Stability of Superoxide Ion in Imidazolium Cation-Based Room-Temperature Ionic Liquids

Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G1-5, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan.
The Journal of Physical Chemistry A (Impact Factor: 2.69). 02/2009; 113(5):912-6. DOI: 10.1021/jp807541z
Source: PubMed


The stability of superoxide ion (O(2)(*-)) generated chemically by dissolving KO(2) in dried dimethyl sulfoxide solutions containing imidazolium cation [e.g., 1-ethyl-3-methylimidazolium (EMI(+)) and 1-n-butyl-2,3-dimethylimidazolium (BMMI(+))] based ionic liquids (ILs) was investigated with UV-visible spectroscopic, NMR, and voltammetric techniques and an ab initio molecular orbital calculation. UV-visible spectroscopic and cyclic voltammetric measurements reveal that the O(2)(*-) species reacts with BMMI(+) and EMI(+) cations of ILs to form hydrogen peroxide. The pseudo first order rate constant for the reaction of BMMI(+) and O(2)(*-) species was found to be about 2.5 x 10(-3) s(-1). With a molecular orbital calculation, the O(2)(*-) species is understood to attack the 2-position (C-2) of the imidazolium ring (i.e., BMMI(+)) to form an ion pair complex in which one oxygen atom is bounded to C-2 and the other to the hydrogen atom of -CH(3) group attached to C-2. Eventually, the ion pair complex of BMMI(+) cation and O(2)(*-) species undergoes a ring opening reaction as evidenced with (1)H NMR measurement.

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    • "Other research groups reported that O ÁÀ 2 is not stable in imidazolium based ILs [41] [46] [54]. Islam et al. (2009) demonstrated that the chemically generated O ÁÀ 2 in imidazolium cation based ILs reacts with the cation to form H 2 O 2 [54]. Whereas, AlNashef et al. (2010) found that the superoxide ion reacts with the imidazolium cation of many ILs to give the corresponding 2-imidazolones and H 2 O 2 [41]. "
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    ABSTRACT: The superoxide ion electrochemically generated by reduction of oxygen, or chemically generated by dis-solving potassium superoxide in ionic liquids, reacts with alkyl imidazolium cations of imidazolium-based ionic liquids at room temperature and atmospheric pressure to give the corresponding 2-imidazol-ones in excellent yields. 2-Imidazolones, for example, 1-butyl-3-methylimidazolone, are useful as intermediates for polymers, agrochemicals, and pharma-ceutical compounds. 1 In addition, they possess interesting biologi-cal activities. Several derivatives of 2-imidazolone are biotin antagonists in biological systems, amongst which are compounds capable of inhibiting the growth of malignant tumors. 2 Moreover, some 2-imidazolone compounds are anti-inflammatory agents and are useful for the treatment of dermatitis, inflammation of joints, and similar conditions which are usually responsive to known anti-inflammatory agents. They also possess antipyretic and analgesic properties and are thus particularly useful in the treatment of inflammatory conditions in which an increase in body temperature and pain, or excessive discomfort are present. These compounds appear to be well tolerated and do not cause undesir-able side effects such as allergic reactions. 3 The simple heterocycle, 1,3-dihydro-2H-imidazol-2-one has proved to be a promising building block for 1,2-diamine skeletons, 4 which are found as structural units in bioactive compounds of medicinal interest, 5 and chelating ligands for metal catalysts. 6 Seo et al. reported on the preparation of versatile chiral synthons for 1,2-diamines, (4S,5S)-, and (4R,5R)-4,5-dimethoxy-2-imidazo-lidinone derivatives, using 1,3-dihydro-2H-imidazol-2-one as the starting material. 7 They showed that both methoxy groups of the synthons could readily be replaced with primary to tertiary alkyl groups and aryl moieties with full retention of configuration in a stepwise manner. Subsequent ring opening provides a versatile route to optically active threo-1,2-diamines. However, it should be noted that the reported synthesis of the chiral synthons involves an efficient, but rather tedious optical resolution step. Kuneida re-viewed the use of 1,3-dihydro-2H-imidazol-2-one as a building block for chiral polyfunctional compounds as well as chiral hetero-cyclic auxiliaries for asymmetric synthesis. 8 Simple five-membered heterocycles such as 2-oxazolone, 1,3-dihydro-2H-imidazol-2-one, and 2-thiazolone show interesting behavior toward polymeriza-tion and condensation, leading to homopolymers and telomers, as well as condensation reagents. 9 Several synthetic procedures for 2-imidazolones have been reported in the literature. 10 Lipshutz et al. showed that N-SEM-protected imidazoles could be sequen-tially derivatized at the 2-and 5-positions in a one-pot operation. Quenching with selected peroxides following initial lithiation led directly to imidazolones. 11 All the reported synthetic procedures are complicated and utilize expensive reagents. In addition, the maximum yield in most cases was less than 80%. 12 Generation of the superoxide ion (O 2 ÅÀ) by electrochemical reduction of oxygen in aprotic solvents and ionic liquids (ILs) has gained increased importance due to its many potential applica-tions, Eq. (1). 13–31 O 2 þ e À ! O 2 ÅÀ ð1Þ We were the first to report the generation of stable O 2 ÅÀ in imi-dazolium-based ILs. 20,21 The O 2 ÅÀ ion was then used for the destruction of chlorinated hydrocarbons, reduction of carbon diox-ide, and the synthesis of ketones and carboxylic acids from the cor-responding secondary and primary alcohols, respectively. It was reported in the literature that the positive charge density on the carbon atom at position 2 (C-2) in the imidazolium ring of ILs is greater than that at the other positions. 26 Various research groups have observed that electro-generated O 2 ÅÀ undergoes ion pairing with the imidazolium cation of ILs and that the degree of associa-tion of O 2 ÅÀ with the 1-ethyl-3-methylimidazolium cation in di-methyl sulfoxide is comparable to that of O 2 ÅÀ with H 2 O. 30 Such ion pairing may ultimately lead to nucleophilic attack on C-2 of the imidazolium cation by O 2 ÅÀ to form a complex or a new prod-uct. 32–34 Islam et al. studied the stability of the superoxide ion generated chemically by dissolving KO 2 in anhydrous dimethyl-sulfoxide containing imidazolium cation-based ILs using UV–visi-ble, NMR, and voltammetric techniques, and an ab initio molecular orbital calculation. 35 UV–visible spectroscopic and cyclic voltammetric measurements revealed that the O 2 ÅÀ species reacted 0040-4039/$ -see front matter Ó
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