Capturing Aromatic Cr5 Pentagons in Large Main-Group Molecular Cages

Chemistry Materials Science Inorganic Chemistry Materials Chemistry
chromium chemistry aromaticity molecular cages Zintl ion synthesis Cr5 clusters

Academic Background

Transition metal clusters have long been a focus of chemical research due to their unique structures and electronic properties. Chromium (Cr), as one of the transition metals, is particularly notable for its interesting Cr-Cr bonding interactions and diverse structural arrangements. However, exploring the synthesis of polynuclear Crₙ (n > 3) clusters remains challenging due to the difficulty in achieving precise matching between the metal core and ligands. Specifically, the experimental synthesis of planar Crₙ configurations (n > 3) has not been realized. To address this gap, the research team successfully isolated and characterized a Cr₅ cluster using the Zintl ion synthesis route, revealing the significant contribution of aromaticity to the stability of the cluster.

Source of the Paper

This paper was co-authored by Wei-Xing Chen, Wen-Juan Tian, Zi-Sheng Li, Jing-Jing Wang, Álvaro Muñoz-Castro, Gernot Frenking, and Zhong-Ming Sun. The research team is affiliated with Nankai University, Shanxi University, the University of Oxford, Universidad San Sebastián in Chile, Nanjing Tech University, Philipps-Universität Marburg in Germany, and the Donostia International Physics Center in Spain. The paper was published in the journal Nature Synthesis in April 2025, with the DOI 10.1038/s44160-024-00711-5.

Research Process

1. Synthesis and Characterization

The research team synthesized two Cr clusters, [Cr₅Sn₂Sb₂₀]⁴⁻ and [(Cr₅)₂Sn₆Sb₃₀]⁶⁻, using the Zintl ion synthesis route under mild and controllable conditions. The specific steps are as follows: - Synthesis of Compound 1a: K₈SnSb₄ and 18-crown-6 were dissolved in ethane-1,2-diamine (en), heated, and stirred. After adding dicyclopentadienyl chromium (CrCp₂), the mixture was further heated, and black block crystals were obtained through crystallization. - Synthesis of Compound 1b: A similar method was used, but 2.2.2-crypt was employed as the cation-sequestering agent, resulting in [K(2.2.2-crypt)]₄[Cr₅Sn₂Sb₂₀]. - Synthesis of Compound 2: By extending the reaction time and adjusting the reaction conditions, the [(Cr₅)₂Sn₆Sb₃₀]⁶⁻ cluster was successfully synthesized.

2. Structural Characterization

The crystal structures were characterized using single-crystal X-ray diffraction (SC-XRD). It was found that the Cr₅ units in [Cr₅Sn₂Sb₂₀]⁴⁻ and [(Cr₅)₂Sn₆Sb₃₀]⁶⁻ exist in a planar pentagonal form, surrounded by Sb or Sn-Sb molecular cages. Additionally, the composition of the clusters was verified using electrospray ionization mass spectrometry (ESI-MS) and energy-dispersive X-ray spectroscopy (EDX).

3. Theoretical Calculations

The research team employed density functional theory (DFT) and adaptive natural density partitioning (AdNDP) analysis to reveal the aromaticity of the Cr₅Sn₂ core. The calculations showed that the electron distribution in the Cr₅Sn₂ unit follows the 4n + 2 Hückel rule, indicating its aromatic nature. Furthermore, nucleus-independent chemical shift (NICS) calculations further confirmed the aromatic properties of the Cr₅Sn₂ core.

Main Results

1. Cluster Structures

  • In [Cr₅Sn₂Sb₂₀]⁴⁻, the Cr₅ unit forms a planar structure with surrounding Sb₂ dumbbells, encapsulated within an Sb₂₀ molecular cage.
  • [(Cr₅)₂Sn₆Sb₃₀]⁶⁻ can be viewed as two [Cr₅Sn₂Sb₁₄] units bridged by Sn₂Sb₂, forming a nanoscale dimer fusion.

2. Aromaticity Analysis

Theoretical calculations revealed that the electron distribution in the Cr₅Sn₂ core is aromatic, which is a key factor in the stability of the clusters. NICS calculations showed that the shielding region of the Cr₅Sn₂ core is further enhanced in the dimer structure.

3. Electronic Structure

Analysis of the electronic structure of the clusters indicated that the unpaired electrons in the Cr₅ unit are mainly localized on the Cr atoms, suggesting paramagnetism. Additionally, the HOMO-LUMO gaps (0.78-0.88 eV) of the clusters indicate semiconductor-like behavior.

Conclusion

This study successfully synthesized and characterized the [Cr₅Sn₂Sb₂₀]⁴⁻ and [(Cr₅)₂Sn₆Sb₃₀]⁶⁻ clusters containing planar Cr₅ units, revealing the crucial role of the aromaticity of the Cr₅Sn₂ core in stabilizing the clusters. These findings not only expand the scope of chromium chemistry but also provide new models for studying metal-metal bonding. Furthermore, these clusters can be regarded as inorganic all-metal counterparts of aromatic rings (e.g., cyclopentadienyl Cp⁻), offering new insights for designing and synthesizing isostructural clusters with variable spin states.

Research Highlights

  1. First Synthesis of Planar Cr₅ Clusters: The planar Cr₅ unit was successfully captured using the Zintl ion synthesis route, filling a gap in chromium cluster research.
  2. Aromaticity Enhances Stability: Theoretical calculations revealed the aromaticity of the Cr₅Sn₂ core, which is a key factor in the stability of the clusters.
  3. Inorganic All-Metal Aromatic Rings: These clusters can be regarded as inorganic all-metal counterparts of aromatic rings, providing new models for studying metal-metal bonding.
  4. Paramagnetic Clusters: The unpaired electrons in the clusters are mainly localized on the Cr atoms, suggesting paramagnetism, which offers potential for designing novel magnetic materials.

Research Significance

The scientific value of this study lies in the first successful synthesis of planar Cr₅ clusters and the revelation of the critical role of aromaticity in their stability. This not only expands the scope of chromium chemistry but also provides new models for studying metal-metal bonding. Additionally, the paramagnetic and semiconductor-like properties of these clusters offer potential applications in magnetic materials and semiconductor devices.