engleski

Halogen bonding in an isostructural series of N-(4- halogenobenzyl)-3-halogenopyridinium halogenides

Isostructurality – the occurrence of different substances adopting very similar (or almost identical) crystal structures – has over previous decades shown to be a particularly useful phenomenon for study of minute differences in crystal structures and properties, as in isostructural materials the majority of various contributions of the overall crystal packing can be taken to be equivalent within an isostructural series. One field of supramolecular chemistry which has particularly benefited from the study of isostructural systems, is the study of halogen bonded materials, since replacing one halogen atom with another in a molecule generally has only a minute effect on molecular geometry. As halogen bond energy greatly increases with the donor atom size, the only significant difference between two crystals within an isostructural series comprising different halogens, will be the strength of the halogen bond. In this contribution, we shall present a study of a series of N-(4-halogenobenzyl)-3- halogenopyridinium halogenides. The cation was selected as a potential donor of two inequivalent halogen bonds – one through the halogen on the pyridine ring (on which the majority of the charge is expectedly located) and the other through the halogen on the N-benzyl substituent. Sixteen compounds were obtained ; two crystallized as hydrates and 14 as solvent free salts, 11 of which belonged to one isostructural series and 3 to another. The halogen bonds with the pyridine halogen show a linear correlation between the relative halogen bond length and angle, which in turn primarily depend on the donor halogen. The parameters of the other halogen bond vary with all three halogens, indicating that the former halogen bond is the dominant interaction. This is also in accord with the calculated electrostatic potential in the σ-holes of the halogens, as well as the measured thermal properties of the solids. The second isostructural group comprises combinations of best halogen bond donors and acceptors, and features a more favourable halogen bond geometry of the dominant halogen bond, reaffirming its significance as the main factor in determining the crystal structure. [1] L. Fotović, N. Bedeković, and V. Stilinović, Cryst. Growth Des. 22 (2022) 1333−1344.

halogen bonding ; isostructurality ; halogenopyridinium halogenides

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