding of aromatic ring reactivity and contribute to the broader field of main group element chemistry.

This study successfully demonstrates a germanium atom transfer reaction via an unprecedented aromatic-to-aromatic nuclear exchange mechanism. The process involves the formation of germabenzenylgermylene as a key intermediate, its successful isolation as an NHC complex further validating the proposed mechanism. The findings expand the synthetic methods available for heavy benzene derivatives and offer potential access to new unsaturated molecular germanium clusters. Future research could explore extending this methodology to other group 14 elements and investigate the reactivity of the isolated germanium clusters.

The study focuses mainly on the reaction with a specific type of dibromodigermene. Further research is needed to explore the scope and limitations of this reaction with various substituted germabenzenyl anions and other dibromodimetallenes. While DFT calculations provide valuable mechanistic insight, the high computational cost limits the exploration of the full reaction coordinate and a larger range of substituents. Complete characterization of the NHC complex of zero-valent germanium remains elusive. The yields of some reaction products are moderate, and optimization of reaction conditions could further improve efficiency.