POTENTIOMETRIC AND SPECTROSCOPIC STUDY OF COPPER(II) AND NICKEL(II) COMPLEXES OF TRANS-DIOXOPENTAAZA MACROCYCLES IN AQUEOUS-SOLUTION

The trans-dioxopentaaza macrocycles 2,6-dioxo-1,4,7,10,13-pentaazacycl opentadecane (L-1) and 2,6-dioxo-1,4,7,11,14-pentaazacycloheptadecane (L-2) have been prepared. Their protonation and metal-binding properti es with Cu2+ and Ni2+ have been investigated in 0.10 mol dm(-3) KNO3 ( aq) at 25.0 degrees C by potentiometric pH-metry, calorimetry (in part ), electronic absorption and IR spectroscopy. Metal-ion complexation p romotes the deprotonation and the co-ordination of the amide nitrogens , resulting in neutral complexes with four nitrogen donors and a MLH-2 stoichiometry at pH 8. Additional complexes with stoichiometry MHL, M L and MLH-1 complete the complexation set. The overall stability const ants of the complexes were calculated. The electronic spectra support a tetragonal co-ordination of Cu2+ and, square-planar co-ordination of Ni2+ in the MLH-2 complexes leaving one amino group unco-ordinated. I t is suggested that the planar trigonal nature of the coordinated amid o nitrogens in trans position to each other in the equatorial plane ar ound Ni2+ or Cu2+ structurally prevents the fifth nitrogen donor from binding at an axial site. The (NiLH-2)-H-2, complex exhibits a yellow- to-blue conversion equilibrium.