Exploring the Structural Diversity of Ge/As Pseudo-Tetrahedral Clusters with Group 12 Organometallics

The use of pseudo-tetrahedral units in molecular architecture design has several challenges including the difficulty in their synthesis and stabilization of these units due to their tendency to react and form less predictable structures. Traditional methods often fail to maintain the integrity of these tetrahedral units when they are incorporated into larger molecular frameworks. Moreover, there is limited understanding of the reactivity and coordination behavior of these pseudo-tetrahedral units with various metal centers especially in solution-phase reactions and the balance between maintaining the tetrahedral geometry and achieving desired chemical properties requires precise control over the reaction conditions and the choice of reactants. Additionally, the ability to selectively form specific clusters from a mixture of potential products adds another layer of complexity. Therefore, there is a need for better understanding of the reaction mechanisms and factors that influence product selectivity. To this end, the Karlsruhe Institute of Technology (KIT) research team comprised of Shangxin Wei, Dr. Benjamin Peerless, Dr. Lukas Guggolz, Dr. Stefan Mitzinger, and led  by Prof. Dr. Stefanie Dehnen, investigated the reactions of binary Ge/As anions with group 12 organometallic compounds. Their work, published in the Angewandte Chemie International Edition, highlights the potential of these pseudo-tetrahedral units to form complex and novel cluster compounds.

The research team synthesized clusters from the reactions of binary Ge/As anions with group 12 organometallic compounds [MPh₂] (M=Zn, Cd, Hg; Ph=phenyl). The binary reactant was obtained by extracting the solid ‘K2GeAs’ with ethane-1,2-diamine (en), which resulted in the coexistence of (Ge₂As₂)^2- and (Ge₃As)^3- anions in solution. This mixture of anions provided a versatile starting point for subsequent reactions. In their first experiment, the team reacted the extracted binary Ge/As solution with [ZnPh₂] in the presence of the sequestering agent crypt-222. This reaction produced a series of single-crystalline [K(crypt-222)]⁺ salts, including [PhZn(Ge₃As)^2-. This finding highlighted the unique bonding environment and structural stability of the cluster, with the Zn atom favoring interaction with the Ge-Ge bond rather than the As atom. Similarly, the reaction of the Ge/As solution with [HgPh₂] under the same conditions yielded [PhHg(Ge₃As)]^2- (2). The structural analysis showed a different coordination mode, with the Hg atom exhibiting an η^2-Ge₂ coordination to the (Ge₃As)^3- unit. This contrast in coordination modes between Zn and Hg was attributed to the differences in Lewis acidity and steric properties of the metal centers, emphasizing the subtle impact of metal identity on the cluster’s final structure. The team then explored the formation of mixed-ligand clusters by reacting the Ge/As solution with [CdPh₂]. According to the authors, the reaction resulted in the isolation of [(Ge₃As)Zn(Ge₂As₂)]^3- , which is a complex where the Zn atom was coordinated by both (Ge₃As)^3- and (Ge₂As₂)^2- anions. The mixed-ligand structure demonstrated the potential for creating diverse molecular architectures by leveraging the coexistence of different pseudo-tetrahedral units in solution. Further experiments with [CdPh₂] led to the formation of [Cd3(Ge₃As)3]^3-, a pinwheel-like structure analogous to previously reported [Cd₃(Ge₃P)₃]^3- clusters. This compound consisted of three (Ge₃As)^3- units coordinated to three Cd atoms, forming a robust and highly symmetric cluster. The team confirmed the elemental composition and structural details through a combination of X-ray diffraction and energy-dispersive X-ray spectroscopy, and showed  pseudo-tetrahedral units are able to form multi-metal clusters. Moreover, the researchers reacted the Ge/As solution with [ZnMes₂] and [Zn(C₆F₅)₂]. The reaction with [ZnMes₂] yielded [MesZn(Ge₃As)]^2-, which featured a (MesZn)⁺ unit attached to the (Ge₃As)^3- pseudo-tetrahedron. The structural analysis showed that the Zn atom in this compound had a more symmetric coordination environment compared to [PhZn(Ge₃As)]^2- , likely due to the steric effects of the mesityl group. They also reported the formation of [Zn₃(Ge₃As)₄]^6-, a chain-like oligomer comprising four (Ge₃As)^3- units and three Zn²⁺ ions which has the potential for creating extended coordination networks using pseudo-tetrahedral units where two of the pseudo-tetrahedra acted as μ-η²:η² bridges, while the other two were terminal η^2- ligands, and formed a linear arrangement of Zn and Ge/As atoms. The researchers also noted that the coordination environment around the Zn atoms was typical of four-coordinate Zn²⁺ ions, with elongated tetrahedral geometries.

In conclusion, Prof. Dr. Stefanie Dehnen and colleagues carefully investigated the reactions of binary Ge/As anions with group 12 organometallic compounds which provided valuable data into the structural and reactive properties of pseudo-tetrahedral units and the findings demonstrate the potential of these units to form a diverse array of novel molecular architectures, which can have far-reaching impacts on several scientific and industrial applications. For instance, the unique structural and electronic properties of the synthesized clusters can be used in catalysis and the ability to create stable, multi-metal architectures with specific coordination environments could lead to the development of highly efficient catalysts for various chemical reactions. Moreover, their findings have implications for the design of electronic materials because the reported novel clusters could be used to create materials with specific electronic properties, potentially leading to advancements in semiconductor technology and the development of new electronic devices. Furthermore, these clusters can be used to construct nanoscale materials with precise structural control, which could be applied in areas such as drug delivery, sensors, and nanoelectronics. Additionally, the synthesized clusters  could be applied to the development of new materials with unique mechanical, thermal, and chemical properties which can advance aerospace, automotive, and energy applications.