# Cocrystals of 1,2-Diiodotetrafluorobenzene with Pyridine Derivatives: Pyridine Nitrogen as a Bifurcated Acceptor with Ortho-Diiodo Halogen Bond Donors

## TL;DR

This paper explores how a specific chemical compound forms unique crystal structures with different nitrogen-containing bases, revealing a rare bonding pattern.

## Contribution

The study identifies a rare bifurcated halogen bond motif involving pyridine nitrogen atoms and ortho-diiodo donors.

## Key findings

- Three weakest bases failed to form cocrystals, while stronger bases yielded new cocrystal phases.
- Quantum calculations show that two iodine atoms provide a more favorable binding site for nitrogen atoms.
- CSD survey confirms that bifurcated halogen bonding with aromatic nitrogen is rare but more common with ortho-diiodo donors.

## Abstract

The behavior of 1,2-diiodotetrafluorobenzene (12tfib) as a halogen bond donor was studied by cocrystallizing it with
a series of aromatic nitrogen bases covering a wide range of pK
a values (2.10 ≤ pK
a ≤ 9.60) and comparing the results with those reported
for other perfluorinated iodobenzenes. The cocrystal screening was
performed by grinding 12tfib and each of the selected
bases in a 1:2 donor:acceptor stoichiometric ratio as well as crystallization
from solution in a small excess of the acceptor (ratio 1:2.5). Of
the 14 bases used in this study, three weakest bases (pK
a below ca. 4) have failed to produce cocrystals, nine
intermediate bases (4.85 ≤ pK
a ≤
6.72) have yielded four new phases, which were only obtainable in
grinding experiments and not as pure samples, four 1:1 cocrystals
and one 1:2 cocrystal, while the two strongest bases (pK
a above ca. 7.5) have yielded 1:2 cocrystals. A total
of eight new solids were studied by single-crystal X-ray diffraction.
The three 1:2 cocrystals comprise discrete halogen-bonded trimers
with 12tfib acting as a ditopic halogen bond donor to
two base molecules. Among the five 1:1 cocrystals, in two, 12tfib was found to act as a monotopic donor; however, in other three, 12tfib was found to be ditopic, with the nitrogen atom of
the base being a bifurcated acceptor of a pair of halogen bonds. This
highly unusual binding motif has been further investigated by quantum
chemical calculations and a detailed CSD survey. The computational
study has found that a binding site comprising two converging iodine
atoms possesses a wide, single minimum potential, which allows for
a more favorable binding of weakly and intermediately basic pyridine
(aromatic sp2) nitrogen atoms than a single iodine atom.
The CSD survey has shown that the aromatic sp2 nitrogen
atom acting as a bifurcated halogen acceptor is indeed an extremely
rare occurrence (appearing in only ca. 2.2% of the total) but considerably
more likely to occur in the presence of ortho-diiodo
halogen bond donors.