Unusual bridging fluorine discovered in one-of-a-kind interhalogen ion

For the first time, chemists have synthesised an interhalogen compound exhibiting a unique central fluorine atom coordinated by four BrF₅ groups.¹ The one-of-a-kind interhalogen ion in [NMe₄][Br₄F₂₁]·BrF₅ represents the first example of a central tetracoordinated fluorine bridging to neither metal nor hydrogen atoms.

Tetracoordinated fluorine atoms are known in ionic compounds, for example in CaF₂. And µ­­₄-fluorine atoms are present in the tetrahydrogen pentafluoride ion and a handful of metal compounds.

Last year, a team led by Florian Kraus from the Philipps University of Marburg in Germany, synthesised [Br₃F₁₆]^–, ² which has a triple-coordinated central fluoride ion. Now, building on their previous work, Kraus and co-workers have used a sterically demanding [NMe₄]⁺ counter ion to form the bulkier µ­­₄-fluorine containing interhalogen ion, [Br₄F₂₁] ^–.

Halogen fluorides are known for their extreme reactivity and the synthesis did not come without its challenges. The team first tried reacting pure BrF₅ with [NMe₄]F but a violent reaction between the two compounds caused a small explosion and a day’s work was lost in an instant. On searching for an alternative method they came across research by Karl Christe from 1989³ that helped them to reliably access [NMe₄][BrF₆] and they went on to attempt a controlled reaction of [NMe₄][BrF₆] with excess BrF₅ at low temperatures. It worked. Graham Saunders, a synthetic fluorine chemist from the University of Waikato in New Zealand, comments ‘technically it’s very difficult work and it’s very elegant’.

Deciphering the compound’s structure also presented a challenge as it initially appeared very complex. However, by focusing on the centres of the ions and BrF₅ molecules, the team discovered its crystal structure was similar to the MgAgAs-type structure. ‘Although the two compounds could not be more chemically different, they are structurally closely related,’ says Marburg team member Martin Möbs. Additional quantum calculations reveal that the µ­­₄-fluorine–bromine bonds are best described as ionic.

‘To encapsulate a fluoride ion in a non-metallic inorganic framework such that it’s bound to four different moieties is really quite extraordinary, because normally you would not anticipate that kind of bonding arrangement,’ comments Thomas Lectka, an organofluorine chemist from Johns Hopkins University, US. ‘It’s an interesting result and I think it will spur more work into the halogens with fluorine,’ adds Saunders.

But while the µ­­₃-fluorine and µ­­₄-fluorine polynuclear anions of BrF₅ have now been isolated, Möbs concludes that ‘the anion in which the fluoride ion is surrounded by two BrF₅ molecules, [Br₂F₁₁]^–, is still missing.’

Correction: The images were updated on 12 February 2024