An international team of scientists has identified a number of halogen-bound complexes that consist of two anions that are stable in solution. The spontaneous formation of this counterintuitive interaction shows that halogen bonds are strong enough to overcome the electrostatic repulsion between two anions.
Halogen bonds are an attractive interaction that arises between an electron-rich nucleophilic species (XB acceptor) and the electrophilic region of a molecule with a halogen substituent (XB donor). Either cationic or neutral donors are normally involved in such interactions, while interactions between anionic halogen species and electron-rich acceptors, especially anionic acceptors, are considered to be electrostatically repulsive.
“Introductory science courses teach us that charged particle interactions attract opposites – this is known as Coulomb’s law,” explains Mark Taylor, an organic and supramolecular chemist from the University of Toronto, Canada, who does not attend the Research involved. As such, “an attractive interaction between two negatively charged ions in solution” is [would be] surprising in view of Coulomb’s law. ‘
However, a team led by Sergiy Rosokha from Ball State University, USA, and Stefan Huber from Ruhr University Bochum, Germany, has shown that halogen bridges between several anionic species are possible. “Halogen bonds are a fundamental non-covalent interaction similar to hydrogen bonds, which has not been explored much so far,” explains Huber. “We noticed some anomalies in the currently fashionable σ-hole model of halogen bonding, particularly some trends that it couldn’t explain, and started investigating unusual cases.”
Rosokha says that anti-electrostatic bonding is possible due to a combination of factors. “First, polar solvents weaken the original anion-anion repulsion and make it possible [for the] close proximity of the counterparts. This leads to their polarization, which further reduces repulsion and can even create an area of positive charge – and thus a source of attraction – on the surface of the highly polarizable iodine substituents. Finally, the stability of such complexes at shorter intervals is aided by molecular orbital interactions similar to those that lead to the formation of covalent bonds. ‘
To characterize this anti-electrostatic bond between two anions, the team focused on the interaction between halides and 1,2-bis (dicyanomethylene) -3-iodo-cyclopropanide, a compound with a universally negative electrostatic potential over its entire surface. While the interaction has already been investigated in the solid,2 Investigating its behavior in solution, where it is “unaffected by crystal forces and counter-ion interference is minimal,” meant breaking new ground.
When this cyclopropenylium-based anionic donor was mixed with halides in polar and moderately polar solvents, new bands appeared in the UV-vis spectra despite the thermodynamic similarities with corresponding interactions using neutral XB donors.
“The variation of this additional absorption with the concentration of the reagents, comparisons with spectral changes observed in related compounds, and computational analysis have shown that the UV-vis spectral changes … are related to the formation of halogen-bound complexes,” explains Rosoha. The team showed that these new absorption bands are independent of the halide used and are influenced by the differences in the molecular orbital energies of the two binding species.
“The identification of such anti-electrostatic halogen-bound complexes also supports the idea that halogen bonds are more than the electrostatic attraction between two isolated species. Instead, this bond provides a channel for electrons to move between them, ”adds Rosokha.
“I would be curious to find out more about the actual binding mechanism and the actual physics behind the stabilizing interaction,” says Matthias Bickelhaupt, who develops chemical theories and methods for the rational design of molecules and chemical processes at VU Amsterdam in the Netherlands. “There is no doubt in the conclusion that orbital interactions play a crucial role in anti-electrostatic halogen-bonded complexes, but it would still be wonderful to explicitly confirm the occurrence of HOMO-LUMO orbital interactions, for example with quantitative molecular orbital theory. ‘
For the team, the next step in understanding anti-electrostatic interactions is to uncover other systems that may be able to form such complexes in the hope that such unusual halogen bond formations could have synthetic or biological applications.
“In the longer term, it would be useful to develop systems that have higher association constants with anions and that work in a variety of solvents,” adds Taylor, who notes that a number of anion-anion interactions take place in aqueous solution. and that halogen bonding has applications in anion recognition in water. “It might ultimately be possible to use halogen bridges between anions to achieve selective molecular recognition in aqueous, salt-rich solutions.”