Synthesis and Characterization of Imidazolium Salts with the Weakly Coordinating [B(C6F5)4]--.4 Anion

Weakly coordinating anions (WCAs) have been a subject of intensive research in the past decades due to their increasing importance in coordination chemistry and catalysis [1 – 6]. These applications have grown out of basic research into the special properties of anions containing hydrocarbon and fluorocarbon functionality such as [SO3CF3], [B(C6F5)4], [OTeF5] [7]. Owing to the importance of WCAs both in fundamental and applied chemistry, a new class of WCAs has recently been developed. Robert et al. reported the anions [(C6F5)3M-LN-M(C6F5)3], with M = B or Al and the linking group LN being azide, dicyanamide, and imidazolide, which were found to be the most stable and effective activators for olefin polymerization reactions [8, 9]. [CB11H12] and related carborane anions developed by Reed et al. were applied in many systems, despite the expensive and time-consuming multistep procedure of their preparation [10]. The [B(C6F5)4] anion, commonly abbreviated as BArF, is much more stable as compared to the other anions and frequently used as a counteranion for electrophilic cations [7, 11]. It can be introduced by salt metathesis reaction of reagents such as Li[B(C6F5)4], which was first reported in 1963 [12], [Mn(NCCH3)6][B(C6F5)4]2 [13], [H(OEt2)2][B(C6F5)4] [14], [Li(OEt2)4][B(C6F5)4] [15], [HNBu3][B(C6F5)4] [16], [Bu4N][B(C6F5)4] [17], the superacid [C6Me3H4][B(C6F5)4] [18], [Cp2Zr(CH3)][B(C6F5)4] [19, 20], [Ag(NCCH3)4] [B(C6F5)4] [21], and [M(NCCH3)6][B(C6F5)4] (M = Cr, Fe, Co, Ni, Cu, Zn) [22, 23] with labile or sometimes even covalently bound halides. Recently, imidazolium-based ionic liquids (ILs) have received considerable attention in different research fields [24 – 30]. The synthesis of ILs bearing [B(C6F5)4] anions and organic cations such as imidazolium could represent an approach to combine the advantages of unreactive and stabilizing WCAs and eco-friendly ILs with low vapor pressure. Since [B(C6F5)4]-based organic salts with imidazolium cations are quite rare [31, 32], we focused on the synthesis of new ionic compounds consisting of imidazolium cations and [B(C6F5)4] anions via metathesis reactions. In this report, we present the precursors [Li(NCCH3)4][B(C6F5)4] (1) and [Ag(NCCH3)4][B(C6F5)4] (2) as metathesis reagents for the conversion of imidazolium bromides [Bmim]Br and [Dbmim]Br (Bmim = 1butyl-3-methylimidazolium; Dbmim = 1,2-dimethyl3-butylimidazolium) to yield the imidazolium-based salts [Bmim][B(C6F5)4] (3) and [Dbmim][B(C6F5)4] (4) with the [B(C6F5)4] anion.