Synthetic ionophores for cations

Abstract

A series of 14-crown-4 derivatives bearing amide substituents have been prepared in order to develop ionophores selective for lithium. Complexation with lithium ions was monitored using (^13)C and IR spectroscopy and liquid membrane electrodes prepared and evaluated using a fixed interference method. The highest selectivities with respect to sodium ions were obtained for di-n-butylamide-oNPOE and a di-n-benzylamide-oNPOE derivatives; log(^POT)(_ Li,Na) = -2.92 and -2.93 respectively. A series of amide and amide-ester N-functionalised coronands based upon [12]-N202 , [15]-N203 and [18]-N204 parent macrocycles has been prepared. Complexation of certain alkali and alkaline-earth cations was monitored by (^13)C NMR and IR specteoscopy, enthalpies of complexation measured in methanolic solution using micro-calorimetry and stability constants measured in aqueous media by potentiometric methods. Strong complexation of Ca2+ in aqueous media was observed with good selectivity over Na+ and K+. Lower free energies of binding for Ha cations were displayed by the tertiary amide derivative (of [12]N(_2)O(_2)), than by its secondary analogue, despite displaying higher enthalpies of complexation. Thus the lower free energies result from significantly lower entropies of complexation. Two sets of oxa-amide and oxa-ester tripodal ligands have been prepared and solvent membranes fabricated. The effects of the nature of the plasticiser, bis (butylpentyl)adipate (BBPA) verses o -nitrophenyl octyl ether (oNPOE), the ligand structure and the ionic strength of the analyte solution on the electrode response to la and Ila cations were studied. The performance of the oxa-amides was superior to that of the oxa-esters particularly at higher ionic strengths, however super-Nernstian responses were observed with the more charge-dense ions in the presence of chloride and/or with the less polar plasticiser BBPA. Measurements of intracellular sodium concentration could be effected with a sensor based upon bis (N,N',N"-tributyl)-4,4',4"-propylidintris(3-oxabutanamide) and oNPOE for which -log KPOT = 2.64 and -log KPOT =3.0, whilst bis (N,N',N"-tributyl)Na,K Na,Mg-2,2',2"-phenylmethylidintris(3-oxabutamide) and oNPOE functioned as a calcium sensor, displaying excellent selectivity over Mg. -log KPOT= 4.3.Ca,MgAdditionally a triamide based upon a cyclohexane triol skeleton was prepared, a membrane fabricated and its performance assessed as a sodium sensor.