Ionic Liquids with Weakly Coordinating [M(III)(OR(F))4](-) Anions.

Ionic liquids (ILs) are defined as salts with melting points below 100 °C. They attracted much attention in the last two decades due to their unique set of properties, including high conductivities, low viscosities, negligible vapor pressure, and high electrochemical resistance. ILs are seen as tunable systems, of which (also in mixtures) up to 10(19) combinations may exist. These properties make ILs interesting candidates for a variety of fundamental to industrial applications. Our addition to this field was weakly coordinating, little interacting anions, the highly fluorinated aluminates [Al(OR(F))4](-) (R(F) = C(CF3)3, C(CH3), (CF3)2, and CH(CF3)2 and later also CH2(CF3)). We have used these anions in a broad spectrum of applications, including the stabilization of reactive cations, (polymerization) catalysis, and conducting salts for cyclic voltammetry or in electrochemical cells. Especially the [Al(Ohfip)4](-) (hfip = CH(CF3)2) anions in combination with asymmetric organic cations turned out to be very well suited for the synthesis of ILs with very low melting points, some even far below 0 °C. Also the analogous borates, [B(OR(F))4](-), were shown to yield ILs, and currently a plethora of such aluminate and borate ILs have been synthesized and thoroughly investigated. In many aspects, at least the [Al(Ohfip)4](-) ILs present almost ideally noninteracting prototype ILs with (nearly) isotropic but weak and flat Coulomb potential. Consequently, their overall interionic interactions are significantly reduced compared with other classes of ILs, resulting in an extraordinarily low degree, or (for short cation chain lengths below six) even complete absence of ion pairing. From thorough analysis of the principles governing the physical properties of this highly fluorinated IL class with minimized interactions, we were able to learn basic principles that could be extended, for example, to the prediction of the principal properties of a wide variety of typical ILs. In this Account, we give a comprehensive review of their syntheses, thermal and toxicological behavior, physical as well as dynamic properties, and use in electrochemical applications. We delineate advantages and limitations of the [M(III)(OR(F))4](-) ILs developed in our lab and give an outlook on those fields, in which there is still a lack of knowledge.