Hydroxyl-functionalized ionic liquids

A new one phase method for the synthesis of uniform monodisperse crystalline Ag nanoparticles in aqueous systems was developed by using newly synthesized mono and dihydroxylated ionic liquids and cationic surfactants based on 1,3-disubstituted imidazolium cations and halogens anions. The hydroxyl functionalized ionic liquids and hydroxyl functionalized cationic surfactants simultaneously act both as the reducing and protective agent. By changing the carbon chain length, alcohol structure and anion of the hydoxy-functionalized 1,3-imidazolium based ionic liquids and the hydroxyl functionalized cationic surfactants the particle size, uniformity and dispersibility of nanoparticles in aqueous solvents could be controlled (Dorjnamjin et al, 2008). 

A hydroxyl functionalized ionic liquid (IL) l-(3-chloro-2-hydroxy-propyl)-3-methylimidazolium tetrafluoroborate (PMIMBF4) was used for the preparation of bulky-modified carbon paste electrode (IL-CPE) by Guo et al. [31] and it was applied to the sensitive detection of adenosine-5 -diphosphate (ADP). Due to the specific characteristics of IL such as high ionic conductivity and strong adsorption ability, IL-CPE showed remarkable redox catalysis effect on the oxidation of ADP. By using (DPV) the oxidation peak current was linearly dependent on the ADP concentration in the range from 10.0 to 1000.0 pmol L with the detection limit as 3.23 pmol L (3o). 

Gao et al. [32] prepared a carbon CILE with a new hydroxyl functionalized ionic liquid (IL) l-(3-chloro-2-hydroxy-propyl)-pyridinium acetate as modifier and used it for the voltammetric determination of adenosine-5 -monophosphate (5 -AMP). Remarkable enhancement of the oxidation peak currents was observed, which was attributed to the specific characteristics of hydroxyl functionalized IL used on the CILE Under the selected condition and by using sensitive DVP the oxidation peak current was proportional to 5 -AMP concentration in the range fi om 3.0 to 2500.0 pmol L with the detection limit as 0.60 pmol L" (3o). The fabricated electrode was further used for the detection in commercial vidarabine monophosphate injection samples with satisfactory results. 

The interest in functionalized ionic liquids is growing because ionic liquids bearing ether, amino or alcohol functionalities have been shown to display special properties, including the ability to dissolve a larger amoimt of metal halide salts and to extract heavy metal ions from aqueous solutions. Imidazolium-based ionic liquids with ether and hydroxyl (see Section 2.2.1), thiourea, thioether and urea (see Section 2.2.8) " have been prepared following the standard quatemization procedure. A straightforward approach has been described for the preparation of imidazolium (as well as pyridinium) cations with ester, ketone or cyanide functionalities 1-methylimidazole reacts with methanesulfonic acid to provide the imidazolium salt 11, which undergoes a Michael-type reaction with methyl vinyl ketone as a ,j8-unsaturated compound to produce the ionic liquid 12 (Scheme 5). ... 

New 2-hydroxypropyl-functionalized imidazolium cation ionic liquids (containing an appended hydroxyl functionality) have been made [116] by use of an atom efficient one-pot reaction befween 1-mefhylimidazole and acid with propylene oxide. Unfortunately the systems were not studied... 

Holbrey, J. D., Turner, M. B., Reichert, W. M., and Rogers, D. R., New ionic liquids containing an appended hydroxyl functionality from the atomefficient, one-pot reaction of 1-methylimidazole and acid with propylene oxide, Green Chem., 5, 731-736, 2003. 

Figure 25.6 Molecular structures of hydroxyl-terminated mesogenic compounds 13-15, ionic liquid 16, and mesogenic compound 17 without functional groups.

One synthesis approach towards y-hydroxylated ionic liquids is a Michael-type addition of a protonated ammonium salt to a a, /l-an sal uni led carbonyl compound such as methylvinyl ketone yielding an oxobutyl functionalized cation. Intrinsically unstable due to retro-Michael reaction, this OS could however be transformed by a heterogeneous catalyzed hydrogenation reaction yielding a hydroxyl functionalized TSIL [24] (Fig. 7). 

Two-dimensional ion conduction was also achieved for the smectic liquid crystals consisting of the mixture of ionic liquids and hydroxyl-functionalized rod molecules [49-51] and smectic LC imidazolium salts having a long alkyl chain [52-55]. One-dimensional ion conduction was performed for aligned columnar LC phases forming ion-channels of fan-shaped imidazolium salts having a tris(alkoxy)phenyl moiety [56]. 

Similarly, ionic liquids bearing a hydroxyl functionality - which leads to unfavorable intermolecular hydrogen bondings - can be used as a liquid at low temperamre. Salts 30-33 are easily synthesized from 1-methylimidazole and 2-chloroethanol they are solid at room temperature. However, because of a weak tendency to crystallize, they enter into a metastable supercooled liquid state when cooled and can therefore be used as solvents at low temperature. The glass transition temperatures are very low, between -72°C (30) and -111°C (33, Table 2.5). The influence of the anion on the glass transition temperatures, as opposed to the melting points, is not clear, as the chlorine salt 33 is a metastable liquid at lower temperature than the hexafluorophosphate and tetrafluoroborate salts 30 and 31. ... 

Adding a polar functional group on the cation will also form water-soluble ionic liquids. Salts 30-33 (Table 2.5), whose imidazolium ring has a hydroxyl functionality, are all soluble in water, even when the hydrophobic hexafluorophosphate anion is used. These salts have been developed for their ability to solubilize inorganic salts (LiCl, HgCl2 and LaCb). ... 

Various primary and secondary alcohols can be oxidized to aldehydes and ketones with the ion-supported [bis(acyloxy)iodo]arene 99 in the ionic liquid [emim]+[BF4] (l-ethyl-3-methylimidazolium tetrafluorobo-rate) in the presence of bromide anion [94], or in water in the presence of ion-supported TEMPO [97]. Under similar conditions reagent 99 can be used for mild, efficient, highly selective and environmentally friendly oxidation of aliphatic and aromatic sulfides to sulfoxides in excellent yields [98]. This reaction is compatible with hydroxyl, nitrile, methoxy, carbon-carbon double bonds and ester functionalities. The analogous pyrrolidinium-derived ion-supported [bis(acyloxy)iodo]arenes are efficient oxidants of alcohols to carbonyl compounds in the presence of TEMPO [99]. 

Yang et al. developed a green chemical functionalization method using water and room temperature ionic liquids as the reaction media. By in situ ROP of E-caprolactone from hydroxyl-modified MWCNTs in ionic liquids of l-butyl-3-methylimidazolium tetrafluoroborate, MWCNT-g-PCL was obtained. When the reaction time was increased from 2 to 8 h, the PCL fraction improved from 30.6 to 62.7 wt%. ... 

A recent review by Tang et al. (2012) describes methods of preparation of ionic liquids in which either the cation or the anion beas functional groups such as hydroxyl, ethers (including PEG -(CH2-CH2-0) -CH3 with up to 16), thiols, and others. They discuss their physical, including electrochemical, properties, toxicity and biodegradability, and applications as solvents for organic and enzymatic reactions, as media for capture of CO and SO and for separation of gaseous mixtures. They list 406 references to literature. 

Other commonly used anions for ionic liquids, such as tetrafluoroborate and hexafluorophosphate, also imdergo hydrolysis at sufficiently rapid rates that would cause difficulties for their use with either aqueous solvents or hydroxylic cations. While the hexafluorophosphate anion imdergoes hydrolysis most rapidly only under acidic conditions (Freire et al. 2010), the preparation of the ionic liquids using hexafluorophosphoric add can lead to significant formation of the (hydrolyzed) phosphate salt (Lall et al. 2000). Similarly, the alternative approach for the prepwation of the phosphate salts (liquid ionic phosphates, LIPs) involving the use of 98% phosphoric acid (Lall et al. 2002) similarly experiences difficulty when cations are used that bear hydroxylic sites. Presumably the "anion exchange" method (Lall et al. 2002) would not experience this difficulty, but this process is extremely ineffident for the preparation of any sizable quantity of material. Similarly, the tetrafluoroborate anion imdergoes hydrolysis sufficient rapidly (Freire, et al. 2010) and can be anticipated to read with free hydroxyhc functions such that it would not be a useful anion for ionic hquids based on carbohydrate and carbohydrate-related materials. 

Surfactants are molecules that have both hydrophilic and hydrophobic sections that impart partial affinity toward both polar and nonpolar surfaces. The hydrophobic section of surfactant molecules consists of nonpolar moieties such as repeating carbon-hydrogen (CHj) units or carbon-fluorine (CFj) units. The hydrophilic sections of surfactant molecules contain ionic functional groups such as ammonium (NH4+) or carboxylate (C02"), or they consist of nonionic polar groups such as the hydroxyl portion of alcohol molecules. Thus, surfactants are at least partially soluble in polar liquids such as water as well as nonpolar media such as hexane. 

Su et al. (Xiao et al., 2008) fabricated a new type of thermotropic liquid-crystalline photosensitive supramolecular ionic self-assembly of polyelectrolyte and functional unit azobenzene IL crystal (azo-ILC), where the thermal and phase behaviors can be modulated by changing the spacer length (methylene units in azo). Ma et al. (Ma et al., 2008) found that the addition of very small amounts of an alcohol or water into tri-n-decylmethylphosphonium chloride and bromide salts (IPlOX) induced the formation of liquid crystalline, where strong association between the hydroxyl groups of alcohol or water and the head groups of IPlOX is indicated. 

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