Closing the Gap: Structural Evidence for the Missing Hexabromide Dianion [Br₆ ]².

Area of Science:

• Inorganic Chemistry
• Halogen Bonding
• Crystal Engineering

Background:

• Polybromide anions (Br_n^-) are known from tribromide (Br₃^-) to undecabromide (Br₁₁^-).
• The hexabromide dianion (Br₆^2-) was previously uncharacterized, representing a gap in the polybromide series.

Purpose of the Study:

• To synthesize and experimentally characterize the first hexabromide dianion.
• To compare experimental findings with quantum-chemical calculations regarding its structure and stability.
• To investigate the role of the counterion in stabilizing the novel anion.

Main Methods:

• Single-crystal X-ray diffraction
• Mass spectrometry
• Nuclear Magnetic Resonance (NMR) spectroscopy
• Infrared (IR) spectroscopy
• Single-crystal Raman spectroscopy
• Quantum-chemical calculations

Main Results:

• The first hexabromide dianion, [Br₆]^2-, was successfully synthesized and characterized.
• Experimental results indicate a hockey-stick-like structure, contrasting with the T-shaped structure predicted by electrostatic interactions alone.
• The anion comprises two tribromide moieties, with one exhibiting significant asymmetry.
• The counterion [C₅H₁₀N₂Br]⁺ was found to stabilize the hexabromide dianion through σ-hole interactions.

Conclusions:

• The formation of the hexabromide dianion fills a significant gap in polybromide chemistry.
• Halogen-halogen bonding plays a crucial role in determining the experimentally observed structure.
• The study highlights the importance of counterion interactions in stabilizing unusual anionic species.