Imidazolium, phosphonium and pyridinium ionic liquids with flourinated anions : synthesis characterization and applications.

Abstract

Four sets of ionic compounds were synthesized, each with an organic spacer between two ligands creating a dication with the bromide as the counter anion. In each of the sets of ionic compounds, the counter anion was exchanged in a metathesis reaction with either all or some of NaBF4, KPF6, CF3SO3Li and CF3COONa in an effort to produce fluorinated ionic liquids. The first set of ionic salts was based on the reaction between 1-methylimidazole and a,a– dibromo-p-xylene to form a bis(methylimidazolium) cation with an aromatic spacer between them. This was followed by metathesis reactions with four fluorinated anion sources, which yielded new fluorinated imidazolium salts [C6H4(CH2(C4H6N2)2]2+ 2[A]- where A = BF4 (A- 2), PF6 (A-3), CF3SO3 (A-4) and CF3COO (A-5). The compounds were characterized by 1H-, 13C-, 19F-, 31P-NMR and IR spectroscopy. Single crystal X-ray diffraction data of compounds A-2, A-3 and A-4 were carried out where the study revealed that the different fluorinated anions affected the spacial arrangement of atoms and the extent of cation-anion interactions and hence, influenced the stability and coordination properties of the imidazolium salts. A trend was observed which related the strength of cation-anion interaction to physical properties such as melting point. A further set of ionic salts, keeping the same organic moiety, the a,a-dibromo-p-xylene but this time changing the ligand to that of triphenylphosphine (PPh3) was synthesized to first produce the dibromide (B-1) and then the fluorinated salts B2-B5 with CF3SO3Li, NaBF4, KPF6 and CF3COONa respectively. Compounds B2-B5 were all novel compounds whose synthesis have not been reported previously. The salts were characterized by 1H, 13C, 19F, 31P NMR, and IR spectroscopy. The organic salts were tested for their antimicrobial activity and showed significant activity against Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Salmonella typhi, Candida albicans, and Streptococcus pyogenes when compared with the activities of the standard antimicrobial chloramphenicol and antifungal nystatin. The crystal structure of B-3, the ditetrafluoroborate is also discussed. In the third set of ionic salts, the solvent free reaction of 1-methylimidazole and alkyl dibromides (RBr2; R = -CH2-, -(CH2)3-, -(CH2)4-, and -(CH2)5-), were synthesized (C1a-4a) in a highly exothermic reaction. This was followed by metathesis reactions with fluorinated anion sources, NaBF4 and KPF6 to produce the fluorinated imidazolium ionic compounds C1b-4b and C1c-4c respectively. All twelve compounds were characterized by 1H-, 13C-, 19F-,31P-NMR, and IR spectroscopy. A trend was observed relating the length of the alkyl chain in the cations and the strength of cation-anion interactions to the melting point of the compounds. The dibromide ionic compounds decreased in melting point as the length of the alkyl spacers increased. The strength of the cation-anion interactions were strongest for the Br- ionic liquids followed by the PF6 - and the BF4 - counteranions as demonstrated by their melting points which increased in the order of M(BF4)2 < M(PF6)2 < MBr2. NMR studies of the synthesised ionic salts were also carried out to determine how the shifts of the alkyl groups and ligands changed as the anion changed. In the last set of compounds, pyridine was explored as the ligand with p- , -dibromoxylene and 1,1-dibromomethane respectively to yield the quarternized salts D1a and D2a respectively, followed by metathesis reactions with the fluorinated anion sources, NaBF4 and KPF6 to produce fluorinated pyridinium ionic liquids D1b-c and D2b-c. Compounds D1b and D2c are novel compounds as the combination of cation and anion had not been synthesised previously. The synthesis of D2a was carried out without the use of solvents and the metathesis reactions carried out with ethanol and water and can be considered as green reactions. 2D NMR studies of the synthesised compounds are reported here.