Synthetic macrocyclic Ionophores

Abstract

14-Crown-4 derivatives bearing either one or two oxymethyl, benzyl- oxymethyl, methoxycarbonylmethyl or carbamoylmethyl substituents have been prepared in an attempt to obtain selective ionophores for lithiumions. Complexation has been monitored by IR, (^13)C NMR, Fast Atom Bombardment Mass Spectrometry, and solvent polymeric membranes have been fabricated and evaluated using the fixed interference method. Improved lithium selectivities of the disubstituted 14-crown-4 ligands compared to the monosubstituted analogues in the potentiometric experiments, bears out the premise that there is a need to suppress competitive 2:1 complex formation with sodium. The most encouraging lithium selectivity was observed with an electrode based on trans- (2S,3S)(-)-2,3-bis(N,N'- dibutylcarbamoylmethyl)-1,4,8,11- tetraoxacyclo- tetradecane which vindicates the choice of axial amide donors to enhance the Li/Na selectivity of the 14-crown-4 skeleton. Triazacyclononane, triazacyclododecane and tetraazacyclododecane amide functionalised ligands were prepared in order to investigate the effect of donor number, macrocyclic ring size and chelate ring size on stability and selectivity of complexation with alkali and alkaline earth metal cations. Complexation has been monitored by IR, i3C NMR, Fast Atom Bombardment Mass Spectrometry and pH-metric titration experiments. The most stable complex was formed between calcium and N,N'-dimethyl- 1,4,7,10-tetraacetamido-1,4,7,10-tetraazacyclododecane, for which log K(CaL) =6.81 (H(_2)0). Once again, measurable log K values ≥ 3.9 in aqueous media for lithium, sodium and calcium complexation vindicate the choice of amide donors to ensure strong coordination with small cations of high charge density ratio.