With additional coordinating groups Sulfonic acid

Mixtures of carriers (ionic additiues). Cotransport of opposite-charged ions is the most obvious way to maintain electroneutrality, but alternative means may be explored as additives to LM. In recent years, many studies have been conducted which examine the use of anionic membrane additives for maintenance of electroneutrahty at cation transport [65, 84-89]. The anionic additives are typically fipophific carboxylic, phosphoric, or sulfonic acids. Cation or neutral macrocychc carriers coupled with anionic additives result in a synergistic transport of cations which exceeds that accomplished by each component individuaUy. This synergism was demonstrated in Ref. [90]. The authors observed a 10- to 100-fold enhancement of copper extraction. Enhanced extraction is achieved by adding the anionic group to the cation coordination macrocycle. 

Despite the ability of carbenes to form stable complexes with main group Lewis acids, the direct synthesis of stable carbene-S02 adducts by coordination of the free carbene with SO2 has been elusive. In contrast, when 2-chloroimidazolium chlorosulfite salt 243 reacted with aqueous KCN, the zwitterionic complex 244 was formed (Scheme 5.33). Complex 244 reacted readily with strong acids HX (X = BF4, SbFg) to give NHC-sulfonic acids 245. Interestingly, while carbene-S02 adducts eluded synthetic isolation, 2-chloroimidazolium salts could be prepared by addition of SO2 to NHC-CI2 charge transfer complexes vide infra). 

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