Synthesis of sequentially deuterated 1-n-butyl-3-methylimidazolium ionic liquids

LIQUIDOS IONICOS

SINTESIS

BIBLIOGRAFIA NACIONAL QUIMICA

2011

Arbitrarily defined as organic salts with melting points below 1001C, ionic liquids (ILs) have gained considerable notoriety in the past decade. Owing to their negligible vapor pressure, thermal stability, and tunable solvent properties, these materials have found applications in a wide range of laboratory and industrial processes.1 As part of efforts aimed at better understanding the liquid structure of imidazolium-based ILs, we have shown that intraionic hydrogen-bonding (H-bonding) interactions in 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl (1), Figure 1) are evidenced by sizable deuterium isotope effects on the chloride ion 35/37Cl NMR signal (i.e. D35/37Cl(H,D)).2 Measurements obtained on the [C4mim]Cl isotopologues [2-d1]-[C4mim]Cl and [2,4,5-d3]-[C4mim]Cl led to D35/37Cl(H,D) values of up to 2 ppm, and corroborated the presence of H-bonds between the anion and the imidazolium cation protons on the C-2, C-4, and C-5 positions. Recent theoretical studies indicate that H-bonding between the anion and protons along the N-alkyl groups could also have an impact on the structure and dynamics of this class of ILs.3,4 Based on our previous results, these anion–cation interactions could be detected experimentally by measurement of deuterium isotope effects on the 35/37Cl NMR signal of the chloride ion. For this purpose, we decided to synthesize a series of [C4mim]Cl deuterium isotopologues sequentially labeled along the N-alkyl group carbons (Figure 1a–e). These could also be easily converted by metathesis to the corresponding tetrafluoroborate and hexafluorophosphate imidazolium salts (i.e. [C4mim]BF4 and [C4mim]PF6), which would be suitable for the study of interionic H-bonds through analysis of deuterium isotope effects on the 10/11B, 19F, and 31P signals of the BF! 4 and BF! 6 anions. In this manuscript we describe a strategy which uses similar routes to minimize the number of distinct reactions needed to prepare these compounds. As detailed below, good yields after the initial deuterium incorporation reaction were achieved in several cases through the combination of well-established chemical transformations into efficient single-step processes.

Journal of Labelled Compounds and Radiopharmaceuticals v. 54, no. 8. -- p. 401-407

Wiley & Sons

2011

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DOI: 10.1002/jlcr.1884