engleski

Halogenated aromatic cations as halogen bond donors

The key feature which enables halogen atoms to act as Lewis acids in order to form halogen bonds, is presence of an area of depleted electron density (σ-hole).[1] Most common strategies for the design of halogen bond donors thus rely on using the most polarisable halogens (I and Br) as donor atoms, bonded to electron-withdrawing molecular residues. This is usually achieved by addition of substituents on the hydrocarbon skeleton (most commonly fluorine, as in perfluorinated iodo and bromo-hydrocarbons [2]), or binding the halogen to an electronegative heteroatom (e.g. nitrogen in halogenoimines [3]). There is, however, another approach for ensuring a large positive electrostatic potential on the halogen atom: making the halogen atom a part of a positively charged species.[4] Here we will present a systematic study of both the possibilities and the limitations of using halogenopyridinium (X-Py) cations as halogen bond donors. For this purpose, we have selected mono-halogenated pyridine derivatives (o-, m- and p- ; chloro, bromo and iodo), both as protonated and as N-methylated pyridinium cations. While both protonation and N- methylation of halogenopyridines lead to a considerable increase in the electrostatic potential of the halogen σ-hole, X-Py cations have not shown to be superior to the commonly used neutral halogen bond donors. A closer inspection of the distribution of the charge, demonstrates that the halogen σhole is generally not the most positive site on the surface of the cation. In spite of this, cations derived from iodo- and even bromopyridines form halogen bonds with remarkable consistency, which makes them reliable halogen bond donors for synthesis of halogen bonded materials.

halogen bond, cationic halogen bond donors, pyridinium cations

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