Effect of structural variations within lipophilic dibenzocrown ether carboxylic acids on the selectivity and efficiency of competitive alkali-metal cation solvent extraction into chloroform

Abstract

Competltive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by a series of lipophilic dlbenzocrown ethers wlth pendant carboxylic acld groups has been Investigated. Both the extraction selectivity and efflciency are Influenced by variations of the crown ether ring size and the attachment site of the ilpophillc group. Preorganlzation of the binding site by proper positioning of the ilpophlilc group enhances extraction seiectlvity. Very high Na' extraction selectivity (Na+/K+ = 32, Na+/LI+ = 66 and no detectable extraction of Rb' or Cs') was obtained with sym-decyldbenzo-16-crown-5-oxyacetic acld. INTRODUCTIONIn his recent review of crown ethers as solvent extraction agents for metal ions, McDowell points out that when a proton-ionizable group is attached to the cyclic polyether framework, the molecule is both a cation exchanger and a coordinator ( I ) . This arrangement has the potential for providing an extraction system with greater selectivity and efficiency than one in which an organophilic acid is simply mixed with a crown ether.In earlier work (2,3), we examined competitive alkali-metal cation extraction from aqueous solutions into chloroform by dibenzocrown ether carboxylic acids 1-3. I t was found that these proton-ionizable ionophores were of insufficient lipophilicity to remain completely in the organic phase during extraction of alkali-metal cations from alkaline aqueous phases. To avoid such complications in extraction behavior, a lipophilic group was attached either to each benzene ring or to the carboxylic acid containing sidearm of the dibenzo-16-crown-5 compound 2 to produce the lipophilic dibenzo-16-crown-5-carboxylic acids 4 and 5, respectively (3-5). Compounds 4 and 5 were found to be sufficiently lipophilic to remain completely in the chloroform phases even when the contacting aqueous solutions of alkali-metal cations were highly alkaline (3,5). Although the overall alkali-metal cation extraction behavior was similar for structural isomers 4 and 5, the former gave enhanced Na+/K+ selectivity and excluded Li+ from the chloroform phase (5). Hence the lipophilic group attachment site was found to exert some influence upon extraction behavior.The efficiencies and selectivities of competitive alkali-metal cation extraction into toluene (6) and of alkali-metal cation transport across chloroform and toluene liquid membranes

Poly[poly(oxypropylene) phosphate] macroionophores for transport and separation of cations in a hybrid: Cation‐exchange polymer and liquid membrane system

“…Alternatively, these two PPOPP components can cooperate in the way such that ionic groups mediate an easy access to those parts of the macroionophore that are responsible for their selective recognition. Such a mechanism was described by Alexandratos et al 42 as operating in some functionalized polymeric sorbents, and by other authors [43][44][45] as the mechanism mediated by ionizable lariat-type crown ethers in liquid membranes. In view of these considerations the basic aim of our investigation was to verify the following hypotheses:…”

Sterically congested, geminal aryl-substituted, proton-ionizable sym-dibenzo-16-crown-5 lariat ethers: synthesis and alkali metal cation extraction

“…Extraction results for the three sym-(aryl)dibenzo-16-crown-5-oxyacetic acids 6, 7, and 19 with geminal 1-naphthyl, 2-naphthyl, and 9-phenanthryl groups, respectively, are presented in Figure 4. As is readily evident, all three ligands are highly selective extractants for Na 6 Thus replacement of the geminal linear alkyl group in the lariat ether carboxylic acids with geminal 1-naphthyl, 2-naphthyl, and 9-phenanthryl units produces a marked enhancement in the selectivity for Na + extraction. Alkali metal cation loading is nearly quantitative for formation of 1:1 ionized lariat ether-metal ion extraction complexes.…”

“…Such high extraction selectivity for Na + strongly suggests simultaneous metal ion complexation by the polyether oxygens and the oxyacetic acid side arm. 6,9 A qualitative measure of acidity for proton-ionizable ligands is pH 0.5 , the aqueous phase pH at which half of the maximal metal ion loading is reached. 15 For lariat ether carboxylic acids 6, 7, and 19, the same pH 0.5 value of 7.0 was observed.…”

“…[4][5] During a metal ion separation process, the presence of lipophilic groups on the crown ether ring is important in reducing loss of the macrocyclic ligand from an organic phase into a contacting aqueous phase. 6 The dibenzo-16-crown-5 ring system provides a convenient scaffold for investigating the influence of structural variation within lariat ethers upon their selectivity and efficiency in metal ion complexation processes due to its synthetic accessibility with pendant groups attached to the central carbon of the three-carbon bridge. [7][8] In earlier studies, we found that the introduction of a lipophilic decyl group geminal to the functional side arm in symdibenzo-16-crown-5 oxyacetic acid ( Figure 1) increased the Na + selectivity by preorganization of the binding site in which the alkyl group oriented the oxyacetic acid side arm over the polyether cavity.…”

“…Various alkyl groups have been attached geminal to the proton-ionizable side arm in symdibenzo-16-crown-5 ethers to form effective and selective extractants for alkali metal cations. [5][6] In comparison, analogues with more sterically demanding geminal aryl groups have received very little attention. We now report the synthesis of three series of proton-ionizable sym-dibenzo-16-crown-5 ethers with geminal 1-naphthyl, 2-naphthyl, and 9-phenanthryl groups ( Figure 2 Variation of the proton-ionizable side arms include oxyacetic acid moieties and N-(X)sulfonyl oxyacetamide units with X = Me, Ph, C 6 H 4 -4-NO 2 , and CF 3 .…”