Abstract
In order to study the effect of halogen bond on tautomerism in β-diketones in the solid-state, we have prepared a series of cocrystals derived from an asymmetric β-diketone, benzoyl-4-pyridoylmethane (b4pm), as halogen bond acceptor and perfluorinated iodobenzenes: iodopentaflourobenzene (ipfb), 1,2-, 1,3- and 1,4-diiodotetraflorobenzene (12tfib, 13tfib and 14tfib) and 1,3,5-triiodo-2,4,6-trifluorobenzene (135titfb). All five cocrystals are assembled by I···N halogen bonds involving pyridyl nitrogen and iodoperfluorobenzene iodine resulting in 1:1 (four compounds) or 1:2 (one compound) cocrystal stoichiometry. Tautomer of b4pm in which hydrogen atom is adjacent to the pyridyl fragment was found to be more stable in vacuo than tautomer with a benzoyl hydroxyl group. This tautomer is also found to be dominant in the majority of crystal structures, somewhat more abundantly in crystal structures of cocrystals in which additional I···O halogen bond with the benzoyl oxygen has been established. Attempts have also been made to prepare an equivalent series of cocrystals using a closely related asymmetric β-diketone, benzoyl-3-pyridoylmethane (b3pm); however, all attempts were unsuccessful, which is attributed to more effective crystal packing of b3pm isomer compared to b4pm, which reduced the probability of cocrystal formation.
Highlights
One of the intriguing aspects of the study of halogen bonding, and supramolecular interactions in general, is the interrelationship between different types of interactions [1,2,3,4,5,6,7,8]
The effect of supramolecular interaction on tautomeric equilibrium in the solid-state has been studied to some extent for aromatic o-hydroxyimines (Schiff bases), where the intramolecular transfer of the hydrogen atom from the hydroxyl to the imino group leads to the formation of keto-amino tautomer [21]
As was described in a study performed by Dudek et al [71], both b3pm and b4pm in the solid-state exist in keto-enol form, with a strong intramolecular resonance assisted O–H···O hydrogen bond formed between the two oxygen atoms
Summary
One of the intriguing aspects of the study of halogen bonding, and supramolecular interactions in general, is the interrelationship between different types of interactions (competition, cooperativity, anti-cooperativity, etc.) [1,2,3,4,5,6,7,8]. The effect of supramolecular interaction on tautomeric equilibrium in the solid-state has been studied to some extent for aromatic o-hydroxyimines (Schiff bases), where the intramolecular transfer of the hydrogen atom from the hydroxyl to the imino group leads to the formation of keto-amino tautomer [21]. It has been demonstrated that hydrogen bond with the oxygen atom can greatly facilitate this proton transfer in the solid-state by stabilizing the keto-amino form [22,23,24,25,26] This has led to the question of whether the halogen bond—an interaction in many ways similar to hydrogen bond [27,28,29,30]—can have a similar effect on the intramolecular hydrogen transfer and stabilization of the keto-amino tautomer. A tautomeric system with a more symmetric intramolecular hydrogen bond (and subsequently with a smaller energy difference between the tautomers) might provide a better model for the study of the effect of halogen bond on tautomerism
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