Progress in Paramagnetic Ionic Liquids

Abstract

Ionic liquids are entirely composed of ions as the name implies, and melt below room temperature (RT) or 100 °C. The ionic character means the extraordinary high ion density (i.e., the order of a molarity), and thus results in the negligible vapor pressure (i.e, wide liquid temperature region and negligible flammability) and high ionic conductivity. Such special fascinations attract considerable attention of researchers in many fields, as promising greener alternatives to the volatile molecular solvents for many areas of synthetic, separation, and electrochemical applications (Wasserscheid & Keim, 2000; Dupont et al., 2002; Buzzeo et al., 2004; Armand et al., 2009; Yoshida & Saito, 2010). Although ionic liquids have been known for nearly a century (Walden, 1914; Gabriel & Weiner, 1888), efforts in exploring new and more versatile ionic liquids have only recently been devoted, after the discovery of the first waterand air-stable RT ionic liquids formed with 1-ethyl-3methylimidazolium (C2MI; Scheme 1) cation reported by Wilkes and Zaworotko (Wilkes & Zaworotko, 1992), and Cooper and O’Sullivan (Cooper & O'Sullivan, 1992), in 1992. The majority of existing ionic liquids is composed of organic quaternary cations (e.g., 1,3dialkylimidazolium, N-alkylpyridinium, and tetraalkylphosphonium) and inorganic small anions (e.g., BF4–, PF6–, and AlCl4–), for which monovalent ions are favorable for stabilizing the liquid state because of the depressed interionic Coulomb interactions. In general, cations are responsible for reducing the melting point, and therefore, low-symmetrical hetero cations with well-delocalized charge and/or long alkyl group(s) have been used. On the other hand, the selection of anions can readily tailor the liquid properties and introduce the desired functionalities. For example, the combination with PF6 and (CF3SO2)2N (bis(trifluoromethanesulfonyl)amide; Tf2N) anions gives the hydrophobic ionic liquids (Bonhote et al., 1996; Suarez et al., 1998), and CH3COO and EtOSO3 anions stabilize the liquid state even at low temperatures (Wilkes & Zaworotko, 1992; Holbrey et al., 2002; Borra et al., 2007). These properties could find a range of synthetic and separation applications. In relation, HCOO and (MeO)(R)PO2 (R = H, Me, MeO) anions give ionic liquids, which can solubilize cellulose (Fukaya et al., 2006; Fukaya et al., 2008). BF4, Tf2N and (FSO2)2N anions give ionic liquids with a wide electrochemical window, which opens the applicative way for electrolytes of capacitors, lithium ion batteries, field-effect transistors, and electrodeposition (Bonhote et al., 1996; Fuller et al., 1997; Sato et al., 2004; Zein El Abedin et al., 2005; Matsumoto et al., 2006; Ono et al., 2009). Highly conducting ionic liquids formed with N(CN)2, C(CN)3, and B(CN)4 anions have served as electrolytes for dyesensitized solar cells (MacFarlane et al., 2001; Wang et al., 2003; Yoshida et al., 2004; Kawano et al., 2004; Wang et al., 2005; Kuang et al., 2006; Yoshida et al., 2007a; Yoshida et al., 2007b).