Quaternary halogenopyridinium cations as halogen bond donors

U ovome istraživanju, uvođenjem različitih (alifatskih) supstituenata na piridinski dušikov atom halogenpiridina (osobito meta izomera) pripravljeno je te strukturno, termički i spektroskopski okarakterizirano 49 novih soli protoniranih i N-alkiliranih halogenpiridina. Usporedbom halogenskih veza u strukturama N-alkil-3-jodpiridinijevih jodida utvđeno da postoji trend da što je duži ugljikovodični lanac supstituiran na dušikov atom jodpiridinskog prstena to su kraće halogenske veze C−I⋯I−. Ustanovljeno je da afinitet prema halogenskoj vezi opada s veličinom halogenidnog aniona kao akceptora, odnosno slijedi isti trend kao i u slučaju vodikove veze. Analizom raspodjele naboja na halogenpiridinijevim kationima utvrđeno je da i protoniranje i N-metiliranje halogenpiridina dovode do značajnog povećanja elektrostatskog potencijala σšupljine halogena što uvelike povećava incidenciju ostvarivanja halogenskih veza u odnosu na neutralne halogenpiridine. To čini halogenpiridinijeve katione (jodpiridinijeve posebno) prilično pouzdanim donorima halogenskih veza, te na taj način opravdava njihovu upotrebu u dizajnu i sintezi supramolekulskih struktura povezanih halogenskom vezom.In this study, 49 novel salts of halogenopyridinium cations were prepared and characterized by single crystal X-ray diffraction, and by thermal and spectroscopic analysis methods. In the crystal structures of N-alkyl-3-iodopyridinium iodides it is observed trend that longer chains as N-substituents generally leading to shorter C−I⋯I− halogen bonds. Based on the trend observed in the relative shortenings of distances of the two interactions, both halogen and hydrogen bonds seem to be decreasing in strength with the size of the halogenide anion. Inspection of the distribution of the charge on the cations revealed that both protonation and N-methylation of halogenopyridines lead to a considerable increase in the electrostatic potential of the halogen σ-hole which increase incidence of the halogen bond in the structures comprising halogenopyridinium cations (as compared to neutral halogenopyridines). This makes halopyridiniom cations (iodopyridinium in particularly) fairly reliable halogen bond donors for deliberate synthesis of halogen bonded supramolecular structures