Halomethyltrimethylammonium halide salts were synthesized by nucleophilic bimolecular substitution reactions (S_N2) between trimethylamine gas and dihalomethane (Scheme 1). 

Scheme 1

1. X: Br 

2. X: I

The halomethyltrimethylammonium tetrafluoroborates and hexafluorophosphates were synthesized through a counterion exchange reaction, using halomethyltrimethylammonium halide and silver tetrafluoroborate or silver hexafluorophosphate, respectively (Scheme 2). 

Scheme 2

3. X: Br, Y: BF₄

4. X: Br, Y: PF₆

5. X: I   , Y: BF₄

The yields of the five synthesized salts were as follows: bromomethyltrimethylammonium bromide in 69.77% yield, bromomethyltrimethylammonium tetrafluoroborate in 12.40% yield, bromomethyltrimethylammonium hexafluorophosphate (new salt) in 31.43% yield, iodomethyltrimethylammonium iodide in 86.56% yield, and iodomethyltrimethylammonium tetrafluoroborate in 80.96% yield.  All salts were characterized by proton NMR, carbon-13 NMR and FTIR.  Bromomethyltrimethylammonium bromide and bromomethyltrimethylammonium tetrafluoroborate were additionally characterized by HRMS and X–ray crystallography.  X-ray crystallography showed that the bromines in the bromomethyl portions of the cations were directly anti to one of the methyl groups in the same cation.