A Research Paper Published in Crystal Growth & Design

Over the last few decades, halogen-bonded multi-component solids, including cocrystals, have been intensively studied. The most widely used halogen bond donors are perfluorinated iodobenzenes, while the most commonly used acceptors to date are those containing aromatic nitrogen atoms, such as pyridine derivatives. Lately, there has been a growing interest in the design of halogen-bond acceptor molecules comprising accessible oxygen atoms. Therefore, the study of halogen-bonding propensity of N,O-ambidentate acceptors – molecules that contain both nitrogen and oxygen acceptor sites – is of considerable importance. The instances when I···O and I···N halogen bonds coexist in a given crystal structure are relatively rare. Following this argument, we have attempted to prepare halogen-bonded cocrystals of structural isomers of benzoylpyridine (bzpy) and a (potentially) tritopic halogen bond donor 1,3,5-triiodo-2,4,6-trifluorobenzene (titfb).

In most of the cocrystals presented in this paper, bzpy is a ditopic acceptor that participates in the halogen bonds both via pyridine nitrogen and carbonyl oxygen, with I···N halogen bond shorter than I···O interactions. The crystal structures of titfb-bzpy cocrystals, as well as ab initio calculations of dimers in the gas phase, thus indicate that carbonyl oxygen atom can act as a reliable halogen-bond acceptor in the presence of pyridine nitrogen. Whenever the I···O halogen bond is present, it leads to the formation of halogen-bonded chains in which titfb acts as a ditopic donor and bzpy as an N,O-ambidentate ditopic acceptor.

The research was performed as a part of the research project New building blocks for the supramolecular design of complex multi-component molecular crystals based on halogen bonds (HaloBond IP-2019-04-1868) funded by the Croatian Science Foundation.