Kinetics of dissociation of M(II)-aza-oxa cryptates (M = Ni, Cu) in weaklyacidic aqueous-acetonitrile media

H+ ion promotes dissociation of nickel(II) and copper(Il) complexes of a he teroditopic aza-oxa cryptand L, derived from tris(2-aminoethyl)amine (tren) . L is a tetradentate ligand and in 50% (v/v) CH3CN-H2O media a tetra acidi c base with ligand protonation constants pK(1) 9.96 +/- 0.004, pK(2) 8.98 /- 0.02, pK(3) 4.22 +/- 0.03 and pK(4) 2.18 +/- 0.02 at 25.0 degreesC; I = 0.10 mol dm(-3). Kinetics for the H+ ion promoted dissociation of both the complexes are first-order in [H+] and complex concentration. Replacement of H2O with D2O affects the rate insignificantly. At 25.0 degreesC; I = 0.10 mol dm(-3) the second order rate constant (dm(3) mol(-1) s(-1)) for the dis sociation of the Ni(II) complex (31.5 +/- 0.9) is larger than that for the Cu(II) complex (18.8 +/- 0.7) but smaller than that for [Ni(tren)(H2O)(2)]( 2+)- a complex thermodynamically much more stable than the Ni(II) cryptate (log K = 8.55 +/- 0.02). The results indicate that 'cryptate effect' signif icantly affects the kinetic lability of transition metal cryptates. (C) 200 1 Elsevier Science B.V. All rights reserved.