Allosteric Binding of Alkali Metal Ions to a Pseudocryptand Formed by a C-Pivot Tripodal Ligand Containing 3-Hydroxy-2(1H)-pyridinone and Ga(III)

Abstract

A novel C-pivot tripodal hexadentate ligand (3,2-HOPOHL) composed of 3-hydroxy-2(1H)-pyridinone as a bidentate ligand, the ethyleneoxy chain as a spacer, and tris(carboxyric acid) as an anchor was synthesized. 3,2-HOPOHL recognized only Na + ion, suggesting that it pre-organized a cavity due to the electrostatic interaction among the 2(1H)-pyridinone rings. UV-VIS spectroscopic analysis indicated that 3,2-HOPOHL formed a stable intramolecular 1:1 Fe(III) complex in aqueous solution. The stability constant (log K) of 3,2-HOPOHL-Fe(III) complex was estimated to be 27.6 from the competitive reaction with EDTA. 1 H-NMR titration of 3,2-HOPOHL-Ga(III) complex with Na + and K + ions in CDCl 3 -CD 3 CN indicated the formation of 1:1 complexes. The binding constants of Na + - and K + -3,2-HOPOHL-Ga(III) complexes were estimated to be 3.3×10 3 and 7.8×10 3 M - 1 , respectively, the ion selectivity of K + toward Na + being more than two-fold.