Synthesis and crystallographic study of six quaternary salts of N-halomethylated and non-N-halomethylated ammonium: Importance of C-H‧‧‧X (X=F, I) and I‧‧‧I− halogen interactions in the supramolecular structures

Abstract

The syntheses of six salts, N-halomethylated (1a,1b,1c) and non-N-halomethylated (2a,2b,2c), are achieved by a four-step sequence involving: (i) Grignard reaction starting from p-(F)C6H4MgBr and Br(CH2)n+1COOEt; (ii) acid-catalyzed dehydration of the resulting tertiary alcohols Br(CH2)n+1C(OH) (C6H4-p-F)2; (iii) bromide substitution by aqueous dimethylamine in the resulting olefins Br(CH2)nCH=C(C6H4-pF)2; and (iv) displacement of an iodide anion by reaction of the resulting amines of (CH3)2N(CH2)nCH=C(C6H4-pF)2 with CH2I2. These compounds were crystallized in a binary mixture of water-isopropanol in slow evaporation. Crystallographic studies show that the crystal growth of these compounds is controlled by C-H‧‧‧X (X=F, I or I−) and I‧‧‧I− interactions with a complete absence of classic hydrogen bonds. The detected H‧‧‧X and I‧‧‧I− distances are very short compared with the values normally observed in literature. The results suggest that, in the lack of other hydrogen bonds, halogen atoms form strong interactions and, in the case of these salts, control completely the supramolecular structure. Moreover, it was found that the length of (CH2)n chains influences the intermolecular interactions, being longer for lengthy chains. Hirshfeld surface (HS) analysis support this observation showing that these halogenated intermolecular interactions conform in high proportion the contacts in the crystal. Additionally, HS allowed the detection of σ-holes were I‧‧‧I− act as Lewis´s acid-base pair, respectively.