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

The Ni(II) complex of 6,6'-bis(hexylbenzoylamino)-2,2'-bipyridine (LH( 2)) in square-planar symmetry underwent a reversible and simultaneous deprotonation of the two amide units. The non-deprotonated complex (Ni LH(2)(2+)) showed high SCN- selectivity by the preferential axial coor dination of two SCN- molecules (log K-SCN = 8.11 +/- 0.03 in CH2Cl2 at 20 degrees 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 u p-hill transport across a CH2Cl2 liquid membrane by coupling with the symport of protons from pH 4.0 to 6.0 aqueous solutions. Equilibrium a t the aqueous/organic interface could be represented by the equation K -app = ([Ni(LH(2))(SCN)(2)](org))/( [NiL](org)[H+](2)(aq) [SCN--](2)(a q)), where K-app is the equilibrium constant (log K-app = 13.6 at 20 d egrees 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 me chanisms of the transport are discussed.