Basic ionic liquids characterization

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

The tendency to form covalent rather than ionic bonds is crucial in the formation of hypervalent compounds. Incorporation of fluoride as a ligand usually stabilizes covalent species. For example, PF5 is covalent. However, PCI5 is characterized by the equilibria shown in equations (18) and (19). In the gas and liquid phases, the molecular form exists, while in the solid state it is ionic, consisting of PCI4+ and PClg" ions. In solution in nonpolar solvents, the molecular form is favored, while the ionic form described in equation (18) predominates in polar solvents. At low concentrations, some PCI4+ and Cl may be present owing to the equilibrium in equation (19). For equation (20) (R = alkyl and aryl), if L is a reasonably basic anion such as Cl, Br, I, or OPh, the ionic phosphonium salt (20) is favored. With L = F or C>-alkyl, the... 

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