Nickel(II) Complex of 6,6′-Bis(hexylbenzoylamino)-2,2′-bipyridine as a Functional Carrier: pH-Induced Affinity-Switching and Mechanism of the SCN−-Selective Up-Hill Transport

Abstract

Abstract The Ni(II) complex of 6,6′-bis(hexylbenzoylamino)-2,2′-bipyridine (LH2) in square-planar symmetry underwent a reversible and simultaneous deprotonation of the two amide units. The non-deprotonated complex (NiLH22+) showed high SCN− selectivity by the preferential axial coordination of two SCN− molecules (log KSCN = 8.11 ± 0.03 in CH2Cl2 at 20 °C), while the deprotonated complex showed little affinity toward anions. Deprotonation-induced affinity-switching of the complex by the external aqueous layer mediated an efficient SCN−-selective up-hill transport across a CH2Cl2 liquid membrane by coupling with the symport of protons from pH 4.0 to 6.0 aqueous solutions. Equilibrium at the aqueous/organic interface could be represented by the equation Kapp = ([Ni(LH2)(SCN)2]org)/([NiL]org[H+]2aq[SCN−]2aq), where Kapp is the equilibrium constant (log Kapp = 13.6 at 20 °C). However, the rates of protonation and deprotonation of the complex at the interface were dependent mostly on the concentration of the complex in the organic layer. Based on these results, detailed mechanisms of the transport are discussed.